Structure–Property Relationships of Poly(glycolic acid) and Poly(2-hydroxybutyrate)

Conformational characteristics of poly(3-hydroxyvalerate) (P3HV) and structure-property relationships of P3HV and poly(3-hydroxybutyrate)

Conformational analysis of biosynthetic and biodegradable poly(3-hydroxyvalerate) (P3HV), an analog of poly(3-hydroxybutyrate) (P3HB), was performed. The only structural difference between the two polymers consists in the side chain, which is either methyl (P3HB) or ethyl (P3HV). Molecular orbital calculations and NMR experiments were conducted on a monomeric model compound to determine the bond conformations of the main and side chains of P3HV. The refined rotational isomeric state (RIS) scheme was applied to derive configurational properties of P3HV. The characteristic ratio of P3HV (2.1-3.0) is smaller than that of P3HB (5.4-5.6), indicating greater conformational flexibility of the P3HV chain. The increased flexibility due to the internal rotation of the ethyl side group of P3HV results in a lower equilibrium melting point (130 °C) compared with P3HB (203 °C). The RIS calculations on P3HV also suggested its potential for rubber-like properties. Periodic density functional theory calculations were used to optimize the crystal structures of P3HB and P3HV and obtain their elastic moduli. Three-dimensional Young's modulus distributions of both crystals are closer to isotropic than anisotropic. In conclusion, the material design and usage of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) are discussed.

Conformational characteristics and conformation-dependent properties of poly(ε-caprolactone)

Structures and properties of isolated and crystalline poly(ε-caprolactone) (PCL) chains have been investigated. Ab initio molecular orbital (MO) calculations were conducted for methyl 6-acetoxyhexanoate (MAH), a model compound of PCL, to yield Gibbs free energies of all its existent conformers. Bond conformations of its seven bonds corresponding to the repeating unit of PCL were calculated therefrom; two C-C bonds close to the ester group show gauche preferences owing to intramolecular C-H⋯O attractions, and the other five prefer trans forms. ¹H and ¹³C NMR vicinal coupling constants of ¹³C-labeled MAH yielded bond conformations consistent with the MO calculations. The rotational isomeric state (RIS) calculations on PCL led to the characteristic ratios (4.1-4.5) that agree with those estimated experimentally from the Stockmayer-Fixman plots. Periodic density functional theory calculations on PCL crystals yielded the optimum structures and Young's moduli in the a-, b-, and c (fiber)-axis directions. The fiber-axis modulus (252 GPa) falls short of that (333 GPa) of polyethylene but exceeds that (182 GPa) of poly(ethylene terephthalate), whereas the three-dimensionally averaged Young's modulus (10.7 GPa) of PCL is the smallest of those of representative polymers investigated so far. The enzymatic selectivity of biodegradable polyesters is discussed herein in terms of their conformational characteristics and surface structures of enzymes.

Fast-Scanning Chip-Calorimetry Measurement of Crystallization Kinetics of Poly(Glycolic Acid)

We report fast-scanning chip-calorimetry measurement of isothermal crystallization kinetics of poly(glycolic acid) (PGA) in a broad temperature range. We observed that PGA crystallization could be suppressed by cooling rates beyond -100 K s^-1 and, after fast cooling, by heating rates beyond 50 K s^-1. In addition, the parabolic curve of crystallization half-time versus crystallization temperature shows that PGA crystallizes the fastest at 130 °C with the minimum crystallization half-time of 4.28 s. We compared our results to those of poly(_L-lactic acid) (PLLA) with nearby molecular weights previously reported by Androsch et al. We found that PGA crystallizes generally more quickly than PLLA. In comparison to PLLA, PGA has a much smaller hydrogen side group than the methyl side group in PLLA; therefore, crystal nucleation is favored by the higher molecular mobility of PGA in the low temperature region as well as by the denser molecular packing of PGA in the high temperature region, and the two factors together decide the higher crystallization rates of PGA in the whole temperature range.

Molecular Design of Aromatic Polythionoesters

As an example of molecular design of new polymers, structures and properties of poly(ethylene thionoterephthalate) (PET[S₂]) and the related polymers have been predicted from calculations of ab initio molecular orbital (MO) theory, rotational isomeric state (RIS) scheme, and periodic density functional theory (DFT). The MO calculations were confirmed by NMR experiments and introduced to the RIS scheme for PET[S₂] to yield its configurational properties, which are compared herein with those of analogous polyester, polythioester, and polydithioester. Configurational properties of randomly thiono-substituted poly(ethylene terephthalate) (PET), PET[S _z O_1-z ], were also evaluated as a function of sulfidity (z). On the assumption that the crystal of PET[S₂] can be expressed as an isomorphic replacement of the PET crystal, the crystal structure was optimized by a periodic DFT simulation and its Young's moduli in the a-, b-, and c-axis directions were, respectively, evaluated to be E_a = 0.94(7.20) GPa, E _b = 19.58(22.26) GPa, and E _c = 142.1(182.4) GPa, where the parenthetic values are those of the PET crystal. There is a possibility that properties of PET[S _z O_1-z ] will be controlled between those of PET and PET[S₂] by adjusting the sulfidity. The potential practical applications of the polythionoesters are also discussed herein. By purely theoretical computations, the structures and properties of the not-yet synthesized polymers were predicted quantitatively; that is, the theoretical molecular design of new polymers has been achieved.