Abstract:
At terahertz frequencies, the torsional-vibrational motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between these processes is still poorly understood, especially at temperatures below the glass transition temperature, which is especially important for molecular mobility in polymers. We have studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K in the range 0.25–4 THz using far-infrared spectroscopy. Three general features were observed in the spectrum of dielectric losses, $\varepsilon''(\nu)$ (a) at temperatures well below the glass transition $(T_g)$, these losses are represented by the low-frequency wing of the absorption peak due to the libration of polymer monomer units. (b) At 0.7 $T_g<T<T_g$, additional temperature-dependent losses are observed, which can be associated with the manifestation of secondary relaxation processes. (c) At temperatures above $T_g$, primary $\alpha$-relaxation processes become the dominant contribution to terahertz losses. The results obtained show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which, apparently, is universal for systems with similar intermolecular interactions.
Keywords:polymers, terahertz IR-spectra, temperature dependence, $\alpha$- and $\beta$-relaxations, hydrogen bonding.
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