Surface phase morphology and composition of the casting films of PVDF–PVP blend

Films of a binary polymer blend comprising of hydrophobic poly(vinylidene fluoride) (PVDF) and hydrophilic poly(vinylpyrrolidone) (PVP) have been prepared by solution-casting. The dependence of surface structure and composition of the films on the PVP content in the blend was investigated by using atomic force microscope (AFM), XRD, XPS, SEM and differential scanning calorimeter (DSC). It has been found that the interaction between the two homopolymers prevents PVDF from crystallization in the blend, the net result of which has a primary effect on the surface properties of the films. PVP has a greater concentration at the surface than in the bulk as long as PVDF crystallizes in the bulk during the film formation process, which leaves a thermodynamically non-equilibrium surface state. On the other hand, with an increase in the PVP content, the interaction between segmental PVDF and PVP in the beginning transforms the crystalline state of PVDF from α to γ phase, and finally results in the disappearance of crystalline PVDF phases. A meager crystallization of PVDF segments could still carry on at the surface of a film with a miscible (or an amorphous) bulk; this occurrence makes the surface more hydrophobic than its bulk phase.