The determination of the order parameters and director alignment in the nematic phase of a liquid crystalline side-chain polymer by the resolution of dielectric loss spectra

Temperature and pressure dependence of the dynamics in a poly(methyl acrylate) side-chain liquid-crystalline polymer

The molecular dynamics of a side-chain polymer liquid crystal with a poly(methyl acrylate) backbone and a (p-alkoxy-phenyl)-benzoate mesogenic group have been studied in the unaligned state as a function of temperature and pressure using dielectric spectroscopy. Polarizing optical microscopy, differential scanning calorimetry, and pressure-volume-temperature (PVT) measurements revealed three transition temperatures separating four phases (glass, smectic, nematic, and isotropic). Different dynamic processes have been identified reflecting librational modes (gamma process), local relaxation of the mesogenic group (beta process), the segmental mode (alpha process) associated with the dynamic glass transition, and a slower process (delta process) reflecting the side-chain dynamics within the liquid crystal order. Pressure exerts a stronger influence on the alpha as compared to the delta process. Starting from the nematic phase, pressure was found to induce the nematic-to-smectic transformation. The associated dynamic changes were in excellent agreement with the PVT results implying that the dynamics are directly coupled to the thermodynamic state. Pressure was found to enhance the stability of the smectic order within the P-T phase diagram.

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.