Carbon Dioxide Sorption and Plasticization of Thin Glassy Polymer Films Tracked by Optical Methods

Our previous publications have demonstrated that thin glassy polymer films respond to highly sorbing penetrants, such as CO 2, quite differently than thick films. These studies focused on CO 2 permeation behavior and revealed that, for thin films, CO 2 permeability at constant CO 2 pressure goes through a maximum followed by a continual decrease in permeability owing to physical aging. So far, thick and thin glassy polymer films have been compared in the context of permeability, but lack of substantial means of obtaining thin film sorption data has prevented adequate comparison of thick and thin films in the context of gas solubility. In this paper, spectroscopic ellipsometry is used to obtain simultaneously the film thickness and CO 2 sorption capacity for thin glassy polymer films. This allows a more comprehensive look at CO 2 permeability, sorption, and diffusivity as a function of both CO 2 pressure and exposure time. The evidence reported here suggests that thin film sorption behavior is substantially different than that of thick film counterparts. Partial molar volume is determined from sorption-induced swelling data. Fractional free volume and diffusivity are calculated as a function of CO 2 pressure. Dual sorption model parameters are presented for Matrimid thin films for different aging times. Dynamic ellipsometry experiments show that refractive index minima, fractional free volume maxima, and CO 2 diffusivity maxima correlate well with observed CO 2 permeability maxima observed for thin Matrimid films. The results support the claim that plasticization and physical aging are competing processes but that aging dominates over long time scales. The CO 2 diffusivity behavior over time is most affected by the competing effects of plasticization and aging, and the evolution of CO 2 diffusivity is shown to be the main contributing factor to changes in CO 2 permeability at constant pressure.