Effect of crosslinker structure and crosslinker/monomer ratio on network parameters and thermodynamic properties of Poly (N-isopropylacrylamide) hydrogels

The effect of polymer composition and polymerization parameters such as the type of crosslinker and comonomer, crosslinker/monomer ratio and polymerization temperature on the polymer-solvent interactions and mechanical properties of tetraallylammonium bromide (TAB)-crosslinked N-isopropylacrylamide (NIPAAm), N,N'-methylenebisacrylamide (BIS)-crosslinked NIPAAm and TAB-crosslinked NIPAAm - itaconic acid (IA) hydrogels were studied. It was observed that the hydrogels prepared by 0.7 mole / L concentration of NIPAAm and crosslinked with BIS exhibited mechanical weakness and, they were broken even under a pressure, being less than 5.0 N at a swelling temperature of 25 C while the PNIPAAm hydrogels crosslinked with ionic-octafunctional crosslinker, TAB, had better compression properties. A decrease in the concentration of NIPAAm, increasing TAB content, addition of a hydrophilic/weakly acidic comonomer, IA and an increase in the functionality of crosslinker resulted in the decreasing values of the poymer-solvent interaction parameter, χ at 25 C. The values of χ, effective crosslinking density (ν_e) and enthalpy changes during the shrinkage process in the ranges of 33^o - 45 C and 37^o- 45 C indicated the exothermal nature of phase transition and the importance of intermolecular interactions between PNIPAAm chains. The partial molar dilution-enthalpies (ΔH₁) and -entropies (ΔS₁) that were calculated from the values of enthalpic and entropic components of χ also indicated a similar trend, referring to a decrease in ordered-structuring of water molecules around hydrophobic isopropyl groups with an increase in the swelling temperature.