Investigations on the crystallization of polyethylene under high pressure: role of mobile phases, lamellar thickening growth, phase transformations, and morphology

At 1.8-5.5 kbar pressure and supercooling at ?10 Deg, formation and growth of identifiable single crystals of polyethylene only proceeded in the mobile hexagonal (h) phase, no matter whether this phase was stable or metastable in the appropriate portion of the pressure-temp. phase diagram. When h was metastable, growth in the thickness direction through chain folding to full extension or to considerably beyond full extension was terminated by transformation into the orthorhombic phase or by layer-to-layer impingement. A primary thickening growth mechanism involving addn. of new materials through chain sliding, interpreted by the sliding diffusion theory, was different from the traditionally considered lamellar thickening through chain rearrangement between lamellas. Phase diagrams needed extension to include metastable and virtual phase lines which had a previously unsuspected influence on crystn. [on SciFinder (R)]