Some new habit features in crystals of long chain compounds part IV. the fold surface geometry of monolayer polyethylene crystals and its relevance to fold packing and crystal growth

Abstract

The obliquity of the fold surfaces of monolayer polyethylene crystals was investigated quantitatively. Two crystal types were used: (a) true lozenges formed at 76°c (high supercooling) previously observed as corrugated; (b) truncated lozenges formed at 90°c (low supercooling) previously observed as hollow pyramidal. Quantitative fold surface assessments were made from external shapes of uncollapsed crystals as observed in the liquid, from morphological features arising from collapse and from diffraction effects—-x-ray and electron—-given by collapsed crystals. The fold surface of (a) was found to be {(314)(110)}, while that of (b) {(312)(110)} with 100 sectors appropriate for sub-cell continuity. It is found that (b) can be accounted for by a preference of the system for one of the two basic modes of regular fold stacking along {110}_s, which preference would be swamped at higher super-coolings leading to the fold surface observed for (a). In the case of (b) a new unexpected arrangement of sectors was discovered where diametrically opposite sectors are parallel, separated by a region of the form of a curved step along the short diagonal, this [chair] form being in equal proportion to the hollow pyramids in the same preparation. The occurrence of this [chair] crystal in conjunction with that of the pyramidal crystal and the existence of corrugated forms in preparation (a) is taken as strong support for rearrangements occurring into more favoured fold packings subsequent to the laying down of the folded ribbons in the original act of growth. A number of further morphological features are described and discussed and some new techniques, observations and deductions concerning the collapse of crystals when dried down from suspension, are reported.