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Electronic Theses, Treatises and Dissertations The Graduate School
2007 Design and Fabrication of Transparent Polycarbonate/Carbon Nanotube Composite Films Yuan-Chen (Craig) Chin
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THE FLORIDA STATE UNIVERSITY
FAMU-FSU COLLEGE OF ENGINEERING
DESIGN AND FABRICATION OF TRANSPARENT
POLYCARBONATE/CARBON NANOTUBE COMPOSITE FILMS
By
YUAN-CHEN (CRAIG) CHIN
A Thesis submitted to the Department of Industrial Engineering in partial fulfillment of the requirements for the degree of Master of Science
Degree Awarded: Spring Semester 2007
The members of the Committee approve the thesis of Yuan-Chen Chin defended on 04/12/2007.
______Young-Bin Park Professor Directing Thesis
______Ben Wang Committee Member
______Zhiyong Liang Committee Member
______Okenwa Okoli Committee Member
Approved.
______Chuck Zhang, Chair, Department of Industrial and Manufacturing Engineering
______C. J. Chen, Dean, College of Engineering
The Office of Graduate Studies has verified and approved the above named committee members.
ii
ACKNOWLEDGEMENTS
I am most appreciative of my advisor, Dr. Young-Bin Park. He always earnestly and persistently urged and supported me in my graduate study despite of the set-backs and frustrations hindering my progress. I also would like to thank my committee members, Dr. Ben Wang, Dr. Zhiyong Liang and Dr. Okenwa Okoli for the encouragement and helpful suggestions to my research. Moreover, department chair, Dr. Chunk Zhang, has always kept supporting me in my extended research. Additionally, I would like to acknowledge the post-doctors, Dr. Jin-Gyu Park, Dr. Qiang Wu for their instruction in the instruments; the administrative staff, Mrs. Stephanie Dickey, Mr. James Horne, and Mr. Qing (Charlie) Liu for their assistance in administration; also, Giang Pham, Eric Rodriguez, Evin Chen, Ashley Liao, Edward Wang, Irene Yeh, Charlie Lin, Kenny Tsai, Daphne Ku, Vavinlen Chen and all my co-workers at High-Performance Materials Institute (HPMI) for their help and encouragement to me. I also would like to acknowledge Mr. Han Gi Chae at Georgia Tech for his helpful discussions on PC crystallization and transparency analysis, and the Army Research Lab for their valuable assistance and contribution in my research. Lastly, I would like to give special thanks to my family for their supporting and emboldening me to finish my study.
iii TABLE OF CONTENTS
LIST OF TABLES ...... vi LIST OF FIGURES ...... vii ABSTRACT...... x CHAPTER 1 MOTIVATION AND OBJECTIVES...... 1 1.1 Introduction...... 1 1.2 Current State of the Transparent Materials...... 1 1.3 General Problem Statements...... 3 CHAPTER 2 BACKGROUND AND LITERATURE REVIEW...... 5 2.1 The Optical Principles and Basics ...... 5 2.1.1 The Optical Principles and Basics ...... 5 2.1.2 Numerical Calculation ...... 8 2.1.3 The Effect of Crystallization on Transparency...... 9 2.1.4 Microphysics...... 10 2.2 Chemistry and Properties of Polycarbonate...... 11 2.2.1 Driving Forces for Polymer Crystallization...... 13 2.2.2 Measurement of Degree of Crystallization...... 13 2.3 Carbon Nanotubes as Toughening Agents...... 14 2.2.1 General Properties of Carbon Nanotubes ...... 14 2.2.2 Toughening Mechanisms of CNT Composites...... 15 2.3 Methods for PC/CNT Composite Fabrication ...... 18 2.3.1 Melt Processing...... 18 2.3.2 Solution Processing...... 19 2.3.3 Electrospinning ...... 20 2.4 Characterization of PC/CNT Composites...... 21 2.4.1 CNT Toughening Mechanism...... 21 2.4.2 Optimal CNT Loading for Enhancing Mechanical Properties ...... 23 2.4.2 Optical Properties...... 25 2.4.3 Haze and Luminous Transmittance ...... 26 2.4.3 Differential Scanning Calorimetry (DSC) Technique ...... 27 2.4.3 Impact and Ballistic Resistance ...... 29 2.5 Summary of Literature Review...... 30 2.6 Research Objectives...... 30 CHAPTER 3 FABRICATION AND CHARACTERIZATION OF NEAT POLYCARBONATE FILMS ...... 32 3.1 Introduction...... 32 3.2 Transparent Neat PC Film Fabrication ...... 32 3.3 Design and Analysis of Experiments for the Degree of Crystallization...... 35 3.3.1 Solution Casting Factor Selection...... 35 3.4 DOE Analysis Results and Discussions...... 40 3.4.1 Driving Forces for Polymer Crystallization...... 40
iv 3.4.2 DOE Analysis ...... 41 3.4.3 Optimization and Discussions...... 45 3.5 Summary...... 47 CHAPTER 4 FABRICATION OF TRANSPARENT PC/MWNT COMPOSITE FILMS...... 49 4.1 Introduction...... 49 4.2 Fabrication of Transparent PC/MWNT Composite Films...... 49 4.3 Summary...... 50 CHAPTER 5 OPTICAL PROPERTIES OF PC/MWNT COMPOSITE FILMS... 53 5.1 Introduction...... 53 5.2 Characterization of CNT Dispersion by Optical Microscopy...... 54 5.3 UV-Visible Spectrometry ...... 55 5.3.1 The Beer-Lambert Law...... 56 5.3.2 UV-Visible Property ...... 58 5.4 Differential Scanning Calorimetry (DSC) Analysis ...... 62 5.5 Scanning Electron Microscopy (SEM) Analysis ...... 66 5.6 PC/MWNT Composite Film Thickness Analysis...... 68 CHAPTER 6 MECHANICAL PROPERTIES OF PC/CNT COMPOSITE FILMS ...... 71 6.1 Review ...... 71 6.2 Mechanical Properties of PC/CNT Films ...... 71 6.2.1 Experimental...... 71 6.2.2 Experiment Results ...... 72 6.2.3 Discussions ...... 72 6.3 Optimal CNT Loading for Enhancing Mechanical Properties ...... 76 6.4 SEM Analysis of CNT Mechanism ...... 80 CHAPTER 7 CONCLUSIONS AND PROPOSED WORK ...... 84 7.1 Conclusions and Contributions...... 84 7.2 Future Work...... 85 REFERENCES...... 86 BIOGRAPHICAL SKETCH ...... 92
v LIST OF TABLES
Table1.1 Selected mechanical properties of transparent glass and ceramic materials...... 2 Table 1.2 Typical properties of polymeric materials found in military ballistic systems... 3 Table 2.1 Properties of LEXAN® polycarbonate...... 12 Table 2.2 Properties of carbon nanotubes (CNTs)...... 15 Table 2.3 Solvent selection (SWNT solubility) [ 43]...... 19 Table 2.4 Solvent selection (PC solubility) [ 43- 51]...... 20 Table 3.1 Factorial design for solution casting optimization...... 37 Table 3.2 Data sheet for solution casting optimization from Design-Expert...... 38 Table 3.3 Model fit summary from Design-Expert...... 42 Table 3.4 The analysis of variance (ANOVA) table from Design-Expert...... 44 Table 5.1 Data sheet for the %T of the neat PC and the PC/MWNT composite films..... 61 Table 5.2 Degree of crystallization of neat PC and PC/MWNT composite films calculated by TA Universal...... 66 Table 5.3 Raw Data for thickness analysis of PC/MWNT composite films...... 69 Table 6.1 Shimadzu tension test data for neat PC, 0.05, 0.1, and 0.15 wt.-% PC/CNT composite films...... 73
vi LIST OF FIGURES
Figure 2.1 The electromagnetic spectrum...... 6 Figure 2.2 A path of incident light affected by the particle size. Internal features such as opaque areas or areas of different refractive index will force the light be either scattered by refraction or reflected as it passes through the crystals...... 7 Figure 2.3 The incident light enters a medium (high index of refraction) from the air (low index of refraction) with smaller refraction angle...... 7 Figure 2.4 Molecular structure of bisphenol A polycarbonate (BPA-PC)...... 12 Figure 2.5 Molecular structure of polycarbonate with two allyl groups on the end...... 13 Figure 2.6 (a) CNT knotting; (b) MWNT bending; (c) SWNT bending...... 16 Figure 2.7 The process of MWNT telescoping; A: the beginning of experiment; B: the upper break tip; C: the lower break tip; D: before breaking; E: partial pullout of internal nanotube bundles...... 17 Figure 2.8 MWNT telescoping...... 17 Figure 2.9 Entangled, random CNT network with bridges, cracks, and gaps under SEM; (a)(c): PC/SWNT; (b)(d): neat PC. (% Elongation: Neat PC: 20%, PC/SWNT: 30%) [45]...... 22 Figure 2.10 Young’s modulus versus CNT % by TMA; 29% increase in modulus at 0.06% is the apparent “optimal” CNT% [ 45]...... 22 Figure 2.11 Schematic of: (a) ideally dispersed CNTs; (b) aggregated CNT particles. ... 24 Figure 2.12 Solvent-induced crystallization in PC; (a) Crystalline structure grows within amorphous PC; (b) Joining of crystalline entities...... 25 Figure 2.13 (a) CNT dispersion under SEM (melt-mixed PC/15%MWNT). Rope-like structures are PC-bounded CNTs, and the bulge is rough PC surface; (b) CNT dispersion under TEM and AFM (melt-mixed PC/2%MWNT)...... 26 Figure 2.14 Temperature versus heat flow for DSC data and applications...... 28 Figure 2.15 DSC heating and cooling curve for (a) pure PP (polypropylene) and (b) 0.8 wt.-% PP/SWNT composite...... 28 Figure 2.16 (a) Changes in ballistic performance and compressive strength relative to the MWNT loading in PC; (b) Impact and Ballistic resistance; (c) Effect of blend composition on the compressive strength of virgin PC and MWNT/PC...... 29 Figure 3.1 Flow chart of neat PC films fabrication...... 34 Figure 3.2 Different levels of degree of crystallization. The degree of crystallization increases from left to right...... 34 Figure 3.3 Cause and Effect Diagram...... 36
vii Figure 3.4 The neat PC films of the 32 runs in DOE...... 39 Figure 3.4 Continued...... 40 Figure 3.5 Diagnostics plots for residual analysis from Design-Expert...... 45 Figure 3.6 Comparison of solution cast neat PC films: (a) Before process optimization (run 5); (b) After process optimization (run 7)...... 46 Figure 4.1 Flow chart of PC/MWNT composite film fabrication...... 51 Figure 4.2 PC/MWNT composite films with MWNT loading from 0.05~1 wt.-%...... 52 Figure 5.1 Optical micrographs of PC/MWNT composite thin films at 0.05, 0.1 and 0.15 wt.-%...... 55 Figure 5.2 Light absorbance of Beer-Lambert Law with a beam of light traveling through a material of thickness D...... 57 Figure 5.3 The percentage of light transmittance (%T) versus the concentration of the material and the light absorbance (A) versus the concentration of the material...... 58 Figure 5.4 The light wavelength versus the percentage of light transmittance for neat PC and 0.1 wt.-% PC/MWNT composite film...... 59 Figure 5.5 The light wavelength ranged from 400 nm~700 nm versus the percentage of light transmittance for neat PC and 0.1 wt.-% PC/MWNT composite film...... 59 Figure 5.6 The light wavelength ranged from 300 nm~700 nm versus the light absorbance for 0.1 wt.-% PC/MWNT composite film...... 60 Figure 5.7 The trends of MWNT concentration (0, 0.05, 0.1, 0.15) versus %T for 300 nm- 700 nm wavelength...... 61 Figure 5.8 Differential Scanning Calorimetry heat flow traces of neat PC and PC/MWNT composite films...... 63 Figure 5.9 Temperature versus heat flow showing the heat capacity and the degree of crystallization: (a) transparent PC; (b) translucent PC; (c) opaque PC...... 64 Figure 5.9 Continued...... 65 Figure 5.10 The SEM images of opaque area of neat PC films showing microvoids...... 67 Figure 5.11 The cross-section view of fractured PC/MWNT films (0.1 wt.-%)...... 67 Figure 5.12 Three positions for thickness analysis in neat PC/PC/MWNT composites films: A: central area, B: internal area, C: peripheral area...... 68 Figure 5.13 Three positions versus thickness (µm) for thickness analysis in neat PC/PC/MWNT composites films...... 69 Figure 5.14 Sample from DOE (run 12) shows that internal area is more transparent than peripheral and central area...... 70 Figure 6.2 Elastic moduli (E) of neat PC and PC/CNT films...... 74 Figure 6.3 Ultimate tensile strengths (σu) of neat PC and PC/CNT films...... 74
viii Figure 6.4 % elongation of neat PC and PC/CNT films...... 75 Figure 6.5 Showing different extension (% elongation) of tensile test for (a) 0.15 wt.-% PC/MWNT composite film, (b) neat PC film...... 76 Figure 6.6 Showing realistic and practical Young’s moduli of PC/MWNT films...... 79 Figure 6.7 SEM images showing: (a) (b) the dispersion of MWNTs from the fracture surface of failed 0.05 wt.-% PC/MWNT films; (c) Fracture surface showing exposed nanotubes; (d) Exposed MWNT with PC-coated connection with PC matrix...... 80 Figure 6.8 SEM images showing the toughening effects of nanotubes of failed 0.15 wt.-% PC/MWNT films: (a) (b) Fracture surface showing PC-coated nanotubes...... 81 Figure 6.9 SEM images of crack tip and fracture surface of failed 0.1 wt.-% PC/MWNT films showing the toughening effects of nanotubes: (c) Crack surface showing nanotube breaking of PC matrix; (b) (d) Crack tip showing nanotube bridging...... 82 Figure 6.10 SEM images of crack tip and fracture surface of failed 0.15 wt.-% PC/MWNT films showing the MWNT pull-out of the PC matrix...... 83
ix
ABSTRACT
Polycarbonate (PC) is a transparent, impact resistant polymer that provides protection against breakage or intrusion. The mechanical toughness of PC is reported to be associated with the molecular motion of main chain molecules. The molecular motion is present upon exposure to impact and can therefore provide efficient dissipation of impact energy. PC has found wide usages in military and commercial applications. This thesis investigates processing and characterization of transparent PC/carbon nanotube (CNT) composite films. The reinforcing capability and efficiency of CNTs in nanocomposites have been studied intensely world-wide due to their exceptional mechanical properties. Not only do they provide stiffness and strength, but they also have been reported to impart fracture toughness when dispersed in polymer matrices. As cracks develop in nanocomposites, CNTs serve as the bridging nanofibrils that effectively retard crack propagation. In some cases, CNTs act as obstacles that obstruct the crack propagation paths, thus increasing the energy needed to be dissipated for further crack opening. This research deals with two important issues in multi-walled carbon nanotube (MWNT)-based composites that are seemingly trade-offs – dispersion of high-loading MWNTs and maintenance of optical transparency. Higher loading of MWNTs is desired for increased reinforcing effects; however, it is limited by the difficulties in achieving uniform dispersion in the polymer resin due to increased viscosity and the tendency of MWNTs to aggregate. At the same time, only a small addition of MWNTs (less than 1 wt.%) makes the resin turn dark, which defeats the advantage of the transparency that should otherwise be retained by PC. PC/MWNT composite films (up to 0.15 wt.% MWNT) were fabricated using a solvent-based film casting method. Several optimization schemes were adopted to determine the most suitable solvent and the concentrations of PC/MWNT in the solvent. Solvent-induced PC crystallization, which was evidenced by milky tinting in the produced films, was minimized by identifying and optimizing the
x process parameters, namely, PC/MWNT/solvent solution viscosity, casting temperature, and film thickness. Design of experiments technique was used to determine the combination of optimal parameters.
xi
CHAPTER 1 MOTIVATION AND OBJECTIVES
1.1 Introduction
Why do we need transparent materials with fine mechanical properties such as impact resistant or high degree of tensile property? When flying fragments and debris are generated from explosives, transparent, impact resistant armor shields provide protection to increase soldiers’ survivability in a battlefield without losing eyesight. In ice hockey games, transparent, impact resistant windows surrounding the rink protect spectators from the collisions of players and pucks. Transparent materials have been widely employed in a number of military and commercial applications, such as ground vehicle protection, air vehicle protection, personnel protection, protection of equipment like sensors, riot gear, face shields, security glass, bullet-proof cars, electromagnetic (EM) windows, visors, etc. With the progress of advanced technology, there are increasing demands for high performance, lightweight, affordable, durable, and easy-to-fabricate transparent, impact resistant materials. The wide range of applications of transparent materials still have unlimited possibility to improve the mechanical properties, electrical conductivity, or optical property.
1.2 Current State of the Transparent Materials
Transparent materials are usually called glazing materials and are generally classified into glass, ceramic, or plastic system. These materials shall be fabricated and
1 designed to meet the specifications validated through test standards. For laminated composites, interior face has to be spall resistant, and the adhesive for bonding interlayers shall be chemically compatible with the surfaces of interlayer materials [1]. The primary requirement for transparent materials is to defeat multi-hit threats and retain visibility in the surrounding areas. Polymeric materials, transparent crystalline ceramics, glass and glass-ceramics are main transparent material systems for general applications. The selected mechanical properties of common transparent materials are summarized in Tables 1.1 and 1.2 which is provided by the U.S. Army materials research [ 2, 3]. Generally, polymeric materials offer lighter weigh (lower density) than glass and ceramic material systems. Although transparent ceramic materials have shown significant ballistic resistance, size limitation, sophisticated processing procedures, and cost are still the primary issues. Not only do polymeric materials reduce weight, but they are also easy to be designed and processed.
Table1.1 Selected mechanical properties of transparent glass and ceramic materials.
Fused Zinc ALONTM Sapphire Spinel Silica Sulfide Density g/cm³ 3.69 2.21 3.97 3.59 4.08 Areal Density at lb/ft² 19.23 11.44 20.68 18.61 21.20 1” thick Young’s Elastic GPa 334 70 344 260 10.7 Modulus Mean Flexural MPa 380 48 742 184 103 Strength Fracture MPa m 2.4 - - 1.7 - Toughness √ Knoop GPa 17.7 4.5 19.6 14.9 2.45 Hardness (HK2)
2 Table 1.2 Typical properties of polymeric materials found in military ballistic systems.
Lexan Simula Plexi Glass G
Polycarbonate Polyurethane PMMA Density g/cm³ 1.2 1.104 1.19 Areal Density at 1” lb/ft² 6.2 5.7 6.2 thick Tensile Strength MPa 66 62 72 Tensile Modulus MPa 2208 689 3102 Shear Strength MPa 45 - 62 Shear Modulus MPa 1000 - 1151 Compressive Strength MPa 83 72 124 Compressive Modulus MPa 1660 1241 3030 Flexural Strength MPa 104 89 104 Flexural Modulus MPa 2586 2020 3280 Max Operating °C 121 149 95 Temperature Glass Transition °C 145 75 100 Temperature
1.3 General Problem Statements
Transparent materials have been studied for half a century and the U.S. Army Research Laboratory (ARL) aspired to advance existent materials so as to reduce the risk of personnel bloodshed in recent several years. For ceramic materials, cost is still a major issue due to the high purity powder requirements, the high processing temperatures, long processing times, complex processing steps, and high machining and polishing costs [2]. Despite the light weight, high production cost makes ceramic materials low-demand materials. Glass materials possess great optical properties and ballistic performance, but reducing weight is always an issue. Even though polymer-based materials are much more economic and lighter than other transparent, impact resistant materials, there is room for improvement in both optical properties and mechanical performance. A combination of new polymeric material systems and laminate architectural design may open the door to
3 advanced transparent materials with optimized properties and cost. The general goal of this research includes: 1. Fabricate transparent PC (polycarbonate) films with appropriate fabrication method. 2. Fabricate transparent MWNT (multi-wall carbon nanotube)-reinforced PC composite films with optimal CNT loading. (“Optimal” CNT loading pertains to the concentration at which maximum mechanical properties are obtained while maintaining see-through-grade optical clarity.) 3. Characterize the optical property and mechanical properties of PC/carbon nanotube composite films.
4
CHAPTER 2 BACKGROUND AND LITERATURE REVIEW
2.1 The Optical Principles and Basics
Before improving the mechanical properties of transparent materials, how to maintain the transparency with additives is the first issue. Understanding the basics of optical principles would help to inspect the issues influencing the optical transparency and find the balance between the optical clarity and the mechanical properties.
2.1.1 The Optical Principles and Basics
The electromagnetic spectrum includes all the electromagnetic waves [4]. The visible light is only a tiny part of the radiation. The electromagnetic radiation of the visible light is in the range 3.4 ×1014 to 7 ×1014 Hz, and the wavelength of the visible light ranges from approximately 400 nm to 700 nm, which depends on observer’s eyes (Figure 2.1). Theoretically, the waves can be distinguished into two properties, longitudinal and electromagnetic; in other word, the waves come into being in both ways, light or sound. We can observe light travel as waves by our naked eyes directly. Sound waves will cause a frequency of wave vibration. We can observe sound waves by using mediums that waves can go through such as water.
5
←Low f (cycles/s) High→
Visible light Radio Gamma waves Infrared Ultraviolet X rays rays
−4 −6 −8 −10 03.0 3×10 3×10 3×10 3×10 ←Long λ (m) Short→
Figure 2.1 The electromagnetic spectrum.
Any opaque object in a material, such as particles or crystalline masses, will influence the transparency (Figure 2.2) [5]. Light will be reflected if the opaque area or particle size is larger than the visible light wavelength. If the opaque area or particle size is smaller than this range, they will not affect the passage of light and the material can appear transparent. In polymer structure, the density of the crystalline areas is a key factor in the optical clarity of crystalline plastics because the refractive index changes with the density of the material. Crystalline plastics have larger crystal density than amorphous plastics or semi-crystalline plastics. If the crystalline areas become larger, then the light will be scattered by refraction as it passes through the crystals and the material will be opaque. Based on Snell’s Law, launching light into different medium interface generates both reflection and refraction [6, 7]. They are on the same plane, and the angle between incident ray and the normal (θ1 ) and the angle between refracted ray and the normal (θ 2 ) are satisfied as follows (see Figure 2.3):
n1 sinθ1 = n2 sinθ 2 , Eq. (2.1)
where the principal values of refractive index are designated as n1 and n2 .
6
Wavelength of Wavelength of visible light (λ) visible light (λ)
Transparent Transparent material material
Particle
Figure 2.2 A path of incident light affected by the particle size. Internal features such as opaque areas or areas of different refractive index will force the light be either scattered by refraction or reflected as it passes through the crystals.
Normal Incident light Reflection light
Normal θ θ Air 1 1
θ Medium 2
Air θ1
Refraction light
Figure 2.3 The incident light enters a medium (high index of refraction) from the air (low index of refraction) with smaller refraction angle.
7 The light transmitting ability depends on the light path and can be quantitatively expressed by the Lambert-Beer law [8]:
log()II 0 −= AL , Eq. (2.2)
where II 0 = the fraction of light transmitted, A= the absorptivity of the material at the wavelength L= the path length. If the loss of light transmitted due to reflections at air-plastic interfaces is taken into account, the Fresnel relation can be used to estimate this loss at each surface [ 8]:
2 2 Treflection = ()()n −1 n +1 Eq. (2.3)
Take the example of PC: the index of refraction of air is 1.0, while the index of
refraction of the PC (n) is ~1.586 [ 8]. Light incident on the PC surface will be refracted at
a greater angle into the surface. Therefore, if we want to calculate the light transmission of a neat PC film which is 0.1 cm in thickness, 88% transmission can be gained by the log relation corresponding to an absorptivity of 0.6 cm −1 from Eq. (2.2), and there is a 5.1% reflection loss at each contact surface from Eq. (2.3). Then, the light transmittance of reflection loss will be counted twice as the light enters PC from the air, and then enters the air from PC. The apparent transmission will be: (100% − 1.5 %) × 88% × (100% − 1.5 %) = %9.94 × 88% × %9.94 = %3.79 For perfect optical clarity (without any light absorption), light transmission will be: %9.94 ×100%× %9.94 = 90%
2.1.2 Numerical Calculation
When we talk about PC, the properties of toughness and transparency are the most significant attributes. The information provided by GE Structured Products shows that PC has 90% light transmission. Based on ASTM D1003 [ 9], it gives us a general idea how to define the percentage total luminous transmittance. The luminous transmittance is to
8 measure the amount of light that passes through a specimen. The total luminous
transmittance Tt can be calculated as follows:
Tt = T2 T1 Eq. (2.4) where:
T1 = incident light, and
T2 = total light transmitted by the specimen. The value can be measured by either hazemeter or spectrophotometer. Haze is the scattering of light as it passes through a transparent material, resulting in poor visibility and glare. Based on Eq. (2.4), if 90% of the incident light is transmitted through the PC specimen, the other 10% of the incident light is scattered either by the test instrument or the specimen. The optical clarity of a plastic specimen is measured by the lack of optical haze. ASTM D1003 tells us:
haze% = ()Td Tt ×100 Eq. (2.5) where:
Td = the diffuse luminous transmittance
= T4 T1
T4 = light scattered by the instrument and specimen, assuming that the light scattered by the instrument is zero.
Therefore, haze % for PC with no crystallization should be greater than zero and smaller than 10%. It indicates that there is up to 10% of the incident light reflected from the surface of PC or absorbed by PC. More details of calculation will be discussed in the following section.
2.1.3 The Effect of Crystallization on Transparency
When light passes through the material, it will be scattered at the interface if the dimensions of the discontinuities such as particles are greater than the wavelength of
9 visible light (0.4-0.7 µm) [10]. In a polymeric structure, crystalline structure has higher density and the index of refraction than amorphous structure. As the degree of crystallinity increases, the density of polymer also increase which induces more light scattered and makes the material translucent. Therefore, transparent polymers are noncrystalline, and translucent polymers are crystalline [ 11]. To conclude, as the dimensions of the dispersed-phase particles become smaller than the wavelength of visible light, scattering decreases; so, a polymer with small crystallites and low degree of crystallinity appears nearly transparent. However, the amount of light scattered or absorbed still depends on the material as well.
2.1.4 Microphysics
Because of the van der Waals interaction between CNTs, the aggregation of CNTs is very easy to be generated during the fabrication process. Therefore, the size of CNT aggregates is one of the major issues for transparent PC/CNT composite films. When the light beam irradiates objects, there are reflection, refraction, light scattering (we can examine the structure of CNT by analyzing the spectrum of Raman scattering [ 12]), and energy absorption. The rate at which the light can penetrate a transparent composite depends on the content of translucent components present in the composite. Optical microscopes use the same principle that bounces light off of the surface of objects to create images. Basically, the high loading levels of particles result in not only poor optical properties, but also poor mechanical properties [ 13]. Molecules and groups of atoms are too small to be observed by the naked eye. The particles which can influence mechanical properties significantly are in the size range from well under a micron (µm) to about 100 microns [14]. The naked eye can see down to the visible light wavelength (400~800 nm or submicron). The resolution of optical microscopy is about 1 micron. In order to examine much smaller objects, such as individual CNTs, scanning electron microscopes (SEMs) were invented in the 1930s. It allowed us to see objects as small as 10 nanometers. SEM bounces electrons off of
10 surfaces to create images instead of light, and has higher resolution due to small size of electrons.
2.2 Chemistry and Properties of Polycarbonate
Polycarbonate (PC) is a thermoplastic that combines the advantages of lightweight, luminous transmittance and optical clarity, high impact resistance, creep resistance, low water absorption, flame-retarding ability, thermoform ability, excellent melt process ability (e.g. molding, thermal forming, etc.), and affordable price. General Electric (GE) is one of the largest producers of PC, and it is sold under the trade name LEXAN®. LEXAN® polycarbonate has 88% light transmission (ASTM D1003); flexural strength of 93 MPa (ASTM D790); glass transition temperature of 150°C (302°F)). PC has high solubility in many common solvents, which allows high processibility. The other properties of PC are summarized in Table 2.1. The aforementioned advantages provide wide uses for security applications. PC has already brought up wide usages in military and commercial applications, such as shatter-proof windows, lightweight eyeglass lenses, windshields, windows and vision blocks for armored vehicles and boats, face shields and goggles, aircraft transparent sensor windows, infrared domes for missiles, and laser ignition windows for medium and large caliber cannons. The most common PC is called bisphenol A polycarbonate (BPA-PC), which is currently used for many optical applications, such as ultra-light eyeglass lenses and CDs because of its high transparency [15]. BPA-PC is a thermoplastic, which means it can be molded at a high temperature, and the process is reversible. The repeating units of bisphenol A group (see Figure 2.4) form long molecular chains, and individual molecules are linear in structure without any chemical linking [ 16]. They show no long-range order (amorphous structure) and are held by van der Waals and hydrogen interactions, which enable the polymer to be softened and melted by heating. Long molecular chains also can transfer stresses and absorb energy, which leads to high impact strength and fracture resistance giving the materials excellent damage tolerance properties. The ductility of PC is reported to be associated with the molecular motion of the molecular chains at low
11 Table 2.1 Properties of LEXAN® polycarbonate.
LEXAN® polycarbonate Property ASTM Std Value Specific gravity D792 1.20 Light transmission D1003 ~90% Density D638 1.2 g/cm³ Tensile modulus D638 2-2.4 GPa Tensile yield strength D638 62 MPa Ultimate tensile strength D638 55-75 MPa Flexural modulus D790 2.4 GPa Flexural strength D790 93 MPa Elongation at break D638 110% Izod impact strength (notched) D256A 12-16 ft-lb/in
temperatures [17]. There is also a thermosetting polycarbonate (see Figure 2.5) [ 18]. It contains two allyl groups on each monomer. The two allyl groups will become parts of different polymer chains and form three-dimensional crosslink between polymer chains, which gives high impact and heat resistance.
O 3 CH
[ O C O C ]n
CH3 Carbonate group Bisphenol A group
Figure 2.4 Molecular structure of bisphenol A polycarbonate (BPA-PC).
12
O O
C C CH 2 CH CH2 O O CH2 CH2 O O CH2 CH CH2 Allyl group Allyl group
Figure 2.5 Molecular structure of polycarbonate with two allyl groups on the end.
2.2.1 Driving Forces for Polymer Crystallization
Crystallization of polymers occurs because molecular chains are held together by crystal bonds (secondary bonds). Crystal bonds have lower energy than noncrystalline structure [ 11]. It indicates that the molecules release energy when they form crystalline structure. In the thermoplastic polymer structure, individual molecules are linear in structure without any chemical linking between them. Weak intermolecular forces such as van der Waals bonds and hydrogen hold molecules in place [ 16]. The amount of crystallinity depends upon several factors. First, increasing stress will induce attraction and intermolecular forces within polymers, increasing crystallinity. Then, the process in which the polymer is cooled from melt (known as “quenching”) provides different level of crystallinity [19]. This processing method with slow crystallization allows the polymer structure to be frozen at various stages of crystallization by quenching to room temperature.
2.2.2 Measurement of Degree of Crystallization
The degree of crystallization is a quantitative scale to measure the amount of crystalline structure as opposed to amorphous. Four general methods have been used to
13 determine the degree of crystallization: differential scanning calorimeter (DSC), specific volume (specific gravity or density), X-ray diffraction, and infrared spectroscopy [11]. For DSC method, the heat flow shows the melting peak, and the degree of crystallization can be determined using the DSC peak area. Then, ASTM D792 (specific gravity), ASTM D1505 (density by density-gradient technique), ASTM D1895 (bulk density), and ASTM D1921 (Sieve-analysis (particle-size) test) provide methods to measure specific gravity and density. X-ray diffraction can develop a characteristic pattern when it diffracted through a crystalline structure. Infrared spectroscopy can detect the vibrations and rotations of the atoms affected by crystalline structure.
2.3 Carbon Nanotubes as Toughening Agents
Carbon nanotubes (CNTs) can be classified as single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT). Since MWNT was discovered by Iijima in 1991 [ 20] and SWNT was discovered by Bethune et al. in 1993 [21], many researches on the physical and mechanical properties of CNTs have been performed. Due to the small fraction required and their exceptional mechanical properties, CNTs are known to be one of the most effective and efficient reinforcing agents.
2.2.1 General Properties of Carbon Nanotubes
The remarkable mechanical properties make CNTs the most reinforcement material (summarized in Table 2.2). The stiffness of SWNT can be 10-100 times higher than that of steel. A perfect individual SWNT has elastic modulus (Young’s modulus) as high as 1 TPa and tensile strength as high as 200 GPa which depends on the structure and diameter [ 22, 23]. Failure strain is around 20~30%. CNTs also exhibit electric-current- carrying capacity 1000 times higher than copper wires (resistance: 10-4 Ω/cm (at 300 K); stability: 1013 A/cm (maximum)), thermal conductivity twice as high as diamond (SWNT: 6000 W/m-K; MWNT: 3000 W/m-K (both at room temp.)), high thermal
14 stability (2800°C in vacuum and 800°C in atmosphere), and extremely high aspect ratio in the order of 1000 [ 24, 25]. The properties of CNTs lead to a potential wide range of applications in nanocomposite materials. This research will take advantage of the excellent mechanical properties of CNTs and develop transparent composite systems with optimal CNT content and high mechanical performance.
Table 2.2 Properties of carbon nanotubes (CNTs).
Mechanical properties of carbon nanotubes (CNTs) Stiffness Elastic modulus: as high as 1 TPa (Dependent on their structure and diameter) Toughness Tensile strength: as high as 200 GPa Failure strain: around 20~30% Electric-current-carrying Resistance: 10-4 Ω/cm (at 300 K) capacity Stability: 1013 A/cm (maximum) Thermal conductivity 6000 W/m-K (at room temperature) (MWNT : 3000 W/m-K ) Thermal stability 2800°C in vacuum (800°C in atmosphere) Aspect ratio ~1000
2.2.2 Toughening Mechanisms of CNT Composites
After undergoing elastic deformation, CNTs show a special plastic deformation behavior to relieve stress by changing its configuration [ 26]. It comes from the 90° rotation of the C-C bond in graphene structure around the central point of CNTs, which is called Stone-Wales deformation [27]. Stone-Wales deformation has a great effect on stress discharging capability of CNTs. It is one of the reasons why CNTs offer more plastic deformation than other materials. Under the observation of high resolution transmission electron microscopy (HRTEM), both SWNTs and MWNTs show high
15 bending levels (see Figure 2.6) [ 28,29]. The combination of elasticity and plasticity makes CNTs excellent candidates as toughening fillers for composite materials.
(b)
(a) (c)
Figure 2.6 (a) CNT knotting; (b) MWNT bending; (c) SWNT bending.
B. I. Yakobson [30- 32] also simulated the fracture mechanism of SWNTs under tensile, compressive, bending and shearing loads. In previous research, when tensile load reached the critical point, there was huge amount of Stone-Wales deformation in CNT structure, and C-C bonds started to break at the same time. It introduced holes on the surface of SWNTs which marked the beginning of fracture. With increasing load, atoms spread out in the axial direction and finally broke. The breaking mechanism of compression is much more complicated [ 32, 33].
16 MWNTs also provide toughness through “telescoping” [ 34- 37]. The HRTEM and SEM studies show that the break of MWNT starts from the external bundles to the internal bundles (see Figure 2.7, 2.8) [ 34, 37].
Figure 2.7 The process of MWNT telescoping; A: the beginning of experiment; B: the upper break tip; C: the lower break tip; D: before breaking; E: partial pullout of internal nanotube bundles.
Figure 2.8 MWNT telescoping.
17 2.3 Methods for PC/CNT Composite Fabrication
There are several issues associated with using CNTs as a toughening agent in PC. Addition of CNTs decreases optical transparency significantly even only at a small loading. Besides, it is difficult to disperse CNTs in the polymer matrix due to inter-tube van der Waals interaction. It forces CNTs to aggregate during the fabrication process. Also, CNTs are chemically stable in nature, making it difficult to establish strong interfacial bonding with polymer matrix molecules. This section introduces several methods to process PC/CNT nanocomposites.
2.3.1 Melt Processing
Melt processing is the method that usually uses extrusion or compression molding to fabricate specimens. Extrusion provides a continuous and efficient means of polymer processing. Polymers can be melted in an extruder to form strands, granules or pellets. Pötschke et al. [38- 42] performed melt processing methods that effectively dispersed CNT into PC matrices. One of the methods started with high MWNT concentration (master batch: 15 wt.-% of MWNT), which was diluted with pure PC. PC and MWNT were mixed in micro compounder for 5-15 min of mixing time and 50-150 rpm of extruder speed (~mean shear rate 150 s-1). MWNT loading ranged from 0.1% to 12.5%. Another melt processing method Pötschke used was to directly melt-mix MWNT or SWNT with PC. They were also mixed and dispersed in micro compounder for a range of mixing time from 1-120 min at 80 rpm of rotor speed. Samples were produced by extrusion and were formed into spun fibers, which could provide effective CNT alignment. CNT loading in PC was around 5 wt.-%. Although the CNT loading in the second method is not as much as the first method, the second method can be done with short mixing time and achieve equivalent degree of dispersion.
18 2.3.2 Solution Processing
Solution processing is another method to fabricate PC/CNT composites. PC and CNT are mixed into chemical solvents and cast under specific temperature conditions [ 44- 46]. A wet annealing based on high-temperature PC/CNT solution casting prior to complete drying yields a uniform and transparent film. The temperature for hot casting is around 150°C. Thin films ranging from 10 to 60 µm in thickness were produced. The CNT loading was relatively small, ranging from 0.025 to 0.25%. Compared with melt processing, solution processing requires longer processing time and fabricates PC/CNT composites with lower CNT loading; however, it provides better CNT dispersion at the same CNT loading. Tables 2.3 and 2.4 show the solubility of CNT and PC in some notable solvents [ 43- 51].
Table 2.3 Solvent selection (SWNT solubility) [43]. (Note: The sonicator bath water temperature rose to ca. 35°C over the course of 1 hour. SWNT dissolved in solvent at room temperature. Filtered through glass wool until no visible particulate remains.)
Concentration Solvent (mg/L) 1,2-Dichlorobenzene 95 Chloroform 31 1-Methylnaphthalene 25 1-Bromo-2-Methylnaphthalene 23 N-Methyl pyrrolidinone 10 Dimethylformamide 7.2 Tetrahydrofuran 4.9 1,2-Dimethylbenzene 4.7 Pyridine 4.3 Carbon disulfide 2.6 1,3,5-Trimethylbenzene 2.3 Acetone <1 1,3-Dimethylbenzene <1 1,4-Dimethylbenzene <1 Ethanol <1 Toluene <1
19 Table 2.4 Solvent selection (PC solubility) [43- 51].
Solvent Available Concentration Applications N-Methyl Pyrrolidone PC/MWNT composite for N/A (NMP) mechanical characterization 1,2-Dichlorobenzene (DCB) ~10 wt.-% PC/SWNT thin films PC/MWNT composite for Chloroform (CHF) 8-15 wt.-% CNT length shortening Tetrahydrofuran (THF) PC solutions for fiber 15-20 wt.-% Dimethylformamide (DMF) spinning Methylene Chloride (MCH) PC solutions, PC cleaning 1,2-Dichloroethane (DCE) Up to 50 wt.-% solvent 1,3-Dioxolane (DXO)
2.3.3 Electrospinning
Electrospinning process uses electric field to create charged thin liquid jets and forms nanofibers on a metal screen. A series of papers have focused on producing submicron polymer nanofibers from polymer solutions [ 52- 56]. The nanofibers mats transform into the spider web structure under the microscopic observation. The fiber thickness and structure formation are affected by the electrostatic spinning voltages, solvent viscosity, and the polymer concentration. By adding small amounts of nanomaterials into transparent matrix materials with electrospinning techniques, the mechanical properties can be improved; meanwhile, the optical properties will not be sacrificed. Electrospinning techniques can produce non- woven mats of fibers with characteristic diameters in the range of 50-300 nm, which is well below the wavelengths of visible light. Theoretically, the light transmission for transparent matrix materials infused nano-additives is approximately 90%. Certain military applications require transparent materials of 85-95% light transmission. 60-70% light transmission is acceptable for the materials with better ballistic performance such as ballistic windshields, and protection armor configurations [1].
20
2.4 Characterization of PC/CNT Composites
In previous research, it has been demonstrated that effective dispersion of anisotropic nanoparticles can essentially improve reinforcement of the polymeric matrices. The dispersion of nanoparticles highly depends on processing techniques, such as solution blending, shear mixing, in situ polymerization, ultrasonic cavitation and high pressure mixing [57]. CNTs as toughening particles tend to require cracks in composites to detour or even obstruct cracks. The energy absorption (i.e. by requiring more energy to create new surfaces as cracks travel longer paths) makes PC/CNT composites become potential high-performance and multifunctional composites.
2.4.1 CNT Toughening Mechanism
The tensile test that performed by Singh et al. [ 45] showed 10% improvement in % elongation of PC/SWNT (0.06 wt.-%) films as compared to neat PC films. Neat PC films displayed bridges along the crack surface, and PC/SWNT films displayed SWNTs pulled out (see Figure 2.9). They also found out that the concentration of CNTs and mechanical properties are not directly proportionate. There is an optimal CNT% about 0.06% for improving the stiffness of PC/CNT composites, and mechanical properties can not be indefinitely increased [ 45] (see Figure 2.7). The Singh group represented an 85% of the improvement in modulus for 0.06 wt.-% of SWNTs by using the rule of mixture. The assumption was that there were perfect dispersion and perfect bonding between the matrix (PC) and the reinforcement (CNT). Also, the rule of mixture is usually used for the estimation of continuous aligned fibers. To estimate short fibers, Halpin-Tsai equation and Fukuda equation should be used [ 69, 72]. Further discussion will be in Chapter 6.
21 (a) (b) (e)
(c) (d)
Figure 2.9 Entangled, random CNT network with bridges, cracks, and gaps under SEM; (a)(c): PC/SWNT; (b)(d): neat PC. (% Elongation: Neat PC: 20%, PC/SWNT: 30%) [45].
Figure 2.10 Young’s modulus versus CNT % by TMA; 29% increase in modulus at 0.06% is the apparent “optimal” CNT% [ 45].
22 2.4.2 Optimal CNT Loading for Enhancing Mechanical Properties
Figure 2.10 in last section shows Young’s modulus versus CNT % (Dynamic mechanical analyzer, DMA), and 29% increase in modulus at 0.06% is the apparent “optimal” CNT%. It also indicates that the theoretical maximum concentration for 0.06 wt.-% PC/CNT calculated using the “rule of mixture” will be 2300 MPa (YCNT= 1 TPa,
TPCpure= 1662 MPa, and converting wt.-% to volume using densities of PC= 1.20 g/cm3 and CNT= 1.34 g/cm3). Based on the rule of mixtures, the properties of composites will depend on fiber volume fraction and matrix volume fraction. For example, the longitudinal modulus for the composite can be calculated by the following equation [ 16]:
Ecomposite = E fiber v fiber + Ematrix (1− v fiber ) Eq. (2.6) where:
Ecomposite = the estimated modulus of the composite,
E fiber = the modulus of the fiber, vcomposite = the volume fraction of the fiber,
Ematrix = the modulus of the matrix. Theoretically, the composite load can be increased by increasing fiber volume fraction because fibers carry up to 10 times of the load as compared to the matrix in polymer matrix composites [16]. However, the most practical limit is approximately 80%. For PC/CNT composites, it has been demonstrated that the optimal CNT loading is much lower than other reinforcement materials. It can be explained from the following reasons. First, as CNT loading increases, it becomes more difficult for the matrix to wet the reinforcement. The interfacial bonding between matrix and reinforcement is not 100% perfect, which will undermine the properties of composites. Then, CNTs are easily aggregated due to inter-tube van der Waals interactions. The aggregation results in weak interfacial bonding between matrix and reinforcement. Figures 2.11(a) illustrates ideally dispersed (or “exfoliated) CNTs in the matrix. In this case, the mechanical properties can be maximized, assuming perfect interfacial strength. However, if the CNTs aggregate, which is a more realistic case, the case is more similar to a matrix containing a larger
23 sized particle, from which poor properties are expected (Figure 2.11(b)). Moreover, the mechanical properties in Figure 2.11(b) is further degraded, since the inter-tube shear strength is weak and therefore the aggregate has a tendency to separate within itself due to shear. This competing mechanism between CNT reinforcement and aggregation effects explains why the mechanical properties of the CNT-reinforced composites can not be improved infinitely (to a theoretical level) by increasing CNT loading. In addition, CNTs are chemically stable in nature, which makes it difficult to build strong interfacial bonding with polymer matrix molecules.
(a) (b)
Figure 2.11 Schematic of: (a) ideally dispersed CNTs; (b) aggregated CNT particles.
In Figure 2.10, 0.06 wt.-% of PC/CNT composite provides 85% of the theoretical improvement of modulus. To conclude, the rule of mixtures works only in the ideal case of perfect dispersion and alignment of continuous fibers and perfect bonding at the fiber- resin interface. The rule of mixtures is generally used for long-fiber-based composite material. For short fibers, such as CNTs, Halpin Tsai and Fukuda equations are likely to provide more accurate estimation of mechanical properties.
24
2.4.2 Optical Properties
CNT dispersion and solvent-induced crystallization in PC are two major issues that influence the transparency of PC/CNT composites [15, 58]. Crystalline entities will be induced by solvent when temperature drops. Despite crystallization brings about the enhancement of modulus, the semi-crystalline PC brings uninformed molecular structure and fragility which will cause the loss of strength and optical transparency (see Figure 2.12). Potschke et al. [39] observed individual CNTs under the scanning electron microscope (SEM). Individual CNTs in CNT network indicated the CNT dispersion was locally good (see Figure 2.13).
(a) (b)
Figure 2.12 Solvent-induced crystallization in PC; (a) Crystalline structure grows within amorphous PC; (b) Joining of crystalline entities.
25 (a)
(b)
Figure 2.13 (a) CNT dispersion under SEM (melt-mixed PC/15%MWNT). Rope-like structures are PC-bounded CNTs, and the bulge is rough PC surface; (b) CNT dispersion under TEM and AFM (melt-mixed PC/2%MWNT).
2.4.3 Haze and Luminous Transmittance
The brief instruction of haze and luminous transmittance is discussed in ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics) [ 9]. We know that a haze or smoky field will occur when light passes through the material because of reflection and refraction. The crystallization of PC is one of the major issues that cause the haze field. The two methods for haze measurements use hazemeters and spectrophotometers, respectively. In general, they require a light source, specimens, a photo-detector, a light trap, and an integrating sphere with an internal reflecting area, an entrance and exit ports. Although the systems of hazemeters and
26 spectrophotometers are similar, hazemeters use unidirectional illumination and diffuse viewing; spectrophotometers use diffuse illumination and unidirectional viewing. The measurement is based on total luminous transmittance which is calculated by the method described in section 2.1. A hazemeter is the most commonly used instrument to measure haze and luminous transmittance. However, a spectrophotometer can provide valuable diagnostic data on the original of the haze. Both ways are acceptable and the practical application might depend on their price.
2.4.3 Differential Scanning Calorimetry (DSC) Technique
Differential scanning calorimetry (DSC) is one of the most convenient tools to identify the thermal behavior of polymers. By calculating the peak area of heat flow curve in DSC analysis, we can measure the crystallinity of polymers (see Figure 2.14) [ 47]. Previous DSC study indicates that crystallization in polymer-CNT composites occurs at higher temperature than pure polymers (see Figure 2.15) [48]. Besides, both the melting and crystallization peaks in polymer-CNT composites are narrower than in pure polymers.
27
Oxidation or Decomposition Glass Melting Transition
Crystallization Cross-linking (Thermoset Polymer Cure)
Heat Flow -> exothermic
Temperature
Figure 2.14 Temperature versus heat flow for DSC data and applications.
Cooling
Heating
Figure 2.15 DSC heating and cooling curve for (a) pure PP (polypropylene) and (b) 0.8 wt.-% PP/SWNT composite.
28 2.4.3 Impact and Ballistic Resistance
Based on the research of lightweight armor materials, PC/CNT nanocomposites achieve enhanced properties of impact resistance [59]. Compressive strength improves when CNT loading increases. If transparency is not the major issue, CNT loading can reach as high as 30-40% and give the optimal improvement of compressive properties. Compressive strength decreases when CNT loading keeps increasing (see Figure 2.16).
(a) (b)
(c)
Figure 2.16 (a) Changes in ballistic performance and compressive strength relative to the MWNT loading in PC; (b) Impact and Ballistic resistance; (c) Effect of blend composition on the compressive strength of virgin PC and MWNT/PC.
29 2.5 Summary of Literature Review
From the literature review, it can be understood and summarized as following: 1. The crystallization and the CNT aggregations are two major issues that influence the optical transparency. 2. It has been demonstrated that CNTs can serve as a potential reinforcement for toughening PC. 3. A number of PC/CNT composite fabrication methods have been demonstrated. Among all processing techniques used, solution casting yields the best CNT dispersion at the same loading. However, solution casting is hindered by solvent- induced crystallization, which results in degradation in transparency. 4. The previous work on characterization of mechanical properties indicates that there may be an optimal CNT loading for maximized properties. 5. Previous work shows improvement of PC in impact resistance by reinforcing with CNTs; however, transparency was not taken into account.
2.6 Research Objectives
The primary objective of this research is to develop transparent PC/CNT composite films with improved mechanical properties. It can be incorporated into sandwich PC structure for transparent, impact resistant applications. Specific objectives include: 1. Fabricate transparent polycarbonate (PC)/CNT composite films with optimal CNT loading and degree of dispersion by using solution processing method. Optimal CNT loading means the maximum concentration of CNTs while transparency is maintained. 2. Characterize mechanical (static and dynamic) and optical properties of neat PC and PC/CNT composite films. 3. Identify and understand CNT-based toughening mechanisms that contribute to enhanced properties.
30 4. Identify factors that affect the transparency of neat PC and PC/CNT composite films.
31
CHAPTER 3
FABRICATION AND CHARACTERIZATION OF NEAT
POLYCARBONATE FILMS
3.1 Introduction
Solution processing method provides simple and efficient operation to mix PC and CNT together, and is easy to control the transparency of thin films. Solvent-induced crystallization and CNT aggregation are two major issues that influence the transparency of PC/CNT composite thin films. In this chapter, design and analysis of experiments (DOE) is presented to attack the problem of the PC crystallization (CNT aggregation will be discussed in the following chapter). DOE is the most commonly used statistics-based tool to find out the most influential factors and generate a regression model for each response. The following step is to use UV-VIS to define the response (% Transmittance) and build a regression model as:
%Trans. = ()xf 1 (Temperature), x2 (Concentration), x3 (Thickness) . (Eq. 3.1)
3.2 Transparent Neat PC Film Fabrication
Based on the literature review, chloroform is one of the most dominant solvents which provide good solubility for both PC and CNT. In the preliminary procedures of transparent PC film fabrication (see flow chart in Figure 3.1), First, we weighed PC
32 pellets based on the target thickness of PC films and dissolved them in chloroform by stirring for 30 minutes to one hour at room temperature. Then, the pure solution was poured into Petri dishes (the number of dishes depends on the thickness of thin films that we need). The Petri dishes were placed on the hot plate, and the temperature was set to 60°C. In this step, temperature was controlled to be lower than 60°C, as the boiling point of chloroform is 61.1°C, and the chloroform evaporation would induce air bubbles on the surface of PC films. Completely vitrified PC films were put in the vacuum oven at 80°C for two hours so as to eliminate all the moisture and the chloroform remaining in the PC films. The last step is to evaluate the transparency of the PC films. Literature review shows evidence that the milky area in PC films indicates crystallization. The degree of crystallization governs the transparency of neat PC films and potentially influences their mechanical properties (see Figure 3.2). We tried to determine the dominant factors in the neat PC film fabrication process which cause the solvent-induced milky area on the films. The milky area directly affects the transparency of neat PC films. Then, we could minimize the solvent-induced milky area (assuming it was the effect of PC crystallization) by using design and analysis of experiments (DOE) method. DOE could be used to build a quantitative model governed by the dominant factors that we defined. The response was the transparency measured by UV-VIS spectrophotometer. Since the PC crystallization is one of the major reasons influencing the transparency, we could minimize solvent-induced crystallization from improving the neat PC transparency.
33
Weigh PC pellets PC dissolution in Solution hot casting solvent (chloroform)
Hot cast at 60°C Mechanical stirring 30min (hot plate)
Mechanism Vacuum oven drying Characterization No
Evaluate transparency Yes
Figure 3.1 Flow chart of neat PC films fabrication.
Figure 3.2 Different levels of degree of crystallization. The degree of crystallization increases from left to right.
34
3.3 Design and Analysis of Experiments for the Degree of Crystallization
3.3.1 Solution Casting Factor Selection
The PC crystalline structure is one of the major factors that influence the transparency of PC/CNT films [60]. DOE provides a systematic method to determine the level of factors which affect the transparency. Based on the preliminary study on solution casting, possible factors were list in the Cause and Effect Diagram (see Figure 3.3), and the following observations were made: Measurement: The level of pouring solution will cause high variation of thickness, and uneven heat transfer will cause temperature errors. Method: The vitrification (or solidification) time of the solution will affect the degree of crystallization, which is related to PC concentration. Machine: The tools for solution casting must be not changed and clean, including hot plate, casting mold (Petri dish), and syringe. Environment: Keep PC dry and reduce the move of personnel that will influence the solvent evaporating. Material: Chloroform is used as the solvent for solution casting. Manpower: The pouring volume will influence the thickness of PC films.
A full factorial 33 design was employed in this research. It simplifies each of the three different potential factors into three levels plus center points and gives measurable levels of each factor. Preliminary solution casting trials using neat PC showed that the dominant parameters were solution viscosity, casting temperature, and film thickness. The factor levels are summarized in Table 3.1. Other factors, such as human error, and casting time were considered noise factors. The 33 full factorial design with five center points, resulting in 32 treatments, was employed (see Table 3.2). Solution viscosity was
35 expressed in terms of solution concentration, as they are directly related; that is, the higher the PC concentration, the higher the solution viscosity. Concentration was controlled by varying the amount of PC added to chloroform. Base on the physics behind the optimization of the solution casting conditions, temperature, concentration and film thickness are dominant to the transparency because of the morphology and molecular structure of PC. High temperature will induce the entanglement of long molecule chains of PC and force the transformation from crystalline structure to amorphous structure. High concentration will induce the quenching process and increase the cooling rate. It will force the amorphous structure to keep its configuration. Then, small thickness of PC films gives large surface of evaporating area. It will also increase the cooling rate and reduce the evaporating time.
Measurement Method Machine Hot Plate Uneven Level Container Vitrification Time Syringe Uneven Heat Transfer
% Transmittance Temperature Thickness Volume Control Moisture Content Concentration
Windy Influ ence Solvent Selection
Environment Material Manpower
Figure 3.3 Cause and Effect Diagram.
36 Table 3.1 Factorial design for solution casting optimization.
Level Factor - 0 + Temperature (°F) 32°F 78°F (room temperature) 123°F (0°C) (25°C) (≈50.6°C) Concentration (g/mL) 0.025 0.05 0.075 Thickness (µm) 15.228 22.842 30.456
Film thickness was controlled by adjusting the amount of PC/chloroform solution poured into the Petri dish. The response was the final quality of the PC film, in terms of optical transparency and degree of crystallization. Thickness was calculated by:
The weight of composite in each dish g)( Thickness = (Eq. 3.2) ⎛ ⎞⎛ ⎞ ⎜ Density of polycarbonate (g / cm3 )⎟⎜ Square measure (cm 2 )⎟ ⎝ ⎠⎝ ⎠ Low Level: 0.333 (2 g in six dishes) / 1.2 ÷ (3 × 2.54)²π = 0.015228 (mm) 0 Level: 0.5 (2 g in four dishes) / 1.2 ÷ (3 × 2.54)²π = 0.022842 (mm) High Level: 0.667 (2 g in three dishes) / 1.2 ÷ (3 × 2.54)²π = 0.030456 (mm)
The responses for measuring transparency were defined by UV-VIS spectrophotometer. The percentage of the light transmittance (%T) at the wavelength of 300 nm was selected to be the responses. Three specimens from each run were tested, and the results of the specimens for each run were averaged to be each single response. Figure 3.4 shows the visual transparency for each experimental specimen at the wavelength of 300 nm.
37 Table 3.2 Data sheet for solution casting optimization from Design-Expert.
38
Figure 3.4 The neat PC films of the 32 runs in DOE.
39
Figure 3.4 Continued.
3.4 DOE Analysis Results and Discussions
3.4.1 Driving Forces for Polymer Crystallization
According to the DOE study performed on solution casting, temperature, PC concentration, and film thickness have been identified as the most significant factors that influence transparency. Other factors, such as voids, nonuniform density, nonuniform heat temperature, uneven placement of Petri dishes, etc. are regarded as noise factors. PC is a thermoplastic polymer and can be heat softened, melted, and reshaped. By increasing
40 the temperature, we can force crystalline structure to scatter and form amorphous structure. The higher the crystallinity, the more heat is required to break up crystalline structure. So, for crystalline thermoplastics, there is no Tg (except high-density polyethylene, HDPE) [ 11], and they provide heat resistance and hardness. When the solution evaporates, the temperature of the films decrease because the energy is absorbed by amorphous molecular chains. The heat is dissipated while PC is forming crystalline structure. During the process, it forces molecular chains to form crystalline structure and maintain configurations while cooling. Therefore, high concentration of PC gives high viscosity of solution and faster evaporating time, which directly reduces the chance that crystalline structure will form. Large film thickness also increases the layers of crystalline structure in semicrystalline PC, and influences transparency. The visual inspection of neat PC thin films fabricated according to the DOE indicates that a combination of high temperature, high PC concentration (viscosity), and small thickness yields high transparency.
3.4.2 DOE Analysis
Based on the sequential model sum of squares, lack of fit tests, and model summary statistics of Design-Expert, a linear model is suggested (Table 3.3). The model generated by Design-Expert contains three main factors: thickness, concentration, and temperature. There is no two factor interaction or quadratic terms in the model. It indicates that these three factors independently affect the optical transparency of neat PC films. The high F-value indicates that linear model will contribute to the best optimization of response estimation. Therefore, two factor-interaction, quadratic, and cubic terms can be dropped from the model. It also indicates that the interaction between each dominant factor was small.
41 Table 3.3 Model fit summary from Design-Expert.
In the final equation of actual factors generated by Design-Expert, the positive coefficients for temperature and concentration mean the direct proportion between them and % Transmittance. On the contrary, the negative coefficient for thickness means the inverse proportion between thickness and % transmittance. In the equation of coded
42 terms, it is easily seen that temperature and thickness have larger effect on % transmittance than concentration.
Final equation in terms of actual factors: %Trans. = .65 25751+ .0 53394()Temperature + .65 02222(Concentration) − .2 06895(Thickness) (Eq. 3.3) Final equation in terms of coded factors: %Trans. = 63.62 + 29.24 ⋅ A + 63.1 ⋅ B − 75.15 ⋅C (Eq. 3.4)
From the ANOVA output (Table 3.4), we found that the model F-value of 18.07 implies the model is significant and fit. There is less than 0.01% chance that a "Model F- Value" of this magnitude could occur due to noise. In this case, temperature and thickness are significant model terms with values of "Prob > F" less than 0.05. P-value of concentration is greater than 0.5, which indicates that it is an insignificant term for the model. Also, the “Lack of Fit F-value” of 0.32 implies the lack of fit is not significantly related to the pure error and the model is fit. R-Squared= 0.6594, so the model explains about 66 percent of the variability in the distance. The "Pred R-Squared" of 0.5759 is not as close to the "Adj R-Squared" of 0.6230 as one might normally expect. This may indicate a large block effect or a possible problem with the model and/or data. Things to consider are model reduction, response tranformation, outliers, etc. "Adeq Precision" measures the signal to noise ratio. A ratio greater than 4 is desirable. The ratio of 14.108 indicates an adequate signal. This model can be used to navigate the design space. The low R-squared requires the validation for the model in order to give more accurate prediction of the response. For the residuals analysis, the normal probability plot of the residuals is a method of checking the normality assumption. In the normal probability plot of the studentized residuals, the trend follows the normal distribution which indicates the normality of residuals. Studentized residuals versus predicted values to check for constant error. The plot of the residuals versus the ascending predicted response values tells us that there is no any expanding variance phenomenon. It indicates the model is fit in predicting new
43 observations. Also, the plot of the residuals versus run has a distribution of normally random scatter.
Table 3.4 The analysis of variance (ANOVA) table from Design-Expert.
44
Figure 3.5 Diagnostics plots for residual analysis from Design-Expert.
3.4.3 Optimization and Discussions
The optimization choices include numerical optimization which can set goals for each response and generate numerical optimal conditions as the solution. Then, use graphical optimization to set minimum or maximum limits for each response then create an overlay graph highlighting an area of operability. Also, use point prediction to discover the predicted responses values with confidence intervals with desired operating
45 conditions. To maximize the % transmittance, the solutions of the optimization were generated from Design-Expert:
The data in which showed the optimized solutions indicate that high temperature and thin thickness give the highest % transmittance. According to visual inspection of neat PC thin films, it also seems high temperature, high PC concentration (viscosity), and small thickness yields high transparency. The optimal conditions were determined to be 75 mg/mL, 123°F (≈50°C), and 15 µm, respectively, which indicates that crystallization can be minimized by increasing PC concentration (viscosity) and temperature, and reducing the thickness of films. Figure 3.6 shows typical hot solution cast neat PC films before and after optimization, showing improved clarity and surface integrity due to minimized crystallization.
(a) (b)
Figure 3.6 Comparison of solution cast neat PC films: (a) Before process optimization (run 5); (b) After process optimization (run 7).
46 In order to reduce the influence from noise factors, several additional steps were added to the film fabrication process. They are list as follows: 1. Pre-dry LEXEN PC pellets in the oven at 120°C (248°F) for two hours before dissolving in the chloroform. The reason is to get rid of most of the moisture from PC pellets. The moisture present in PC pellets will induce air bubbles during the process. 2. During the hot solution casting process, temperature must be controlled at 60°C which is below the boiling point of chloroform (61.1°C). It will help to accelerate the evaporation of chloroform but prevents the solution from boiling. 3. Agitating the PC solution in the Petri dish during the process helps the solvent to evaporate identically and conduct heat uniformly.
3.5 Summary
The design and analysis of experiments (DOE) method was used to find the optimal fabrication condition under which crystallization was minimized. The optimal condition reduced the influence of solvent-induced crystallization on the transparency of PC thin films, and established a guideline for fabricating PC/CNT composite thin films. One phenomenon observed was that crystallization occurs within the PC film as the solution cools down rapidly. It was also found that increasing the temperature of the solution helps prevent crystallization. This method has been used in previous research which is known as hot solution casting [45]. It is a wet annealing process based on high- temperature PC or PC/CNT solution casting prior to complete drying, yielding a uniform and transparent film. The thickness of acceptable transparent thin films is produced in the range of 10~60 µm, which verifies the optimal thickness of 15 µm determined from DOE. The regression model generated using Design Expert will enable prediction of crystallization-induced transparency of PC/CNT composite films under given film casting conditions. The next step is to use differential scanning calorimeter (DSC) to
47 define the degree of crystallization and build a model that helps predict the degree of crystallization as a function of temperature, concentration and thickness.
Degree of crystallization = ()xf 1 (Temperature), x2 (Concentration), x3 (Thickness) (Eq. 3.5)
48
CHAPTER 4 FABRICATION OF TRANSPARENT PC/MWNT COMPOSITE FILMS
4.1 Introduction
Based on previous research, PC/CNT composite films are fabricated using various techniques, such as melt processing, solution processing, and electrospinning. Solution processing was employed in this research since we optimized process for neat PC casting. The solution-based processing also provides an effective, lab-scale method to assess CNT dispersion and build well mixed network in PC/CNT composite. However, a major challenge was encountered, where the CNTs aggregated and precipitated at high CNT loadings (another is the crystalline PC during solution casting process [Chapter 3,60]). This chapter aims at developing a systematic method to fabricate transparent, CNT- toughened PC composite films without any functionalization of CNTs.
4.2 Fabrication of Transparent PC/MWNT Composite Films
Multi-walled carbon nanotubes (MWNTs) were used, as they yield better dispersion in organic solvents than SWNTs at a lower cost. The MWNTs used in this research were purchased from Aldrich© (purity: 95%, I.D.: 5~10 nm, O.D.: 10~30 nm, length: 0.5~500 µm, BATCH#: 02926TB). In order to fabricate PC/CNT composite films, solution casting was used with granular-structured LEXAN® 103-112 polycarbonate (PC) (GE Plastics). LEXAN® has flexural strength of 93 MPa with 90% light
49 transmission. Chloroform from Fisher Scientific was used as the solvent, which has a boiling point of 61.1°C and high PC solubility. Because thicker films were required to be more efficient to mechanical properties of laminate PC/CNT composite design, the thickness of 30 µm was used (twice as the optimal thickness in previous DOE). First, we weighed 4 grams of PC pellets and MWNTs with the MWNT concentration in the range of 0.05~1 wt.-%, and dissolved them in chloroform (see Figure 4.1). PC was completely dissolved by stirring for 30 minutes to one hour at room temperature. The PC/MWNT/chloroform mixture was sonicated for 2~2.5 hours using Sonicator 3000 from Misonix Corporation. The sonication power was maintained below 40 watts in order to prevent the structure of MWNTs from being damaged. The sonicated solution was poured into Petri dish, which was subsequently heated on a hot plate (hot casting [45]). The casting temperature was set below the boiling point of chloroform. The thickness of the PC/CNT films fabricated was in the range of 25-35 µm. The formation of crystalline entities within the PC film is due to film temperature elevation or plasticizer ingress, which causes the loss of strength and optical clarity [58, 60].
4.3 Summary
High-temperature PC/MWNT solution casting prior to complete drying yields a uniform and transparent film; however, CNT dispersion is difficult to control when the CNT loading increases. Therefore, the CNT loading has to be small, ranging from 0.05~0.1 wt.-% (see Figure 4.2), so as to prevent CNT aggregating together. Without any chemical bonding, the technical roadblock is that CNT dispersion becomes worse during the hot casting process because the concentration of CNTs in chloroform increases as chloroform evaporates in air, and CNTs start to aggregate together. One technique that was adopted to resolve this problem was to achieve uniform evaporation throughout the evaporation process (by swaying the Petri dish side-to-side gently until all the chloroform evaporated). Subsequent to chloroform evaporation, the thin films were demolded from the dish and vacuum-dried at 60°C. The PC/MWNT composite films fabricated had
50
Weigh MWNT MWNT and PC Sonication and PC pellets dissolution in solvent (chloroform)
2~2.5 hrs Mechanical stirring 30min (hot plate)
Solution hot casting
Hot cast at 60°C
Mechanism Characterization Vacuum oven drying
No
Evaluate transparency Yes
Figure 4.1 Flow chart of PC/MWNT composite film fabrication.
51 thickness in the range of 10~40 µm. The optical transparency and various mechanical properties of PC/MWNT composite films have been characterized, and the results are presented in Chapter 5.
0.05 wt.-% MWNT 0.1 wt.-% MWNT
0.15 wt.-% MWNT 0.25 wt.-% MWNT
0.5 wt.-% MWNT 1 wt.-% MWNT
Figure 4.2 PC/MWNT composite films with MWNT loading from 0.05~1 wt.-%.
52
CHAPTER 5 OPTICAL PROPERTIES OF PC/MWNT COMPOSITE FILMS
5.1 Introduction
In addition to solvent-induced crystallization, CNT dispersion is another major factor that governs the level of the transparency of PC/MWNT films. In the preliminary experiment, the MWNT loading ranged from 0.05~1 wt.-%, and the visual dispersion is showed in Figure 4.2. At the preliminary level, optical microscopy helps optimize PC/MWNT film fabrication (we can observe rapid increase of CNT aggregations at 0.15 wt.-% MWNT). Three methods were used to find out the relationship between the two major issues that influence the transparency of PC/MWNT composite films, solvent- induced PC crystallization and CNT aggregations. The methods include UV-VIS spectrometry, differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The light transmittance was determined by using UV-VIS-NIR spectrophotometer, and differential scanning calorimetry (DSC) was used to examine PC crystallinity. The characterization for optical properties of PC/MWNT composite films was performed as following: 1. Use UV-VIS-NIR spectrophotometer to determine the optical transparency and particle size of the specimens: a) Transparent area on neat PC, 0.05, 0.1, and 0.15 wt.-% PC/MWNT films. b) Translucent area on neat PC, 0.05, 0.1, and 0.15 wt.-% PC/MWNT films. c) Opaque area on neat PC, 0.05, 0.1, and 0.15 wt.-% PC/MWNT films. 2. Use differential scanning calorimetry (DSC) to measure the PC crystallization and the influence of CNTs to the PC crystallization. a) Transparent area on neat PC, 0.05, 0.1, and 0.15 wt.-% PC/MWNT films.
53 b) Translucent area on neat PC films. c) Opaque area on neat PC films. 3. Use SEM to confirm the crystallization and semi-crystallization of neat PC films and the particle (CNT aggregations) size in PC/MWNT composite films. a) Transparent, translucent, and opaque area on neat PC films. b) 0.05 wt.-% PC/MWNT composite films. c) 0.1 wt.-% PC/MWNT composite films. d) 0.15 wt.-% PC/MWNT composite films.
5.2 Characterization of CNT Dispersion by Optical Microscopy
The dispersion of PC/MWNT composite films was observed using optical microscopy (OLYMPUS BX-40). Figure 5.1 represents the CNT dispersion of 0.05, 0.1, and 0.15 wt.-% of MWNTs in PC observed at 100X and 500X magnifications, respectively. The results showed acceptable dispersion for 0.05 and 0.1 wt.-% of MWNTs; however, CNTs tended agglomerate into considerable size when the loading of MWNT increased to 0.15 wt.-% or more. Previous research suggests the visible length scale of particles by the naked eye is around 400~700 nm. The images in Figure 5.1 show that the particles of both 0.05 and 0.1 wt.-% are in the range from 500~5000 nm; however, the aggregated CNTs in 0.15 wt.-% are much larger than 50 µm. So, the optimized CNT dispersion should be achieved below 0.15 wt.-% CNT loading. The CNT aggregations of the CNT loading greater than 0.15 wt.-% will definitely influence the transparency of PC/MWNT composite films. Nevertheless, the further optical properties of PC/MWNT composite films would have to be studied by SEM and UV-VIS spectrometry. The CNT loadings investigated for the following analysis were 0.05, 0.1, and 0.15 wt.-% MWNT.
54
0.05 wt.-% (x100) 0.1 wt.-% (x100) 0.15 wt.-% (x100)
500 μm 500 μm 500 μm
0.05 wt.-% (x500) 0.1 wt.-% (x500) 0.15 wt.-% (x500)
100 μm 100 μm 100 μm
Figure 5.1 Optical micrographs of PC/MWNT composite thin films at 0.05, 0.1 and 0.15 wt.-%.
5.3 UV-Visible Spectrometry
The UV-VIS properties of the solution cast films including neat PC, 0.05, 0.1 and 0.15 wt.-% MWNT PC/MWNT composite films were characterized using Cary® 5000 UV-VIS-NIR spectrophotometer (ASTM D1003). UV-VIS-NIR spectrophotometer can test light wavelength ranging from NIR (near-infrared rays) which is around 3000 nm to UV (ultraviolet rays) which is around 175 nm. Since the visible length scale of particles by the naked eye is around 400~700 nm, UV-VIS-NIR spectrophotometer can define the optical properties via the wavelength curve within this range versus percentage light transmittance (% T) or light absorbance (A).
55 5.3.1 The Beer-Lambert Law
The Beer-Lambert Law is an “empirical relationship” defining the absorption of light by the material when light travels through it [7, 16, 61- 62]. Basically, the absorption of light and light transmittance are in a relationship of an inverse proportion. Besides, the concentration of the material and the length of the material which light travels through are directly proportional to the light absorbance (see Figure 5.2). To reform the concept here, the absorption of light and the light absorbance are two different measurements that the absorption of light is expressed as a percentage and the light absorbance (A) is defined by the Beer-Lambert Law, as: ⎛ 100 ⎞ A = Log ⎛ I ⎞ = Log 1 = α ⋅C ⋅ D = Log⎜ ⎟ (Eq. 5.1) 10 ⎜ I ⎟ 10 ()T ⎜ ⎟ ⎝ 0 ⎠ ⎝100 − %Absorption ⎠ Where,
I0 = incident radiation, I = transmitted radiation, α (or E) = absorption coefficient (for solid state) or the molar absorptivity (for liquid or gaseous state) of the material, cm-1 (or m-1), the material represents different absorption coefficient based on the light wavelength; for example, the absorption coefficient of polycarbonate for 248 nm laser is 4.17×l04 cm-1 [ 62], C = concentration of the material, D = light path length, and T = light transmittance. Also, the light transmittance T is defined as:
T = I and %T = 100× I (Eq. 5.2) I 0 I 0 where,
I0 = incident radiation, I = transmitted radiation. The absorption of light is expressed as percentage and defined as: %Absorption = 100 − %T (Eq. 5.3)
56 Absorbance is often more useful than transmittance as there is a linear relationship between absorbance A and the concentration of the absorbing species (see Figure 5.3). Still, absorbance and transmittance can be converted by Eq. 5.1.
I0 (incident light) α C I (transmitted light)
D
Figure 5.2 Light absorbance of Beer-Lambert Law with a beam of light traveling through a material of thickness D.
57
%T A
Concentration Concentration
Figure 5.3 The percentage of light transmittance (%T) versus the concentration of the material and the light absorbance (A) versus the concentration of the material.
5.3.2 UV-Visible Property
The UV-VIS property was analyzed by the Cary WinUV Scan software. The wavelength for the test started from 1000 nm and stopped at 300 nm. The samples were cut into 45 mm×45 mm pieces and 30 µm ± 5 µm in thickness. The solid sample holder of Cary 5000 provides a 2.5 cm×1.5 cm slot for light incident radiation to get through. In the graph preferences, the x-axis displays the light wavelength, and the y-axis displays % Transmittance (%T) or Absorbance (A). Figures 5.4 and 5.5 show the %T and absorbance of the PC/MWNT composite films (0.05, 0.1, and 0.15 wt.-%) and the neat PC films (transparent area, translucent area, and opaque area) respectively. The %T of the PC/MWNT composite films dropped while the MWNT loading increased. The %T of the transparent neat PC films was consistently in the range of 96%-99%, but the %T of the translucent and the opaque neat PC films was well below 30% at the wavelength of 300 nm. Figure 5.5 represents the light wavelength versus the light absorbance (A) of the PC/MWNT composite films (0.05, 0.1, and 0.15 wt.-%) and the neat PC films (transparent area, translucent area, and opaque area). The absorbance (A) is inversely
58 proportional to %T. The absorbance of the 0.15 wt.-% PC/MWNT composite films slightly increased from 0.08 to 0.17 within the range of 300nm~800nm.
Neat PC (Transparent) 0.05 wt.-% PC/MWNT 0.10 wt.-% PC/MWNT 0.15 wt.-% PC/MWNT Neat PC (Translucent) Neat PC (Opaque)
Figure 5.4 The light wavelength versus the percentage of light transmittance for neat PC and 0.1 wt.-% PC/MWNT composite film.
Neat PC (Transparent) 0.05 wt.-% PC/MWNT 0.10 wt.-% PC/MWNT 0.15 wt.-% PC/MWNT Neat PC (Translucent) Neat PC (Opaque)
Figure 5.5 The light wavelength ranged from 400 nm~700 nm versus the percentage of light transmittance for neat PC and 0.1 wt.-% PC/MWNT composite film.
59
In Figure 5.6, the light wavelength is focused in the range of 400 nm~700 nm. At 400 nm, the %T for the neat PC films was 97% which was comparable to the %T of GE LEXEN (90%); however, the %T for the 0.15 wt.-% PC/MWNT composite films dropped to 83%. The neat PC films showed constant %T over the visible wavelength range, while the %T of the PC/MWNT composite films descended steeply when the wavelength reached below 500 nm. It indicates that the PC/CNT composite films probably have maximum particle size that ranges from 400 nm to 500 nm. It appeared to be very close to the maximum length of MWNT (500 nm). The raw T% data of each film type are summarized in Table 5.1, and Figure 5.7 represents the trend of the %T with increasing MWNT concentration. The %T at 300 nm shows a decreasing trend with increasing MWNT concentration. It also indicates that the aggregation of MWNTs grows with the increasing MWNT concentration.
98.0 96.7 96.5 92.9 86.6 86.4 85.3 79.0 Neat PC (Transparent) 50.2 0.05 wt.-% PC/MWNT 47.6 47.1 0.10 wt.-% PC/MWNT 43.1 0.15 wt.-% PC/MWNT Neat PC (Translucent) Neat PC (Opaque)
Figure 5.6 The light wavelength ranged from 300 nm~700 nm versus the light absorbance for 0.1 wt.-% PC/MWNT composite film.
60 Table 5.1 Data sheet for the %T of the neat PC and the PC/MWNT composite films.
Neat PC (Transparent) 0.05 wt.-% PC/MWNT 0.1 wt.-% PC/MWNT Wavelength Batch 1 Batch 2 Avg. Batch 1 Batch 2 Avg. Batch 1 Batch 2 Avg. (mm) 1000 99.97 99.55 99.76 99.97 99.79 99.88 98.45 95.87 97.16 900 97.57 97.11 97.34 99.19 96.98 98.085 95.59 93.2 94.395 800 89.1 88.49 88.795 90.01 88.14 89.075 86.38 84.51 85.445 700 95.98 95.16 95.57 96.56 94.81 95.685 91.88 90.13 91.005 600 96.45 95.63 96.04 96.91 94.69 95.8 91.18 90.01 90.595 500 96.49 95.44 95.965 96.23 93.86 95.045 89.4 88.34 88.87 400 98.07 96.76 97.415 96.23 92.81 94.52 86.5 86.24 86.37 300 96.23 94.65 95.44 89.4 82.3 85.85 75.2 72.05 73.625 Neat PC (Transparent) 0.05 wt.-% PC/MWNT 0.1 wt.-% PC/MWNT Wavelength Batch 1 Batch 2 Avg. Batch 1 Batch 2 Avg. Batch 1 Batch 2 Avg. (mm) 1000 99.97 99.55 99.76 99.97 99.79 99.88 98.45 95.87 97.16 900 97.57 97.11 97.34 99.19 96.98 98.085 95.59 93.2 94.395 800 89.1 88.49 88.795 90.01 88.14 89.075 86.38 84.51 85.445 700 95.98 95.16 95.57 96.56 94.81 95.685 91.88 90.13 91.005 600 96.45 95.63 96.04 96.91 94.69 95.8 91.18 90.01 90.595 500 96.49 95.44 95.965 96.23 93.86 95.045 89.4 88.34 88.87 400 98.07 96.76 97.415 96.23 92.81 94.52 86.5 86.24 86.37 300 96.23 94.65 95.44 89.4 82.3 85.85 75.2 72.05 73.625
120
100
80 700 nm 600 nm
60 500 nm % T 400 nm 40 300 nm
20
0
012345 Neat PC 0.05 wt.-% 0.1 wt.-% 0.15 wt.-%
Figure 5.7 The trends of MWNT concentration (0, 0.05, 0.1, 0.15) versus %T for 300 nm- 700 nm wavelength.
61
5.4 Differential Scanning Calorimetry (DSC) Analysis
The usage and the properties of PC rely on the crystallinity of molecular chains, as they affect the optical transparency and mechanical properties. Differential scanning calorimetry (DSC) is one of the most typical analytical methods to define the degree of crystallization. DSC provides sample heating, cooling, or isothermal temperature and measures the heat flow during the process. As discussed, solvent-induced crystallization results in tinting of PC-based films (both neat PC and PC/CNT films), degrading optical transparency. To quantify crystallization-influenced optical transparency, DSC was used to measure the degree of crystallization of the films produced previously. The following equation will be used to calculate the degree of crystallinity [63 65]:
Actual Heat Degree of Crystallization = (Eq. 5.4) S tan dard Heat
In order to measure the degree of crystallization of semi-crystalline PC, transparent, translucent, and opaque area on the neat PC films were selected to test on DSC. The measurement calibration was set for PC from -100°C to 400°C. Experiment was ramped at 10°C/min to 300°C and then cooled down to 0°C at -20°C/min. The procedure ran for two cycles, and equilibrated for 1 min at both 0°C and 300°C. Figure 5.8 represents the heat flow for the different areas of the neat PC films. The mass for the samples will be 10±1.5 mg. The opaque and translucent PC films both have downward peak curves when the temperature reached the melting temperature of PC (around 250°C). The greater area of the downward peak area for the opaque PC films indicates higher degree of crystallization. When crystalline structure transforms into amorphous structure, the heat is dissipated in order to release the bonding between molecular chains. Therefore, the heat flow went down when the heat was generated by the crystalline structure in PC.
62 1
0 PC_Transparent endothermic -1 PC_Translucent -2 PC_Opaque -3 0.05 wt.-% PC/MWNT 0.1 wt.-% PC/MWNT
Heat Flow (W/g) Heat -4 0.15 wt.-% PC/MWNT -5
-6
exothermic 0.030 22.3 50 55.6 100 89 122 150 1 56 189 200 2 22 256250 289 300 Temparature (°C)
Figure 5.8 Differential Scanning Calorimetry heat flow traces of neat PC and PC/MWNT composite films.
As shown in Eq. 5.5, the heat capacity can be calculated using the heat flow, the sample mass and the heating rate (HR) [ 66 67]: 1 1 C =HeatFlow× × P HeatingRate mass (Eq. 5.5) mcal 18.4 J 60sec min 1 J = × × × × = sec 1cal 1min °C mg g °× C
Based on the DSC data, the heat capacity of the sample can be calculated from the area within the downward peak curve of heat flow versus temperature. TA University was used to calculate both the heat capacity and the peak area (degree of crystallization) (see Figure 5.9). The standard heat (theoretical heat of fusion) of PC was taken as 130J/g [ 19]. The crystallinity calculated by TA Universal was collated and shown in Table 5.2. It obviously shows crystallinity in both the translucent PC (3.81% of degree of crystallization) and opaque PC (14.01% of degree of crystallization). It indicates that the crystallization of PC is one of the reasons causing the milky area on PC. For PC/MWNT
63 composite films, there is almost no crystallinity shown in the test, which indicates that the transparency for PC/MWNT composite films is influenced by MWNT aggregations. This information helps us to identify the MWNT aggregations with the UV-VIS.
(a)
Figure 5.9 Temperature versus heat flow showing the heat capacity and the degree of crystallization: (a) transparent PC; (b) translucent PC; (c) opaque PC.
64
(b)
(c)
Figure 5.9 Continued.
65 Table 5.2 Degree of crystallization of neat PC and PC/MWNT composite films calculated by TA Universal.
Peak Temperature Actual Heat Standard Heat Degree of (°C) (J/g) (J/g) Crystallization (%) Transparent 219.07 3×10-4 130 2.3×10-4% PC Translucent 220.59 4.95 130 3.81% PC Opaque 227.84 18.22 130 14.01% PC 0.05 wt.-% 212.73 0.014 130 0.01% PC/MWNT 0.1 wt.-% 212.48 0.034 130 0.01% PC/MWNT 0.15 wt.-% 212.36 0.894 130 0.69% PC/MWNT
5.5 Scanning Electron Microscopy (SEM) Analysis
The morphology of neat PC films and CNT dispersion in PC/MWNT composite films were observed under the JEOL 7401F field-emission scanning electron microscopy (SEM). Three different areas of neat PC films were observed under SEM including 1) opaque area, 2) semi-transparent (translucent) area, and 3) transparent area. Figure 5.10 shows the SEM images of opaque area of neat PC films. Microvoids in the images indicate that not only is the milky color of neat PC films due to PC crystallization, but is also due to the microvoids. Contrasting the UV-VIS results, transparent area has almost 99% of the %T; however, the %T of translucent area and opaque area is below 30%. It indicates that microvoids are key factors for optical transparency of PC with solution casting process. When solvent evaporates rapidly, there is not enough time for PC to fill the microvoids on the surface of the films. Therefore, the incident light will be refracted by the microvoids and hardly penetrate the films. In summary, opacity is primarily due to: 1. Microvoids formed during solution casting (due to solvent evaporation).
66 2. Solvent-induced crystallization. The effect of solvent-induced crystallization has been further investigated through DSC analysis. A cross-section view of 0.1 wt.-% PC/CNT films shows larger MWNT aggregates ranging between 1 and 5 µm (see Figure 5.11).
Figure 5.10 The SEM images of opaque area of neat PC films showing microvoids.
Figure 5.11 The cross-section view of fractured PC/MWNT films (0.1 wt.-%).
67 5.6 PC/MWNT Composite Film Thickness Analysis
Uneven thickness may affect the properties of PC/CNT composite films. The purpose in this section is to examine the influence of film thickness to PC crystallization and CNT dispersion. The thickness of PC/MWNT composite films was measured at three different positions including: A: central area, B: internal area, and C: peripheral area (Figure 5.12). Two specimens for each film (neat PC, 0.05, 0.1, 0.15 wt.-% PC/MWNT), and four samples from each position were tested. The raw data are shown in Table 5.3. The result shows that the central and peripheral areas are slightly thicker than the internal area (Figure 5.13). It was found from the previous DOE that increasing thickness would induce more crystallization. In Figure 5.14, the central and peripheral areas of the pure PC film show crystallized PC as compared to the internal area of the film.
× A A ×
B B × ×
× C C×
Peripheral area
Internal area
Central area Figure 5.12 Three positions for thickness analysis in neat PC/PC/MWNT composites films: A: central area, B: internal area, C: peripheral area.
68 Table 5.3 Raw Data for thickness analysis of PC/MWNT composite films.
PC/MWNT Composite Film Thickness Raw Data (Unit: µm) Position Film Mean Std. Dv. Sample I Sample II PC 28.325 3.362 31.45 30.95 32.05 31 27.05 24.4 25.2 24.5 0.05 wt.-% 25.243 0.813 24.65 24.75 24.65 25.85 24.1 26.4 25.54 26 A 0.1 wt.-% 32.438 6.819 38.1 39.25 39.35 37.7 23.05 27.9 25.45 28.7 0.15 wt.-% 33.069 4.093 29.8 26.8 30.75 31.2 34 38.75 36.65 36.6 29.768 5.24 PC 27.75 3.423 29.05 27.85 25.3 32.3 25.55 21.65 30.3 30 0.05 wt.-% 21.181 3.579 27.55 20.5 16.35 17.8 21.3 23.75 23 19.2 B 0.1 wt.-% 26.05 4.816 22.1 29.7 28.25 31.8 20.5 19 27.35 29.7 0.15 wt.-% 26.831 5.616 17 21 27.9 25.2 33.15 33.15 28.65 28.6 25.453 4.955 PC 37.113 7.545 48.5 31.5 31.65 39.75 44.65 40.65 34.15 26.05 0.05 wt.-% 29.25 3.934 23.5 27.5 31 28.9 35.05 34.1 26.1 27.85 C 0.1 wt.-% 35.631 1.984 38 33.4 35.65 35 32.95 38.6 36.25 35.2 0.15 wt.-% 30.373 3.805 26.05 27.5 29.77 38 33 27.73 31.4629.47 33.092 5.662
Av e rage
40
35
30
25 Neat PC
20 0.05% 0.10% 15 0.15% Thickness (µm) Thickness 10
5
0 A123 B C Position
Figure 5.13 Three positions versus thickness (µm) for thickness analysis in neat PC/PC/MWNT composites films.
69
Figure 5.14 Sample from DOE (run 12) shows that internal area is more transparent than peripheral and central area.
70
CHAPTER 6 MECHANICAL PROPERTIES OF PC/CNT COMPOSITE FILMS
6.1 Review
Singh et al. [45] is one of the groups who reported that there is an optimal CNT% for maximized mechanical properties of PC/CNT composites, and that mechanical properties can not be increased indefinitely with the addition of CNTs. In this chapter, Halpin-Tsai equation and Cox equation were used to estimate short CNT-based composite modulus instead of rule of mixtures. In addition, mechanical properties of PC/CNT thin films including neat PC, 0.05, 0.1, and 0.15 wt.-% PC/MWNT composite films were characterized by the Shimadzu AGS-J materials testing system.
6.2 Mechanical Properties of PC/CNT Films
6.2.1 Experimental
The tensile properties of the solution cast films (neat PC, 0.05, 0.1 and 0.15 wt.-% PC/MWNT composite films) were characterized by using the Shimadzu AGS-J materials testing system (ASTM D882-02). The specimens were 10 mm in width, and gauge length was set to 25-35 mm. The thickness of the specimens ranged from 25~50 µm. The specimens were prepared with laser cutting machine. Crosshead speed was 1 mm/min, and data sampling rate is 0.05 seconds. No extensometer was used, and the change of
71 stroke was obtained from crosshead displacement. GE LEXAN PC sheets were also tested as the baseline (thickness: 0.25 mm).
6.2.2 Experiment Results
The measured properties of the PC/MWNT composite films included elastic modulus, ultimate tensile strength, and % elongation. The results are summarized in Table 6.1. GE LEXAN PC sheets showed consistent data range which could be used to compare with solution cast neat PC films. Solution cast PC films had an average of 1.71 GPa of elastic modulus and 49.9 MPa of ultimate tensile strength, which were close to the result of GE LEXAN PC sheets (elastic modulus: 1.68 GPa, ultimate tensile strength: 46.2 MPa). The 0.05 wt.-% PC/MWNT films showed a 10.9% improvement in elastic modulus while the 0.1 wt.-% PC/MWNT showed 13.5% as compared to neat PC films (see Figure 6.1). In comparison, elastic modulus slightly decreased by 2% when the CNT loading was increased to 0.15 wt.-%. The ultimate tensile strengths for neat PC, 0.05, and 0.1 wt.-% PC/MWNT were similar and were 49.9, 47.2, and 49.1 MPa, respectively (see Figure 6.2). However, ultimate tensile strength of 0.15 wt.-% PC/MWNT dropped by 14.4% as compared to neat PC films. It indicates a significant drop of tensile strength for 0.15 wt.-% PC/MWNT composite films. 0.05, 0.1 and 0.15 wt.-% PC/MWNT all decreased in % elongation as compared to neat PC (see Figure 6.3). Some of PC/MWNT composite films showed extremely high elongation especially at high MWNT concentration. This phenomenon rarely happened with neat PC films.
6.2.3 Discussions
The complexities of CNT-reinforced polymer are due to the unique properties of the CNTs. The difficulties in analyzing and modeling the mechanical properties of CNT- reinforced polymers are associated with [ 71]: 1. The structure and chemical stability of the CNTs.
72 2. The dispersion of the CNTs within the polymer. 3. The interfacial bonding between the CNTs and the polymer. 4. The understanding of the influence of the CNTs to the molecular chains of the polymer.
Table 6.1 Shimadzu tension test data for neat PC, 0.05, 0.1, and 0.15 wt.-% PC/CNT composite films.
Shimadzu Tension Test GE LEXAN Neat PC Specimen E UTS % E UTS % No. (GPa) (MPa) Elongation (GPa) (MPa) Elongation 1 1.56 44.1 0.224 1.63 48.4 0.148 2 1.68 48.1 0.203 1.76 50.6 0.076 3 1.67 48.0 0.228 1.68 48.6 0.151 4 1.73 48.1 0.237 1.61 45.5 0.100 5 1.74 43.0 0.193 1.76 51.5 0.109 6 ------1.59 47.2 0.236 7 ------1.63 50.1 0.238 8 ------1.73 54.0 0.198 9 ------1.84 53.6 0.197 10 ------1.91 49.2 0.068 Mean 1.68 46.2 0.217 1.71 49.9 0.152 St. Dev. 0.069 2.5 0.019 0.106 2.7 0.063
0.05 wt.-% PC/MWNT 0.1 wt.-% PC/MWNT 0.15 wt.-% PC/MWNT Specimen E UTS % E UTS % E UTS % No. (GPa) (MPa) Elongation (GPa) (MPa) Elongation (GPa) (MPa) Elongation 1 1.93 47.1 0.329 1.98 47.4 0.119 1.67 42.8 0.111 2 1.88 48.4 0.253 2.01 50.1 0.167 1.66 41.5 0.101 3 1.96 48.8 0.145 1.92 45.7 0.155 1.73 42.5 0.111 4 1.88 45.8 0.218 2.12 56.1 0.144 1.64 44.0 0.153 5 2.24 52.0 0.160 2.12 54.8 0.211 1.65 42.7 0.252 6 1.87 48.9 0.096 1.90 49.5 0.152 1.53 39.8 0.674 7 1.80 46.7 0.103 1.82 47.1 0.269 1.68 40.1 0.113 8 1.84 44.8 0.118 1.86 50.5 0.079 1.73 42.4 0.169 9 1.86 45.1 0.127 1.81 44.9 0.476 1.76 43.7 0.511 10 1.75 44.8 0.143 1.92 44.7 0.458 1.76 47.8 0.373 Mean 1.90 47.2 0.169 1.95 49.1 0.223 1.68 42.7 0.257 St. Dev. 0.137 2.2 0.079 0.117 3.9 0.118 0.068 1.5 0.207
73 Young's modulus (E)
E (MPa)
GE LEXAN Neat PC 0.05 CNT 0.1 CNT 0.15 CNT
Figure 6.2 Elastic moduli (E) of neat PC and PC/CNT films.
Ultimate tensile strength (σuts)
(MPa) uts σ
GE LEXAN Neat PC 0.05 CNT 0.1 CNT 0.15 CNT
Figure 6.3 Ultimate tensile strengths (σu) of neat PC and PC/CNT films.
74 Break strain
)
% elongation (100% elongation %
GE LEXAN Neat PC 0.05 CNT 0.1 CNT 0.15 CNT
Figure 6.4 % elongation of neat PC and PC/CNT films.
The increase of CNT loading adversely affects CNT dispersion in PC and causes CNT aggregates to form during the solution casting process. The aggregates lower both the transparency of the film and the interfacial bonding between PC and CNTs, thus allowing the CNTs to act as microcracks rather than reinforcement or toughening agent. Comparing with the results from optical microscopy and SEM, 0.15 wt.-% PC/MWNT composite films show both increasing MWNT aggregations and decreasing modulus. However, the increase of MWNT concentration seems to enhance the % elongation (see Figure 6.5) although the variance is large.
75
Figure 6.5 Showing different extension (% elongation) of tensile test for (a) 0.15 wt.-% PC/MWNT composite film, (b) neat PC film.
6.3 Optimal CNT Loading for Enhancing Mechanical Properties
Literature reveals several methods to estimate theoretical mechanical properties. The Singh group used the rule of mixtures to estimate long entangled SWNT network in PC [45]. Basically, the rule of mixture is usually used for long aligned fibre-reinforced composites. Fukuda equation and Halpin-Tsai equation are commonly applied for the estimation of the modulus of short fibre-reinforced composite. In 1951, Cox [68] first proposed a shear-lag model for estimating the modulus of the short glass-fibre-reinforced composites. Then, Fukuda and Kawaka [ 69] corrected Cox’s model by considering the length and orientation of fibres, and some corrections was made by Jayaraman and Kortshot [ 70]. Wang [ 71] used specific Weibull distribution to analyze the length of CNTs which helped to estimate parameters accurately. Halpin and Tsai [ 72] also proposed the model for estimating the bulk modulus of particulate composites. In our research, short range, multi-walled carbon nanotubes (MWNTs) purchased from Aldrich© (purity: 95%, I.D.: 5~10 nm, O.D.: 10~30 nm, length: 0.5~500 µm, 95±%, Batch#
02926TB) were used. The suggested modulus for MWNT is 1 TPa (Ef= 1 TPa). For PC, the elastic modulus was 1676 MPa, obtained from previous tensile test with GE LEXAN
76 (Em= 1676 MPa), and Poisson ratio νm= 0.37 [ 63]. The elastic modulus for PC/MWNT composite films can be estimated by the following equation [71]:
Ecom = χ χ 21 E V ff + Em (1−V f )
1 l max ⎡ tanh()βl 2 ⎤ χ1 = 1− ()lf ldl ∫l min ⎢ ⎥ lmean ⎣ βl 2 ⎦
1 l max ⎡ tanh()βl 2 ⎤ = 1− abl b exp()− al b dl ∫l min ⎢ ⎥ lmean ⎣ βl 2 ⎦
21 ⎡ Em ⎤ 21 2× ⎡ 2G ⎤ ⎢ 12 +ν ⎥ m ⎢ ()m ⎥ β = ⎢ 2 ⎥ = 2 ⎣⎢ E f r ln()R r ⎦⎥ ⎢ E f r ln()R r ⎥ ⎢ ⎥ ⎣ ⎦ 21 ⎡ ⎤ Em = ⎢ 2 ⎥ ⎣⎢ E f ()1+ν m r ln()R r ⎦⎥
π 2 2 2 2 χ 2 = []()cosθ −ν 12 ()sinθ ()(cosθ g θ )dθ ∫0 where,
Ecom = Young’s modulus of nanocomposites,
χ1 = length effect factor for discontinuous fibers,
χ 2 = orientation effect factor, equal to 1/5 for 3D plane randomness, 3/8 for 2D randomness,
E f = Young’s modulus of reinforced fiber,
Em = Young’s modulus of resin matrix,
V f = reinforced fiber volume fraction, R= average separation for the reinforced fibre norm and the length, r= reinforced fiber radius.
From the equation above, the length effect factor was influenced by CNT loading and rope diameter. The estimation of the modulus can be enhanced as the CNT loading increases. On the contrary, the estimation of modulus reduces while the diameter of CNT
77 ropes increases. For 0.1 wt.-% PC/MWNT, the theoretical modulus calculated by the rule of mixtures is 2.3 GPa. In our research, the practical modulus for 0.1 wt.-% PC/MWNT is 1.95 GPa. The 15% reduction indicates the interfacial bonding between PC and MWNTs weaken the mechanical properties. The Singh group [45] reported that there is an optimal CNT% for maximized mechanical properties of PC/CNT composites, and that mechanical properties can not be increased indefinitely with the addition of CNTs. CNT aggregation is the primary reason that limits the improvement of mechanical properties. In order to predict the modulus of PC/MWNT composite films in the research, Halpin-Tsai equations were used. Halpin-Tsai equations are used for the estimation of modulus in random discontinuous short fiber composites:
⎡3 1+ 2()l f d f η V fL 5 1+ 2η V fT ⎤ E = ⎢ + ⎥ E composite 8 1−η V 8 1−η V m ⎣⎢ fL fT ⎦⎥
( E f E m ) − 1 ηL = () E f E m + 2 () l f d f
( E f E m ) − 1 ηT = () E f E m + 2 where, l f = length of the fibers, d f = diameter of the fibers,
V f = volume fraction of the fibers,
Em = elastic modulus of the matrix,
E f = elastic modulus of the fibers. The theoretical stiffness can be estimated by the assumption of the following information: The length of the fibers: MWNT: 0.5-500 µm, The diameter of the fibers: MWNT: 10-30 nm, The volume fraction of the fibers: 0.05-0.15 wt.-% of MWNT, The elastic modulus of the matrix: PC: 1.714 GPa, The elastic modulus of the fibers: MWNT: 500 GPa
78 In Figure 6.6, the measured moduli of 0.05 and 0.1 wt.-% PC/MWNT composite films were comparable to the theoretical stiffness estimated by Halpin-Tsai equations. Realistically, the maximum achievable elastic modulus is approximately 80% of the theoretical value [ 45]. However, the modulus for 0.15 wt.-% PC/MWNT composite films shows 17% reduction as compared to the practical value. It indicates the MWNT dispersion was getting worse while the MWNT loading increased to 0.15 wt.-%. The dispersion of MWNTs can be determined qualitatively by the microscopy images and the SEM images.
Young's modulus (E)
Nea PC CNT CNT E LEXAN E (MPa) CNT alpin T ai
GE LEXAN Neat PC 0.05 CNT 0.1 CNT 0.15 CNT
Figure 6.6 Theoretical and measured Young’s moduli of PC/MWNT films.
79 6.4 SEM Analysis of CNT Mechanism
The nanostructure and morphology of the fracture surfaces of PC/MWNT composite films obtained from tensile testing on Shimadzu AGS-J were studied using the JEOL 7401F field-emission scanning electron microscope (SEM). Figure 6.7 shows the dispersion of MWNT from the fracture surface of 0.05 wt.-% PC/MWNT composite films. The MWNTs were dispersed well and exposed of the fracture surface uniformly. The exposed MWNTs with PC-coated connection with PC matrix indicate toughening mechanism of MWNT breaking.
(a) (b)
(c) (d)
Figure 6.7 SEM images showing: (a) (b) the dispersion of MWNTs from the fracture surface of failed 0.05 wt.-% PC/MWNT films; (c) Fracture surface showing exposed nanotubes; (d) Exposed MWNT with PC-coated connection with PC matrix.
80
The PC-coated nanotubes (instead of the pulled-out nanotubes) on the fracture surface indicate good interfacial adhesion between PC and MWNTs, which results in enhanced PC toughening as well as modulus and strength (see Figure 6.8(a, b)). Micrographs in Figure 6.8(b) show MWNTs with large diameters. It indicates that there is PC-coating with MWNTs. Not only do PC-coated CNTs larger than the data provided by the manufacturer indicate good interfacial bonding, but they also indicate that the adhesive force is greater than cohesive force within PC.
(a) (b)
Figure 6.8 SEM images showing the toughening effects of nanotubes of failed 0.15 wt.-% PC/MWNT films: (a) (b) Fracture surface showing PC-coated nanotubes.
Figures 6.9 shows the SEM images obtained from the fracture surfaces of 0.1 wt.- % PC/MWNT composite films. The images suggest two primary toughening mechanisms-nanotube bridging and PC-CNT interactions. The nanotubes retards crack propagation through the CNT-breaking (see Figure 6.9(c)) or CNT-bridging (see Figure 6.9(b, d)). CNT-breaking and CNT-bridging indicate the energy-dissipating mechanism.
81 In addition, CNT pull-out of PC matrix was also found on the fracture surface (Figure 6.10). CNT pull-out indicates that microvoids in PC matrix undermined the interfacial bonding between MWNTs and PC. In this case, we want PC-coated MWNTs to be broken from the surface rather than pulled out from the PC matrix.
(a) (b)
(c) (d)
Nanotube bridging
Figure 6.9 SEM images of crack tip and fracture surface of failed 0.1 wt.-% PC/MWNT films showing the toughening effects of nanotubes: (c) Crack surface showing nanotube breaking of PC matrix; (b) (d) Crack tip showing nanotube bridging.
82
(a) (b)
Figure 6.10 SEM images of crack tip and fracture surface of failed 0.15 wt.-% PC/MWNT films showing the MWNT pull-out of the PC matrix.
83
CHAPTER 7 CONCLUSIONS AND PROPOSED WORK
7.1 Conclusions and Contributions
The PC/CNT composite films were fabricated using solution casting and were characterized for optical transparency, tensile properties, and microstructure. The design of experiments (DOE) technique showed that high solution viscosity, high temperature, and small film thickness minimize solvent-induced crystallization, which directly affects transparency. CNT dispersion is another issue that affects transparency. Upon visual assessment, MWNT loading up to 0.1 wt.% yielded optimized, optically transparent composite films. The analysis from the UV-VIS spectrophotometer indicates that the PC crystallization has larger influence than MWNT concentration. In the SEM images, the microvoids were observed in opaque PC, which leads to more light refraction and affects the optical transparency. 0.15 wt.-% PC/MWNT decreases the %T by half as compared to 0.05 and 0.1 wt.-% PC/MWNT, which indicates significant MWNT aggregation at 0.15 wt.-% MWNT loading. 0.1 wt.% PC/MWNT films showed 13.5% increases in Young’s modulus as compared to neat PC films. The SEM study confirmed the toughening mechanisms of CNTs, evidenced by PC wetting on CNT surfaces, nanotube bridging, and nanotube breaking. The microstructures of the fabricated composite films were characterized using optical and scanning electron microscopes (SEM). Even at 0.1 wt.-% MWNT loading, the nanotubes were observed to form a uniform, entangled network structure throughout the composite, which is the key factor that provides toughening of the polymer matrix. In addition, the crack bridging phenomena by the nanofibrils were observed. The contribution of this research can be generalized as follows:
84 1. Developed and optimized a hot solution casting method to fabricate PC/MWNT composite films using the DOE method, and developed a quantitative model for predicting the transparency based on the dominant processing parameters (temperature, concentration, and thickness). 2. Determined the optimal untreated MWNT loading (0.1 wt.-%) for maximized transparency and mechanical properties using hot solution casting method. 3. Identified the relationship between the optical transparency and the MWNT loading by using UV-VIS method, and separated the effects of crystallization and CNT aggregation on optical transparency by combining the UV-VIS, DSC, and SEM techniques.
7.2 Future Work
CNT dispersion with pure mixing of PC and CNT is limited. There are several ways are considered to improve the optical and mechanical properties: 1. Add the factor of CNT concentration in DOE analysis so as to find the relationship between CNT concentration and the degree of crystallinity. 2. Use functionalized CNTs to improve dispersion and mechanical properties, and therefore increase the optimal CNT loading. 3. Align CNTs in the through-thickness direction using magnetic or electric field to improve transparency. 4. Fabricate PC/CNT composite films using extrusion for improved CNT dispersion and potential scale-up production. 5. Understand the underlying physics behind the existence of optimal CNT loading and further investigate the effects of CNT dispersion on transparency and mechanical properties.
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91
BIOGRAPHICAL SKETCH
Yuan-Chen Chin was born in June of 1976, is currently a master’s student in Industrial Engineering at FAMU-FSU College of Engineering and expecting graduation in April 2007. He received her Bachelor’s of Engineering in Industrial Engineering in June 1999 from Tung-Hai University in Taichung, Taiwan. His research interests include nanotube-reinforced polymer based composites and optical and mechanical properties and applications. He is looking forward to pursue a career in the field of Industrial Engineering and nanocomposite technologies. For more information, please contact to [email protected].
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