What can polymer crystal structure tell about polymer crystallization processes?

Contrary to most or all other materials, crystallization of chiral but racemic polymers such as isotactic polypropylene is accompanied by a conformational rearrangement which leads to helical geometries: the building units of the crystal are helical stems, approximate to 10-20 nm long, which can be either right-handed or left-handed. Helical hand cannot be reversed within the crystal structure: it is therefore a permanent marker and an indicator of molecular processes tin particular segregation/selection of helical hands) which take place during crystal growth, and more precisely during the crucial step of efficient helical stem deposition. The issue of proper helical hand selection during polymer crystal growth is mainly illustrated with isotactic polypropylene. Its various crystalline polymorphs (alpha, beta, gamma and smectic) display virtually all possible combinations of helical hands, azimuthal settings and even non-parallel orientation of helix axes in space. Furthermore, a specific homoepitaxy which generates a lamellar branching in the ct phase quadrites and alpha/gamma composite structures makes it possible a) to determine the helical hand and associated azimuthal setting of every stem in the crystalline entities and b) to determine the impact on the crystal structure and morphology of mistakes in helical hand of the depositing stem. Analysis of these morphologies demonstrates that the crystallization of isotactic polypropylene land by implication of other achiral, helical polymers) is a highly sequential and substrate-determined process, i.e. that. the depositing stem probes the topography of the growth face prior to attachment. These observations appear difficult to reconcile with crystallization schemes in which molecules (helical segments) are prearranged in a kind of pseudo-crystalline bundle land as such, are not subjected to the high constraints of crystal symmetry) before deposition as a preassembled entity on the substrate.