Study of the Formation of Poly(vinyl alcohol) Films

The film formation of poly(vinyl alcohol) of different molecular weights from concentrated solution has been, observed in real time by means of low-field nuclear magnetic resonance (NMR) methods. The drying of films was followed with a depth resolution of 50 mu m up to the formation of the final film of typically 300 mu m thickness, and the molecular mobility was determined with spatial resolution by analyzing the NMR relaxation times (T-2, T-1) behavior. A gradient in the molecular dynamics was observed from T-1 data during evaporation process up to an intermediate time when the film shrinkage rate decreases significantly; T-2 indicates dynamical heterogeneity as well, persisting up to complete removal of water. The relaxation times suggest an increase of local molecular order which is more pronounced toward the air/film interface. Wide-angle X-ray diffraction confirms the formation of an ordered region at this interface with a crystallinity higher-depending on molecular weight-than at the bottom side of the film.