Stereocomplex poly(lactic acid) (sc-PLA)
crystallites exhibit higher mechanical strength and thermal stability
than PLA homocrystallites. In sc-PLA, a hydrogen bond that governs
the physical properties is formed between two enantiomeric PLA chains
during crystallization. Therefore, the investigation of this hydrogen
bond is essential for a better understanding of the properties of
sc-PLA. Here, we report on the terahertz spectroscopy of sc-PLA films
with low crystallinities. The dielectric function in the terahertz
regime allows us to estimate the crystallinity of sc-PLA because various
intra- and intermolecular vibration modes lie in the terahertz frequency
range. By comparison with X-ray diffraction measurements, the validity
of the crystallinity evaluation based on terahertz spectroscopy is
verified. Furthermore, a remarkable red-shift of the peak in the imaginary
part of the dielectric function occurs as the crystallinity increases.
By comparison with Fourier transform infrared spectroscopy, it is
considered that this red-shift is caused by the anharmonic nature
of the vibrational potential corresponding to the intermolecular libration
mode of the sc-PLA crystallites. The hydrogen bond formation associated
with the sc-PLA crystallization increases the anharmonicity of the
vibrational potential of sc-PLA, and then the red-shift of the peak
occurs. Our findings suggest that structural changes due to the stereocomplexation
can be sensitively probed by terahertz spectroscopy.