Effects of mechanical and chemical properties on transport in fluoropolymers. I. Transient sorption

Abstract

This is the first part of a study of chemical structure-physical property-performance relationships among several fluoropolymers and liquid penetrants, focusing on their diffusivities and solubilities. Transient sorption experiments conducted with perfluoroalkoxy, fluorinated ethylene propylene, and ethylene tetrafluoroethylene exhibited Fickian diffusion. The first two exhibited concentration-independent diffusivity, while the last displayed concentration-dependent diffusivity. Conversely, polyvinylidene fluoride (PVDF) and ethylene chlorotrifluoroethylene (ECTFE) exhibited non-Fickian behavior, including acceleration as saturation approached. Suspected causes for the unusual sorption behavior were structural characteristics due to processing (e.g., skin or orientation with respect to processing direction), intrinsic chemical structure, and morphological deformation induced by swelling. Stress-strain tests indicated that mechanical properties depend similarly on the presence of penetrants to transport properties. Weak swelling agents exhibited Fickian transport and a negligible change of mechanical properties. In contrast, strong swelling agents induced non-Fickian diffusion and caused significant changes of mechanical properties. A new kinetic model fit the transient sorption results, particularly the observed acceleration in PVDF and ECTFE. The model accounts for structural changes due to swelling and relaxation resulting from free-volume creation, which is assumed to be proportional to the amount of liquid sorbed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 455–476, 1997