This study deals with the influence of processing induced crystalline orientation on the macroscopic deformation and failure behavior of thin samples of polyethylene and polypropylene. Distribution and structure of flow-induced orientations were characterized by optical microscopy, X-ray diffraction techniques, and transmission electron microscopy. Hermans' orientation functions were either determined from the flat plate wide-angle X-ray diffraction patterns or calculated from full pole figures. The deformation behavior of the oriented samples was studied in both impact and tensile testing conditions and was found to be strongly anisotropic and related to the oriented structure. For all polymers studied, an increase of extended chains (shish) in the loading direction is proposed to cause an increase in the yield stress, and a lamellar structure oriented perpendicular to loading direction leads to an increase in strain hardening. In the extruded samples, where a low level of extended chains and a high level of oriented lamellae were found, the resulting combination of yield stress and strain hardening leads to homogeneous deformation. Brittle-ductile transitions in impact toughness of the molded samples could also be explained from differences in yield stress and strain hardening. Toughness enhancement was found to be most efficient with increasing strain hardening, and the effect was less pronounced in the polypropylene samples.

The influence of crystallinity and lamellar thickness on the intrinsic deformation behavior of a number of semicrystalline polymers is studied: a poly(ethylene terephthalate) and two different molecular weight grades of polyethylene and polypropylene. The crystallinity and lamellar thickness are altered by varying the rate of crystallization from the melt and by cold crystallization (annealing) at elevated temperatures above T g but below the melting point. Crystallinity and lamellar thickness are determined by wide-angle X-ray diffraction and small-angle X-ray scattering measurements. Uniaxial compression tests are performed to obtain the large strain intrinsic deformation behavior, e.g., yield stress, strain softening, and strain hardening modulus. The yield stress is found to be proportional to lamellar thickness, whereas the strain hardening modulus is shown not to depend on crystallinity or lamellar thickness. Over the strain range experimentally covered, the strain hardening modulus appears to be well described by a simple neo-Hookean relation and appears to be related to the chain entanglement density. An affirmation for this relation arises from the observation that slowly melt crystallized samples exhibit a lower strain hardening, resulting from a lower chain entanglement density, which is expected to be caused by reeling in of the molecular chains in such a slow crystallization process. The similarity in the results observed on all polymers tested supports the conclusion that the crystalline phase does not contribute to strain hardening, which is primary controlled by the chain entanglement density.

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