On the Anomalous Freezing and Melting of Solvent Crystals in Swollen Gels of Natural Rubber

Abstract

Abstract The anomalously large solvent freezing point depression, ΔTf, observed in crosslinked rubbers swollen in solvent has been a subject of study for over thirty years without clear resolution. While a sizeable ΔTf is accounted for by the lowering of the thermodynamic potential of solvent molecules in a polymer solution derived from the Flory theory, the additional ΔTf observed for crosslinked rubbers has been attributed to various physical effects such as restriction of solvent crystals to small size by the network mesh or difficulty in nucleation of the solvent crystals. In this paper, we identify two points of misunderstanding in the literature on this problem, and attempt to clarify the analysis of ΔTf for solvent swollen rubbers. The first point relates to the application of the Flory calculation to solvent freezing, where nucleation is a concern, rather than for solvent melting, for which it was intended. We present new calorimetric data on both the freezing and melting of solvent crystals in crosslinked and uncrosslinked NR swollen in benzene to illustrate this point. The second point relates to a reconsideration of the hypothesis of Kuhn et al., that the anomalous ΔTf-can be accounted for by small crystallite size. Although Boonstra and coworkers argued against Kuhn's hypothesis based on the results of x-ray line-broadening studies of solvent crystallite size, d, in the frozen gel, we have shown that their interpretation is suspect due to the use of an incorrect value for the solid-liquid surface energy of the solvent in the calculation of the melting- or freezing-point depressions.