Infrared spectroscopic study of CH3•••O=C interaction during poly(L-lactide)/poly(D-lactide) stereocomplex formation

The nature of the peculiarly strong interaction between the poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains was investigated by real time infrared spectroscopy during the isothermal melt crystallization process of the PLLA/PDLA stereocomplex. A very small low-frequency shift (about 1 cm(-1)) of v(as)(CH3) and a larger low-frequency shift (about 5 cm(-1)) of v(C=O) were observed. The typical butterfly pattern in the two-dimensional (2D) asynchronous correlation spectrum and the second-derivative spectra reveal that there is a peak shift for v(C=0). The red shifts of the stretching vibration modes of the methyl and carbonyl groups suggest that the interaction between the PLLA/PDLA stereocomplex is ascribed to (CH3O)-O-...=C hydrogen bonding. Another interesting result is that the peak shift of the v(C=O) band already occurs in the induction period, which indicates that the (CH3O)-O-...=C interaction is the driving force for forming the racemic nucleation of the PLLA/PDLA stereocomplex. Moreover, the 2D correlation analysis indicates that the structural adjustment of the CH3 group occurs prior to that of the C-O-C backbone during the stereocomplexation process of PLLA/PDLA. The (CH3O)-O-...=C interaction may be the reason for this sequence of structural changes.