The nucleation of droplets of polyethylene, polyethylene oxide, polym''Ymethylene, nylon 6, poly (3,3bis-chloromethyloxacYclobutane), isotactic polypropylene, and isotactic polystyrene, suspended in silicone oil, has been investigated. Under conditions of slow cooling (0.1 °C/min), two different phenomena were observed; occasional droplet solidification at low degrees of subcooling, the number of droplets solidified and the temperature at which nucleation occurred being a function of the thermal melt history; and a sudden catastrophic, history-independent, nucleation phenomenon at large subcooling. The latter event may be identified as a critical phenomenon occurring at a subcooling D.Tc which is only slightly dependent upon the rate of cooling.For polyethylene and isotactic polypropylene, the t:..T.'s correspond closely with values obtained by other authors using different suspending media and have been identified with homogeneous nucleation. Homogeneous nucleation may also be explored by the method of kinetic-rate measurements, with the advantage that rate constants can be evaluated directly rather than by theoretical derivation. Interfacial energies calculated from nucleation theory using the first (slow-cooling) method, have been compared in this work with those obtained from the second (isothermal) method. Such comparison shows that the isothermal method gives values of the same order as, but somewhat lower than, the slow-cooling method.For the other polymers, most of which are glass-forming, it is not entirely clear that the catastrophic event corresponds with homogeneous nucleation. Whereas critical supercooling and solidification rates resemble those for homogeneous nucleation, there is evidence that the interfacial energies recovered are too low and the circumstances surrounding the catastrophic event suggest that the suspending medium may be catalyzing nucleation.
Nucleation rates for the formation of ice from water have been calculated from observations of the frequency with which liquid droplets solidify. The stochastic nature of these solidifications has been considered and statistical methods employed for weighting observations and predicting experimental errors. The dependence of the solidification frequency on droplet volume and temperature has also been considered and methods were found for incorporating these dependencies into the rate calculations. The influence of the droplets' environment on the nucleation process was found in most cases to be negligible. The nucleation rates obtained from isothermal and continuous cooling experiments were in good agreement with each other, and their temperature dependence was in accord with the functional relationship predicted by theory. The rate's temperature dependence indicated a transition from a heterogeneous to a homogeneous nucleation mechanism; this finding was supported by the rate's dependence on droplet radius, which indicated a surface catalysis for the heterogeneous data. The average experimental kinetic constant was found to be greater than the theoretical value by a factor of 10 15 • 6 • This discrepancy could be explained by invoking a temperature dependent ice-water interfacial free energy.
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