Crystal Structure of Poly(lactic acid) Stereocomplex: Random Packing Model of PDLA and PLLA Chains As Studied by X‑ray Diffraction Analysis

The crystal structure model of stereocomplex between poly­(l-lactic acid) (PLLA) and poly­(d-lactic acid) (PDLA) with the L/D ratio of 50/50 was proposed by several groups [the space group P1 model by Okihara et al. (1991) and R3c or R3̅c model by Cartier et al. (1997)], among which the model of the space group R3c (or R3̅c) had been currently recognized as the most preferable one. However, the thus apparently established model cannot explain such an important experimental fact that the stereocomplex is formed not only for the PLLA/PDLA blend of the L/D ratio 50/50 but also for the blends with the L/D ratio of 70/30–30/70. We have proposed here a new model with the space group P3, which can cover the SC structures of the L/D ratio in the range of 70/30–50/50–30/70. This model can show the more quantitative reproducibility of the observed X-ray diffraction data for both the hkl and 000l reflection profiles. In particular the experimental observation of the 0003 reflection is inconsistent with the previous model of the space group R3c, which requests the appearance of only the 000l with l = 6, 12, etc. In the newly proposed P3 model the molecular chains take the regular (3/1) helical conformation; however, the packing mode is not regular. The unit cell consists of the statistically disordered arrangement of the right-handed (R) and left-handed (L) chains of the upward (u) or downward (d) directionality. For example, in the case of the stereocomplex with L/D ratio 70/30, a pair of the left-handed downward (Ld) and right-handed upward (Ru) chains is located at one lattice site at the statistical occupancy of 70/30 ratio and another pair of the left-handed upward (Lu) and right-handed downward (Rd) chains is at the adjacent lattice site at the same 70/30 ratio. The unit cell contains the three sets of these two pairs, which are connected to each other by the symmetric relation of the 3-fold rotation axis. The proposed statistical packing model of the upward and downward chains can explain such various observations as the spherulite formation or the existence of the lamellae with the chain folding structure in the spherulite, the solvent-induced change from the stereocomplex to the α form, and so on.