Magnetic resonance and calorimetric studies of 4’-n-pentyl-4-cyanobiphenyl confined to Vycor glass, a random network of pores with average diameter ∼70 Å, are reported. Under such severely constrained ocnditions, the nematic-to-isotropic transition is replaced by a continuous evolution of orientational order in the pores. Our findings are explained by a Landau type of model where the liquid crystal is confined to independent pore segments, suggesting that the glassylike orientational order is to a large extend locally determined.

High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Å-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density ${\rho }_S.$ The excess specific heat peaks associated with the nematic-isotropic $\mathrm{}(N-I)\mathrm{}$ and the nematic–smectic-A $(N-\mathrm{Sm}A)$ transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as ${\rho }_S$ is increased. Two distinct regimes of ${\rho }_S$-dependent behaviors are observed with a crossover between them at ${\rho }_S\cong 0.1{\mathrm{g}\mathrm{}\mathrm{cm}}^{\mathrm{-}3}.$ For lower silica densities, sharp second-order $C_p$ peaks are observed at the $N-\mathrm{Sm}A$ transitions, characterized by effective critical exponents that decrease monotonically with ${\rho }_S$ from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order $C_p$ features are observed at the $N-I$ transition. For higher silica densities, both the $N-\mathrm{Sm}A$ and the $N-I$ transitions exhibit a single rounded $C_p$ peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CB+aerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silica densities above and below the crossover density.

We present a study of the specific heat and effective thermal conductivity in anisotropic and randomly oriented multi-wall carbon nanotube (MWCNT) and randomly oriented single-wall carbon nanotube (SWCNT) composites from 300 to 400 K. Measurements on randomly oriented MWCNTs and SWCNTs were made by depositing a thin film of CNTs within a calorimetric cell. Anisotropic measurements were made on MWCNTs grown inside the highly ordered, densely packed nanochannels of anodic aluminum oxide. The specific heat of randomly oriented MWCNTs and SWCNTs showed similar behavior to the specific heat of bulk graphite powder. However, the specific heat of aligned MWCNTs is smaller and has weaker temperature dependence than that of the bulk above room temperature. The effective thermal conductivity of randomly oriented MWCNTs and SWCNTs is similar to that of powder graphite, exhibiting a maximum value near 364 K indicating the onset of phonon-phonon scattering. The effective thermal conductivity of the anisotropic MWCNTs increased smoothly with increasing temperature and is indicative of the one-dimensional nature of the heat flow.

Using an ac calorimetry technique we measured the specific heat at the nematic-isotropic, smectic-A–nematic, and smectic-A–isotropic phase transitions, for the alkylcyanobiphenyl liquid crystal nCB series confined to the 0.2-μm-diam pores of Anopore membranes. Studies were also performed as a function of nematic director alignment within the pores. Novel effects regarding specific-heat amplitudes, transition-temperature shifts, and broadening and rounding of phase transitions are found. They are strongly dependent on the director orientation and on the order of the phase transition.

Assembling quantum dots (QDs) into nanoscale configurations over macroscopic dimensions is an important goal to realizing their electro-optical potential. In this Rapid Communication, we present a detailed study of a pentylcyanobiphenyl liquid crystal (LC) and a CdS QD colloidal dispersion by probing the dielectric property $\overline{\epsilon }$ and relaxation as a function of an applied ac electric field $E_{\text{ac}}$. In principle, dispersing QDs in a nematic LC medium can direct the dots to align in nearly one-dimensional chainlike structures along the nematic director and these assemblies of QDs can be directed by external electric fields. In a uniform planar aligned cell, the Fréedericksz switching of the $\text{LC}+\text{QDs}$ appears as a two-step process with the same initial switching field as the bulk but with the final $\overline{\epsilon }$ value larger than that for an aligned bulk LC. The relaxation of $\overline{\epsilon }$ immediately following the removal of $E_{\text{ac}}$ follows a single-exponential decay to its original value that is slower than the bulk but becomes progressively faster with increasing $E_{\text{ac}}$, eventually saturating. These results suggest that the arrangement of the QDs is mediated by the LC.

Electric field induced director orientation of a nematic liquid crystal $\left(\mathrm{LC}\right)+\text{carbon}$ nanotube (CNT) system reveals insights on switching behavior for this anisotropic composite. Once the field goes off, the $\mathrm{LC}+\mathrm{CNT}$ system relaxes back to the original orientation through a mechanical rotation, revealing the intrinsic dynamics. LC molecules and CNTs cooperatively form local pseudonematic domains in the isotropic phase due to strong LC-CNT interactions. These field-responsive anisotropic domains do not relax back to the original orientation on switching of the field off, which could find potential applications in memory devices.