The Apparent Second-Order Transition Point of Polystyrene

Abstract

(1) At high temperatures (above 80°C) the specific volume of polystyrene is an explicit function of the temperature. dV/dT=0.00043. (2) At low temperatures (below 40°C) the specific volume of polystyrene depends upon its past thermal history dV/dT=0.00024, although V is not explicitly fixed by the temperature alone. (3) When a sample of polystyrene is cooled at constant rate, it contracts according to the higher expansion coefficient until some critical region of temperature, and then contracts according to the smaller coefficient. (4) The position of the critical temperature region depends upon the rate of cooling. The faster the cooling, the higher the ``transition point.'' (5) In the intermediate temperature range (say 40°C to 80°C), complex time effects can be observed. A sample heated from 20°C to this intermediate range will first expand, according to the small expansion coefficient, and then contract slowly toward a smaller volume. The rate of this contraction is greater at higher temperatures. (6) All the observations are in fair accord with the interpretation of the ``second-order transition point'' of polystyrene as the temperature at which rate of volume change becomes comparable with the experimental time scale.