BIREFRINGENCE GRADIENT DEVELOPMENT DURING DRYING OF SOLUTION CAST
FUNCTIONAL FILMS AND THEIR MECHANICAL, OPTICAL
AND GAS BARRIER PROPERTIES
A Dissertation
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Doctor of Philosophy
Orcun Yucel
December, 2013
BIREFRINGENCE GRADIENT DEVELOPMENT DURING DRYING OF SOLUTION CAST
FUNCTIONAL FILMS AND THEIR MECHANICAL, OPTICAL
AND GAS BARRIER PROPERTIES
Orcun Yucel
Dissertation
Approved: Accepted:
Advisor Department Chair Dr. Mukerrem Cakmak Dr. Robert Weiss
Committee Member Dean of the College Dr. Mark D. Soucek Dr. Stephen Z. D. Cheng
Committee Member Dean of the Graduate School Dr. Robert Weiss Dr. George R. Newkome
Committee Member Date Dr. Matthew Becker
Committee Member Dr. Gerald W. Young
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ABSTRACT
For the first time, the development of optical anisotropy gradient as a result of solvent evaporation for poly (amide-imide) (PAI) solution in Dimethylacetamide
(DMAc) was investigated. Experiments were carried out using real time optical measurement with spectral birefringence technique coupled with off-line optical techniques such as Abbe refractometer and optical compensator method. Drying process induced temporal evolution of non-uniform out of plane birefringence profile through the thickness direction while in plane birefringence remained zero. The highest birefringence was observed at the substrate-solution interface at early stages of drying. Beyond a critical time, the formation of highly oriented layer was observed at the air-solution interface. This oriented layer progresses through the thickness direction as the solvent concentration is disproportionately reduced in these regions. Abbe refractometer results confirmed the anisotropy is preserved at longer drying times, air-solution interface birefringence becoming higher compared to substrate-solution interface. Overall, observations obtained by real-time measurement system agreed with off-line measurements.
In additon, multifunctional single and triple-layer films exhibiting flexibility, high modulus and high gas barrier properties were developed using a soluble polyamide-imide
(PAI) in dimethylacetamide (DMAc) with ammonium-modified montmorillonite (MMT,
Cloisite 30B) mineral clay. The drying behavior and associated anisotropy development
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were determined real-time, using a newly developed real-time measurement system. Out- of-plane birefringence development occurred earlier for thinner neat samples caused by increased depletion rate of solvent. Addition of organoclay content resulted in a decrease in evaporation rate of solvent due to planar orientation of well exfoliated nanoplatelets as shown by TEM images and WAXS. This is in agreement with developed out-of-plane anisotropy during drying. Planar orientation of nanoplatelets resulted in excellent helium- barrier properties. Mechanical properties were optimized at 3wt% clay content.
In a similar way, multifunctional nanocomposite films exhibiting flexibility, high modulus and high gas barrier properties were developed using a soluble polyamide-imide
(PAI) in dimethylacetamide (DMAc) with graphene-oxide nanosheets (GO). Addition of
GO content resulted in increase in evaporation rate of solvent. This was attributed to increase in hydrophobicity of the films with increased GO content as shown by contact angle measurements. Overall He permeability of dried hybrid films decreased over 40% even with very small GO content.
Multi-layered optical retarder film exhibiting low birefringence dispersion and high optical clarity was developed using a solutions of polysulfone (PSF), polycarbonate- co-polymer (PCC) and a-tactic polystyrene (PS) in N-methyl pyrrolidone (NMP). The uniaxial and biaxial deformation behavior and associated anisotropy development were determined real-time, using a newly developed real-time measurement system. Machine
Direction (MD) stretching resulted in negative retardation values at high deformation rates. This behavior was reversed upon inception of Transverse Direction (TD) stretching. Optimum Rth and R0 values were achieved at 1mm/sec stretch rate to
compensate ECB-LCDs. Birefringence dispersion of films was found to be flattened.
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ACKNOWLEDGEMENTS
I owe my deepest gratitude to my advisor, Distinguished Professor Mukerrem
Cakmak for his guidance, patience, encouragement and support throughout this study. It was a remarkable experience which will benefit me for the rest of my life.
I would like to extend my sincere gratitude to the committee members, Professor
Robert Weiss, Matthew Becker, Mark D. Soucek and Gerald W. Young for discussions and directions during the preliminary stage of this research.
I would like to thank Dr. Dan Jones in the Lockheed Martin Corporation and John
Harvey and Matt Graham in Akron Polymer Systems for their help with the PAI preparation and characterization of the dried films.
Also I would like to thank all the past and present members in Professor
Cakmak’s research group during my research. It was an enjoyable journey working with all of you. Thank all of you for hosting so many happy gatherings.
Finally, I would like to dedicate this dissertation to my parents, for their unconditional love, understanding and support.
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TABLE OF CONTENTS
LIST OF TABLES……………………………………………………………………..…xi
LIST OF FIGURES…………………………………………………………………...…xii
CHAPTER
I. INTRODUCTION ...... 1 II. LITERATURE REVIEW ...... 6 2. 1 OPTICAL METHODS ...... 6
2.1.1 Basics of Theory of Light ...... 6
2.1.2 Interaction of the Light with the Medium ...... 8
2.1.3 Retardation and Birefringence ...... 10
2.1.4 Determination of Optical Parameters ...... 13
2.1.5 Optical Dispersion & Chromaticity ...... 22
2.2 LIQUID CRYSTAL MATERIALS ...... 24
2.2.1 Liquid Crystals ...... 24
2.2.2 Physical Properties of Liquid Crystals ...... 28
2.2.3 Optics of Liquid Crystals ...... 30
2.3 SPECTRAL BIREFRINGENCE TECHNIQUES ...... 32
2.3.1 Single Wavelength Methods ...... 33
2.3.2 Dual Wavelength Methods ...... 34
2.3.3 Multi Wavelength Methods ...... 35
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2.3.4 Continuous (on-line) Wavelength Methods ...... 37
2.4 LIQUID CRYSTAL DISPLAYS (LCDS) ...... 40
2.4.1 Basics of LCDs ...... 40
2.4.2 Addressing Methods for LCDs ...... 42
2.4.3 Types of LCDs ...... 44
2.4.4 Components of LCDs ...... 51
2.5 WIDE ANGLE OPTICAL RETARDER ...... 53
2.5.1 Basics of Retarder Films ...... 53
2.5.2 Optical Retarder Film Designs ...... 57
2.5.3 Advantages of Multilayer Optical Retarders ...... 63
2.5.4 Methods of Manufacturing for Multilayer Retarder Films ...... 64
2.6 SOLUTION CASTING PROCESS ...... 66
2.6.1 Process Overview ...... 66
2.6.2 Residual Solvent ...... 68
III. TEMPORAL EVOLUTION OF OPTICAL GRADIENTS DURING DRYING IN CAST POLYMER SOLUTIONS ...... 70 3. 1 INTRODUCTION ...... 70
3.2 EXPERIMENTAL ...... 74
3.2.1 Materials ...... 74
3.2.2 Optical Measurements ...... 74
3.2.3 Real-time Weight, Thickness, Temperature & Birefringence Measurements. 76
3.3 RESULTS AND DISCUSSION ...... 76
3.3.1 Refractive Index Measurements through Abbe Refractometer ...... 76
3.3.2 Birefringence Calculations through Compensator Technique ...... 78
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3.3.3 Drying Data through Real-time Measurement System ...... 81
3.3.4 Comparison of Optical Techniques ...... 82
3.4 CONCLUSIONS ...... 85 IV. ENHANCED GAS BARRIER AND MECHANICAL PROPERTIES IN ORGANOCLAY REINFORCED MULTI-LAYER POLYAMIDE-IMIDE NANOCOMPOSITE FILM ...... 86 4. 1 INTRODUCTION ...... 86
4.2 EXPERIMENTAL ...... 89
4.2.1 Materials ...... 89
4.2.2 Polymer Solution Preparation ...... 89
4.2.3 Rheology ...... 90
4.2.4 Simultaneous Solution Casting Process ...... 90
4.2.5 Real time Weight, Thickness, Temperature & Birefringence Measurements .. 91
4.2.6 Characterization ...... 92
4.2.7 Mechanical & Permeability Testing ...... 93
4.3 RESULTS AND DISCUSSION ...... 93
4.4 CONCLUSIONS ...... 108
V. ENHANCED GAS BARRIER AND MECHANICAL PROPERTIES IN GRAPHENE-OXIDE REINFORCED POLY(AMIDE-IMIDE) NANOCOMPOSITE FILMS ...... 109 5. 1 INTRODUCTION ...... 109
5.2 EXPERIMENTAL ...... 112
5.2.1 Materials ...... 112
5.2.2 Polymer Solution Preparation ...... 113
5.2.3 Simultaneous Solution Casting Process ...... 113
5.2.4 Real time Weight, Thickness, Temperature & Birefringence Measurements 114
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5.2.5 Characterization ...... 115
5.2.6 Mechanical & Permeability Testing ...... 116
5.3 RESULTS AND DISCUSSION ...... 116
5.4 CONCLUSIONS ...... 126
VI. MULTI-LAYER OPTICAL RETARDER FILM WITH LOW BIREFRINGENCE DISPERSION AND ITS MECHANO-OPTICAL BEHAVIOR ...... 127 6. 1 INTRODUCTION ...... 127
6.2 EXPERIMENTAL ...... 129
6.2.1 Materials ...... 129
6.2.2 Solution Preparation and Casting ...... 130
6.2.3 Rheology ...... 132
6.2.4 Thermal Analysis ...... 132
6.2.5 Refractometry ...... 133
6.2.6 Microscopy ...... 133
6.2.7 Online Birefringence and Stress-Strain Measurements through Biaxial
Stretcher ...... 133
6.3 RESULTS AND DISCUSSION ...... 136
6.4 CONCLUSIONS ...... 148
VII. SUMMARY AND RECOMMENDATIONS ...... 149 REFERENCES ...... 152 APPENDICES ...... 165 APPENDIX A: SUPPORTING INFORMATION FOR CHAPTER III ...... 166
A.1 Birefringence Profile Correlation with Percent Solid Content ...... 166
A.2 Integration Process for Birefringence Calculation through Compensator Method
...... 167
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A.3 Lowest Energy Conformational State Calculation ...... 169
APPENDIX B: DESIGN AND BUILT OF REAL-TIME MEASUREMENT SYSTEM FOR TRACKING WEIGHT, THICKNESS, BIREFRINGENCE ...... 170
B.1 System Design ...... 170
B.1.1 Main Body ...... 170
B.1.2 Thermal System ...... 172
B.1.3 Optical System ...... 174
B.2 Aerodynamics Testing ...... 177
B.3 Thermal Expansion Testing & Substrate Selection ...... 180
APPENDIX C: THE EFFECT OF CO-SOLVENT & ORGANOCLAY CONTENT ON OPTICAL PROPERTIES OF THIN FILMS ...... 183
C.1 Introduction ...... 183
C.2 Experimental ...... 185
C.2.1 Materials ...... 185
C.2.2 Polymeric Solution Preparation ...... 185
C.2.3 Rheology ...... 186
C.2.4 Simultaneous Solution Casting Process ...... 186
C.2.5 On-line Weight, Thickness, Temperature & Birefringence Measurements .. 187
C.3 Results and Discussion ...... 187
C.4 CONCLUSIONS ...... 197
APPENDIX D: THE EFFECT OF MUTUAL DIFFUSION COEFFICIENT ON THICKNESS EVOLUTION DURING DRYING OF POLYMER FILMS...... 198 D.1 Introduction ...... 198
D.2 Experimental ...... 201
D.2.1 Materials ...... 201
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D.2.2 Polymeric Solution Preparation ...... 201
D.2.3 Rheology ...... 201
D.2.4 Simultaneous Solution Casting Process ...... 202
D.2.5 Thermal Analysis ...... 202
D.2.6 On-line Weight, Thickness, Temperature & Birefringence Measurements .. 202
D.3 Results and Discussion ...... 203
D.4 Conclusions ...... 214
APPENDIX E: SUPPLEMENTARY INFORMATION FOR CHAPTER VI ...... 215
E.1 Sequential (SEQ) Biaxial Stretching using Uniaxial Constrained Width (UCW)
...... 215
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LIST OF TABLES
Table Page
2.1 BIREFRINGENCE VALUES FOR LIQUID CRYSTALS WITH POSITIVE 30 DIELECTRIC ANISOTROPY ...... 32
2.2 PROPERTIES OF COMMONLY USED NEMATIC LIQUID CRYSTALS IN 53 DISPLAYS ...... 45
2.3 PRIMARY MANUFACTURING METHODS FOR COLOR FILTERS (SHARP CORP.)...... 52
2.4 RELATIONSHIP BETWEEN THE REFRACTIVE INDICES FOR DIFFERENT RETARDER PLATES...... 55
2.5 OPTICAL AND PROCESSING PROPERTIES OF CELLULOSE ACYLATE SAMPLES 81...... 58
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LIST OF FIGURES
Figure Page
1.1 REAL-TIME SOLUTION DRYING PLATFORM. FROM RIGHT TO LEFT IT IS NUMBERED AS: 1) HOT AIR BLOWER, 2) CONNECTION OF BLOWER TO THE TUNNEL, 3) VERTICAL BAFFLES 4) DRYING SAMPLE AND SENSOR LOCATION AND 5) OPEN END OF THE TUNNEL...... 2
2 1.2 ILLUSTRATION OF SINGLE-LAYER RETARDATION FILM ...... 4
1.3 ILLUSTRATIONS OF QUARTER WAVE RETARDERS TO OBTAIN CIRCULARLY POLARIZED LIGHT (ANCHOR-API OPTICS)...... 4
2.1 ELECTROMAGNETIC PROPAGATION OF LIGHT...... 6
2.2 DIFFERENT TYPES OF POLARIZATION OF LIGHT; (A) NATURAL LIGHT, (B) PLANE POLARIZED, (C) CIRCULARLY POLARIZED, (D) ELLIPTICALLY POLARIZED...... 8
2.3 ILLUSTRATION OF REFLECTION AND REFRACTION ON A PLANE SURFACE...... 9
2.4 PROPAGATION OF LIGHT THROUGHOUT THE BIREFRINGENT MEDIUM. 10
2.5 PHASE LEAD AND PHASE LAG PHENOMENA AS A FUNCTION OF PATH 9 DIFFERENCE ...... 11
2.6 SCHEMATIC ILLUSTRATION OF CROSS-POLARIZED OPTICAL MICROSCOPE WITH BEREK COMPENSATOR...... 13
2.7 EXPERIMENTAL SETUP FOR OPTICAL BENCH MEASUREMENTS...... 15
2.8 VIEW AS SEEN THROUGH THE EYEPIECE...... 15
2.9 EXPERIMENTAL SETUP FOR INTENSITY METHOD...... 17
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2.10 SCHEMATIC OF UNIAXIAL STRETCHING MACHINE WITH OPTICAL 16 PROPERTY MEASUREMENT SETUP ...... 19
18 2.11 TYPICAL SCHEMATIC OF BIAXIAL STRETCHING MACHINE ...... 21
2.12 OPTICAL DISPERSION CURVE FOR POLYCARBONATE...... 23
21 2.13 CLASSIFICATION OF LIQUID CRYSTAL (LC) MESOPHASES ...... 24
22 2.14 SOME CALAMITIC LIQUID CRYSTAL MESOPHASES ...... 25
2.15 TYPICAL CALAMITIC MESOGENS...... 26
2.16 SOME EXAMPLES OF CORE UNITS (A), LINKING GROUPS (B), TERMINAL MOIETIES (C) AND LATERAL SUBSTITUENTS (D) EMPLOYED IN 25 MESOGENIC MOLECULES ...... 27
2.17 SPLAY, TWIST AND BEND ELASTIC CONSTANTS OF 5CB AS A FUNCTION 27 OF SHIFTED TEMPERATURE ...... 29
2.18 DIELECTRIC PERMITTIVITIES FOR 6OCB 28...... 30
2.19 THE DYNAMIC BIREFRINGENCE APPARATUS WITH SINGLE WAVELENGTH METHOD: (A) MERCURY LIGHT SOURCE, (B) CONDENSING LENS, (C) MONOCHROMATIZING FILTER, (D) POLARIZER, (E) SLITS, (F) SAMPLE, (G) ANALYZER, (H) PHOTOMULTIPLIER, (I) TWO- CHANNEL AMPLIFIER, (J) TWO-CHANNEL RECORDER, (K) ECCENTRIC DRIVE, (L) MOTOR, (M) VARIABLE-SPEED TRANSMISSION, (N) LINEAR 31 VARIABLE DIFFERENTIAL TRANSFORMER ...... 33
37 2.20 EXPERIMENTAL SETUP FOR SPECTROPHOTOMETRIC METHOD ...... 36
38 2.21 SCHEMATIC ILLUSTRATION OF SPECTRAL-CONTENT ANALYSIS ...... 36
2.22 EXPERIMENTAL SETUP FOR SPECTROGRAPHIC BIREFRINGENCE 45 TECHNIQUE ...... 38
2.23 SCHEMATIC REPRESENTATION OF OPTICAL SETUP USED FOR ON-LINE 46 BIREFRINGENCE MEASUREMENT ...... 38
2.24 SCHEMATIC OF ON-LINE STRETCHING SPECTRAL BIREFRINGENCE 48 INSTRUMENT ...... 39
22 2.25 TWISTED NEMATIC EFFECT IN LCDS ...... 41
2.26 BASIC STRUCTURE OF ACTIVE-MATRIX LCDS (BRITTANICA)...... 44
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58, 59, 60 2.27 SCHEMATIC REPRESENTATION OF A CHOLESTERIC LCD ...... 45
62 2.28 SCHEMATIC ILLUSTRATION OF VA-LCDS WITH ON OR OFF STATE . ... 47
64, 65, 66, 67, 68, 69 2.29 SCHEMATIC ILLUSTRATION OF ECB-LCDS ...... 48
62 2.30 SCHEMATIC ILLUSTRATION OF OCB-LCDS WITH ON OR OFF STATE . . 49
54 2.31 SCHEMATIC REPRESENTATION OF TN-LCD ...... 49
77, 78 2.32 BASIC SCHEMATIC OF IPS-LCDS ...... 51
2.33 ILLUSTRATION OF UNIAXIAL POSITIVE AND NEGATIVE A RETARDER FILMS...... 56
2.34 ILLUSTRATION OF UNIAXIAL POSITIVE AND NEGATIVE C RETARDER FILMS...... 56
2.35 ILLUSTRATION OF BIAXIAL POSITIVE AND NEGATIVE B RETARDER FILMS...... 56
61 2.36 BASIC SYNTHESIS REACTION TO OBTAIN LC DIOXETANES ...... 58
2.37 STRUCTURE OF PATTERNED OPTICAL RETARDER FOR TRANSFLECTIVE 61 LCDS ...... 59
84 2.38 SCHEMATIC ILLUSTRATION OF MULTILAYER COMPENSATOR ...... 60
2.39 SCHEMATIC ILLUSTRATION OF MULTILAYER OPTICAL RETARDER IN 85 LCD ...... 61
86 2.40 BASIC SCHEMATIC FOR HYBRID RETARDER ...... 62
89 2.41 SCHEMATIC ILLUSTRATION OF COEXTRUSION PROCESS ...... 64
90 2.42 SCHEME OF MULTI-SLOT DIE ...... 65
91 2.43 EXPLANATION OF SOLUTION CASTING PROCESS ...... 66
91 2.44 SCHEMATIC OF BELT-MACHINE ...... 67
2.45 DOCTOR BLADE DIE (LEFT) AND SLOT DIE (RIGHT)...... 67
2.46 SCHEMATIC ILLUSTRATION OF DRYING PROCESS DURING SOLUTION 91 CASTING ...... 68
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91 2.47 CLAMP/TENTER DRYER ...... 68
94 2.48 THERMAL STABILITY OF DENSE SPPO FILMS ...... 69
3.1 SOLUTION CASTING AND SAMPLE PREPARATION PROCEDURE FOR RETARDATION CALCULATION USING COMPENSATOR METHOD...... 75
3.2 OPTICAL MEASUREMENTS THROUGH ABBE REFRACTOMETER. (A). TEMPORAL EVOLUTION OF PRINCIPAL REFRACTIVE INDICES DURING DRYING OF NEAT PAI-DMAC SOLUTION (AIR INTERFACE CHANGE (B), SUBSTRATE INTERFACE CHANGE (C)). INSET MOLECULAR DRAWINGS DEPICT PLANAR LOW ENERGY CONFORMATION STATE FOR SINGLE PAI REPEATING UNIT...... 77
3.3 CROSS-POLARIZED OPTICAL MICROGRAPHS WITH 30 ORDER BEREK COMPENSATOR INSERTED AT 45° ORIENTATION CAPTURED DURING DRYING OF PAI-DMAC SOLUTION AT DIFFERENT DRYING TIMES ((A) AIR SURFACE, (S) SUBSTRATE SURFACE). (ΔDOL= HIGH ORIENTATION LAYER) ...... 78
3.4 CALCULATED BIREFRINGENCE VALUES FROM OPTICAL MICROGRAPHS USING COMPENSATOR METHOD AT DIFFERENT DRYING TIMES (T=TIME IN MINUTES, GRAY COLOR REGIONS DEPICTS UNMEASURED BIREFRINGENCE VALUES DUE TO SOLID-RICH LAYER GROWTH (ΔDOL))...... 79
3.5 OFF-LINE MEASURED REFRACTIVE INDEX AND CALCULATED BIREFRINGENCE VALUES ALONG WITH SOLID LAYER THICKNESS GROWTH. (A) GROWTH OF SOLIDIFIED LAYER AT AIR/SUBSTRATE- COATING INTERFACE (% OVERALL COATING THICKNESS). (B) AVERAGE REFRACTIVE INDEX COMPARISON CALCULATED FROM OFF-LINE TECHNIQUES INCORPORATING COMPENSATOR METHOD AND ABBE REFRACTOMETER. (C) BIREFRINGENCE COMPARISON BETWEEN REFRACTOMETER READINGS AND COMPENSATOR METHOD DURING DRYING OF NEAT PAI/DMAC SOLUTION...... 80
3.6 DRYING DATA FROM REAL-TIME MEASUREMENT SYSTEM. (A) REAL- TIME DRYING DATA FOR PAI-DMAC SOLUTION WITH INITIAL SET WET THICKNESS OF 1MM WITH 0.2 M/SECOND AIR SPEED AT 25% RH. (B) COMPARISON OF REAL-TIME BIREFRINGENCE DEVELOPMENT FOR DIFFERENT INITIAL SET THICKNESSES OF PAI-DMAC COATINGS...... 82
3.7 COMPARISON OF BIREFRINGENCE VALUES FROM REAL-TIME MEASUREMENT SYSTEM AND ABBE REFRACTOMETER (OPEN SYMBOLS DEPICTS AIR-INTERFACE REFRACTOMETER MEASUREMENTS WHERE
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CLOSED SYMBOLS DEPICTS SUBSTRATE-INTERFACE MEASUREMENTS)...... 83
3.8 MOLECULAR MODEL REPRESENTING THE PAI CHAIN ORIENTATION DURING DRYING...... 84
4.1 CHEMICAL STRUCTURES OF GENERIC SOLUBLE POLY(AMIDE-IMIDE) (PAI) REPEATING UNIT AND ORGANIC MODIFIER FOR CLOISITE 30B MONTMORILLONITE. (A) PAI, (B) C30B...... 89
4.2 HYBRID PROCESS SCHEMATIC WITH MULTI-LAYER DOCTOR BLADE. AIRFLOW AND CASTING DIRECTION IS REPRESENTED WITH DASHED RED ARROWS...... 91
4.3 CLOSE-UP PICTURE OF THE SAMPLE POSITIONS AND REAL-TIME MEASUREMENT SENSORS. ONLY SAMPLE PLATFORM IS COVERED BY GLASS MATERIAL TO ALLOW OPTICAL MEASUREMENT...... 92
4.4 VARIATION OF SHEAR VISCOSITY OF HYBRID SOLUTIONS AT DIFFERENT ORGANOCLAY LOADINGS WITH SHEAR RATES...... 94
4.5 TYPICAL DRYING TEST RESULT OF PAI CAST FILM FROM 8WT.% DMAC SOLUTION. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: BLADE GAP 559 µM, AIRFLOW RATE 0.5M/SEC AT ROOM TEMPERATURE. (STAR DENOTES OFFLINE BIREFRINGENCE MEASUREMENT) ...... 95
4.6 TYPICAL DRYING TEST RESULT OF POLYAMIDE-IMIDE NANOCOMPOSITE FILM FROM 8WT.% DMAC SOLUTION (3WT.% C30B). CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: BLADE GAP 1 MM, AIRFLOW RATE 0.25M/SEC AT ROOM TEMPERATURE. (STAR DENOTES OFFLINE BIREFRINGENCE MEASUREMENT) ...... 96
4.7 CALCULATED BIREFRINGENCE GRADIENT THROUGH THICKNESS OF PAI-DMAC COATING WITH 3WT.% ORGANOCLAY CONTENT...... 97
4.8 EFFECT OF INITIAL WET THICKNESS ON REAL-TIME DRYING FOR SINGLE-LAYER PAI-ORGANOCLAY COATINGS...... 98
4.9 EFFECT OF INITIAL WET THICKNESS ON REAL-TIME DRYING & OUT-OF- PLANE ANISOTROPY DEVELOPMENT FOR SINGLE-LAYER PAI- ORGANOCLAY COATINGS. (20% RH) ...... 99
4.10 EFFECT OF ORGANOCLAY LOADING ON REAL-TIME THICKNESS AND MASS FLUX CHANGE FOR SINGLE-LAYER PAI-ORGANOCLAY COATINGS. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS:
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BLADE GAP 559 µM, AIRFLOW RATE 0.5M/SEC AT ROOM TEMPERATURE...... 99
4.11 EFFECT OF ORGANOCLAY LOADING ON REAL-TIME DRYING & OUT-OF- PLANE ANISOTROPY DEVELOPMENT FOR SINGLE-LAYER PAI- ORGANOCLAY COATINGS. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: BLADE GAP 559 µM, AIRFLOW RATE 0.5M/SEC AT ROOM TEMPERATURE. (45 %RH) ...... 100
4.12 MOLECULAR REPRESENTATION OF REAL-TIME ANISOTROPY DEVELOPMENT DURING DRYING OF PAI-DMAC COATING WITH 229 µM INITIAL WET THICKNESS...... 101
4.13 REAL-TIME ANISOTROPY DEVELOPMENT FOR MULTI-LAYER FILM (MIDDLE LAYER IN BETWEEN NEAT PAI LAYERS). (750 µM INITIAL WET THICKNESS, STAR DENOTES OFFLINE BIREFRINGENCE MEASUREMENT)...... 102
4.14 DRYING MODEL AND OUT-OF-PLANE BIREFRINGENCE DEVELOPMENT FOR MULTI-LAYER NANOCOMPOSITE FILM UNDER AIRFLOW CONDITION (SCHEMATICS ARE NOT IN ACTUAL SCALE, REPRESENTATIVE PURPOSES ONLY. RED COLOR INDICATES FRONT LASER READING)...... 103
4.15 WAXS DIFFRACTION PATTERNS OF ORGANOCLAY REINFORCED MULTI-LAYER FILM WITH CLOISITE 30B...... 104
4.16 CHANGE IN PAI DOMAIN SIZE WITH DRYING CONDITIONS: 30NM, 32NM AND 40 NM FOR 25 °C (A), 30 °C (B) AND 50 °C (C) DRIED SAMPLES. (ARROWS DEPICT THICKNESS DIRECTION) ...... 104
4.17 TEM MICROGRAPHS OF CROSS-SECTION CUT ORGANOCLAY REINFORCED MULTI-LAYER PAI FILMS (ARROWS SHOWS THICKNESS DIRECTION, 3WT.% (A), 5WT.% (B) AND 7WT.% (C) ORGANOCLAY LOADINGS FROM LEFT TO RIGHT) ...... 105
4.18 MECHANICAL PROPERTIES OF ORGANOCLAY REINFORCED MULTILAYER FILM WITH DIFFERENT NANOCLAY LOADINGS...... 106
4.19 THE EFFECT OF ORGANOCLAY ADDITION ON HE PERMEABILITY FOR DRIED MULTILAYER NANOCOMPOSITE FILMS...... 106
5.1 CHEMICAL STRUCTURES OF GENERIC PAI REPEATING UNIT AND CHEMICALLY REDUCED GRAPHENE-OXIDE NANOSHEET. (A) PAI, (B) GO...... 112
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5.2 SCHEMATIC ILLUSTRATION OF SOLUTION CASTING PROCESS...... 114
5.3 SAMPLE POSITIONS AND SENSOR LOCATIONS IN REAL-TIME MEASUREMENT SYSTEM...... 115
5.4 DRYING DATA FOR 10 WT.% PAI-DMAC SOLUTION. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: 559 µM INITIAL WET COATING THICKNESS, AIRFLOW RATE 0.2M/S AT ROOM TEMPERATURE. (STAR DENOTES OFFLINE BIREFRINGENCE MEASUREMENT) ...... 116
5.5 DRYING DATA FOR PAI NANOCOMPOSITE SOLUTION WITH 0.01WT.% GO CONTENT. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: 559 µM INITIAL WET COATING THICKNESS, AIRFLOW RATE 0.2M/S AT ROOM TEMPERATURE. (STAR DEPICTS OFFLINE BIREFRINGENCE MEASUREMENT) ...... 117
5.6 EFFECT OF CAST INITIAL WET THICKNESS ON PERCENT WEIGHT AND REAL-TIME THICKNESS DATA FOR 0.01 WT.% PAI-GO COATINGS...... 118
5.7 EFFECT OF INITIAL WET THICKNESS ON BIREFRINGENCE FOR 0.01 WT.% PAI-GO COATINGS...... 119
5.8 EFFECT OF GO CONTENT ON PERCENT WEIGHT CHANGE AND REAL-TIME THICKNESS DATA FOR 0.01 WT.% PAI-GO COATINGS...... 119
5.9 EFFECT OF PERCENT HUMIDITY ON REAL-TIME DRYING FOR NEAT PAI- DMAC SOLUTION DUE TO HYGROSCOPICITY OF DMAC. CASTING AND DRYING CONDITIONS ARE AS FOLLOWS: BLADE GAP 559 µM, AIRFLOW RATE 0.2M/SEC AT ROOM TEMPERATURE...... 120
5.10 EFFECT OF GO CONTENT ON HYDROPHOBICITY OF DRIED PAI-GO NANOCOMPOSITE FILMS. CALCULATED CONTACT ANGLES ARE: 57° FOR NEAT, 80°, 86° AND 93° FOR 0.01, 0.05 AND 0.1 WT.% GO CONTENT. 121
5.11 EFFECT OF GO CONTENT ON BIREFRINGENCE FOR PAI-GO COATINGS...... 121
5.12 TEM MICROGRAPHS SHOWING EXFOLIATION AND PLANAR ORIENTATION OF GO NANOSHEETS: (A) RAW GO SHEETS, (B) CROSS- SECTION CUT 0.1 WT.% GO REINFORCED PAI FILMS, (C) CROSS-SECTION CUT NEAT PAI FILM WITHOUT FILLER (ARROW SHOWS THICKNESS DIRECTION) ...... 122
5.13 EFFECT OF GO CONTENT ON TENSILE STRENGTH FOR PAI-GO NANOCOMPOSITE FILMS...... 123
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5.14 EFFECT OF GO CONTENT ON MODULUS FOR PAI-GO NANOCOMPOSITE FILMS...... 123
5.15 THE EFFECT OF GO CONTENT ON HE PERMEABILITY FOR DRIED PAI-GO NANOCOMPOSITE FILMS...... 124
5.16 MOLECULAR MODEL REPRESENTING REAL-TIME ANISOTROPY DEVELOPMENT DURING THE COURSE OF SOLUTION EVAPORATION (INSET GRAPH SHOWS DATA FOR 229 µM INITIAL WET THICKNESS). ... 125
6.1 SCHEMATIC OF MULTI-LAYER SOLUTION CASTING PROCESS...... 130
6.2 HYBRID PROCESS SCHEMATIC WITH MULTI-LAYER SLOT-DIE. AIRFLOW DIRECTION IS REPRESENTED WITH DASHED ARROWS...... 131
6.3 POSITIONS OF REAL-TIME MEASUREMENT SENSORS FOR BIXIAL STRETCHING MACHINE...... 134
6.4 OPTICAL DISPERSION PROFILE FOR THE PROPOSED RETARDER FILM AND ITS AVERAGE REFRACTIVE INDEX...... 137
6.5 VISCOSITY VARIATIONS OF PSF AND PCC WITH DIFFERENT SOLID CONTENTS...... 137
6.6 DSC THERMOGRAPHS FOR NEAT PS, PCC AND PSF GRADES...... 138
6.7 DSC THERMOGRAMS FOR SOLUTION CAST PSF, PS AND PCC FILMS...... 138
6.8 BOUND SOLVENT RATIOS FOR POLYSULFONE FILMS WITH DIFFERENT INITIAL CONCENTRATIONS...... 139
6.9 MICROGRAPHS WITH MAGNIFICATION 50X TRANSMISSION (RIGHT) AND REFLECTION (LEFT) INDICATING PHASE SEPARATION DUE TO %RH HUMIDITY FOR PSF FILMS...... 139
6.10 SURFACE LSM MICROGRAPHS OF MULTI-LAYER OPTICAL RETARDER FILMS WITH 5X MAGNIFICATION (LEFT) AND 10X MAGNIFICATION (RIGHT) USING REFLECTION MODE. TOP PICTURES SHOW 3D TOPOGRAPHICAL VIEW (488NM LEFT, 543NM RIGHT) OF THE SURFACE PROFILES...... 140
6.11 MECHANO-OPTICAL BEHAVIOR OF SOLUTION CAST PSF FILMS AT 140 °C AT 100 MM/MIN...... 141
6.12 MECHANICAL BEHAVIOR OF SOLUTION CAST PSF-PCC-PS MULTILAYER OPTICAL FILMS...... 142
xx
6.13 TRUE HENCKY STRAIN IN MD & TD VS. RETARDATION BEHAVIOR OF PSF-PCC-PS SOLUTION CAST FILMS STRETCHED AT DIFFERENT STRETCH RATES...... 142
6.14 TRUE HENCKY STRAIN IN MD & TD VS. BIREFRINGENCE BEHAVIOR OF PSF-PCC-PS SOLUTION CAST FILMS STRETCHED AT DIFFERENT STRETCH RATES...... 143
6.15 THE EFFECT OF DEFORMATION RATE ON IN-PLANE BIREFRINGENCE DURING MD DEFORMATION OF PSF-PCC-PS SOLUTION CAST FILMS. .. 144
6.16 CALCULATED RTH VALUES FOR MULTI-LAYER OPTICAL RETARDER. 144
6.17 THE EFFECT OF STRETCH RATE ON IN-PLANE & OUT-OF-PLANE RETARDATION DEVELOPMENT FOR MULTI-LAYER PSF-PCC-PS FILMS (SOLID ARROWS REPRESENT INCREASING STRAIN ON MD WHEREAS DASHED ARROWS REPRESENT INCREASING STRAIN ON TD)...... 145
6.18 COMBINED EFFECT OF STRETCH RATE ON IN-PLANE & OUT-OF-PLANE RETARDATION DEVELOPMENT FOR MULTI-LAYER PSF-PCC-PS FILMS (HOLLOW SYMBOLS REPRESENT TD STRAIN)...... 146
6.19 THE EFFECT OF STRETCH RATE ON IN-PLANE & OUT-OF-PLANE RETARDATION DEVELOPMENT FOR SINGLE-LAYER PCC (LEFT) AND PS (RIGHT) FILMS (HOLLOW SYMBOLS REPRESENT TD STRAIN, SOLID ARROWS REPRESENT INCREASING STRAIN ON MD WHEREAS DASHED ARROWS REPRESENT INCREASING STRAIN ON TD)...... 147
6.20 DISPERSION CHARACTERISTICS OF THE MULTI-LAYER PSF-PCC-PS OPTICAL RETARDER...... 147
7.1 EFFECT OF INITIAL WET COATING THICKNESS ON TOTAL DRYING TIME FOR 1WT.% CLAY LOADED PAI SYSTEM...... 147
7.2 EFFECT OF INITIAL WET COATING THICKNESS ON TOTAL DRYING TIME FOR 0.01WT.% GO LOADED PAI SYSTEM...... 147
xxi
CHAPTER I
INTRODUCTION
Solution casting process is increasingly being used in the production of functional films including optical film retarders, electrically conductive transparent films, flexible photovoltaic films and engineered films for gas barrier applications. It is a complex process that involves multiple physical changes taking place simultaneously as a result of evaporation of solvent. These physical changes include evaporation induced weight loss and accompanying thickness reduction while the polymers undergoing reorientation with their primary chain axes oriented in the plane of the film. Therefore understanding the physics and kinetics of drying is crucial to optimize processing parameters including casting and drying conditions.
For the first time, an innovative real-time measurement system 1 for tracking birefringence, weight, thickness and surface temperature during the course of drying of a polymer coating and/or film is developed. This instrument can be used to systematically study the effect of material variables including solvent types and mixtures and solid concentrations, and system variables including temperature, air speeds, and initial thickness. A fully automated spectral birefringence measurement system was also incorporated into the design of this machine for optical characterization of the drying polymer solutions to form thin films (Figure 1.1).
1
Figure 1.1 Real-time solution drying platform. From right to left it is numbered as: 1) Hot air blower, 2) Connection of blower to the tunnel, 3) Vertical baffles 4) Drying sample and sensor location and 5) open end of the tunnel.
Evaporation of the solvent molecules occurs from the air surface that results in gradient in refractive index through the thickness of the coating. The existence of this optical anisotropy and its gradient through the thickness for PAI solution in DMAc was experimentally addressed through optical compensation and Abbe refractometer method.
This was determined by real time tracking of refractive indices on both substrate and air surfaces and their anisotropy by polarized Abbe refractometry and their gradient by rapid microtomy coupled with optical retardation method. Final results were compared with data from custom built real-time measurement system for comparison.
Real-time drying behavior of single/multi-layer organoclay-filled soluble PAI cast films was also investigated using a series of clay loadings and initial wet thicknesses to observe the effects on drying behavior and anisotropy development using spectral birefringence technique. The influence of organoclay loading on gas barrier and mechanical properties of multi-layered PAI-nanoclay nanocomposite films are discussed.
Single layer brittle nanocomposite film was reinforced with flexible pristine PAI and polyurethane (PU) layers through simultaneous multi-layer solution casting process to enhance gas barrier properties, tensile strength and flexibility.
2
In a similar way, real-time drying behavior of GO-filled soluble PAI cast films using a series of GO loadings and initial wet thicknesses was investigated. The effect of
GO content on drying behavior and optical birefringence was determined. To the best of our knowledge, application of GO was not reported previously for any soluble PAI system. The resulting gas barrier and mechanical properties of single-layer PAI/GO nanocomposite films are also discussed as influenced by GO content.
Liquid Crystal Displays (LCDs) are today’s one of the leading high-information- content display technologies that is widely commercialized. Their light weight, portability, low power consumption and high optical quality have been the most important advantages compare with old cathode ray tube (CRTs) displays. Their cheap process costs and high raw material availability make LCDs more feasible than new display technologies such as organic light emitting diode displays (OLEDs).
Although LCDs are widely commercialized, they suffer from several problems such as wide-viewing angle, high contrast ratio and brightness especially at an oblique angle. Unstable optical dispersion curves are also one of the other disadvantages of
LCDs. The underlying reason for these problems is based on the orientation state of liquid crystal materials (LC) inside LCDs which introduce in-plane and out-of-plane retardations to the incoming bright light generated at the back plate. In order to compensate these problems, retarder film concept was introduced to the industry.
3
Figure 1.2 Illustration of single-layer retardation film 2.
It was originally a Japanese innovation 2 in display industry to improve contrast
on STN (Super-Twisted-Nematic) and TN (Twisted-Nematic) displays (Figure 1.2).
Retarder films have an ability to generate retardation at certain magnitudes to the light that is passing through due to its optically anisotropic nature. The mechanism is similar to the one for simple quarter wave retarders. Figure 1.3 shows the illustrations of propagation of light through a quarter wave retarder film to obtain circularly polarized light.
Figure 1.3 Illustrations of quarter wave retarders to obtain circularly polarized light (Anchor-api optics).
4
Design for optical compensation films incorporates several techniques to
efficiently compensate polarized light coming from LC layer in the display. Multilayer
retarder films has an advantage to provide wide viewing angle, better contrast and
flattened optical dispersion curve due to incorporation of optical films with different
optical nature. Therefore a multi-layer optical compensator design with reduced
birefringence dispersion was proposed and produced through hybrid process which
present in our group 3. The uniqueness of such a multilayer retarder film is in its ability to
compensate the given retardation by liquid crystal layer in LCDs with flattening of the
optical dispersion curve. The biaxial stretcher developed by our group was used to
investigate the changes in optical properties during uniaxial and biaxial deformation
through spectral birefringence system 4. By tuning the mechano-optical behavior of the multilayer film, independent control on out-of-plane retardation (Rth) and in-plane
retardation (Rin) values was achieved. In addition, true stress-strain behavior and the
coupling between true mechanical responses and structural development was investigated
for multilayer optical compensation films.
5
CHAPTER II
LITERATURE REVIEW
2.1 Optical Methods
2.1.1 Basics of Theory of Light
The excitation of light in space mainly constitutes two vectors called E and H, which are the electric vector and the magnetic induction respectively. In homogeneous isotropic media, vectors E and H are mutually perpendicular and normal to the direction of the light propagation 5. Figure 2.1 shows the orientation of these vectors.
Figure 2.1 Electromagnetic propagation of light.
If the medium is homogeneous, the wave equations for the propagation light reduce to a simplified form 6. Equations 2.1 and 2.2 represent this simplified form. Here,
ε and µ represent the corresponding dielectric constant and magnetic permeability and c stands for the speed of light.
6
= 0 (2.1) 2 휀휇 2 ∇ 퐸 − 푐 퐸̈ = 0 (2.2) 2 휀휇 2 푐 ̈ ∇ 퐻 − 퐻
Generally, the vibration of each wave has no certain direction. However, in some cases, in which light passes through a special environment that introduces polarization, all of the waves in the light beam can have a combination of certain directions (linear,
elliptical, circular). This phenomenon mainly introduces the polarization concept which
plays a vital role in today’s industrial world.
Usually the electromagnetic waves have a disorderly orientation of the vectors E and H. Such a light is called the non-polarized light or natural light. The light in which
the direction of vibration is somehow ordered is called the polarized light. If vibration of
the vector E occurs only in one plane such a light is called the plane-polarized light. The
plane of polarization is the plane in which the vector H and the wave normal lie. If the
vibration of the vector E is such that the amplitude remains constant while the orientation
of the light vector changes uniformly so that the tip of the vector traces out a circle, the
light is said to be circularly polarized. If both amplitude and orientation vary which will
result in elliptic behavior, the light is said to be elliptically polarized. Figure 2.2 shows
the different types of polarization of light.
7
Figure 2.2 Different types of polarization of light; (a) Natural light, (b) Plane polarized, (c) Circularly polarized, (d) Elliptically polarized.
There are also various methods to obtain the polarized light. These methods can
be polarizer sheets in the case of Polaroid films, Fresnel’s thumb which incorporates a
special crystal or reflection of light through water surface. These cases change the degree
of polarization of the light to the linearly polarized state. To obtain circularly and elliptically polarized light we can use variation of other methods.
2.1.2 Interaction of the Light with the Medium
A normal beam of light in isotropic material consists of many individual waves, each vibrating in a direction perpendicular to its path. Measurable intensities therefore refer to a superposition of many millions of waves 7.
This corresponding intensity of the propagating light also decreases while passing
through the material which can be transparent polymers, liquid crystals or inorganic
crystals. For these cases the speed of light in the medium also plays an important role in
characterizing the material. This parameter is mainly controlled by the refractive index of
the medium which is also known as the optical density of the material. It also determines the angle of reflectivity during reflection and refraction.
8
Figure 2.3 illustrates the propagation of light through a medium. In this figure,
AO represents an incident ray where N-N' is the normal at the point of incidence. OB and
OC are the reflected and transmitted rays respectively. The direction of the transmitted ray does not coincide with that of the incident ray and the transmitted ray is said to be
refracted.
Figure 2.3 Illustration of reflection and refraction on a plane surface.
The angles between the incident, reflected and refracted rays with the normal to
the surface at the point of incidence are known as the angles of incidence, reflection and
refraction. They are denoted as i, R and r, respectively. Reflection and refraction in
isotropic media obey the following laws:
• AD, BO, OC are coplanar,
• i = R,
• = where n12 is the relative refractive index. sin 푖 sin 푟 12 Among푛 these, the latter law is known as Snell's law. If light enters a medium from
vacuum, the above ratio is called the absolute refractive index of the medium. If c is the
velocity of light in a vacuum, the absolute refractive indices can be given as nl = c/vl, n2
= c/v2, n12 = vl / v2 = n2/ nl. The frequency of the propagating light, f = v/λ, unchanged
9
when the light travels through various media which results in a wavelength change. If λ1 and λ2 denote the wavelengths in the two media, then the absolute refractive indices can be given by Equation 2.3.
nl = λ0/λ1, n2 = λ0/λ2, n12 = λ1/λ2 = vl/v2 (2.3)
Absolute refractive indices can also change depending on the nature of the medium. For homogeneous mediums n is everywhere the same. In an inhomogeneous or heterogeneous medium the index varies with position. In an isotropic medium n is the same at each point for light traveling in all directions and with all polarizations, so the refractive index can be defined as a scalar function of position 8.
2.1.3 Retardation and Birefringence
One of the main characteristics of the propagation of light in a medium is that this can result in a change in the properties of the electromagnetic nature of the light.
Electromagnetic waves in any medium can be represented by two waves which are plane polarized in two mutually perpendicular directions. In an isotropic medium, velocities of both waves are equal. In an anisotropic medium these velocities are different and they are travelling through the medium with respect to the refractive index values in each direction which can be represented by a 3X3 tensor of refractive indices for 3D. Figure
2.4 shows this behavior.
Figure 2.4 Propagation of light throughout the birefringent medium. 10
The directions of the plane of polarization of these two waves coincide with the principal directions of the tensor of refractive indices. One of them experiences the reflection even in the case of incident ray is normal to surface. Thus, it is called the extraordinary ray. The second ray behaves as usual and is called ordinary ray. Each of the two rays, ordinary and extraordinary, which are emerging from the anisotropic medium, is plane polarized.
The planes of vibration corresponding to the two rays are mutually perpendicular.
The vibrations of the extraordinary ray lie in the principal plane of the face through which the image enters or leaves. The two rays travel with different velocities through the anisotropic medium, except in the direction of the optic axis where the velocities are equal.
Since each ray travels in space at different speeds, this fact results in a path difference which is well known as retardation, denoted as R or Γ. It is inversely proportional to the wavelength λ and directly proportional to the distance d of the medium. Figure 2.5 illustrates the retardation of propagating light passing through a medium.
Figure 2.5 Phase lead and phase lag phenomena as a function of path difference 9.
11
the resulting phase difference between the waves is:
2π R δ = (2.4) λ therefore optical phase difference (retardation) is the expected outcome that is measured by the experimental techniques such as compensation method and intensity method.
Double refraction or birefringence can also be calculated using this retardation values.
Since it contains information regarding anisotropy, it directly represents the molecular orientation and structure development within the polymer. Depending on the number of optic axes, the anisotropic materials can be divided into uniaxial and biaxial crystals. For an isotropic cubic material such as glass or salt crystals, there is no birefringence, however in a uniaxial crystal only one birefringence will be observed whereas in the case of a biaxial crystal, there will be three birefringences due to the difference of refractive indices. Since polymers are anisotropic materials in general, there will be birefringence in most cases.
Convenient ways to measure birefringence include utilization of cross-polarized optical microscopes, Abbe refractometers and optical benchs. In the former one, a necessary compensator such as a Berek compensator which measures the retardation after the propagation of light through the material that is being analyzed needed. Cross- polarized optical microscope apparatus is widely used for birefringence determination for polymeric fibers which can be produced either wet or dry spinning processes. For birefringence determination for polymeric films generally optical bench method is widely used which results in in-plane and out-of-plane birefringence values.
12
2.1.4 Determination of Optical Parameters
There are basically two types of methods of measuring retardation and birefringence. First method is based on the compensator technique 10: the phase shift introduced by the unknown material to the light components passing through it is compensated by a well known calibrated compensator. The function of the compensator is to reverse the effect of the unknown material on the state of polarization of the light sent to the sample and, by doing so, precisely determine how much compensation was needed. This technique is manual and may take as long as several minutes to perform the experiment depending on the optical complexity of the material. Therefore, it is not suitable for automated measurement. Cross-polarized optical microscope with Berek compensator which is suitable for polymeric fiber birefringence determination or optical bench apparatus with Babinet compensator which is suitable for polymeric film birefringence determination can be an example for this case. Figure 2.6 shows the schematic explanation of cross-polarized optical microscope with Berek compensator.
Figure 2.6 Schematic illustration of cross-polarized optical microscope with Berek compensator.
13
The resulting birefringence ∆n12 for the fibers can be calculated easily with the corresponding retardation and thickness data. Equation 2.5 shows the birefringence calculation route. In this equation retardation can be calculated with the help of the black and red dials which can be read using a cross-polarized optical microscope and the necessary retardation tables which can be obtained using laboratory notes.
= (2.5) Γ 12 푑 For crystalline polymers, birefringence∆ 푛of the sample can be determined in a different manner. The contribution of crystalline and amorphous parts and form birefringence should be taken into account. Equation 2.6 shows the resulting equation for birefringence determination. Here, X the crystal fraction can be determined by DSC and crystalline phase orientation factor can be determined by WAXS neglecting form birefringence. The necessary intrinsic birefringence values can be obtained from literature.
= + (1 ) + (2.6) 0 0 12 푐 푐 푎 푎 푓표푟푚 The order of the compensator∆푛 is푥 푓also∆푛 important− 푥 e.g.푓 ∆for푛 Bere∆푛k compensator there are two orders which are 4th and 30th, and each of these modes requires different retardation calculation steps. After the determination of the birefringence, an additional parameter, Herman’s orientation factor can also be calculated. It is basically the ratio of the birefringence of the material to its intrinsic birefringence.
Equations 2.7 and 2.8 represent the relations for orientation factor calculation
11 steps . Here, n1, n2 and n3 represent the refractive indices in machine (MD), transverse
(TD) and normal (ND) directions.
14
= ° = ° (2.7) 퐵 푛1−푛3 ∆푛13 푓1 ∆ ∆푛 = ° = ° (2.8) 퐵 푛2−푛3 ∆푛23 2 On the other hand, in the case 푓of polymeric∆ films,∆푛 optical bench apparatus with a
Babinet compensator and a goniometer is more appropriate. Figure 2.7 shows a typical
schematic for optical bench experimental setup.
Figure 2.7 Experimental setup for optical bench measurements.
The polarizing axis is adjusted at 45° to the fast axis of compensator and the analyzer set in crossed position. Figure 2.8 shows the illustration of this crossed position.
Here, the fibrillars (thin vertical lines) are centered on the central black fringe and the compensator counter should read 20 and the drum should read 0 in this position.
Figure 2.8 View as seen through the eyepiece.
15
Finally by rotating the sample and reading the fringe patterns on the eyepiece, we
can measure the corresponding in-plane and out-of-plane birefringences. We can calculate in-plane and out-of-plane birefringences with the corresponding ∆n23 values.
Equations 2.9, 2.10 and 2.11 show the formulation of in-plane and out-of-plane birefringences. By calculating the difference between in-plane and out-of-plane birefringences we can obtain ∆n23.
= (2.9) 휆0푅0 12 0 ∆푛 − 푑 / / = /2 2 1 2 (2.10) 휆0 푅0−푅휙�1−푠푖푛 휙 푛� � 2 2 ∆푛13−휙 − 푑0 � 푠푖푛 휙 푛� �
= 푛 (2.11) ∑1 ∆푛13−휙 ∆푛13 − 푛 where, λ0 is the wavelength of the light which is 565 nm, d0 is the thickness of the film,
R0 is the optical retardation when the tilt angle is zero,
Rφ is the optical retardation when the tilt angle is φ,
is the average refraction index of the film which is 1.49 for PP,
φ푛� is the tilt angle, n is the total number of experiments.
The corresponding orientation functions can also be calculated using Herman’s
orientation function formulation. As an example for PP, ∆nC = 0.0291, the corresponding
orientation functions can be calculated as follows:
= ° (2.12) 퐵 ∆푛13 푓1 ∆푛퐶 = ° (2.13) 퐵 ∆푛23 푓2 ∆푛퐶
16
For multiphase materials such as semi-crystalline polymers, both amorphous and crystalline phase contributions should be introduced with the form birefringence values.
Equations 2.14 and 2.15 show the relations for biaxially oriented systems. and 0푐 0푐 푐푏 푎푏 are the cb and ab crystallographic direction birefringences for the crystalline∆푛 phase and∆푛
is the intrinsic birefringence for the amorphous phase. The remaining parameters 0푎푚 ∆are푛 the necessary orientation factors for different axes and x is the crystalline fraction
within the polymer.
= [ + ] + (1 ) + (2.14) 푏푐 0푐 푏푐 0푐 0푎푚 푏푎 13 1푐 푐푏 1푎 푎푏 1 푓표푟푚 ∆푛 = 푥[푓 ∆푛 + 푓 ∆푛 ] + (1 − 푥)∆푛 푓 + ∆푛 (2.15) 푏푐 0푐 푏푐 0푐 0푎푚 푏푎 23 2푐 푐푏 2푎 푎푏 1 푓표푟푚 The second∆푛 techn푥ique푓 ∆ is푛 called푓 intensity∆푛 method− 푥 12:∆ typically푛 푓 an optical∆푛 setup, in
which a polarizer, the unknown sample and an analyzer photo detector that are aligned, is
utilized. Along with the compensation method, these optical measurement techniques are
referred to as interferometric methods. Figure 2.9 shows the typical experimental setup
for birefringence measurement using intensity method. Considering the optical setup seen
in Figure 2.9, the light intensity at the detector can be calculated using Mueller matrices
13, 14, 15.
Figure 2.9 Experimental setup for intensity method.
17
In this technique, variants are faster than one wavelength of the dominant
wavelength that is probing the medium. Due to the sine functionality of intensity, it is
enough to follow transient phenomena as long as the retardation (phase shift) introduced
to the light is less with retardation given in the Equation 2.16. If the retardation (Γ)
becomes larger than one wavelength (λ), it becomes no longer determinable with such optical setup.
πΓ I = C sin 2 (2.16) λ
Birefringence determination during uniaxial or biaxial deformation is also very
important as far as the resulting data are concerned. A uniaxial stretching instrument that
allows for the real time determination of true stress, true strain and optical retardation
data during the uniaxial deformation process has been developed in our group 16.
Basically it allows us to gain a fundamental understanding of the molecular
mechanisms – orientation, crystallization, and relaxation – that take place during
deformation without the need to stop and examine the samples, thus revealing a more
realistic behavior of the materials in their actual use. Figure 2.10 shows the schematic of
uniaxial stretching machine.
18
Figure 2.10 Schematic of uniaxial stretching machine with optical property measurement setup 16.
The machine is essentially composed of three parts: the uniaxial stretching
machine with environmental chamber, the spectral birefringence system, and a laser-
based width measurement system. The machine simultaneously records real-time measurements of optical retardation, sample width at the stationary mid-symmetry plane, and force. This machine consists of a laser micrometer, mounted at an oblique angle, which continuously monitors the width of the sample at the mid-symmetry plane by a precision measurement of the sample shadow generated by a very narrow laser light sheet
(~0.2 mm).
Film specimens used consist of a dumbbell-shape with the initial dimensions: 75 mm long, 40 mm wide and 30 mm wide in the narrowest region. The benefits of using dumbbell-shaped specimens are that they ensure that measurements are taken at the region experiencing almost all of the deformation, or at the narrowest region of the sample. The distance between the clamps is about 30 mm. A preheating time of 15 minutes or more is required to thermally equilibrate the sample at a desired temperature in the environmental oven. Using transverse isotropy, one can calculate the real time thickness using the initial thickness as can be seen in Equations 2.17 and 2.18. Here,
19
whereby D0 is the initial film thickness, Dt is the real time film thickness, W0 is the
initial width of the film and Wt is the real time width of the film.
W D t = t W D 0 0 (2.17)
W D D = t 0 t W 0 (2.18)
Combining the transverse isotropy effect with the incompressibility assumption
(D0W0L0 = DtWtLt), the local true stresses and local true strains can be calculated using the Equations 2.19 and 2.20. Here, L0 represents the initial length of the film, Lt is the real time length of the film and Ft is the time variation of force.
2 Lt W0 true strain = − 1= − 1 (2.19) L0 Wt
F F true stress = t = t (2.20) (W D ) 2 t t Wt D0 Wo
Since the sample width is continuously measured during the deformation process
and the development of crystallinity is concentrated near the end of the stretching, the
film can be assumed to exhibit incompressibility. This assumption has been verified
before with off-line thickness and width measurements of the film in comparison with the
real-time thickness values calculated from width measurements 17. In the case of
birefringence and retardation measurements during biaxial stretching, an Iwamoto biaxial
stretching machine was modified by Hassan and Cakmak 18 in order to obtain
measurements of true stress, true strain and in-plane and out-of-plane birefringences.
20
In addition to having conventional biaxial stretching machine components, it also
contains an optical system, a vision system and a motor controller for experimental setup.
This allows the machine to acquire true strain and instantaneous retardation values of the
film when the polarized light beam is at 90° and 45° with the film surface which occurs at
the geometric center of the sample. Figure 2.11 shows the typical experimental setup for biaxial stretching machine with on-line optical property measurement setup.
Figure 2.11 Typical schematic of biaxial stretching machine 18.
The true strain is determined from measuring the displacement of a dot pattern
printed in the center of the sample. The variations in the film thickness at the center of the
sample are determined based on the X and Y strains when incompressibility is assumed
(LX0LY0T0 = LXtLYtTt). In this case, LX0 and LY0 are the original distances between the
dots in the X and Y directions, respectively, LXt and LYt are the X and Y distances
between the dots during material deformation, respectively, T0 is the samples’ initial
thickness, which is measured using a thickness gage and Tt is the instantaneous thickness
in the center of the film sample, which is going to be calculated.
21
As far as software is concerned, a LabView based program has been developed in
order to control the machine, to acquire data and to analyze the data. The traditional
modes of deformation that are handled by this machine include the following:
• SIM mode (simultaneous biaxial),
• SEQ mode (sequential biaxial),
• UCW mode (uniaxial Constrained Width).
Samples having 14 x 14 cm dimensions are cut from as-cast films, and an array of
24 yellow dots is printed on these samples. The samples are then placed and clamped in pneumatic clamps for a minimum thermal equilibration time of 5 minutes prior to the start of the deformation process.
The local true strains are determined from the image analysis of the pre-painted dot matrix in the center of the sample, through the use of a high speed CCD camera coupled with an automated image analysis system. The global engineering strains are determined from the gage separation distance.
In-plane and out-of-plane birefringences are measured in the geometric center of
the sample at the same location where true strains are determined. The true stresses are
calculated by dividing the load cell force acting on one of the grips on each side by the
cross-sectional area consisting of the width multiplied by the instantaneous thickness calculated in the geometric center of the sample.
2.1.5 Optical Dispersion & Chromaticity
Basically optical dispersion of a material is nothing but the wavelength-dependent nature of its refractive index. This frequency-dependent refractive index can result in
22
different optical properties for the material during the propagation of light. Detailed description of optical dispersion can be found elsewhere 6. Figure 2.12 also shows the typical optical dispersion behavior of polycarbonate.
Figure 2.12 Optical dispersion curve for polycarbonate.
There are also several formulations which describe the optical dispersion behavior of materials. Koch 19 depicted that the following formula holds for hydrogen, oxygen and air where a, b and λ0 are constants.
1 = + (2.21) 2 푏 2 2 푛 − 푎 휆 −휆0 An extended version of Koch’s formula can be given by replacing n2-1 term with
2(n-1). Cauchy 20 described this formulation as follows.
1 = 1 + (2.22) 퐵1 2 1 휆 For high density materials such푛 as− liquids퐴 and� solids,� Sellmeir’s dispersion formula4 can be used for the variation of refractive index as shown in Equation 2.23.
1 = (2.23) 2 휌���푘� 푘 2 2 푛 − ∑ 푣���푘�−푣
23
Direct consequence of the optical dispersion within the material is the multi-
reflection of incident polychromatic light which results in poor image sharpness.
Therefore the system is said to suffer from chromatic aberration 10. Optical films, lenses
and prisms have this weakness which decreases the overall optical quality of the product.
In order to obtain an achromatic system very thin lenses can be employed.
2.2 Liquid Crystal Materials
2.2.1 Liquid Crystals
Liquid crystals are complex materials in the nature and commonly they are described as partially ordered, anisotropic fluids and thermodynamically classified in between the three dimensionally ordered solid state crystals and the isotropic liquids 21.
General class of liquid crystal mesophases (crystal and liquid phases) can be divided into
two categories that are thermotropic and lyotropic mesophases. Figure 2.13 shows the
common states of liquid crystal matter and classification of liquid crystal phases.
Figure 2.13 Classification of liquid crystal (LC) mesophases 21.
The key factor which determines the state of the mesophase in thermotropic class
of liquid crystals is temperature of the medium. On the other hand for lyotropic case the
24
key parameter is the presence of a suitable (isotropic) solvent. Therefore lyotropic
mesophases are always mixtures, whereas many of the reported thermotropic liquid
crystals are single compounds. These thermotropic phases can be classified as calamitic
(rod-like), discotic (disk-like) and lath-like (sanidic).
Figure 2.14 also shows mesophases of calamitic molecules. In simplest case the
molecules possess only orientational but no positional long range order. Liquid crystals
of this type are called nematic. If a nematic liquid crystal is made of chiral molecules, i.e.
the molecules differ from their mirror image; a cholesteric liquid crystal is obtained. In
smectic A (SmA) phase liquid crystals the molecular orientation is perpendicular to the
layers, whereas the director is tilted in the SmC phase 22 liquid crystals.
Figure 2.14 Some calamitic liquid crystal mesophases 22.
As depicted by the author 23 the variation of the chain conformations within the
liquid crystal polymers (LCPs) can be in the form of flexible chains, semi-flexible chains
and rigid rod-like chains. These characteristics can be controlled by the molecular architecture which contains main-chain LCPs with monomeric LCs in the main chain of
25
flexible links and side-chain LCPs with monomeric LCs attached as a pendant side chain
to the main chain. Therefore the molecular architecture has a direct impact on the
properties of LCPs 24.
Especially anisotropic optical properties are very remarkable for liquid crystals
and by tuning these properties various optical devices can be developed. Orientational
order, therefore birefringence can be manipulated effectively using magnetic or electric
fields which leads to significant magneto-optical, electro-optical and opto-optical effects
22. The reason behind soft nature with high sensitivity to external fields stems from the
chemical structure of the liquid crystal polymers. Figure 2.15 shows typical calamitic
mesogens such as MBBA and DOBAMBC. They have generally a rigid core
incorporating phenyl and biphenyl groups containing also alkyl or alkoxy endgroups. In
discotic mesogens six flexible endgroups are commonly attached to a rigid, disk-like
core.
Figure 2.15 Typical calamitic mesogens.
Nematics and SmAs are uniaxial, SmCs weakly biaxial in nature. Cholesterics
give rise to Bragg reflections if the helix pitch is in the magnitude of the light
wavelength. As mentioned above these properties are carried by a fluid, soft material, and
therefore are extremely sensitive against external perturbations. Molecular architecture
26
also leads to important trends in melting points, transition temperatures and mesophase
morphology. Hird 25 described a general template to represent calamitic systems. Figure
2.16 shows the variation of different units in the liquid crystal structure in these systems.
Figure 2.16 Some examples of core units (a), linking groups (b), terminal moieties (c) and lateral substituents (d) employed in mesogenic molecules 25.
The most successful applications of liquid crystal displays can vary from wrist watches and pocket calculators to flat screens of laptop computer which take advantage of electro-optical effects. More recently, it turned out that orientational order can be also affected by optical fields leading to rather sensitive opto-optical effects and nonlinear optical properties, which are important e.g. for all-optical switching and other photonic devices in future optical information technologies.
27
2.2.2 Physical Properties of Liquid Crystals
The long-range orientational ordering of the nematic directors can be described by
well established Maier-Saupe theory. Details about this theory can be found elsewhere 26.
This molecular statistical theory predicts the long-range ordering of LCPs using the following formulation. Equations 2.25 and 2.26 also describe the orientation distribution function and nematic potential for the system with v is the nematic interaction parameter.
= (3 1) (2.24) 1 2 푠 2 〈푐표푠 휃〉 − ( ) ( ) = (2.25) 1 푢 푐표푠휃 4휋푧 푘푏푇 푓( 푐표푠휃) 푒푥푝 �− � = (3 1) (2.26) 푢 푐표푠휃 1 2 푘푏푇 −푣푠 2 푐표푠 휃 − As a result of orientational order, most physical properties of liquid crystals are
anisotropic and must be described by second order rank tensors. Examples are the heat
diffusion, the magnetic susceptibility, the dielectric permittivity or optical birefringence.
Additionally, there are new physical qualities, which do not appear in simple liquids as
e.g. elastic or frictional torques (rotational viscosity) acting on static or dynamic director
deformations, respectively.
One of the properties of liquid crystals is their ability to support torsional strain 27.
In nematic liquid crystals there is no positional organization and centers of mass of the
liquid crystal molecules are randomly distributed. However, orientational organization of
the molecules induces macroscopic anisotropy for physical properties which results in
increased torsional or curvature elasticity.
Dunmur 27 suggested various experimental methods such as electric or magnetic
field induced Freedericksz transitions, light scattering or torsion pendulum to determine
the elasticity of liquid crystals. These measurements play an important role especially in
28
estimating the behavior of liquid crystals in device designs. As an illustration, Figure 2.17
shows the variation of elasticity constant of 4,4'pentylcyanobiphenyl (5CB) with the
temperature of the medium. It can be an example of the elasticity behavior of nematic
alkylcyanobiphenyls. It is obvious that all elastic constants such as bend, splay and twist
are decreasing with increasing temperature. K1 and K3 are measured with Freedericksz
transitions and K2 is measured by using dynamic light scattering.
Figure 2.17 Splay, twist and bend elastic constants of 5CB as a function of shifted temperature 27.
The static dielectric permittivity is also another important material property that
characterizes the response of a medium to the application of an electric field 28. The determination of dielectric constant can be done by using either the distribution of the electric charges or the intermolecular interactions in molecules.
As an example, Figure 2.18 shows the variation of dielectric constants with temperature for 4-hexyloxy-4'-cyanobiphenyl (6OCB). It is obvious that 6OCB which has negative dielectric anisotropy, can be well oriented by strong magnetic and electric fields.
29
Figure 2.18 Dielectric permittivities for 6OCB 28.
2.2.3 Optics of Liquid Crystals
Key optical parameters for liquid crystals are intensity of light scattering, refractive index and birefringence. Thick nematic liquid crystal samples with thicknesses above 50 µm generally look milky. This indicates that light is being scattered. The scattering is due to thermal fluctuation which causes a random variation of the director in all points in the liquid crystal 7. De Gennes 29 showed the relation of scattered intensity
Iscat of typical liquid crystal layer (Equation 2.27) where d is the thickness of the liquid
crystal slab, Δε is the dielectric anisotropy at optical frequencies and λ is the wavelength
of the light.
2 (2.27) ∆휀 푑 푠푐푎푡 4 Birefringence of liquid crystals is also퐼 another∝ 휆very important property which has
a direct impact on contrast ratio and viewing angle in liquid crystal displays. This
30
phenomenon occurs due to refractive index variation of a medium in each direction in 3-
D space. For this purpose, the principal refractive indices ( , ) of a nematic liquid
푒 표 crystal can be interpreted using the equations of Vuks 30. 푛� 푛�
(2.28)
(2.29)
where,
(2.30)
and; α = 1/3 (αl + 2αt), p is the density, M is the molar mass, εo is the permittivity of a
vacuum and is the average refractive index.
Moreover,푛� α can be defined as the average molecular polarizability, and
퐼퐼 ⊥ are the average polarizabilities parallel and perpendicular to the director, αl 훼and� αt are훼� the longitudinal and transverse molecular polarizabilities respectively and S is the second rank orientational order parameter.
The extraordinary and ordinary refractive indices can be determined by using an
Abbe refractometer. Table 2.1 shows some birefringence values for liquid crystals which have positive dielectric anisotropy. Ohtsuka 30 suggested that the optical anisotropy of a
nematic system increases with the increase of the π-electron conjugation within the
molecule. Thus the replacement of a cyclohexane ring by an aromatic ring makes the
birefringence larger. Low birefringence is required in liquid crystal layer in displays for
obtaining wide viewing angle performance by setting d×Δn to a smaller value.
Phenylbicyclohexanes are typical materials in this category 30.
31
Table 2.1 Birefringence values for liquid crystals with positive dielectric anisotropy 30.
2.3 Spectral Birefringence Techniques
As known from elsewhere, birefringence is an important measure for degree of anisotropy for the materials that are being examined. There are several techniques in literature to measure birefringence using either single wavelength, double wavelength or multi-wavelength methods but none of them are as successful as the spectral birefringence technique recently developed by Valladeres and Cakmak et al. 16.
The key point in this technique is its ability to be fast in tracking the structural changes that are being developed within the material in the order of milliseconds.
Because of the fact that morphological changes in polymers during processing occur very fast, spectral birefringence technique is a perfect candidate for characterization studies.
As depicted by Hassan et al. 18, historical progress of polarized light technique can be listed as:
32
• Single wavelength methods;
• Dual wavelength methods;
• On-line dual wavelength methods;
• Multi-wavelength method;
• Discrete multi-wavelength method; and
• Continuous wavelength method (spectral birefringence).
2.3.1 Single Wavelength Methods
Onagi et. al. 31 employed single wavelength method by using filtered parallel mercury vapor light which is polarized 45° to the stretching direction of the sample
(Figure 2.19). Monochromatic light simply passes through the sample which is clamped in vertical direction and through the analyzer oriented perpendicular to the polarizer and finally detected by a photomultiplier. The output is amplified and recorded whereas the reading which is a function of birefringence is adjusted against using a Babinet compensator. Stein et al. 32 also used a very similar apparatus for dynamic birefringence
studies with the extension of relaxation times.
Figure 2.19 The dynamic birefringence apparatus with single wavelength method: (A) mercury light source, (B) condensing lens, (C) monochromatizing filter, (D) polarizer, (E) slits, (F) sample, (G) analyzer, (H) photomultiplier, (I) two-channel amplifier, (J) two-channel recorder, (K) eccentric drive, (L) motor, (M) variable-speed transmission, (N) linear variable differential transformer 31.
33
Another attempt is also made by Asada et al. 33 which uses a similar experimental design of Onogi et al. 31 but sodium light as the light source. The experimental setup includes a cone-and-plate rheometer with a transparent cone and plate made of quartz with an optical system. The intensities of crossed and parallel polarized light can be found using the following expressions where k=1 for no absorption and scattering, R is the retardation and λ is the wavelength:
× = × (2.31) 2 휋 푅 ⊥ 휆 퐼 푘 푠푖푛 � � × = × 1 (2.32) 2 휋 푅 퐼∥ 푘 � − 푠푖푛 � 휆 �� The latter technique was also significantly successful in measurement of rheological and optical properties of a sheared polymeric system. However all of these techniques employed un-automated calibration procedures with inability to measure retardations larger than one wavelength.
2.3.2 Dual Wavelength Methods
Venkateskaran 34 used dual wavelength photometric birefringence technique to follow birefringence changes in annealing of pre-stretched PET films. The need for dual wavelength method was to detect any inversion in the trend of refractive indices which can be monitored using the intensity profiles of the two wavelengths.
Both He-Ne red (λ = 632.8nm) and green (λ = 543.5nm) lasers were employed as two different light sources instead of one. When the light passes through the optically anisotropic sample the corresponding intensity of the light can be defined in a sinusoidal function which can be expressed in Equation 2.33.
34
× ( ) ( ) = (2.33) 퐼0 2 휋 푅 푡 2 휆 Galay and Cakmak 35, 36 also studied퐼 푡 the푠푖푛 inversion� point� of the birefringence using
two different wavelengths instead of one. It was a demonstration of two-color laser
optical measurement system capable of analyzing fast structural changes takes place
within the polymer which can be correlated with the retardation and therefore
birefringence data. They also employed similar two different laser sources with two
different wavelengths. Using several dichoric filters individual wavelengths can be
differentiated from the beams leaving the analyzer. Birefringence can also be calculated
th using the following expression at the N critical point, Δn12 (n=N) namely, with an
intensity profile with k intensity peaks.
( = ) = ( ) , × (2.34) 휆 12 12 푓 2 푑 2.3.3 Multi Wavelength Methods∆푛 푛 푁 ∆푛 − 푘 − 푁
Full spectrum of light was first used by Yang 37 for the measurement of highly oriented polymeric fibers. First technique to be compared with the new method was
Photographic Fringe Method in which the light coming from the analyzer was dispersed by a front-surface grating and the spectrum was photographed.
The name of the new technique was “Spectrophotometric Method” which is based on measuring the distribution of intensity-wavelength using a spectrophotometer. Figure
2.20 shows the experimental setup of spectrophotometric method. Birefringence can be calculated using the change of the wavelength of the incident polarized light.
35
Figure 2.20 Experimental Setup for Spectrophotometric Method 37.
Redner 38 modified spectrophotometry technique by adding a photodiode which measures sixteen different wavelengths to determine corresponding retardations.
Although it was automatic version of the spectrophotometric method, his system needed
to be calibrated and it was not possible to measure intensities for different wavelengths at
the same time which results in a time lag. Figure 2.21 shows schematic illustration of
Spectral-Content Analysis.
Figure 2.21 Schematic illustration of Spectral-Content Analysis 38.
Hongladarom and Burghardt 39, 40, 41, 42, 43 improved the Spectral-Content Analysis significantly by eliminating the calibration procedure. The light intensities coming from
36
crossed (┴) and parallel (║) polarizers are recorded as a function of wavelength using a
diode array spectrograph. These intensities can be further expressed in following
equations for a sample with birefringence Δn, thickness d and wavelength of the light λ:
× × = ( ) (2.35) ⊥ 퐼0 2 휋 푑 ∆푛 퐼 2 푠푖푛 × 휆× = ( ) (2.36) ∥ 퐼0 2 휋 푑 ∆푛 To help spectrographic technique퐼 in2 eliminating푐표푠 휆 the wavelength dependence of
the light source intensity, spectra for crossed and parallel polarizers are normalized.
Equation 2.37 shows this correlation.
= ; = ⊥ ∥ (2.37) ⊥ 퐼 ∥ 퐼 ⊥ ∥ ⊥ ∥ As mentioned by the author although푁 퐼 this+퐼 technique푁 퐼 + shows퐼 good correlation with
the experimental data for birefringence, still a potential source of ambiguity was present.
This was due to the dispersion in the birefringence Δn(λ) and a cubic equation was used
to represent. As described by Hassan 18, there was also no true extinction for different
wavelengths in this technique which results in an error in calculating the normalized
intensity.
2.3.4 Continuous (on-line) Wavelength Methods
In order to monitor and collect data for very fast structural and/or morphological
changes of polymers take place during processing and/or related processes, the
corresponding frequency of measurements should also be high enough with automated
calibration and data acquisition characteristics. Therefore an advanced version of
spectrographic birefringence technique including the improvements made by de Boer 44
was developed by Serhatkulu 45 to monitor birefringence development of PET and PETG films using on-line spectral birefringence technique. Figure 2.22 shows the basic
37
experimental setup. It had a white light source (420 – 695 nm) which was polarized 45°
to the MD of the film inside the chamber. Using a photodiode array spectrograph
spectrum was collected and sent to the computer for retardation data calculation.
Figure 2.22 Experimental Setup for Spectrographic Birefringence Technique 45.
Cakmak 46, 47 monitored on-line measurement of birefringence during crystallization using two monochromatic He-Ne lasers with 632.8 nm and 543.5 nm wavelengths. His technique was very successful in measuring retardation values from very small values (PLA films) to very high values (PET films) employing sub-second intervals. Figure 2.23 shows the optical setup used for kinetics of PET crystallization.
Figure 2.23 Schematic Representation of Optical Setup Used for On-line Birefringence Measurement 46.
38
+45° plane polarized two laser beams were made parallel using a dichoric-filer
mirror before entering to the heat chamber. Outgoing beams were then first passed
through -45° plane polarized analyzer and then passed through the chopper that chops the
beams at a specified frequency. Finally beams were split into individual wavelengths and recorded using photodiodes.
Cakmak and coworkers 48 also developed real-time stretching spectral birefringence instrument in order to monitor structural changes takes place during uniaxial stretching polymers with on-line measurement of true stress and birefringence. It has been a great success for this technique because of its ability to simulate and characterize real-time processing conditions that takes place during industrial processes.
This spectral birefringence technique was able to capture rapid changes with high resolution and precision in retardation (and birefringence) which can be due to relaxation, crystallization and/or melting depending on the processing conditions. Figure 2.24 also shows the schematic illustration of the new technique.
Figure 2.24 Schematic of on-line Stretching Spectral Birefringence Instrument 48.
39
2.4 Liquid Crystal Displays (LCDs)
2.4.1 Basics of LCDs
One of the most important applications in which liquid crystals are employed is
liquid crystal displays. LCDs are widely used in industry due to their ease and low cost of
production as well as the light weight and low power consumption. As described by
Rezniko 49 in these devices, thin layers of liquid crystals are usually sandwiched between
two glass plates with transparent electrodes. The work of the display was based on the
electro-optical response of the liquid crystal; when its molecules reorient under the action
of the electric field, it results in change in optical properties (e.g., transmission or
reflectance) of the device.
Today, the LCD application spectrum has expanded from wristwatches and
pocket calculators to portable computers and to larger-screen TVs. Although the first
reported basic liquid crystal display is reported in 1968 by the researchers at RCA 50, the main cornerstone in the development of LCD technology was the invention of the twisted nematic effect by Schadt and Helfrich in 1971.
According to this work, by the application of external electric field, liquid crystals can be oriented which results in a change in the propagation of light. The details about this work can be found elsewhere 51. Figure 2.25 shows the basic operation principle of liquid crystal display which is composed of a thin liquid crystal material sandwiched between a set of polarizers.
40
Figure 2.25 Twisted nematic effect in LCDs 22.
Mcdonald 22 described the design of the display as follows: a nematic liquid
crystal is filled between two glass plates, which are separated by thin spacers, coated with
transparent electrodes and orientation layers inside. The liquid crystal molecules are fixed
with their alignment more or less parallel to the plates, pointing along the rubbing
direction, which include an angle of 90º between the upper and the lower plate which
gives a homogeneous twist deformation in alignment. Polarization of a linearly polarized
light wave is then guided by the resulting quarter of a birefringent helix, if the orientation
is not disturbed by an electrical field.
If, however, an AC voltage of a few volts is applied, the resulting electrical field
forces the molecules to align themselves along the field direction and the twist
deformation is unwound. The light wave is not affected and it cannot pass crossed
polarizers. The modulator appears dark.
The illumination LCDs is also another important issue and it can be either transmissive, reflective or transflective (combination of the first two) modes. The selection of these modes depends on the type of electronic displays. As an example,
41
transmissive mode is widely used for especially laptop screens and LCD-TVs; on the other hand, reflective mode is primarily employed in wrist watches, PDAs and rear- projection LCD-TVs.
Several modeling efforts also depicted 52, 24 before which can help us to predict the optical light path right after refraction from the liquid crystal layer incorporated within the display itself. This may help us for further enhancements on the LCD designs.
2.4.2 Addressing Methods for LCDs
In LCDs, the information to be displayed is achieved by pixel addressing process and there are mainly three addressing methods in display industry depending on the information content to be displayed. These can be listed as:
• Direct addressing,
• Multiplex addressing,
• Active matrix addressing.
Direct addressing is the most basic addressing method where each pixel is driven directly with a dedicated electrical contact and a driver for each segment of the digit 53. It was widely employed especially for first commercial LCDs which were designed for operation in simple, low-information-content displays such as digital clocks etc. All segments are placed on one plate of the display with a common counter electrode at the opposite and can be addressed separately. It was explained by Kelly 53 that direct addressing allows the off-state voltage to be zero and the on-state voltage to be several times larger than the threshold voltage. Therefore, a good contrast can be attained as well as low power consumption. This type of addressing is generally applied for twisted- nematic (TN)-LCDs.
42
In multiplex addressing M electrode columns and N electrode rows are used
which allow MxN pixels to be created driven by M+N connections made at the end of
each row and column 54. This type of addressing allows high-information-content LCDs with acceptable contrast and viewing angles. Also number of addressable lines in a multiplexed LCD is limited, although to a much lesser extent than using direct addressing
55. Generally super twisted nematic (STN)-LCDs are used for this type of addressing.
Rezniko 49 depicted that “Active-matrix displays usually contains a matrix of thin-
film transistors (TFT)s in addition to the liquid crystal and embedded polarizers. These
transistors store the state of each pixel on the display while all the other pixels are being
updated”. This method provides a much brighter, sharper and ultra high-information-
content display than a passive matrix of the same size.
The major LCD modes which contain active-matrix structure can be listed as
Twisted-Nematic (TN)-LCDs, Electrically-Controlled Birefringence (ECB)-LCDs, In-
Plane Switching (IPS)-LCDs, Vertical Alignment (VA)-LCDs etc. As described by Boer
56, “For active matrix addressing, generally either thin film transistors (TFT) or thin film
diodes (TFD) can be used. TFTs are cousins of the transistors used in semiconductor
chips. In terms of switching speed and operating voltage, they are inferior to state-of-the-
art MOS transistors in crystalline Si. However, they are quite adequate as a simple
ON/OFF pixel switch in AMLCDs with a 60 Hz refresh rate”. On the other hand, a TFD can be either a rectifying diode or a bi-directional diode or a nonlinear resistor.
Figure 2.26 also illustrates the schematic of typical active-matrix displays
commonly used in industry. LCDs with active matrix addressing are switched on by a
43
voltage pulse at the TFT or diode. The charge at the pixel, which corresponds to a
capacitor, should remain constant until the pixel is addressed again in the next frame 53.
Figure 2.26 Basic structure of active-matrix LCDs (Brittanica).
2.4.3 Types of LCDs
The majority of LCDs produced in industry have active matrix addressing due to high-information-content of the display. There are also different types of AMLCDs that are using different nematic liquid crystals which results in different LCD classes. The major nematic LCD classes will be listed below. Table 2.2 shows commonly used nematic liquid crystals in displays and their properties. In this table Cr and N represents melting and clearing point transition temperatures. Also, dielectric anisotropy (∆ε) and conductivity (Ω-1cm-1) values are given.
44
53 Table 2.2 Properties of commonly used nematic liquid crystals in displays .
Cholesteric Reflective Displays
One of the distinguished phases among liquid crystal classes is the cholesteric mesophase. It is generally obtained by alignment of chiral molecules or mixtures containing chiral components. The major property of this mesophase that is different from the nematic liquid crystals is the helical structure of the molecules which results in no reflection symmetry 57. Figure 2.27 shows the basic structure of cholesteric LCDs.
Figure 2.27 Schematic representation of a cholesteric LCD 58, 59, 60.
45
In the off-state, since there is no electric field application, liquid crystals have chiral nematic phase 53. Therefore, scattered light can be transmitted through the second
polarizer and the display appears white in the off-state. Whereas, in the on-state application of the electric field results in phase change from chiral nematic to nematic state. Helix is completely unwound which produces a clear state. Therefore, analyzer absorbs linearly polarized light and the black information can be displayed at the appropriate pixels against a white background to produce an image with positive contrast
53.
For extreme viewing angles, the reflection band shifts to a shorter wavelength
which results in broader reflection band. Therefore the color of the LCD changes with the
viewing angle. This problem can be partially solved by dispersing a small amount of
polymer in the liquid crystal 57. But this problem is not so severe because of the fact that majority of cholesteric LCDs employ only single color instead of multiple colors in conventional LCDs.
Kent Displays has also developed a "no power" display that uses Ch-LCD structure. The major drawback to the Ch-LCD is slow refresh rate, especially with low temperatures. Typical retardation values required for the optical retarder layer can be given as λ/4 where λ denotes the wavelength of the light 61.
Vertically Aligned (VA)-LCDs
As depicted by Greener 62, a vertically aligned liquid crystal display (VA-LCD)
offers an extremely high contrast ratio for normal incident light. Figure 2.28 show the
schematic illustration for VA-LCD. In the off-state, light in the normal direction does not
see the birefringence of the liquid crystal layer which results in a dark state that is close
46
to that of orthogonally crossed polarizers. However, an obliquely propagated light can be
retarded due to the liquid crystal layers and this leads to light leakage. On the other hand,
in the on-state, liquid crystal structure is rotated which results in increased birefringence
during the propagation of light that creates a white display.
62 Figure 2.28 Schematic illustration of VA-LCDs with on or off state .
VA-LCDs provide some of the same advantages as IPS panels, particularly an
improved viewing angle and improved black level. However, they suffer from poor
contrast ratios in a viewing angle range. As depicted by Kim 63, typical out-of-plane retardation values required for the optical retarder layer can be given as less than 200 nm.
Electrically Controlled Birefringence (ECB)-LCDs
The first electrically controlled birefringence LCDs were developed by the
Siemens Corporation in Germany which was based on direct addressing 64. VA-LCDs are
also derived from ECB-LCDs by changing the alignment behavior of the liquid crystals.
These related display types based on the same electro-optical effect, but slightly different
display configurations are denominated by a multitude of names 53. Figure 2.29 shows the
schematic of the ECB-LCDs.
47
64, 65, 66, 67, 68, 69 Figure 2.29 Schematic illustration of ECB-LCDs .
One or two linear, elliptical or circular polarizers are necessary depending on the corresponding design of the display and whether the LCD is working on either transmissive or reflective mode 69. In the off-state of the LCD, the resulting birefringence
is zero due to the perpendicular orientation of liquid crystals to the substrate. In the on-
state however, resulting birefringence increases with increasing applied electric field
strength for orientation of the liquid crystals. Sakamoto 70 described the out-of-plane retardation value requirement for the optical retarder which is around 300 nm.
Optically Compensated Bend (OCB)-LCDs
Optically compensated bend liquid crystal displays use a nematic liquid crystal cell based on the symmetric bend state. The brightness of the display can be easily controlled by the applied voltage in order to align the liquid crystals. Figure 2.30 shows the basic structure of OCB-LCDs.
48
62 Figure 2.30 Schematic illustration of OCB-LCDs with on or off state .
In the off-state liquid crystals are aligned in bend position with respect to the center line. In the on-state liquid crystals almost 90° to the substrate except the liquid
crystals that are very near to the wall. OCB-LCDs offer fast response time which is
crucial for LCD-TV applications. However, due to the birefringence of liquid crystals and
cross polarizers, viewing angle suffers from contrast decrease. Lyu 71 described the retardation value requirement for the optical retarder which is around -30 nm.
Twisted Nematic (TN) & Super Twisted Nematic (STN)-LCDs
The first TN-LCD was developed by F. Hoffman-La Roche in Switzerland in
1970 72. Typical TN-LCD contains a nematic liquid crystal mixture of positive dielectric
anisotropy, an alignment layer on both substrate surfaces such as rubbed polyimide and
crossed polarizers with a cell gap of 5-10 µm 53. Figure 2.31 shows a basic TN-LCD.
Figure 2.31 Schematic representation of TN-LCD 54.
49
In the off-state, liquid crystals are in bend position and LCD display is seen as
dark. A certain amount of light still leaks through all TN-LCDs in the off-state due to
non-ideal polarizers, non-uniform nematic alignment, liquid crystal cell thickness
variation, temperature dependence, dispersity of Δn (birefringence) and the
polychromaticity of light 53. Therefore this light leakage can be easily resulted in a
contrast decrease. In the on-state, due to the applied electric field the liquid crystals are perpendicular to the substrate.
During the development of LCDs, classical TN-LCD structure was not sufficient for high-information-content displays such as notebook displays. Therefore new classes
of TN-LCDs so called super twisted nematic LCDs were developed. The first STN-LCD
was developed by the researchers from Bell Laboratories, Murray Hill, New Jersey, USA
73, 74, 75. As depicted by Mi 76, typical retardation value requirement for the optical retarder
which is less than 100 nm.
In Plane Switching Mode (IPS)-LCDs
LCD based on the response of the nematic liquid crystal to electric field in same plane of the display was developed first by the Fraunhofer Institut in Germany in 1990s
53. Due to the fact that twisted nematic to aligned nematic phase change of the liquid
crystal occur in the same plane with the display, IPS-LCDs have several advantages in compare with TN-LCDs other displays. The main reason behind is the fixed optical axis of the liquid crystals. Figure 2.32 shows the basic schematic of IPS-LCDs. As can be
seen from the figure, in either off or on-state the optical axis of the liquid crystals remains in the same plane.
50
Figure 2.32 Basic schematic of IPS-LCDs 77, 78.
In the off-state, aligned nematic liquid crystal mixture does not rotate the plane polarized light which results in effective absorption of light. In on-state, with the
application of electric field, nematic liquid crystal mixture reorients itself with the
molecular long axis parallel to the applied field 53.
Generally IPS-LCDs offer high contrast ratios but they have a poor oblique angle
contrast. Jeon 79 described the out-of-plane retardation value requirement for the optical retarder film which is 20-100 nm.
2.4.4 Components of LCDs
During the manufacture process of LCDs, highly engineered plastics and high-
tech materials are included so that the mechanical, thermal and structural properties of
these materials become very important as the number of LCD applications is increasing.
In order to achieve a complete understanding of technical issues, first an overview of
where these materials are used should be taken into account 2:
Alignment Layer
The main purpose of this layer is to align the liquid crystal phase on top of it to a
certain angle with respect to the polarizers. These layers are generally made up of special
51
polymers such as polyimides for stability issues. The strength of anchoring is important
because it must compete with the elastic energies of the liquid crystals in the presence
and absence of the applied fields.
Retardation Film
This layer is generally referred to as a phase compensating sheet or compensator.
An important feature of the film is that it offers uniform retardation over the wavelength
spread of the visible spectrum. It is a Japanese innovation and it has been applied
primarily for TN and STN displays to increase viewing angle.
Color Filters
They are mainly RGB color filters for full color active and passive matrix displays. Table 2.3 shows the main manufacturing methods to produce color filters. The purpose of these filters is to give the necessary color to the incoming polarized light coming from liquid crystal layer.
Table 2.3 Primary manufacturing methods for color filters (Sharp corp.).
Glass Substrate
Flat-glass substrates are the major components of the LCDs. The glass should
have a high degree of flatness over large areas for TFT printing especially for active
matrix LCDs. The most popular glass for these displays is borosilicate glass supplied
from major suppliers.
52
Polarizing Sheets
Typical optical properties of these films include a transmittance of 40-43% with a degree of polarization closely approaching 100%. Although there are some concerns about the deterioration of the transmission and of the polarizing efficiency, this industry is mature and technology results in improved polarizing efficiencies.
Spacers
These small spheres used as spacers are primarily used for PDLC displays. These spacers are especially useful if soft plastic substrates are used during manufacturing process.
2.5 Wide Angle Optical Retarder
2.5.1 Basics of Retarder Films
Optical retarder films play a very important role in changing the final optical properties of the liquid crystal displays. These optical properties are mainly contrast ratio, viewing angle and color shift performance. Because of the fact that today’s LCD manufacturing technology is good enough to catch up good contrast ratios for high- information-content displays, the corresponding viewing angle values are suffering due to the light leakage in oblique angles of the propagation of light and given birefringence.
Therefore efficient optical retarders for LCDs are necessary to obtain good viewing angle characteristics.
Optical retarder films are mainly characterized in terms of birefringence which is related to the refractive index n of the retarder film. As well known from literature 18, the refractive index of an isotropic polymer film in each direction (MD, TD and ND) will be the same regardless of the polarization state of the light wave that is propagating through.
53
On the other hand, as the material becomes oriented, either uniaxially or biaxially, the
refractive index becomes dependent on the direction of anisotropy. This resulting
anisotropy can be quantified using in-plane and out-of-plane retardation (Rin & Rth) and
birefringence (Δnin & Δnth) values.
Assuming cartesian coordinates x, y and z represent MD, TD and ND
respectively, Equations 2.38-41 show this relation where d is the thickness of the film.
= = (2.38)
∆푛푖푛 ∆푛푥푦 푛푥 − 푛푦 = = + 2 (2.39) �Δ푛푥푧+Δ푛푦푧� 푡ℎ 2 푥 푦 ⁄ 푧 ∆푛 = � × � �푛 푛 � − 푛 (2.40)
푖푛 푖푛 푅 = ∆푛 ×푑 (2.41)
푡ℎ 푡ℎ Experimentally, retardation푅 values∆푛 of푑 R0 and R45 can easily be obtained using the
16 method depicted by Hassan where R0 = Rin = retardation of the film for normal
incidence and R45 is the retardation of the film tilted through 45° angle. If we assign
numbers 1, 2 and 3 to the cartesian coordinates x, y and z, retardation measured at 0 degree provides the in-plane birefringence.
= (2.42) 푅0 ∆푛12 푑 Using the same retardation values of R0 and R45 and using Stein`s equation, the
out-of-plane birefringence can be calculated using Equation 2.43 when the machine direction is vertical.
/
1 2 = 푠푖푛245 (2.43) 0 45 1 푅 −푅 �1− 푛�2 � 푠푖푛245 23 푑 ∆푛 − � 푛�2 � is the average refractive index of the film which can be assumed to be a constant value.
푛�
54
= (2.44) 푛1+푛2+푛3 The third birefringence can be calculated푛� automatically3 using the orthogonality
relation.
+ + = 0 (2.45)
12 23 31 Finally out-of-plane retardation∆푛 Rth can∆ 푛be calculated∆푛 using the well known
relation.
( ) = × (2.46) Δ푛13+Δ푛23 푡ℎ 2 Another important parameter of푅 birefringence/retardation� � 푑 behavior of polymeric
materials is intrinsic birefringence. It is a physical property of a material in molecular level and represents theoretical birefringence of the material assuming polymer chains
aligned in one direction. Therefore materials can be classified in two main groups namely
positive birefringent and negative birefringent materials. The former class shows lower
refractive index perpendicular to the stretch direction whereas the latter shows higher
refractive index.
Retarder films used in industry can also be classified in to two categories
including biaxial films where all three refractive indices differ and two optical axes exist,
and uniaxial films having only one optical axis where two of the three refractive indices
are the same 80. Table 2.4 shows this classification depends on refractive indices in each
direction. Figures 2.33-35 illustrate the classification of different retarder films.
Table 2.4 Relationship between the refractive indices for different retarder plates.
Positive A nx>ny=nz Negative C nz Negative A nx Positive C nz>nx=ny Negative B nx 55 Figure 2.33 Illustration of uniaxial positive and negative A retarder films. Figure 2.34 Illustration of uniaxial positive and negative C retarder films. Figure 2.35 Illustration of biaxial positive and negative B retarder films. 56 2.5.2 Optical Retarder Film Designs There are mainly three different categories of wide viewing angle retarder designs. The first and most commonly used one is single layer optical retarders which can be specific polymers such as polyimides, polyamides, polycarbonates etc. The second category of retarders are basically multilayer retarders which are composed multiple combination of polymers that can be produced by specific processes. Final category of optical retarders includes a mixture of either single-layer or multiple-layer retarder film plus the polarizer sheet. During the processing, these films can be stacked onto each other and after a uniaxial stretching process the polarizer sheets can be obtained. These different classes of retarders are described below. Single Layer Wide-Angle Optical Retarder Typical single layer optical retarder can be the one that is patented by Fuji Photo Film Co. in Japan 81. In this patent, after several chemical reactions of raw cellulose (hardwood pulp) steps, we can easily obtain final raw cellulose acylate resin. After a suitable melt casting process, the melt casted cellulose acylate films can be biaxially stretched 82, 83 to introduce necessary in-plane and out-of-plane birefringence. This results in a positive retardation value which can be used to compensate the birefringence of the propagated light after liquid crystal layer in the LCD. Table 2.5 shows the content of the cellulose acylate after chemical reactions, its optical and processing properties and final thickness of the film. 57 Table 2.5 Optical and processing properties of cellulose acylate samples 81. Another important example can be patterned single layer wide-angle optical retarders for especially transflective LCDs. These types of retarders have been primarily developed by Philips Research Laboratories in Netherlands 61. Main advantages of patterned retarders are high reflectivity and transmission, good contrast ratio, low chromaticity at all gray levels and wide viewing angles. The primary material that is being used is liquid crystal di-oxetanes. They can be obtained from LC diacrylates which can be illustrated in Figure 2.36. 61 Figure 2.36 Basic synthesis reaction to obtain LC dioxetanes . In this application, for alignment of liquid crystals a polyimide layer Precursor OPTMER AL-1051 was used. Surfactants and initiators were obtained from Merck and Union Carbide UVI respectively. Finally all other remaining chemicals were obtained 58 from Aldrich. After the photoinitiation steps, final patterned retarder can be obtained as can be seen from Figure 2.37. Here, bright fields represent birefringent domains whereas dark fields represent isotropic domains. Final polymer film has a 1µm thickness which is far below conventional optical retarders which have approximately 150 µm thickness. Figure 2.37 Structure of patterned optical retarder for transflective LCDs 61. Multi-Layer Wide-Angle Optical Retarder In literature there are many multi-layer optical retarder designs which are aimed to compensate the corresponding retardation in light due to the liquid crystal layer in LCDs. One example can be multi-layer films which contain spaced-apart rows of 84 different materials . In this retarder design, each consecutive layer should contain TiO2, SiO2, Al2O3 or different dielectric material containing polymer. In addition to this the first and second layers should have an independent optical retardation value of at least 1 nm for the light of wavelength λ. The wavelength of the light can be in between 400 nm to 700 nm or 1,200 nm to 1,600 nm. Also, the combined thickness of the first and second layers should be about 10 microns or less. Figure 2.38 shows the schematic of this design. Each consecutive layer contains different refractive index material. 59 Figure 2.38 Schematic illustration of multilayer compensator 84. Another example can be a multi-layer compensator comprising different polymer layers which have different out-of-plane birefringence values 85. In this design, the individual layers should have an out-of-plane birefringence not more negative than -0.01 and the overall in-plane retardation of the multilayer compensator should be greater than 20 nm and the out-of-plane retardation of the multilayer compensator should be more negative than -20 nm. As an illustration, layer A can be a polymer film other than one containing a chromophore group in the backbone such as triacetyl cellulose (TAC), cellulose acetate butylate (CAB), cyclic polyolefin, polycarbonate, polysufonate etc. These polymeric materials can be made into a film form by solvent casting, extrusion or other methods. Polymer to be used in the B layer should not have chromophores off of the backbone such as polyarylates possessing the fluorine group. The thickness of each consecutive group can be around 10 µm. The corresponding glass transition temperatures of the polymers should be above 180 °C for desired results. The polymer used in layer B can be synthesized by a variety of techniques such as condensation, addition, anionic, cationic or other common methods of synthesis. Because of the fact that intrinsic birefringence of the polymers is determined by factors such as polarizabilities of functional groups and their bond angles with respect to 60 the polymer chain, the actual birefringence of a polymer depends on the process of forming it. Therefore by applying a uniaxial or biaxial stretching procedure, the desired birefringence values can be easily obtained. Figure 2.39 shows the basic structure of retarder film. Figure 2.39 Schematic illustration of multilayer optical retarder in LCD 85. Hybrid Retarders Final class of optical retarder designs is so called hybrid retarders which contain both the retarder film and polarizer sheet together. The main purpose in these hybrid designs is to reduce the manufacturing cost of the LCDs. One example for this design can be a composite retarder plate which contains at least one retarder film and at least one liquid crystal phase sheet which can be combined with an optically compensatory polarizing plate by absorption technique 86. The liquid crystal phase sheet includes a transparent substrate, and a liquid crystal polymer layer provided on the transparent substrate. At least one of the refractive indices nx, ny and nz of the retarder film should be 61 different from the other refractive indices when nx and ny are main refractive indices (nx ny) in in-plane directions and nz is a refractive index in a direction of the thickness of the≥ composite retarder plate respectively. The retarder film, the transparent substrate and the liquid crystal polymer layer have different birefringence characteristics changing with the wavelength of the light. Figure 2.40 shows the basic schematic for hybrid retarder. Figure 2.40 Basic schematic for hybrid retarder 86. Another example can be the hybrid retarder by the Nitto Denko Corp. in Japan 87. This design is based on a solution casted multilayer compensator which includes one or more polymeric first layers and one or more polymeric second layers which can be combined with a polarizer sheet using a PVA solution. In this design, the individual layers should have an out-of-plane birefringence not more negative than -0.01 and not more positive than +0.01 and the overall in-plane retardation of the multilayer compensator should be greater than 20 nm and the out-of-plane retardation of the multilayer compensator should be more negative than -20 nm or more positive than +20 nm. The in-plane retardation of the one or more first layers is 30% or less of the overall in-plane retardation of the multilayer compensator. Cellulose ester is used as the low birefringent first layer which can be followed by birefringent second layer such as polyimides, polycarbonates, polysulfonates etc. 62 Plasticizers are used to improve drying characteristics of the wet film. Surfactants are also used to improve the uniformity of the coated film. If the birefringent second layer comprises an amorphous polymer, rapid drying is needed during solution casting process to retain its birefringence values. Finally, the multilayer composite film can be stretched uniaxially or biaxially depending on the desired in-plane and out-of-plane birefringence values to give the necessary compensating characteristics. In addition to this an optional polarizer sheet can also be attached to the multilayer retarder for the sake of cost effective manufacturing process. 2.5.3 Advantages of Multilayer Optical Retarders Among previously mentioned optical retarder designs for LCDs, multilayer optical retarders have some superior advantages. One of these advantages is the well known optical dispersion curve flattening phenomenon. It can be also known as birefringence dispersion. Optical dispersion is basically the variation of the refractive index of the material with frequency of the light that is propagating 6. It is obvious that the corresponding refractive index of PMMA is changing with the wavelength of the light that is propagating through. In typical retarder film applications, this birefringence dispersion cause color saturation and other optical issues decreasing the overall optical performance of the films. Because of the fact that optical retarder films for LCDs are designed for certain wavelength range depending on the desired birefringence, the variation of the refraction index of the overall optical compensator should be minimal. Therefore multilayer nature of the retarder induces flattening of the dispersion curve, i.e. minimization of the 63 refractive index, by tuning different polymers which have different optical characteristics in the same compensator. This can include employment of consecutive positive and negative birefringent polymeric layers for further enhancement. 2.5.4 Methods of Manufacturing for Multilayer Retarder Films Melt Coextrusion This method is based on the co-extrusion process first reported by Walter Schrenk 88. This process provides an easy symmetry control of the layer thickness during multilayer extrusion. If we use two different polymeric materials, first, streams of first and second thermoplastic material are divided into substreams being combined at locations which are disposed on an arc of a circle with the interdigitated substreams. A more advanced multilayer co-extrusion process can be the co-extrusion of PMMA grades in a microextruder 89. In this example, photoreactive blends of PMMA and up to 25 wt.% of trans-cinnamic acid (CA) or trans-methyl cinnamate (MC) was used at temperatures between 190 to 200 °C in a co-rotating twin-screw extruder. Figure 2.41 shows the schematic illustration of multilayer co-extrusion process. Figure 2.41 Schematic illustration of coextrusion process 89. 64 This process allows the production of films with an individual layer thickness of as little as 10 nm, with a thickness accuracy of approximately 20 %. With the help of the multiplier, assembly of n consecutive multiplying elements produces an extrudate with n the layer sequence (AB)x where x = 2 . On the other hand, an important prerequisite for the multi-layering process is comparable rheological behavior of the materials. Solvent Casting Another important alternative for multilayer polymeric film manufacture is by solvent casting. For conventional processes, normally a single doctor blade or die is used for solution casting incorporating with the necessary dryer equipment. In order to produce multilayer polymeric films, a proper multi-slot die should be used during solution casting process which allows multiple polymeric solutions casted simultaneously on top each other. A proper die design for this purpose was given by the Eastman-Kodak company 90. It is based on the fact that applying all of the coating materials simultaneously while maintaining a distinct relationship between the different layers after they have been cured or dried on the support. Figure 2.42 shows the illustration of multi- slot die. Figure 2.42 Scheme of multi-slot die 90. 65 2.6 Solution Casting Process 2.6.1 Process Overview Solvent casting process is one of the oldest technologies which have been used for the manufacture of high-quality films with excellent flatness and dimensional stability. The main logic behind this process includes four individual steps known as raw material preparation, dope preparation, film casting and solvent recovery. Figure 2.43 shows a brief illustration of these steps that take place in solution casting process 91. After obtaining raw materials, next step is dope preparation. Notice that selection of different polymer grades with different solvents including additives plays a very important role in determining the final characteristics of the solution casted film. This stems from the fact that polymer-solvent combinations directly affect the rheology, drying characteristics and thermodynamics of the polymer solution that is being casted. After the film casting procedure with drying steps, high quality films can be obtained with an optional solvent recovery system. Figure 2.43 Explanation of Solution Casting Process 91. 66 Film casting step in solution casting process usually includes a belt machine. Figure 2.44 shows schematic of a belt machine. It consists of two drums one of which is connected to a drive that requires extremely accurate speed control to prevent speed variations. The other drum is connected to a servo system which adjusts belt tension to minimize vibrations during solution casting 91. Figure 2.44 Schematic of belt-machine 91. Although there are different kinds of configuration of dies for the solution casting process, the two kinds of dies are shown in Figure 2.45. Detailed information about different configurations can be found elsewhere 92. Notice that the selection criteria for casting dies depend on the process characteristics of solution casting process. Figure 2.45 Doctor blade die (left) and slot die (right). 67 Drying process is also crucial for the sake of decreasing the residual solvent bound to the casted film due to the fact that decreased diffusion coefficient of the solvent and chemical likeliness of the solvent and the polymer. Residual solvent generally acts as a plasticizer and decreases the overall glass transition temperature of the polymer film in compare with the neat polymer grade Tg. Figure 2.46 shows temperature balance in film formation process 91. Figure 2.46 Schematic illustration of drying process during solution casting 91. Depending on the requirement of efficient drying a second drying step right after the first basic drying can be applicable in order to remove the majority of the solvent. For this purpose, generally clamp or tenter driers are widely used which can be seen in Figure 2.47. Heated rollers can be used for special high efficiency drying and IR radiators are used sometimes in the last part of the drying process 91. Figure 2.47 Clamp/tenter dryer 91. 2.6.2 Residual Solvent In literature there are almost no references related with the residual solvent concept in solution casting process. Kaczmarek 93 depicted the importance of Atomic 68 Force Microscopy (AFM) technique in determination of polymer-support interactions in solution casting using an poly(methyl methacrylate) (PMMA) based system with the influence of solvent residue. Also Kruczek 94 revealed the fact that polymer-solvent interactions existing in casting solution primarily determine the concentration of residual solvent and surface morphology of the films using an sulfonated poly(2,6-dimethyl-1,4- phenylene oxide) (SPPO) based system. A complementary thermogravimetric analysis (TGA) was also done revealed the fact that TGA spectra contains three consecutive weight loss stages which are due to removal of water, decomposition of sulfonic groups and beginning of splitting of main chains before final decomposition. Figure 2.48 shows this behavior. Figure 2.48 Thermal stability of dense SPPO films 94. Since the references depicted above were mainly based on specific polymer- solvent cases, research on this area is needed for different polymer-solvent systems focused on the analysis of residual solvent content and the variation of solvent casting process parameters. These concepts play an important role in the final quality of the optical films which can be applied major industries including electronics. 69 CHAPTER III TEMPORAL EVOLUTION OF OPTICAL GRADIENTS DURING DRYING IN CAST POLYMER SOLUTIONS 3.1 Introduction* The solution casting process is increasingly being used in the production of functional films including optical film retarders, electrically conductive transparent films and flexible photovoltaic devices. Doctor blade coating and slot die casting of polymer solutions on substrates gained significant importance in film industry due to uniform thickness distribution for roll-to-roll continuous film manufacturing 95. Solution casting in general is a complex process in which solvent evaporation triggers multiple physical changes simultaneously. These physical changes include evaporation-induced weight loss and the accompanying reduction in thickness while the polymers undergo reorientation with their primary chain axes oriented in the plane of the film 96. During the drying process diffusion of the solvent occurs from substrate-coating interface to air-coating interface which proceeds through the thickness direction of the wet coating. This results in development of a gradient in physical parameters such as concentration and refractive index 97, 98, 99. The concentration profile of polyvinyl alcohol (PVA) solution in water with tracer polystyrene (PS) fluorescent particles was tracked * Yucel O., Unsal E., Cakmak M., Macromolecules, August 2013 (Accepted). 70 using confocal microscopy and particle image velocimetry 100. It showed evaporation driven flow inside the polymer solution toward the substrate with resulting concentration gradient at different drying temperatures. A number of theoretical and experimental studies have been carried out on the drying behavior of glassy polymers and reported skin layer formation during drying 101, 2. As a result of this phenomenon, the PVOH solutions was found to develop crystallized skin in the early stages of drying evidenced by specially developed low field nuclear magnetic resonance (NMR) methods with Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence 102, 103, 104, 105. The development of concentration gradient leads to skinning. This skinning phenomenon stems from rapid evaporation of solvent near the air surface coupled with slow diffusion of solvent molecules from interior to the top surface. This occurs by formation of a skin layer at air-polymer coating interface at severe drying conditions 106. Skinning acts as a barrier leading to entrapment of the residual solvent inside the dried film. Trapping skinning effect becomes less evident for thicker coatings as the larger amount of solvent molecules remaining in the film suppresses the formation of the skin as they steadily diffuse to the surface from interior 107. The extent of bulk averaged optical anisotropy in solvent cast polymer films after drying reported previously 108, 62. It was found that level of birefringence was controlled by drying conditions, surface tension of the substrate and boiling point of the solvent chosen. Several theoretical studies carried out to develop proper models to predict out-of- plane birefringence generation 109, 110, 111. The generated birefringence was attributed to the residual drying stresses produced during the film casting and drying process along with other parameters such as polymer and solvent type, film thickness, drying rate and 71 molecular weight. Prest and Luca also discussed the effect of coating thickness, polymer concentration for plasticized systems and molecular weight on birefringence using principal refractive index measurements 112. The drying induced refractive index increase of cyclo-olefin polymer (COP) and polystyrene (PS) films was correlated with density along with film thickness decrease 113. Refractive index change at different temperatures and concentrations of PVA in PVA/PMMA blend was also reported 114. The addition of zinc-oxide (ZnO) nanoparticles on spin-coated and bulk polymerized ZnO/PMMA composite showed increased refractive indices with increased nanoparticle concentration 115. Poly(propylene glycol) (PPG) – salt complexes showed decrease in the refractive index value due to density change of the polymer-salt composite 116. Okajima and Koizumi first reported the application of Abbe refractometer to the measurement of principal refractive indices of the dried polymer films 117. This technique can be employed for refractive index measurements in machine direction (MD), transverse direction (TD) and normal direction (ND) as described by Samuels 118. It was shown the refractometer measures the refractive indices at the surface facing the refracting prism 119, 120. This technique was used to obtain the refractive indices of each layer in two component laminates 121, 122 and principal refractive indices in biaxially stretched Poly(Ethylene Terephthalate) (PET) films 123. It was shown to be useful for direct measurement of surface anisotropic refractive indices on inside as well as outside surfaces of the stretch-blow molded PET bottles as this process imparts significant through thickness deformation (thus orientation) gradient as a result of radial deformation 124. 72 A number of non-contact techniques can be utilized in order to measure birefringence using single wavelength 33, 31, double wavelength 46, 36, 35 or multi- wavelength methods 42, 39, 40. Among these, spectral birefringence technique is unique as it can measure the rapid changes on the order milliseconds in the whole visible range (400nm to 700 nm) 56, 45. Recently, an improved version of spectral birefringence technique for on-line tracking of birefringence measurements during drying of solution cast polymer coatings was introduced by our group 1. The custom built drying equipment can measure real-time in and out of plane birefringences, weight, thickness and surface temperature changes during the drying of polymer solutions. The in-plane (Δn12) and out- 125 of-plane birefringence (Δn31) are calculated using equation shown below (Equation 3.1 & 3.2); = = (3.1) 푅0 12 푀퐷 푇퐷 ∆푛 푑푚 푛 − 푛/ 1 2 = 푠푖푛2휃 = (3.2) 0 휃 1 푅 −푅 �1− �푛��2� � 푠푖푛2휃 31 푑푚 푇퐷 푁퐷 ∆푛 � � � 푛���2� � 푛 − 푛 where dm is the real-time thickness value, R0 is the measured 0° retardation Rθ is the retardation value measured at θ degrees (where θ = 45° in our case), and is the average refractive index of the polymer coatings in three principal refractive index푛� directions i.e. nMD, nND and nTD. Because of the existence of the refractive index gradient due to fast evaporation rate of the solvent at coating-air interface of the drying polymer coating, the use of average refractive index throughout the thickness direction may be in question in determining the real time out of plane birefringence using spectral birefringence method that uses equation 2. We will address this issue in this chapter as well. 73 Evaporation of the solvent molecules occurs from the air surface that results in gradient in refractive index through the thickness of the coating. In this chapter, we experimentally address the existence of this optical anisotropy and its gradient through the thickness for PAI solution in DMAc by real time tracking of refractive indices on both substrate and air surfaces and their anisotropy by polarized Abbe refractometry and their gradient by rapid microtomy coupled with optical retardation method. 3.2 Experimental 3.2.1 Materials Poly (amide-imide) (PAI or L8®) 10 weight percent solution in DMAc was provided by Akron Polymer Systems (APS). Before solution casting process, PAI solutions were mixed further using Thinky Planetary Centrifugal Vacuum Mixer ARV- 310 (Rotation + Revolution) for improved dissolution, uniformity and degassing. Refractive index values of PAI and DMAc are 1.688 and 1.435 at 633nm respectively. 3.2.2 Optical Measurements Principal refractive index measurements on both surfaces were performed by Bellingham+ Stanley limited 60/HR Abbe refractometer with eyepiece polarizer. To track the refractive indices against the substrate surface, we directly cast the solution on Abbe refractometer prism and determine the in and out of plane refractive indices (MD, ND, TD) as a function of time with eyepiece polarizer without the use of additional immersion contact liquid. These measurements were carried out using a white light source with a band pass filter (633 nm) to generate the monochromatic light. In order to track the refractive indices of solution-air surface, we cast the solution of same thickness (1 mm.) onto a PET film substrate and periodically measured this surface by placing a 74 freshly cut sample from large cast film against the measuring prism of the Abbe refractometer with an immersion solvent (Refractive index of 1.72)26. Since the solution gels very rapidly (on the order of 10s of minutes) it has sufficient structural integrity for this measurement. Birefringence profile through the thickness was measured by Leitz Laborlux 12 POL S cross-polarized microscope using 30 order Berek compensator. A large film from PAI solution was cast on a PET substrate film surface (Figure 3.1 step 1). 1 mm wide slice was cut using a special cutting tool that keeps two fresh razor blades parallel during cutting as shown in Figure 3.1 (step 2), and retardation measurements were carried out within 5 minutes of cutting by laying down the sample in ND-TD plane to minimize changes (Figure 3.1 steps 3-4). For each time interval, we made freshly cut slices from the large cast film to reduce solvent losses from cut surfaces for through thickness optical retardation and birefringence calculation. In this cutting the substrate PET layer was kept attached to eliminate the possibility of evaporation from substrate surface during measurements. Validation of birefringence of the final dried PAI films was performed using an optical bench apparatus with a 7 order Babinet compensator. Figure 3.1 Solution casting and sample preparation procedure for retardation calculation using compensator method. 75 3.2.3 Real-time Weight, Thickness, Temperature & Birefringence Measurements. Solution cast films were dried in the chamber of real-time measurement system described in detail elsewhere1. This system measures, weight, thickness and in and out of plane birefringences of cast solution on a glass substrate with controlled air speed and temperature. Dried films peeled from glass substrate and final thicknesses were measured using Mutitoyo CUA-154 micrometer. Dimensions of cast films were 3” by 7” and retardation and birefringence values were calculated in the center portion using 633 nm light. Real time weight measurements incorporate both global and local weight change while the former was directly acquired from precision balance and latter was calculated using both real-time thickness data and density of the coatings. The solutions were dried at room temperature with no air speed. Relative humidity (RH) within the chamber was not controlled and varied between 20 to 25 % RH. 3.3 Results and Discussion 3.3.1 Refractive Index Measurements through Abbe Refractometer Evaporation of solvent during drying of polymer coatings leads to formation of a concentration gradient as surface regions are depleted of solvent quickly (Figure 3.2-a). Measured refractive indices for two principal directions (MD=TD and ND) and resulting birefringences are shown in Figure 3.2-b and 3.2-c for air and substrate interfaces of the PAI coating (inset drawings show low energy conformational state for generic PAI single repeating unit. 76 Figure 3.2 Optical Measurements through Abbe refractometer. (a). Temporal evolution of principal refractive indices during drying of neat PAI-DMAc solution (air interface change (b), substrate interface change (c)). Inset molecular drawings depict planar low energy conformation state for single PAI repeating unit. Measurements on air-liquid and substrate-liquid surfaces indicate that the solution is optically isotropic at early stages. Beyond about 4 hours at air-liquid interface and 6 hours at substrate liquid interface, refractive indices start to increase with parallel development of optical anisotropy with the in-plane (MD) refractive indices becoming higher than out-of-plane (ND) direction refractive indices. This was attributed to the solvent evaporation induced planar orientation of highly aromatic and rigid PAI chains resulted in a higher refractive index value in MD rather than ND. Highly oriented layer development started at the liquid-air interface and progressed through the liquid-substrate interface. Refractive index values in TD and MD was measured to be similar using Polarized Abbe refractometer. Totally dried films also exhibited slightly higher out-of- plane birefringence (measure of Δn31=Δn23) 77 3.3.2 Birefringence Calculations through Compensator Technique In order to capture the development of through thickness anisotropy at early stages of drying, we use optical compensation technique on samples cut and laid flat as graphically illustrated in steps 2, 3 and 4 in Figure 3.1. The photographs shown in Figure 3.3 are taken with sample oriented in ND-TD plane with substrate attached. The optical path in these films is the same and color profiles shown in the pictures represent retardation gradient from air to substrate surface. These gradients were quantified and are shown in Figure 3.4 at a series of time intervals. Figure 3.3 Cross-polarized optical micrographs with 30 order Berek compensator inserted at 45° orientation captured during drying of PAI-DMAc solution at different drying times ((a) air surface, (s) substrate surface). (∆Dol= high orientation layer) The cast solution started to solidify (self-supporting) after 40 minutes of drying therefore samples were collected after this drying time. At the earlier stages of drying the birefringence is slightly higher at the substrate-solution interface, which we attribute to shear induced planar orientation of rigid PAI chains near the substrate surface. 78 Figure 3.4 Calculated birefringence values from optical micrographs using compensator method at different drying times (T=time in minutes, gray color regions depicts unmeasured birefringence values due to solid-rich layer growth (ΔDol)). Starting from 140 minutes of drying, a rapid increase in the air-solution interface was observed, followed by the formation of a highly oriented layer at 200 minutes at air- liquid interface as shown in Figure 3.3 as depicted. The high value of retardation at this layer, caused orientation of PAI chains, prevented reading of the retardation as they went out of range of the 30 order Berek compensator (indicated as shaded area as “oriented layer” in Figure 3.4). After 200 minutes, the highest birefringence occurred at air-solution interface that progressively increases as the solvent concentration is disproportionately reduced in these regions. These observations are in good agreement with the Abbe refractometer results at air-solution interface. Growth rate of the distinct highly oriented layer thickness at air-solution interface directly measured from pictures shown in Figure 3.3 for the first 360 minutes of drying. Figure 5-a shows the increase in ratio of this highly oriented layer thickness to the real time total thickness (ΔDol/dm) from 24 % to 40 % as the highly oriented layer development proceeds towards the substrate (Figure 3.3). 79 As shown in Figure 3.4, before oriented layer was observed, birefringence profiles were integrated through thickness direction to obtain birefringence values for each drying time. In order to extract MD refractive index data from calculated birefringence values, constant ND refractive index (nND) was assumed during drying. Using Equation 3.3, average refractive index (nAVG) values were calculated. We have shown that nAVG values calculated from compensator method are in good agreement with air interface and substrate interface Polarized Abbe refractometer readings (Figure 3.5-b). × = (3.3) 2 푛푀퐷+푛푁퐷 퐴푉퐺 푛 3 Figure 3.5 Off-line measured refractive index and calculated birefringence values along with solid layer thickness growth. (a) Growth of solidified layer at air/substrate-coating interface (% overall coating thickness). (b) Average refractive index comparison calculated from off-line techniques incorporating compensator method and Abbe refractometer. (c) Birefringence comparison between refractometer readings and compensator method during drying of neat PAI/DMAc solution. 80 Similarly, Figure 3.5-c depicts the comparison of compensation method (with integrated birefringence values) along with Polarized Abbe refractometer (air and substrate interfaces) during the initial drying period for PAI-DMAc coating. Birefringence values calculated through compensator readings located in between air- interface and substrate-interface refractometer readings as expected. 3.3.3 Drying Data through Real-time Measurement System A real-time measurement system developed by our group1 was used to investigate the drying behavior of PAI-DMAc solution by tracking solution weight, thickness and in and out of plane birefringences using spectral birefringence method where two beams of polarized white light is transmitted at 0° and 45° to normal direction of cast solution sampling the optical anisotropy gradient through the thickness. This technique, thus, determines average anisotropy development (measured by Δn12 and Δn23) of polymer coatings using equation 3.2. This equation requires a bulk average of the refractive index of the polymer coating at a particular drying time which is estimated linearly through the concentration change (Equation 3.4). = × + × (3.4) 푠표푙푖푑 푠표푙푖푑 푠표푙푣푒푛푡 푠표푙푣푒푛푡 Using the above푛� equation,푛 real휒 -time thickness푛 averaged휒 anisotropy development of PAI solution was monitored during drying (Figure 3.6-a). Out-of-plane birefringence (Δn23) increased rapidly at a critical drying time where total coating thickness leveled off. Throughout the drying process the in-plane birefringence (Δn12) remains zero indicating in-plane isotropy maintained. Beyond this critical point, solid wt.% continued to increase due to diffusion of remaining solvent within the bulk. 81 Figure 3.6 Drying data from real-time measurement system. (a) Real-time drying data for PAI-DMAc solution with initial set wet thickness of 1mm with 0.2 m/second air speed at 25% RH. (b) Comparison of real-time birefringence development for different initial set thicknesses of PAI-DMAc coatings. Initial wet thickness effect on real-time anisotropy development was also investigated using this technique by varying cast thicknesses of 170 µm, 350 µm, 650 µm and 1mm (Figure 3.6-b). Out-of-plane birefringence development occurred earlier for thin samples as expected from increased depletion rate of solvent content. Birefringence values were relatively smaller as initial coating thickness increased. This is in agreement with literature96,108,181,216. 3.3.4 Comparison of Optical Techniques Optical measurements were compared with online spectral birefringence method that uses average refractive index values for the bulk of the polymer film (calculated real time through concentration change as the weight is continuously monitored). Figure 3.7 shows the comparison of birefringence values measured by spectral birefringence method with Abbe refractometer for 650 μm initial cast polymer solution. Open and closed symbols represent the birefringence values at air-solution interface and substrate-solution 82 interface respectively. In order the check the validity of equation 3.2, two different birefringence calculations were performed. Figure 3.7 Comparison of birefringence values from real-time measurement system and Abbe refractometer (open symbols depicts air-interface refractometer measurements where closed symbols depicts substrate- interface measurements). Birefringence profile shown with black curve in Figure 3.7 represents birefringence values calculated from percent solid averaged refractive index (Equation 3.4). Birefringence profile shown with blue curve in Figure 3.7 represents arithmetic average of air-solution interface and substrate-solution interface refractive indices acquired from Abbe refractometer readings. Overall, both real-time birefringence data located in be between air-interface and substrate-interface refractometer readings with only 5 percent error. We have shown that birefringence data calculated by real-time measurement system represent reasonable thickness averaged anisotropy during drying. 83 Figure 3.8 Molecular model representing the PAI chain orientation during drying. As shown in Figure 3.8, a molecular model was also developed to represent the physical changes occur during drying to match birefringence data depicted in Figure 3.7. First the system is at isotropic state (step 1) for 0-200 minutes of drying. It was followed by partial planar orientation of PAI chains between 200-600 minutes of drying (step 2), and finally after drying (>600 minutes) total planar orientation of PAI chains was observed (step 3). 84 3.4 Conclusions We have performed a comprehensive investigation on the formation of solvent evaporation induced optical anisotropy gradient during drying of the PAI-DMAc solution. To the best of our knowledge, application of either Abbe refractometer or compensator method was not reported previously for investigation of any wet polymer coating during drying for birefringence gradient determination through the thickness of the film. Abbe refractometer readings revealed that birefringence development occurred earlier at air-solution interface in comparison with substrate-solution interface. The evolution of optical anisotropy gradient and development of highly oriented layer were investigated using compensator method in the first 360 minutes of drying. It was observed that casting procedure introduced higher birefringence on the substrate- liquid interface at earlier stages. As solvent evaporates, this profile was reversed and resulted in higher birefringence on air-solution interface. After a critical time, we observed the formation of a distinct highly oriented layer. In addition, observations from real-time measurement system and off-line measurements showed good agreement. This will allow us to use the real-time measurement system to quantitatively study the thickness averaged optical anisotropy in drying films and coatings. 85 CHAPTER IV ENHANCED GAS BARRIER AND MECHANICAL PROPERTIES IN ORGANOCLAY REINFORCED MULTI-LAYER POLYAMIDE-IMIDE NANOCOMPOSITE FILM 4.1 Introduction† The solution casting process is increasingly being used in the production of functional films including optical film retarders, electrically conductive transparent films and flexible photovoltaic films. Solution casting in general is a complex process with solvent evaporation triggers multiple physical changes simultaneously. These physical changes include evaporation-induced weight loss and the accompanying reduction in thickness while the polymers undergo planar reorientation with their primary chain axes 96, 112. During the drying process diffusion of the solvent occurs from substrate-coating interface to air-coating interface which proceeds through the thickness direction of the wet coating. At low substrate velocities a drying front appears that is characterized by a concentration gradient and a peak in the evaporation flux 97. A number of theoretical and experimental studies have been carried out on the drying behavior of glassy polymers and reported skinning during drying 126, 107. Edwards depicted the theoretical background of skinning in polymers 101. As a result of this † O. Yucel, E. Unsal, J. Harvey, M. Graham, D.H. Jones and M. Cakmak (Manuscript in preparation) 86 phenomenon, the PVOH solutions was found to develop crystallized skin in the early stages of drying evidenced by specially developed low field nuclear magnetic resonance (NMR) methods with Carr−Purcell−Meiboom−Gill (CPMG) pulse sequence 102, 103, 104. Polyimides (PI) are well-known as high-performance polymeric engineering materials that have outstanding thermal and chemical stability, mechanical performance and low dielectric constant 127, 128, 129, 130. Despite the fact that they have remarkable material properties, their application in the polymer industry is limited due to processing issues such as insolubility in commercially available solvents 131, 132, 133. PAI is a good alternative as soluble PI since they offer high thermal stability combined with mechanical properties and easy processing 134. Once their thermal and anti-corrosion properties are combined within the framework of a hybrid material, the final structure would be used particularly for aerospace applications. The homogeneous dispersion of organically modified MMT mineral clay within the PI matrix provides a hybrid material with excellent properties such as decreased gas permeability, increased tensile strength, modulus and decreased coefficient of thermal expansion (CTE) 135, 136, 137. Homogeneous exfoliation of organically modified MMT clay mineral within the soluble PAI matrix also provides an nanocomposite material with excellent gas barrier and electrical properties with increased structural strength that are crucial for aerospace applications 138. Various studies reported the effect of organoclay content on mechanical properties of nanocomposite films including nanocomposites of polyamide 6 (PA6) 139, polyvinyl chloride (PVC) 140 and PI 141. For the PA6, maximum tensile strength was observed at 2.5wt% MMT loading and further increase in organoclay content resulted in a stiffer structure. Tensile strength for PVC/organoclay 87 nanocomposite films was maximized at 3wt% due to strong interfacial interaction between the organoclay and the PVC matrix. In the case of PI/MMT nanocomposite films tensile strength was maximized at 1wt% MMT. At higher organoclay contents, tensile modulus and strength decreased due to brittleness caused by aggregation of the platelets 141. Distortion of the polymeric matrix by organoclay platelets causes mechanical failure when uniaxial stress is applied to the material leading to total fracture even before neck formation. This brittleness limits wider application of these films in areas such as flexible electronics or the aerospace industry. In order to improve mechanical properties with flexibility, PAI-organoclay composite layer can be sandwiched between two neat PAI layers. As shown by Toyota researchers 135, 136, organically modified clay minerals can easily penetrate through the polymer matrix and results in a dramatic increase in gas barrier properties. Therefore a multi-layer embodiment of the solution casting process resolves problems associated with the manufacturing of PAI-clay composite films that allows production of a film with improved gas barrier properties Helium permeability for polymer-MMT nanocomposite films reported previously 142, 143, 144, 145. Studies showed that for epoxy-MLS nanocomposite films gas diffusivity decreased with an increase in MMT loading. In addition, nanoplatelets in polymers were more effective in improving gas barrier properties than fiber-like or spherical fillers due to blocking of gas diffusion path. In the case of MMT-polypropylene (PP) nanocomposite, gas permeability decreased with the addition of fillers compared to the unfilled PP film. For Nylon-6/MMT system permeability decreased 60% which was attributed to the high compatibility between the clay and nylon-6 resin. 88 In this chapter, we present real-time drying of single/multi-layer organoclay-filled soluble PAI cast films using a series of clay loadings and initial wet thicknesses to investigate the effects on drying behavior and anisotropy development using spectral birefringence technique. The influence of organoclay loading on gas barrier and mechanical properties of multi-layered PAI-nanoclay nanocomposite films are discussed. 4.2 Experimental 4.2.1 Materials DMAc and tetrahydrofuran (THF) were purchased from Sigma-Aldrich and used without further purification. Cloisite® 30B (C30B) natural MMT organoclay mineral modified with a quaternary ammonium salt was obtained from Southern Clay Products. C30B belongs to the structural family of 2:1 phyllosilicates and composed of silicate layers of 1.85 nm in thickness and 30 nm to 100-200 nm in lateral dimensions. Amorphous PAI (L8®) was specially synthesized by Akron Polymer Systems in solid fiber form and used without further purification (Figure 4.1). Figure 4.1 Chemical structures of generic soluble poly(amide-imide) (PAI) repeating unit and Organic Modifier for Cloisite 30B montmorillonite. (a) PAI, (b) C30B. 4.2.2 Polymer Solution Preparation Solid PAI fibers were dried at 120°C overnight before preparing final polymeric solutions. PAI solution was prepared using DMAc as the solvent with 8 wt% polymer in 89 solution. Thinky Planetary Centrifugal Vacuum Mixer ARV-310 (Rotation + Revolution) was used for mixing, with improved dissolution, uniformity and degassing. The solutions were heated or cooled as necessary to prevent heat of mixing. Nanocomposite solutions were prepared using the following protocol; first, 3 wt% clay dispersion in DMAc was prepared in a small glass vile by sonication for better nanoplate exfoliation. Hielscher UP400S Ultrasonic Processor at 0.5 cycles and 80% amplitude was used for 60-70 minutes. This was mixed with a 9.3 wt% neat PAI solution in DMAc. Final composition of the dispersion was 8 wt% PAI and 3 wt.% clay with respect to the polymer. Similarly, 5 wt.% and 7 wt.% clay solutions with respect to polymer were prepared. 4.2.3 Rheology Bohlin Instruments Gemini-Nano cone-and-plate rheometer was used for rheological measurements. Shear viscosities and shear stress were measured at room temperature using a 4°/40 mm apparatus. Viscosity values measured by varying shear rates from 0.001 to 100 sec-1. A custom built evaporation chamber was used as necessary to decrease rate of evaporation of DMAc from the sides of the cone. 4.2.4 Simultaneous Solution Casting Process Real-time monitoring of single-layer and multi-layer polymer coatings were performed by doctor blade casting method using custom-made 76 mm (single-layer) and 102 mm (multi-layer) wide doctor blades. Solutions with different initial wet thicknesses were cast on a flat borosilicate glass substrate using Cheminstruments EC-200 hand caster. For multi-layer PAI-organoclay hybrid film the following solution casting procedure was followed; 100 µm 1st layer wet thickness for neat PAI solution, 550 µm 2nd layer wet thickness for nanocomposite PAI-organoclay solution and 100 µm 3rd layer 90 wet thickness for neat PAI solution. Casting speed was 30 cm/min. Averaged dried film thicknesses were measured using Mutitoyo CUA-154 micrometer. Figure 4.2 Hybrid process schematic with multi-layer doctor blade. Airflow and casting direction is represented with dashed red arrows. For mechanical and permeability tests of the films, commercial size solution casting platform present in our group was employed (Figure 4.2). It incorporates six heating zones, dual carriers, forced laminar air flow, programmable carrier motion, solid granite casting surface for enhanced film roughness and safety lower explosive limit (LEL) sensors. This process includes replaceable, custom-built single and multilayer doctor blades. A custom-made six-inch multi-layer doctor blade was used for casting the multi-layer polymer coating on a flat PET (Mylar) substrate. The films were dried at room temperature and at varied %RH. 4.2.5 Real time Weight, Thickness, Temperature & Birefringence Measurements Cast films were dried in the real-time measurement system 1 for 10 hrs at room temperature (Figure 4.3). Drying parameters were controlled by the air heater/blower attached to the upstream part of the airflow tunnel including three vertical baffles inside. The sample platform is located in the middle of the tunnel and solution cast glass substrate is placed on the graphite sample holder on an electronic balance. Film thickness 91 was monitored from the top through a glass window using laser-displacement sensors. Two light sources and an incident side polarizer are located underneath the stage for retardation measurements (for 0° and 45°). Optical sensors are located above the tunnel. Online weight measurements includes both global and local weight percent change during drying of coatings; the former was directly acquired from precision balance whereas the latter was calculated using both real-time thickness data and densities of DMAc (0.94 g/cm3) and PAI (1.18 g/cm3). For all experiments the air speed was kept constant at 0.5 m/sec. The temperature of the chamber was controlled and adjusted at room temperature. Figure 4.3 Close-up picture of the sample positions and real-time measurement sensors. Only sample platform is covered by glass material to allow optical measurement. 4.2.6 Characterization The morphology and microstructure of the multi-layer organoclay reinforced PAI films were characterized using JEOL (JSM-7401F) scanning electron microscope (SEM) and JEOL model 1230 transmission electron microscopes (TEM). For SEM experiments, multi-layered films were mounted in epoxy, cured and cross-sectioned by diamond knife using a Reichert Ultracut S/FC S ultramicrotome (Leica, Germany). Cut samples were mounted on an aluminum substrate using highly-conductive, adhesive carbon tape. Later, mounted samples were sputter coated with silver using a Peltier Cooled Sputter Coater 92 K575x (Emitech, UK) under argon atmosphere. SEM images of coated cross-section cut films were photographed at 20 kV. For TEM experiments, the specimens with thickness of about 100 nm were microtomed at room temperature using a Reichert Ultracut S/FC S ultra microtome (Leica, Germany) with diamond knife using JEOL TEM at 120 kV. Cut films were placed on 300-mesh copper grid prior imaging. WAXS experiments were performed by Bruker AXS generator equipped with a copper target tube and a two-dimensional detector. The X-ray beam was monochromatized to CuKα radiation (λ = 1.54Å) and the X-ray generator was operated at 40 kV and 40 mA. Distance between detector and sample was 12.23 cm with 2θ set to 20° and χ set to 45°. One-quadrant WAXS patterns of C30B powder and PAI-clay multi- layer film were obtained. Clay powder was placed between layers of conventional tape and the diffraction pattern of tape was subtracted from powder scattering. In the case of PAI-organoclay multi-layer film, a stack of 10 cut layers was mounted to the sample holder using conventional tape. 4.2.7 Mechanical & Permeability Testing Instron 5969 tensile testing equipment was used for mechanical tests. For this purpose 6mm x 25.4mm samples were prepared and stretched at 25.4mm/min crosshead rate. Helium permeability was tested on 6-inch diameter film specimens in a VersaPerm Mk VI which detects 10 ppm of helium. 4.3 Results and Discussion Rheology and chemistry of individual polymer solutions during multi-layer solution casting and drying process exhibit importance. Layer interactions can be either in the form of phase separation and/or complete delamination of the wet-cast layers. 93 Rheological properties of polymeric solutions were investigated using cone-and-plate rheometry (Figure 4.4). Three consecutive shear rates 2.98s-1, 5.96s-1 and 8.94s-1 were chosen to simulate the solution casting process with casting speeds of 10 cm/min, 20 cm/min and 30 cm/min. Shear viscosity and shear stress values increased with organoclay loading at low shear rates. Beyond a critical shear rate of 0.35 s-1 this behavior was reversed. To the best of our knowledge, existence of the critical shear rate phenomenon was not reported before for PAI-organoclay system. The behavior at lower shear rates was reported for PEO-based polymeric systems filled with solid particles, including nanoclay 146, 147 and attributed to the increased interaction between nanoclay particles. At high shear rates, the organoclay platelets preferentially orient their surfaces in the shear plane due to the increased rotational shear, leading to shear thinning. Similar results were also reported for PLA/OMMT nanocomposites prepared by melt-compounding 148. Figure 4.4 Variation of shear viscosity of hybrid solutions at different organoclay loadings with shear rates. Real-time drying behavior of neat PAI solution was monitored using the real-time measurement system for temporal changes in all of the monitored parameters (Figure 4.5). We observed mainly three different stages: Stage 1; During this period, both local and global weight decreases rapidly as solvent evaporation takes place with high mass 94 flux of DMAc. Coating thickness decreases before leveling off at longer times. The sample remains isotropic during this stage as both in in-plane (∆n12) and out-of-plane (∆n23) birefringences are zero (1 = Air flow direction and casting direction, 2 = transverse to air flow direction, 3 = Thickness direction). Figure 4.5 Typical drying test result of PAI cast film from 8wt.% DMAc solution. Casting and drying conditions are as follows: Blade gap 559 µm, Airflow rate 0.5m/sec at room temperature. (Star denotes offline birefringence measurement) Stage 2; As the cast solution continues to dry, a drying front characterized by a rapid local thickness reduction develops upstream and moves in the downstream direction across the length of the sample. The out-of-plane birefringence along with in-plane birefringence remains constant near zero at this location. Thickness data starts to level off at this stage. Stage 3; The drying front proceeds downstream to the edge of the coating while the temperatures remain nearly constant and the local weight steadily decreases at a much slower pace while out-of-plane birefringence stays constant after a dramatic increase, at which thickness of the coating levels off. This depicts a substantial end of evaporative cooling. Real-time drying behavior of PAI-organoclay coating was also monitored and showed similar three-regime behavior (Figure 4.6). Due to the evaporation driven 95 orientation of organoclay platelets parallel to the substrate of the film, out-of-plane birefringence starts to develop earlier during stage 1, well before the dramatic increase of ∆n23 seen at the end of stage 2 for the neat PAI coating when the overall thickness levels off. Figure 4.6 Typical drying test result of polyamide-imide nanocomposite film from 8wt.% DMAc solution (3wt.% C30B). Casting and drying conditions are as follows: Blade gap 1 mm, Airflow rate 0.25m/sec at room temperature. (Star denotes offline birefringence measurement) In order to capture the development of through thickness anisotropy at early stages of drying we used optical compensation technique. The photographs shown in Figure 4.7 taken with sample oriented in ND-TD plane with substrate attached. The optical path in these films is the same and color profiles shown in the pictures represent retardation gradient from air to substrate surface. These gradients were quantified and are shown at a series of time intervals. The cast solution started to solidify (self-supporting) after 40 minutes of drying therefore samples were collected after this drying time. At the earlier stages of drying the birefringence is higher at the substrate-solution interface, which we attribute to shear induced planar orientation of nanoplatelets near the substrate surface. Starting from 140 minutes of drying, a rapid increase in the air-solution interface 96 was observed, followed by the formation of a highly oriented layer at 240 minutes at air- liquid interface. The high value of retardation at this layer, caused by orientation of PAI chains and nanoplatelets, prevented reading of the retardation as they went out of range of the 30 order Berek compensator (indicated as shaded area as “oriented layer” in Figure 4.7). After 180 minutes, the highest birefringence occurred at air-solution interface that progressively increases as the solvent concentration is disproportionately reduced in these regions Figure 4.7 Calculated birefringence gradient through thickness of PAI-DMAc coating with 3wt.% organoclay content. An understanding for the effect of initial wet polymer coating thickness was developed. Three different initial wet thicknesses were chosen; 229 µm, 356 µm and 559 µm respectively with 1 wt.% organoclay loading. The extent of overall drying time was larger for initially thicker coatings due to higher solvent content per unit drying surface (Figure 4.8). 97 Figure 4.8 Effect of initial wet thickness on real-time drying for single-layer PAI- organoclay coatings. Real-time drying data for 559 µm thickness showed a distinct solvent evaporation controlled behavior incorporating two distinct regimes indicating a transition from rubbery to glassy state. The decrease in the mass diffusion rate of DMAc from the polymer matrix attributed to the increased diffusion path that correlates directly with the thickness of the coating and number of organoclay particles in the diffusion path (Figure 4.9-a). This behavior was similar to the drying kinetics of neat polymer-solvent systems without organoclay particles. The rubbery to glassy transition for neat polymer-solvent systems without filler was reported by other researchers 149. Nanoclay reinforced polymer coatings with smaller initial wet thicknesses on the other hand, showed similar behavior that does not include distinct regimes with larger out-of-plane birefringence (Δn23) values (Figure 4.9-b). The increase in interaction between nearby PAI chains and nanoclay platelets resulted in smaller Δn23 values for thicker coatings. This is in agreement with literature 96, 112, 108. 98 Figure 4.9 Effect of initial wet thickness on real-time drying & out-of-plane anisotropy development for single-layer PAI-organoclay coatings. (20% RH) The critical solid concentration at which out-of-plane birefringence develops (a measure of out-of-plane anisotropy) reveals the mechanisms of simultaneous solution evaporation under constant air flux. This critical concentration value was increased with initial wet coating thickness. This was attributed to the decrease of total degrees of freedom in the polymer matrix as the PAI chains and organoclay platelets orients parallel to the substrate. Thicker PAI-organoclay coatings needed more time for polymer chains and platelets to orient and develop out-of-plane anisotropy. Figure 4.10 Effect of organoclay loading on real-time thickness and mass flux change for single-layer PAI-organoclay coatings. Casting and drying conditions are as follows: Blade gap 559 µm, Airflow rate 0.5m/sec at room temperature. 99 Similarly, the effect of organoclay content on the drying behavior of nanocomposite PAI coatings, three different organoclay loadings (3wt.%, 5wt.% and 7wt.%) were chosen. Total drying time was increased with organoclay content due to reduced evaporation rate of solvent (Figure 4.10). This was attributed to the presence of the organoclay platelets that blocked the diffusion path of the solvent in a greater extent. The critical solid concentration at which out of plane anisotropy developed decreased from 40 wt.% to 10 wt.% (Figure 4.11). Further increase in organoclay content did not affect this critical concentration; rather, showed larger Δn23 values. Percent relative humidity (%RH) of the environment was found to be another parameter to directly influence the total Δn23. The effect of relative humidity in the environment on out-of- plane anisotropy was attributed to the solvent’s water uptake since DMAc is hygroscopic. Addition of water at the beginning of the drying process decreases the overall solid content and slows the evaporation rate of DMAc; which affects the overall evaporation dynamics followed by Δn23. Figure 4.11 Effect of organoclay loading on real-time drying & out-of-plane anisotropy development for single-layer PAI-organoclay coatings. Casting and drying conditions are as follows: Blade gap 559 µm, Airflow rate 0.5m/sec at room temperature. (45 %RH) A proper molecular model was proposed to represent orientation development for PAI chains and organoclay platelets during drying of the polymer coating (Figure 4.12). 100 Due to the relatively large aspect ratio of nanoclay platelets (2nm X 100nm) in comparison with PAI chains, their respective size was not depicted at actual scale. Initially (i), polymer chains and organoclay platelets were randomly oriented. This was followed by stages (ii) & (iii); polymer chains and organoclay platelets began to orient parallel to the substrate due to solvent evaporation induced chain collapse starting from coating-air interface. Finally, aromatic PAI chains and nanoplatelets exhibited planar orientation that can be shown by dramatic increase in Δn23. Figure 4.12 Molecular representation of real-time anisotropy development during drying of PAI-DMAc coating with 229 µm initial wet thickness. Figure 4.13 shows real-time thickness evolution with anisotropy development for organoclay reinforced multi-layer PAI film. Organoclay containing middle layer was sandwiched between neat PAI layers. Diffusion of DMAc from each layer resulted in a collapse of the wet thickness due to solvent evaporation. In-plane (IP, measure of ∆n12) anisotropy development was negligible. Out-of-plane (OP, measure of Δn23) anisotropy 101 was maximized when the multi-layer film was completely dry indicating planar orientation of the aromatic PAI chains and organoclay platelets. Figure 4.13 Real-time Anisotropy Development for Multi-layer Film (Middle layer in between neat PAI layers). (750 µm initial wet thickness, star denotes offline birefringence measurement). Figure 4.14 illustrates a model for the drying mechanism. Immediately following casting, the out-of-plane birefringence of the cast multi-layer film was zero which indicates that the casting procedure produced isotropic sample (step a). In a second stage, the drying and thickness reduction was dominated by uniform solvent evaporation through the first layer (step b). First layer became solid rich (step c). After a short period of time, a drying front developed characterized by a rapid reduction in the thickness measured at the front laser with a slight increase in out-of-plane birefringence (step d). Drying proceeded through the second layer, became solid (polymer and nanoclay rich) and a glassy region appeared at the upstream edge with no change at the front laser position after the drying front passes through (step e). Drying proceeded through the third layer and the drying front approached the center position where birefringence was measured (step f). When the drying front passed through the center area, out-of-plane birefringence developed rapidly and a sharp decrease was observed in the middle laser 102 reading (step g). After the drying front passed through the center position, the birefringence remained constant, without any further increase, while the middle laser reading stayed almost constant (step h). Figure 4.14 Drying model and out-of-plane birefringence development for multi-layer nanocomposite film under airflow condition (Schematics are not in actual scale, representative purposes only. Red color indicates front laser reading). In order to improve tensile properties due to the brittleness of PAI/organoclay nanocomposite films simultaneous multi-layer solution casting was employed. Organoclay containing middle layer was sandwiched between top and bottom layers of neat PAI solution. Interaction between layered silicates and polymers can form different structures depending on the level of exfoliation 150. This has direct influence on mechanical and gas barrier properties. In general, better level of exfoliation results in better permeability performance for industrial applications and better tensile strength. 103 Figure 4.15 WAXS Diffraction Patterns of Organoclay Reinforced Multi-layer Film with Cloisite 30B. The level of exfoliation of the organoclay reinforced multi-layer PAI films was examined using WAXS by comparing diffraction patterns of clay powder and nanocomposite multi-layer films. Diffraction patterns showed fully exfoliated nanoplatelets within PAI matrix (Figure 4.15). The evidence for complete exfoliation was the disappearance of the diffraction peak at 2θ = 4.8° (d = 1.83 nm) which was specific to clay. It suggests the loss of order and structure regularity in the organoclay accompanying a high degree of exfoliation. The nearly complete exfoliation was attributed to the sonication process during preparation of PAI-organoclay solution, as applied for full exfoliation in other polymer systems 151, 152. Similar exfoliation techniques to produce PI hybrid films in order to fully exfoliate organoclay particles can be found elsewhere. Figure 4.16 Change in PAI domain size with drying conditions: 30 nm, 32 nm and 40 nm for 25 °C (a), 30 °C (b) and 50 °C (c) dried samples. (Arrows depict thickness direction) 104 As depicted by Malakpour 153, the PAI domains appeared as spherical domains, aligned almost parallel to the film surface indicating confinement due to the existence of planar organoclay particles. Morphology of spherical PAI domains were further investigated by varying drying temperatures of 25, 30 and 50 °C (Figure 4.16). Spherical domains for PAI were observed with an increase in domain size: average diameter of 30 nm for 25°C, 32 nm for 30°C and 40 nm for 50°C dried films. Figure 4.17 TEM Micrographs of cross-section cut organoclay reinforced multi-layer PAI films (Arrows shows thickness direction, 3wt.% (a), 5wt.% (b) and 7wt.% (c) organoclay loadings from left to right) The level of exfoliation of nanoplatelets and their planar orientation within the PAI matrix were shown by TEM images (Figure 4.17). This was in accordance with the real-time out-of-plane birefringence data (Δn23) measured during solvent evaporation from the polymer coating. Increase in organoclay content resulted in aggregation of nanoplatelets that caused decrease in level of exfoliation with bending. Nanoplate bending phenomenon at higher organoclay contents was attributed to the internal drying stresses due to high interfacial surface area between organoclay nanoparticles and aromatic PAI chains. 105 Figure 4.18 Mechanical properties of organoclay reinforced multilayer film with different nanoclay loadings. Mechanical properties of dried multi-layer nanocomposite films were investigated, and revealed that films incorporating a small amount of organoclay particles (3wt.%) within the PAI matrix has similar tensile strength as neat PAI film (Figure 4.18- a). However, increasing organoclay content within the films (5-7wt.%) resulted in a decrease in ultimate tensile strength due to an increase in brittleness (loss of toughness) in single-layer hybrid films. Even at high organoclay loadings, overall flexibility was satisfied. Figure 4.19 The effect of Organoclay addition on He Permeability for dried multilayer nanocomposite films. Others studies reported deterioration in thermal and mechanical properties of thermally imidized PI-organoclay hybrid films for organoclay content of more than 106 1.0wt% 151, 154. In the case of PAI-organoclay nanocomposite films prepared by solution intercalation 153, optimum mechanical strength was achieved at 10 wt.% nanoclay loading. As a measure of stiffness, Young`s modulus of the films was also measured and showed a decrease with increasing organoclay content (Figure 4.18-b). Even at low organoclay content, overall modulus was decreased followed by more significant loss at high organoclay contents. Helium permeability values of dried multi-layer films indicate the effectiveness of the organoclay addition to improve permeability supported by planar orientation of organoclay platelets parallel to the film surface (Figure 4.19). Further addition of nanoclay did not produce a dramatic decrease in permeability values indicating that the optimum organoclay content had been attained. The improved properties also indicate improved exfoliation of the organoclay platelets within the PAI matrix 155. 107 4.4 Conclusions A multi-layer PAI-clay hybrid film in which an exfoliated-clay-containing middle layer was sandwiched between neat PAI layers was developed using organically modified natural MMT mineral. The level of out of plane anisotropy was high due to the evaporation induced planar orientation of nanoplatelets. This was supported by interaction PAI chains with primary chain axes oriented in the plane of the film. X-ray diffraction showed full exfoliation as a complete disruption of the characteristic (001) reflection in clay. As a result, hybrid, multi-layer films with enhanced gas-barrier properties and flexibility without sacrificing low thermal expansion were obtained. Overall, permeability values decreased over 50% in compare with neat PAI films. Therefore hybrid multi-layer PAI films are widely adaptable to aerospace and other industrial applications. 108 CHAPTER V ENHANCED GAS BARRIER AND MECHANICAL PROPERTIES IN GRAPHENE- OXIDE REINFORCED POLY(AMIDE-IMIDE) NANOCOMPOSITE FILMS 5.1 Introduction‡ Solution casting process is increasingly being used in the production of functional films, including optical film retarders, electrically conductive transparent films and flexible photovoltaic films. Roll-to-roll counterparts of coating techniques such as knife- over-edge, slot-die and slide coating are also common in film manufacturing industry. It is a complex process in which solvent evaporation triggers multiple physical changes taking place simultaneously. These physical changes include evaporation-induced weight loss and reduction in film thickness while polymer chains undergo reorientation with their primary chain axes oriented in the plane of the film 96, 112. During the drying process, mass diffusion of the solvent occurs from substrate-coating interface to air-coating interface which proceeds through the thickness direction of the wet coating. PIs are well-known as high-performance polymeric engineering materials that have outstanding thermal and chemical stability, mechanical performance and low dielectric constant 127, 156, 128, 129, 130. Despite the fact that they have remarkable material properties, their application in the polymer industry is limited due to processing issues ‡ O. Yucel, E. Unsal, J. Harvey, M. Graham, D.H. Jones and M. Cakmak (Manuscript in preparation) 109 such as insolubility in commercially available solvents 131, 132, 133. PAIs are good alternatives as soluble PIs since they offer high thermal stability combined with mechanical properties and easy processing 134. Once their high temperature nature and strength with anti-corrosion properties are combined within the framework of a hybrid material, the final nanocomposite structure would be used useful, particularly for aerospace applications. The scientific invention of graphene leaded substantial investigation of its mechanical, electrical and optical properties within a polymer nanocomposite framework 157. It can be produced through several techniques such as chemical vapor deposition (both of discrete monolayers onto a substrate and agglomerated powders), micro- mechanical exfoliation of graphite, and growth on crystalline silicon carbide 158. The existence of complex refractive index contribution on single layer graphene sheets was also reported by several studies 159, 160. Optical properties in the visible range were also determined through Fresnel coefficient calculation 161. Remarkably after the discovery of Toyota researchers comprising outstanding material properties of organoclay reinforced polyimide films 135, 136, similar analogy was applied for different nanoparticles. The homogeneous dispersion of chemically reduced graphene-oxide (GO) nanosheets within the polymer matrix provides a hybrid material with excellent properties such as decreased gas or water permeability 162, 163, increased tensile strength and modulus 164 and increased electrical conductivity threshold with improved thermal conductivity properties 165. Similar homogeneous exfoliation of chemically reduced GO nanosheets within the PI matrix also provides a nanocomposite material with excellent gas barrier and reduced thermal expansion coefficient (CTE) 110 properties with increased structural strength, which are crucial for aerospace applications 166, 167. For similar reason, the level of exfoliation of GO nanosheets within PI matrix also depicted by Yoonessi to show strong interfacial interaction between GO and PI matrix employing high-resolution transmission electron microscopy (HR-TEM) 168. Various studies reported the effect of GO content on mechanical properties of nanocomposite films of PI 169, 170, 171. They have shown that addition of chemically reduced GO nanosheets to the polymer matrix increased both tensile strength and tensile modulus. At relatively high GO content, tensile modulus and strength was decreased due to brittleness, caused by aggregation of the nanosheets. In agreement with the concept of hybrid films to improve mechanical properties, single-layer PAI/GO nanocomposite films with relatively high GO loadings produced by solution casting, were inherently brittle at high stress levels. This brittleness limits wider application of these films in areas such as flexible electronics or the aerospace industry. Distortion of the polymeric matrix by GO nanosheets causes mechanical failure when uniaxial stress is applied to the material leading to total fracture even before neck formation. Helium permeability for polymer-nanocomposite films reported previously revealed the fact that for montmorillonite (MMT)/epoxy embodiment, with the increase of MMT loading gas diffusivity decreased significantly. Earlier studies 142, 143, 144, 145 reported that epoxy-MMT nanocomposite films demonstrated that gas diffusivity decreased significantly with an increase in MMT content comprising nanoscale platelets in polymers were more effective in improving gas barrier properties than fiber-like or spherical fillers. In the case of hybrid films of GO nanosheets within polyimide matrix, water vapor permeability decreased 80% at 0.001 GO content (wt.%) 166 and oxygen 111 permeability decreased appreciably with the addition of chemically modified GO compared to the unfilled PI film 172. In this chapter, we present real-time drying of GO-filled soluble PAI cast films using a series of GO loadings and initial wet thicknesses to investigate the effects on drying behavior and optical birefringence. To the best of our knowledge, application of GO was not reported previously for any soluble PAI system. The resulting gas barrier and mechanical properties of single-layer PAI/GO nanocomposite films are also discussed as influenced by GO content. 5.2 Experimental 5.2.1 Materials Dimethylacetamide (DMAc) was purchased from Sigma-Aldrich and used without further purification. Chemically reduced GO nanosheet dispersion in DMAc was obtained from Angstron Materials Inc. Amorphous PAI (L8®) was specially synthesized by Akron Polymer Systems in solid fiber form and used without further purification (Figure 5.1). Figure 5.1 Chemical structures of generic PAI repeating unit and chemically reduced graphene-oxide nanosheet. (a) PAI, (b) GO. 112 5.2.2 Polymer Solution Preparation Solid PAI fibers were dried at 120°C overnight before preparing final polymeric solutions. PAI solution was prepared using DMAc as the solvent with 10 wt% polymer in solution. Thinky Planetary Centrifugal Vacuum Mixer ARV-310 (Rotation + Revolution) was used for mixing with improved dissolution, uniformity and degassing. The solutions were heated or cooled as necessary to prevent heat of mixing. Nanocomposite solutions were prepared using the following protocol; first, 3 wt% GO dispersion in DMAc was prepared in a small glass vile by sonication for better nanoplate exfoliation. Hielscher UP400S Ultrasonic Processor at 0.5 cycles and 80% amplitude was used for 60-70 minutes. This was mixed with a 9.3 wt% neat PAI solution in DMAc. Final composition of the dispersion was 10 wt% PAI and 3 wt.% GO with respect to the polymer. Similarly 0.01 wt.%, 0.05 wt.% and 0.1 wt.% PAI-GO solutions with respect to polymer were prepared. 5.2.3 Simultaneous Solution Casting Process Real-time monitoring of polymer coatings was performed by doctor blade casting method using custom-made 76 mm wide doctor blade (Figure 5.2). Solutions with different initial wet thicknesses were cast on a flat borosilicate glass substrate using Cheminstruments EC-200 hand caster. Three different initially set wet thicknesses were used; 229, 356 and 559 µm. Casting speed was 30 cm/min. Averaged dried film thicknesses were measured using Mutitoyo CUA-154 micrometer. Air speed was 0.25 m/sec. The temperature of the chamber was controlled and set at room temperature while humidity within the chamber was not controlled and varied between 20-50 %RH. 113 Figure 5.2 Schematic illustration of solution casting process. For mechanical and permeability tests of the films, commercial size solution casting platform present in our group was employed. It incorporates six heating zones, dual carriers, forced laminar air flow, programmable carrier motion, solid granite casting surface for enhanced film roughness and safety lower explosive limit (LEL) sensors. This process includes replaceable, custom-built single and multilayer doctor blades. A custom- made six-inch multi-layer doctor blade was used for casting the multi-layer polymer coating on a flat PET (Mylar) substrate. The films were dried at room temperature and at varied %RH. 5.2.4 Real time Weight, Thickness, Temperature & Birefringence Measurements Cast films were dried in the real-time measurement system 1 for 10 hrs at room temperature. Drying parameters were controlled by the air heater/blower attached to the upstream part of the airflow tunnel including three vertical baffles inside. The sample platform is located in the middle of the tunnel and solution cast glass substrate is placed on the graphite sample holder on an electronic balance (Figure 5.3). Film thickness was monitored from the top through a glass window using laser-displacement sensors. Two light sources and an incident side polarizer are located underneath the stage for retardation measurements (for 0° and 45°). Optical sensors are located above the tunnel. Online weight measurements includes both global and local weight percent change during drying of coatings; the former was directly acquired from precision balance 114 whereas the latter was calculated using both real-time thickness data and densities of DMAc (0.94 g/cm3) and PAI (1.18 g/cm3). For all experiments the air speed was kept constant at 0.25 m/sec. The temperature of the chamber was controlled and adjusted at room temperature. Figure 5.3 Sample positions and sensor locations in real-time measurement system. 5.2.5 Characterization The morphology and microstructure of the multi-layer organoclay reinforced PAI films were characterized using JEOL (JSM-7401F) scanning electron microscope (SEM) and JEOL model 1230 transmission electron microscopes (TEM). For SEM experiments, multi-layered films were mounted in epoxy, cured and cross-sectioned by diamond knife using a Reichert Ultracut S/FC S ultramicrotome (Leica, Germany). Cut samples were mounted on an aluminum substrate using highly-conductive, adhesive carbon tape. Later, mounted samples were sputter coated with silver using a Peltier Cooled Sputter Coater K575x (Emitech, UK) under argon atmosphere. SEM images of coated cross-section cut films were photographed at 20 kV. For TEM experiments, the specimens with thickness of about 100 nm were microtomed at room temperature using a Reichert Ultracut S/FC S ultra microtome (Leica, Germany) with diamond knife using JEOL TEM at 120 kV. Cut films were placed on 300-mesh copper grid prior imaging. 115 5.2.6 Mechanical & Permeability Testing Instron 5969 tensile testing equipment was used for mechanical tests. For this purpose 6mm x 25.4mm samples were prepared and stretched at 25.4mm/min crosshead rate. Helium permeability was tested on 6-inch diameter film specimens in a VersaPerm Mk VI which detects 10 ppm of helium. 5.3 Results and Discussion Real-time drying behavior of the polymeric coatings right after solution casting was monitored using the highly instrumented evaporation chamber 1. As shown in Figure 5.4, temporal changes in all of the monitored parameters were recorded using real-time measurements for neat PAI coating of 10 wt% solution in DMAc with 559 µm initial wet thickness. We have observed mainly three different regimes; in the first stage evaporative cooling induced temperature decrease of the polymer coating observed while surface temperature leveled off with transition to the second stage. Both local and global weights decreased rapidly as evaporation takes place with high mass flux of solvent with fast coating thickness shrinkage. Figure 5.4 Drying data for 10 wt.% PAI-DMAc solution. Casting and drying conditions are as follows: 559 µm initial wet coating thickness, airflow rate 0.2m/s at room temperature. (Star denotes offline birefringence measurement) 116 The sample remained isotropic during this stage as both in in-plane (∆n12) and out-of-plane (∆n23) birefringences were zero (1 = Air flow direction and casting direction (MD), 2 = transverse to air flow direction (TD), 3 = Thickness direction (ND)). In the second stage, a drying front characterized by a relatively slow local thickness reduction followed by rapid thickness decrease, develops upstream and moves in the downstream direction across the length of the sample. The out-of-plane birefringence along with in- plane birefringence remained constant until rapid thickness reduction. We have shown that while coating thickness rapidly decreased and leveled off, ∆n23 data increased with planar orientation of aromatic PAI chains. In the final stage, the drying front proceeded downstream to the edge of the coating while coating temperature and thickness remained constant. Out-of-plane birefringence was constant after the critical point at which coating thickness leveled off. This indicates end of evaporative cooling and drying. Figure 5.5 Drying data for PAI nanocomposite solution with 0.01wt.% GO content. Casting and drying conditions are as follows: 559 µm initial wet coating thickness, airflow rate 0.2m/s at room temperature. (Star depicts offline birefringence measurement) Real-time drying behavior of PAI coating with chemically reduced GO was also monitored and shows similar three-regime behavior (Figure 5.5). However due to the evaporation induced orientation of GO nanosheets parallel to the substrate of the film out-of-plane anisotropy development was slightly faster with rapid increase in out-of- 117 plane birefringence which was steeper at the end of stage 2 in comparison with neat and organoclay filled PAI coatings. In order to develop an understanding for the effect of initial coating thickness on the real-time drying behavior of nanocomposite coatings, three different initial wet thicknesses were chosen; 229, 356 and 559 µm respectively. Figure 5.6 Effect of cast initial wet thickness on percent weight and real-time thickness data for 0.01 wt.% PAI-GO coatings. Overall drying time was larger for initially thicker coatings due to more solvent content to diffuse per unit drying surface (Figure 5.6). Since the GO reinforced polymer coating with the largest initial wet thickness (559 µm thickness) held more solvent per unit drying area at the same temperature and air speed, real-time thickness data also showed a distinct solvent mass-flux controlled two distinct regimes. This change in drying behavior appears to be due to a translation like the transition from a rubbery to glassy state seen in amorphous polymers. 118 Figure 5.7 Effect of initial wet thickness on birefringence for 0.01 wt.% PAI-GO coatings. The decrease in the mass diffusion rate of DMAc from the polymer matrix stems from an increased diffusion path that correlates directly with the thickness of the coating and number of GO nanosheets. This behavior was similar to the drying kinetics of neat polymer-solvent systems without nanoparticles. The rubbery to glassy transition for neat polymer-solvent systems without nanofillers was reported by other researchers 149. GO reinforced polymer coatings with smaller initial wet thicknesses on the other hand showed similar behavior that does not include distinct regimes with larger out-of-plane birefringence (Δn23) values related due to reduction in coating thickness (Figure 5.7). Figure 5.8 Effect of GO content on percent weight change and real-time thickness data for 0.01 wt.% PAI-GO coatings. 119 To investigate the role of presence of GO nanosheets on the drying behavior of filled nanocomposite PAI coatings, solutions of different GO loadings were prepared using three different concentrations of 0.01 wt.%, 0.05 wt.% and 0.10 wt.% with initial wet coating thickness of 559 µm. We have shown that total drying time decreased with increased GO content (Figure 5.8). This was attributed to the hygroscopicity of DMAc as shown by Figure 5.9. Increased humidity percent in the drying environment caused PAI solution to dry slower due to water intake from air. Particularly at early drying stages water intake was more obvious from balance readings as shown by increase in coating weight. Figure 5.9 Effect of percent humidity on real-time drying for neat PAI-DMAc solution due to hygroscopicity of DMAc. Casting and drying conditions are as follows: Blade gap 559 µm, Airflow rate 0.2m/sec at room temperature. In a similar way, contact angle measurements were conducted for dried PAI hybrid films (Figure 5.10). Calculated contact angles increased from 57° to 93° with series of GO content where 57° was calculated for neat films. This proves that GO content increased the hydrophobicity of hybrid PAI-GO films. We have shown that increased GO content repelled moisture from air during drying of PAI hybrid solutions that makes the coating less hygroscopic for the films dried at same drying conditions. 120 Figure 5.10 Effect of GO content on hydrophobicity of dried PAI-GO nanocomposite films. Calculated contact angles are: 57° for neat, 80°, 86° and 93° for 0.01, 0.05 and 0.1 wt.% GO content. Generated out-of-plane anisotropy started earlier as GO content was increased for the PAI-GO nanocomposite solutions with highest birefringence achieved for neat PAI solution as shown by Figure 5.11. This was attributed to the decrease in light transmission due to absorption by GO nanosheets as increased with GO content. Details on absorption coefficient of graphene-oxide, its optical constants and complex refractive index determination can be found elsewhere 161, 160, 159. Figure 5.11 Effect of GO content on birefringence for PAI-GO coatings. 121 The level of exfoliation of the GO nanosheets within the nanocomposite PAI films was investigated using TEM microscopy. We have shown that GO nanosheets oriented in plane, parallel to the film surface (Figure 5.12). The micrographs were in accordance with the real-time out-of-plane birefringence data (Δn23) measured during solvent evaporation from the polymer coating. The nearly complete exfoliation was due to sonication process during the preparation of the PAI-GO hybrid solution, as applied for full exfoliation in other polymer systems 151, 152. Similar exfoliation techniques to produce PI hybrid films to fully exfoliate organoclay particles which can be tracked by disappearance of the wide-angle x-ray scattering (WAXS) diffraction peaks can be found elsewhere. Figure 5.12 TEM Micrographs showing exfoliation and planar orientation of GO nanosheets: (a) raw GO sheets, (b) cross-section cut 0.1 wt.% GO reinforced PAI films, (c) cross-section cut neat PAI film without filler (arrow shows thickness direction) Mechanical properties of dried GO nanoreinforced films were investigated, and revealed that films incorporating very small amount of GO nanosheets (0.01 wt.%) within the PAI matrix have decreased the ultimate tensile strength by 20 % (Figure 5.13). As shown in Figure 5.14, further increasing GO content within the films first resulted in a minima in Young`s modulus and increased with higher GO loadings. The chemically reduced GO nanosheets are composed of layers of atomic thickness with very high 122 surface area. Therefore the effect of GO nanosheets on tensile properties was caused by strong interfacial interactions between GO and PAI as well as the fine dispersion and certain extent of planar orientation of GO nanosheets in PAI matrix, and thus allowing an efficient stress transfer from polymer matrix to the filler 173, 170. Figure 5.13 Effect of GO content on tensile strength for PAI-GO nanocomposite films. However, in contrast with mechanical properties of PI-GO nanocomposites, brittleness of PAI-GO hybrid films was reached at very small GO content which acts as physical cross-linking points in the matrix and restricts the mobility of polymer chains and reversed as the GO content increased 171. This was attributed to the solution casting process rather than solution intercalation technique to form hybrid films which induces high level of planar orientation particularly at high GO content. Figure 5.14 Effect of GO content on modulus for PAI-GO nanocomposite films. 123 Helium permeability values of dried hybrid films indicate the effectiveness of the chemically reduced GO content to improve permeability comprising planar orientation of nanosheets parallel to the film surface (Figure 5.15). The level of exfoliation of nanosheets is another concern which directly affects permeability values. In general, better exfoliation results in better permeability performance for industrial applications and better tensile strength. Improved barrier properties also indicate improved exfoliation of the GO nanosheets within the PAI matrix. Further increase in GO content did not exhibit a dramatic decrease in permeability values. From the mechanical and permeability data, improvement in mechanical strength almost linearly correlated with barrier properties. Figure 5.15 The effect of GO content on He Permeability for dried PAI-GO nanocomposite films. A proposed molecular model also shows orientation development for PAI chains and GO nanosheets during drying of the coating (Figure 5.16). Due to the relatively large aspect ratio of nanosheets their molecular size indirectly depicted at artificial scale. Initially (i), polymer chains and organoclay platelets are randomly oriented. Two stages (ii & iii) follow, consecutively, in which polymer chains and organoclay platelets begin to orient themselves to the plane parallel to the coating surface (chain collapse 124 phenomenon), starting from the liquid-air interface. Finally, polymer chains with aromatic rings and GO nanosheets exhibit planar orientation which can be tracked by the dramatic increase in Δn23. Figure 5.16 Molecular Model representing real-time Anisotropy Development during the course of Solution Evaporation (Inset graph shows data for 229 µm initial wet thickness). 125 5.4 Conclusions In summary, a flexible PAI-GO hybrid film in which exfoliated nanosheets resulted in enhanced barrier properties was developed using chemically reduced graphene-oxide. To the best of our knowledge, application of GO was now reported previously for any soluble PAI system. It was observed that mass diffusion of solvent (DMAc) during drying of wet PAI-GO coatings induces planar orientation of aromatic rings along with GO nanosheets due to interfacial interaction. Further increase in GO content resulted in a decrease in total drying time for wet coatings. We have shown that addition of GO content resulted in reduction on hygroscopicity of DMAc caused by hydrophobic PAI-GO matrix as shown by contact angle measurements. In contrast with PI-GO films, brittleness of PAI-GO hybrid films was reached at very small GO content. We have shown that overall He permeability of dried hybrid films decreased over 40% even with very small GO content. Therefore these hybrid films are widely adaptable to aerospace and other industrial applications. 126 CHAPTER VI MULTI-LAYER OPTICAL RETARDER FILM WITH LOW BIREFRINGENCE DISPERSION AND ITS MECHANO-OPTICAL BEHAVIOR 6.1 Introduction Optical compensation films are widely used in LCD technology to compensate excessive retardation generated by liquid crystal layer within the display. For this purpose combination of rigid rod polyimides (PI) and/or low birefringent polymers are used to introduce desired retardation values that depend on the type of LCD. Although these birefringent compensation layers help to improve the overall image quality, they have serious color distortion problems at wide viewing angles which decreases the satisfaction of the consumers and other end users. This phenomenon is caused by the variation of the optical dispersion curve of the retarder film with the wavelength of the light passing through the liquid crystal layer of the display. Details for dispersion of birefringence and its influence on optical characteristics of polymer films can be found elsewhere 174. Different techniques were used to prevent birefringence dispersion including employment of copolymers 175, liquid crystal molecules 176, 177, 178, 179, photopatterning 61 and designs with multilayered retarders 84, 87, 85. Reverse birefringence dispersion properties of polystyrene and its blends with and without uniaxial deformation was also reported and showed homo-polymer a-tactic 127 polystyrenes with out-of-plane birefringence (Δnth) less than 0.003, reverse birefringence dispersion can be easily achieved 180, 181. Polymers with reverse dispersion characteristics mainly show an increase of birefringence for longer wavelengths in the visible region which results in improved birefringence dispersion characteristics. Display technology incorporates different type of LCDs depending on the type of the nematic liquid crystal material used in display assembly. Compensation films with predesigned retardation values determined by the type of the liquid crystal material are used to improve optical properties. Typical retardation values for IPS (In-plane switching), TN (twisted nematic) and ECB (electrically compensated bend) displays that needs to be compensated were reported as 20-100 nm for IPS 79, less than 100 nm for TN 76 and around 300 nm for ECB displays 70. Control of the level of retardation and birefringence with uniaxial deformation is well known in literature 182. Novel custom built uniaxial stretching machine developed by our group was also reported that incorporates on-line monitoring of birefringence during stretching of polymer films 16. Cakmak and coworkers was also developed real-time spectral system to monitor structural changes takes place during uniaxial stretching with on-line measurement of true stress and birefringence 48. It was capable of capturing rapid changes with high resolution and precision in retardation (and birefringence) which can be due to relaxation, crystallization and/or melting depending on the processing conditions. Real-time spectral birefringence technique was also applied in our group for birefringence and retardation measurements during biaxial stretching acquiring true stress, true strain, in-plane and out-of-plane birefringences of the film under deformation 128 18. It incorporates an optical system, a vision system and a motor controller for precise measurements. Out-of-plane retardation (Rth) and in-plane retardations (Rin) can be independently controlled by adjusting deformation either in MD or TD. In this work, an innovative approach was used that incorporates optical dispersion curve flattening by multilayered consecutive positive and negative birefringent layers. Optical properties were tuned by given uniaxial and biaxial deformation depending on the level of desired optical anisotropy needed. PSF, PCC and PS were used as independent layers of the multi-layered optical retarder film. Multi-layer solution casting process using custom built multi-layer doctor blade through hybrid process was employed. Right after completion of multi-layer solution casting and drying process, the final optical analysis of the multilayer optical films coupled with uniaxial and/or biaxial deformation were analyzed while the sample was located inside the heating and stretching chamber of the highly instrumented custom built biaxial stretching machine. After stretching the corresponding out-of-plane or in-plane retardations can be generated In addition to this, true stress-strain behavior coupled with true mechanical responses and structural development has not been studied for multilayer optical compensation films. Therefore this study would provide a beneficial feedback for multilayer optical film production. 6.2 Experimental 6.2.1 Materials PSF p1700nt grade for optical applications was obtained from Solvay Chemicals Inc.; PCC LEXAN OQ1060 copolymer (polycarbonate and polyester groups which were block copolymerized) with reduced oxygen transmission properties for advanced optical 129 applications was obtained from SABIC Innovative Plastics (formerly GE Plastics) and PS Styron 675 injection molding grade was obtained from Dow Chemical Company. PSF resin is soluble in commercially available, water-miscible, dipolar, aprotic solvents such as dimethylacetamide (DMAC), dimethylformamide (DMF), and N-methyl pyrrolidone (NMP). PSF, PCC and PS pellets were dissolved prior solution casting process using high purity NMP purchased from Sigma-Aldrich Corporation. Moisture-proof packages were provided therefore no drying procedure was employed prior solution mixing. 6.2.2 Solution Preparation and Casting 30wt.% PCC-NMP and PS-NMP solutions with 20wt.% PSF-NMP solution were prepared using Thinky® Planetary Centrifugal Vacuum Mixer ARV-310 (Rotation + Revolution) with improved material dissolution, uniformity and deaeration. Heating/cooling cycles were performed to eliminate heat of mixing results in degradation and decrease of molecular weight of polymer chains. In order to manufacture wide angle multi-layer optical retarder films for LCD applications, polymer films were produced by multi-layer solution casting process employing a custom made 6” wide multi-layer doctor blade which was used to cast the liquid over a flat PET (Mylar) substrate. Figure 6.1 shows the illustration of the multi-layer solution casting procedure. Figure 6.1 Schematic of multi-layer solution casting process. 130 For mechanical tests of the films, commercial size solution casting platform present in our group was employed (Figure 6.2). It incorporates six heating zones, dual carriers, forced laminar air flow, programmable carrier motion, solid granite casting surface for enhanced film roughness and safety lower explosive limit (LEL) sensors. This process includes replaceable, custom-built single and multilayer doctor blades. A custom- made six-inch multi-layer doctor blade was used for casting the multi-layer polymer coating on a flat PET (Mylar) substrate. The films were dried at room temperature and at varied %RH. An industrial scale manufacture three-manifold slot die option is applicable (with max. three different polymeric solutions). Figure 6.2 Hybrid process schematic with multi-layer slot-die. Airflow direction is represented with dashed arrows. Sequential manufacturing scheme was used including solution casting and drying the PSF layer first and then casting consecutive 2nd and 3rd layers using simultaneous multi-layer solution casting on top of the dried support layer. Initial wet thickness for the support layer was 130 µm for pristine PSF solution. Right after drying, consecutive layers were casted by; 130 µm 2nd layer wet thickness for pristine PCC solution and 130 µm 3rd 131 layer wet thickness for pristine PS solution. Casting speed was adjusted to 20 cm/min for viscosity match of individual solutions. Right after solution casting of PSF-PCC-PS three layer optical retarder, multi- layer polymer coating was dried using conductive and convective heating including hot air at 100 °C with minimum rate of air flow of 0.5 m/sec respectively. After drying phase, multi-layer optical film had a good optical quality after visual examination. Overall total dry thickness of the film was around 60 µm with approximately 20 µm thickness for each layer. 6.2.3 Rheology Rheological characteristics of the multi-layer solution casting system were investigated. For this purpose Bohlin Instruments Gemini Nano cone-and-plate rheometer was used. In order to determine shear viscosities of each polymer grade with different solvent and solute content combinations, 4º/40 mm apparatus was employed mainly to observe the viscosity change with shear rates from 0.001 to 100 sec-1. 6.2.4 Thermal Analysis Thermal properties of solution cast PSF, PCC and PS films were measured using universal Q200 TA Instruments DSC and universal Q50 TA Instruments TGA. For TGA experiments, heating rate of 20 °C/min under a dry nitrogen atmosphere was employed. For DSC experiments, samples were placed as stack of cut films inside aluminum hermetic pans to improve heat transfer efficiency. Heating rate of 10 °C/min under a dry nitrogen atmosphere was applied after samples were cooled down to -20 °C. The polymer grades have amorphous nature, no crystallization peak was observed. 132 6.2.5 Refractometry AO Instruments Mark II Digital Abbe Refractometer was used with monochromatic Na lamp for refractive index determination of solution cast films. Each individual layer has approximately 20 µm of thickness for the designed multi-layer compensator film. Averaged refractive index of the multi-layer film was calculated as value of 1.59. 6.2.6 Microscopy Confocal and optical microscopy was used to observe morphological changes occur during course of drying such as phase separation caused by existence of copolymers and high moisture content. Zeiss LSM 710 series laser excited confocal microscope with Leitz Laborlux 12 POLS optical microscope were used for surface characterization. Normal and/or advanced illumination settings were used for images where necessary. 488nm and 543nm laser excitations were used for LSM application. 6.2.7 Online Birefringence and Stress-Strain Measurements through Biaxial Stretcher Highly instrumented biaxial stretcher designed by Hassan and Cakmak 18 was used in order to characterize stress and stain-optical behavior of single layer solution cast polymer films. Cast polymer solutions were kept in drying chamber approximately for 12 hours which was followed by sample preparation. Samples with dimensions of 14 x 14 cm were cut from as-cast films, and an array of 24 yellow dots was printed in the center portion of these samples. Cut films were placed using pneumatic clamps inside the heating chamber of the biaxial stretcher. For each polymer film, stretch temperature of Tg+20 °C (±5 °C) was applied with a minimum thermal equilibrium time of 10 minutes 133 before the start of the biaxial deformation process. Local true strain values are calculated by capturing real-time displacement of the 24 yellow dot pattern using a high speed CCD camera coupled with an automated image analysis system (Figure 6.3). Figure 6.3 Positions of Real-time Measurement Sensors for Bixial Stretching Machine. Incompressibility assumption, which is based on X and Y axis strain values, was taken into account in order to determine thickness variation at the center of samples. Equation 1 represents the formulation of incompressibility assumption. LX0LY0T0 = LXtLYtTt (6.1) where LX0 and LY0 are the original distance between dots in X and Y directions, LXt and LYt are the distance between dots in X and Y directions during deformation, T0 is the initial thickness measured by a thickness gage and Tt is the instantaneous thickness in the center of the cast film which is to be calculated. The true Hencky strains were also calculated from the true strain values using the following equation: εH = ln(ε t +1) (6.2) True stresses values were also calculated by dividing the load cell force by the cross-sectional area multiplied by the instantaneous thickness calculated previously at the geometric center of the film. After stretching, the cast films were immediately cooled 134 down using exhaust fans to “freeze” the deformed structure. In-plane and out-of-plane birefringences were also measured using two polarized light beams normal and 45° to the film surface at the geometric center of the sample. The calculation procedure at 546 nm (green light wavelength) can be summarized using the following equations: (i) in-plane birefringence Δn12: ( ) = ( ) (6.3) 푅0 푡 ∆푛12 푑 푡 (ii) out-of-plane birefringence Δn23, when the machine direction is horizontal, was found by using Stein’s equations 183: 1 / 2 Sin 2φ R − R 1− 0 φ 2 1 ∆ = − n n23 2 d 0 Sin φ 2 n (6.4) (iii) out-of-plane birefringence Δn13, was then calculated using: ∆n13 = ∆n12 + ∆n23 (6.5) where d(t) is the local thickness of the sample at time t, R0 and Rϕ are the retardations at the geometric center of the film when the light beam is oriented normal and 45° with the film surface, ϕ is the tilting angle (taken as 45°) and is the average refractive index of the material. Corresponding out-of-plane retardation 푛�(Rth) values were also calculated by using the following equation; 135 ( n + n ) R = { x y − n } *thickness (6.6) TH 2 z where 1=MD=X, 2=TD=Y, 3=ND=Z which can be combined with orthogonality relations: (6.7) (6.8) 6.3 Results and Discussion Multi-layer films were produced by sequential solution casting of three different polymeric systems including: very low-birefringent support layer (PSF) and consecutive birefringent layers of positive (PCC) and negative (PS) of nature. PSF was chosen for mechanical stability and processability issues. In comparison with previously depicted multi-layer optical retarders 90, 86, 184, 185, this design comprise sequential multi-layer solution casting procedure including non-crystallizable PCC as the middle layer. Birefringence dispersion profile of the resulting multi-layer film flattened by merging normal dispersion profile obtained from PCC layer and negative dispersion profile obtained from PS layer (Figure 6.4). The characteristics of negative optical dispersion profile for a-tactic PS systems can be found elsewhere 180, 181. 136 Figure 6.4 Optical dispersion profile for the proposed retarder film and its average refractive index. Individual polymer solutions were prepared with 20wt.% PSF and 30wt.% PS and PCC initial concentrations for multilayer solution casting. These concentrations were chosen according to match overall viscosity of the three layer system (Figure 6.5). Shear viscosity values of PSF solutions were increased with solid content as a homopolymer. PCC solutions exhibit similar behavior except an increase in shear viscosity at low shear rates for diluted solutions. This was attributed to the copolymer nature of the polycarbonate. Figure 6.5 Viscosity variations of PSF and PCC with different solid contents. Thermal properties of the individual layers were investigated for the retarder film to maintain mechanical and structural integrity during uniaxial/biaxial deformation. As shown in Figure 6.6, glass transition temperature (Tg) difference for neat polymer grades was high, around 80 °C for PS and PSF. Each grade did not show any crystallization peak as expected due to amorphous nature of the polymers including PCC. 137 Figure 6.6 DSC thermographs for Neat PS, PCC and PSF Grades. In a similar way, glass transition temperatures of the individual solution cast films of PSF, PCC and PS were determined (Figure 6.7). Calculated Tg values of cast films were closer and Tg difference was decreased significantly from 80 °C to 16 °C. This was attributed to the bound solvent content which was determined by TGA (PCC & PSF films with 13%wt., PS film with 9 %wt. bound NMP content). Bound solvent acts as a plasticizer and affect thermal properties of the films 186, 187, 188. Figure 6.7 DSC thermograms for solution cast PSF, PS and PCC films. The effect of initial concentration of polymer on bound solvent ratio in dried films was also investigated using TGA. Figure 6.8 shows the bound solvent ratio for 138 polysulfone films casted with 10, 15 and 20 wt.% initial concentrations. Highest bound solvent content was observed at 20 wt.% concentration with 9.5 wt.% solvent content. It was linearly proportional with initial solid content. Thus, initial multi-layer solution casting formulations can be tuned for desired Tg match between individual layers. Figure 6.8 Bound Solvent Ratios for polysulfone films with different initial concentrations. During simultaneous multilayer solution casting, lack of structural integrity can be either in the form of phase separation or a complete delamination of the individual cast layers. Since PSF is well known for liquid-liquid phase separation characteristics 189, 190, the introduction of a non-solvent (water due to humidity) even at very small amounts immediately promotes phase separation with two liquid phases of different composition (Figure 6.9). Therefore low %RH drying environment was preferred. Figure 6.9 Micrographs with magnification 50x transmission (right) and reflection (left) indicating phase separation due to %RH humidity for PSF films. 139 Multi-layer optical retarder was manufactured by using two successive steps including, first solution casting of PSF support layer and drying; second multi-layer solution casting of PCC and PS layers simultaneously on top of the dried support layer. By employing these successive steps optical, mechanical and structural characteristics were maximized and high quality multi-layer optical retarder films were manufactured. De-lamination was not observed high optical clarity as seen in micrographs captured by confocal microscopy (Figure 6.10). Surface topographical micrographs show surface smoothness without cracks that may form due to the drying stresses that are developing during the course of drying. Figure 6.10 Surface LSM Micrographs of Multi-layer Optical Retarder Films with 5X Magnification (Left) and 10X Magnification (Right) using Reflection Mode. Top pictures show 3D Topographical View (488nm left, 543nm right) of the Surface Profiles. Uniaxial stretching along the machine direction (MD) was applied for dried films which were produced by solution casting. The effect of uniaxial stretching on stress- optical behavior of solution cast and dried polysulfone (PSF) films with 20 wt.% initial solid concentration was monitored using uniaxial spectral birefringence system 16. Polysulfone films were stretched with 1.5X stretch ratio at 140 °C with film thickness of 40 µm (Figure 6.11). 140 Figure 6.11 Mechano-optical Behavior of Solution Cast PSF Films at 140 °C at 100 mm/min. Yielding was observed after true strain value of 0.8 and higher supported by Tg increase of the film during stretching due to solvent evaporation. In-plane birefringence (Δn12) values calculated showed regime I behavior until yielding obeying the stress- optical rule (SOR) due to a decrease in overall free volume that polymer chain matrix. After yielding occurred regime II behavior was observed indicating high degree of orientation. Biaxial stretching on the other hand is a better and more enhanced process to introduce the desired in-plane and out-of-plane anisotropy by employing deformation along the machine direction (MD) and transverse direction (TD) to the polymeric films which were produced by either a melt casting or solution casting configuration. Therefore analyzing the effect of uniaxial and/or biaxial stretching on stress-optical behavior of solution cast and dried multilayer optical retarder films has great importance. Highly instrumented biaxial stretcher with spectral birefringence system 18 was employed for this purpose for multilayer polymer films stretched at different deformation conditions. Stretching temperature was chosen as 60 °C in order to catch the processing window that is +15 degree C higher than the Tg of the multilayer film. The stretch ratio was limited to 1.3X with 1, 2 and 4 mm/sec stretch rates for the sake of uniform film structure and mechanical uniformity. Biaxial strain was applied sequentially to the multi- 141 layer films which consist of applying first MD direction deformation followed by TD direction deformation (Figure 6.12). Both MD and TD direction stresses were increasing with an increase in stretch rates. As expected, yielding behavior was more noticeable especially at high deformation rates. Figure 6.12 Mechanical Behavior of Solution Cast PSF-PCC-PS multilayer optical films. The effect of biaxial stretching on in-plane and out-of-plane anisotropy was also monitored by calculating IP and OP birefringence values. For this purpose, 0 degree and 45 degree retardation values were obtained from the biaxial stretcher (Figure 6.13). The theoretical background and design of the system is very similar to the multiple- wavelength spectral birefringence systems as described before. Figure 6.13 True Hencky strain in MD & TD vs. Retardation behavior of PSF-PCC-PS solution cast films stretched at different stretch rates. 142 Corresponding in-plane (IP) and out-of-plane (OP) birefringence values were then calculated by LabVIEW based software which also controls the biaxial stretcher (Figure 6.14). During the calculation steps, average refractive index of the multi-layer optical retarder film was also used along with the real-time thickness and retardation values. An increase in stretch rate dramatically increases OP birefringence (negatively) and IP birefringence (positively) values. Figure 6.14 True Hencky strain in MD & TD vs. Birefringence behavior of PSF-PCC-PS solution cast films stretched at different stretch rates. Rate effect was noticeable for MD deformation resulting increased Δn12 values with deformation rate (Figure 6.15). MD stretching had almost no effect on IP & OP birefringence values at low stretch rates. In order to compensate the optical anisotropy due to the liquid crystal layer within LCD displays, the out-of-plane retardation (Rth) values of optical retarder films becomes important. 143 Figure 6.15 The effect of deformation rate on in-plane birefringence during MD deformation of PSF-PCC-PS solution cast films. Therefore corresponding Rth values after stretching were also calculated which shows similar behavior with 45 degree retardation values (Figure 6.16). Right after completion of TD deformation for 1mm/sec stretch rate, Rth reached a value around 300 nm which is good enough for compensating optical anisotropy for ECB-LCDs. For IPS- LCDs and so on, TD stretch ratio of 1.1X or lower should be chosen right after 1.3X MD deformation. Figure 6.16 Calculated Rth Values for Multi-layer Optical Retarder. 144 Rth vs. R0 plots at each stretch rate was also prepared in order to develop understanding on the effect of stretch rate for in-plane and out-of-plane anisotropy development (Figures 6.17 and 6.18). In compare with the mechano-optical behavior of single-layer solution cast films, an increase in stretch rate in MD caused significant increase in in-plane retardation values which is a measure of in-plane anisotropy at the same time, resulting the fact that overall the multi-layer film acted as single-layer positive birefringent PCC film. Out-of-plane retardation value was minimized at high stretch rates causing reduced orientation on the plane parallel to the cast film surface. Figure 6.17 The effect of stretch rate on in-plane & out-of-plane retardation development for multi-layer PSF-PCC-PS films (Solid arrows represent increasing strain on MD whereas dashed arrows represent increasing strain on TD). Slopes of Rth vs. R0 curves at relatively high deformation rates showed similar trends during both MD and TD stretching. Therefore in order to reach large Rth and R0 values for this particular multi-layer compensation film, MD and TD strains should be introduced at relatively higher stretch rates. For lower Rth and R0 values, smaller deformation rates would be sufficient. 145 Figure 6.18 Combined effect of stretch rate on in-plane & out-of-plane retardation development for multi-layer PSF-PCC-PS films (Hollow symbols represent TD strain). In order to compare performance of multi-layer film with single-layer case, similar Rth vs. R0 graphs were plotted. Slopes were much more linear in this case without rate effect which shows decrease in Rth as the deformation rate increases (Figure 6.19). Sequential multi-layer solution casting scheme was chosen including solution casting and drying the polysulfone support layer first and then casting consecutive 2nd and 3rd layers using custom built multi-layer doctor blade. Right after completion of multi-layer solution casting and drying process, the final optical analysis of the multilayer optical films coupled with uniaxial and/or biaxial deformation were analyzed while the sample was located inside the heating and stretching chamber of the highly instrumented custom built biaxial stretching machine. According to the Rth values, right after completion of TD deformation for 1mm/sec stretch rate, Rth reached a value around 300 nm which is good enough for compensating optical anisotropy for ECB-LCDs. For IPS-LCDs and so on, TD stretch ratio of 1.1X or lower should be chosen right after 1.3X MD deformation. 146 Figure 6.19 The effect of stretch rate on in-plane & out-of-plane retardation development for single-layer PCC (left) and PS (right) films (Hollow symbols represent TD strain, solid arrows represent increasing strain on MD whereas dashed arrows represent increasing strain on TD). Multi-layer embodiment of PSF, PCC and PS polymer chains that represents positive and negative birefringent in sequence helped decrease the birefringence dispersion of the multilayer retarder film (Figure 6.20). This would result in improved optical characteristics of the final retarder film for LCD applications with improved contrast ratio and viewing angle. Figure 6.20 Dispersion characteristics of the multi-layer PSF-PCC-PS optical retarder. 147 6.4 Conclusions Multi-layered optical retarder design was used and high quality multi-layer optical films were produced without phase separation and de-lamination. Design comprise low birefringent support layer with consecutive positive and negative birefringent retardation layer. Higher deformation rates generated lower Δn23 birefringence with higher Δn12 birefringence. Optimum Rth and R0 values were achieved at 1mm/sec stretch rate to compensate ECB-LCDs. Birefringence dispersion of multi-layer films was found to be flattened. 148 CHAPTER VII SUMMARY AND RECOMMENDATIONS In summary, the evolution of optical anisotropy gradient and development of highly oriented layer were investigated for the first time using compensator method in the first 360 minutes of drying. It was observed that casting procedure introduced higher birefringence on the substrate-liquid interface at earlier stages. As solvent evaporates, this profile was reversed and resulted in higher birefringence on air-solution interface. After a critical time, we observed the formation of a distinct highly oriented layer. In addition, observations from real-time measurement system and off-line measurements showed good agreement. This will allow us to use the real-time measurement system to quantitatively study the thickness averaged optical anisotropy in drying films and coatings. Hybrid films from organoclay nanoplatelets and graphene-oxide nanosheets are produced incorporating PAI-DMAc solution. The level of out of plane anisotropy was high due to the evaporation induced planar orientation of nanoplatelets and nanosheets. X-ray diffraction showed full exfoliation as a complete disruption of the characteristic (001) reflection in clay. As a result, hybrid, multi-layer films with enhanced gas-barrier properties and flexibility without sacrificing low thermal expansion were obtained. Overall, permeability values decreased over 40-50% in compare with neat PAI films. Therefore hybrid PAI films are widely adaptable to aerospace and other industrial applications. 149 Multi-layered optical retarder design comprising low birefringent support layer with consecutive positive and negative birefringent retardation layer are produced. Higher deformation rates generated lower Δn23 birefringence with higher Δn12 birefringence. Optimum Rth and R0 values were achieved at 1mm/sec stretch rate to compensate ECB-LCDs. Birefringence dispersion of multi-layer films was found to be flattened. Further investigation on the effect of initial wet thickness scale-up on drying time for organoclay loaded PAI based system would be beneficial for the real-time effect of clay content on drying characteristics. As shown in Figure 7.1, the increase in initial wet coating thickness is not linear with total drying time due to increase in interaction between organoclay platelets and three phase system due to water in-take from air due to humidity. For this purpose a study based on compensator method should be carried out to determine the oriented layer development at different organoclay loadings such as 1 wt.%, 3wt.%, 5wt.% and 7wt%. This would result in better insight on diffusion process and interaction of clay platelets with three phase system. Figure 7.1 Effect of initial wet coating thickness on total drying time for 1wt.% organoclay loaded PAI system. 150 In a similar way, the effect of initial wet coating thickness on total drying time for GO loaded PAI based system can be plotted for 0.01wt.% GO content (Figure 7.2). A similar study based on compensator method should be carried out to determine the oriented layer development at different content. This would result in better insight on diffusion process and interaction of GO nanosheets with three phase system. The complex refractive index of GO nanosheets would be taken into account to eliminate excessive absorption of light during measurements. Figure 7.2 Effect of initial wet coating thickness on total drying time for 0.01wt.% GO loaded PAI system. 151 REFERENCES 1. Unsal, E.; Drum, J.; Yucel, O. Real-time measurement system for tracking birefringence, weight, thickness, and surface temeperature during drying of solution cast coatings and films. Rev. Sci. 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Polymer Science 1991, 33 (5), 994-1000. 164 APPENDICES 165 APPENDIX A SUPPORTING INFORMATION FOR CHAPTER III A.1 Birefringence Profile Correlation with Percent Solid Content Abbe refractometer technique was used by procedure depicted by Samuels 118 to measure air and substrate-interface birefringence profiles during drying of PAI-DMAc coating. In addition, percent solid change data was supplemented by real-time measurement system acquired from precision balance. Substrate-interface birefringence increase was in agreement with percent solid data (Figure A1a and A1b). As shown in Figure A1c and A1d, peak in anisotropy development was observed at 15 wt.% for air- interface and 25 wt.% for substrate-interface. Figure A1. Optical properties of neat PAI-DMAc solution during drying with initial set wet thickness of 1mm. (a) Air-interface refractive index change. (b) Substrate-interface refractive index change. 166 Figure A1. (c) Percent solid change with refractive indices for air-interface. (d) Percent solid change with refractive indices for substrate-interface. A.2 Integration Process for Birefringence Calculation through Compensator Method Compensator technique for measuring birefringence of melt spun fibers as a rendition of overall molecular orientation is well known in literature. Previously, Berek compensator technique was used with monochromatic light to measure melt spinning stresses built up for polycapronamide fibers and Poly(L-lactic acid) (PLA) fibers 191, 192. Using similar procedure, birefringence profile through the thickness was measured for freshly cast PAI-DMAc solution. Compensator readings were converted into birefringence values for the first 360 minutes of drying using optical tables. After 200 minutes, formation of a highly oriented layer was observed at air-liquid interface. Therefore air-liquid interface birefringence data was unable to read after 200 minutes. nAVG calculation step for a particular drying time included data until 200 minutes. For this purpose a simple integration procedure was applied for each particular drying time. Figure A2 shows calculated birefringence profile for 160 minutes of drying time. 167 Figure A2. Birefringence profile calculated for T=160 minutes through compensator technique. Arithmetic average for all of the data points for a particular drying time through thickness was not used. Instead, thickness increments were used during integration for freshly cut samples for nAVG calculation (Gray lines in Figure S2). The differences between these two methods were shown in Table S1. This resulted in more accurate birefringence profile calculation for a particular drying time. Table A6. Arithmetic averaged and integrated birefringence values. Drying time Arithmetic Integrated Coating Thickness (minutes) Average Δn23 Δn23 (µm) 40 1.44E-03 1.51E-03 444.0 60 1.36E-03 1.39E-03 499.5 80 1.35E-03 1.39E-03 518.0 100 1.57E-03 1.60E-03 518.0 120 1.54E-03 1.57E-03 499.5 140 1.84E-03 1.83E-03 518.0 160 1.42E-03 1.40E-03 499.5 180 1.68E-03 1.65E-03 462.5 200 2.83E-03 2.76E-03 351.5 168 A.3 Lowest Energy Conformational State Calculation Studies based on lowest energy potential energy surface of a polymer chain reveals important details based on molecular packing and physical properties 193, 194, 195. Investigation of anisotropy development through compensator method revealed that substrate-interface birefringence was higher at early drying times. We attribute this phenomenon to shear induced planar orientation of rigid PAI chains near the substrate surface. To achieve this goal, we first searched for energy minima on the potential energy surface for a single generic PAI chain to find lowest energy conformers. Theoretical calculations were performed using conformers package using Materials Studio program. Figure A3. Top view and side view for predicted lowest energy conformer for single PAI chain. As shown in Figure A3, planar orientation of generic PAI chain was predicted for lowest energy potential energy surface. We have used universal forcefield with atom based summation method for electrostatic and van der Waals terms. Geometry optimization was used with systematic grid scan with perturbation in torsion angles with 10 degrees of increments. 169 APPENDIX B DESIGN AND BUILT OF REAL-TIME MEASUREMENT SYSTEM FOR TRACKING WEIGHT, THICKNESS, BIREFRINGENCE B.1 System Design The novel thickness-temperature-birefringence measurement system can be divided into three different parts, main body, thermal system and optical system namely. The main design criteria were as follows during the course of system development: • Provide laminar airflow with constant flow rate and temperature • Monitor weight with minimal fluctuations from outside influences • Monitor thickness with minimal fluctuations from outside influences • Monitor film surface temperature without contact • Monitor film thickness without contact B.1.1 Main Body The main body was made from stainless steel for superior toughness and corrosion resistance and designed as a wind tunnel in order to satisfy laminar air flow conditions throughout the evaporation chamber. The overall dimensions of the body was 7 x 1 x 2 feet (L x W x H) with heat insulation inside to reach isothermal conditions as quickly as possible by preventing excessive heat loss during heating and drying stages (Figure B.1). Vibration pads were also located to prevent excessive noise. 170 Figure B.1 Main body of the novel thickness-temperature-birefringence measurement system. Hot air generator was placed on the right hand side of the evaporation chamber in order to provide necessary hot air with uniform flow rate and temperature. Generator was purchased from Taketsuna Co., model TSK-52HT with custom temperature and air speed control (minimum is ambient room temperature and maximum is 500°C whereas minimum is no air flow and maximum is 6 m/s over the sample holder). At the end of the downstream there is an opening for exhaust stream which has solvent containing air. Three vertical baffles were also placed right in front of the upstream connected to the hot air generator in order to maintain uniform laminar flow and calibrate where necessary in combination with an anemometer. At the middle of the evaporation chamber there is also another opening for the sample holder and high precision balance. This opening was covered by glass surfaces from all directions to maintain laminar airflow. The balance was purchased from Sartorius AG with sensitivity of 0.01g and placed on top of a heavy iron base to satisfy vibration free environment. It was further surrounded by a polycarbonate enclosure to prevent any disturbances coming from the surrounding environment. The sample holder was made from graphite as the manufacturing materials since it has superior performance for thermal conductivity (Figure B.2). It has a very high 171 thermal conductivity with very low thermal expansion coefficient which is crucial during heating and cooling cycles within the evaporation chamber. Moreover, the overall sample platform weight is very small in compare with aluminum or stainless steel so that it is easy to handle. The platform has four graphite rod-shaped legs with a circular opening at the center for the transmission of light beams. Figure B.2 3D technical drawings for the sample holders built with different materials. The legs contact directly with the high precision balance surface for corresponding weight measurements during the course of drying of polymeric solution. By using four small holes throughout the bottom glass surface and PC enclosure, legs do not touch anything. Finally, sample holder made completely from graphite was used for all of the experiments of which solution casted borosilicate glass substrate containing an uncoated center portion for optical measurements put on top. B.1.2 Thermal System Surface temperature measurements were carried out by four different pyrometers purchased from Micro-Epsilon Corp., model no., thermoMETER-CT. These sensors were 172 selected due to their high temperature resistance during the course of temperature measurements and their high precision for contactless measurement. Corresponding focal length of each sensor were adjusted in such a way to maximize temperature readings by facing towards the drying polymeric solution. Emissivity values were also adjusted according to the substrate that is being used. Figure B.3Top view of the top anti-reflective coating glass surface after placement of pyrometers. As described before in the middle section of the highly instrumented evaporation chamber, sample holder was placed just on top of the high precision balance and it was isolated from the environment with the addition of anti-reflective coated special glass surfaces in all directions. The reason for anti-reflective coating was to maximize the transmission of light for the optical system which will be described in the following section. Four small holes were placed on the top anti-reflective coated glass surface for the placement of pyrometers which are facing directly to the polymer coating which is casted on borosilicate glass substrate located on top of the sample holder (Figure B.3). Each sensor was connected to a signal reconditioning box which is directly connected to the main computer by serial cables respectively. The temperature readings from these sensors were also verified with thermocouples for the overall accuracy of the system. 173 B.1.3 Optical System There are two optical breadboards for the optical system to work efficiently. Optical sensors including laser displacement sensors for real-time thickness measurement and fiber-optic cables attached to the spectrometers for 0° and 45° light beams for birefringence measurements were attached to the top optical breadboard (3 x 2 ft). The optical lens system with corresponding polarizers was located on the smaller breadboard at the bottom position (2 ft by 10 inch). Both of these breadboards were bolted directly to the heavy iron base to eliminate any vibration related noise generation. Figure B.4 Overall schematic illustration of the placement of the sensor system on the novel thickness-temperature-birefringence measurement system. For optical measurements, spectral birefringence method described by Hassan 18 was employed. It was originally based on the work of Cakmak and coworkers 48 who developed real-time stretching spectral birefringence instrument in order to monitor structural changes takes place during uniaxial stretching polymers with the addition of 45° light beam for out-of-plane birefringence measurements. It has been a great success 174 for this technique because of its ability to simulate and characterize real-time processing conditions that takes place during industrial processes. This spectral birefringence technique was able to capture rapid changes with high resolution and precision in retardation (and birefringence) which can also be implemented on real-time drying of polymeric solutions. The optical system mainly consists of visible range high intensity light sources purchased from Dolan-Jenner (MI-152) with a wavelength range of 400 nm to 700 nm, linear polarizers in order to generate 45° plane polarized light beams, bifurcated fiber optic cables and dual-channel fiber-optic spectrometers purchased from Avantes Inc. During the course of optical measurements, polymeric solution that is being examined was casted on 1mm thick borosilicate glass in order to prevent heat conduction issues and placed on top of the graphite sample holder with the center portion remained uncoated for optical system. The remaining area was coated with black wet paint in order to increase the overall precision of the laser displacement sensors. Plane polarized light passed through the sample was then collected by bifurcated fiber optic cables which are connected to the spectrometers. Three separate non-contact displacement sensors were purchased from Keyence Co. (Model: LK-G 152) for the determination of real-time thickness change during the course of solvent evaporation within the highly instrumented evaporation chamber. The corresponding locations of each laser (red) with pyrometers can be shown in Figure B.5. 175 Figure B.5 Exact locations of laser displacement sensors on top of the solution casted glass substrate. Laser-A was employed to monitor real-time displacement change of the surface of the borosilicate glass substrate and this value was used to subtract from Lasers-B & C to eliminate thermal expansion related correction issues. Lasers-B & C was designed to monitor real-time thickness change of the polymer coating located upstream (Laser-C) and downstream (Laser-B) of the evaporation chamber. In birefringence calculations, real-time corrected thickness data from Laser-B was used in general since it was located to the middle portion of the glass substrate from which data was collected. A complementary high definition USB camera was also used and positioned next to the sensors where needed for image and video capture. All these sensors were connected to a computer and the real-time data was acquired using Labview software developed by our group. Data acquisition rate can be adjusted 1 to 4 data points/second which is responsive enough to characterize solution casting and real-time drying process. The design of the novel thickness-temperature-birefringence measurement system can be further enhanced by installing complementary GC/MS system, an ultrasound system and with an optical UV curing kit in order to monitor real-time orientation change during the course of UV curing. 176 B.2 Aerodynamics Testing In order to maintain laminar air flow conditions a proper aerodynamics testing was also accomplished inside the wind tunnel. For this purpose an industrial grade airflow thermal anemometer was purchased from Kanomax Corp. which is capable of recording airflow rate and temperature for desired time interval. Three vertical baffles which were located in front of the upstream inside the wind tunnel was configured for optimum laminar and steady state conditions (Figure B.6). Figure B.6 Baffle configurations during the aerodynamics testing. Two different airflow rates were measured during the course of aerodynamics testing which can be listed as 1 m/s and 1.5 m/s flow rates. These flow rates were set using a smaller-sized industrial scale hot air generator purchased from Taketsuna Co. (TSK-11) for testing purposes and recorded the resulting airflow rates and temperatures just close enough to the top surface of the sample holder on which borosilicate glass substrate including the polymeric coating was put during experimental data collection. 177 Figure B.7 Airflow and temperature distribution graphs for 1 m/s & 75 °C airflow rate setting. Figure B.7Airflow and temperature distribution graphs for 1 m/s & 75 °C airflow rate setting. As can be noticed from Figure B.7, variations for airflow rate and temperature can be noticeable for specific baffle configurations. Left/Center/Right setting on x-axis mainly represents the position of the anemometric measurement on top of the sample 178 holder e.g. right means the data was collected on the top right portion of the sample holder which is closest to the incoming airflow from upstream. Figure B.8 Airflow and temperature distribution graphs for 1.5 m/s & 75 °C airflow rate setting. Figure B.8 Airflow and temperature distribution graphs for 1.5 m/s & 75 °C airflow rate setting. In order not to affect airflow dynamics anemometer probe was hold with test hole on the side of the tunnel using a magnetic holder attached to the main frame of the 179 evaporation chamber. Optimal laminar airflow conditions with homogenized airflow and temperature distributions was satisfied when all of the three baffles were aligned parallel to the airflow direction. Moreover, for high airflow rates the optimal conditions was also observed when all the three baffles were configured parallel to the incoming airflow. B.3 Thermal Expansion Testing & Substrate Selection The original sample holder for the novel thickness-temperature-birefringence measurement system was made from aluminum initially. The design was also improved further using graphite as the main material during manufacture of sample holder. Because of the fact that thermal expansion coefficient of graphite (7.9x10-6 m/m K) is three times less than that of for aluminum (22.2x10-6 m/m K), sample holder displacements due to thermal fluctuations were minimized. Figure B.9 Displacement of empty sample holders made from different materials during a heating cycle (1st laser indicates laser displacement sensor Laser-B whereas 2nd laser indicates Laser-A). Figure B.9 summarizes this fact during a heating cycle from room temperature to 100 °C respectively at 1 m/s airflow rate. As expected, sample holder made from aluminum only showed the greatest thermal expansion whereas the sample holder made 180 completely from graphite showed the lowest thermal expansion values which were measured using installed laser displacement sensors. The sample holder with aluminum base and graphite legs showed a transition in between those two. On the other hand, thermal conductivity of the material from which the sample holder is made is also very important due to the fact that only convective heating is applied from the hot air with a lack of conductive heating. Linear thermal conductivity of the material should be high enough in order to compensate temperature difference of the sample holder (and glass substrate indirectly) from upstream of the airflow to the downstream. Although graphite has lower heat conductivity in thickness direction (5.7 W/m K), in parallel direction its heat conductivity is extremely high (1950 W/m K) in compare with the aluminum (237 W/m K). Therefore graphite was selected as the main material for the manufacture of the sample holder. Substrate selection was also another issue since the material should be thermally conductive, thermal expansion free and good enough for laser displacement sensors for laser performance. For this purpose a test polymer solution including 20 wt.% polysulfone in NMP (N-Methylpyrrolidone) was prepared using a conventional rotary mixer purchased from THINKY Co. (ARV-310). For test purposes unheated evaporation chamber was used with a hot air flow rate of 1 m/s at 75 °C and solution was casted with a wet thickness of 550 µm at 30 cm/min coating speed using a conventional hand coater. As can be seen from Figure B.10, hard-anodized aluminum surface was more realistic. 181 Figure B.10 Substrate performance during a heating cycle of the evaporation chamber. Since metal substrates were not realistic enough to represent real-time evaporation data a polymeric coating, a borosilicate glass substrate was selected as the main substrate on top of which polymer was coated. It also had great optical performance for real-time birefringence measurements. A sample data can be seen in Figure B.11 with real-time thickness and weight data of the polymer coating that is being analyzed. Figure B.11 Test data with borosilicate glass substrate. 182 APPENDIX C THE EFFECT OF CO-SOLVENT & ORGANOCLAY CONTENT ON OPTICAL PROPERTIES OF THIN FILMS C.1 Introduction Polymer nanocomposites are widely used in many industrial applications varying from consumer electronics to transportation vehicles and construction materials for their distinct properties due to large interfacial area per unit volume. As depicted in literature 196, polymer-clay nanocomposites can improve mechanical durability including elastic modulus and strength 197, 198, 199, gas barrier properties 200, 201, 202, anti-flammability 203. In the original work of Toyota Researchers, clay nanocomposites were prepared by in-situ polymerization 197, whereas they can also be produced using traditional melt-casting or employing relatively new solution casting processes without reaction. Polysulfone (PSF) thermoplastic polymers are well known in polymer industry for their toughness with thermal and mechanical stability at high temperatures which makes it perfect candidate for various engineering applications. Their grades with high optical clarity are also used in LCD display industry for several purposes. Among these applications, particularly in preparation of membranes for gas separation and ultrafiltration for water purification it has been widely used due to controllable pore size during the course of phase inversion process. For the former, various studies have been reported regarding properties of neat 204, co-polymers 205 and blends 206 of polysulfone 183 membranes whereas the effect of different solvents was also depicted previously 207. Morphology and properties of polysulfone membranes prepared with different co- solvents and non-solvent additives was also discussed properly 208. For the latter, chemical modification is favorable to increase permeability of PSF membranes 209. The application of polysulfone as nanocomposite gas separation membranes was also reported 210 previously which includes traditional phase inversion process of the polysulfone-montmorillonite (MMT) dispersion. Moreover, preparation and application of PSF/clay nanocomposite coatings employing solution casting without phase inversion process was also depicted 211, 212, reveals the fact that polysulfone-clay nanocomposite materials applied on cold-rolled steel have superior anti-corrosion properties over neat PSF. However real-time evaporation dynamics and anisotropy development for polysulfone-clay nanocomposite materials have never been reported. Therefore a fundamental understanding of the effect of different initial nanocomposite concentration during drying stage of solution casting process possesses a critical importance. The addition of co-solvent into polymer solutions during the process of solution casting is a well-known technique in coating industry to improve overall drying performance. Therefore co-solvent study based on real-time solution evaporation would possess similar importance for the drying characteristics on a real-time basis since no specific study has been reported in literature. For this purpose PSF/N-Methylpyrrolidone (NMP) and Tetrohydrofuran (THF) solutions were investigated. With the help of novel thickness-temperature-birefringence measurement system for real-time solution casting and drying process reported by our group 213, 1, complex 184 mechanisms underlying evaporation dynamics and co-solvent effect can be identified which can help improving processing conditions and nanocomposite content. C.2 Experimental C.2.1 Materials N-Methylpyrrolidone (NMP) and Tetrahydrofuran (THF) solutions were purchased from Sigma-Aldrich Corporation and used without further purification. Cloisite® 30B natural montmorillonite (MMT) nanoclay mineral modified with a quaternary ammonium salt was purchased from Southern Clay Products (Figure C.1) in order to prepare polysulfone/clay nanocomposite films. Figure C.1 Chemical structure of C30B modified by ammonium salt (where T is Tallow, ~65% C18, ~30% C16, ~5% C14). UDEL p1700-nt based polysulfone (PSF) resin was obtained from Solvay Advanced Polymers. After drying to less than 0.1% moisture content, the pellets were provided in moisture-proof bags. Thus, the resin was used without further pre-drying immediately before processing. C.2.2 Polymeric Solution Preparation Polysulfone (PSF) pellets were used as is without further drying. Pristine PSF solutions were prepared using NMP and THF as the solvents with 20 wt. % polymer in solution. During mixing phase THINKY Planetary Centrifugal Vacuum Mixer ARV-310 185 (Rotation + Revolution) was used with improved material dissolution, uniformity and deaeration. Heating/cooling cycles were performed if needed to eliminate heat of mixing. To prepare nanocomposite PSF-nanoclay solution, first pristine 20 wt. % PSF solutions were prepared followed by 1, 3 and 5 wt.% (of polymer solution) of clay addition to the mixture. Nanocomposite solutions were prepared by mixing clay solutions for around one hour employing THINKY Planetary Centrifugal Vacuum Mixer ARV-310 (Rotation + Revolution) with improved material dissolution, uniformity and deaeration. Heating/cooling cycles were performed if needed to eliminate heat of mixing. After bulk mixing phase, final nanocomposite PSF-nanoclay solutions were obtained with 20 wt. % PSF with 1, 3 and 5 wt.% of clay in solution. C.2.3 Rheology Bohlin Instruments Gemini-Nano cone-and-plate rheometer was used for rheological measurements. At room temperature shear viscosities and shear stress values of each polymer solution were measured with the help of corresponding target and actual shear rates. 4°/40 mm apparatus was used to observe the viscosity change with shear rates from 0.001 to 100 sec-1. Gap distance between the cone and plate was 150 µm. C.2.4 Simultaneous Solution Casting Process Polymer films were produced by hand blade casting (BC), a custom made 3” wide doctor blade was used to cast the liquid over a flat borosilicate glass substrate. Cheminstruments EC-200 hand caster was used for this purpose. Casting configuration was as follows: 559 µm wet thicknesses for pristine PSF solutions and PSF-clay nanocomposite solutions with varying clay loadings. Casting speed was 30 cm/min which was the lowest speed setting for the hand blade caster. 186 C.2.5 On-line Weight, Thickness, Temperature & Birefringence Measurements Cast PSF-clay nanocomposite films were dried within the chamber of highly instrumented Thickness-Temperature-Birefringence Measurement System for at least 10 hrs at 75 °C. For the co-solvent study, cast pristine PSF solutions with either NMP and/or THF were dried within the chamber of highly instrumented Thickness-Temperature- Birefringence Measurement System for at least 10 hrs at 55 °C. Air speed was 0.5 m/sec. The chamber was preheated prior to begin experiment. Dried cast films were obtained by peeling from borosilicate glass substrate followed by visual examination for the quality of the film. If there were no defects, thicknesses of cast films were measured using Mutitoyo CUA-154 micrometer with an average value of at least three measurements. Average dimensions of cast films were approximately 3” by 7” and corresponding retardation and birefringence measurements were conducted using the center portion of the cast films. C.3 Results and Discussion The drying characteristics of polysulfone-clay nanocomposite solutions were investigated which can reveal important information regarding evaporation dynamics at different organoclay loadings. Therefore for the first time, real-time drying and anisotropy development for filler contained PSF-NMP system was analyzed; in this way coating formulations can be further enhanced knowing at which particular time interval out-of-plane anisotropy is maximized. 187 Figure C.2 Rheological properties of 20 wt.% PSF-NMP solutions with different nanoclay loadings. Rheological properties of polymeric solutions with different organoclay loadings was also compared (Figure C.2). It is important to note that particularly at low shear rates both shear viscosity and shear stress values were increased with organoclay loading. Similar behavior for PEO-organoclay filled system with a decrease in particle size of organically modified clay was also depicted before 146 due to stronger polymer-clay interactions as the particle size is reduced. As we increase organoclay content per unit volume similarly, polysulfone-clay interactions become stronger which leads to this increase. In the case of 1 wt.% organoclay loading over 10 hours of drying time, in-plane birefringence (Δn12) was zero indicating no in-plane anisotropy whereas out-of-plane anisotropy (Δn23) started to develop after three hours at which surface temperature increase stopped due to almost zero mass flux of solvent (Figure C.3). 188 Figure C.3 Real-time drying & anisotropy development for 20wt.% polysulfone/NMP coating with 1wt.% organoclay content. For 3 wt.% organoclay loading, in-plane birefringence (Δn12) was similarly zero indicating no in-plane anisotropy whereas out-of-plane anisotropy (Δn23) started to develop at an earlier drying time, after two hours at which, again surface temperature increase stopped due to almost zero mass flux of solvent (Figure C.4). In this case however, the magnitude of Δn23 was increased to 0.001 which is almost three times larger than the organoclay loading of 1 wt.%. Therefore addition of organoclay within the polysulfone matrix increased the overall mass flux rate of the solvent. Figure C.4 Real-time drying & anisotropy development for 20wt.% polysulfone/NMP coating with 3wt.% organoclay content. 189 Further increase in organoclay content resulted in faster out-of-plane anisotropy development with almost same Δn23 values in compare with 3 wt.% (Figure C.5). Figure C.5 Real-time drying & anisotropy development for 20wt.% polysulfone/NMP coating with 5wt.% organoclay content. Addition of organoclay particles within the polymer matrix clearly slowed down the rate of evaporation per unit time (Figure C.6). Bound solvent content was higher in compare with neat polysulfone-NMP coating as can be seen from percent weight change. Figure C.6 Real-time weight change for neat and Cloisite 30B loaded polysulfone solutions. It was attributed to the fact that addition of clay nanoparticles increases the bound solvent ratio by preventing free evaporation during the course of drying. 190 Figure C.7 Real-time mass flux and temperature change for neat and Cloisite 30B loaded polysulfone solutions. This trend was also followed by mass flux date per unit time showing the effect of addition of organoclay particles on the rate of evaporation (Figure C.7). On the other hand real-time thickness data supports the fact that an increase in clay loading clearly hinders coating thickness reduction which was decreasing with less organoclay content (Figure C.8). This result was on the contrary of the former one surprisingly. Figure C.8 Real-time thickness change of neat and Cloisite 30B loaded polysulfone solutions. 191 Moreover in all of the cases including the neat polysulfone coating, drying data showed two-regime drying characteristics during solution evaporation. Another regime right after inception can also be identified where bulk thickness change was positive due to water uptake from the environment. It can be reduced by employing low %RH environment. In-plane and out-of-plane anisotropy change also tracked using the highly instrumented evaporation chamber and expressed by both in-plane and out-of-plane birefringences (Figure C.9). Organoclay loading has almost no influence on the in-plane anisotropy whereas out-of-plane anisotropy increases with clay loading. This was also expected since oriented nanoclay platelets within the plane of the polymer surface induce out-of-plane orientation. Figure C.9 Real-time anisotropy development of neat and Cloisite 30B loaded polysulfone solutions during drying. Since there is no aromatic ring structure on the backbone of polysulfone, increased out-of-plane anisotropy was primarily attributed to the planar orientation of organoclay platelets i.e. forced orientation due to the presence of nanoclay platelets is 192 dominant throughout the chain orientation process. A molecular model was also incorporated representing the polysulfone chains and organoclay platelets (Figure C.10). Planar orientation started at the coating-air interface and later developed in the bulk as the drying time increased. Out-of-plane anisotropy was maximized when almost all of the organoclay particles oriented to the plane parallel to the coating surface at elevated temperature. Figure C.10 Molecular Model representing real-time Anisotropy Development during the course of Solution Evaporation. The critical concentration at which out-of-plane anisotropy increased dramatically was also tracked indicating the fact that increased organoclay content within the polysulfone matrix resulted in relatively smaller solid concentrations. 193 Figure C.11 The effect of organoclay loading on overall out-of-plane birefringence. In order to understand the co-solvent effect on evaporation dynamics of polymer solutions during drying phase, Polysulfone/N-Methylpyrrolidone and tetrohydrofuran (PSF/NMP & PSF/THF) solutions were prepared. Totally six different PSF/solvent combinations varying with different NMP/THF loadings were investigated. These solvents were chosen already in order to study the effect of different rate of evaporations of solvents only. As can be found from literature THF has a boiling point of 66 °C whereas NMP has a boiling point of 203 °C respectively. The drying temperature was also reduced to 55 °C in order to match rate of evaporation difference of two solvents and to keep the final film quality in a good condition without bubble formation during the course of heat application. 194 Figure C.12 Real-time weight change data for PSF/NMP-THF solutions. Figure C.12 shows the real-time weight change of the polysulfone solutions during the drying phase within the highly instrumented evaporation chamber. An increase of THF content in the solution clearly increases the resulting evaporation flux which can be tracked by decreasing drying time. Also bound solvent ratio is much more less in the case of low boiling point solvent is the majority of the polymer solution. On the other hand, real-time thickness data (Figure C.13) exhibit greater importance since the rate of change of the bulk thickness was not same for all the cases. As depicted before 149, 214, corresponding mass flux during the course of drying can represent different regimes which is related to the rubbery state assumption stating the fact that varnish temperature is greater than the glass transition temperature of the polymer film. In the case of especially higher NMP concentrations (e.g. pure NMP and 80NMP/20THF wt.%), clearly two different regimes are obvious which are related with the real-time mass flux changes and can be expressed with thickness change under the assumption of the cast varnish dimensions do not change. For high low boiling point 195 solvent concentrations on the other hand (THF), drying regime is pretty simple and can be represented by constant rate of decrease of the varnish thickness. Figure C.13 Real-time thickness evolution during drying phase of polysulfone solutions. Evaporation induced in-plane and out-of-plane anisotropy change can also be tracked using the highly instrumented evaporation chamber and can be expressed by both in-plane and out-of-plane birefringences. As can be seen in Figure C.14, the rate of evaporation change has almost no influence on the in-plane anisotropy of the system whereas out-of-plane anisotropy increases with the rate of evaporation increase. This can be expected since polymer chains has less time to relax and immediately freezes due to high evaporation rate which results in increase out-of-plane anisotropy. Figure C.14 Real-time evaporation induced anisotropy change during drying phase of PSF/NMP-THF solutions. 196 C.4 Conclusions The effects of co-solvent ratio and organoclay content for nanoclay composite films were investigated for the first time during the drying phase of the wet films. By employing typical tape casting procedure, cast thin coatings of PSF-THF, and PSF- organoclay nanocomposites in NMP solution revealed the fact that an increase in the rate of evaporation and clay-content induces out-of-plane anisotropy increase. This causes polymer chains preferentially to orient along the plane parallel to the film surface. Especially this should help for anti-corrosion applications since increased OP anisotropy with organoclay platelets would diminish significantly the diffusion pathway of O2 and other related gas particles. 197 APPENDIX D THE EFFECT OF MUTUAL DIFFUSION COEFFICIENT ON THICKNESS EVOLUTION DURING DRYING OF POLYMER FILMS D.1 Introduction Solution casting is an important process in industry not for production of functional optical films such as optical retarders and photovoltaic films but also for applications such as adhesive tapes and magnetic media. Understanding the kinetics of drying of a polymer film therefore is very crucial in order to optimize processing parameters including casting and drying conditions. Simultaneous physical changes also occur on molecular level due to evaporation of solvent which results in reorientation of polymer chains with their primary chain axes oriented in the plane of the film 215, 216. Various studies have been reported in literature in order to develop a proper physical model for drying of a typical polymeric coating using hot air 149, 217, 218, infrared- heating (IR) 219 and using natural convection conditions 220, 214, 221, 222, 223. General assumptions and descriptive mass and heat transfer equations can be found elsewhere 224, 225, 226. These models generally assume Fickian diffusion over the thickness domain while solvent is evaporating and model verification steps do not include any experimental real- time thickness data of the polymer solution. Instead, since the spontaneous solvent evaporation itself forms a moving boundary value problem, generally a coordinate 198 transformation as in the case of Landau transformation is used to represent moving boundary of the varnish/air interface. As depicted by the author 149, the coupled mass and heat transfer equations governing the system of evaporation of the solvent from polymer coating can be expressed as follows; + ( ) = + (D.1) 푠 푠 푠 푑푇 푎푖푟 푎푖푟 푓표푖푙 푓표푖푙 푝 푝 푑푡 푡ℎ 푎 푡ℎ 푎 푉 푚 where �휌 푐 푒is the�휌푐 �substrate���푒 푡 � thermalℎ capacity�푇 − per푇� unitℎ area�푇 of the− sample푇� − 퐿 (J/m휑 2°C), 푠 푠 푠 푝 ( 휌) is푐 the푒 average polymeric coating thermal capacity including the real-time change 푝 of�휌푐��� �the푒 푡thickness of the coating by including the term e(t) and finally and which 푉 푚 stands for vaporization heat (J/g) and the solvent flux (g/m2s). The evaporation퐿 휑 solvent flux can also be represented by Equation 5.2. Overall heat transfer coefficients between air and foil were also taken into account using and terms. 푎푖푟 푓표푖푙 ( ) ℎ푡ℎ ℎ푡ℎ = (D.2) 푃푉푆 푖푛푡푒푟푓푎푐푒푀푆 푔 휑푚 ℎ푚 � 푅푇 − �휌푆 �∞� In this expression, ( ) is the saturating vapor pressure of the 푉푆 푖푛푡푒푟푓푎푐푒 polymer/solvent mixture, 푃 is the solvent molar mass, is the overall solvent mass 푆 푚 transfer coefficient (m/s) and푀 is the solvent vaporℎ concentration in air far from the 푔 푆 interface. There are also various�휌 experimental�∞ studies for the calculation of overall transfer coefficients the geometries that was interested 227, 228, 229. It is also important to note that heat and mass transfer equations are coupled with ( ) which is a function of temperature and solvent volume fraction. Therefore 푉푆 푖푛푡푒푟푓푎푐푒 local푃 solvent partial density was expressed 149 as Equation 5.3 including the mutual diffusion coefficient DSP. 199 ( , ) = 0 (D.3) 휕휌푆 휕 휕휌푆 휕푡 휕푧 푆푃 푆 휕푧 Several experimental setups− also�퐷 depicted휔 푇 to monitor� real-time solvent mass evolution and other processing parameters 149, 223, 230, 231. But none of them was as successful as the recent real-time measurement system developed by our group to analyze system parameters in solution casting process for the sake of tracking coating thickness, temperature and solvent mass evolution inside a drying chamber consecutively which comprise one-to-one simulation of industrial driers 213, 1. Once real-time behavior of coating thickness and solvent mass evolution are related to the process parameters, final model would be used particularly for industrial applications. In this work, simultaneous single-layer solution casting was employed with different initial concentrations of polysulfone (PSF) - NMP solutions to further understand the effect of solution properties on the kinetics of polymer solution drying. Evaporation induced anisotropy due to chain collapse phenomenon during drying of the wet single-layer polysulfone films was also measured using novel real-time measurement system. As shown by Guerrier 149 before, drying kinetics of polymer films under isothermal conditions show two distinct regimes including a fast regime controlled by coupled heat and mass transfers followed by a slow regime depends on the physicochemical properties of the solution. Therefore if the kinetics of PSF/NMP system at elevated temperatures can be understood, problems of simulating the drying kinetics of polymer solutions inside conventional driers might be solved. In this chapter, we will report for the first time the effect of solvent mutual diffusion coefficient on real-time thickness and solvent mass evolution. Real-time overall mutual diffusion coefficient data 200 showed excellent correlation during transition from fast regime to slow regime. The analysis of the physical properties during the drying the film will be described. D.2 Experimental D.2.1 Materials N-Methylpyrrolidone (NMP) solution was purchased from Sigma-Aldrich and used without further purification. UDEL p1700-nt based polysulfone (PSF) resin was obtained from Solvay Advanced Polymers. After drying to less than 0.1% moisture content, the pellets were provided in moisture-proof bags. Thus, the resin was used without further pre-drying immediately before processing. D.2.2 Polymeric Solution Preparation PSF pellets were used as is without further drying. Pristine PSF solutions were prepared using NMP as the solvent with 20, 25 and 30 wt. % polymer solid in solution. Initial concentrations of 35 wt.% and higher were not prepared due to viscosity related casting issues since the corresponding viscosity values were maximized at 30 wt.% of polysulfone-NMP solution. Rheological analysis of the polymeric solutions will be reported in detail in the following section. During mixing phase, THINKY Planetary Centrifugal Vacuum Mixer ARV-310 (Rotation + Revolution) was used with improved material dissolution, uniformity and deaeration. Heating/cooling cycles were performed if needed to eliminate heat of mixing. After bulk mixing phase, final PSF/NMP solutions with different concentrations were obtained. D.2.3 Rheology Bohlin Instruments Gemini-Nano cone-and-plate rheometer was used for rheological measurements. At room temperature shear viscosities and shear stress values 201 of each polymer solution were measured with the help of corresponding target and actual shear rates. 4°/40 mm apparatus was used to observe the viscosity change with shear rates from 0.001 to 100 sec-1. Gap distance between the cone and plate was 150 µm. D.2.4 Simultaneous Solution Casting Process Polymer films were produced by hand blade casting (BC), a custom made 3” wide doctor blade was used to cast the liquid over a flat borosilicate glass substrate. Cheminstruments EC-200 hand caster was used for this purpose. Casting speed was 30 cm/min which was the lowest speed setting for the hand blade caster. Initial wet thickness of pristine PSF/NMP solution to form the films was 559 microns which gives around 80 microns of final thickness right after drying inside the evaporation chamber. D.2.5 Thermal Analysis The thermal properties of solution cast polysulfone films were measured using universal Q50 TA Instruments TGA. For TGA experiments, samples with approximate weight of 5-10 mg were cut and placed in the platinum pan connected to the microbalance. Decomposition characteristics of samples which relate with bound solvent were examined at a heating rate of 20 °C/min under a dry nitrogen atmosphere. Investigations were made while heating the cut films from room temperature to 800 °C. TA Universal Analysis software was used to analyze the TGA results. D.2.6 On-line Weight, Thickness, Temperature & Birefringence Measurements Cast films were dried within the chamber of highly instrumented Thickness- Temperature-Birefringence Measurement System for at least 10 hrs at 75 °C. Air speed was 0.5 m/sec. The chamber was preheated prior to begin experiment. Dried cast films 202 were obtained by peeling from borosilicate glass substrate followed by visual examination for the quality of the film. If there were no defects, thicknesses of cast films were measured using Mutitoyo CUA-154 micrometer with an average value of at least three measurements. Average dimensions of cast films were approximately 3” by 7” and corresponding retardation and birefringence measurements were conducted using the center portion of the cast films. D.3 Results and Discussion Polymeric solutions with higher solid concentrations have higher viscosities compare with the more dilute ones; therefore initial polysulfone concentrations of 20 wt.%, 25 wt.% and 30 wt.% were chosen so that no wet thickness reduction due to high viscosity and leaking due to low viscosity during casting was observed (Figure D.1). Figure D.1 Rheological properties of PSF-NMP solutions with various concentrations. In order to investigate the effect of mutual diffusion coefficient on real-time thickness evolution for evaporation induced wet film drying; we need to have information regarding mutual diffusion in systems based on polysulfone (PSF). There are several proposed theoretical approaches 232, 233, 234 depending on the concentration domain of the polymer solution for the analysis of diffusion. Some approximated relations were also 203 depicted 235, 236 within the framework of self- and mutual-diffusion coefficient relationship. Since it was difficult to expand all of this information over the whole concentration domain, except the diffusion coefficient at very weak and strong solvent concentrations due to difficulty in obtaining both theoretically and experimentally, an analytical expression is crucial in order to correlate diffusion coefficient data with real- time thickness evolution during drying. There was a lack of experimental information in literature except the work of Mikhailov 237 and coworkers which includes real-time mutual diffusion coefficient data for PSF/NMP system at around 75 °C. Figure D.2 The Relation between Mutual Diffusion Coefficient and percent Solid Weight in Polysulfone (PSF) / N-methylpyrrolidone (NMP) system 237. Figure D.2 shows the empirical relation between mutual diffusion coefficient and percent solid weight in polymeric solution. Mutual diffusion coefficients were calculated by Matano-Boltzman analysis using interference micro method. The diffusion coefficient shows a maximum around 40 wt. % polymer concentration and nearly almost zero at very high polymeric solution concentration. A third degree polynomial fit also shows the correlation precisely with root-mean-square value of 0.99 which can be described in Equation D.4. 204 = 4 × 10 7 × 10 + 3 × 10 + 2 × 10 (D.4) −10 3 −10 2 −10 −12 By employing푦 the above푥 empirical− relation,푥 drying kinetics푥 of 20 wt.% PSF-NMP solution was revealed which can be divided into four different regimes depending on the real-time extrapolated overall mutual diffusion coefficient (Dv) and mass flux data (Figure D.3). The first regime can be characterized by tracking the mass flux data until it reaches the global maximum where Dv has a constant rate of increase. The second regime on the other hand continues until the rate of change of mass flux after reaching global maximum stays constant which can be identified by either seeking the inflection point the mass flux curve or looking at the increased rate of change of Dv values. Third regime continues until reaching the inflection point in the extrapolated mutual diffusion coefficient curve with a decreasing rate of change in the mass flux. Finally at fourth regime polymer solution is almost dried and both mass flux and mutual diffusion coefficient values stays constant and converging to the final “dried” values. It is also important to note that, since the concentration of solvent was decreased significantly after 2nd regime, due to decreased evaporation flux surface temperature further increased. Similar behavior was also depicted before for PBMA-MEK system 149. Although empirical mutual diffusion coefficient data set for PSF/NMP reveals the fact that diffusion was maximized at around 30 wt.% - 40 wt.% solid content, bound solvent ratio in the film right after successive drying stages does not depend on mutual diffusion coefficient data. If it was the case then polysulfone coatings with 30 wt.% - 40 wt.% initial solid content should have lower bound solvent ratios compared with the rest. In 205 order to make a supplement for this hypothesis, Thermogravimetric Analysis of the dried films were carried out (Figure D.4). Figure D.3 Real-time data during drying phase of 20 wt.% polysulfone solution. It revealed the fact that lower bound solvent ratios can be achieved at 20 wt.% initial concentration in compare with 25 wt.% and 30 wt.% initial concentrations. Bound solvent ratio for those initial concentrations were almost similar with 13.5 wt.% ratio. 206 Figure D.4 Thermogravimetric Analysis of the dried polysulfone films with different initial concentrations. Therefore, rather than bound solvent ratios, mutual diffusion coefficient for polysulfone/NMP wet coating had a direct effect on the rate of evaporation of the solvent. For this purpose real-time thickness study was carried out. Figure D.5 Real-time thickness & mass evolutions during drying phase of polysulfone solutions. Due to the fact that initial cast wet thicknesses of each polysulfone solution were the same, final weight of dried films casted at higher initial concentrations was higher as expected (Figure D.5). However, the rate of evaporation of NMP was faster for the 207 polymer coating of 30 wt.% initial polysulfone concentration; it was followed by 25 wt.% and 20 wt.% initial concentrations. This was in accordance with the empirical mutual diffusion coefficient data depicted before revealing the fact that, diffusion coefficients for polysulfone/NMP system has a direct effect on the rate of evaporation of the solvent. Moreover, the rate of change of real-time thickness patterns with different initial polysulfone concentrations shows distinct behavior for each case. Polysulfone solution with 25 wt.% initial concentration seems to have a transition thickness evolution in compare with 20 & 30 wt.% initial polysulfone solutions. As expected, final thickness of the dried film with initial polysulfone concentration of 30 wt.% was thicker than the rest of the cases. Figure D.6 Real-time birefringence development during drying phase of polysulfone solutions. As known from literature 215, 216, 213, 1, evaporation induced solvent loss in a polymeric solution during drying phase right after solution casting results in anisotropy on molecular level which can be tracked by out-of-plane and in-plane birefringences respectively. Thus, during drying phase of polysulfone solution with different initial concentrations, corresponding birefringence levels were also monitored (Figure D.6). 208 The level of in-plane and out-of-plane anisotropy strictly depends on several factors such as chain rigidity, molecular weight (Mw), solvent-polymer interaction parameter etc. Among these parameters level of aromaticity of polymer chains is an important measure for the level of out-of-plane anisotropy as can be seen in the case of polyamic acids (PAA) and polyamideimide-soluble polyimides (PAI) 213, 1, which shows immediate chain collapse in the plane parallel to the casting surface during solution evaporation. In the case of polysulfone however, the level of out-of-plane anisotropy is not high enough in compare with PAA & PAI solution due to the lack of closed ring and bulky heterocyclic structures. Moreover, in-plane birefringences are almost zero which indicates no in-plane anisotropy whereas out-of-plane birefringence is increasing with increasing initial concentration. This can be due to immediate freezing of polymer chains into glassy state from dilute solution in a relatively oriented state. Figure D.7 The effect of initial wet polysulfone solution concentration on overall out-of- plane birefringence. In addition to this, the correlation between solid wt.% and Δn23 also plotted (Figure D.7) which revealed the fact that regardless of the initial polymer solution concentration the out-of-plane anisotropy starts to develop at 40 wt.%. 209 Figure D.8 Effect of initial wet solution concentration on real-time solvent content of PSF/NMP coatings. In order to support TGA thermograms, real-time percent solvent within the polysulfone coatings was also plotted (Figure D.8). Similarly coatings with higher initial solid concentrations had higher solvent content. Figure D.9 Real-time variation of mass flux rates with time for polysulfone solutions with different initial concentrations. This was primarily attributed to the reduced mass flux rated due to smaller mutual diffusion coefficient. This behavior is especially noticeable if we analyze the mass flux variation with initial solution concentration (Figure D.9). As the initial concentration 210 increases, the following global maximum also diminishes. On the other hand, extrapolated mutual diffusion coefficient values have higher values at relatively low initial concentrations (Figure D.10). This seems reasonable since final dried films contains more bound solvent which forms greater free volume for solvent molecules to diffuse within the polymer matrix during solution evaporation. Figure D.10 Real-time variation of mutual diffusion coefficient with time for polysulfone solutions with different initial concentrations. Figure D.11 Real-time variation of surface temperature with time for polysulfone solutions with different initial concentrations. Temperature vs. time data was also used to show the fact that surface temperature readings were relatively maximized for coatings which have higher initial solution concentration due to lower rate of solvent evaporation with higher amount of bound 211 solvent at the end of drying (Figure D.11). For proof of concept aforementioned relations can also be seen if Figures D.4 & D.8. Figure D.12 Real-time data during drying phase of 25 wt.% polysulfone solution. Drying kinetics data was also revealed for the rest of the systems (25 wt.% & 30 wt.%). If we compare Figures D.12 & D.13 with Figure D.3, first regime for 30 wt.% polysulfone solution was diminished and replaced by an immediate second regime behavior right after solution casting at the beginning of the drying phase whereas the for 25wt.% polysulfone solution the domain for the first regime is narrowed. This shows that polymer solution behaves like a pure solvent at the beginning especially at 20 wt. % case. 212 Figure D.13 Real-time data during drying phase of 30 wt.% polysulfone solution. 213 D.4 Conclusions The effect of solvent mutual diffusion coefficient on real-time thickness and solvent mass evolution was also investigated for specific PSF/NMP system with different initial concentrations. Real-time overall mutual diffusion coefficient data showed excellent correlation during transition from fast regime to slow regime. The analysis of the physical properties during the drying the film can be divided into three different regimes; • Regime I: Evaporation is similar to the evaporation of pure solvent. • Regime II: After thermal equilibrium, flux is nearly constant. • Regime III: After solvent mass fraction at interface decreased (skin layer formed), with constant weak decrease of evaporative flux. Moreover, OP birefringence showed slight increase with solid wt.% while IP birefringence was constant. Since the concentration of solvent was decreased significantly after 2nd regime for 20 wt.% PSF-NMP system, due to decreased evaporation flux surface temperature further increased. Similar behavior was also depicted before for PBMA-MEK system. This tendency was not observed for 25-30 wt.% PSF-NMP systems as expected because of high initial concentrations. 214 APPENDIX E SUPPLEMENTARY INFORMATION FOR CHAPTER VI E.1 Sequential (SEQ) Biaxial Stretching using Uniaxial Constrained Width (UCW) Solution cast films of PCC c8924 & PS 675 were stretched with 1.5X-1.5X stretch ratio by stretching in machine direction (MD) first while being constrained in the transverse direction (TD) and then stretching in the transverse direction (TD) while keeping MD constrained in order to achieve biaxial stretching. Temperature was constant throughout the stretching experiments which were 80 °C for PS and 60 °C for PCC. Figure E.1 shows mechano-optical behavior of solution cast films of PCC which depicts the fact that, at constant temperature (60 °C), MD stretching induces slight positive birefringence whereas TD stretching has a similar effect on Δn12 which gives slight negative birefringence. Since these in-plane birefringence levels are too low for orientation of polymer chains, random orientation of polycarbonate-co-polymer chains can be assumed at the beginning of the deformation process and upon TD stretching due to applied biaxial strain. This is also interesting since PC resin originally has positive birefringence. It is noticeable that as the stretch ratio increases, the rate of positive Δn12 also increases for MD stretching whereas for TD stretching there is no rate effect, which can be seen from the slopes of the regression lines for the raw data (Figure E.2). 215 Figure E.1 True hencky strain in MD & TD vs. Birefringence behavior of PCC solution cast films stretched at different stretch rates. On the other hand, if we look at the ΔnOP (out-of-plane birefringence) values of Δn13 & Δn23 during deformation process, Δn13 & Δn23 decreases upon MD stretching and increases dramatically upon TD stretching which gives biaxial orientation. Because of the fact that out-of-plane birefringences are determined by the parallelism between the chain axis and film plane, positive birefringence indicates noticeable parallelism of polycarbonate copolymer chains during biaxial orientation. The rate of generation of out- of-plane positive birefringence was also increased with an increase in deformation rate during TD deformation whereas deformation rate had no effect during MD deformation. Figure E.3 also shows true stress – true hencky strain relationship for PCC solution cast films. Stress values were relatively low for machine direction (MD) deformation whereas for transverse direction (TD) deformation strain hardening phenomenon was observed. 216 Figure E.2 The effect of deformation rate on in-plane birefringence during MD deformation of PCC solution cast films. Rate effect due an increase in deformation rate was also noticeable which was higher at high stretch due to the fact that polycarbonate-co-polymer chains could find limited time for relaxation whereas for low stretch rates relaxation time was increased which gives lower stress levels. Yielding behavior was noticeable especially for deformations at high stretch rates in both MD & TD. Moreover, a transition from hard & tough material type to soft & tough type was observed as the deformation rate was increasing with the introduction of TD deformation due to biaxial stretching. Figure E.3 True stress – true hencky strain behavior in MD and TD of PCC c8924 solution cast film stretched at various stretch rates. 217 Solution cast films of a-tactic PS which is negative birefringent in nature also shows a distinct strain-optical behavior in compare with PCC c8924 grade which is positively birefringent in nature (Figure E.4). At constant temperature (80 °C), MD stretching induces slight negative Δn12 whereas TD stretching diminishes this orientation. Figure E.4 True Hencky strain in MD & TD vs. Birefringence behavior of a-tactic PS solution cast films stretched at different stretch rates. Out-of-plane birefringence during MD deformation also increases where for TD deformation this behavior reverses with overall negative birefringence as expected. Rate effect is also significant which shows in the case of increase in Δn12 (negatively) & Δn13 and Δn23 (positively) birefringences during MD deformation (Figures E.4 & E.5). Figure E.5 The effect of deformation rate on in-plane birefringence during MD deformation of a-tactic PS solution cast films. 218 Figure E.6 True stress – True Hencky strain behavior in MD and TD of PS 675 solution cast film stretched at various stretch rates. Figure E.6 also shows true stress – true hencky strain relationship for a-tactic PS cast films. Stress levels are pretty low for machine direction (MD) deformation whereas for transverse direction (TD), strain hardening phenomenon was observed. Yielding behavior was also noticeable especially for increased deformation rates. Therefore PS chains are easily deformable even at high stretch rates if the stretch ratio is low enough. Since out-of-plane birefringence can be represented by the parallelism between the chain axis and film plane and have a linear relationship with stress due to the stress-optical rule (SOR) at the initial stage of stretching, increase in TD and MD tensile stress values was expected due to dramatic increase in OP birefringences with an increase in stretch rates. In addition to this, optical uniaxial symmetry started to disrupt for the films stretched at higher stretch rates which can be monitored using the fact that two out-of-plane birefringences was increasing monotonously with deformation. 219