What can polymer crystal structure tell about polymer crystallization processes?

Contrary to most or all other materials, crystallization of chiral but race mic polymers such as isotactic polypropylene is accompanied by a conformati onal 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 hand s) which take place during crystal growth, and more precisely during the cr ucial step of "efficient" helical stem deposition. The issue of proper heli cal 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 h ands, azimuthal settings and even non-parallel orientation of helix axes in space. Furthermore, a specific homoepitaxy which generates a lamellar bran ching in the ct phase "quadrites" and alpha/gamma composite structures make s it possible a) to determine the helical hand and associated azimuthal set ting of every stem in the crystalline entities and b) to determine the impa ct 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 ach iral, helical polymers) is a highly sequential and "substrate-determined" p rocess, 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 prea rranged in a kind of pseudo-crystalline bundle land as such, are not subjec ted to the high constraints of crystal symmetry) before deposition as a pre assembled entity on the substrate.