Tuning the electro-optic and viscoelastic properties of ferroelectric liquid crystalline materials
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This work deals with determination of the viscoelasticity coefficient in chiral smectic liquid crystals possessing the helical structure and is the continuation of the work, in which the elasticity coefficient was presented [Phase Transitions 89 (2016) 368]. The measurements have been performed using optical detection in the small deformation limit. The viscosity coefficient was measured on two commercially available pure chiral materials, namely, 4-(n-hexyloxy phenyl)-1-(2-fuethyl butyl) biphenyl-4-carboxylate and 4-(2-methylbutyl) phenyl-4-n-octylbiphenyl-4-carboxylate, and on resulting binary mixture composed from those materials in 50:50 weight ratio. All three liquid crystalline materials exhibit a tilted ferroelectric phase over a reasonably broad temperature range. Each of the two pure liquid crystalline materials have its own disadvantages. However, by design of the binary mixture in definite weight concentration, we tried to improve the behavior and to tune the properties; specifically, the effect of the viscoelastic properties on the mixture composition has been established.
KeywordsLiquid crystals Ferroelectric liquid crystals Electro-optic behavior Viscoelasticity Viscosity
Two commercially available materials 4-(n-hexyloxy phenyl)-1-(2-fuethyl butyl) biphenyl-4-carboxylate (denoted Ce3) and 4-(2-methylbutyl) phenyl-4-n-octylbiphenyl-4-carboxylate (denoted as Ce8) and resulting binary mixture (denoted here as Ce3/8) composed from those materials in 50:50 weight ratio have been used for the investigations. All three FLC materials exhibit ferroelectric properties within a relatively broad temperature range. The mesomorphic properties of pure FLC materials and of the binary mixture were determined by complementary methods: differential scanning calorimetry, electro-optics, and dielectric spectroscopy. The characteristic textures and their changes were observed using polarized optical microscope (POM) equipped by the temperature chamber with temperature controller. The synthetic details and the mesomorphic behavior of those materials were presented earlier (Bone et al. 1984). Ferroelectric liquid crystal, denoted as Ce3, possesses the cholesteric (N*) and the tilted ferroelectric smectic C* (SmC*) phases with phase transition temperatures as follows: Cr 65.0 °C SmC* 77.5 °C N* 162.0 °C Iso. Absence of the orthogonal smectic A* phase for this material caused definite difficulties while obtaining a homogeneous alignment. The second material studied was also ferroelectric liquid crystal, denoted as Ce8; it possesses a very rich polymorphism between the isotropic and crystal phases: Cr 39.6 °C SmG 56.0 °C SmJ 65.0 °C SmF 67.0 °C SmC* 86.0 °C SmA* 124.0 °C N* 145.5 °C BP 147.0 °C Iso. The resulting binary mixture possesses the following sequence of mesophases: Cr 39.6 °C SmG 56.0 °C SmJ 65.0 °C SmF 67.0 °C SmC* 85.8 °C SmA* 123.2 °C N* 148.7 °C BP 150.0 °C Iso.
For the measurements, the materials under the study were introduced by means of capillarity action in standard planar cells produced by Linkam Co. (UK) of 5 μm thick with ITO electrodes coated with a planar alignment polymer layers. The cells were placed in a modified Mettler hot stage. Their temperature was stabilized using Digi-Sense TC-9500 temperature controller with the accuracy of about 0.1 K.
The measurement of optic axis deviation a in a weak electric field is essential for determination of the elastic constant K when the electro-optic method is used. This quantity was measured by detection of electro-optical response with a photodiode connected to a lock-in amplifier SR850 (Stanford Research) followed by the calibration procedure as described by Dardas et al. 2011. This calibration procedure allows expressing the experimental results as angular quantities independent on experimental conditions. The remaining quantities can be found using further methods and techniques presented in more details by Diamant et al. (1957), Dąbrowski et al. (1992), Kuczyński et al. (2002), Dardas and Kuczyński (2004), Jeżewski et al. (2008) Kuczyński (2010), Kuczyński et al. (2010) and Kuczyński et al. (2012). The relaxation time was determined from the measurement of electro-optic response versus frequency in order to determine the rotational viscosity of the system.
Results and discussion
The main objective of this work was to check the viscoelastic behavior of a binary ferroelectric mixture and to compare the values with those obtained on the pure FLC materials—the original components of the mixture. In the specific system studied here, the experimental results reveal that if we know the viscoelastic properties, specifically the viscosity, of the pure components, it is possible to predict it for the resulting binary mixture, e.g., the data presented in this work clearly show that the viscoelastic properties of new binary Ce3/8 mixture can be pre-determined by well-known viscoelastic properties of Ce3 and Ce8 materials. Very promising, from the potentials application point of view in information visualization devices is the fact, that knowing the coefficients of pure liquid crystal material with the ferroelectric properties, we could to anticipate the viscoelastic properties in newly designed liquid crystalline mixtures. In case of ferroelectric liquid crystalline Ce3/8 binary mixture, the estimated coefficients of elasticity (Dardas 2016) and, in this work, the viscosity are practically between the elasticity and viscosity coefficients of the original Ce3 and Ce8 pure materials. Having available a database of viscoelastic properties of FLC materials, it will be quite possible to produce a binary or a multicomponent mixture with the desired viscoelastic behavior. In the future, it would be worthwhile to verify this hypothesis on different subsequent FLC materials and to expend the area of ferroelectric materials/mixtures to other polar mesophases, like antiferroelectric and ferrielectric smectic mesophases. An essential difference in the evolution of the rotational viscosity seems to have now its grounds on helicoidal features and the nature of the neighboring liquid crystalline phases.
This work was supported by the National Science Centre (NCN) Poland under Grant 2017/25/B/ST3/00564.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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