Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4′-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases
Authors:T. Hatakeyama, S. Yamamoto, and H. Hatakeyama
The melting and crystallization behaviour of water restrained in polysulfone (PSF) hollow fibre has been investigated by DSC.
The PSF hollow fibre was prepared by wet spinning. The morphology of the fibre was observed by scanning electron microscopy.
The amount of water was varied by successive evaporation and it was observed that melting and crystallization shifted to temperatures
lower than those observed for free water. DSC and NMR studies showed that water in hollow fibres is in a restricted state.
Authors:J. Binner, G. Dimitrakis, D. Price, M. Reading, and B. Vaidhyanathan
of the β to α phase transition in silver iodide was investigated
by conventional and modulated temperature DSC and dielectric property measurements.
On cooling, the high temperature phase remained stable 2.5C below its
normal transition temperature even at a very slow cooling rate 0.2C h–1.
Dielectric property measurements under conventional and microwave heating
suggested an anomalous effect of the latter on the β to α phase
transition in this material.
A simple method of determination of kinetic parameters by analysis of DTA(t) function was developed for the case of systems undergoing transitions without mass change and when kinetic equation describing
transition rate is known. The presented method also permits the determination of transition rate dα/dt (or α(t )) when the kinetic equation of transition is unknown. The developed method was tested using DTA data of crystallization
of 2CaOAl2 O3 1.95SiO2 glass pure and doped with Cr3+ and Nd3+ .
Authors:A. Habitante, P. Sobral, R. Carvalho, J. Solorza-Feria, and P. Bergo
The aim of this work was to study the glass transition, the glass transition of the maximally freeze-concentrated fractions,
the ice melting and the gelatinization phenomenon in dispersions of starch prepared using glycerol-water solutions. The starch
concentration was maintained constant at 50 g cassava starch/100 g starch dispersions, but the concentration of the glycerol
solutions was variable (Cg= 20, 40, 60, 80 and 100 mass/mass%). The phase transitions of these dispersions were studied by calorimetric methods, using
a conventional differential scanning calorimeter (DSC) and a more sensitive equipment (micro-DSC). Apparently, in the glycerol
diluted solutions (20 and 40%), the glycerol molecules interacted strongly with the glucose molecules of starch. While in
the more concentrated glycerol domains (Cg>40%), the behaviour was controlled by migration of water molecules from the starch granules, due to a hypertonic character
of glycerol, which affected all phase transitions.
An adiabatic calorimetry was used for some investigations of equilibrium and non-equilibrium phase transitions. For one of
the substances studied (4,4′-di-n-heptyloxyazoxybenzene) it was possible to determine temperature dependence of an order parameter and number of clusters of
high temperature phase in a region of a phase transition. For another substance (liquid 3,4 dimethylpiridine) an anomaly on
the specific heat curves was interpreted as being responsible for a decay of molecules’ clusters.
Non-equilibrium phase transitions were investigated for some liquid crystal substances. The process of transformation between
metastable and stable phases was described quantitatively. The conclusions obtained concern the stability of metastable phases.
A model is proposed for AC calorimetry (ACC) at the first order phase transition point. The model is compared with the results of ACC around the melting point of ann-paraffin (C20H42). The observed frequency dependence of ACC is consistent with the model. A harmonic component of the temperature modulation with a frequency equal to twice the heating frequency was observed at the phase transition point. It is shown that the harmonic component can be explained on the basis of the proposed model.