Mechanism of the stereocomplex formation between enantiomeric poly(lactide)s

Abstract

Poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) crystallize into a stereocomplex with a melting point 50 °C higher than the crystals of the enantiomers. The racemic crystal is formed by packing β-form 31-helices of opposite absolute configuration alternatingly side by side. Single crystals of the stereocomplex exhibit triangular shape. The drastic difference of the powder patterns evidences the different packing of the β-form in the stereocomplex and in crystals of the pure lactides. By force field simulation of the stereocomplex and the PLLA unit cells and of their powder patterns, the reasons for the different packing could be clarified. Between the β-helices in the stereocomplex, van der Waals forces cause a specific energetic interaction-driven packing and, consequently, higher melting point. Helices of identical absolute configuration pack different from pairs of enantiomer β-helices. Packing favors R-type helication. A well-defined 103-helix has not been found. Good agreement with the experimental powder patterns proves the correctness of the simulations. On the basis of morphology, packing calculations, and atomic force microscopy, we propose a model of stereocomplex crystal growth, which explains the triangular shape of single crystals. Thus, for polymer components beyond chain folding length, the stereocomplex formation by simultaneous folding of the two types of chains is plausible. The triangular type of crystallizing offers favorable position for the polymer loops during the crystal growth. Our study of the PLA complexation mechanism may offer a chance to predict other polymeric stereocomplexes and their properties.