Relationship between Swollen Network Structure of Rubber Vulcanizates and Freezing Point Depression of Swelling Solvent

Abstract

Rubber vulcanizates in the state of swelling equilibrium with benzene were cooled, and the freezing point depressions (FPD) were measured. It was found that the FPD of the crosslinked rubber sample was larger than that of the uncrossliked one at the same concentration. For polydimethylsiloxane the FPD of bimodal network was larger than that of unimodal network. Here the bimodal network means the network prepared by the end-linking of two prepolymers having different average molecular weights; it has a wider distribution in mesh size of network (chain weights between crosslinking points) than the unimodal network, and may be regarded as a model of the inhomogeneous network. Thus, the result suggests that the inhomogeneity of the network structure can be detected by the cryoscopic method.
For uncrosslinked rubber systems the larger FPD was observed with increasing concentration, decreasing molecular weight, and decreasing χ-parameter. Though we may qualitalively explain these features by the Flory-Huggins lattice model, we observemore complex behavior: the compressive moduli of swollen rubber vulcanizates decreased with lowering temperature, and fell rapidly from a certain value to zero when the sample freezes. This may be considered to be associated with a phase transition phenomenon.
With lowering temperature, the intrinsic viscosity of rubber solutions decreased and the content of benzene in swollen samples decreased. Swollen samples became turbid in the vicinity of 283K. The result may suggest that with lowering temperature the concentration fluctuation becomes large as the solvent becomes poor. By further cooling, it is considered that crystallization of benzene occurs first and then the swollen samples freezes and contracts.