Polymer Chains Fold Prior to Crystallization

Crystallization of long polymer chains changes their own structures from a random coil state to a partially folded structure in the thin crystal lamellae. There have been continuous debates in the folding mechanism and structure of long polymer chains in the melt and solution-grown crystals. In this talk, I would like to summarize our recent efforts on understanding folding structure of a semicrystalline polymer in the melt-grown crystals by using solid-state NMR spectroscopy. NMR spectroscopy does not require three-dimensional long-range order and thus is capable of being applied to structural study even in a glassy state. First, we focus on the folding structure or local chain trajectory of poly(L-lactic acid) (PLLA) in a glassy state. Comparisons of the folding structure between the crystalline and glassy states will provide a new mechanism of the folding event and polymer crystallization. Second, we prepared various ¹³C enriched PLLAs with different molecular weights (M_w = 2.5k – 300k g/mol) across critical entanglement length (M_c = 16k g/mol). We observed molecular weight dependence of chain folding structure of PLLA. Based on the experimental results, I would like to discuss a possible mechanism for chain folding in the melt-grown crystal.