Over the past few decades, research studies have established that the mechanical properties of hydrogels can be largely impacted by the addition of nanoparticles. However, the exact mechanisms behind such enhancements are not yet fully understood. To further explore the role of nanoparticles on the enhanced mechanical properties of hydrogel nanocomposites, we used chemically crosslinked polyacrylamide hydrogels incorporating silica nanoparticles as the model system. Rheological measurements indicate that nanoparticle-mediated increases in hydrogel elastic modulus can exceed the maximum modulus that can be obtained through purely chemical crosslinking. Moreover, the data reveal that nanoparticle, monomer, and chemical crosslinker concentrations can all play an important role on the nanoparticle mediated-enhancements in mechanical properties. These results also demonstrate a strong role for pseudo crosslinking facilitated by polymer–particle interactions on the observed enhancements in elastic moduli. Taken together, our work delves into the role of nanoparticles on enhancing hydrogel properties, which is vital to the development of hydrogel nanocomposites with a wide range of specific mechanical properties.

Current studies investigating properties of nanoparticle-reinforced polymers have shown that nanocomposites often exhibit improved properties compared to neat polymers. However, over two decades of research, using both experimental studies and modeling analyses, has not fully elucidated the mechanistic underpinnings behind these enhancements. Moreover, few studies have focused on developing an understanding among two or more polymer properties affected by incorporation of nanomaterials. In our study, we investigated the elastic and thermal properties of poly(acrylamide) hydrogels containing silica nanoparticles. Both nanoparticle concentration and size affected hydrogel properties, with similar trends in enhancements observed for elastic modulus and thermal diffusivity. We also observed significantly lower swellability for hydrogel nanocomposites relative to neat hydrogels, consistent with previous work suggesting that nanoparticles can mediate pseudo crosslinking within polymer networks. Collectively, these results indicate the ability to develop next-generation composite materials with enhanced mechanical and thermal properties by increasing the average crosslinking density using nanoparticles.

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Recent studies have expanded our understanding of the effects of nanoparticles on hydrogel mechanical properties. However, further studies are needed to validate the generality of these findings, as well as to determine the exact mechanisms behind the enhancements afforded by the incorporation of nanoparticles. In this study, we performed rotational rheological characterizations of chemically crosslinked poly(acrylamide) hydrogels incorporating silica nanoparticles to better understand the role of nanoparticles on the enhanced properties of hydrogel nanocomposites. Our results indicate that incorporating nanoparticles can lead to enhancements in hydrogel elastic moduli greater than the maxima obtainable through purely chemical crosslinking. Moreover, we find that the increases in elastic moduli due to the addition of nanoparticles not only depend on particle concentration, but also on the monomer and chemical crosslinker concentration. Finally, our data indicates a strong role for pseudo-crosslinking mediated by noncovalent interactions between the nanoparticles and hydrogel polymers on the observed reinforcements. Collectively, our results shed further insight into the role of nanoparticles on enhancements of mechanical properties of hydrogels and may thereby facilitate engineering specific mechanical properties in a wide range of hydrogel nanocomposite systems.

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