The role of transient hexagonal phase in crystallization of polymers

Crystn. studies of polyethylene (I) under hydrostatic pressure substantiate the findings by D. C. Bassett et al. that crystn. occurs initially in the hexagonal (h) phase, even within the thermodynamically stable orthorhombic (o) region. Present studies demonstrate that this is, in fact, a necessary condition for crystn., at least in the temp. and pressure range in question; in fact, once a crystal transforms from h to o phase, further growth of crystals stops. Morphol. features show that crystal growth includes both lateral and thickening growth; in the latter case the chains thicken continuously from folded to extended form and beyond. It is the thickness in a crystal which governs the phase inversion from h to o phase. Thickness dependence on phase inversion is explicitly shown in the trans-1,4-polybutadience, where the mobile h phase, similar to I, can exist, even at atm. pressure. [on SciFinder (R)]