Supercooling and Solvent Depletion-Driven Poly(vinyl Alcohol) Film Formation Mechanism as Elucidated by In Situ Synchrotron Radiation X-Ray Scattering

The microstructure evolution of semicrystalline poly(vinyl alcohol) (PVA) during drying within a wide temperature window ranging from 95 to 50 °C was captured in situ by synchrotron radiation X-ray scattering (SRXS), specifically small- and wide-angle X-ray scattering. A portable film-casting apparatus with a high-pressure reactor was designed and manufactured. Its high safety features, coupled with its detachable and installable design, facilitate its integration with SRXS for researching the drying process of polymer solutions. The time-resolved microstructure parameters, such as the crystallinity and lamellar thickness, as well as the water fraction during the PVA film formation process help us to elucidate four sequential regimes during drying: (1) regime I, the evaporation stage, where the water evaporates at a constant rate without any crystallization occurring; (2) regime II, the rapid-crystallization stage, where PVA crystallinity increases dramatically and the long period declines rapidly due to the evaporation of the water in the interlamellar amorphous layer; (3) regime III, the diffusion-limited stage, where the PVA chain mobility and diffusion of water in the amorphous layers are gradually hindered as the PVA concentration increases, resulting in a slowdown of the decline in the long period; (4) regime IV, the annealing stage, during which the long period exhibits a secondary rapid reduction with the insertion of new-formed lamellae into the lamellar stacks. The synergistic and competing effects of the temperature and water fraction significantly influence the crystallization kinetics and chain mobility: lowering the temperature results in an increasing supercooling degree but a decrement of chain mobility as evidenced by low-field NMR experiments, where the former facilitates the nucleation but the latter impedes the crystallization kinetics. The above effects lead to a nonmonotonic temperature-dependent onset PVA weight fraction (ωPVA) of crystallization at the very beginning of regime II.