Modulus of Plasticized Ionic Rubbers

The rubbery modulus of a crosslinked ionizable rubber is explored in the absence of the crosslinking effect of ions, that is, only through the effect of ions on conformation. The crosslinking effect is eliminated by plasticizing the rubber with a liquid of high dielectric constant. Experimentally, a rubber is prepared by mixing poly (acrylic acid) and poly (vinyl alcohol) and cross-linking thermally. The relationship between the rubbery moduli of the unionized and the ionized forms of the plasticized material is studied. A theory for the modulus of such a two-component rubber is derived and the predictions are compared with experiment. It is shown that the predicted ratio of the moduli in the acidic and ionic forms is in the same direction as determined experimentally, but that the calculation underestimates the magnitude of the effect. Several reasons for this discrepancy are given.