Strength and salt/pH dependent-sorption capacity modulation of N-(alkyl)acrylamide-based semi-IPN hybrid gels reinforced with silica nanoparticles

A simple methodology was described for design of anionically modified thermosensitive organic-inorganic hybrid gels using silica nanoparticle-mediated semi-interpenetrating polymer networks. A thermosensitive copolymer poly(N-isopropylacrylamide-co-methacrylic acid), P(NIPA-MA), with linear polyacrylamide (LPA) chains was synthesized by conducting free radical polymerization under warm and cold conditions using NIPA and MA in the presence of SiO₂ nanoparticles. Simple mixing of aqueous monomer solution with linear LPA chains and silica suspensions led to the formation of ionic thermosensitive hybrid gels. The effect of changes in solution environment on the swelling and elastic properties was investigated. Sodium salts of CH₃COO⁻, NO₃⁻, and SCN⁻ were employed to understand specific ion effects and to gain an insight into the influence of kosmotrope and chaotrope solutes on swelling and elasticity at various ionic strength and temperature. The identity of counterion had a strong influence on swelling and compression modulus and was found to follow the reverse order of classical Hofmeister series (HS). The phase transition temperature decreased with increasing salt concentration, and transition changed from discontiuous to continuous. At low salt concentrations, the gels showed discontinuous phase transition at 50 °C, while transition temperature shifts towards 35–45 °C with increasing salt concentration. Dilution of the solution to 10⁻³ M resulted in a 2.0-fold reduction in compressive modulus of hybrid gels in NaAc solutions while a 3.0-fold reduction in NaSCN solutions. Compression modulus increased with shrinkage of hybrids due to increasing salt concentration. The effect of pH change of swelling medium on the swelling, phase transition profile and elasticity was evaluated. The continuity of pH-dependent swelling curve depended on swelling temperature. pH-triggered oscillation kinetics have been investigated to understand whether they can be used as a pH oscillator since the phase transition pH is close to 5.7. The swelling of NP-loaded hybrids followed a Fickian type diffusion. NP-loaded semi-IPN hybrid gels with typical feature of anionic structure were applied for adsorption of cationic dye methyl violet (MV). Different adsorption isotherm models were investigated to describe interfacial adsorption of MV which was strongly dependent on composition of hybrids. Langmuir isotherm model was found to be most appropriate to describe adsorption process. The superior properties imparted to the present system prepared with anionic modification, semi-IPN structure and NP-loading make semi-IPN hybrid gels suitable candidates for controlled release systems and for adsorbents of cationic dyes.