Investigating linear and nonlinear viscoelastic behavior using model silica-particle-filled polybutadiene

We explore filler reinforcement (i.e., increase of elastic modulus G' due to incorporation of fillers) and Payne effect (i.e., decrease of G' at large strain amplitudes) in terms of the matrix molecular weight, filler loading, and time scales used to probe the viscoelasticity of filled melts. Use of monodisperse non-cross-linked 1,4-polybutadiene (PBD) along with a silica filler allows illustration of different mechanisms of filler reinforcement in the elastic and liquid regimes. The greater filler reinforcement for matrices of higher molecular weight indicates the filler association through chain adsorption and bridging. The role of matrix-mediated filler-filler interactions is explicitly illustrated in terms of the increased elastic modulus in the terminal region due to replacement a fraction of the matrix chains with longer chains of the same kind. The Payne effect is seen to be time-dependent and comprised of an instantly recoverable and a slowly recovering component. Measuring G' at both high and low strain amplitudes through stepwise ramping allows us to reveal that (a) these multiphase materials exhibit lower G' at large gamma if the corresponding shear stress exceeds a certain level required to break down the filler networking and (b) the filler-filler dissociation is partially immediately repairable upon switching to a small gamma. Large step-strain experiments reveal a two-step stress relaxation process, where the initial rapid drop of the relaxation modulus G(t) is due to disintegration of the filler networking under sufficient shear stress. The disintegration in both large-amplitude oscillatory shear and large step-strain shear is not expected to be uniform across the sample thickness as commonly assumed.