Evaluation The Hydrogen Bonding in Stereocomplex Poly(lactic acid) by Terahertz Spectroscopy*

Recently, terahertz spectroscopy has attracted attention as a powerful tool for investigating the evolution of physical bonds in polymers, because most macromolecular vibrational modes that sensitively depend on the physical bond formation, lie in the terahertz frequency range. Here, we report on the terahertz spectroscopic investigation of the hydrogen bond formation between the two enantiomeric molecular chains in the stereocomplex (sc) PLA [1]. A remarkable red-shift of the macromolecular librational mode of the sc-PLA crystals at 46 cm^-1 was observed as the crystallinity increased. It has been pointed out that the librational motion was induced by the hydrogen bond and its vibrational potential was anharmonic [2]. Thus, we considered that the hydrogen bond formation associated with the crystallization increased the anharmonicity of the librational potential, resulting in the spectral red-shift. This result suggests that the crystallization process of sc-PLA can be sensitively probed by terahertz spectroscopy.

[1] J. Hirata et al., Macromolecules 53, 7171 (2020).
[2] F. Kollipost et al., J. Chem. Phys. 141, 174314 (2014).