THE THEORETICAL MODULUS OF BIAXIALLY ORIENTED POLYMER-FILMS

Young's moduli of balanced, biaxially oriented polymer films composed of single crystal elements are calculated for several hypothetical structures, in the ideal case of 100% crystallinity. The numerical data used in the computations are the mechanical properties of the single-crystal unit, in the form of the modulus or compliance tensor. Structural characteristics of equibiaxially oriented polyethylene samples indicate that the appropriate model is an aggregate of single crystals, for which the tensile modulus is calculated by Reuss-averaging the properties of the structural unit over all directions. It is predicted, in accordance with experimental observations, that the modulus of such equibiaxially oriented films is low in comparison with the axial theoretical modulus of single crystals, due to the large contribution of the shear compliance of the crystals. By contrast, calculations presented in this study indicate that structures in which a uniform distribution of strain is achieved may exhibit a very high modulus. Such materials include laminates and certain composites. © 1990, American Chemical Society. All rights reserved.