DIELECTRIC-RELAXATION IN LIQUID-CRYSTALLI NE COMB-SHAPED POLYMERS WITH CYANOBIPHENYL GROUPS IN THE SIDE-CHAINS

Dielectric losses in mesomorphic comb-shaped polymers with a polysilox ane backbone were examined as a function of temperature within the fre quency range 30 Hz to 1 MHz. Mesogenic cyanobiphenyl groups were attac hed to the backbone via spacers of three or five methylene groups or v ia (CH2)2O(CH2)2. Two relaxation processes (alpha and delta) were obse rved, which were related to the reorientation of the lateral and longi tudinal components of the dipole moment of mesogenic group. The fact t hat alpha- and delta-processes were positioned close to the glass tran sition on the temperature scale was attributed to the interplay betwee n the segmental motion of the backbone and interactions between mesoge nic side groups: segmental mobility is possible only when mesogens are free to move. An external aligning field, leading to homeotropic orie ntation, enhanced the delta-transition and suppressed the alpha-transi tion, the relaxation time and activation energy of the delta-transitio n remaining unaffected.