Studies Of First-order Transitions Research Articles

Specific heat under uniaxial stress

An experimental approach for the adaptation of conventional calorimetric techniques for measurements under uniaxial stress is reviewed. Special attention is focused on the description of the use of the a.c. method for measuring specific heat under uniaxial stress. The application of isoperibolic calorimetry to specific heat measurements under uniaxial stress is described in detail. The method has been tested at low stress by studying the influence of uniaxial stress on the change of dimensionality of the order parameter of strontium titanate. Measurements of the specific heat and heat power exchanged by a deuterated dihydrogen potassium phosphate sample at the ferroelectric–ferroelastic phase transition are also included. These measurements show the advantages of this method for the study of first-order transitions under uniaxial stress.

Calorimetric study of first order transitions in poly(vinyl chloride)

A calorimetric study of first order transitions in the specific heat of PVC is reported. The results show that various types of commercial PVC exhibit a melting range which starts at about 150 °C and extends into the thermal decomposition region. A heat of fusion of 750 cal/mole was calculated from the melting energy and the change in density upon melting. A similar melting range starting at 180 °C was found in PVC polymerized at −30 °C. Once melted, this primary cristallinity does not reappear after annealing. It can be restored by dissolution and subsequent careful precipitation.