Temperature Evolution of the Interaction of Relaxation Processes with Local Dynamics at Terahertz Frequencies in Polymers with Hydrogen Bonds

Abstract

At terahertz frequencies, the torsional vibration 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 especially important for the molecular mobility in polymers. We studied polymers with hydrogen bonds (polyamide-6 and polyvinyl chloride) at temperatures from 90 to 400 K using far IR spectroscopy in the range of 0.25–4 THz. The following three common features were observed in the spectrum of dielectric losses ε''(ν): (i) at temperatures well below the glass transition temperature (T_g), these losses are represented by the low-frequency wing of the absorption peak due to libration of monomer units of the polymers; (ii) in the range of 0.7T_g < T < T_g, additional temperature dependent losses are observed, which may be associated with the manifestation of secondary relaxation processes; (iii) at temperatures above T_g, the primary α relaxation processes predominantly contribute to terahertz losses. The obtained results show that the evolution of terahertz losses with temperature is caused by a change in the structure of hydrogen bonds, which seems to be common for systems with similar intermolecular interactions.