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 (R) AF or Hyflon (R) 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 Tg, 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 Tg increases from 160 degrees C to 172 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.