Poly(lactic acid). Группа авторов

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.

6.6 PHB AND OTHER BIODEGRADABLE POLYESTERS

      In addition to PLA, several aliphatic polyesters are becoming good targets for biodegradable and carbon‐neutral multipurpose polymers, though they do not have necessarily the asymmetric carbon atoms on the skeletal chains. Because of the relatively high biodegradability, these various polyesters have become increasingly important in commodity field, including medical and pharmaceutical applications, food packaging, and even automobiles. To understand and improve their physical properties, it is needed to know how the combination between the ester parts and the aliphatic (and aromatic) segments affects the structure, phase transition, and crystallization behaviors in the different manner from those of the above‐mentioned PLA case. In this section, several representative aliphatic polyesters are listed, and their structural characteristics are briefly described.

      6.6.1 Poly(3‐Hydroxybutyrate) (PHB)

       6.6.1.1 Crystal Structure of α Form

PHB crystallizes usually as the α form. The orthorhombic unit cell parameters are a = 5.76 Å, b = 13.20 Å, and c (chain axis) = 5.96 Å, in which the two 2/1 helical chains of TTGG conformation are packed with the P2₁2₁2₁ space group symmetry [79]. Figure 6.23 shows the crystal structure of the α form [80, 81]. In the IR spectra of the α form, the anti‐symmetric CH stretching mode of CH₃ groups [ν _as(CH)] was found to appear at appreciably high frequency side, 3009 cm⁻¹, which was interpreted as a result of the intermolecular CH₃…O(═C) interactions between the neighboring chains coming from the anomalously short H…O distance (2.62 Å) [80, 82].

FIGURE 6.23 (a) Chain conformation and (b) chain packing mode obtained for the PHB α and β forms.

      Source: Reproduced from Phongtamrug and Tashiro, Macromolecules 2019, 52, 2995–3009 and Wang and Tashiro, Macromolecules 2016, 49, 581–594.

       6.6.1.2 Crystal Structure of β Form

The PHB β form is produced by strongly stretching an oriented α form sample [83]. It is actually impossible to obtain the pure β sample even when the sample is stretched up to the occurrence of fracture. Therefore, the observed X‐ray diffraction pattern contains necessarily the two patterns of the α and β forms. Figure 6.24b shows the X‐ray diffraction pattern of the pure β component, which was obtained by subtracting the diffraction pattern of the pure α form from the original pattern [81]. The equatorial line consists of the highly oriented spots, while the layer lines are quite diffuse. The unit cell parameters were determined as a = b = 9.22 Å, c (chain axis) = 4.66 Å and γ = 120°. The repeating period 4.66 Å corresponds to the almost extended chain conformation (see Figure 6.23a). After the many trial‐and‐error processes, the structure model of the space group P3₂21 was found to reproduce the observe diffraction profiles along all the layer lines reasonably. Figure 6.23b shows the thus‐obtained crystal structure of the β form. The upward and downward chains are located at one lattice site at 50% probability. Figure 6.24c compares the observed diffraction profiles with those calculated for this model.

      At this stage, we can compare the chain conformations of the various crystalline forms of PLA and PHB as follows. Roughly speaking, PLA takes the TTG chain conformation, though the T and G values vary slightly among the different crystal forms (α, δ, and β forms). On the other hand, the PHB α form takes the conformation of TTGG sequences, but the positions of these torsional angles are different from those of PLA chain although the local chemical structure is similar to each other. Besides, the strong stretching of the PHB chains causes the large change of the torsional angle from G to T. The rough comparison of the torsional angles is made