Crystallization and morphology of metallocene polyethylenes with well-controlled molecular weight and branching content

The crystallization and morphology of some metallocene polyethylenes with well-controlled molecular weight and branching content were investigated by DSC, WAXD, PLM and SALS. The banded spherulites observed in linear PE are not seen in crystallization of branched PEs. The small spherulites with small lamellae or fringed micelle crystals are formed when branching content is higher, as suggested by PLM and SALS. The expansion of the unit cell was observed by WAXD as the molecular weight and branching content increased. At even higher branching content (more than 7 mol%), a shrinkage of the unit cell was seen, probably due to a change of crystal morphology from lamellar-like crystals to fringed micelle-like crystals. Crystallization temperature, melting point and crystallinity are greatly decreased for branched PEs compared with linear PEs. The equilibrium melting temperature cannot be determined via the Hoffman-Weeks approach for branched PEs since T_m is always 5-6°C higher than T_c and there is no intercept with the T_m = T_c line. Our results show a predominant role of branches in the crystallization of polyethylene.