Physical aging of glassy perfluoropolymers in thin film composite membranes. Part II. Glass transition temperature and the free volume model
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Section snippets
Introduction and Background
Glassy polymers have been widely explored for membrane gas separation, due to their rigid structure and strong size sieving ability [1], [2]. However, when these polymers are made into thin film composite (TFC) membranes, their gas permeability often decreases with time due to physical aging, which limits their potential for industrial applications [3], [4], [5], [6]. For example, Part I of this study investigates TFC membranes with selective layers of glassy perfluoropolymers such as Teflon®
Materials and preparation of TFC membranes
The materials and the preparation of TFC membranes were described in detail in the Part I of this study [7]. Briefly, TFC membranes were prepared by coating of perfluoropolymer solutions on top of polyethersulfone (PES) porous supports (PES-2 from Ultura™ High Recovery Membrane Technology, Long Beach, CA) using an automatic draw machine (DP-8301, The Paul N. Gardner Company, Pompano Beach, FL). Pure-gas permeance in the TFC membranes was determined using a constant pressure/variable volume
Tg of perfluoropolymers determined using nano-TA
Fig. 2 shows exampled deflection curves of nano-TA for Teflon AF1600 in TFC membranes after being aged for about 744 h. The deflection approaches a maximum at the apparent Tg of the thin film polymer. More specifically, Fig. 2a shows the deflection curves at five different positions of a 50-nm-thick Teflon AF1600, which demonstrate the reproducibility of nano-TA. Fig. 2b shows deflection curves of Teflon AF1600 with different film thicknesses in TFC membranes after aging for 744 h. The deflection
Conclusion
Thin film glassy polymers are subjective to physical aging, which decreases gas permeability over time. There is a critical need to understand the structure and property relationship to design advanced membrane materials with good stability over time, together with high permeability and selectivity. The Part I of this study reports gas permeance in two-layer TFC membranes with perfluoropolymers as the selective layers. In this Part II study, a new nano-thermal analysis was used to determine the
Acknowledgments
MY and HL gratefully acknowledge the financial support of this work by Korean Carbon Capture and Sequestration R&D Center (KCRC), and the partial support from the U.S. National Science Foundation (NSF) under the CAREER Award no. 1554236. YD and SM acknowledge the financial support from NSF under the Award no. CBET-1264276 and IIP-1432952.
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