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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.