On the thermodynamic driving force for nucleation at large undercoolings

Abstract

The molar free energy difference ΔGf, between amorphous and crystalline phases, is computed using heat capacity data over a large temperature range between the melting (Tm0) and the glass transition temperatures. These values are used to assess the validity of the approximate correction factors, fH1=T/Tm0 and fH2=2T/(T+Tm0), suggested by Hoffman [Thermodynamic driving force in nucleation and growth processes. J Chem Phys 1958;29(5):1192–93], and widely used in the literature for a variety of polymers to account for the change in free energy with temperature, T. Surprisingly, for polyethylene and isotactic polypropylene, which are industrially important polymers, it is found that the performance of these correction factors is worse than uncorrected estimates. For isotactic polystyrene and polyethylene oxide, it is found that the correction factor fH2 offers a good approximation for the free energy of fusion at moderate to large undercoolings, although Hoffman's criterion recommends the use of fH1. Empirical correction factors that provide a better fit with the experimental data, and a computer program used to determine them are also provided.