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.
6.6.1.3 Transition Mechanism to the β Form
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.
6.6.2 Polyethylene Adipate (PEA)
PEA (─[─OCH2CH2OCO(CH2)4CO─] n ─) crystallizes to the α form with the unit cell parameters of a = 5.47 Å, b = 7.23 Å, c (chain axis) = 11.72 Å and β = 113.5° [87–89]. The two chains of almost zigzag type are packed in the unit cell with the P21/a space group symmetry.
The spherulite of PEA α form gives a beautiful ring pattern as shown in Figure 6.26. The stepwise WAXD/SAXS measurements at 1 μm step using a synchrotron X‐ray beam of 1 μm size revealed that the lamellae grow radially with the 180° twisting around the a′ axis at 7 μm pitch. On the other hand, the SEM observation of the fractured surface of the spherulite clarified that the spherulite consists of the discontinuous aggregation of the lamellar blocks in the radial direction [90]. These two observations, the lamellar twisting phenomenon and the discontinuous growth of lamellae, must be combined together consistently. In the growing spherulite of PEA α form, the molecular chains are attached on the front surface of the lamella, and the lamella grows with the small twisting of the plane sheet of the extended chain stems. Once when the 180° twisting is completed, a new lamella starts to generate discontinuously on the adjacent lamellar surface and grows with the twisting phenomenon. So far, as a general phenomenon of the ring‐pattern spherulite, the twisted lamellae have been believed to grow continuously from the center. But the study of the PEA spherulite gave us a warning about such an unsound universal concept of the continuously growing process of the twisted lamellae.
FIGURE 6.26 (a) (A) polarized optical microscopic image and (B) and (C) SEM (scanning electron microscope) images measured for PEA spherulite.
Source: Reproduced Woo et al., Macromolecules 2012, 45, 1375–1383.]
(b) Illustration of the stacking structure of the twisted lamellae.
Source: Reproduced from Tashiro et al., Polymer Journal 2019, 51, 131–141.
6.7 FUTURE PERSPECTIVES
Among many kinds of biodegradable polyesters, PLA is expected to be one of the most promising candidates. The present chapter described mainly the latest details of the crystal structure and phase transition of the various crystalline forms of this polymer. Some characteristics of such related polyesters as PHB and PEA are also compared briefly.
At this moment, there are many unsolved problems about these polymers. For example, we need to know the relation between the crystal structures and the morphology of the bulk PLLA sample. The complex hierarchical structure of the polymer makes it difficult to understand the structure–property relation from a wide range of structure. For example, the details of the structure evolution process in the isothermal crystallization from the melt are still ambiguous for these polyesters. Another important aspect of the biodegradable polyesters is related to the microscopically viewed investigation of their biodegradation process in the presence of bacteria. As described briefly about the phase transition of PHB, the role of taut tie chains passing through the stacked lamellae (or the rigid amorphous fraction, RAF) must be also revealed in association with the structure‐property relation of the bulk samples [72].
In the long history of the biodegradable polyesters, many papers have reported their structural information. However, reliability of the crystal structure and chain conformation reported in the literature remain elusive. Different from the single crystals of low‐molecular‐weight compounds, the low accuracy of the analyzed structure might be fatal for the semicrystalline polymer substances because the poor X‐ray diffraction data of small and broad diffraction spots are collected at best. It is important to always check whether the structure information is consistent with other experimental data, as exemplified from several case studies presented in this chapter.
