Stress-induced changes in microstructure of a low-crystalline polypropylene investigated at uniaxial stretching

Novel asymmetric metallocene catalysts lead to low isotaetie polypropylenes (iPP) with randomly distributed stereo irregularities. The polypropylenes are low crystalline and show elastic mechanical behavior due to physical crosslinking. The morphology of such iPP, which is responsible for the observed mechanical properties, is still sparsely resolved. In the present work a low isotactic, low crystalline metallocene iPP containing randomly distributed stereoerrors was investigated. The influence of the chain microstructure in the elastic properties was studied using two complementary investigation methods, X-ray diffraction and scanning force microscopy (SFM). For a better understanding of the unique mechanical properties, microscopic changes in morphology and strain-induced variation in chain orientation were monitored during uniaxial stretching using SFM and wide angle X-ray scattering measurements. For quantitative analysis and discussion the polymer chain orientations were calculated. The correlation between the orientation, the arrangements of the amorphous and crystalline phases observed by SFM, and the mechanical properties of the material at different elongation ratios allowed an interpretation of the macroscopic behavior on the microscopic scale. It was shown that the deformation behavior of low isotaetie polypropylene with randomly distributed stereo-errors is in agreement with existing structural models, which proposed that small crystalline domains act as physical crosslinks for the amorphous matrix.