Şükrü Özgan

Thermal and mesomorphic properties of ternary mixtures of some hydrogen-bonded liquid crystals

The thermal and mesophase behaviour of ternary mixtures formed from hydrogen-bonded nematic liquid crystals 4-hexylbenzoic acid, 4-(octyloxy)benzoic acid and 4-(decyloxy)benzoic acid were investigated by differential scanning calorimetry (DSC) and phases identified by polarised optic microscopy (POM). The DSC and POM results clearly indicate that the produced mixtures display liquid crystalline properties. The phase transition temperature values obtained in the DSC experiments are in line with the POM observations. Investigation of the thermal properties of ternary mixtures reveals that the nematic range of the produced mixtures is affected in a way that is dependent on both the mixture ratio and the difference in the alkyloxy chain lengths. Furthermore, the phase transition temperature values of ternary mixture increase with increasing heating rate, and the calculated activation energy values show that the reaction arising during the phase transitions of the mixture is regular.

Stable, metastable and unstable solutions of a spin-1 Ising system based on the free energy surfaces

Abstract Stable, metastable and unstable solutions of a spin-1 Ising model with bilinear and biquadratic interactions are found by using the free energy surfaces. The free energy expression is obtained in the lowest approximation of the cluster variation method. All these solutions are shown in the two-dimensional phase space, especially the unstable solutions which in some cases are difficult to illustrate in the two-dimensional phase space, found by Keskin et al. recently.

Dynamic behaviour of the modified Pople–Karasz model

The dynamic behaviour of the modified Pople–Karasz model is studied by the direct relaxation method and the path probability method. First the equilibrium behaviour of the model is given briefly in order to understand the dynamic behaviour. Then the direct relaxation method, which is based on the detailed balance conditions, and the path probability method are applied to the model and the dynamic equations or the rate equations are obtained. Dynamic equations are solved either by means of the flow diagram or by using the Runge–Kutta method, or both. The stable, metastable and unstable solutions are shown in the flow diagrams explicitly, and the role of the unstable state as separators between the stable and metastable is described. Moreover, the “flatness” property of the metastable state and the “overshooting” phenomenon are seen explicitly from the relaxation curves of the order parameters.

THE EFFECT OF MOLECULAR ORIENTATION TO SOLID–SOLID AND MELTING TRANSITIONS

The thermodynamics of solid–solid and solid–liquid transitions are investigated with an account of the number of molecular orientation. The variations of the positional and orientational orders with the reduced temperature are studied. It is found out that orientational order parameter is very sensitive to the number of allowed orientation. The reduced transition temperatures, volume changes and entropy changes of the phase transitions and theoretical phase diagrams are obtained. The entropy changes of melting transitions for different numbers of allowed orientation of the present model are compared with the theoretical results and some experimental data. The quantitative predictions of the model are compared with experimental results for plastic crystals and agreement between predictions of the model and the experimental results are approximately good. Also, different numbers of allowed orientation D correspond to different experimental results HI, HBr, H2S for D = 2; HBr, CCl4, HI for D = 4; C2H12 for D...