Weak intermolecular interactions during the melt crystallization of Poly(L-lactide) investigated by two-dimensional infrared correlation spectroscopy

The isothermal crystallization behavior of poly(L-lactide) (PLLA) at 150 degreesC from the melt was monitored by infrared (IR) spectroscopy. The two-dimensional (2D) correlation analysis of time-dependent IR spectra collected during the melt crystallization process revealed details about the intermolecular interaction of the CH3 and C=O groups and the conformational changes in the C-O-C backbone that are not easily detected by conventional one-dimensional spectra. It was found that the intermolecular interaction of the CH3 group appears during both the induction period and the growth period of PLLA melt crystallization, while the intermolecular coupling of the C=O group can only be observed during the crystallization period. The order formation of C-O-C backbone during the induction period of PLLA melt crystallization can also be clearly observed in the 2D synchronous spectra. These observations not only provide direct evidence that the distorted 10(3) helix conformation of PLLA chain in alpha crystals is due to the interchain interactions between CH3 groups, but also show that the weak interchain interactions play an important role in controlling the nucleation and growth of polymer crystallization.