Relationship between the molecular dynamics of polystyrene and its modifications and the THz absorption parameters in IR spectra

The IR spectra of polystyrene (PS) and its modifications obtained by replacing hydrogen atoms in the benzene ring with methyl groups and / or chlorine and bromine atoms have been measured and analyzed. All these spectra have an anomalously wide asymmetric absorption band with a maximum in the terahertz range from 40 to 80 cm$^{-1}$, depending on the nature of the substituent. It can be attributed to absorption associated with libration (rotational vibrations) of phenolic rings of monomer units, i.e., to absorption by the Poly mechanism. The relationship between the spectral parameters of this absorption and the molecular characteristics of the studied polymers makes it possible to directly analyze the role of the molecular structure and intermolecular forces in the dynamics of their macromolecules. It was found that the heights of potential barriers to libration of monomeric units of PS and its modifications, estimated from the analysis of Poly absorption, are close to the activation energies of low-temperature $\delta$-relaxation in these glassy polymers. Comparison of the barriers to libration of monomeric units of macromolecules of PS and its modifications with the activation barriers of local segmental mobility in the same PS confirms that the beta process as an elementary act of segmental dynamics is caused by correlated librational movement of the chain section, which is statistically independent of neighboring regions. In this sense, the universal $\delta$ process is a high-frequency precursor of the $\beta$ process in polymers like PS.