Thermodynamics of polydimethylsiloxane solutions

Abstract

Solvent chemical potentials and heats of dilution have been obtained for systems containing polydimethylsiloxane and solvents comprising normal and branched alkanes, aromatic hydrocarbons and dimethylsiloxane oligomers. Using the Flory–Huggins approximation to eliminate the combinatorial contribution to ΔS_M, the results are expressed as concentration-dependent _{χ, χH} and _χS parameters (X=X+XS). The _χ and _χS parameters have large positive values typical of polymer-solvent systems, which correspond to negative non-combinatorial contributions in ΔS_M. Current theories associate such results with a large difference in degrees of thermal expansion between polymer and solvent. This difference is unimportant in the present systems. Accordingly the new Flory theory gives poor predictions, as do other models consistent with the Prigogine corresponding states approach. The failure of theory is discussed in terms of: an error in the Flory–Huggins approximation when the polymer is of large chain-diameter; the Flory Q₁₂ parameter; and the van der Waals combining rules. None of the explanations is entirely satisfactory. The negative non-combinatorial contribution to ΔS_M is, however, consistent with negative values of the excess volume found in these systems which are also unexplained by current theory.