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Free volume in a big group of glassy polymers distinguished by a wide range of gas permeability (P in the range 1-10 4 Barrer) was studied using the positron annihilation lifetime (PAL) method. To take into account an additional channel of decay of positronium (Ps), caused by the reaction with sorbed O2, measurements were performed both under ambient conditions and under a nitrogen atmosphere. The finite-term and continuous lifetime analyses (PATFIT and CONTIN programs) were used for the treatment of the positron annihilation decay curves. It was shown that better statistical fitting parameters can be obtained for four component PAL spectra of a vast majority of the glassy polymers studied. The presence of two Ps lifetimes (τ 3 ) 1.7-3.3 ns and τ4 ) 2.7-10.9 ns) indicates that size distribution of free volume elements (FVE) in glassy polymers is typically bimodal. The presence of smaller and larger FVE (R3 ) 2.5-3.5 Å, R4 ) 3.5-7.0 Å) was proved by PATFIT analysis of the data. Continuous analysis clearly indicated two broad peaks, especially for more permeable polymers. As permeability and free volume of the polymers decrease, the two peaks start to approach and overlap each other, ending with one broader or asymmetric peak for the less permeable polymers. In the presence of oxygen both lifetimes τ 3 and τ4 are shifted to shorter values; this shift is more pronounced for the polymers with larger radii R4 of FVE. A novel correlation of diffusion selectivity of glassy polymers with the size of FVE is demonstrated.

Sorption and permeation parameters of light gases, C 1-C12 hydrocarbons, and C1-C7 perfluorocarbons were determined in a random, amorphous, glassy copolymer containing 65 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) and 35 mol % tetrafluoroethylene (TFE) (TFE/BDD65 or AF1600). AF1600 results were compared to those of another copolymer, AF2400, which contains 87 mol % BDD. As the amount of bulky, packing-disrupting BDD increases, solubility coefficients increase systematically, primarily due to increases in the nonequilibrium excess volume of the glassy polymer. Permeability and diffusivity also increase with increasing BDD content. AF1600 is easily plasticized by larger, more soluble penetrants and is susceptible to penetrant-induced conditioning. As penetrant size increases, permeability and diffusion coefficients decrease. The rates of decrease of permeability and diffusivity with increasing penetrant size, which characterize permeability and diffusivity selectivity, are intermediate between those of conventional glassy polymers and exceptionally high free volume glassy materials such as poly(1-trimethylsilyl-1-propyne) (PTMSP). Positron annihilation lifetime spectroscopy (PALS) results suggest unusually large free volume elements and a bimodal distribution of free volume element size: this is consistent with similar results obtained earlier for other high free volume glassy polymers such as AF2400 and PTMSP. Inverse gas chromatography and PALS estimates of free volume element size distributions were consistent.

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The development and implementation of the extracorporeal membrane oxygenation (ECMO) technique for the treatment of patients in critical conditions make it possible to effectively and safely support gas exchange processes in the blood for a long time. One of the main components of the ECMO unit is a gas permeable membrane which is a barrier separating the blood from the gas phase. Since the 1950s, the development of this technology has been aimed at improving the safety and duration of use of membranes, which led to the creation of oxygenators that provide life support for several weeks. This review is devoted to the development of the extracorporeal membrane oxygenation technology including the choice of materials, methods to improve their hemocompatibility, and approaches to the design of the membrane contactor.

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