DIELECTRIC-RELAXATION IN LINEAR AND CROSS -LINKED POLYMERS WITH MESOGENIC GROUPS IN SIDE-CHAINS ORIENTED BY AN EXTERNAL-FIELD

The relaxation of dipole polarization in linear and cross-linked sampl es of comb-shaped polymer poly 4-(4-cyano-4-biphenyloxy)butyloxycarbon yl]ethylene in nematic and isotropic states was studied. Two overlappi ng regions of dielectric losses (alpha and delta) were observed near t he glass transition temperature. These regions are associated. with th e motion of the longitudinal and transverse components of the dipole m oment of a mesogenic fragment. The difference between the parameters o f dipole relaxation in the samples oriented by a constant electric or mechanical shear field is likely to be associated with the different o rientation of mesogenic groups., For each of the relaxation processes, the parameters of the relaxation time distribution as well as the ord er parameter in the oriented samples were determined as a function of temperature. The development of a three-dimensional network with a low cross-link density was shown to exert no marked effect on the charact eristics of relaxation. Dielectric studies provide support for the exi stence of the phenomenon of the so-called orientational ''memory'', wh ich was earlier observed for the cross-linked samples of the polymer s tudied.