Processing and Characterization of Polymer Based Nanocomposites. Master of Science. Rick A. Pollard

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2 Processing and Characterization of Polymer Based Nanocomposites A thesis submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science in the Department of Materials Science and Engineering of the College of Engineering and Applied Science 2012 by Rick A. Pollard B.S., Shawnee State University, 2009 Committee Chair: Stephen J. Clarson, Ph.D.

3 Abstract The Kentera polymer treatment system was investigated as a potential candidate for improving the properties of polymer nanocomposites. Polymer treatments have the ability to improve dispersion and interfacial bonding of fillers within a polymer matrix. This investigation focused on one particular Kentera polymer and its ability to compatibilize carbon nanotubes with polycarbonate. In this study untreated carbon nanotubes, CNT s, and non-wrapping polymer treated CNT s were mixed with polycarbonate, PC, in order to produce polymer nanocomposites. The nanocomposites were created by diluting 15 wt.% masterbatches into virgin PC, and then compounding them using a Killion single screw extruder. These compounded samples were then pelletized for further processing. Thermal and rheological analysis was performed on the pelletized samples. Differential scanning calorimetry was used to perform the thermal analysis. The results showed the glass transition temperature of the PC was unaffected by the addition of untreated or treated CNT s. The rheological behavior of these nanocomposites was characterized using a capillary rheometer. The results showed that the polymer treatment of the CNT s helped to plasticize the nanocomposites. The rheological data shows a significant decrease in viscosity between the treated and untreated nanocomposites. Also, depending on how the materials are processed for end-use products, such as compression or injection molding, a significant change in rheological behavior is observed between the 0.5 wt.% and 2 wt.% nanocomposites. The pelletized samples were then injection molded into ASTM standard tensile, flex, and impact bars, which were tested according to ASTM standards. The addition of CNT s seemed to ii

4 only improve the flexural properties of the PC. Also, a shear rate study was conducted using three different injection velocities. This investigated how increasing the shear rate during molding can affect the mechanical properties of the polymer nanocomposites. The results showed that increasing the shear rate did not significantly affect the mechanical properties of either the untreated or treated nanocomposites. Overall, the Kentera polymer treatment supplied minimal mechanical property improvement to the nanocomposites, but did provide very interesting rheological data. This Kentera polymer treatment makes PC-CNT nanocomposites easier to process by lowering the viscosity of the PC matrix. iii

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6 Acknowledgements I would like to thank Dr. Stephen J. Clarson, Dr. Jude O. Iroh, and Dr. Larry M. Miller for serving on my thesis review committee. I would also like to thank Shawnee State University for allowing me to use their Plastics Processing and Testing Laboratories throughout my work. Finally, I would like to personally thank Dr. Jill Kunzelman (PolyOne Corporation), Dr. Srinagesh Potluri (Zyvex Technologies), Dean Marinchek (Shawnee State University) and Dan Prager (Shawnee State University) for their kind help throughout this project. v

7 Table of Contents Section Title Page 1.0 Introduction Polycarbonate Carbon Nanotubes Polymer Nanocomposites Polymer Treatments Extrusion Injection Molding Experimental Materials Kentera Polymer Treatment Sample Preparation Mechanical Properties Capillary Rheology Shear Rate Study Differential Scanning Calorimetry Optical Microscopy Results Weight % Loading Effects on Tensile Properties Weight % Loading Effects on Flexural Properties Capillary Rheology 35 vi

8 3.4 Shear Rate Calculations Shear Rate Effects on the Mechanical Properties Differential Scanning Calorimetry Optical Microscopy Discussion Weight % Loading Effects on Mechanical Properties Comparing Treated and Untreated Mechanical Properties Processing Effects on Rheology Shear Rate Effects on Mechanical Properties Weight % Loading Effects on Glass Transition Temperature Optical Microscopy and Tensile Breaking Strain Summary and Conclusions Future Work 66 Bibliography 68 Appendix I Melt Flow Rate Data 70 Appendix II Percent of Treatment Compared to 70 Overall Wt.% Loading Appendix III Tensile Testing Data 71 Appendix IV Flexural Testing Data 75 vii

9 List of Figures Figure No. Title Page Figure 1.1 Chemical reaction of bisphenol A and phosgene to produce 3 polycarbonate. Figure 1.2 Chemical reaction of bisphenol A and diphenyl carbonate to 3 produce polycarbontate. Figure 1.3 Structure of a single-wall carbon nanotube, SWNT. 4 Figure 1.4 Structure of a multi-wall carbon nanotube, MWNT. 4 Figure 1.5 Wrapping polymer treatment attached to a CNT. 7 Figure 1.6 Non-wrapping conjugated polymer treatment attached to a 7 CNT. Figure 1.7 Schematic representation of an extruder. 9 Figure 1.8 Image of the two strand capillary die used during 10 compounding. Figure 1.9 Typical Injection Molding Cycle (a = Fill, b = Pack, c = 12 Cooling, d = Ejection). Figure 2.1 The chemical structure of the Kentera polymer treatment 14 system. Figure 2.2 The Kentera polymer treatment applied to the CNT s for 15 this investigation. Figure 2.3 Image of injection molded mechanical test bars. 16 Figure 2.4 Tensile testing of PC-CNT nanocomposite molded samples. 20 Figure 2.5 Flexural testing of PC-CNT nanocomposite molded 21 samples. Figure 2.6 Schematic of a Capillary Rheometer. 24 Figure 3.1 Tensile Modulus vs. Wt.% Loading of CNT s. 29 Figure 3.2 Tensile Stress at Yield vs. Wt.% Loading of CNT s. 30 Figure 3.3 Tensile Breaking Strain vs. Wt.% Loading of CNT s. 31 Figure 3.4 Flexural Modulus vs. Wt.% Loading of CNT s. 33 Figure 3.5 Flexural Maximum Stress vs. Wt.% Loading of CNT s. 34 viii

10 Figure 3.6 Viscosity vs. Shear Rate Curves for the PC and PC-CNT 36 Nanocomposites at 580 F. (T) = Treated. (UT) = Untreated. Figure 3.7 Tensile Modulus of the 0.5 wt.% Nanocomposites. 40 Figure 3.8 Tensile Modulus of the 2 wt.% Nanocomposites. 40 Figure 3.9 Tensile Stress at Yield of the 0.5 wt.% Nanocomposites. 41 Figure 3.10 Tensile Stress at Yield of the 2 wt.% Nanocomposites. 41 Figure 3.11 Tensile Break Strain of the 0.5 wt.% Nanocomposites. 42 Figure 3.12 Tensile Break Strain of the 2 wt.% Nanocomposites. 42 Figure 3.13 Flexural Modulus of the Nanocomposites. 43 Figure 3.14 Flexural Maximum Stress of the Nanocomposites. 43 Figure 3.15 DSC plot for the virgin PC. 44 Figure 3.16 DSC plot for the reprocessed PC. 45 Figure 3.17 DSC plot for the untreated 0.5 wt.% nanocomposite. 46 Figure 3.18 DSC plot for the treated 0.5 wt.% nanocomposite. 47 Figure 3.19 DSC plot for the untreated 2 wt.% nanocomposite. 48 Figure 3.20 DSC plot for the treated 2 wt.% nanocomposite. 49 Figure 3.21 DSC plot for the untreated 4 wt.% nanocomposite. 50 Figure 3.22 DSC plot for the treated 4 wt.% nanocomposite. 51 Figure 3.23 Glass Transition Temperature vs. Wt.% Loading of CNT s. 52 Figure 3.24 Figure 3.25 Figure 3.26 Figure 3.27 Optical microscopy of fractured tensile surfaces for virgin PC. Optical microscopy of fractured tensile surfaces for reprocessed PC. Optical microscopy of fractured tensile surfaces for untreated 0.5 wt.% nanocomposite. Optical microscopy of fractured tensile surfaces for treated 0.5 wt.% nanocomposite ix

11 Figure 3.28 Optical microscopy of fractured tensile surfaces for 56 untreated 2 wt.% nanocomposite. Figure 3.29 Optical microscopy of fractured tensile surfaces for treated 56 2 wt.% nanocomposite. Figure A.1 Tensile testing data for the virgin PC, Lexan Figure A.2 Tensile testing data for the reprocessed PC, Lexan Figure A.3 Tensile testing data for the untreated 0.5 wt.% PC-CNT 72 nanocomposites. Figure A.4 Tensile testing data for the treated 0.5 wt.% PC-CNT 72 nanocomposites. Figure A.5 Tensile testing data for the untreated 2 wt.% PC-CNT 73 nanocomposites. Figure A.6 Tensile testing data for the treated 2 wt.% PC-CNT 73 nanocomposites. Figure A.7 Tensile testing data for the untreated 4 wt.% PC-CNT 74 nanocomposites. Figure A.8 Tensile testing data for the treated 4 wt.% PC-CNT 74 nanocomposites. Figure A.9 Flexural testing data for the virgin PC, Lexan Figure A.10 Flexural testing data for the reprocessed PC, Lexan Figure A.11 Figure A.12 Figure A.13 Figure A.14 Figure A.15 Figure A.16 Flexural testing data for the untreated 0.5 wt.% PC-CNT nanocomposites. Flexural testing data for the treated 0.5 wt.% PC-CNT nanocomposites. Flexural testing data for the untreated 2 wt.% PC-CNT nanocomposites. Flexural testing data for the treated 2 wt.% PC-CNT nanocomposites. Flexural testing data for the untreated 4 wt.% PC-CNT nanocomposites. Flexural testing data for the treated 4 wt.% PC-CNT nanocomposites x

12 List of Tables Table No. Title Page Table 2.1 Extruder processing conditions for the compounding of the nanocomposites. 17 Table 2.2 Injection molding processing conditions for producing the 17 sample test bars. Table 2.3 Variables and Definitions used in Equations 2.11 and Table 3.1 Constant Variables used for Equations 2.11 and Table 3.2 Calculated Shear Rates 37 xi

13 1.0 Introduction Developing new polymer based materials in a research laboratory is an amazing feat, but individuals often forget that this is just the beginning for these materials. These materials need to be scaled up, develop ideas for end-use products, and develop a process for manufacturing this product. For polymer or plastic materials, this can be accomplished through polymer processing techniques. Examples of these processing techniques include: extrusion, injection molding, compressing molding, and blow molding. Polymer nanocomposites are a growing research field in today s society. Scientists are enhancing the properties of traditional plastic resins by adding filler materials to maximize their potential. Polymer nanocomposites consist of fillers that are dimensionally on the nano-scale, and can range in different shapes from particles to fibers. Recently, carbon nanotubes, CNT s have been the filler of choice by many laboratories. The addition of any type of filler can potentially alter the mechanical, rheological, and thermal properties of the base resin. Altering the rheological and thermal properties of the matrix material can affect processing. The addition of fillers may have an effect on the viscosity of the polymer, which will cause the polymer to flow differently during processing. Also, the fillers can potentially affect the polymer s glass transition temperature, T g. This can affect the processing temperatures and the environments in which the polymer nanocomposites can be exposed. Other challenges are getting fillers to disperse and bond to the polymer matrix (1) (2) (3). If you do not get good dispersion throughout the polymer matrix, then the mechanical properties of the polymer nanocomposites will be inconsistent. Inconsistency in mechanical properties will have major affects on end-use products, causing them to fail randomly. Depending on the 1

14 application, these random mechanical failures could create a dangerous environment for the consumer. To help with these issues, polymer treatments can be applied to modify the filler s surface. The idea of polymer treatments is to reduce the attraction between individual filler particles, and improve the interaction between the filler and the polymer matrix. This will improve both dispersion and bonding between the filler and polymer matrix, which can potentially improve the mechanical properties. This investigation consists of processing and characterizing polycarbonate nanocomposites that contain either polymer treated multi-walled carbon nanotubes, MWNT s, or untreated MWNT s. The treated MWNT s were treated using a non-wrapping polymer treatment system. The original efforts of this polymer treatment system were to make it compatible with thermosetting resins (4). Recently, the compatibility of this polymer treatment system for MWNT s with thermoplastic matrices was investigated by Kunzelman and coworkers. Their investigation consisted of compression molding the treated nanocomposite samples followed by characterization (4). To the best of my knowledge, the CNT treatment that I report here has only been used, to date, for lower shear rate processing techniques such as compression molding. Thus, choosing a commercially important thermoplastic, PC, and determining the effects of the CNT treatment on higher shear rate processing techniques, such as injection molding, and properties is both new and novel. This work consists of four main topics: characterizing the rheological properties of the nanocomposites, characterizing the thermal properties of the nanocomposites, characterizing the mechanical properties of the nanocomposites, and an injection molding shear rate study. 2

15 1.1 Polycarbonate Polycarbonate, PC, is an engineering thermoplastics resin which is used throughout the polymer field. PC is synthesized by condensation polymerization of bispehnol-a, BPA, and phosgene in methylene chloride-water solution (5). Figure 1.1 shows the condensation reaction between BPA and phosgene. PC can also be created by melt transesterification of BPA and diphenyl carbonate (6). This reaction can be seen in Figure 1.2. PC is an amorphous polymer and is naturally colorless, which allows it to be used for transparent applications. Some physical properties of PC include: high heat resistance, stiffness, strength, and dimensional stability. PC is used for a variety of applications including CD s, cell phones, outdoor signs, helmets, and face shields (6). PC products are manufactured by conventional processing techniques (5). These include: extrusion, injection molding, compression molding, and rotational molding (6). Figure 1.1 Chemical reaction of bisphenol A and phosgene to produce polycarbonate. Image from reference (5). Figure 1.2 Chemical reaction of bisphenol A and diphenyl carbonate to produce polycarbontate. Image from reference (5). 3

16 1.2 Carbon Nanotubes Carbon nanotubes, CNT s, are produced from carbon atoms which are arranged in a similar structure to graphene sheet. The difference is instead of being a flat sheet, the CNT s resemble a cylinder type structure that is capped at both ends (7). There are two types of CNT s, single-wall carbon nanotubes, SWNT, and multi-wall carbon nanotubes, MWNT. MWNT s contain several single-wall CNT s nested together to form multiple layers (7). MWNT s typically possess an outside diameter of 2-20 nm, inside diameter of 1-3 nm, and a length of μm (7). Figure 1.3 and Figure 1.4 show the structures of a SWNT and a MWNT, respectively. CNT s possess extremely high tensile modulus, tensile strength, thermal, and electrical properties. CNT s have a tensile strength 100 times that of steel, while being one-sixth the weight of steel. CNT s are thermally stable up to 2800 C in vacuum, and produce an electriccurrent-carrying capacity about 1000 times higher than copper wire (8). CNT s can be produced through arc evaporation, laser ablation, pryolysis, PECVD, and electrochemical methods (7). Figure 1.3 Structure of a SWNT. From Reference (9). Figure 1.4 Structure of a MWNT. From Reference (9). 4

17 1.3 Polymer Nanocomposites Polymer nanocomposites are an exciting and highly researched group of materials. These materials contain nano-sized particles embedded within a polymer matrix. The particles must have at least one dimension in the nanometer scale. Platelets and carbon nanotubes are two types of nanoparticles used for producing nanocomposites (8). The nanoparticle selected for this project was carbon nanotubes, CNT s. CNT s contain an extremely high strength to weight ratio, which allows them to produce strong but light weight nanocomposites (10). For CNT s to improve the mechanical properties of the polymer matrix two things need to occur: the CNT s need to be dispersed homogeneously throughout the matrix material, and there needs to be good interfacial bonding between the CNT s and the polymer matrix material (1) (2) (3). Strong bonding at the interface is required to transfer the load from the polymer material to the reinforcing MWNT (11) (12). This can be achieved if the surface energy of the filler, CNT, exceeds the cohesive energy of the polymer matrix (13). Weak interfacial bonding will result in de-lamination giving instant mechanical failure. Weak interfacial bonding is caused by a non-wetting phenomenon between the CNT and polymer matrix, which is caused by the lack of functional groups on the CNT s (14). Getting the MWNT s to disperse evenly is a significant processing challenge. The reason for this problem is that the CNT s are received as entangled aggregates before extrusion compounding, and the individual CNT s are bound together by van der Waals forces (3). This is a problem because the nanotubes want to stay grouped together, instead of separating and dispersing throughout the polymer matrix. Aggregates inside the polymer matrix can initiate cracks and weaken the nanocomposite (15). If separation and homogeneous dispersion of the CNT s does not occur, then there can be major issues with end-use products. 5

18 Polymer reinforced nanocomposites can be used for many applications such as interior and exterior automotive accessories, structural components for electronic devices, and food packaging. Thermal and electrical applications are other possibilities for CNT nanocomposites. As stated earlier, the issues hindering these applications are the dispersion and interfacial bonding of nanoparticles within a polymer matrix (8). 1.4 Polymer Treatments The addition of treatments to the surface of the CNT s are being researched and are intended to improve dispersion during processing, such as injection molding. These treatments consist of functionalizing the CNT by attaching polymeric chains to its surface (3). There are two styles of polymer treatments for CNT s: wrapping and non-wrapping (10). Polymer wrapping means the treating polymer completely envelops the CNT surface. Non-wrapping polymer treatments are where the polymer backbone extends along the length of the CNT without any portion of the polymer treatment covering more than half of the diameter of the CNT. Non-wrapping treatments are produced from functional conjugated polymers. Nonwrapping polymer treatments contain a rigid backbone, which results in parallel π-stacking phenomena between the polymer and the CNT (10). Figure 1.5 and 1.6 show examples of wrapping and non-wrapping polymer treatments, respectively. 6

19 Figure 1.5 Wrapping polymer treatment attached to a CNT. From reference (10). Figure 1.6 Non-wrapping conjugated polymer treatment attached to a CNT. From Reference (10). 7

20 1.5 Extrusion Extrusion is one of the most common methods of polymer processing. An extruder consists of a screw inside a barrel, which is used to heat and convey plastic granules. The granules are fed into a hopper and conveyed through different sections of the screw. The depth of the screw decreases along the length of the screw, which is used to compact the plastic material (16). An extruder screw consists of three different zones: feed, compression, and metering. The polymer melt then passes through a die, which is used to produce an extrudate with a desired shape (16). Figure 1.7 shows an extruder system. Extruders can be used for multiple processing methods such as profile production, blown film, and granule compounding (16). For this project, granule compounding was used. This process uses a capillary die to produce continuous strands. These strands are pulled by rollers into a pelletizer, which produces granules (16). Here, the extruder was used to help mix the CNT s within the PC, and produce nanocomposite pellets for the injection molding machine. Figure 1.8 shows an image of the two strand capillary die used during compounding of the nanocomposite samples. 8

21 Figure 1.7 Schematic representation of an extruder. From reference (16). 9

22 Figure 1.8 Image of the two strand capillary die used during compounding. 10

23 1.6 Injection Molding Injection Molding is a similar process to extrusion. An injection molding machine also consists of a screw which is used to melt and convey the plastic pellets. The difference is injection molding is an intermittent process, unlike the continuous process of extrusion. The plastic melt is conveyed forward through a check valve until a certain amount is collected, this is called the shot size. This material is then forced through a nozzle and into a closed mold. Once the material has time to cool the mold opens and ejects the part, and the cycle is repeated. Injection molding is a very versatile process and can be used to produce a wide range of end-use products at very high production rates (16). Figure 1.9 illustrates the typical cycle for an injection molding machine. 11

24 Figure 1.9 Typical Injection Molding Cycle (a = Fill, b = Pack, c = Cooling, d = Ejection). From Reference (16). 12

25 2.0 Experimental 2.1 Materials The polycarbonate, PC, matrix material, Lexan 2240, was supplied by SABIC Innovative Plastics. Two masterbatches consisting of 15 wt.% MWNT and a lower viscosity polycarbonate, Lexan OQ1022, were compounded by PolyOne Corporation. Melt flow data of Lexan 2240 and Lexan OQ1022 is provided in Appendix I. One masterbatch contained the untreated MWNT s and the other contained the polymer treated MWNT s. The MWNT s used in the masterbatch had an average of 5 15 walls, an outer mean diameter of nm and a length of μm, and were supplied by Arkema. The treatment, Kentera, on the MWNT was applied by Zyvex Technologies. The nanocomposite mixtures were made by diluting the masterbatches with varying amounts of Lexan For both the untreated and treated masterbatches concentrations from 0.25 wt.% to 4 wt.% were pre-mixed by hand in 5-10 lb lots. 2.2 Kentera Polymer Treatment The non-wrapping polymer treatment used is a system from the family of polyphenylene ethynylene, PPE. The functionalization approach of Kentera is non-covalent, which means it attaches by van der Waals forces to the CNT (4). Figure 2.1 shows the chemical structure of the Kentera polymer treatment system that was applied to the CNT s. The rigid backbone of Kentera is responsible for adhering to the CNT. R 1 and R 2 represent side chains extending off the backbone of the structure. These side chains provide functional groups, which can be tailored to improve compatibility and adhesion between the CNT s and the polymer matrix (4). Figure 2.2 shows the structure of the Kentera polymer treatment selected for this research project. Appendix II discusses the percentage of polymer treatment and the percentage of CNT s that make up the overall wt.% loading of the treated nanocomposites. 13

26 Figure 2.1 The chemical structure of the Kentera polymer treatment system. From reference (4). 14

27 Figure 2.2 The Kentera polymer treatment applied to the CNT s for this investigation. 15

28 2.3 Sample Preparation These pre-mixed composite concentrations were dried in a dryer at 200 F for a minimum of 10 hours. The concentrations were let down using a 1.5 Killion KN-150 single screw extruder and pelletized using a Cincinnati Milacron LPQ912 pelletizer. Lexan 2240 was also extruded and pelletized using the same techniques, which was used as the control for this investigation. The reason for the control, or reprocessed PC, is to ensure the PC was exposed to the same thermal history as the PC-CNT nanocomposites. The nanocomposite materials were again placed in a dryer at 200 F for a minimum of 10 hours. Tensile, flexural, and impact bars were injection molded using a 100 ton Cincinnati Milacron ACT machine. The tool steel mold was built in house and it contained three cavities (one for each type of test bar). Figure 2.3 shows an image of the molded runner system and test bars. The extruder and injection molding processing conditions are presented in Table 2.1 and Table 2.2, respectively. Figure 2.3 Image of injection molded mechanical test bars. 16

29 Table 2.1 Extruder processing conditions for the compounding of the nanocomposites. Table 2.2 Injection molding processing conditions for producing the sample test bars. 17

30 2.4 Mechanical Properties An Instron 4204 with Bluehill software was used to conduct Tensile and Flexural testing on the virgin PC, reprocessed PC, and PC-CNT nanocomposite samples. The tensile and flexural mechanical properties were measured in accordance with ASTM D638 and ASTM D790, respectively. The tensile modulus, tensile stress at yield, tensile breaking strain, flexural modulus, and flexural max stress were all investigated throughout this project. The Bluehill software produced stress-strain curves to detect the tensile modulus, tensile stress at yield, and tensile breaking strain. These detected values were then plotted against wt.% loading of CNT s in the polymer nanocomposites. The normal stress, ζ, is the ratio of the applied load, F, to the original cross sectional area, A, of the test specimen. The normal strain, ε, refers to the change in length, ΔL, divided by the original length, L, of the test specimen (17). Equation 2.1 and 2.2 represent the normal stress and normal strain, respectively. The tensile or elastic modulus, E, is defined as the slope of the stress-strain curve, and is represented in Equation 2.3. The tensile stress at yield is the first point on the stress-strain curve where an increase in strain occurs without an increasing stress value. The tensile breaking strain is the maximum strain the material can withstand without failing (17). Equation 2.1 Equation 2.2 Equation

31 The Bluehill software was also used to detect flexural modulus and flexural max stress values from stress-strain curves. Again, these values were then plotted against wt.% loading of CNT s. Equation 2.4 and 2.5 represent the flexural stress and flexural strain, which are used to calculate the flexural modulus. The flexural modulus calculation is shown in Equation 2.6. The variables used in these equations are defined below. The flexural modulus is the initial slope of the stress-strain curve. The flexural max stress is the stress that was detected at a flexural strain value of 5% (17). Figure 2.4 and Figure 2.5 show images of tensile and flexural testing of the PC-CNT nanocomposites samples, respectively. Equation 2.4 Equation 2.5 Equation 2.6 δ = Deflection W = Applied Load b = Width of Test Specimen d = Height of Test Specimen L = Length Between Beams 19

32 Figure 2.4 Tensile testing of PC-CNT nanocomposite molded samples. 20

33 Figure 2.5 Flexural testing of PC-CNT nanocomposite molded samples. 21

34 2.5 Capillary Rheology A Dynisco LCR7000 capillary rheometer was used to determine how the apparent viscosity, η, was related to the shear rate,. A capillary rheometer uses a piston to force melted polymer materials through a die of a specified size, at known velocities, and at a set temperature. Then the pressure is recorded using a pressure transducer, which is located above the capillary die (18). Knowing the barrel dimensions and piston velocity the volumetric flow rate,, can be calculated. The volumetric flow rate is then used to calculate the shear rate and shear stress, η. The shear rate and shear stress are then used to calculate the apparent viscosity (18). The LCR7000 capillary rheometer uses software to perform these calculations automatically. Equations show the relationships between these different variables. Figure 2.6 shows an image of a capillary rheometer. The piston velocity can be altered to produce different shear rates during testing. The shear rate range used for this experiment was 10 1/sec 10,000 1/sec. The samples used were the virgin PC, reprocessed PC, 0.5 wt.% nanocomposites, and the 2 wt.% nanocomposites. The temperature used for testing was 580 F, which was the nozzle temperature of the injection molding machine. This created a rheological representation of the polymer nanocomposite samples during molding. 22

35 Equation 2.7 Equation 2.8 Equation 2.9 Equation 2.10 P = Pressure R b = Radius of Barrel L c = Length of Capillary Die = Velocity of the Piston R c = Radius of Capillary Die 23

36 Figure 2.6 Schematic of a Capillary Rheometer. Image from reference (18). 24

37 2.6 Shear Rate Study A shear rate study was conducted to compare the effects of the untreated CNT nanocomposites to the polymer treated CNT nanocomposites. This investigation consisted of injection molding the materials at three different injection velocities to create three different shear rates during processing. This study was conducted using the 0.5 wt.% and 2 wt.% concentrations. The objective was to see the effects that shear rate had on the mechanical and rheological properties of these PC-CNT nanocomposites. The tensile and flexural testing was performed according to ASTM D638 and ASTM D790, respectively. The shear rate at the gate of the runner system was calculated, which allowed the viscosity of the material to be determined during processing. The gate location was selected for the calculations, because it contains the maximum shear rate that the nanocomposites will be exposed to during molding. Equations 2.11 and 2.12 were used to calculate the shear rates during molding. Table 2.3 lists and defines the variables used in the equations. Equation 2.11: Equation 2.12: 25

38 Variables Q I.V. I.D. Definitions Volumetric Flow Rate Injection Velocity Inside Diameter of Barrel Shear Rate at Gate A T H Area Inside Barrel Thickness of Gate Height of Gate # Number of Gate Locations Table 2.3 Variables and Definitions used in Equations 2.11 and

39 2.7 Differential Scanning Calorimetry Differential Scanning Calorimetry, DSC, was performed on the pelletized polymer nanocomposite samples, using a TA Instruments Q20 V24.4 Build 116. A DSC compares the amount of energy required to maintain a temperature difference of zero between a test sample and a reference specimen (19). This information is then plotted by software, and the glass transition temperature, T g, of the nanocomposites can be detected. The samples tested during this experiment were the virgin PC, reprocessed PC, 0.5 wt.%, 2 wt.%, and 4 wt.% concentrations. The tested samples ranged from mg, and were heated at a rate of 10 C/min. The objective was to see the effects untreated and treated MWNT s had on the T g of PC. The T g values detected by the DSC were plotted against the wt.% loading of CNT s. 2.8 Optical Microscopy Optical microscopy was performed on the fractured surfaces of the tensile test bars. The optical microscopy images were then related to the tensile testing data. The optical microscope used for this investigation was an Olympus CX41. A Moticam 1000 digital camera with Motic Images Plus 2.0 software was used to photograph the PC-CNT nanocomposite test bar samples. 27

40 3.0 Results 3.1 Weight % Loading Effects on Tensile Properties The tensile properties: tensile modulus, stress at yield, and breaking strain were plotted against wt.% loading of CNT s. The tensile modulus plot, Figure 3.1, shows an initial drop in modulus from the virgin PC to the reprocessed PC. Both the untreated and treated PC-CNT nanocomposites possessed modulus values lower than the virgin PC for all wt.% loadings. The tensile modulus values continued to drop from wt.% loadings and began to increase again from 1 4 wt.% loadings. The untreated and treated nanocomposites behaved similar for increasing wt.% loadings of CNT s. The stress at yield plot, Figure 3.2, shows a drop between the virgin PC and reprocessed PC. The untreated and treated PC-CNT nanocomposites showed improvements in stress at yield compared to the virgin PC. The treated PC-CNT nanocomposites increased rapidly at 0.25 wt.% loading and flattened out from wt.% loadings, and then drastically dropped at 4 wt.% loading. The untreated PC-CNT nanocomposites continued to drop at 0.25 wt.% loading, but increased gradually from wt.% loadings. For the treated nanocomposites, increasing wt.% loadings of CNT s had less, or negative, effect on the PC. The untreated samples behaved totally opposite as increasing wt.% loadings of CNT s had a positive effect on the PC. The breaking strain plot, Figure 3.3, shows significant drops in breaking strain between the PC and the nanocomposites. Both the treated and untreated nanocomposites showed a negative correlation as wt.% loading of CNT s was increased. The untreated 0.25 wt.% loading showed a slight increase in breaking strain compared to the reprocessed PC, but the breaking strain value was still lower than the virgin PC. 28

41 Modulus, psi Modulus vs. Weight % Loading PC Virgin PC Reprocessed 0.25% 0.5% 1% 2% 4% Wt. % Loading Figure 3.1 Tensile Modulus vs. Wt.% Loading of CNT s. 29

42 Stress at Yield,psi 7000 Stress at Yield vs. Weight % Loading PC Virgin PC Reprocessed 0.25% 0.5% 1% 2% 4% Wt. % Loading Figure 3.2 Tensile Stress at Yield vs. Wt.% Loading of CNT s. 30

43 Breaking Strain, % 250 Breaking Strain vs. Weight % Loading PC Virgin PC Reprocessed 0.25% 0.5% 1% 2% 4% Wt. % Loading Figure 3.3 Tensile Breaking Strain vs. Wt.% Loading of CNT s. 31

44 3.2 Weight % Loading Effects on Flexural Properties The flexural properties: flexural modulus and flexural maximum stress were plotted against wt. % loading of CNT s. The flexural modulus plot, Figure 3.4, shows that the flexural modulus of PC was improved by the addition of CNT s. Both the treated and untreated PC- CNT s nanocomposites showed very similar plots. The treated and untreated nanocomposites flexural modulus gradually increased from 0.25 wt.% samples to the 4 wt.% samples. The flexural maximum stress plot, Figure 3.5, showed a similar result as the flexural modulus. The flexural maximum stress of PC was improved by the addition of CNT s for both the treated and untreated samples. The only difference was between the treated 4 wt.% nanocomposite and the untreated 4 wt.% nanocomposite. The untreated nanocomposites flexural maximum stress continued to increase, while the treated nanocomposites flexural maximum stress dropped drastically. 32

45 Flexural Modulus (ksi) Flexural Modulus vs. Wt. % Loading Virgin PC Reprocessed PC 0.25% 0.5% 1% 2% 4% Wt. % Loading Figure 3.4 Flexural Modulus vs. Wt.% Loading of CNT s. 33

46 Flexural Maximum Stress (ksi) Flexural Maximum Stress vs. Wt. % Loading Virgin PC Reprocessed PC 0.25% 0.5% 1% 2% 4% Wt. % Loading Figure 3.5 Flexural Maximum Stress vs. Wt.% Loading of CNT s. 34

47 3.3 Capillary Rheology The viscosity, η, of the virgin PC, reprocessed PC, and the PC-CNT nanocomposites is plotted against the shear rate,, in Figure 3.6. The test measurement temperature for this plot is 580 F, which is the same temperature as the nozzle temperature chosen for injection molding of the nanocomposites. Figure 3.6 shows that there is a decrease in viscosity, η, by one order of magnitude for the untreated samples, and by two orders of magnitude for the treated samples. The graph also shows a drop in viscosity from the virgin PC to the reprocessed PC, which is common and is due to thermal history. Both the treated and untreated 0.5 wt.% samples show a more Newtonian plot compared to the 2 wt.% samples. This results in a transition where the 2 wt.% plots crossover the 0.5 wt.% plots, which corresponds to the 2 wt.% samples having lower viscosities at higher shear rates. The shear rate study, Section 3.4, will show that all processing during this investigation was completed at shear rates that are higher than this transition area. 3.4 Shear Rate Calculations Table 3.1 shows the constant variables that were used for calculating the shear rates,, at different injection velocities. The injection velocities used during molding were 0.7, 1.2, and 1.7 inches/sec. The calculated shear rates from these injection velocities are present in Table 3.2. Using the calculated shear rates from Table 3.2 and the plot from Figure 3.6, the viscosity at the gate was determined for the PC-CNT nanocomposites during processing. 35

48 Figure 3.6 Viscosity vs. Shear Rate Curves for the PC and PC-CNT Nanocomposites at 580 F. (T) = Treated. (UT) = Untreated. 36

49 Table 3.1 Constant Variables used for Equations 2.11 and 2.12 Table 3.2 Calculated Shear Rates 37

50 3.5 Shear Rate Effects on the Mechanical Properties The mechanical properties: tensile modulus, stress at yield, breaking strain, flexural modulus, and maximum flexural stress are plotted in Figures , respectively. These mechanical properties are plotted against the shear rates calculated in Table 3.2. Each figure shows the data and the standard deviation error bars of the virgin PC, the treated samples, and the untreated samples. Figure 3.7 and 3.8 show the tensile modulus results of the 0.5 wt.% and 2 wt.% samples, respectively. The tensile modulus plots are linear, which shows that increasing the shear rate does not significantly affect the modulus of the 0.5 wt.% and 2 wt.% nanocomposites. Figures 3.9 and 3.10 represent the stress at yield for the virgin PC and the nanocomposite samples. The treated samples resulted in higher stress at yield values compared to those of the untreated samples. The stress at yield graphs are again mostly linear, except for the untreated 0.5 wt.% nanocomposite from Figure 3.9. The untreated 0.5 wt.% nanocomposite shows fluctuation as the shear rate is increased. Figures 3.11 and 3.12 show the breaking strain for the virgin PC, 0.5 wt.% and 2 wt.% nanocomposites. The breaking strain plots are linear, except for the untreated 0.5 wt.% sample. Both the 0.5 wt.% and 2 wt.% comparisons show that incorporating untreated CNT s resulted in a higher breaking strain, while the treated samples became less ductile. The untreated samples seem to be tougher materials, but they also contained significantly larger standard deviations compared to the treated samples. Figures 3.13 and 3.14 represent the flexural testing of the virgin PC and nanocomposites. Figure 3.13 shows the flexural modulus and Figure 3.14 represents the maximum flexural stress of the same samples. The flexural properties were similar for both the untreated and treated 38

51 samples of equal wt.%. The flexural properties do not seem to correlate with increasing shear rate, as again the plots are linear. 39

52 Modulus, psi Modulus, psi Modulus (psi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.7 Tensile Modulus of the 0.5 wt.% Nanocomposites Modulus (psi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.8 Tensile Modulus of the 2 wt.% Nanocomposites. 40

53 Stress at Yield, psi Stress at Yield, psi Stress at Yield (psi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.9 Tensile Stress at Yield of the 0.5 wt.% Nanocomposites Stress at Yield (psi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.10 Tensile Stress at Yield of the 2 wt.% Nanocomposites. 41

54 Break Strain, % Break Strain, % Break Strain (%) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.11 Tensile Break Strain of the 0.5 wt.% Nanocomposites Break Strain (%) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.12 Tensile Break Strain of the 2 wt.% Nanocomposites. 42

55 Flexural Maximum Stress (ksi) Flexural Modulus (ksi) Flexural Modulus (ksi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.13 Flexural Modulus of the Nanocomposites Flexural Maximum Stress (ksi) vs. Shear Rate (1/sec) Shear Rate, 1/sec Figure 3.14 Flexural Maximum Stress of the Nanocomposites. 43

56 3.6 Differential Scanning Calorimetry The individual DSC plots for the virgin PC, reprocessed PC, 0.5 wt.%, 2 wt.%, and 4 wt.% samples are shown in Figures In each figure, the T g values are given for each sample. The computer software calculates a minimum, maximum, and an expected T g value for each sample. T g is plotted against wt.% loading of CNT s in Figure The T g, C, of the PC decreased slightly as wt.% loading of CNT s increased for both the untreated and treated samples. The untreated samples dropped a total of 4.22 C, while the treated samples dropped a total of 8.7 C. Figure 3.15 DSC plot for the virgin PC. 44

57 Figure 3.16 DSC plot for the reprocessed PC. 45

58 Figure 3.17 DSC plot for the untreated 0.5 wt.% nanocomposite. 46

59 Figure 3.18 DSC plot for the treated 0.5 wt.% nanocomposite. 47

60 Figure 3.19 DSC plot for the untreated 2 wt.% nanocomposite. 48

61 Figure 3.20 DSC plot for the treated 2 wt.% nanocomposite. 49

62 Figure 3.21 DSC plot for the untreated 4 wt.% nanocomposite. 50

63 Figure 3.22 DSC plot for the treated 4 wt.% nanocomposite. 51

64 T g, C T g vs. Weight % Loading Virgin PC Reprocessed PC 0.5% 2% 4% Wt.% Loading Figure 3.23 Glass Transition Temperature vs. Wt.% Loading of CNT s. 52

65 3.7 Optical Microscopy Images of fractured tensile bars were observed for virgin PC, reprocessed PC, 0.5 wt.% nanocomposites, and 2 wt.% nanocomposites. The optical microscopy images for these samples are present in Figures The virgin PC and reprocessed PC images show stretching and surface deformation, which corresponds to the tensile breaking strain data. The untreated and treated 0.5 wt.% nancomposites show less deformation, which relates to the drop in breaking strain presented in Section 3.1. As wt.% loading of CNT s increased there was a change in trend between the treated and untreated samples. The untreated 2 wt.% sample continued to show slight surface deformation, while the treated 2 wt.% sample hardly showed any surface deformation. Again, this corresponds to the breaking strain data which showed the treated samples became extremely brittle as wt.% loading of CNT s increased. 53

66 Figure 3.24 Optical microscopy of fractured tensile surfaces for virgin PC. Figure 3.25 Optical microscopy of fractured tensile surfaces for reprocessed PC. 54

67 Figure 3.26 Optical microscopy of fractured tensile surfaces for untreated 0.5 wt.% nanocomposite. Figure 3.27 Optical microscopy of fractured tensile surfaces for treated 0.5 wt.% nanocomposite. 55

68 Figure 3.28 Optical microscopy of fractured tensile surfaces for untreated 2 wt.% nanocomposite. Figure 3.29 Optical microscopy of fractured tensile surfaces for treated 2 wt.% nanocomposite. 56

69 4.0 Discussion 4.1 Weight % Loading Effects on Mechanical Properties The virgin and reprocessed PC outperformed the nanocomposites in tensile modulus and in break strain. Higher tensile modulus values for the virgin and reprocessed PC were surprising and unexpected results. The drop in tensile modulus is believed to be caused by the lower wt.% loadings, , that were selected for this investigation. Similar discoveries were observed by Kunzelman et. al when they used lower wt.% loadings of CNT s to produce nanocomposites. It seems as if introducing lower wt.% loadings can behave as contaminants in the system, and affect certain mechanical properties negatively. Introducing CNT s at lower wt.% loadings, negatively affected the tensile modulus for the Lexan According to Figure 3.1, one could believe as wt.% loading continues to increase, 5 10 wt.% loadings, the tensile modulus may continue to improve and surpass the virgin PC. The stress at yield properties were improved for both the untreated and treated nanocomposites compared to the Lexan The difference is that less treated CNT s were needed to maintain the stress at yield properties of the untreated nanocomposites. For example, wt.% treated nanocomposites maintained stress at yield properties of equal value compared to the untreated 2 wt.% nanocomposites. Above 2 wt.% loadings the treated nanocomposites became less ductile and the untreated nanocomposites continued to improve. To improve stress at yield properties of Lexan 2240; use wt.% loadings of treated CNT s or use higher wt.% loadings of untreated CNT s. The addition of CNT s significantly lowered the breaking strain of Lexan The decrease in break strain of the nanocomposites was expected. The addition of fillers to a 57

70 polymer matrix can cause the material to become less ductile. Except for the untreated 0.25 wt.% nanocomposite this is exactly what was seen throughout this investigation. The flexural modulus and flexural maximum stress of Lexan 2240 were improved by the addition of CNT s. The addition of treated or untreated CNT s provided very similar test results. The only difference between them was the treated 4 wt.%, which became brittle and provided a significant drop in flexural maximum stress. The flexural properties were improved by increasing the wt.% loading of CNT s, and the treatment had little to no effect on the improvement of the flexural properties to the PC. 4.2 Comparing Treated and Untreated Mechanical Properties The main difference between the untreated and treated PC-CNT nanocomposites was the standard deviation values of the tensile properties. The untreated nanocomposites contained higher standard deviation values than the treated nanocomposites. This shows that the treated PC-CNT nanocomposites provided more consistent test results, by improving dispersion during molding. The inconsistencies of the untreated nanocomposites are believed to be due to poor dispersion of the CNT s. Figure 3.3 shows an example of this standard deviation phenomena. In Figure 3.3 the untreated nanocomposites possessed much higher breaking strain values than did the treated nanocomposites. So it would seem as if the untreated nanocomposites would be tougher more ductile materials. The error bars show that the untreated samples have the capability of being very brittle as well. The treated nanocomposites are by far the more brittle material, but they seem to be the more reliable of the two materials when it comes to getting consistent results. 58

71 The mechanical properties of the treated nanocomposites were equal or higher in value of their wt.% counterparts. The breaking strain was the lone defeat seen for the treated samples, which was expected due to the improved dispersion. Increasing the dispersion would result in a less ductile material, as seen in Figure 3.3. It seems as if treating the CNT s allowed for more consistent dispersion during molding. 4.3 Processing Effects on Rheology Using the nozzle temperature, 580 F, of the injection molding machine for the set temperature of the capillary rheometer, I was able to create a rheological representation of the PC-CNT nanocomposites during molding. The virgin PC maintained the highest viscosity curve throughout increasing shear rates. The reprocessed PC showed a drop in viscosity, which was caused by the material being exposed to a second heat profile. The drop between the virgin PC and reprocessed PC was expected, as stated in section 3.3, but the PC-CNT nanocomposites showed some interesting characteristics. In Figure 3.6, the PC-CNT nanocomposites show significant drops in viscosity compared to the virgin PC and the reprocessed PC. For the untreated nanocomposites this drop can be explained due to the fact the CNT masterbatch was created using a lower viscosity PC. Mixing the lower viscosity PC, Lexan OQ1022, with the matrix PC, Lexan 2240, could cause this decrease in viscosity. The treated nanocomposites show a similar trend but the viscosity is further reduced. The initial (or primary) drop is believed to be the same as the untreated samples, which is caused by the lower viscosity PC. The secondary decrease in viscosity is because of the treatment applied to the CNT s. Figure 2.2 shows the chemical structure of the Kentera polymer treatment used on the CNT s. In this figure there are four side chains per repeat unit located off the backbone of the 59

72 treatment polymer. This Kentera polymer used here contains long hydrocarbon chains for the R 2 side groups and long ether chains for the R 1 side groups. The secondary decrease in viscosity is believed to be caused by these two long hydrocarbon chains present in the polymer treatment. These long hydrocarbons would create a waxy effect on the nanocomposites, resulting in a shear thinning effect during processing. This shear thinning effect is present in Figure 3.6. The treated nanocomposites viscosity curves are one order of magnitude lower than those of the untreated nanocomposites. The treatment, therefore, helped to plasticize the nanocomposites, which allowed the materials to flow easier during molding. Figure 3.6 also shows interesting characteristics from a processing standpoint. There is a crossover point for the nanocomposite untreated samples at 100 1/sec, and 200 1/sec for the treated samples. At shear rates below these crossover positions, the 2 wt.% samples have a higher viscosity than the 0.5 wt.% samples. At higher shear rates, the 2 wt.% samples have viscosities below the 0.5 wt.% samples. The processing shear rate zone for injection molding occurs above this crossover but some polymer processing, such as compression molding, occurs below these crossover positions. This is important because the PC-CNT nanocomposites viscosity will behave differently depending on the selected processing method. The shear rates reached during injection molding usually occur between 1000 and /sec (20). In this processing zone, the viscosity difference between 0.5 wt.% and 2 wt.% samples was expected. This decrease was expected because more lower viscosity PC is used when mixing the 2 wt.% samples, which should result in larger drops in viscosity. The calculated shear rates fall into the processing zone for injection molding, which makes our data useful for individuals seeking to use CNT s during injection molding. 60