Investigation on surface molecular conformations and pervaporation performance of the poly(vinyl alcohol) (PVA) membrane.

A simple method of changing pre-treatment temperature in the course of film formation was used to tune the surface structures of PVA membranes. Surface structure and property of the resulting membranes were characterized by X-ray photoelectron spectroscopy (XPS), sum frequency generation (SFG) vibrational spectroscopy, and contact angle measurements. The results show that PVA have different molecular conformations at the membrane surface while those membranes were prepared at different pre-treatment temperature. At higher pre-treatment temperatures, polar acetoxyl residues and hydroxyl groups of the PVA chains oriented in a more orderly fashion, as induced by the faster evaporation of water. When the membranes were in air, CH(3) groups adjacent to the acetoxyl groups covered the surface in order to minimize the surface free energy, while backbones of the PVA were rarely observed. These surfaces exhibited a hydrophilic nature upon contact with water due to rapid surface reconstruction. Conversely, at lower pre-treatment temperatures, the backbone CH(2) groups dominated the surface, forming a less hydrophilic surface. When the PVA membranes were employed to separate ethanol/water mixtures, it was found that the PVA membranes with more hydrophilic surface exhibited higher water selectivity. Our investigation indicates that molecular conformations on the membrane surface have considerable influence on pervaporation performance.