Dipole dynamics and macroscopic alignment in molecular and polymeric liquid crystals by broad-band dielectric relaxation spectroscopy

A liquid crystal monomer (LCM) was synthesized and attached as a side chain to a siloxane polymer (SCLCP). The molecular dynamics of LCM and SCLCP were investigated by broad-band dielectric relaxation spectroscopy (DRS) over 10 decades of frequency. The surface treatment of the electrodes was found to have a pronounced effect on the macroscopic alignment and dynamics. In the isotropic state the relaxations in LCM and SCLCP are well-described by the Kohlrausch-Williams-Watts (KWW) functional form. In the LC state, the overall dielectric response is a weighted sum of two dispersions that depend on the macroscopic alignment. The hometropic (H) alignment is dominated by the (slower) δ process, while the planar (P) alignment is dominated by the (faster) α_m process. Both processes are symmetric and can be described by Cole-Cole (CC) or Fuoss-Kirkwood (FK) functional forms. The δ process governs the dynamics in the SCLCP near the glass transition, similar to the segmental α process in non-LC glass formers. Excellent agreement was observed with the various aspects of the seminal work by Attard and Williams.