Surface-engineered polystyrene nanoparticles as multifunctional crosslinkers for hydrogels

Nanocomposite hydrogels incorporating nanoparticles offer a promising route to overcome the classic tradeoff between high water content and mechanical robustness. This study presents the synthesis of polyacrylamide hydrogels reinforced with N,N’-methylenebisacrylamide (MBA)-grafted polystyrene nanoparticles (PS@MBA). The MBA graft provides copolymerizable vinyl groups on the PS surface, enabling covalent incorporation of the rigid PS cores into the polyacrylamide network during in situ free-radical polymerization. The resulting nanocomposites were characterized by tensile testing, oscillatory rheology, and equilibrium swelling measurements. The PS@MBA-reinforced hydrogels exhibit dramatically improved mechanical properties: the tensile modulus and strength increase substantially with nanoparticle loading, and the composite gels sustain much higher shear stress before yielding. The elastic modulus and viscous modulus of the hydrogels increased with increasing PS@MBA concentration, indicating the formation of a more rigid and energy-dissipative network. The yield stress and creep-recovery tests further confirmed the enhanced mechanical robustness and resilience of the nanocomposite hydrogels at higher nanoparticle loadings. Although the reinforced gels swell slightly less, they retain a high-water uptake characteristic of hydrogels. These results demonstrate that MBA-grafted nanoparticles act as effective “macro-crosslinkers” binding multiple polyacrylamide chains and dissipating stress within the network. This covalent nanoparticle integration imparts exceptional toughness and strength without severely compromising swelling.