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

Abstract

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 50–400nm. In this Part II study, apparent glass transition temperature (Tg) 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 Tg, and the rate of changes is more significant for thinner selective layers. For example, N2 permeance decreases from 1000 gpu to 550 gpu while apparent Tg increases from 160°C to 172°C after aging for 2000h in a membrane with 100-nm-thick Teflon AF1600. The measured Tg 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 Tg increase during physical aging. Polymers with good stability of permeability should have low physical aging rate and high fractional free volume.