The role of fillers in the mechanical properties of hydrogel composites

We investigate the mechanical properties of attractive filler-hydrogel composites, characterizing their response under small- and large-scale deformations. We reveal the physical mechanism for the strong reinforcement of the shear moduli of the composites in the linear regime. The attraction between the fillers and the hydrogel induces an increase in the gel density around the fillers. The new unit of fillers and densified regions around them percolates at a volume fraction that coincides with the onset of reinforcement. The percolation leads to suppressed gel displacements and enhanced stress-coupling throughout the system, resulting in stiffening at low volume fraction of fillers in a variety of attractive filler-hydrogel composites. We identify the role of the percolated network of effective fillers in the nonlinear properties of the composites, showing that the fillers induce an earlier onset of nonlinearities, including yielding and strain hardening. In both pure gels and composites, the onset of plasticity occurs at the deformation at which the gel stiffness is maximal. We link the gel's plastic response to residual stresses stored in the gel matrix.