Stretch-Induced Structure Evolution of Poly(vinyl alcohol)–Glycerol Gel Films: An In Situ Synchrotron Radiation X-ray Scattering Study

High-temperature stretching is a potential approach for the industrial-scale fabrication of poly(vinyl alcohol) (PVA) nanofibril films. In this study, in situ small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS) are employed to investigate the structural evolution of PVA gel films containing glycerol during stretching at different temperatures. The SAXS and WAXS analyses reveal that uniaxial stretching triggers the melting–recrystallization of PVA lamellar crystals and the formation of nanofibrils. The key factors for structural evolution will change with the stretching temperature. When stretched at low temperatures, the destruction of crystals is dominated by stress. The reconstructed nanofibrils withstand the stress and cause feedback on the structural evolution, slowing the crystal destruction. At high temperatures, crystal destruction is dominated by melting, and the crystallinity decreases rapidly under stress, while stretch-induced recrystallization and reconstruction of nanofibrils are more readily achieved. Besides, the long period of lamellar crystals and nanofibrils increases with the increase of stretching temperature. The PVA porous nanofibril films with favorable nanofiber networks and porous structures can be obtained by biaxial stretching followed by solvent extraction and drying.