Physical aging of glassy perfluoropolymers in thin film composite membranes. Part II. Glass transition temperature and the free volume model

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, N₂permeance decreases from 1000 gpu to 550 gpu while apparent T_gincreases from 160 °C to 172 °C after aging for 2000 h in a membrane with 100-nm-thick Teflon AF1600. The measured T_gvalues 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_gincrease during physical aging. Polymers with good stability of permeability should have low physical aging rate and high fractional free volume.