Broadband dielectric spectroscopy on the collective and molecular dynamics in (low molecular weight and polymeric) ferroelectric liquid crystals

Abstract

Broadband dielectric spectroscopy (10⁻² Hz to 10¹⁰ Hz) is employed to study the collective and molecular dynamics in (low-molecular weight and polymeric) ferroelectric liquid crystals. Two collective loss processes are observed for frequencies smaller than 1 MHz: the Goldstone-mode which is assigned to fluctuations respectively modulations of the phase of the helical superstructure and the soft-mode which corresponds to fluctuations of the amplitude of the ferroelectric helix. In the frequency range from 10⁶ Hz to 10¹⁰ Hz one relaxation process is observed, the β-relaxation. This process is assigned to fluctuations of the chiral mesogene around its long molecular axis. At the phase transition S_A to S*_C the relaxation peak of the β-relaxation does not split or broaden, and the temperature dependence of its relaxation rate does not show any deviation from an Arrhenius-like behavior. Its dielectric strength does not decline. These experiments prove that the β-process is a local relaxation which is not involved in the collective dynamics taking place for frequencies below 1 MHz. Chiral mesogenic molecules can be readily incorporated into a variety of different polymeric architectures (e.g. side chain polymers, main chain polymers, combined main chain/side group systems, functionalized copolymers, crosslinked systems, etc.). This enables one to tailor materials according to the current technological requirements.

Keywords:

• Ferroelectric liquid crystals
• dielectric spectroscopy
• polymers