Journal
MACROMOLECULES
Volume 53, Issue 16, Pages 7171-7177Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.macromol.0c00237
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Funding
- JSPS KAKENHI [JP18H02040, JP18H01190]
- MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant [JPMXS0118067246]
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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 scPLA 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 scPLA, 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.
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