Pretransition Effects Research Articles

Infrared Spectroscopic Study of Orientational Order and Phase Transformations in Liquid Crystalline CBOOA

Abstract An infrared study of the different phases of liquid crystalline CBOOA (N-p-cyanobenzilidene p-n-octyloxyaniline) has been made in the range 30-3100 cm−1. From a study of the -C≡N vibration in aligned samples, the absolute orientational order parameter, S, is determined in the smectic A and nematic phases. The variation of S is compared with the relative values determined earlier from diamagnetic anisotropy, NMR and birefringence studies. The farinfrared spectrum of the solid (Phase I) exhibits a mode whose intensity decreases gradually as the crystal-smectic A transition is approached. Also, a band observed at 296 cm−1 in the solid (Phase I) practically disappears in the fluid phases. These results are discussed in terms of possible pretransition effects and intramolecular changes characteristic of crystal-mesophase transitions. Dichroic effects are evident in the far-infrared absorption of the liquid crystalline phases and these are considered in relation to the influence of molecular structure ...

Absorption Spectra of p-Methoxy Benzylidene p-n-Butylaniline in the 80–400 cm−1 Range at Temperatures Between -200 and 70°C

Abstract The absorption coefficient of all phases of p-methoxy benzylidene p-n-butylaniline was measured in the 80 to 400 cm−1 range. An analysics of the obtained data is given. There is evidence that there are two modifications (metastable and stable) in the solid phase featuring different absorption spectra. Pretransition effects were observed to accompany all phase transitions.

Short range order in the isotropic phase of nematic liquid crystals

A theory is developed of the magnetic birefringence in the isotropic phase of nematic liquid crystals taking into account orientational correlations between neighboring molecules. This leads to a better description of the pre-transition effects in the isotropic phase than given by the mean field treatment.

Transitions in mesophase forming systems. IV. Transformation behavior and pretransition effects in p-azoxyanisole

Transitions in mesophase forming systems. IV. Transformation behavior and pretransition effects in p-azoxyanisole

Transitions in mesophase forming systems. I. Transformation kinetics and pretransition effects in cholesteryl myristate

Transitions in mesophase forming systems. I. Transformation kinetics and pretransition effects in cholesteryl myristate

Pretransition Effects in Nematic Liquid Crystals: Model Calculations

Abstract Calculations of the properties of some lattice models and continuum models of a hard rod liquid with and without attractive energies are presented. The theory is of the “molecular field” variety in that short-range-order is neglected. Both the long-range-order parameter and the density are varied to obtain the state of minimum Gibbs free energy at given pressures and temperatures. Among the properties reported are expansivity, compressibility and specific heat. Each of these displays pretransition effects qualitatively similar to those seen on the low temperature side of nematic liquid crystal-isotropic liquid phase transitions. There are important quantitative discrepancies however. It appears that the discrepancies are due more to the approximate nature of solutions than to the idealizations of the models. The “near critical” nature of liquid crystal phase transitions cannot be well described in terms of long-range-order alone. One gross error of the hard-rod models can, however, be remedied. T...

Statistical Mechanics of Rodlike Particles. III. A Fluid of Rigid Spherocylinders with Restricted Orientational Freedom

A scaled particle treatment is applied to a fluid of rigid spherocylinders of radius a and cylindrical length l, with cylindrical axes permitted to point in only three mutually perpendicular directions. Singling out one of these, by means of some arbitrary device such as an infinitesimal applied field, and denoting by s the fraction of rods in each of the directions perpendicular to this “preferred” direction in any given configuration of the system, an expression is derived for the configurational Helmholtz free energy Ac as a function of s, and an equation determining s̄, that value of s which minimizes Ac, is obtained. At densities below some critical value ρc, the only solution to the latter equation is s̄ = 13, but for ρ > ρc, an anisotropic solution is also possible. A first-order anisotropic–isotropic transition is observed and the transition densities, free volume fractions, and entropy change are determined at two values of a and for ratios l / a from 2 through 18. Thus, that transition which Zwanzig observed in a fluid of very long rods (l / a → ∞) is also observed in fluids of relatively short rigid rods. The transition free volume fractions, the entropy of transition, the degree of order S in the anisotropic phase at the transition, and the relative density change Δρ / ρaniso are all seen to increase with increasing l / a, and values obtained for the latter two quantities are compared with experimentally determined values. The isothermal compressibility and the expansivity of the system are also calculated at free volume fractions (densities) near the anisotropic→isotropic transition, but no pretransition effects are observed.

Interpretation of Mesophase Transitions

New data have provided a means for quantitatively testing the statistical theory of Frenkel for explaining the pretransitional effects adjacent to first-order transitions in mesophase systems. This theory has been frequently suggested as a qualitative explanation for pretransitional changes in physical properties that occur within 10°C of mesophase–isotropic transitions. The one previous quantitative test used compressibility and volume expansion data on p-azoxyanisole, which forms a single mesophase of the nematic type. It was concluded that the Frankel theory was of limited applicability; that (a) the equations did not apply closely, within about l°C of the transition, and (b) did not apply to pretransition effects below the transition. Calculations made here using new data on p-azoxyanisole show, in contrast, that the Frenkel theory (a) applies well near the nematic–isotropic transition, (b) the theory may be applied equally well on both sides of the transition, (c) that asymmetrical pretransitional effects are accurately treated, and (d) in contrast to an earlier result, specific-heat anomalies near the transition are accurately predicted and differ by a factor of 2 from previous estimates.

Phase Boundary Scattering in the Study of Thermodynamic Transitions in Solids

VARIOUS physical properties have been used to study the sharpness of phase transitions in solids. For transitions that are thermodynamically discontinuous, pretransition effects above and below the transition temperature, Tc, can sometimes be detected by means of special properties which are particularly sensitive to structural defects or to anomalous changes of structure with temperature1. For the study of lambda transitions which are thermodynamically continuous—at least in a formal geometrical sense—a considerable diversity of properties has, however, been successfully used in the anomalous region above and below the transformation peak temperature Tc.