Dielectric Relaxation and Thermally Stimulated Discharge Currents in Liquid-Crystalline Side-Chain Polymethacrylates with Phenylbenzoate Mesogens Having Tail Groups of Different Length Natalia Nikonorova* and Tamara Borisova Institute of Macromolecular Compounds of the Russian Academy of Sciences, Bolshoy pr. 31, 199004 St. Petersburg, Russia Evgueni Barmatov Chemistry Department, Moscow State University, 119899 Moscow, Russia Polycarpos Pissis Department of Physics, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece Ricardo Diaz-Calleja Department of Applied Thermodynamics, Polytechnic University of Valencia, 4606 Valencia, Spain Received May 28, 2002; Revised Manuscript Received January 27, 2003 ABSTRACT: Molecular dynamics of six side-chain liquid-crystalline polymethacrylates with phenylben- zoate mesogenic groups having tail groups of different lengths was investigated by dielectric spectroscopy (DS) and by thermally stimulated discharge current (TSDC) methods. In the temperature range from room temperature to 160 °C, three processes of relaxation of dipole polarization, the  1, R, and δ processes, were revealed. These correspond, from low to high temperatures, to the mobility of ester groups adjoining the backbone, to the segmental motion, and to the mesogenic group orientation about its short axis. For a quantitative analysis, dielectric spectra were described by a superposition of one or of two Havriliak- Negami functions and of a conductivity term. It was shown that the molecular mobility of the observed relaxation processes does not depend on the mesogen tail length. The comparison of the DS and TSDC data with each other gives good agreement between the temperature position of dielectric loss peaks at the equivalent frequency of TSDC measurements and the temperature position of depolarization current maximum. By using thermal windowing techniques, the activation parameters for the observed processes were determined. 1. Introduction The classical method of dielectric spectroscopy (DS) makes it possible to investigate processes of relaxation of dipole polarization in polymers and, in particular, in side-chain liquid-crystalline polymers (SCLCPs), which are not only of scientific but also of practical interest. The decoding of a dielectric spectrum allows us to identify the observed relaxation processes and to con- nect them with the mobility of certain kinetic units bearing a polar group. The possibility for decoding of dielectric spectra is based on the systematic, consecutive change in one element of the chemical structure of the macromolecule and on the comparison with each other of the dielectric behavior of systems with similar chemi- cal structures. 1-3 The method of thermally stimulated depolarization currents (TSDC) is also successfully used in the research of dipole polarization mechanisms in polymers. The results obtained by TSDC and DS can be compared with each other because the phenomena studied in both methods are determined by the dynamic behavior of the macromolecules. The relaxation processes displayed on both the ǫ | ) (f) (DS) and I ) (T) (TSDC) plots are due to the orientation mobility of kinetic units contain- ing a polar group. The temperature position of depolar- ization current peaks, T m , formally corresponds to the temperature position of tgδ or ǫ′′ peaks observed at the equivalent frequency f e , which is in the range 10 -2 - 10 -4 Hz. 4,5 Combination of DS and TSDS methods allows us to extend the interval of frequencies of measurements by TSDC and, in some cases, to resolve relaxation processes partly overlapping at higher fre- quencies. In contrast to DS, the TSDC method is not isothermal, which is, in many respects, a disadvantage. However, using the thermal windowing technique, it is possible to determine the kinetic activation parameters of the observed relaxation processes at different temperatures. In the present work a series of side-chain liquid- crystalline polymethacrylates PMm were investigated by TSDC and DS methods. Their general formula is as follows: Here m ranges from 1 to 6. The phase transitions temperatures for PM1-PM6, glass transition temper- ature T g determined by DSC, and T m (temperature of the main global TSDC peak obtained from the I ) (T) * Corresponding author: e-mail nikon@imc.macro.ru; Fax (+007) 812-3286869; Tel (+007) 812-3288535. 5784 Macromolecules 2003, 36, 5784-5791 10.1021/ma020832c CCC: $25.00 © 2003 American Chemical Society Published on Web 06/25/2003