A STUDY OF POLYIMIDE THIN FILMS PHYSICAL AGING AND PLASTICIZATION BEHAVIORS ZHOU CHUN NATIONAL UNVERISTY OF SINGAPORE 2003 A STUDY OF POLYIMIDE THIN FILMS PHYSICAL AGING AND PLASTICIZATION BEHAVIORS ZHOU CHUN (B.Eng., BUAA) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNVERISTY OF SINGAPORE 2003 ACKNOWLEGEMENT First of all, I would like to express my deepest appreciation and thanks to my supervisors, Professor Neal Chung Tai-Shung, Professor Goh Suat Hong, Dr. Wang Rong, and Dr. Liu Ye for their intellectually-stimulating guidance and invaluable encouragement throughout my candidature. I am grateful for the Research Scholarship from the National University of Singapore (NUS) that enables me to pursue my M.Sci. degree. I am also indebted to the Institute of Materials Research and Engineering (IMRE) of Singapore for the equipment and the Top-up Award. Thanks are also due to my fellow students and the researchers in our group, Mr. C. Cao, Dr. S.L. Liu, Ms. M.L. Chng, Mr. D.F. Li, Mr. Y.C. Xiao, Mr. Y. Li, Mr. J.Y. Xiong, Mr. K.Y. Wang, Mr. L. Shao, Mr. Z. Huang, Ms. P.S. Tin, Ms. W.F. Guo, Ms. L.Y. Jiang, Ms. M.M. Teoh, and Ms. H.M. Guan for all the handy helps, technical supports, invaluable discussion and suggestions. Special thanks are due to Dr. K.P. Pramoda in IMRE for her very kind help in characterization assistance. Last but not least, I am most grateful to my parent, brother, and my finacee, Miss N. Li, for their absolute love, encouragement and support. This thesis would not have existed without them. i TABLE OF CONTENTS Page ACKNOWLEDGEMENT i TABLE OF CONTENTS ii SUMMARY vi NOMENCLATURE viii LIST OF FIGURES xi LIST OF TABLES xix CHAPTER INTRODUCTION 1.1 Membrane and membrane-based gas separation 1.2 Transport mechanism of membranes 1.2.1 General principles 1.2.2 Solution-diffusion model 1.2.3 Sorption in glassy polymers – Dual mode sorption model 1.3 Membrane material selection and tailoring 1.4 CO2 plasticization and physical aging of glassy polymer 1.5 Why thin films? 1.6 Goals and organization of this research 10 CHAPTER LITERATURE REVIEW 2.1 12 The aging phenomenon of glassy polymers and the effect on gas ii separation membranes 12 2.1.1 Introduction 12 2.1.2 Non-equilibrium behavior of glassy polymers 13 2.1.2.1 Glass transition 13 2.1.2.2 Relaxation time distribution and cooperative relaxation 16 2.1.2.3 Secondary transition and the temperature range of physical aging 19 2.1.3 An overview of the effect of physical aging on gas separation 22 membranes 2.1.4 Experimental techniques in physical aging study of glassy 30 membranes 2.1.4.1 Mechanical properties, DSC, and PALS 31 2.1.4.2 Solid state NMR and ESR 36 2.2 CO2 plasticization and anti-plasticization of gas separation 39 membranes 2.2.1 CO2 plasticization 39 2.2.2 Suppression of CO2-induced plasticization 40 CHAPTER EXPERIMENTS 42 3. Material synthesis and preparation of dense films 42 3.1.1 Materials 42 3.1.2 Preparation of dense membranes 43 3.1.3 Drying procedure and thermal history 44 3.2 Permeation measurements 46 3.3 Thickness acquisition by Scanning electron microscope (SEM) 50 3.4 Aging monitoring and CO2 plasticization experiments of 6FDA- 52 iii Durene dense membranes 3.4.1 Aging experiments 52 3.4.2 CO2 plasticization experiments 52 3.5 Chemical cross-linking modification of 6FDA-Durene dense 52 membranes for the improvement of the resistance of CO2-induced plasticization and suppressed aging process 3.5.1 Mechanism and procedure of the chemical cross-linking modification 53 3.5.2 FTIR Characterization of cross-linked 6FDA-Durene films 54 CHAPTER GOVERNING EQUATION FOR PHYSICAL 56 AGING OF THICK AND THIN FULOROPOLYIMIDE FILMS 4.1 Introduction 56 4.2 Derivation of the proposed equation 58 4.3 Results and Discussion 61 CHAPTER Accelerated CO2 Plasticization of Ultra-thin 68 Polyimide Films and the Effect of Surface Chemical Cross-linking on Plasticization and Physical Aging 5.1 Introduction 68 iv 5.2 Results and discussion 72 5.2.1 Effect of chemical cross-linking on physical aging 72 5.2.2 The accelerated CO2 plasticization for thin films and the plasticization 74 resistance induced by cross-linking 5.3 Conclusions CHAPTER CONCLUSIONS 6.1 Experimental observation and theoretical aspects of the physical aging of 81 82 82 thick and thin polyimide films 6.2 Accelerated CO2 plasticization of thin polyimide films and an effective 83 cross-linking modification to suppress plasticization and retard physical aging 6.3 Comprehensive review of the effect of physical aging on glassy gas 83 separation membranes and remaining problems REFERENCES 85 v SUMMARY A systematic research, which covers the characterization of the intrinsic gas permeation properties, the physical aging process monitoring, the CO2 plasticization behavior evaluation of the dense 6FDA-Durene polyimide films of different thickness, and finally the chemical cross-linking modification to withstand the plasticization of CO2 for CO2 separation and retard the physical aging process, has been presented in this thesis. We attempted to study the effect of film thickness on the physical aging and the CO2 plasticization behavior of the glassy polyimide membrane, because the asymmetric membrane with a thin and dense separating layer has been widely applied in industrial scale applications and is therefore of great interest, academically and industrially. In addition, we proposed an easy and feasible chemical modification method to improve the physical aging and CO2 plasticization resistance of the membrane. The knowledge of this has been proven to be critical for membrane based gas separation processes. Specifically, this work investigated (i) the aging profile of 6FDA-Durene polyimide dense films with different thickness, thus to correlate the aging of hollow fiber containing a thin and dense selective layer with the aging of dense films of comparable thickness; (ii) the CO2 plasticization behaviors of 6FDA-Durene films with different thickness; (iii) the effects of chemical cross-linking modification of 6FDA-Durene on the aging and CO2 plasticization behaviors. vi Finally, an accelerated physical aging process of the 6FDA fluoro-polyimide was observed and employed to validate a proposed equation, derived from the molecular mobility of polymer segments below the glass transition temperature of the polymer, that serves to correlate the change of permeability as a function of time during the physical aging process. Strongly thickness-dependent aging process was found by employing pure O2 and N2 tests to monitor the change of gas permeation properties as a function of aging time. Interestingly, an accelerated CO2 plasticization indicates that the conventionally defined “plasticization pressure” as an inherent material properties measured from thick dense films is also strongly thickness dependent. Experimental results suggest that chemically modified ultra-thin films show characteristics of retarded aging process and significantly suppressed plasticization. vii NOMENCLATURE A Effective area of the film (cm2) b Langmuir affinity constant (atm-1) C Local penetrant concentration in the film (cm3 (SPT)/cm3 (polymer)) C1 Local penetrant concentration at the downstream side (cm3 (STP) / cm3 (polymer)) C2 Local penetrant concentration at the upstream side (cm3 (STP) / cm3 (polymer)) CD Henry sorption concentration (cm3 (STP) / cm3 (polymer)) CH Langmuir sorption concentration (cm3 (STP) / cm3 (polymer)) cH’ Langmuir sorption capacity (cm3 (STP) / cm3 (polymer)) D Diffusion coefficient (cm2/s) DD Average local measure of mobility of a penetrant in the Henry site (cm2/s) DH Average local measure of mobility of a penetrant in Langmuir (H) environments (cm2/s) dp/dt Rate of pressure in the low-pressure downstream chamber (mmHg/sec) K CH’b / kD kD Henry’ law constant ((cm3 (STP)) / cm3 (polymer) atm) l Membrane thickness (cm) N Permeation flux (cm3/cm2-sec) p Pressure (cm Hg) P Permeability coefficient of a membrane to gas (1 barrer =1 x 10-10 viii Hampsch H. 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Phys., 37, pp.12511265, 1999b. 106 [...]... modification, and other polymer membrane characterizations such as SEM, and FTIR Chapter 4 presents a governing equation for physical aging of thick and thin Fluoropolyimide Films Chapter 5 addresses the discussion of an accelerated CO2 Plasticization observation of ultra -thin polyimide films and the effect of surface chemical cross-linking on plasticization and physical aging Chapter 6 summarizes... contain bulk groups of (CF3), by which the efficient packing is inhibited and local segment mobility is reduced For the advantages and prospects in large-scale application in industry, the 6FDA-Durene has been chosen to study in this work 1.4 CO2 plasticization and physical aging of glassy polymer An important application of gas separation membranes is to remove acid gas from natural gas Natural gas... example, Bos extensively studied the plasticization behavior of thick dense films of commercial polyimide Matrimid 5218 (Bos, 1996) Up to date, there are few reports on the plasticization behavior of thin dense films, which is similar to the case of the plasticization of thin layer of asymmetric membranes, and is suitable for the study that seeks to understand and suppress the plasticization behavior... inclusive of this introduction Chapter 2 presents a comprehensive literature review of the physical aging and CO2 plasticization behaviour of glassy polymers, and the effect of chemical modification to suppress both the physical aging and the CO2 plasticization Chapter 3 describes the experimental approaches for this research including dense membrane preparation, characterization of dense membranes, chemical... CHAPTER ONE INTRODUCTION 1.1 Membrane and membrane-based gas separation Membrane-based separation has appeared to be one of the promising and rapidly growing areas in separation technology (Rousseau, 1987) because it is more economical and energy-saving thus outweighs the traditional approaches like cryogenic distillation that requires a phase change of the feed mixture Most available membrane-based... perspective of cooperative relaxation of polymers, Matsuoka (Matsuoka and Hale, 1997) derived exactly the same relationship between the relaxation time τ (for a certain relaxation unit and thus is identical to the mobility M of the unit) and the aging time t He deduced from the point that the relaxation time τ is always changing during the physical aging as shown in Eq (2) (Matsuoka and Hale, 1997)... membrane research, i.e the aging or failure phenomenon of gas separation membranes, which in most cases are made of glassy polymers because of the excellent combination of separation permeability and selectivity offered (Koros and Fleming, 1993) Depending on different origins, this phenomenon can be broadly categorized as chemical aging and physical aging For glassy polymers, the term chemical aging. .. separation processes are in the forms of gas separation, reverse osmosis, microfiltration, ultrafiltration (Fane, 1984), liquid membranes, pervaporation (Okada and Matsuura, 1991), dialysis and electrodialysis The work presented here is engaged in the membrane-based gas separation A membrane, principally a selective barrier, achieves a separation by allowing certain components in a fluid mixture to pass... temperature of 273.15K Tg Glass transition temperature of penetrant (K) V Volume of the low-pressure chamber (cm3) x Distance from the upstream side of the film to downstream (cm) xS Local concentration of component 1 at the retentate side of permeator y Local concentration of component 1 at the permeate side of permeator α A/ B Separation factor of a gas pair α*/B A Ideal separation factor of a gas pair (permselectivity)... Brinson, 1997) In the meantime, physical aging can also dramatically deteriorate the gas permeability of glassy polymeric membranes The solutions to these two issues are vital to the wide application of membrane-based gas separation Additionally, the fact that the penetrants might act as “lubricant” to the segmental adjustment of chains is also worthy of consideration 1.5 Why thin films? Besides the property-oriented . A STUDY OF POLYIMIDE THIN FILMS - PHYSICAL AGING AND PLASTICIZATION BEHAVIORS ZHOU CHUN NATIONAL UNVERISTY OF SINGAPORE 2003 A STUDY OF POLYIMIDE THIN FILMS. CHAPTER 6 CONCLUSIONS 82 6.1 Experimental observation and theoretical aspects of the physical aging of thick and thin polyimide films 82 6.2 Accelerated CO 2 plasticization of thin polyimide. great interest, academically and industrially. In addition, we proposed an easy and feasible chemical modification method to improve the physical aging and CO 2 plasticization resistance of