Chain Fodling Patterns of Semicrystalline Polymer Fromed via Different Crystallization Pathways: Roles of Chain Network

Crystallization of polymer chains has been a debatable matter due to a lack of experimental techniques to access chain-level structure during and after crystallization. Our group developed a novel strategy to trace chain trajectory of isotope labeled polymer chains by~$^{13}$C-$^{13}$C Double Quantum (DQ) NMR.~$Isotactic$-polypropylene ($i$PP) shows polymorph depending on crystallization kinetics, and metastable mesomorphic and~$\beta$~forms experience phase transitions into stable~$\alpha$~form via melting and re-crystallization by increasing temperature. In this study, we investigated chain trajectory of~$i$PP in metastable crystalline forms obtained by rapid quenching, nuclear agent, stable~$\alpha$~forms obtained via phase transition from metastable forms and quiescent crystallization from the melt. Comparing experimental DQ buildup curves with spin-dynamics simulation, it was revealed that~$\beta$~and mesomorphic forms adopt adjacent re-entry cluster with average folding number of 4-5, which are very close to that in the stable~$\alpha$~form after phase transition and in the melt-grown crystals. The results indicate that available kinetics does not influence average folding number during crystallization, and that invariance of chain network dominate chain-folding process.