Derivation of accurate glass transition temperatures by differential scanning calorimetry

Glass transition temperatures, T_g, cannot be directly determined from differential scanning calorimetry (d.s.c.) curves because of kinetic effects which are especially serious for well annealed samples. Using an anionic polystyrene as an example it is shown how d.s.c. data can be transformed to enthalpy curves which give T_g with an accuracy of ±1K. As the rate of cooling through the glassy region is lowered T_g is found to decrease by 2.2 K per decade decrease in cooling rate. Quantitative data are given for the specific heat increment at T_g.