Structure and properties of biaxial-oriented crystalline polymers by solid-state crossrolling

The effect of biaxial orientation by solidstate crossrolling on the morphology of crystalline polymers including polypropylene (PP), high density polyethylene (HDPE) and Nylon 6/6 was investigated with polarized optical microscopy, atomic force microscopy, wide-angle X-ray scattering, and small-angle X-ray scattering techniques. It was found that crossrolling gradually changed the initial spherulitic structure into a biaxially oriented crystal texture with chain axis of crystals becoming parallel to the rolling direction for all three polymers. The effect of microstructure change on the macromechanical properties was studied in tension at both ambient temperature and-40°C. In tension at room temperature, the localized necking deformation of HDPE and PP control changed upon orientation into homogeneous deformation for the entire sample length. This was attributed to that the oriented crystal morphology eliminated the stress concentration, which existed in the original spherulitic structure from lamellae orientation in the polar and equatorial regions. At ambient conditions, the elastic moduli of HDPE and PP were found to decrease slightly with orientation whereas the modulus of Nylon 6/6 increased with increasing orientation. This was due to the fact that the amorphous chains of HDPE and PP are in a rubbery state and orientation increased the shear relaxation in the orientation direction but the amorphous chains of Nylon 6/6 are in the glassy state inhibited the shear relaxation. Both the yield stress and strain hardening exponent increased with increasing orientation for all three polymers. In tension at-40°C, orientation changed the failure mechanism of all three polymers from brittle fracture into ductile failure, as the original spherulitic structure was changed into an oriented structure with chain axis of crystals becoming parallel to the tension direction, which allowed chain slip deformation of crystals and resulted in oriented samples showing ductile failure.