Effect of carbon nanotubes on the isotropic to nematic and the nematic to smectic- A phase transitions in liquid crystal and carbon nanotubes composites.

A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I -N) and the continuous nematic to smectic-A (N -SmA) phase transitions of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (CNTs) as a function of CNT concentrations is reported. Thermal scans were performed for all samples having CNT weight fraction from Φ(w) = 0.0005 to 0.0060 over a wide temperature range well above and below the two transitions in pure 8CB. Both the I -N and the N -SmA transitions evolve in character and have their transition temperatures qualitatively offset by ∼ 1.10 K lower as compared to that in pure 8CB for all 8CB+CNTs samples. The enthalpy change associated with each phase transition is essentially the same as that of pure 8CB and remains unchanged with increasing Φ(w). However, there is an evidence that the thermal transport properties of the composites differ from the pure LC upon cooling below a Φ(w)-dependent temperature within the nematic phase. In addition, a new C(p) feature is resolved for intermediate Φ(w) samples that appears to be correlated to this onset temperature.