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We have investigated structural changes in a poly(vinyl alcohol) (PVA) film during uniaxial stretching in water by conducting simultaneously the tensile stress−strain measurement with small-angle X-ray scattering (SAXS) using our newly developed drawing apparatus for the in situ SAXS measurements. Below the strain of 70%, the crystalline lamellae orient to the direction perpendicular to the stretching direction and the intervening amorphous regions are elastically expanded with the film drawing in proportion to the macroscopic deformation. Beyond the strain of 70%, the molecular chains in the intermediate amorphous region are relaxed with the lamellar breakup. Above 180% strain, the structural transition of the lamellar structure to the microfibrillar one takes place, as suggested by appearance of the transversal streak with an intensity maximum on each streak and the mechanical transition. Moreover, interfibrillar interaction of the adjacent microfibrils decreases with the film stretching by the pulling-out of the tie chains, which are interpenetrating to the adjacent microfibrils, leading to the macroscopic plastic deformation of the PVA film and stress relaxation of most of the microfibrils, which is shown by the continuous longitudinal long period decrease. In the final stage of deformation, the networking with a long-range connectivity composed of the microfibrils and the interfibrillar extended amorphous chains proceeds associated with the sliding between the adjacent microfibrils with successive drawing. However, the network of the interfibrillar extended amorphous region is considered to be an origin of the strain-induced hardening, which occurs above 180% strain up to a break, because most of the microfibrils are relaxed with strain.

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