Molecular Heterogeneity of Metallocene Short-Chain Branched Polyethylenes and Their Fractions

A comparison on the molecular heterogeneity of two whole metallocene short-chain branched polyethylenes (SCBPEs) with different hexene comonomer (butyl branching) content and their Cross-Fractionated (CF) fractions has been made. As elucidated by differential scanning calorimetry (DSC) thermal segregation experiments and the subsequent isothermal crystallization kinetics investigations, the whole metallocene SCBPEs were found to possess both inter- and intramolecular heterogeneity. The crystallization kinetics of whole SCBPEs are substantially different for samples with or without a pre-multiple-step annealing treatment. For the CF fractions, the inter-molecular heterogeneity is not evident, but intra-molecular heterogeneity exists. Nevertheless, another self-induced SCBPE fraction, as a model sample, shows good homogeneity both inter- and intramolecularly. The crystal morphology observed via transmission electron microscopy (TEM) shows that multiple-step crystallized whole polymers exhibit large scale (molecularly) phase separation, whereas the CF fractions demonstrate microscopic (segmental) segregation only.