Study of the Morphology and Optical Properties of Propylene/Ethylene Copolymer Films Christopher M. Fratini Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry Herve´ Marand, Chair Alan R. Esker Harry W. Gibson Thomas C. Ward Garth L. Wilkes April 14, 2006 Blacksburg, Virginia Keywords: Polypropylene, Copolymer, Morphology, Film, Haze, Clarity, Transparency, Gloss, Light Scattering, Surface Roughness Copyright 2006, Christopher M. Fratini Study of the Morphology and Optical Properties of Propylene/Ethylene Copolymer Films Christopher M. Fratini (ABSTRACT) The development of a new catalyst system by The Dow Chemical Company has resulted in the production of isotactic polypropylene and propylene/ethylene copolymers with a unique defect and comonomer distribution. This work investigated the morphology and optical properties of cast and compression molded films made from the homopolymer and copolymers with up to 20 mol% ethylene comonomer. The defect distribution of the Dow Chemical copolymers resulted in mate- rials with lower crystallinity than Ziegler-Natta or metallocene-made materials of similar ethylene content. These materials exhibited a γ-phase crystal content ranging from 0–95%, depending on ethylene content, processing condition, and catalyst type. The γ-phase crystal content of quiescently crystallized copolymer films was found to significantly influence their bulk optical properties, presumably through a change in the spherulite birefrin- gence. The bulk haze, clarity, and transparency of a homopolymer film were degraded through annealing treatments, which decreased the fraction of γ-phase crystallinity and increased the thick- ness of existing lamellae, resulting in an increased intensity of scattered light and a corresponding degradation in the optical properties of the film. The haze, clarity, transparency, and gloss of the copolymer films were found to improve at higher comonomer content and higher cooling rates. The variation in the length scale and degree of disorder in the bulk morphology of films processed under different conditions was shown to correlate with the optical quality of the films, with smaller scale morphologies scattering less light and resulting in films with better optical properties. It was also shown that no single metric can completely describe the optical quality of a poly- mer film; the relative importance of haze, transparency, and gloss, which depends on the intended application of the film, was discussed. The influence of surface scattering from the films was con- trolled through the compression molding of films using substrates of different surface roughness. The contribution of light scattered from the surface of the films was isolated and found to play a significant role in the degradation of optical quality. iii For Lisa, Kyle, and Gianna . iv Acknowledgments I would like to acknowledge and thank the following people for their contributions to this work. Dr. Herve´ Marand, for providing guidance, motivation, support, and insight throughout the course of my residence at Virginia Tech, and for introducing me to the fascinating world of physical polymer chemistry. Drs. Alan Esker, William Ducker, Harry Gibson, Thomas Ward, and Garth Wilkes for serving on my advisory committee, for teaching informative and stimulating classes, and for the many helpful discussions over the past five years. Wilson Cheung, Li Tau, Patricia Ansems, and Steve Chum from The Dow Chemical Company, which provided the funding and materials used in this research, for the opportunity to work on this project and to contribute to the research program at Dow Chemical. Drs. Brian Okerberg, Julie Uan-Zo-li, and Zhenyu Huang for their friendship and assistance during our time together in the Marand group. Steve McCartney of the Materials Research Institute for the training and use of SEM and AFM instruments, and for entertaining conversation while waiting for AFM images to scan. Melvin Shaver, John Miller, and Scott Allen at the Physics Machine Shop and David Simmons and Darrell Link at the ESM Machine Shop for fabricating many of the parts employed in the modification of laboratory instruments used in this work. Tom Wertalik of the Chemistry Glass Shop for crafting the optical cell used in this work, and for v useful discussions regarding many other topics relevant to this research. Drs. Ross Angel of the Geology Department and Lucian Zelazny of the Crop and Soil Science Department for the education in x-ray diffraction and the use of their instruments. Jim Coulter, James Hall, Larry Jackson, and Travis Heath of the Chemistry Electronics Shop for fabricating several devices used in this project and for keeping our computers running. The dozens of others who have helped me along my way through Virginia Tech. Finally, and most importantly, I thank my family for enduring my absence over the years and for their constant support and love. Lisa, Kyle, and Gianna, I could not have achieved this goal without you. vi Contents 1 Introduction 1 2 Literature Review 4 2.1 Morphology of Isotactic Polypropylene and its Copolymers . 4 2.1.1 Polymer Crystallization . 4 2.1.2 Polymorphism of i-PP . 12 2.1.3 Influence of the Catalyst System . 16 2.1.4 Copolymer Crystallization . 16 2.1.5 i-PP Morphology . 17 2.2 Optical Properties of Polymer Films . 20 2.2.1 Interaction of Light with Matter . 20 2.2.2 Small Angle Light Scattering . 25 2.2.3 Haze . 28 2.2.4 Clarity, Transparency, and Visually Perceived Film Quality . 31 2.2.5 Gloss . 34 2.3 Surface Roughness . 38 vii 2.3.1 Characterization . 38 2.3.2 Measurement of Surface Roughness . 40 3 Experimental 44 3.1 Materials . 44 3.2 Film Fabrication . 45 3.3 Film Thickness . 49 3.4 Density Measurements . 50 3.5 Refractive Index Measurements . 51 3.6 Wide-Angle X-ray Diffraction (WAXD) . 51 3.7 Polarized Optical Microscopy (POM) . 52 3.8 Scanning Electron Microscopy (SEM) . 52 3.9 Atomic Force Microscopy (AFM) . 53 3.10 Haze and Clarity . 53 3.11 Transparency . 54 3.12 Gloss . 54 3.13 Small Angle Light Scattering (SALS) . 54 4 Results and Discussion 56 4.1 Optical Properties of P/E Films . 56 4.1.1 Extrusion Cast Films . 56 4.1.2 Compression Molded Films . 63 viii 4.1.2.1 Preparation of Pressed Films . 63 4.1.2.2 Haze . 68 4.1.2.3 Clarity . 71 4.1.2.4 Comparison of Clarity and Transparency . 73 4.1.2.5 Gloss . 82 4.2 Morphology of Pressed Films . 83 4.2.1 Crystallinity . 83 4.2.1.1 Room Temperature Physical Aging . 83 4.2.1.2 Refractive Index . 85 4.2.1.3 Polymorphism . 87 4.2.1.4 Bulk Density and Crystallinity . 89 4.2.2 Bulk Morphology . 93 4.2.3 Lamellar Morphology . 98 4.3 Control of Optics by Variation of Bulk Morphology . 104 4.3.1 Spherulite Size . 110 4.3.2 Spherulite Birefringence . 112 4.4 Control of Optics by Variation of Surface Roughness . 126 4.4.1 Introduction and Sample Preparation . 126 4.4.2 Correlation of Surface Roughness with Optical Properties . 128 4.5 Comparison of Dow Chemical P/E copolymers to Ziegler-Natta and Metallocene P/E copolymers . 132 4.6 Cast Films Revisited — Optical Characterization . 143 ix 4.7 Conclusions . 154 5 Future Work 158 5.1 On the Dow Chemical P/E copolymers . 158 5.2 On the Surface Roughness . 159 5.3 On the Optical Properties . 160 A Temperature Controller BASIC Program Listing 180 B Morphology and HV SALS patterns of PE, ZN, and MET copolymers 182 C Derivation of a Model for the Birefringence of an iPP Spherulite with γ-α Branching 194 x List of Figures 2.1 The fringed micelle model of polymer crystallization. 6 2.2 The chain-folded lamellar model of polymer crystallization. 6 2.3 Growth stages of a polymer spherulite. 7 2.4 Spherulite of polyvinylidene fluoride. 8 2.5 Determination of the sign of ∆n. ........................... 10 2.6 LH model of polymer crystallization. 11 2.7 Unit cell of α-phase i-PP. View is down the c-axis. 13 2.8 Unit cell of γ-phase i-PP. 15 2.9 Epitaxial α-α and α-γ branching in i-PP. 19 2.10 Schematic drawing of hazemeter. 30 2.11 Schematic drawing of transparency meter. 32 2.12 Schematic drawing of gloss meter. 35 4.1 Schematic of VT hazemeter. 57 4.2 VT haziness vs. Dow Chemical haze for the cast films. 59 4.3 Haziness of cast extruded film separated into total and bulk contributions. 60 xi 4.4 VT haziness/mil vs. Dow Chemical haze/mil for the cast films. 61 4.5 Haziness/mil for Dow Chemical cast films. 61 4.6 Sample temperature profile for slow cool. 65 4.7 Sample temperature profile for bench top cool. 65 4.8 Sample temperature profile for ice/water quench. 66 4.9 Sample temperature profile for cooling at 1◦C/min. 66 4.10 Sample temperature profile for cooling at 10◦C/min. 67 4.11 Pressed film thickness as a function of MW . ..................... 68 4.12 Haze – total and bulk scattering contributions for cast film with 7.7%E. 69 4.13 Haze of pressed films vs. ethylene content and cooling rate. 70 4.14 Bulk haze/mil of pressed films. 71 4.15 Clarity – total and bulk scattering contributions for cast film with 7.7%E. 73 4.16 Total and bulk normalized clarity of pressed films. 74 4.17 Schematic drawing of the Haze Gard Plus clarity mode. 75 4.18 Transparency – total and bulk scattering contributions for cast film with 7.7%E.