Relationship between the Molecular Dynamics of Polystyrene and Its Modifications and the THz Absorption Parameters in IR Spectra

IR spectra of polystyrene (PS) and its modifications, obtained by substitution of hydrogen atoms in the benzene ring by methyl groups and/or chlorine and bromine atoms, have been measured and analyzed. All these spectra exhibit an anomalously wide asymmetric absorption band peaking in the THz range from 40 to 80 cm(-1), in dependence of the substituent nature. It can be assigned to absorption due to libration (rotational vibrations) of phenol rings of monomer units (i.e., Poley absorption). The relationship between the spectral parameters of this absorption and molecular characteristics of the polymers under study makes it possible to analyze directly the role of the molecular structure and intermolecular forces in the dynamics of their macromolecules. It is found that the heights of the libration potential barriers of monomer units of PS and its modifications, which have been estimated by analyzing Poley absorption, are close to the activation energies of low-temperature delta relaxation in these glassy polymers. A comparison of libration barriers of monomer units of macromolecules of PS and its modifications with the activation barriers of local segmental mobility in the same compounds confirms that the beta process (as an elementary event of segmental dynamics) is due to correlated librational motion of a chain portion which is statistically independent of neighboring portions. In this sense, universal delta process is a high-frequency precursor of the beta process in polymers similar to PS.