New trends in polymer crystallization studies: Part I ‐ from alkanes to polyethylene: The onset of chain folding in polymer crystallization

Abstract

AbstractWorks on strictly uniform ultra‐long n‐alkanes enabled the exploration of the onset of chain folding with increasing molecular length. It was established that folding sets in beyond a certain chain length, more specifically dependant on crystallization temperature (Tc), starting in an initially irregular chain deposition, reorganizing subsequently into a strictly quantized conformation of integral fractional fold lengths through either thickening or thinning of the crystal. In the final reorganized stage the folds are regular and sharp. The isothermal crystallization rates for extended chain crystals were found to go through a maximum followed by a minimum with decreasing Tc where chain folding takes over. This remarkable rate inversion, observed for C246 H494 and C198 H398, so far, occurs both for solution and melt crystallization and could be verified for both primary nucleation and crystal growth. We interpret it in terms of a “self poisoning” phenomenon, where chain depositions, which occur transiently in the “wrong” conformation, are blocking the nucleation and growth of the crystal, a phenomenon also reflected in the reversal of the temperature dependence of isothermal refolding on crystallization from solution. Rate reversals of all these kinds promise to be basic to our understanding and are giving rise to recent alternative explanations from elsewhere which are being quoted and discussed.