Diffusion and viscosity of a calamitic liquid crystal model studied by computer simulation

We report a molecular dynamics simulation study on an ensemble of rod-like particles, each composed of nine soft spheres held rigidly along a line. We have calculated translational mean square displacements and velocity autocorrelation functions in the fluid phases exhibited by the model, i.e., smectic A, nematic and isotropic. These quantities have then been used to compute diffusion coefficients. In addition, we have calculated viscosities in the nematic and isotropic phases. Despite its crude nature, the model is capable of providing a faithful reproduction of many features of the transport behavior observed in real liquid-crystalline materials. The simulation results have been compared with the predictions of the modified affine transformation theory, finding only a fair agreement.     

Orientational dynamics in the isotropic phase of a calamitic liquid-crystal model

We report a molecular dynamics simulation study on the isotropic phase of an idealized calamitic liquid crystal model with a length-to-width ratio of approximately 5-6. The study focuses on the characterization of single-particle and collective orientational dynamics on approaching the phase transition to the nematic phase. Recent experimental and simulation works have suggested that a power law behavior exists at relatively short times in the decay of the time derivative of the orientational correlation functions. Qualitatively, our simulation data are consistent with these findings. Both single-particle and collective time correlation function derivatives possess, in their respective log-log plots, a linear region at very short times, whose slope is essentially independent from the thermodynamic state. Nevertheless, the single-particle orientational correlation functions are better described by a function which is the sum of a fast exponential, an intermediate stretched-exponential and a slow exponential, while the collective orientational correlation functions are satisfactorily described by a sum of two exponentials, at higher density, or by just one exponential, at lower density.     

Measurements of Room-Temperature Phosphorescence Spectra of Polycyclic Aromatic Compounds Using Pullulan Films Containing β-Cyclodextrin and 2-Bromoethanol

A clear film was easily prepared by air-drying anaqueous solution of pullulan (6%w/w) containing -cyclodextrin(CD, 1%w/w) and 2-bromoethanol (1%v/w). The resultingpullulan film was used as a substrate for simple measurements ofroom-temperature phosphorescence (RTP) spectra of polycyclicaromatic compounds (PACs). Only a drop (ca. 10 L) of a 100 gmL^-1 sample solution in 95% ethanol was spotted onto the surfaceof the disk film (7–8 mm diameter) and the solvent wasallowed to evaporate at room temperature. The sample-spottedfilm was pasted on a glass plate (75 × 20 × 1 mm) with smallamounts of a starch glue. The plate was mounted into a solidsample holder, or alternatively inserted diagonally into a 1-cm cell holder.Without a dry gas flush during the measurements, RTPspectra based on the CD inclusion complexes of PACs wereobtained from six typical two- and three-ring compounds,including naphthalene, acenaphthene, fluorene, phenanthrene,carbazole, and dibenzofuran. Only anthracene did notproduce a discernable phosphorescence signal by the presenttechnique. This technique was directly applied to the spectralidentification of acenaphthene in commercially available kerosene.     

The pressure effect on the CDW-transition temperatures in 1T-TaS2

The pressure dependence of the two CDW transition temperatures (T_d1? 350 K and T_d2 ～ 190 K) in 1T-TaS₂ has been investigated by measuring the resistivity up to 5 kbar. It is found that the thermal hysteresis of the phase transition at T_d2 is very sensitive to the pressure and is interpreted by the phenomenological Landau theory including cubic term.