Confirmation of the X-ray-Analyzed Heterogeneous Distribution of the PDLA and PLLA Chain Stems in the Crystal Lattice of Poly(lactic acid) Stereocomplex on the Basis of the Vibrational Circular Dichroism IR Spectral Measurement
Kohji Tashiro*†Orcid, Hai Wang†, Naoto Kouno†, Jun Koshobu‡, and Keisuke Watanabe‡
Macromolecules 2017, 50, 20, 8066–8071 Publication Date:October 4, 2017
In the previous report we proposed, on the basis of the detailed X-ray diffraction data analysis, the crystal structure of PLLA/PDLA stereocomplex (SC) with the space group P3, which is different from the R3c or R3̅c models proposed in the literature. The most important point of the change of the structure model is to explain the experimental data that the SC can be formed in the region of the L/D ratio of 70/30–30/70, not only for the L/D = 50/50 ratio. In our P3 structural model, one crystal site is occupied by the Ru and Ld (or Rd and Lu) (3/1) helices at the statistical probability determined by the L/D ratio, where R and L indicate the right-handed and left-handed helical chains and u and d show the upward and downward directionality along the chain axis, respectively. In other words, the D and L chain stems can coexist in a heterogeneous way in the SC crystalline lattice. This structural model has been supported experimentally from the totally different point of view on the basis of the vibrational circular dichroism IR spectral measurement. When the right-handed (r) and left-handed (l) circularly polarized IR beams are incident to the optically active chemical species, the ratio between the r and l band intensity changes depending on the relative content of the R and L species (IR circular dichroism). The IR circular dichroism of PLLA/PDLA blend sample was found to change its dichroic ratio depending on the L/D content in the sample. The most important finding is that the dichroic ratio change was detected not only for the amorphous IR bands but also for the crystalline bands, indicating definitely that the D and L chain stems can coexist in the crystalline region at the various L/D ratios as proposed by the X-ray structure analysis.
Chirality, Crystal structure, Lattices, Infrared light, Optical properties