Effect of hockey-stick-shaped molecules on the critical behavior at the nematic to isotropic and smectic-Ato nematic phase transitions in octylcyanobiphenyl

Abstract

In the field of soft matter research, the characteristic behavior of both nematic-isotropic (N-I) and smectic-A nematic(Sm-A N) phase transitions has gained considerable attention due to their several attractive features. In this work, a high-resolution measurement of optical birefringence (Δn) has been performed to probe the critical behavior at the N-I and Sm-A N phase transitions in a binary system comprising the rodlike octylcyanobiphenyl and a laterally methyl substituted hockey-stick-shaped mesogen, 4-(3-n-decyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-dodecyloxycinnamate. For the investigated mixtures, the critical exponent β related to the limiting behavior of the nematic order parameter close to the N-I phase transition has come out to be in good conformity with the tricritical hypothesis. Moreover, the yielded effective critical exponents (α', β', γ') characterizing the critical fluctuation near the Sm-A N phase transition have appeared to be nonuniversal in nature. With increasing hockey-stick-shaped dopant concentration, the Sm-A N phase transition demonstrates a strong tendency to be driven towards a first-order nature. Such a behavior has been accounted for by considering a modification of the effective intermolecular interactions and hence the related coupling between the nematic and smectic order parameters, caused by the introduction of the angular mesogenic molecules.

Quantum-dots-dispersed bent-core nematic liquid crystal and cybotactic clusters: Experimental and theoretical insights

“…The obtained fitting plots (pure LC: red solid line, LC nanocomposite: blue solid line) along with the fit parameter values, are shown in Figure 3. The four-parameter fitting is considered superior to the Haller's method that involves lesser number of fit parameters [39,44,45]. We obtain ξ = 0.324, S * * = 0.109, T * * = 134.232 • C and β = 0.251 for the pure LC.…”

“…The dielectric measurements were carried out in the frequency range of 20 Hz -2 MHz using an Agilent E4980A precision LCR meter. The measuring voltage was kept at V rms = 0.2 V. For transmission dependent birefringence measurements and the related order parameter calculations, the sample was placed between two crossed Glan-Thompson polarizers (GTH10M, Thorlabs, Inc.) and perpendicularly illuminated with a He-Ne Laser (∼ 633 nm) [39,40]. The rubbing direction r (i.e.…”

“…At T = 0, S(0) = 1, which implies 1 = S * * + A. The birefringence, (∆n), can then be expressed as [39],…”

Alignment and Deformation Properties of the Third Type of Hockey Stick‐Shaped Molecules in Vertical Alignment and Fringe‐Field Switching Modes

“…Intensive studies have been made to further diminish the molecular symmetry of BLCs, contributing to the continued improvement of their unconventional mesomorphism. However, the practical applications with respect to the Fréedericksz transition has been limited so far due to relatively high rotational viscosity, low nematic temperature and difficulty in molecular alignment …”

Critical behaviour of a polar-polar binary system from optical birefringence measurements

“…T SN /T NI , where T SN and T NI are the SmA-N and nematic-isotropic (N-I) transition temperatures, respectively) corresponds to the tricritical limit but experimental outcomes have yielded relatively higher values, ranging between 0.942 and 0.994, thus implying a non-universal nature. [9][10][11][12] Afterward, Halperin, Lubensky and Ma (HLM) [13] divulged that the coupling between the nematic director fluctuations and the SmA order parameter also offers an important contribution, thus making the SmA-N phase transition always fluctuation-induced weakly first order. The discontinuity was expected to be weak and diminishes with decrease in the McMillan ratio, but never vanishes.…”