Dynamics of nanofiller–polymer synergy in gel formation via macromolecular crowding: Structure-property relationship of halloysite-integrated hybrid gels with abundant carboxyl groups

Abstract

Halloysite (Hal) nanotube-reinforced hybrids were designed via mimicing macromolecular crowding for effective adsorption of cationic dyes by copolymerization of N-isopropylacrylamide (NIPA) and pH-responsive methacrylic acid (MA) through thermally initiated free-radical mechanism. A perspective on the synthesis of functional PNIPA-based next-generation hybrids was established using dynamics of nanofiller–polymer synergy in gel formation via incorporation of linear polyacrylamide chains. Since the design of hybrids with high polymer fraction is relatively difficult in gel systems in which the components are simply blended, the optimum gel formation was achieved by combining the advantage of one-pot strategy with the macromolecular crowding created using water-soluble polymer. By interacting with linear polyacrylamide chains via hydrogen bonding interactions, Hal-lumens participate in the formation of hybrid networks and are homogeneously distributed in the polymer matrix, thus significantly improving the physicochemical properties. The reversibility and rapid on-off properties of hybrids with varied Hal-loadings were analyzed depending on pH and ionic strength of surrounding medium. Design of hybrids functionalized with specific groups depends on both the amount of these groups and the nature of their distribution in the network. The swelling of hybrids in pH 9.8 is lower compared to results in 10⁻⁵ M KCl solution, while there was an increase in the compressive modulus with a decrease in swelling pH. The effect of potassium salts of different anions on salt-induced swelling and elastic response of hybrids was investigated to understand the interactions between salt ions and hybrid network. The expansion of hybrids was found to follow the reverse order of classical Hofmeister series. Depending on Fick's law, hybrid gels followed non-Fickian kinetics in salt solutions, while Fickian swelling was observed in pH 9.8. The influence of Hal-addition on cationic dye methyl violet (MV) adsorption of hybrids was investigated. While the addition of Hal increased the adsorption capacity, 93.6 % efficiency for MV was obtained in the presence of 5.40 % Hal. The obtained results provided a novel routine for designing alkyl acrylamide-based functional hybrid gels using green one-dimensional tube-like nanoparticles.