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 ν as (CH 3 ) and a larger low-frequency shift (about 5 cm -1 ) of ν(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 ν(C O). 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 CH 3 ···O C hydrogen bonding. Another interesting result is that the peak shift of the ν(C O) band already occurs in the induction period, which indicates that the CH 3 ···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 CH 3 group occurs prior to that of the C−O−C backbone during the stereocomplexation process of PLLA/PDLA. The CH 3 ···O C interaction may be the reason for this sequence of structural changes.