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 -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 -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 -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 -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.

Crosslinked blends of poly(lactic acid) and polyacrylates: AFM, DSC and XRD studies

“…Pure PLA (Fig. 8a) exhibited characteristic peaks at 2θ= 12.5, 14.7, 16.6, 19.1°and 22.3°which coincided with the data published previously [25][26][27]. The main peak at 16.6°c orresponds to interplanar spacing of 5.9 Å [27].…”

Preparation and properties of multi-branched poly(D-lactide) derived from polyglycidol and its stereocomplex blends

“…Preparation, structures, properties, and applications of this structure have been extensively studied [17,[23][24][25][26][27]. Formation of stereocomplex structure is usually accompanied by homo-crystallization, predominantly when high molecular weight PLAs are employed [2,10].…”

Isothermal Crystallization Kinetics of Poly(lactic acid) Stereocomplex/Graphene Nanocomposites

“…It can be seen that both patterns show the same three peaks around 12, 21, and 24º which are related to the formation of PLA stereocomplexes crystallized in a triclinic unit cell. 21,23 The degree of crystallinity calculated for the stereocomplex PLA and scPLA/GNPO + nanocomposite was 36.4 and 41.3%, respectively. This increase in the degree of crystallinity suggests that the graphene acted as a nucleating agent.…”