Hydrogen‐Bond Mediated Structural Evolution and Mechano‐Optical Anisotropy During Drying and Uniaxial Stretching of Poly(Vinyl Alcohol) Films: Effect of Molecular Weight

ABSTRACT This study presents a systematic investigation of the structural and mechano‐optical evolution of 88% hydrolyzed poly(vinyl alcohol) (PVA) films from solution casting to uniaxial stretching. The effects of molecular weight, drying conditions, and deformation temperature were examined using real‐time birefringence, small‐ and wide‐angle x‐ray scattering (SAXS/WAXS), and calorimetry. Polymer chains adopt a random in‐plane orientation beyond a critical concentration during drying, resulting in optically uniaxial films characterized by positive out‐of‐plane birefringence and near‐zero in‐plane birefringence. Lower drying temperatures, thinner initial film thicknesses, lower solid content, higher air speeds, and higher molecular weight all promoted stronger out‐of‐plane birefringence, reflecting enhanced in‐plane alignment and limited chain relaxation due to early vitrification. In uniaxial stretching, the low‐temperature behavior is governed primarily by hydrogen bonding, with chain entanglement playing a minor role. At higher temperatures (≈130°C), weakened hydrogen bonds and partial melting facilitate chain mobility, resulting in maximum extensibility and orientation, as evidenced by peak strain‐optical constants and four‐point SAXS patterns. These patterns indicate that lamellar rotation and interlamellar shear are key mechanisms for stress dissipation. These findings highlight hydrogen bonding as a critical factor controlling orientation, crystallization, and mechanical anisotropy in PVA, offering insights for tailoring structure–property relationships in hydrogen‐bonded polymers.