Manifestation of the Relationship Between Molecular Relaxation Processes and Vibrational Dynamics of Polymers at Terahertz Frequencies in Their IR Spectra

Abstract

The temperature dependence of low-frequency IR spectra of glassy polymers in the range ν = 0.24–4 THz (8–135 cm⁻¹) at temperatures from 90 to 400 K has been studied. Their analysis made it possible to detect absorptions corresponding to both individual and correlated torsional vibrations, which couple to the manifestation of relaxation dynamics. The temperature evolution of IR spectra at terahertz frequencies shows 3 universal features: low-temperature, corresponding to the anharmonicity of torsional-vibrational motion, high-temperature, due to the contribution of the α-relaxation (glass transition), and intermediate, associated with unfreezing of the conformational mobility of chains during the β-transition. The results obtained were shown to be applicable to polymers with both hydrogen and van der Waals bonds between the macromolecules.

Keywords:

• β-relaxation
• far-infrared spectra
• glassy polymers
• glass transition
• molecular dynamics
• vibrational density of states and boson peak