Temperature dependence of the morphology and mechanical properties of poly(vinyl alcohol) drawn films prepared by gelation/crystallization from solutions by X-ray and solid state 13C NMR

Abstract

The morphology and mechanical properties of PVA were studied using atactic poly(vinyl alcohol) (at-PVA) dry gel films prepared by crystallization from solutions in dimethyl sulfoxide (Me 2SO) and water (H 2O) mixtures, in which Me 2SO/H 2O composition was set to be 60:40. The hot homogenized solution was quenched by pouring it into an aluminum tray controlled at −80°C, thus generating a gel. The crystal lattice modulus along the chain axis was measured by X-ray diffraction. The values were 200–220 GPa, which is independent of temperature up to 170°C. However, the values became lower with further increase in temperature and the value at 200°C became 133 GPa. This tendency is different from the previous reports showing rapid decrease in the crystal modulus at higher temperature than 120°C. The difference means that crystallites within the specimens prepared by quenching the solution (Me 2SO/H 2O=60:40) at −80°C are in much more stable state than those of the previous specimens. In spite of the temperature independence of the crystal modulus along the chain axis and the crystallinity, the storage modulus similar to Young's modulus decreases gradually with increasing temperature. To study the mechanical properties and molecular orientation of the present specimens with the stable crystallites, the orientational behavior of crystallites was estimated in terms of the orientation distribution function of crystallites, and Young's modulus was calculated by using the generalized orientation factors of crystallites and amorphous chain segments estimated from the orientation functions of crystallites and amorphous chain segments. In doing so, the theoretical calculation was carried out by using a three-dimensional model, in which the oriented crystalline layers are surrounded by an anisotropic amorphous phase and the strains of the two phases at the boundary are identical. The theoretical values were in good agreement with the experimental ones. The 13C NMR measurements suggested that the inter-molecular hydrogen bonds may be broken by drawing and the intra-molecular hydrogen bonds are more preferably formed in the mm and mr sequences with draw ratio but no marked difference in the structure exists between the crystalline and non-crystalline regions, as judged from the hydrogen bonding in the triad sequences. Even so, the spin–lattice relaxation time, T 1C, decreased drastically with increasing temperature reflecting drastic activity of chain mobility. Accordingly, the drastic decrease in Young's modulus (the storage modulus) is thought to be due to the fact that chain mobility in the amorphous phase becomes more pronounced as temperature increases.