Interpretation of the Free Volume Changes by Physical Aging on the Thin 6FDA-based Polyimide Films

Abstract

The change in refractive index for glassy 6FDA=based polyimides was monitored by ellipsometry with aging time was used to quantitatively illustrate a correlation with physical aging process. The refractive index increased linearly, attributed to the densification of the glassy polyimide, with respect to aging time, on a logarithmic scale this result is consistent with the change in gas permeability change during physical aging process reported in previous paper. Correlation between the volumetric aging rate, γ computed by Lorentz-Lorenz equation and permeability reduction rate, −(∂logP/∂logt), showed there is linear relationship between glassy polymers depend on specific gas regardless of polymer type. A data of hypothetically generated fractional free volume by aging time was fitted to the Struik model to obtain initial free volume. The direct relationship between gas permeability and free volume via simple equation, P=Ae^(-B/f), was predicted by computing the value of A and B.