Funct. Mater. 2021; 28 (4): 645-651

doi:https://doi.org/10.15407/fm28.04.645

Helix elastic modulus in a ferroelectric liquid crystal with varying helical pitch

E.Popova, V.Mikhailenko, V.Semynozhenko, A.Krivoshey, A.Fedoryako

State Scientific Institution "Institute for Single Crystals", National Academy of Sciences of Ukraine, 60 Nauki Ave., 61001 Kharkov, Ukraine

Abstract: 

Method for determination of the helix elastic modulus (Kh) in ferroelectric liquid crystals (FLCs) from dielectric spectra is proposed. According to the method, temperature dependencies of the dielectric strength of the Goldstone mode and the Goldstone frequency as well as main ferroelectric parameters were measured for the induced FLC mixtures with helical pitch (p0) known from the selective light reflection measurements. From these data, the helix elastic modulus was calculated using an existing theoretical approach. It appears that, near the SmA*-SmC* phase transition, there is remarkable dependence of the helix elastic modulus on the helical pitch; more specifically, the stiffness of helix decreases with p0. However, at lower temperatures the Kh dependence on p0 tends to weaken. From these results, it is concluded that, well away from the SmA*-SmC* phase transition, the Kh obtained for FLCs with known p0 can be used as the constant for determination of p0 which cannot be determined from the selective light reflection. It is also shown that mechanism of the dielectric relaxation in the studied FLC mixtures is not of Debye type and there are several characteristic relaxation times in a FLC cell.

Keywords: 
ferroelectric liquid crystals, the Goldstone mode, helix elastic modulus, helical pitch.

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