We report high-sensitivity and high-temperature resolution experimental data for the temperature dependence of the optical birefringence of a nonpolar monolayer smectogen 4-butyloxyphenyl-4'-decyloxybenzoate (10[over ¯].O.4[over ¯]) liquid crystal by using a rotating-analyzer technique. The birefringence data cover nematic and smectic-A phases of the 10[over ¯].O.4[over ¯] compound. The birefringence data are used to probe the temperature behavior of the nematic order parameter S(T) in the vicinity of both the nematic-isotropic (N-I) and the nematic-smectic-A (N-SmA) transitions. For the N-I transition, from the data sufficiently far away from the smectic-A phase, the average value of the critical exponent β describing the limiting behavior of S(T) is found to be 0.2507±0.0010, which is in accordance with the so-called tricritical hypothesis, which predicts β=0.25 and excludes higher theoretical values. The critical behavior of S(T) at the N-I transition is discussed in detail by comparing our results with the latest reports in the literature and we conclude that by comparing with the previously reported results, the isotropic internal field assumption by the Vuks-Chandrasekhar-Madhusudana model is adequate to extract the critical behavior of S(T) from the optical birefringence data. We observe that there is no discontinuous behavior in the optical birefringence, signaling the second-order nature of the N-SmA transition. The effect of the coupling between the nematic and smectic-A order parameters on the optical birefringence near the N-SmA transition is also discussed. In a temperature range of about 4K above and below the N-SmA transition, the pretransitional evidence for the N-SmA coupling have been detected. From the analysis of the optical birefringence data above and below the N-SmA transition by means of various fitting expressions we test the validity of the scaling relation λ=1-α between the critical exponent λ describing the limiting behavior of the nematic order parameter and the specific heat capacity exponent α. We then show that the temperature derivative of the nematic order parameter S(T) near T(NA) exhibits the same power-law divergence as the specific heat capacity with an effective critical exponent of 0.2303±0.0035.

Critical behavior of a nonpolar smectogen from high-resolution birefringence measurements.

A simple method to determine high-accuracy refractive indices of liquid crystals and the temperature behavior of the related optical parameters via high-resolution birefringence data

Refractive index measurements as a function of temperature have been performed for an induced nematic binary system by means of thin prism technique. The temperature dependence of the birefringence (Δn) has been assessed from the measured refractive index data. A direct extrapolation method has been employed to determine the orientational order parameter for the investigated mixtures and the order parameter so obtained has also been compared with the mean field values. The Haller type fitting expression results in a relatively lower value of the order parameter critical exponent (β) compared to the theoretically predicted values. Therefore, a four-parameter power law expression, consistent with the mean field theory as well as the first-order character of the nematic–isotropic (N-I) phase transition have been used to explore the critical behavior of the order parameter near the N-I transition.

Order parameter and its critical exponent for some binary mixtures showing induced nematic phase

The influence of multi-walled carbon nanotube doping on liquid crystalline phase transitions of a smectogen octylcyanobiphenyl: A high-resolution birefringence study

This paper presents a detailed comparative study of published order-parameter experimental data of nematic liquid crystals. Throughout a simple linear transformation, it is shown that three distinct nematic mesophases exhibit the same and unique universal behavior for all temperatures below the nematic-isotropic phase transition. This paper generalizes a previous one [M. Sim\~oes, D. S. Sime\~ao, S. M. Domiciano, and A. de Campos, Phys. Lett. A 372, 5346 (2008)], in which only the nematic mesophase delimited by the crystalline phase at low temperature was studied. Here we have assembled order-parameter experimental data of 61 different compounds, forming a set with data of 96 experiments, aiming to show that the nematic universal behavior is independent of what is the actual phase or mesophase bordering the nematic phase at low temperatures; our results show that all experimental data of all nematic mesophases coalesce along a common line that extends over the entire range of mesophases.

Universality of the nematic mesophase

In this paper we study the behaviour of the order parameter of the nematic phase for all temperatures of the phase: from the neighbourhoods of the nematic crystalline phase transition region to the neighbourhoods of the nematic isotropic phase transition region To do this, we use experimental birefringence data and thermal anisotropy conductivity data in a recursive process that makes an independent and local calculation of the order parameter exponent β(T) for each temperature value T With these data, and with this procedure, we construct the profile of β(T) and reveal that it remains constant along the entire range of the phase

D.S. Simeão

Papers

Global behaviour of the order parameter in nematic phase