Diffusion of Liquid Water in Free-Standing Polymer Films Using Pressure-Contact Time-Resolved Fourier Transform Infrared Attenuated Total Reflectance Spectroscopy

Abstract

In this work, an in situ pressure-contact Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy apparatus was designed and developed for measuring liquid transport in free-standing polymer films. This new technique allows time-resolved infrared data of liquid transport in free-standing films to be collected accurately without solution casting the film directly onto the ATR element. Liquid water transport in free-standing films of a rubbery polymer (poly(isobutylene); PIB), a glassy polymer (poly(methyl methacrylate); PMMA), and a cross-linked polymer (epoxy/amine resin) was investigated using this technique. The in situ pressure-contact FTIR-ATR spectroscopy apparatus showed similar results to those obtained using the conventional solution-cast FTIR-ATR apparatus for PIB. Non-Fickian behavior was observed for both the solution-cast and free-standing PMMA due to the nonequilibrium state of the polymer. However, the rate of water sorption did not match between the two; the former was attributed to diffusion-relaxation and the latter was attributed diffusion-compression due to the differences in the apparatus. The solution-cast PMMA data were regressed to a previously developed diffusion-relaxation model, while a new model that incorporates diffusion-compression was developed and regressed to the free-standing PMMA data. Liquid water diffusion in the cross-linked polymer exhibited Fickian behavior and was attributed to the suppressed polymer strain response to the diffusant.