Nonlinear viscoelasticity of nanofilled polymers: interfaces, chain statistics and properties recovery kinetics

The mechanisms by which nano-scale fillers promote both reinforcement and nonlinear viscoelastic behavior in polymer melts are explored. Three fumed silica fillers having the same specific surface area but different surface treatments were used to prepare composites with a poly(vinyl acetate) matrix of various molecular weights. Filler concentrations were limited to a maximum of 12.5 vol.% to minimize filler agglomeration and networking. Dynamic storage and loss moduli at about 45 degreesC above the glass transition temperature were obtained over a range of shear strain amplitudes. The shape of the loss factor vs. strain function is found to have a strong characteristic dependence on the filler surface treatment. The recovery of properties following their degradation by a large strain perturbation was investigated using a timed sequence of dynamic moduli measurements at low strain amplitudes. Trapped entanglements and their effects on matrix moduli due to Langevin chain statistics is shown to be a viable mechanism for the initial high reinforcement, and its subsequent reduction with strain. Consistent with this, recovery kinetics suggests that the basic mechanism is controlled by chain diffusion. For simple filler (non-polymeric) surfaces, the self-diffusion of matrix chains is the dominant mechanism for recovery. In the case of a filler with surface-tethered polymer chains, the self-diffusion process is supplemented by the inter-diffusion of the tethered and matrix chains. Relative reinforcement is found to be the highest for the lowest molecular weight matrix, regardless of filler type. The proposed mechanism is based entirely on the behavior of the filler-matrix interface and the physics of polymeric melts, and no theories of filler agglomeration or network formation are invoked. (C) 2003 Elsevier Science Ltd. All rights reserved.