Dielectric relaxation spectroscopy and molecular dynamics of a liquid-crystalline polyacrylate containing spiropyran groups

The dielectric properties of a liquid-crystalline (LC) copolymer containing photochromic spiropyran groups have been determined over wide ranges of frequency and temperature. The observed loss curves contain a component from the dipole reorientations of the mesogenic groups and a further component from a conductivity-related process. An electrical cleaning method was used to reduce the contribution from the latter process and a subtraction method was devised to separate the overall loss curves into the two components. Using electrical/ thermal treatments it was found possible to align the copolymer homeotropically but not planarly. The dielectric loss spectra for unaligned and homeotropically aligned samples were very different. The data were analysed to yield two relaxation parameters for unaligned and aligned samples and were interpreted in terms of the anisotropic motions of dipolar mesogenic groups as have been discussed previously. In addition, the temporal stability of alignment of homeotropic samples at temperatures close to the clearing temperature was determined using the dielectric method.