Studies of nanocomposites of carbon nanotubes and a negative dielectric anisotropy liquid crystal

Abstract

It has been widely recognized that the combination of carbon nanotube (CNT) and liquid crystals (LCs) not only provides a useful way to align CNTs, but also dramatically enhances the order in the LC phases, which is especially useful in liquid crystal display (LCD) technology. As the measure of this phase behavior, the complex specific heat is presented over a wide temperature range for a negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and CNT composites as a function of CNT concentration. The calorimetric scans were performed under near-equilibrium conditions between 25 and 95 °C, first cooling and then followed by heating for CNT weight percent ranging from ϕ(w) = 0 to 0.2. All 9OO4/CNT mesophases have transition temperatures ~1 K higher and a crystallization temperature 4 K higher than that of the pure 9OO4. The crystal phase superheats until a strongly first-order specific heat feature is observed, 0.5 K higher than in the pure 9OO4. The transition enthalpy for the nanocomposite mesophases is 10% lower than that observed in the pure 9OO4. The strongly first-order crystallization and melting transition enthalpies are essentially constant over this range of ϕ(w). Complementary electroclinic measurement on a 0.05 wt. % sample, cooling towards the smectic-C phase from the smectic-A, indicates that the SmA-SmC transition remains mean-field-like in the presence of the CNTs. Given the homogeneous and random distribution of CNTs in these nanocomposites, we interpret these results as arising from the LC-CNT surface interaction pinning the orientational order uniformly along the CNT, without pinning the position of the 9OO4 molecule, leading to a net ordering effect for all phases. These effects of incorporating CNTs into LCs are likely due to "anisotropic orientational" coupling between CNT and LC, the change in the elastic properties of composites and thermal anisotropic properties of the CNTs.

Temperature-dependent electro-optical and elastic properties of carbon nanotube doped polar smectogen octylcyanobiphenyl

“…By taking into account the MWCNT contents, our findings concur quite well with those of Sigdel and Iannacchione [42]. More recently Kalakonda et al [51] presented a comparative discussion, based on specific heat capacity data, on the effect of MWCNTs on the phase transition behavior of 8CB and 9:O:4 (4-butyloxyphenyl-4 ' -nonyloxybenzoate) LC compounds. By taking into account the fact that, apart from the algebraic sign of Δε for 8CB and 9:O:4 compounds, 9:O:4 does not exhibit anti-parallel coupling between dipoles, thus resulting in a so-called normal monolayer smectic A, SmA m , phase while 8CB exhibits a partially bilayer Sm A phase, SmA d [72] and different chemical natures of those compounds as well, the authors claimed that the interactions between CNTs and host LC are quite different for 8CB and 9:O:4.…”

“…Similar downshifts in the transition temperatures of 8CB-MWCNT composites for different MWCNT contents than ours have recently been reported by Sigdel and Iannacchione signaling disordering effects of MWCNTs on liquid crystalline phases [42]. On the other hand, in literature substantial increases in the transition temperature T NI have been reported for some MWCNT-LC composites [12,13,51] raising the fact that MWCNT-LC systems could be considered as, thermodynamically true solutions of MWCNT in LC solvent instead of dispersions [52]. Additionally, it is seen from Fig.…”

“…γ 1 /K 11 , and also to the decrease in the dielectric anisotropy Δε. Thus the increase in the voltage-off response time of doped 8CB samples can be regarded as an evidence for the disordering effect of 8CB in the composite system [51]. Also note that, voltage-on response time is faster than voltage-off response time, as expected, for all samples.…”

“…Due to disordering effect of 8CB in the composites, the E a values are lowered as compared to pure 8CB. As discussed by several authors [42,51] by taking into account the chemical structures of 9:O:4 and 8CB, the presence of CNTs will probably promote the nematic ordering with different extrapolation lengths of the local anchoring field. For 8CB-CNT composites, at the surface of CNTs, the LC orientations have been conjectured to be quasi-random due to the degeneracy of possible orientations that would give rise to quenched random-field disordering to some extent [51,75,76].…”

The peculiar electrical response of liquid crystal-carbon nanotube systems as seen by impedance spectroscopy

“…Interestingly enough, it has been recently found [33,34] that CNT mixtures in LCs with negative dielectric anisotropy are reversible upon switching, at least for low CNT concentrations.…”