No abstract

Glassy perfluoropolymers have become an exciting materials platform for membrane gas separation as they define the upper bounds for some gas separations, such as He/H 2 , He/CH 4 , and N 2 /CH 4 . However, due to the difficulty in synthesis, only a few glassy perfluoropolymers are commercially available, including Teflon AF and Hyflon AD derived from dioxoles and Cytop derived from dihydrofuran. In this study, two perfluoropolymers based on dioxolanes, poly(perfluoro-2-methylene-1,3-dioxolane) (poly(PFMD)) and poly(perfluoro-2-methylene-4-methyl-1,3-dioxolane) (poly(PFMMD)), were synthesized by radical polymerization and characterized thoroughly for physical properties such as glass transition temperature (T g ), d-spacing between polymer chains, and fractional free volume (FFV). The gas permeability and solubility were determined at 35 °C for a series of pure gases in these perfluorodioxolanes and compared with the commercial perfluoropolymers. Poly(PFMD) and poly(PFMMD) exhibit separation properties of He/H 2 , He/CH 4 , H 2 /CH 4 , H 2 /CO 2 , and N 2 /CH 4 near or above the upper bounds in Robeson's plots, and superior to the commercial perfluoropolymers, despite their similar T g and FFV. The underlying reasons for the superior gas separation properties in these dioxolane-based perfluoropolymers are discussed.

Physical aging of thin film glassy polymers continuously decreases gas permeability, presenting a great challenge in designing membrane systems for long-term gas separation. Most studies on the effect of physical aging on membrane applications use freestanding thin films, which are often annealed above the polymer glass transition temperature (T g) before gas permeability is determined. However, industrial membranes are often thin film composites (TFCs) comprising the thin film on top of porous polymeric supports, and they may not be annealed above the T g. The objective of this study is to investigate the effect of physical aging of the selective layer on gas permeance and selectivity of TFC membranes to establish industrial relevance. Two-layer TFC membranes consisting of perfluoropolymers (including Teflon ® AF1600 and Hyflon ® AD) at various film thicknesses (50-400 nm) on polyethersulfone porous support were prepared and determined for permeances of CH 4 , N 2 , H 2 and CO 2 at 35 o C for over 1000 hr. Gas permeances decrease with time, and the decrease is more significant for larger penetrants and for membranes with thinner selective layers. For example, CH 4 permeance decreases by 54% and 27% after aging for about 1400 hr in TFC membranes comprising 50-nmand 370-nm-thick Teflon AF1600, respectively. The decrease of gas permeances over time in these TFC membranes is compared with that of freestanding films. This study is one of only a few to present the results of physical aging in industrial TFC membranes and to provide useful insights for practical membrane applications.

Thin film composite (TFC) membranes for gas separation often comprise a thin selective layer of a glassy polymer, which, however, suffers from physical aging, i.e., gas permeance decreases with time. This study aims to provide a mechanistic understanding of the effect of physical aging on permeance reduction in TFC membranes. The Part I study reports gas permeances in two-layer TFC membranes comprising perfluoropolymers of Teflon ® AF or Hyflon ® AD with thicknesses of 50-400 nm. In this Part II study, apparent glass transition temperature (T g) of thin selective layers was determined in situ over time using a nano-thermal analysis (nano-TA). Physical aging decreases gas permeances and increases apparent T g , and the rate of changes is more significant for thinner selective layers. For example, N 2 permeance decreases from 1000 gpu to 550 gpu while apparent T g increases from 160 o C to 172 o C after aging for 2000 h in a membrane with 100-nm-thick Teflon AF1600. The measured T g values are used to derive polymer fractional free volume and physical aging rate. A simplified free volume model is used to successfully correlate the gas permeance reduction with T g increase during physical aging. Polymers with good stability of permeability should have low physical aging rate and high fractional free volume.

Lattice fluid theory is used to develop transport property−structure correlations for glassy perfluoropolymers with dioxolane pendant rings, a new class of membrane materials for gas separation. Poly(perfluoro-2-methylene-1,3-dioxolane) (poly-(PFMD)) and poly(perfluoro-2-methylene-4-methyl-1,3-dioxolane) (poly(PFMMD)) exhibit lower permeability but much higher selectivity than commercial fluoropolymers, such as Teflons AF. Their enhanced separation performance is due to the combined effect of solubility-and diffusivity-selectivity. Moreover, poly(PFMD) and poly(PFMMD) exhibit enhanced CO 2 -philicity as compared to Teflons AF, which can be ascribed to the higher oxygen/carbon ratio exhibited by the former materials. To provide rational guidelines to maximize the solubility-selectivity, the enthalpic and entropic contributions to sorption coefficient were calculated and compared for several polymers of practical interest for gas separation. In the absence of localized penetrant−polymer interactions, gas sorption is controlled essentially by the free volume and solubility-selectivity is controlled by the polymer cohesive energy density.