Role of mobile phases in the crystallization of polyethylene. Part 1. Metastability and lateral growth

Abstract

The formation, lateral growth, and melting of crystals of polyethylene were studied. At pressure (P) 2-5 kbar and temp. (T) supercoolings ?10 Deg, all crystal growth occurred in the hexagonal (h) phase, and only in this phase, regardless of whether in the h or orthorhombic (o) stability regime of the P-T phase diagram. In the latter case, the h crystals represented a metastable form and transformed into the stable o phase at some stage of growth. The lower melting temp. of the h phase in the appropriate portion of the P-T phase diagram (below the triple point) was consistent with the aforementioned observations and introduced a newly recognized nongrowth region in the phase diagram. The lateral growth of h crystals was readily interpretable by an activated growth mechanism when referred to the supercooling in the h form. Here, the barrier attributable to nucleation was largely unaffected by P, but the preexponential was retarded by increasing P. The implications of these findings for polymer crystn. are discussed. [on SciFinder (R)]