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

Thin glassy polymer films respond to highly-sorbing penetrants, such as CO₂, quite differently than thick films. Studies focused on CO₂ permeation behaviour have revealed that, for thin films, CO₂ permeability at constant CO₂ pressure goes through a maximum followed by a continual decrease in permeability owing to physical aging. This behaviour was not observed for thicker films over comparable experimental timescales. Thick and thin glassy polymer films can be compared in the context of gas permeability, but lack of substantial means of obtaining thin film sorption data has historically prevented adequate comparison of thick and thin films in the context of gas solubility. In this work, spectroscopic ellipsometry is used to obtain simultaneously the film thickness and CO₂ sorption capacity for thin glassy polymer films, allowing a more comprehensive look at CO₂ permeability, sorption, and diffusivity as a function of both CO₂ pressure and exposure time. The ellipsometry results suggest that thin film sorption behaviour 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 CO₂ pressure. Dynamic ellipsometry experiments show that refractive index minima, fractional free volume maxima, and CO₂ diffusivity maxima correlate well with observed CO₂ permeability maxima observed for thin Matrimid films. The permeability and optical data support the claim that plasticization and physical aging are competing processes but that aging dominates over long time scales. The CO₂ diffusivity behaviour over time is most affected by the competing effects of plasticization and aging, and the evolution of CO₂ diffusivity is shown to be the main contributing factor to changes in CO₂ permeability at constant pressure.