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Birefringence Gradient Development During Drying of Solution Cast 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 ii 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 iii 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. iv 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. v 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 vi 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 .............................................................................
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