Role of Nanoparticle–Polymer Interactions on the Development of Double-Network Hydrogel Nanocomposites with High Mechanical Strength

Extensive experimental and theoretical research over the past several decades has pursued strategies to develop hydrogels with high mechanical strength. Our study investigated the effect of combining two approaches, addition of nanoparticles and crosslinking two different polymers (to create double-network hydrogels), on the mechanical properties of hydrogels. Our experimental analyses revealed that these orthogonal approaches may be combined to synthesize hydrogel composites with enhanced mechanical properties. However, the enhancement in double network hydrogel elastic modulus due to incorporation of nanoparticles is limited by the ability of the nanoparticles to strongly interact with the polymers in the network. Moreover, double-network hydrogel nanocomposites prepared using lower monomer concentrations showed higher enhancements in elastic moduli compared to those prepared using high monomer concentrations, thus indicating that the concentration of hydrogel monomers used for the preparation of the nanocomposites had a significant effect on the extent of nanoparticle-mediated enhancements. Collectively, these results demonstrate that the hypotheses previously developed to understand the role of nanoparticles on the mechanical properties of hydrogel nanocomposites may be extended to double-network hydrogel systems and guide the development of next-generation hydrogels with extraordinary mechanical properties through a combination of different approaches

Thermal properties of pAAM hydrogel nanocomposites.

<p>(a) Percent relative thermal diffusivities of pAAm hydrogels as a function of 4 nm silica nanoparticle concentration (grey circles), compared to the traditional effective medium theory (solid line), and the modified effective medium theory (dashed line). The values for relative thermal diffusivity were calculated by normalizing the values for pAAm-SiNP nanocomposite gels (α) to those for neat pAAm gels (α₀). Error bars indicate the standard deviation of triplicate measurements. (b) Correlation between percent relative enhancement in thermal diffusivities and the square root of percent relative enhancement in the elastic moduli for pAAm-SiNP composites (prepared using 4 nm SiNPs). Relative enhancements in thermal diffusivity and elastic modulus were calculated as previously described. Error bars indicate the standard deviation of triplicate measurements.</p

Experimental Investigation of Mechanical and Thermal Properties of Silica Nanoparticle-Reinforced Poly(acrylamide) Nanocomposite Hydrogels

<div><p>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.</p></div

Compressive modulus of pAAM hydrogel nanocomposites.

<p>Percent relative compressive moduli of pAAm hydrogels as a function of 4 nm silica nanoparticle concentration. The values for relative compressive modulus were calculated by normalizing the values for pAAm-SiNP nanocomposite gels (E) to those for neat pAAm gels (E₀). Error bars indicate the standard deviation of triplicate measurements.</p

Swelling properties of pAAM hydrogel nanocomposites.

<p>Percent swelling ratios of pAAm hydrogels as a function of 4 nm silica nanoparticle concentration at various time points – 2 hours (white bars), 6 hours (light grey bars), 12 hours (dark grey bars), and 24 hours (black bars). The values for swelling ratio were calculated by normalizing the values obtained for pAAm gels either containing or not containing SiNPs at various time points to those obtained at time = 0 min. Error bars indicate the standard deviation of triplicate measurements.</p

Viscoelastic properties of pAAm hydrogel nanocomposites.

<p>Percent relative elastic moduli of pAAm hydrogels as a function of silica nanoparticle (a) concentration (prepared using 4 nm SiNPs) and (b) size (prepared using a final concentration of 2% w/v SiNPs). The values for relative elastic modulus were calculated by normalizing the values for pAAm-SiNP nanocomposite gels (G’) to those for neat pAAm gels (G’₀). Error bars indicate the standard deviation of triplicate measurements.</p