Increased gt Conformer Contents of PLLA Molecular Chains Induced by Li-TFSI in Melt: Another Route to Promote PLLA Crystallization

Much attention has been paid on promotion of Poly(L-lactic acid) (PLLA) crystallization. The incorporation of nucleation agents is a commonly used way to enhance the PLLA crystallization. In this work, we reported another efficient strategy to accelerate PLLA crystallization by the conformational pre-ordering of PLLA molecular chains in melt. The organic salt bis(trifluoromethysulfonyl)imide lithium salt (LiTFSI) was incorporated into PLLA and the effects of LiTFSI on the crystallization of PLLA were investigated. It was found that LiTFSI is thermodynamically miscible with PLLA and the anion of LiTFSI interacts with PLLA molecular chains strongly. Small amount of LiTFSI can accelerate the crystallization of PLLA drastically and 0.5 wt % addition gives the best nucleation effects for PLLA. The in situ Fourier transform infrared spectroscopy results revealed that the pre-ordering structure of PLLA chains was formed in the melt with the incorporation of LiTFSI. It was clearly demonstrated that the relative population of gt conformer (corresponding to 10(3) helical conformation of alpha-form PLLA crystal) in PLLA/LiTFSI composites was higher than that of neat PLLA because of the interactions between LiTFSI and PLLA. Therefore, a lower energy barrier for PLLA nucleation was achieved by trace of LiTFSI because of the enhanced gt conformer contents. This work offers an alternative strategy to enhance PLLA crystallization by small molecules induced conformational pre-ordering.