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 $\mathrm{}\left(\mathrm{PVT}\right)\mathrm{}$ measurements revealed three transition temperatures separating four phases (glass, smectic, nematic, and isotropic). Different dynamic processes have been identified reflecting librational modes (γ process), local relaxation of the mesogenic group (β process), the segmental mode (α process) associated with the dynamic glass transition, and a slower process (δ process) reflecting the side-chain dynamics within the liquid crystal order. Pressure exerts a stronger influence on the α as compared to the δ 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 $\mathrm{PVT}$ 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.