Mechanochemical Formation of Polyacrylamide Nanocomposites: Impact of Nanoparticle Surface Chemistry and Substrate Functionalization

Osteoarthritis is a degenerative joint condition, with treatments currently limited to symptom management and invasive surgical procedures. This project investigates the fundamental mechanisms for a potential treatment route that would allow soft material nanocomposites to form within the joint through mechanochemical reactions. To gain fundamental insight into the different chemical pathways for this to occur, this work compares a top-down vs bottom-up approach. The top-down method focuses on the influence of applied oscillatory mechanical stresses on bulk hydrogel polymerization. Polyacrylamide (PAM) is used as a well-studied control system. PAM-nanocomposites were achieved through co-polymerization, with gold nanoparticles with different capping ligands directly added to the PAM precursor solution. A rheometer was used to simultaneously apply the small angle oscillatory shear and monitor gelation time. Initial results indicate decreased gelation time for PAM-nanocomposites with gold nanoparticle capping ligands of citrate and CTAB (cetyltrimethylammonium), relative to no nanoparticles present. Accelerated gelation times, relative to no nanoparticles present, were observed for PVP (polyvinylpyrrolidone) and PAA (polyacrylic acid) capping ligands. Comparing molecular structure vs chemical functionalities, these results suggest structure (e.g. polymer-based capping ligands, PVP and PAA) over functionality (e.g. hydrogen bonding for PAA and citric acid, but not CTAB and to a lesser degree PVP) influences gelation time. The complementary bottom-up method uses a 3-metharcyloxypropyltrimethoxysilane functionalized silica surface and AFM as a nanoscale single point sliding contact to grow a surface bound PAM hydrogel film. Future work will continue to quantify these pathways, examining reactions within a stress-assisted Arrhenius model

Role of Intermolecular Interactions between Nanoparticle Capping Ligands and Hydrogel Surfaces During Sliding

Modern treatments for osteoarthritis increasingly involve the use of nanoparticles as drugdelivery systems, but there is little known about the influence of nanoparticle chemical composition and surface chemistry between nanoparticles and soft materials in sliding contact. This work implements cartilage-mimicking polyacrylamide (PAM) hydrogels as a well-studied, fundamental platform. In situ (in a fluid environment) macroscale friction tests as a function of shear rate were conducted with a rheometer with a tribology adapter, controlling for contact pressure. Comparing different nanoparticle compositions, citrate capped metal (gold) nanoparticles exhibited a 50% increase in friction relative to water. With no difference in solution viscosity, this difference is likely driven by hydrogen bonding between the citrate ligands and PAM surface. In contrast, carbon based nanoparticles (nanodiamonds) with no capping ligands exhibited a 50% decrease in friction relative to water. Here, a higher solution viscosity for the nanodiamonds is likely dictating the sliding mechanism. Additional tests exploring gold nanoparticles with controlled capping ligands further support the impact of intermolecular interactions between nanoparticle capping ligands and the PAM surface in controlling sliding mechanisms. Post-sliding characterization of the PAM with confocal Raman microscopy surfaces indicate no damage to the hydrogel, and the presence of uncapped nanoparticle aggregates. Next steps will focus on the extent to which nanoparticles might be embedded within the PAM surface as a result of sliding