Morphology and mechanical properties of swollen gels and dry gel films of poly(vinyl alcohol) prepared by crystallization from solutions under simultaneous biaxially stretching

Simultaneous biaxial stretching was carried out by using poly(vinyl alcohol) (PVA) gel films which were prepared by crystallization from solutions in dimethyl sulfoxide (Me2SO) and water (H2O) mixtures. The Me2SO/H2O composition was set to be 70:30. The maximum biaxially draw ratio, 3 x 3, is much lower than 8.7 x 8.7 of ultra-high-molecular-weight polyethylene (UHMWPE) dry gel films. Even so, Young's modulus of the PVA film was almost equal to that of the UHMWPE. To address this problem, theoretical analysis was carried out 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. 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. The experimental values were lower than the calculated one. Such a disagreement between the experimental and calculated values was discussed in relation to the morphology estimated by high-resolution solid state C-13 NMR spectroscopy. Furthermore, the ultimate value of Young's modulus was estimated theoretically by assuming an ideal simultaneous biaxially stretched film with 100% crystallinity and the perfect orientation of the c-axes parallel to the film surface. The predicted value at absolute temperature was less than 13.5 GPa, suggesting the impossibility to produce high-modulus and high-strength PVA sheets. Incidentally, the stress-strain curves repeated up to 40 times of swollen gels with large amount of solvents >80% passed through the same hysteresis route indicating rubber elasticity. Such an interesting phenomenon was investigated in relation to crystallization by small angle light scattering under Hv polarization condition and X-ray diffraction. (C) 2000 Elsevier Science Ltd. All rights reserved.