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The above‐mentioned P3 model was built up by introducing the rigid 3/1 helical conformational chains. Because of no flexibility of the rigid chain form, some pairs of the H…H distances between the CH₃ units of the neighboring chains are found to be too short compared with the sum of the van der Waals radius of the H atom. The energy calculation of the model clarified two key points about the chain packing mode. First, the packing of Rd–Ld pairs in the R3c unit cell is energetically stable with the H…H distance of about 2.6 Å. But the chain conformation is strictly constrained to possess the 3/1 helical symmetry required by the space group. Once the crystallographic 3₁ helical symmetry is erased from the chains, as seen in the P3 model, the packing of such too rigid helical chains is energetically unacceptable and the chain conformation is spontaneously deformed so that the total energy becomes lower. Second, in the stereocomplex of the asymmetric R/L content, the population of the R/R (L/L) pairs increases statistically in addition to the R/L pairs. By modifying the chain conformation and the chain packing mode, an energetically stable structure can be attained for the crystal structure composed of only the Ru and Rd (or Ld and Lu) chain stems with the same unit cell parameters as mentioned above. These R/R (L/L) pairs are considered to exist as the locally stable structures in the stereocomplex. When the R/L ratio is beyond the critical value (refer to Figure 6.17c), the large domains of R (or L) chain stems transform to the packing structure of the α form with more stable energy.

FIGURE 6.21 (a) IR circular dichroism ΔAbs and IR spectra measured for a series of the solution‐cast PLLA/PDLA blend samples with the various L/R ratios, and (b) a series of PLLA/PDLA blend samples before and after annealing. The crystalline band at 1306 cm⁻¹ shows a clear change of ΔAbs depending on the L/R ratio.

Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8066−8071.

Figure 6.22 illustrates the formation process of the stereocomplex crystal region in the amorphous film containing the random distribution of L and D (or R) chains. The domains of only L or D components and the region of randomly mixed L and D components are coexistent. These amorphous domains composed of only D or L components crystallize partially into the δ (or α) form, when the film is heated above the glass transition temperature. At a higher temperature, the α crystallites are melted. In the cooling process, these molten chains are aggregated to form the crystalline region of the stereocomplex. It is hard to imagine that they change drastically their spatial distribution to create the structure consisting of the regularly‐ and alternately‐packed R and L chain components. It is more natural to consider that the heterogeneously distributed PDLA and PLLA chains co‐crystallize together without large change of their spatial arrangements.

FIGURE 6.22 A model of stereocomplex formation from the solution cast L/D blend sample. Here “D” is equivalent to “R.”

Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8048−8065.