Re-organization Pathways of Semicrystalline Polymer: Solution- and Melt-Grown Crystals

Chain-folding structure is intrinsic for flexible and long semicrystalline polymer chains and it distinguishes polymer crystals from small organic ones as well as metal and ceramics. Chain-folding structure determines surface free energy who thermodynamically plays vital roles into crystallization and melting of polymer crystals. In this work, we investigate re-organization process of poly(L-Lactic Acid) (PLLA) solution (SGC) and melt-grown crystals (MGC) by using DSC, in-situ Small Angle X-ray Scattering and solid-state (ss) NMR Spectroscopy. ¹³C Double quantum (DQ) NMR combined with ¹³C selective isotope labeling identifies that SGC adopts an adjacent re-entry cluster with adjacent re-entry fraction f = n/n+1 > 0.94, where n is consecutive adjacent re-entry number whereas MGC possesses f= 0.5 (hairpin structure probably formed through entanglements). By combined uses of in-situ SAXS and one-dimensional exchange NMR, it is demonstrated lamellar thickening proceeds via thermally activated crystalline chain dynamics in the latter whereas lamellar thickening does not occur in the former despite the chain dynamics being similarly activated. Therefore, the latter experiences solid-phase re-organization whereas the former requires melting and re-crystallization which drastically changes local chain trajectory as evidenced by a large reduction of f value to 0.5, accompanying a sudden increase in long period. Upon further heating, long period in the recrystallized one follows the same line with MGC. Through these systematic studies, it is concluded that re-organization pathways of mobile polymer crystals are determined not by internal crystalline structure and dynamics but by the crystal-amorphous interfacial structure (chain folding) and probably interlamellar link.