Water sorption in semicrystalline poly(vinyl alcohol) membranes: In situ characterisation of solvent-induced structural rearrangements

Abstract

As with polymeric materials, fundamental technological properties are affected by polymer microstructure, the identification and subsequent assessment of structural transformations associated with the presence of solvent in the polymer network are of substantial interest, not least when long-term structural stability is concerned. Solvent-mediated morphological rearrangements are studied on the basis of water vapour sorption into physically crosslinked poly(vinyl alcohol) membranes. In a series of dynamic vapour-phase sorption experiments under well-defined boundary conditions, the semicrystalline polymer structure, as characterised by means of its degree of crystallinity, is analysed, providing information about time-dependent morphological behaviour as well as the spatial distribution of crystallinity across film thickness. Structural information thus obtained is then interpreted in view of the corresponding water uptake of the polymer network. Morphological rearrangements in the presence of solvent seem thus to be governed by two opposing effects. Upon water uptake, stretching of polymer chains may, on the one hand, induce part of the crystalline entities to unfold; at the same time, however, macromolecules equally gain mobility and may hence reorganise into more favourable ordered arrays. When compared to the swift disintegration of crystalline structures proceeding without any appreciable time lag to water diffusion into the film, the solvent-mediated organisation of mobilised polymer chains into three-dimensional ordered arrays involves more protracted realignments. The extent of molecular rearrangements affecting the structural stability of the polymer network could finally be shown to diminish upon repeated sorption and desorption.