A model for the viscoelastic Behavior of nanofilled hydrogel composites under oscillatory shear loading

A molecular model is proposed to predict the viscoelastic response of the hydrogels reinforced with nonpercolated nanoparticles with emphasis on the role of polymer-particle energetic interactions. The structure of the polymer layer adsorbed on the surface of the particles is analyzed using a scaling theory for reversible polymer adsorption. The energetic affinity between the monomers and filler surface is assumed to be weak and short range. Because of thermal fluctuations or the applied deformation, the transient junctions between the polymer segments and filler particles break after a finite period of time. The dynamics of the polymer segments is studied by a Maxwell type kinetic model. The results show that monomer-filler energetic affinity can strongly affect the overall viscoelastic response of the hydrogel nanocomposite, even within the range of weak coupling between the monomers and filler surface. The model can also predict the strain amplitude dependence of the storage shear modulus based on the kinetics of adsoption/desorption of the interfacial monomers. The model predictions are compared with the experimental results for poly(L-lactide-co-ethylene oxide-co-fumarate) hydrogels reinforced with hydroxyapatite nanoparticles. The filled hydogel/ apatite nanocomposites are useful as a model matrix to mimic the soft elastic and hard mineral phases of the bone matrix and as scaffolds for hard tissue regeneration.