Dielectrics and thermodynamics of a macromolecule's growth

Abstract The heat capacity, the exothermic heat (evolved) and the dielectric properties (in the range 10 4 −10 10 Hz) have been measured during the growth of a network macromolecule at a fixed temperature. From the latter, the dielectric relaxation spectra at different times of reaction have been obtained. The heat capacity of an initially monomeric liquid mixture first irreversibly increases as the macromolecule grows and then undergoes a step-decrease to a value which is mainly due to vibrational contributions. This mimics the behaviour of heat capacity of a liquid on its supercooling through the glass transition temperature. The number of covalent bonds, n , formed during the progress of reaction has been determined from the heat evolved. n in the macromolecule structure at its vitrification is less when the isothermal reaction temperature is low. From the dielectric measurements in the MHz frequency range, a dielectric relaxation process is observed, the characteristic time of which irreversibly increases as the reaction progresses. Measurements performed in the GHz frequency range reveal a second relaxation process, the rate of which changes little, while its strength decreases as the macromolecule grows. These findings have a remarkable resemblance to the evolution of relaxation properties observed on cooling a chemically and structurally stable liquid. The results are discussed and it is suggested that, for each macromolecular state characterized by its molecular weight, there is a corresponding state of its unreacted components at low temperatures with similar kinetic and thermodynamic properties.