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As clarified by the IR spectral data analysis, the β form starts to appear above a constant stress point (critical stress σ*) and increases its content with stress [81]. Another important point is obtained from the SAXS data analysis. As shown in Figure 6.25a, the α form sample shows the meridional 2‐point SAXS pattern. The long period of the stacked lamellae is about 200 Å. When the sample is stretched to 30–60% of the original length, the new long‐period peak of about 400 Å period appeared. At the same time the diffuse scattering peak was detected along the equatorial line, the averaged period of which is about 90 Å. By taking all of the thus‐collected experimental data into consideration, the transition mechanism of the β form crystal regions is described as illustrated in Figure 6.25b. Here the role of the tie chains is emphasized, which pass through the neighboring lamellae and the intervening amorphous region.

The original α crystallites form the regular lamellar stacking structure. As the sample is stretched, the short tie chain segments between the neighboring lamellae start to be tensioned strongly, and the high stress is locally generated in these tie chain parts (sometimes, the fraction of these rigid chains in the amorphous region are called RAF (rigid amorphous fraction) [84, 85]. When the local stress exceeds a critical value σ*, the transformation to the zigzag chain conformation of the β form starts to occur in the highly strained tie chain parts. The thus‐created β crystalline bundles exist at the various positions with the averaged period 90 Å along the equatorial line. The α crystalline regions connected to the strained tie chains are also induced to transform to the β form. As a result, the repeating period of the β crystalline parts becomes quite long, compared with the part of the original α form. By increasing the stress furthermore, the highly tensioned tie chain segments cannot bear against the high local stress, and they are finally broken to generate the radicals. These radicals react with the neighboring chain segments and accelerate the breakage of the surrounding chain segments. As a result, micro‐voids are generated. These micro‐voids are fused into larger macro‐voids, resulting finally in the rupture of the whole sample [72, 86].

FIGURE 6.25 (a) Images of the higher‐order structure of the α and β forms of the oriented PHB sample. (b) The illustration of the higher‐order structure change in the tensile deformation of the oriented PHB sample.

Source: Reproduced from Phongtamrug and Tashiro, Macromolecules 2019, 528, 2995–3009.