Dynamics of side-chain liquid-crystalline polymers: a dielectric spectroscopy investigation

We have studied the dynamics in two side-chain liquid-crystalline derivatives of poly(norbornene diethylester) with dielectric spectroscopy within the temperature range 190-433 K and the pressure range 1-3000 bars. Optical microscopy, x-ray scattering, and differential scanning calorimetry (DSC) revealed the formation of a nematic- and a smectic-A phase, respectively, in the polymers with the shorter and longer spacers. Multiple relaxation processes exist originating from the backbone and mesogenic dipoles. In the smectic-A phase two relaxation processes exist above the DSC glass temperature (alpha and alpha(')) which merge with decreasing temperature or with increasing pressure, thus suggesting a common molecular mechanism. The faster process is the segmental (alpha) relaxation associated with the dynamic glass transition, whereas the slower process reflects mainly the side-chain dynamics within the smectic layers. Pressure was found to increase the glass temperature in the nematic and smectic phases and the dT(g)/dP was 18.7 and 16.9 K/kbar, respectively. However, the effect of pressure in inducing the isotropic-to-smectic transition is more drastic as dT(SI)/dP=26.4 K/kbar.