Poly(lactic acid). Группа авторов

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Poly(lactic acid) - Группа авторов


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[67, 72]. The agreement between the observed and calculated values is relatively good. The crystallite modulus changes depending on the conformational regularity of the PLLA chain: the mesophase (6.7 GPa) < the δ form (12.9 GPa) < the α form (14.7 GPa) < the β form (15.4 GPa). For reference, the X‐ray‐measured crystallite modulus of PLLA/PDLA stereocomplex is 20 GPa, in which the PLLA or PDLA chain takes a 3/1 helical conformation similar to that of the β form [73].

      The crystallite modulus changes depending on the mechanical deformation mechanism of the molecular chain, i.e., the degree of change in the internal coordinates (bond lengths, bond angles, and torsional angles) induced by the external force [67–71]. For example, a planar‐zigzag polyethylene chain is deformed by the stretching of C─C bonds and the deformation of C─C─C bond angles, resulting in a Young’s modulus of 300 GPa [67, 69]. The deformation of a helical chain is induced mainly by the change of the torsional angles, giving 1–2 order lower modulus. The deformation of PLLA helical chains also occurs through the torsional angles around the skeletal C─C and C─O bonds. The disordered chain is more easily deformed due to the easier torsional angle changes. In addition, the methyl groups jutting from the main chain significantly increase the effective cross‐sectional area of the chain. The synergetic effect of these two factors results in the above‐mentioned order of the modulus among the meso, δ, α, and β forms. The crystallite modulus of PLLA chain (in the order of 15 GPa) is lower than those of isotactic polypropylene (34 GPa) and polyoxymethylene (70 GPa) [66, 70], which can be understood from the difference of the chain conformation; the latter two polymers have a tighter and more rigid helical conformation than the PLA chain.

      The anisotropic mechanical property is also important. The Young’s modulus in the plane perpendicular to the chain axis is governed mainly by the H…H interatomic interactions between the neighboring chains [67]. The Young’s modulus of PLLA in the lateral direction is in the same order as that of polyethylene, isotactic polypropylene, and so on [66, 67,71 ].

      6.5.1 Reconsideration of the Crystal Structure

Schematic illustration of (a) and (b) Various chain packing modes of PLA stereocomplex with L/D 50/50 ratio. (c) The structures of the R and L chain stems projected along the chain axis.

      Source: Reproduced from Tashiro et al., Macromolecules 2017, 50, 8048−8065.


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