Evaluation of Crystallinity and Hydrogen Bond Formation in Stereocomplex Poly(lactic acid) Films by Terahertz Time-Domain Spectroscopy

Abstract

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.

Study of Crystallinity and Conformation of Poly(lactic acid) by Terahertz Spectroscopy

“…In this study, we investigated the PLA film samples with different crystallinity induced by a mixed solvent using THz spectroscopy. Previous reports have discussed the physical mechanism between THz dielectric function spectroscopy and crystallinity, demonstrating that it can estimate the crystallinity . The relationship between the THz absorption coefficient and the imaginary part of the dielectric function was further analyzed.…”

“…The spectral changes suggest that THz absorption spectroscopy has the ability to express the crystallinity of PLA, so we discuss it further. Hirata et al 21 estimated the crystallinity of SC-PLA using the imaginary part of the complex dielectric function in the frequency range of 1−2.5 THz. They proposed that the imaginary part of the complex dielectric function describes the electromagnetic wave losses in the THz frequency range because of macromolecular vibrations in crystals.…”

“…The vibrational modes of polymers in the THz band originate from the crystal region, and the shape of the THz spectra changes with the crystallinity. Many studies on the crystallinity of natural macromolecules such as starch and cellulose have been reported, ,, demonstrating the potential of THz spectroscopy for crystallinity characterization. Therefore, we believe that it is also feasible to estimate the crystallinity of PLA by THz spectroscopy.…”

Role of stereocomplex in advancing mass transport and thermomechanical properties of polylactide

“…1), interacting via hydrogen bonding. 32,33 The unique crystal form and chain conformation of the SC crystals endow the stereocomplex PLA (SC-PLA) based material with extraordinary performance, compared to homochiral (HC) PLA (HC-PLA). Since 1987, it has been demonstrated that SC can address real-life challenges of PLA-based materials in terms of their thermal, 34,35 mechanical, 36,37 barrier 38,39 and hydrolytic degradation 40 performance.…”

Insights into Stereocomplexation of Poly(lactic acid) Materials: Evolution of Interaction between Enantiomeric Chains and Its Role in Conformational Transformation in Racemic Blends

“…Presently, the interaction between opposite enantiomers of other polymers is also considered as the driving force for their stereocomplexation . In fact, not only the higher melting point of PLA SC but also the structural evolution in the PLA racemic melt was influenced by the interaction between the enantiomers. ,− Nevertheless, owing to the ultrasmall time window, the details of the structural evolution and role of intermolecular interactions in the very initial racemic melt are not very clear.…”