High pressure crystallization of ethylene-octene copolymers

Polyethylenes generally crystallize in the orthorhombic form at atmospheric pressure, whereas the homopolymer is known to produce chain-extended crystals through the intervention of a hexagonal phase at pressures in excess of 3kbar. The crystallization of ethylene-octene copolymer with 4 hexyl branches per 1000 carbon atoms, at relatively low pressures, produced high melting points inconsistent with thin crystals. SEM studies demonstrate the presence of significant amounts of spherulites containing crystals ca. 100nm in thickness, consistent with the thermal behavior. Thick crystals suggest the formation of a phase that is capable of dissolving large number of hexyl groups during crystallization. The phase diagrams of ethylene-octene copolymers have been constructed using polarized light intensity studies at elevated pressures. It has been established that the triple point is very sensitive to comonomer (hexyl branch) content and moves rapidly to low pressures as comonomer content is increased. It is believed that the triple point in the systems discussed here is between the melt, monoclinic and orthorhombic phases. On dropping pressure to atmospheric, the metastable (monoclinic) phase decays to orthorhombic phase, necessitating the expulsion of excess hexyl groups to an amorphous phase. The process is akin to a eutectoid transformation and results in conversion of the thick monoclinic crystals to thin orthorhombic crystals, which are detected by SAXS and show the expected low melting points at atmospheric pressure. The influences of crystallization temperature and of hexyl branch content on the band spacing have been elucidated for ethylene-octene copolymers, generating valuable new information. A given band spacing is found at increasingly lower crystallization temperatures as hexyl group content is increased. There appears to be a relatively linear relation between the observed temperature shift and the hexyl group content. Studies of a copolymer containing 4 hexyl groups per 1000 carbons permit a concurrent study of all three growth regimes. The reciprocal band spacing versus crystallization temperature plot shows two changes of slope coincident with the two changes of regime. Hence there appears to be a very clear connection between regime plots, and hence growth rate, with band spacing, which has not been reported before.