(709i) Thermodynamic Model for Predicting Swelling of Poly(N-isopropyl acrylamide) Hydrogels in Solvent Mixtures

We are establishing an engineering thermodynamic model to predict swelling behavior of hydrogels in aqueous solvents. Hydrogels are three dimensional cross-linked hydrophilic polymers with water contained within it. Depending on its structural properties and surrounding conditions, i.e. temperature, pressure, solvent concentration, it can absorb and desorb large amounts of solvent from its coexisting liquid phase without dissolving in it [1]. This interesting property is called swelling behavior. Polymer chain of hydrogels are cross-linked with cross-linking agent to give enough mechanical strength. The thermodynamic equilibrium properties, i.e., activity coefficients, are expressed [2] as a combination of Gibbs energy of the solvent components along with Helmholtz energy of the elastic network. Extensive research, including UNIQUAC equation model [3] and PC-SAFT model [4], has been successfully accomplished to model such systems. Other models were explicitly established for hydrogel swelling, but they discounted phase equilibria [4, 5]. With the ultimate goal to model hydrogel swelling in brine solutions in combination with e-NRTL (Non-Random Two Liquid Theory) model [6], we use polymer NRTL activity coefficient model [7] as the thermodynamic framework to correlate swelling behavior of hydrogels in aqueous solution with organic solvents. The model combines local interactions defined by NRTL model and configurational entropy of mixing defined by Flory-Huggins equation. The theory applies local composition concept to solvent molecules and polymer segments. Each binary pair requires two binary interaction parameters that correlates composition dependence of the solution nonideality. In the current study, we investigate poly(N-isopropyl acrylamide) PNIPAAm hydrogels which is non-ionic and synthetic in nature. As for solvents, we study aqueous solutions of methanol, ethanol, acetone, etc. The model correlates well with the degree of swelling [3] of PNIPAAm hydrogels as well as the solvent compositions in these solvent mixtures at 298.15 K and at atmospheric pressure. The model results compare well with available experimental data [3, 4, 8, 9, 10, 11].

References:

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