The effects of chain microstructure on crystallization in polyethylene

The effects of chain microstructure on crystallization have been studied in crosslinked and branched PE. Microstructure, in the form of crosslinks, branches, and molecular weight, controls the crystallization process of the PEs through changes in the relative rates of surface nucleation and lateral spreading. In crosslinked PE, the growth kinetics conform to regime III growth due to the elimination of reptation by the crosslinks. The reappearance of regime II growth is due to the reduction in the rate of surface nucleation and is accompanied by a change in the nucleation mechanism. The structure of the crosslinked PEs has been investigated using WAXD, SAXS. DSC, and dynamic mechanical measurements The lamellar thicknesses of the crosslinked PEs show a strong crosslink density dependency and a relative insensitivity to crystallization temperature. Dynamic mechanical measurements have been used to investigate the effects of crosslinking on the mechanical properties and structure of the PEs.and show the formation of a significant interfacial region in the highest crosslink density samples. A structure for crosslinked PE has been proposed based on the results of the growth kinetics, x-ray studies, and dynamic mechanical measurements. The growth behavior of branched PEs of controlled molecular weight, branch content, and branch size has been examined using both linear and bulk growth kinetics. The growth rate decreases with increasing branch content at constant molecular weight. The regime I-II and regime II-III transition temperatures show a decrease with branch content due to changes in the relative rates of surface nucleation and lateral spreading, as well as effects due to molecular weight. Kinetic theory has shown that the crystallization behavior can be explained by effects due to reptation. The regime II-III transition temperature is believed to be due to a reduction in the rate of secondary nucleation accompanied by a change in nucleation mechanism, just as is the case in crosslinked PE. The structures of the branched PEs have been investigated using WAXD, SAXS, and DSC and show a branch content, molecular weight, and crystallization temperature dependency. SAXS results show the lamellar thicknesses of the branched PEs to be controlled by the chain length between branches. The melting behavior of isothermally crystallized branched samples has been correlated with the structure and crystallization behavior of the branched PEs with the aid of SAXS and growth kinetics.