Effect of pre-shear on flow-induced crystallization of branched polypropylene

Abstract

In this work, the effect of pre-shearing time on the crystallization of long-chain branched polypropylene (LCB-PP) subjected to simultaneous cooling and shearing was investigated using a polarized optical microscope equipped with a parallel-plate shear device and a photodetector. Crystallinity and orientation were evaluated using small-angle X-ray scattering, wide-angle X-ray diffraction and Fourier-transform IR instrument. Increasing the pre-shear time was found to decrease crystallization temperature, crystallinity and orientation (reaching practically constant values at long pre-shear times), suggesting that increasing the pre-shear time causes the chain branches to be more aligned along the backbone, reducing their efficiency, the LCB-PP melt becomes less elastic and the flow-induced crystallization is minimized. This effect is significant because the crystallization temperature has decreased from 137.0°C to 122.8°C due to pre-shear with followed mixed shear and temperature induced crystallization compared to the crystallization temperature of 118.5°C achieved at purely thermally induced crystallization conditions.