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 106 Hz and in a temperature range from 170 K to 430 K was employed to study the molecular dynamics in polymethacrylates with novel calamitic side groups. As mesogenic units, derivatives of p‐methoxyphenyl benzoate were used where the hydrogen in the meta position of the benzoate ring is substituted by a methoxy group or by bromine. The substituted samples do not show any mesophases. The following results were obtained: (i) The β‐relaxation which corresponds to a rotational fluctuation of the mesogen around its long axis is drastically suppressed by the lateral substitution. Moreover the activation parameter of this process depends strongly on the actual mesophase structure of the material under study. (ii) The β‐relaxation is related to the dynamic glass transition of the system. It is shown that the value of the glass transition temperature of the liquid‐crystalline polymethacrylate can be understood as competition of internal plasticization and molecular ordering. (iii) The α‐relaxation corresponds to a rotational fluctuation of the side group around its short axis. From comparison of the relaxation rates of the α‐ and δ‐relaxation, the temperature dependence of the order parameter of the liquid‐crystalline polymethacrylate is estimated by dielectric spectroscopy.