Effects of synthesis-solvent polarity on the physicochemical and rheological properties of poly(N-isopropylacrylamide) (PNIPAm) hydrogels

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) microgels were synthesized via free radical polymerization using four synthesis-solvents with varying polarity index: dioxane, tetrahydrofuran (THF), toluene, and cyclohexane. Characterization by FTIR, NMR, and XRD confirmed the chemical structure of synthesized PNIPAm microgels. Microgels synthesized in polar solvents have a lower degree of porosity and display lower swelling values (around 275% and 314% for THF and dioxane, respectively). In contrast, microgels synthesized in the nonpolar solvents toluene and cyclohexane have a higher degree of porosity and a higher degree of swelling (about 900% for cyclohexane). From the dynamic rheology study, the best mechanical performance was obtained with the microgels synthesized in dioxane and toluene due to their unique crosslinked structure. Considering their higher porosity, degree of swelling, thermal stability, and mechanical properties, this study confirms that microgels synthesized in nonpolar solvent like toluene can be an attractive option for multiple applications.