Infrared Spectroscopic Study of CH₃···OC Interaction during Poly(<scp>l</scp>-lactide)/Poly(<scp>d</scp>-lactide) Stereocomplex Formation

Abstract

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 νas(CH3) and a larger low-frequency shift (about 5 cm-1) of ν(CO) 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 ν(CO). 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 CH3···OC hydrogen bonding. Another interesting result is that the peak shift of the ν(CO) band already occurs in the induction period, which indicates that the CH3···OC 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 CH3···OC interaction may be the reason for this sequence of structural changes.