Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

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Abstract
By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (10−1 Hz-109 Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below 106 Hz, two ferroelectric modes, Goldstone- and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between 106 and 109 Hz, one dielectric loss process exists, the β-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for the β-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free energy at the non-ferroelectric-ferroelectric phase transition. The experiment also leads to a modified understanding for the molecular origin of ferroelectricity in FLCs.