Folding of Polymer Chains in the Early Stage of Crystallization

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated C-13-C-13 double quantum (DO) buildup curves demonstrated that individual iPP chains adopt adjacent reentry sequences with an average folding number < n > = 3-4 in the mesomorphic form, assuming an adjacent re-entry fraction < F > of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.