Stoichiometry and Packing Structure of Poly(lactic acid) Stereocomplex as Revealed by Solid-State NMR and 13 C Isotope Labeling.

Poly(l-lactic acid) (L)/poly(d-lactic acid) (D) blends form a stereocomplex (SC) at a mixing ratio of 7/3-3/7. The stoichiometry and packing structure of L/D in the SC are controversial topics because the SC is semicrystalline and because the enantiomeric pair has the same chemical structure. In this study, both the stoichiometry and packing structure of 33% 13 C CH 3 -labeled ( l ) L/nonlabeled D blends at mixing ratios of 7/3-3/7 were investigated by using solid-state (SS) NMR. The 13 C CO signals in natural abundance provided the fractions of the SC (Φ SC ), α, and amorphous regions of l -L/D blends. Moreover, the 33% 13 CH 3 -labeled signals could determine the fraction of only l -L in the SC (Φ L ) and amorphous region. These two data sets allowed us to determine the stoichiometry of l -L/D in the SC (Φ L-SC /Φ D-SC ) to be 1/1. 13 C- 13 C double-quantum (DQ) buildup curves of l -L in the SC followed one universal curve even at different mixing ratios. Comparison of the experimental and simulated DQ curves led to the conclusion that all SC crystals adopt a regular packing structure at varied mixing ratios.