Dipole Dynamics in Liquid Crystalline Epoxies by Broad-Band Dielectric Relaxation Spectroscopy

Abstract

Dipole dynamics of a series of thermotropic liquid crystalline (LC) epoxy prepolymers and an LC epoxy/amine network were studied by broad-band dielectric relaxation spectroscopy (DRS) over a wide range of frequency and temperature. Two relaxation processes were observed in the prepolymers below the melting point: the β process, associated with the rotational motions of the LC rigid rods about their molecular axis, and the γ process, whose origin lies primarily in the localized motions of the terminal group. The α process, associated with the segmental motions, emerges as a low-intensity process above the calorimetric glass transition and quickly shifts into the gigahertz frequency range with increasing temperature. The δ process, associated with the end-over-end motions in the mesophase, is largely absent because of a negligible longitudinal dipole moment in these LC epoxy molecules. An in-situ investigation of the formation of an LC epoxy/amine network was conducted by DRS and polarized optical microscopy (POM). The initial stage of cure in the mesophase was characterized by the formation of nematic droplets, which quickly coalesce to form larger structures. An interesting observation was the spontaneous formation and the subsequent change of homeotropic to planar orientation during curea trend analogous to that observed by X-ray diffraction. Three relaxations characterize the dynamics of cured networks: the α process due to segmental motions in the cross-linked network, the LC β process also observed in neat prepolymers, and the βOH process, associated with the localized motions of (primarily) hydroxyl groups.