Dynamics of Silica-Nanoparticle-Filled Hybrid Hydrogels: Nonlinear Viscoelastic Behavior and Chain Entanglement Network

The understanding of nanoscale fillers in polymeric materials is of the utmost importance for the performance of elastomers. The dynamics of a filler–polymer matrix is studied from the nonlinear viscoelastic behavior of an entangled composite’s network by using silica nanoparticle (SNP)/poly(acrylamide) (PAM) as a model, which allows us to elucidate the nature of the filler association and its role in the nonlinear viscoelastic properties at large strain amplitudes, termed as the Payne effect. The systems show reduced elastic modulus and turn flowable behavior at sufficiently high strains, then partially recover upon switching to a small strain. The Payne effect seem to be time-dependent and is affected by the filler network’s reversible breakdown and rearrangement. This nonlinear viscoelastic property, in particular the decrease of modulus at high strain amplitude, is interpreted to be a reason for the breakdown of the filler network. The greater reinforcement for nanocomposites indicates the filler association through chain immobilization on bridging. The concept of a layer of “glassy bridge” is used to demystify the interparticle connection and correlates it to the Payne effect. The SNP-filled nanocomposite’s reinforcement mechanisms include the plasticization of glassy-layer-bridging neighboring clusters, trapped entanglement at the filler–polymer interface, and polymer chains dynamic adsorption–desorption transformation.