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SPECTROSCOPIC STUDY OF DIFFUSION IN A GLASSY POLYMER

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ABSTRACT Title of dissertation: SPECTROSCOPIC STUDY OF DIFFUSION IN A GLASSY POLYMER Juchen Guo, Doctor of Philosophy, 2007 Dissertation directed by: Professor Timothy A. Barbari Department of Chemical and Biomolecular Engineering Understanding the diffusion of small molecules in glassy polymer films is very important to applications where selectivity is important, including membrane separations, barrier materials, the controlled-release of pharmaceuticals, and chemical sensors. The non-equilibrium nature of glassy polymers results in sorption and diffusion behavior that can be considerably more complicated than that observed in rubbery polymers. Time-resolved, Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy has been used increasingly to study diffusion in polymers and has proven to be very accurate and reliable. FTIR-ATR spectroscopy is capable of identifying changes in the local environment for both the penetrant and the polymer, resulting in information at the molecular level during the transport process.In this study, FTIR-ATR spectroscopy was used to study the diffusion of a small molecule, acetonitrile, in a glassy polymer, cellulose acetate (CA) from the vapor phase. By monitoring the IR absorbances of the nitrile group in acetonitrile and the carbonyl group in cellulose acetate, the kinetics of sorption/desorption and the rates of penetrant-induced swelling/deswelling, respectively, were studied. An additional physical mechanism, resulting in a time delay prior to the appearance of a Fickian-like concentration profile, was uncovered with this technique. A dual mode transport model with local equilibrium relaxation was proposed and successfully used to capture this phenomenon, revealing that a finite hole-filling rate in the dual mode framework is necessary to fully describe transport in glassy polymers. A modified dual mode transport model, taking into account penetrant-induced plasticization in addition to local equilibrium relaxation, was also used and compared to the original version. Differences in the two models were most apparent when describing desorption and were ascribed to differences in the redistribution of molecules between the two modes at the start of the desorption process. Swelling and deswelling rates in the acetonitrile/CA system were predicted using the dual mode model, with and without the modification. Predictions were excellent for swelling, but the inability to predict deswelling was attributed to limitations inherent in the two models. This work revealed that local equilibrium must be relaxed to fully describe diffusion in glassy polymers. The model developed here should find use in sensor applications of FTIR-ATR spectroscopy, where transient behavior is the key to performance.SPECTROSCOPIC STUDY OF DIFFUSION IN A GLASSY POLYMER by Juchen Guo Dissertation Submitted to the Faculty of the Graduate School of the University of Maryland, College Park In partial fulfillment of the requirements for the degree of Doctor of Philosophy 2007 Advisory Committee: Professor F. Joseph Schork, Chair Professor Raymond A. Adomaitis Professor Timothy A. Barbari Professor Robert M. Briber Professor Srinivasa R. Raghavan Copyright by Juchen Guo 2007ii Dedicated to my parents, Guo Shanyong and Chen Xuena, who love me much more than I can pay them back.iii ACKNOWLEDGEMENTS I am truly grateful to my advisor, Dr. Timothy Barbari, who advised, mentored and supported me during my PhD study. From 2001 to 2006, I learned from him not only how to be a good researcher, but also how to succeed in life. Without his generosity and support, I could not have achieved what I have done. Thanks for everything from the bottom of my heart. In addition, I would like to thank my dear colleagues and friends, Chong Yung, Jianfeng Du, Yuesheng Ye and Quanzeng Wang, who supported and helped me in many ways throughout my study. Also, I am very thankful to my lifelong friends Zhen, Yuejiao, Lizhi, Ji, Jing, Rui and Jingying. They share in my happiness and sorrow, and their friendships are invaluable treasures to me. However, none of this would have been possible if it were not for my Mom and Dad. It is their endless love and faith that has supported me to come this far. There are no words that can describe how much I love you two. I hope that what I accomplished makes you happy and proud. My sister, Juqian, my brother-in-law, Fan and my lovely niece Sophie, I love you all very much. I will never forget the joy you brought to me. Without your love, I would not have been able to persevere throughout the past six years.iv Table of Contents Table of Contents iv List of Tables ix List of Figures xi 1 Introduction 1 1.1 Diffusion in Polymers 1 1.1.1 Characteristics of Fickian Diffusion 1 1.1.2 Characteristics of Non-Fickian Diffusion 2 1.1.3 Free Volume Theory for Diffusion Above the Glass Transition 4 1.1.4 Dual Mode Model for Diffusion Below the Glass Transition 8 1.2 Motivation 11 1.3 A Brief Review of Previous Work Utilizing FTIR-ATR Spectroscopy 12 1.4 Infrared Spectroscopy 17 1.4.1 Molecular Interactions with Electromagnetic Radiation 17 1.4.2 Attenuated Total Reflection (ATR) 19 1.4.3 Quantitative Analysis 22 1.5 Experimental 24 1.5.1 Materials and Sample Preparation 24 1.5.2 Experimental Procedure 25 1.5.3 Gravimetric Experiments 29 1.5.4 FTIR-ATR Experiments 30 1.6 Overview 33v 2 Dual Mode Sorption, Immobilization and the Relaxation of Local Equilibrium 35 2.1 Introduction 35 2.2 Transport Model 36 2.2.1 Simple Fickian Model 36 2.2.2 Development of Dual Mode Transport Model with Local Equilibrium 38 2.3 Results and Discussion 42 2.3.1 Converting Infrared Absorbance to Concentration 42 2.3.2 Sorption Isotherm for Acetonitrile in Cellulose Acetate. 42 2.3.3 Simple Fickian Diffusion. 45 2.3.4 Dual Mode with Local Equilibrium 52 2.3.5 Dual Mode Relaxing Local Equilibrium 52 2.3.6 Prediction of Mass Uptake Using Dual Mode with Local Equilibrium Relaxation 55 2.4 Conclusion 69 3 Moving Boundary Model for Diffusion in a Glassy Polymer Undergoing a Penetrant-Induced Glass Transition 71 3.1 Introduction 71 3.2 Experimental Methods 72 3.3 Moving Boundary Model 72 3.3.1 Model Formulation 72 3.3.2 Numerical Solution Procedure 76 3.4 Results and Discussion 77 3.4.1 Penetrant-Induced Glass


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