Читать книгу Poly(lactic acid) - Группа авторов - Страница 146

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.