Dielectric Relaxation of Liquid Crystalline Polyacrylates and Polymethacrylates

Abstract

A series of liquid crystalline polyacrylates and polymethacrylates was studied by frequency and temperature-dependent dielectric measurements. Depending on the molecular structure, between two and five different relaxation processes were observed. Three of them are active below the glass transition. They can be attributed to reorientations of dipoles located at the center of the mesogenic group (β-relaxation), at the ends of the spacer group (γ1-relaxation), and at the end group (γ2-relaxation). At the glass transition the centers of gravity of the mesogenic groups and the polymer main chain become mobile. Dipole reorientations associated with this motion contribute to the dielectric relaxation (a-relaxation). At higher temperatures the reorientation of the long axis of the mesogenic groups can be observed (δ-relaxation). For the polyacrylates it passes continuously through all phases; in the case of the polymethacrylates it sets in together with a phase transition.