Juern W. P. Schmelzer

Method to estimate crystal/liquid surface energy by dissolution of subcritical nuclei

Abstract Within the framework of the classical nucleation theory (CNT) the nucleus–liquid surface energy ( σ ) is one of the most important parameters to predict or analyze nucleation kinetics in undercooled liquids. However, the few methods proposed to determine σ are based on the direct use of nucleation rate data and, thus, do not allow an independent test of the theory. In the present paper, this parameter is estimated without direct use of nucleation rate data. Instead, we provide further experimental evidence for the dissolution of sub-critical nuclei at several development temperatures and use this effect to estimate the nucleus–liquid surface energy. The results of the present analysis demonstrate that, in agreement with a new approach for the description of nucleation processes, not only the surface energy but also the driving force for nucleation have quite different values from those of the evolving macrophases. The possible implications of this result for the understanding of crystal nucleation in liquids are discussed.

Homogeneous crystal nucleation in polymers

The pathway of crystal nucleation significantly influences the structure and properties of semi-crystalline polymers. Crystal nucleation is normally heterogeneous at low supercooling, and homogeneous at high supercooling, of the polymer melt. Homogeneous nucleation in bulk polymers has been, so far, hardly accessible experimentally, and was even doubted to occur at all. This topical review summarizes experimental findings on homogeneous crystal nucleation in polymers. Recently developed fast scanning calorimetry, with cooling and heating rates up to 106 K s-1, allows for detailed investigations of nucleation near and even below the glass transition temperature, including analysis of nuclei stability. As for other materials, the maximum homogeneous nucleation rate for polymers is located close to the glass transition temperature. In the experiments discussed here, it is shown that polymer nucleation is homogeneous at such temperatures. Homogeneous nucleation in polymers is discussed in the framework of the classical nucleation theory. The majority of our observations are consistent with the theory. The discrepancies may guide further research, particularly experiments to progress theoretical development. Progress in the understanding of homogeneous nucleation is much needed, since most of the modelling approaches dealing with polymer crystallization exclusively consider homogeneous nucleation. This is also the basis for advancing theoretical approaches to the much more complex phenomena governing heterogeneous nucleation.