Temperature-modulated calorimetry (TMC) allows the experimental evaluation of the kinetic parameters of the glass transition
from quasi-isothermal experiments. In this paper, model calculations based on experimental data are presented for the total
and reversing apparent heat capacities on heating and cooling through the glass transition region as a function of heating
rate and modulation frequency for the modulated differential scanning calorimeter (MDSC). Amorphous poly(ethylene terephthalate)
(PET) is used as the example polymer and a simple first-order kinetics is fitted to the data. The total heat flow carries
the hysteresis information (enthalpy relaxation, thermal history) and indications of changes in modulation frequency due to
the glass transition. The reversing heat flow permits the assessment of the first and higher harmonics of the apparent heat
capacities. The computations are carried out by numerical integrations with up to 5000 steps. Comparisons of the calculations
with experiments are possible. As one moves further from equilibrium, i.e. the liquid state, cooperative kinetics must be
used to match model and experiment.
Authors:B. Wunderlich, A. Boller, I. Okazaki, and S. Kreitmeier
Temperature-modulated differential scanning calorimetry (TMDSC) is based on heat flow and represents a linear system for the measurement of heat capacity. As long as the measurements are carried out close to steady state and only a negligible temperature gradient exists within the sample, quantitative data can be gathered as a function of modulation frequency. Applied to the glass transition, such measurements permit the determination the kinetic parameters of the material. Based on either the hole theory of liquids or irreversible thermodynamics, the necessary equations are derived to describe the apparent heat capacity as a function of frequency.
Authors:A. Boller, I. Okazaki, K. Ishikiriyama, G. Zhang, and B. Wunderlich
The quality of measurement of heat capacity by differential scanning calorimetry (DSC) is based on the symmetry of the twin
calorimeters. This symmetry is of particular importance for the temperature-modulated DSC (TMDSC) since positive and negative
deviations from symmetry cannot be distinguished in the most popular analysis methods. Three different DSC instruments capable
of modulation have been calibrated for asymmetry using standard non-modulated measurements and a simple method is described
that avoids potentially large errors when using the reversing heat capacity as the measured quantity. It consists of overcompensating
the temperature-dependent asymmetry by increasing the mass of the sample pan.