Influence of Solvent Quality on the Swelling and Deswelling and the Shear Modulus of Semi-Dilute Solution Cross-Linked Poly(vinyl acetate) Gels

We systematically examine the influence of varying temperature (<i>T</i>) over a large range in model poly(vinyl acetate) gels swollen in isopropyl alcohol. The theta tempearture<i> Θ</i> of our gels, at which the second virial coefficient <i>A₂</i> vanishes, is found to be equal to within numerical uncertainty to the corresponding high molecular mass polymer solution value without cross-links, and we quantify the swelling and deswelling of our model gels relative to their size at <i>T </i>= <i>Θ</i>, as customary for individual flexible polymer chains in solution. We also quantify the “solvent quality” dependence of the shear modulus <i>G </i>relative to <i>G</i>(<i>T</i> = <i>Θ</i>) and compare to the hydrogel swelling factor, <i>α</i>. We find that all our network swelling and deswelling data can be reduced to a scaling equation of the same general form as derived from renormalization group theory for flexible linear polymer chains in solutions so that it is not necessary to invoke either the Flory-Huggins mean field theory or the Flory-Rehner hypothesis that the elastic and mixing contributions to the free energy of network swelling are separable to describe our data. We also find that changes of <i>G</i> relative to <i>G</i>(<i>T</i> = <i>Θ</i>) are directly related to <i>α</i>. At the same time, we find that classical rubber elasticity theory describes many aspects of these semi-dilute solution cross-linked networks, regardless of the solvent quality, although the perfector clearly reflects the existence of network defects whose concentration depends on the initial polymer concentration of the polymer solution from which the networks were synthesized.