On chain-extended and chainfolded crystallization of polyethylene

Abstract

Various polyethylenes have been crystallized from the melt at pressures between 0 and 6 kbar. Their atmospheric melting points fall, in each case, into two non-overlapping regions, with a sharp changeover at a pressure in the region of 3 kbar which is a decreasing function of molecular weight. This separation is used in conjunction with differences in spherulitic texture to distinguish chainfolded polymer (which predominates at lower pressures) from chain-extended polyethylene produced at higher pressures. The thickness of chain-extended lamellae increases with crystallization temperature at a given pressure, with crystallization pressure at a constant super-cooling and with molecular weight (between 2·10⁴ and 5·10⁴) for crystallization at the same temperature and pressure. It is then shown from differential thermal analysis of crystallization as it occurs that chain-folded and chain-extended polyethylene result from two distinct processes and that when both occur at the same pressure and temperature, chain-extended growth is the faster. This and the fact that it is more difficult to crystallize shorter molecules as chain-extended lamellae disprove nearly all previously suggested explanations of chain-extended growth in polyethylene.