Two quenched isotactic polypropylene samples with different molecular weight were used to explore the sudden stress whitening activated at large strains during stretching at elevated temperatures via the ultrasmall-angle Xray scattering technique. This kind of whitening was confirmed to be caused by the creation of cavities within the highly oriented samples. The critical strain for initiating the stress whitening increases with the increase of stretching temperature and molecular weight whereas the critical stress for the stress whitening depends only on molecular weight irrespective of stretching temperature. Thus, this peculiar stress whitening behavior under large strain deformation can be understood as a consequence of disentanglement of the highly oriented amorphous network initiated by the breaking of interfibrillar tie chains. During large strain deformation, two independent processes can occur. Besides the disentanglement induced cavitation, the stacked lamellar structure within microfibrils can be destroyed in the high molecular weight sample. In such case, the critical stress for lamellae destruction is lower than that of breaking of interfibrillar tie chains.

The crystallization behavior of a butene-1/ethylene random copolymer with 9.88 mol % ethylene counits was investigated by means of differential scanning calorimetry, wide- and small-angle X-ray scattering, and polarized optical microscopy. Unlike in its homopolymer counterpart which crystallizes always into a metastable form II from the melt state, the random copolymer was found to crystallize either into form II or stable form I′ directly from its melt state after being cooled down. The occurrence of either crystalline form only depended on the temperature where the crystalline material was molten before cooling down. Even though the material was brought to temperatures higher than the equilibrium melting temperature, heterogeneities of segmental segregation character were preserved which promoted massive nucleation of form I′ crystallites, which makes it possible that the material is able to crystallize into pure form I′. Only if when the melt temperature was high enough where all heterogeneities of the above-mentioned character were erased can the material be crystallized into pure form II. The effect is found independent of the previous crystalline form, meaning that the helical conformation of chains in the heterogeneous melt does not affect the nucleation of stable form I′.