Thermal analyses of polymers. VII. Calorimetric and dilatometric aspects of the glass transition

For the determination of Tg by DTA with the use of small samples, slow heating rates, and extrasensitive amplification, it can be shown that the previously reported basel ne changes for polystyrene, polychloroprene, poly(methyl methacrylate), and polycarbonate can be resolved as endotherms. Measurements with small samples more accurately determine the calorimetric changes than with large samples, even at slow heating rates. The use of large samples for DSC or DTA measurements may be the reason for apparent heating rate dependence. The measurements of this study indicate little heating rate dependence if very small samples are used. Moreover, the slower the cooling from the melt, the sharper the endotherms, indicating the importance of thermal history. The data raise the question about the validity of assuming that a baseline or abrupt change in Cp via a DTA measurement is necessarily that of a real second‐order change or Tg. Preliminary dilatometric results tend to show that the changes for polystyrene, polychloroprene, poly(methyl methacrylate), and polycarbonate may be first order at 94°C, 32°, 114°, and 156°C, respectively. Only the change at 75°C for polystyrene appears second order by volume—temperature data.