Analysis and Observation of Polymer Crystal Structures at the IndividualStem Level

Abstract

Several theories or schemes of polymer crystallization that differ from classical nucleationand growth processes have been put forth recently. They assume some form of structure developmentin the polymer melt prior to crystallization. This ordering assists crystallization or initiatesthe build-up of crystal precursors that ultimately form the full-grown crystal by accretion and reorganization.These schemes are evaluated by analysis of the resulting crystal structure (by adopting a strictlystructural standpoint). More precisely, the outcomes of selection processes that take place duringcrystallization of syndiotactic and isotactic chiral but racemic polyolefins are visualized. Bothtypes of polymers can form right-handed or left-handed helical stems in a crystal lattice (inother words there is a conformational choice), but the hand of each helical stem must obey thecrystallographic symmetry rules corresponding to the phase (either chiral or antichiral) that is produced.Direct observation of the helical hand of stems building up a single layer and embedded in theircrystallographic environment is not normally achievable. It can however be approached using a combinationof epitaxial crystallization on a foreign substrate and Atomic Force Microscopy (AFM). Indeed,selective dissolution of the substrate makes it possible to reach the first layer deposited on thatsubstrate and image it (for example by Atomic Force Microscopy). The stems that build up isochirallayers can be shown to have a common helical hand. In one favorable case (the fully antichiralcrystal form I of syndiotactic polypropylene), the hand of individualstems has been determined. These observations and analyses indicate that the helical hand of stemsis highly dependent on the substrate or growth face topography; in other words they indicate thatthe depositing stem probes and adapts to the surface structure prior to successful attachment. Theseobservations strongly support a crystallization process controlled by the growth front ratherthan by earlier events that may take place in the polymer melt. In a different approach, useof polyolefins that bear a chiral side-chain and adopt preferred helical conformations in solutionand in the melt has been suggested as a way to investigate the relationship (if any) betweenthe helical hand in the melt and that of the resulting crystal structures. On cooling from the melt,they form liquid crystalline phases that later convert to the final crystal structure. In one documentedcase at least, the final crystal structure is antichiral, whereas the liquid crystalline structureis chiral. These systems, although highly specific and possibly not representative of more commonpolymers, provide an opportunity to investigate molecular processes that may take place if some typeof preordering takes place in the polymer melt.