Poly(lactic acid). Группа авторов

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Poly(lactic acid) - Группа авторов


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      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.

       6.6.1.3 Transition Mechanism to the β Form

      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].

Schematic illustration of (a) The higher-order structure of the alpha and beta 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.

Schematic illustration of (a) (A) polarized optical microscopic image and (B) and (C) SEM (scanning electron microscope) images measured for PEA spherulite. (b) The stacking structure of the twisted lamellae.

      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.

      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


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