Equilibrium swelling of (N-isopropyl acrylamide + hydrophobic comonomer) gels in aqueous solutions of ethanol

The equilibrium swelling behavior of nine N-isopropyl acrylamide (NIPAAm) based copolymeric hydrogels containing the hydrophobic comonomer octadecyl acrylate (ODA) in aqueous solutions of ethanol was investigated experimentally at 298 K. The gels were produced by free radical polymerization in the solvent ethyl acetate using methylenebisacrylamide (MBA) for crosslinking. The mole fraction of octadecyl acrylate in the polymerizing substance yODA was varied between 0.01 and 0.3. The mole fraction of the crosslinker was either 0.01 or 0.02. The swelling behavior of such hydrogels in aqueous solutions of ethanol depends on its ODA content. As long as the concentration of ODA in (NIPAAm + ODA)-hydrogels is small (yODA < 0.05) the swelling behavior is dominated by NIPAAm, i.e., the gels are swollen in aqueous solutions at low as well as at high concentrations of ethanol, but they are collapsed at intermediate concentrations. When the ODA-content in a (NIPAAm + ODA)-hydrogel is high (for example, yODA = 0.3) the gels are collapsed in water and aqueous solutions of ethanol and swell only slightly when the mass fraction of ethanol increases to more than 0.9. (NIPAAm + ODA)-hydrogels with an intermediate concentration of ODA (0.05 ≤ yODA < 0.3) reveal a transition from a shrunken to a swollen state when the mass fraction of ethanol in the aqueous solvent is increased. In addition to the experimental work the swelling behavior was modeled with a previously described thermodynamic model. The calculation results are in good agreement with the experimental data.