Physical aging of thin 6FDA-based polyimide membranes containing carboxyl acid groups. Part II. Optical properties

The change in refractive index with time for thin films (∼350 nm) formed from glassy 6FDA-based polyimides was monitored by ellipsometry to quantitatively track the 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 decrease in gas permeability during physical aging reported in part I of this series. An excellent correlation was formed between the volumetric aging rate r, computed from the refractive index change by the Lorentz–Lorenz equation, and the permeability reduction rate, − ( ∂ log P / ∂ log t ) ; this relationship depends on the type of gas but appears to be the same for all polymer structures examined. The change in fractional free volume was examined from the refractive index data using parameters determined by group contribution methods. The free volume versus aging time results are well-described by the self-retarding relaxation model of Struik; however, this model does not explain the strong effect of thickness on aging rate. The change in free volume correlates well with the change in gas permeability of these thin films.