Effect of acetate distribution on surface segregation in poly(vinyl alcohol-co-vinyl acetate) copolymer films

The level of acetate-group surface segregation in poly(vinyl alcohol-co-vinyl acetate) (PVA-PVAc) films was found to depend markedly on the functional group distribution along the backbone (blockiness). PVA-PVAc polymers with both random and blocky distributions were prepared at levels between 2 and 12 mol % acetate and cast into films from aqueous solution. Films from polymers with blocky distributions showed significantly higher levels of acetate at the surface than in the bulk, while polymers with random distributions of acetate functionality exhibited little or no surface segregation. The level of acetate surface segregation was determined by both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Blocky PVA-PVAc films containing 4 mol % were seen to have approximately 40 mol % PVAc at the outermost surface by XPS at low take-off angle. Bulk acetate levels and acetate group distributions were determined by NMR. Variations in the weight average molecular weight (M(W)) between 40k and 155k, modifying the casting solvent, or annealing the films above T-g did not effect the level of surface segregation, suggesting that the segregation is not simply a kinetic phenomenon.