Changes in the lamellar structure of polypropylene during biaxial orientation by hydrostatic extrusion have been examined. Examination of etched surfaces in the transmission electron microscope has revealed that up to a draw ratio of about 1.5 × 1.5 the primary mechanism is lamellar rotation into the plane of orientation, At higher draw ratios breakup of the lamellae is observed with formation of a granular structure. Thermal measurements suggest that localized melting and recrystallization may accompany the process of lamellar breakup. These morphological observations confirm the previous interpretation of SAXS and permit a more detailed model of the hierarchical structure to be proposed. Stress strain measurements show a gradual change from sharp necking to uniform deformation as the draw ratio is increased. On the other hand, the structural change at a draw ratio of about 1.5 × 1.5 Is accompanied by a large increase in the tensile impact strength.
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Pan et al. (1987) studied this question.