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Ranjkesh, Amid,Kiani, Somaye,Strzeż,ysz, Olga,Zakerhamidi, Mohammad Sadegh,Yoon, Tae-Hoon Elsevier 2018 Journal of molecular liquids Vol.268 No.-
<P><B>Abstract</B></P> <P>The temperature dependence of optical birefringence, order parameter, effective geometry parameter, and normalized polarizability was determined for nematic liquid crystal mixtures by measuring the refractive indices. The effects of the adjustable parameters obtained from temperature-dependent refractive indices were analyzed appropriately for the crossover temperature and order parameter values. Besides, a four-parameter power law expression method was used for the optical birefringence measurement to address inappropriate fits in the weak first-order phase transition region. From this method, the effective second order phase transition temperature <I>T</I> <SUP>⁎⁎</SUP> was determined by showing the slightly higher values than phase transition temperature. The value of the order parameter critical exponent <I>λ</I> obtained from this method showed a good agreement with the mean field theory by demonstrating closeness to the tricritical hypothesis (<I>β</I> = 0.25). As a consequence, the measurement of the temperature-dependent optical birefringence was successfully verified the pre-transitional phenomena accurately, and it can facilitate an insightful description of that region.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Temperature dependence of high birefringence (∆<I>n</I>) liquid crystals was analyzed. </LI> <LI> The temperature dependent order parameter was sorted based on ∆<I>n</I> values. </LI> <LI> Lower (∆<I>n</I>)<SUB>o</SUB> and <I>β</I> values were valuable factors for increasing <I>T</I> <SUB>co</SUB> at the same <I>T</I> <SUB>NI</SUB>. </LI> <LI> A four-parameter power law expression shows the high critical exponent value. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Linear dichroism and order parameters of nematics doped with azo dyes
Ranjkesh, Amid,Choi, Jun-Chan,Joo, Kyung-Il,Park, Hee-Won,Zakerhamidi, Mohammad Sadegh,Kim, Hak-Rin Informa UK (TaylorFrancis) 2017 Molecular Crystals and Liquid Crystals Vol.647 No.1
<P>We investigated solvent-solute effects using dye-doped nematic liquid crystals (NLCs) where three Sudan dyes of different concentrations were used as solutes doped into high- and low-polar NLCs as anisotropic solvents. The polarized spectroscopic absorption, dichroic ratio (R), and order parameter (S) were measured, which showed that the rod-like Sudan black B had a higher S and R due to a higher anisotropic solvent-solute effect produced by transition moment and global charge transfer along the molecular axis induced by high polar NLC. The order parameters measured by the Vuks method also showed a higher solvent-solute effect for the high-polar NLC.</P>
Ranjkesh, Amid,Choi, Jun-Chan,Park, Ji-Sub,Park, Min-Kyu,Kiani, Somaye,Zakerhamidi, Mohammad Sadegh,Kim, Hak-Rin Elsevier 2017 Journal of molecular liquids Vol.230 No.-
<P>The temperature dependencies of three low birefringence (Delta n) liquid crystals (LCs) at similar clearing temperatures (T-NI) were investigated to characterize the order parameter S(T) and crossover temperature (T-co). Three different methods were used to determine the S(T) property, including the Vuks model, the Haller approximation model, and a model based on the effective geometry parameter (alpha(eg)). In our low Delta n LCs with relatively analogous T-NI, three parameters (B, (Delta n)(o), and beta) that originated from the modified four-parameter model were analyzed to increase the T-co condition. A larger absolute value for the B parameter and a smaller beta parameter were also needed to reduce the temperature dependence of Delta n. In particular, a lower (Delta n)(o) value was found to be beneficial for increasing the Too value, which resulted in a higher T-co value for the low Delta n LCs, compared with that of a high An LC with a similar T-NI value. This led to a narrower temperature range between T-co and T-NI for the low Delta n LCs compared to that of the high Delta n LC, which could result in appropriate temperature properties for LCs for practical display applications, including a lower temperature dependence of Delta n and wider operational temperature regime. (C) 2017 Published by Elsevier B.V.</P>