Time-dependent heat capacity in the glass transition region (original) (raw)

1971, Journal of polymer science

Timedependent, apparent heat capacities of glucose, poly(viny1 chloride), polystyrene, selenium, poly(methy1 methacrylate), and poly(2,6-diiethyl-1,4-phenylene ether) in the glass transition region were determined by differential thermal analysis. The thermal history was set by linear cooling a t rates between 0.007 and 16OoC/min. Linear heating for analysis was carried out at rates between 0.3 and 6OO0C/min. Average activation energies of 52,81,90,54,77, and 108 kcal/mole, respectively, were evaluated by using the hole theory of glasses previously developed. Within experimental limitations all data could be described quantitatively by the theoretical expressions using only one parameter, the number of frozen-in holes, to describe the thermal history. Experimental and theoretical limitations are discussed. Measurements of heat capacity of amorphous materials in the glass transition region show nonequilibrium effects due mainly to time-dependent configurational rearrangements of the molecules. At temperatures sufficiently below the glass transition T,, the configuration is virtually frozen in, and the heat capacity of the glass behaves like an equilibrium property. Frequently, the heat capacity of glasses is similar to heat capacity of equilibrium crystals of chemically identical structures down to temperatures as low as 50°K. At temperatures sufficiently above T,, the configurational rearrangements are so fast that their time dependence is not measurable, and an equilibrium heat capacity exists for the melt. The heat capacity contribution due to changes in mode of motion (such as vibrations changing to rotation or translation) is much smaller than that due to configurational rearrangements (such as hole formation) in the T, region. This paper will be concerned with the time-dependent apparent heat capacity of six materials in the glass transition region: glucose (CSHIZO~), selenium (Se), poly(viny1 chloride) (PVC), polystyrene (PS), poly(methy1 methacrylate) (PMMA), and poly(2,6-dimethyl-l,4phenyl ether) (PPO). Early observations of heat capacity as a function of time and temperature established that a maximum and a minimum can occur in the transition region of several organic, inorganic, and polymeric Presently