DIELECTRIC-RELAXATION OF POLY(P-HYDROXYBE NZOIC ACID) - TORSIONAL VIBRATIONS OF ESTER GROUPS

Dielectric relaxation of poly(p-hydroxybenzoic acid) due to torsional vibrations of the dipoles of ester groups in the main chain is conside red. A dynamic model of harmonically bound rotators interacting with t he crystal lattice is applied. The model contains two sets of force co nstants, which determine either intrachain correlations between rotato rs along a given chain or interchain interactions in the crystal latti ce. The correlation functions for the cosines of rotational angles of different rotators in the chain are calculated. These correlation func tions are manifested in the dielectric relaxation of poly(p-hydroxyben zoic acid). The fluctuations of the components of dipole moments in a plane normal to the chain axis (i.e., the transverse components) and t he dipole moment fluctuations in the plane of the chain (i.e., the lon gitudinal components) are considered. The transverse component is rela ted to the simplest crankshaft type of the internal rotational motion of ester groups. This type of motion was considered previously. The or igin of the longitudinal component is the superposition of internal ro tations and bending vibrations. The amplitudes of the transverse and l ongitudinal fluctuations depend on intrachain and interchain interacti ons. Their contributions to the dielectric relaxation of poly(p-hydrox ybenzoic acid) are comparable with each other. The fluctuations of the total dipole moment are calculated with both transverse and longitudi nal components taken into account. The obtained results are compared w ith the experimental data and predictions based on the molecular mecha nics calculations performed previously. Consideration of only transver se fluctuations of the dipole moment is insufficient to adequately des cribe the experimentally observed dielectric relaxation of poly(p-hydr oxybenzoic acid). On the contrary, consideration of a superposition of both transverse and longitudinal contributions agrees well with the e xperimental data if the force constants estimated from the torsion pot ential (which was calculated by the AM1 method) are used. In this case , the activation energy corresponding to activation of the mobility of ester dipoles in noncrystalline regions of the sample of poly(p-hydro xybenzoic acid) is obtained from the experimental data on dielectric r elaxation.