Structure Formation in Polymer Systems

Abstract

One of the major problems in polymer physics is the understanding of polymer crystallization. For chains with a regular chemical constitution formation of crystals is in principle possible. These macromolecular crystals are generally built up by chains in a helical low-energy conformation with a uniform orientation of the helix axes. Principally, in order to form the crystalline equilibrium state all chains have to be extended over their full length. Starting from a highly entangled melt of coiled macromolecules this ideal crystalline state is never reached. Cooling a melt below the equilibrium melting point generally leads to the formation of metastable structures which are only in parts crystalline. One observes layer-like crystallites with a thickness in the order of 10 nm which are separated by amorphous layers, thus setting up a lamellar two-phase structure /1/. Thicknesses of amorphous and crystalline layers change with the crystallization temperature; they are determined by the kinetics of transformation. Lowering the transformation rate by increasing the crystallization temperature generally results in thicker crystallites. Under normal conditions transformation starts from nuclei which expand into growing spherulites. There are however special cases where the transformation from the melt to the partially-crystalline solid state occurs continuously without a passage over a heterogeneous state. This second mode of polymer crystallization has been observed for some preoriented systems where chains were stretched in the melt /2/.