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

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)