Shear-induced crystallization in a blend of isotactic polypropylene and high density polyethylene

Shish-kebab morphologies were observed with relatively low shear rate and low temperature in the phase-separated isotactic polypropylene (iPP) and high density polyethylene (HDPE) blend. Both components are crystallizable polymers. In our experiments, relatively low shear rates and low temperatures were used, so that the entangled network chains cannot be broken up or disentangled, and the shish nuclei must be formed from oriented and stretched network chains instead of a bundle of pulled-out chains. The effects of shear rate, shear time and temperature on the formation and morphology of shish-kebabs were studied by in situ optical microscopy and shear hot stage under various thermal and shear histories. Optical microscopic measurement showed that the length of iPP cylindrites is much longer than the dimension of phase domains, which implies that iPP cylindrites grow through both iPP and HDPE phase domains. An unexpected 'core-shell' structure was observed in the melting procedure, which could be explained by the difference of crystallinity between 'core' and 'shell'. It is most important that two kinds of shish-kebabs, the interface morphology and transcrystallites were observed by scanning electron microscopy (SEM). SEM observation also revealed that the width of iPP shish is about 1-2 μm and the width of HDPE shish is about 100 nm. The difference in the shish width probably resulted from the lower molecular weight, higher polydispersity, less inter-chain interaction force, and faster nucleation and growth rate of HDPE relative to the iPP chains.