Measurement of heat capacity to gain information about time scales of molecular motion from pico to megaseconds

Abstract

Molecular motion has primarily a time scale of picoseconds. Quantum mechanical models of small-amplitude vibrations and local, large-amplitude motions can be fitted quantitatively to equilibrium heat capacities. Macroscopic calorimetry, thus yields indirectly the characteristic frequencies of molecular motion in the range of 10 11–10 13 Hz. At lower temperature, much of the large-amplitude molecular motion is restricted to cooperative movements and slows to macroscopic times so that calorimetry can directly measure the kinetics of these changes which may extend the time scales to megaseconds and beyond. In this light, instrumentation and interpretation of differential scanning temperature-modulated calorimetry (DSTMC) are discussed. Six basic thermal effects govern the thermal analysis of linear macromolecules: (1) the vibrational heat capacity; (2) heat capacities arising from large amplitude molecular motion; (3) reversible transitions; (4) annealing; (5) secondary crystallization and (6) primary crystallization. Of these only (1)–(3) are reversible. To these six thermal effects that do not change the molecular integrity, chemical reactions, evaporation, and condensation have to be added for a full thermal analysis. The latter effects have not been treated in this paper.