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 (α and ${\alpha }^{\prime })$ which merge with decreasing temperature or with increasing pressure, thus suggesting a common molecular mechanism. The faster process is the segmental (α) 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 ${\mathrm{dT}}_g/dP$ 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 ${\mathrm{dT}}_{\mathrm{SI}}/dP=26.4\mathrm{}\mathrm{K}/\mathrm{k}\mathrm{b}\mathrm{a}\mathrm{r}.$