INFLUENCE OF LATERAL SUBSTITUENTS IN THE MESOGENS ON THE PROPERTIES OF LIQUID-CRYSTALLINE SIDE-GROUP COPOLYMETHACRYLATES - DIELECTRIC-RELAXATION SPECTROSCOPY ON HOMOPOLYMERS

Broad-band dielectric spectroscopy in the frequency range from 10(-2) Hz to 10(6) Hz and in a temperature range from 170 K to 430 K was empl oyed to study the molecular dynamics in polymethacrylates with novel c alamitic side groups. As mesogenic units, derivatives of p-methoxyphen yl benzoate were used where the hydrogen in the meta position of the b enzoate ring is substituted by a methoxy group or by bromine. The subs tituted samples do not show any mesophases. The following results were obtained: (i) The beta-relaxation which corresponds to a rotational f luctuation of the mesogen around its long axis is drastically suppress ed by the lateral substitution. Moreover the activation parameter of t his process depends strongly on the actual mesophase structure of the material under study. (ii)) The alpha-relaxation is related to the dyn amic glass transition of the system. It is shown that the value of the glass transition temperature of the liquid-crystalline polymethacryla te can be understood as competition of internal plasticization and mol ecular ordering. (iii) The delta-relaxation corresponds to a rotationa l fluctuation of the side group around its short axis. From comparison of the relaxation rates of the alpha- and delta-relaxation, the tempe rature dependence of the order parameter of the liquid-crystalline pol ymethacrylate is estimated by dielectric spectroscopy.