Читать книгу Poly(lactic acid) - Группа авторов - Страница 143

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 as shown below, where T ≈ 160–180°, T′ ≈ 150–160°, and G ≈ 50–80°.

FIGURE 6.24 The 2D X‐ray diffraction patterns of PHB. (a) The mixture of the α and β forms, (b) the β form pattern obtained by the subtraction of the X‐ray pattern of the pure α form from (a). (c) Comparison of the X‐ray diffraction profiles along the various layer lines observed for PHB β form with those calculated for the model with the space group P3₂21.

Source: Reproduced from Phongtamrug and Tashiro, Macromolecules 2019, 52, 2995–3009.

PLA	…─C(CH3)─C(O)─O─C(CH3)─C(O)─O─C(CH3)─
(α, δ, β)	T′ T G T′ T G
PHB	…─CH2─C(O)─O─C(CH3)─CH2─C(O)─O─C(CH3)─…
α	G G T T′ G G T T′
β	T T′ T T′ T T′ T T′

It must be noted that the whole shapes of the chains are affected remarkably even when the changes of the torsional angles are not very large as seen well in the cases of PLA chains.