RESUMO
Within the entire mesomorphic range, high-precision dielectric anisotropy data with the high-temperature resolution is presented for a highly polar smectic-A liquid crystal 8CB (octylcyanobiphenyl) as well as 8CB nanocomposites doped with both pristine multi-walled carbon nanotubes (p-MWCNTs) and carboxyl group (-COOH) functionalized MWCNTs (f-MWCNTs). The temperature variation of the nematic order parameter across both the nematic-isotropic (N-I) and the nematic-smectic-A (N-Sm-A) phase transitions of the neat 8CB and 8CB+MWCNT nanocomposites has then been derived from the dielectric anisotropy data within the framework of the Maier-Meier theory. With the inclusion of MWCNTs, both the N-I and the N-Sm-A transition temperatures have been noted to shift to lower temperatures as compared to the 8CB host. Also, for all 8CB+MWCNT nanocomposites, regardless of the surface functionalization, it has been well documented that the N-I transition is weakly first order, whereas the N-Sm-A transition remains continuous within the experimental resolution. For all investigated samples, the temperature dependence of the nematic order parameter has been shown to be quasitricritical, within the experimental resolution. From the attentive inspection of the dielectric anisotropy data in the vicinity of the N-Sm-A transition, the upper limits for a possible latent heat ΔH_{NA} for the 8CB host and all 8CB+MWCNT composites, for the first time, have been derived. The so-derived ΔH_{NA} values for all investigated samples have also been compared with those extracted from the optical birefringence data, and an excellent consistency has then been noted. The N-Sm-A pretransitional anomaly has been investigated and the effective specific heat capacity exponent α values have been yielded from the power-law analysis of our high-resolution ΔÉ(T) data across the N-Sm-A transition for all investigated samples. It has been well documented that the incorporation of MWCNTs to the 8CB host leaves the N-Sm-A transition essentially bulklike. We discussed that, to some extent, the compliance of MWCNTs does not drive the N-Sm-A transition to 3D-XY-like behavior, that is the strength of de Gennes coupling will remain the same for all nanocomposites. This issue can be ascribed to the weaker nature of MWCNT disorder. In this work, for the first time, high-resolution ΔÉ(T) data has been shown to be very adequate and offer an easy way to investigate the N-Sm-A transition as compared to calorimetric methods.
RESUMO
We report optical birefringence data for a series of mixtures of the liquid crystals octylcyanobiphenyl (8CB) and decylcyanobiphenyl (10CB). Nematic order parameter S data in the nematic and smectic A phases have been derived from phase angle changes obtained in temperature scans with a rotating analyzer method. These S values have been used to arrive at values for possible entropy discontinuities at the smectic A to nematic phase transition temperature T_{NA}. The 10CB mole fraction dependence of the obtained entropy discontinuities could be well fitted with a crossover function consistent with the mean-field free-energy expression with a nonzero cubic term arising from the coupling between the smectic-A order parameter and the orientational order parameter director fluctuations in the Halperin-Lubensky-Ma theory. The obtained results are in good agreement with existing results from adiabatic scanning calorimetry. By exploiting the fact that the temperature derivative of the order parameter S(T) near T_{NA} exhibits the same power law divergence as the specific heat capacity, we have extracted the effective critical exponent α values for the compositions under study. The critical exponent α has been observed to reach the tricritical value α_{TCP}=0.5 for the 10CB mole fraction of x=0.330.
