The mechanical anisotropies (Young's Modulus) of orthogonal- biaxially stretched PVA films, such as simultaneous, strip and two-step, are investigated experimentally and theoretically, and discussed in terms of the orientation of crystallites being intrinsically anisotropic in mechanical properties.
The theoretical results were obtained by means of the calculation, on the basis of both the homogenous stress and the homogenous strain hypothesis, for averaging the aggregation of structural units composed of crystalline and noncrystalline phases.
The comparison between the experimental results and those calculated for each of three types of orthogonal-biaxially stretched films indicates the following:
1) The experimental data of Young's modulus for simultaneous-biaxially stretched films are located between those calculated theoretically in two extreme cases on the basis of the homogeneous stress and the homogeneous strain hypotheses. On the calculation, the preferential orientation of the plane-normal vectors around the crystalline chain axis was considered.
2) The calculated Young's modulus of strip-biaxially stretched films based on the homogeneous stress hypothesis are rather close to the observed.
3) The change of Young's modulus of two-step-biaxially stretched films accompanies with the draw ratio λ₃ and λ₂ along two principal stretching directions. The calculated Young's modulus based on the homogeneous stress hypothesis follows to the change of the observed Young's modulus with draw ratio λ₃×λ₂.