Abstract
The nonlinear rheology of a novel 3D hierarchical graphene polymer nanocomposites was investigated in this study. Based on an isotactic polypropylene, the nanocomposites were prepared using simple melt mixing, which is an industrially relevant and scalable technique. The novel nanocomposites stand out as having an electrical percolation threshold (≈0.94 wt%) comparable to solution mixing graphene-based polymer nanocomposites. Their nonlinear flow behavior was investigated in oscillatory shear via Fourier-transform (FT) rheology and Chebyshev polynomial decomposition. It was shown that in addition to an increase in the magnitude of nonlinearities with filler concentration, the electrical percolation threshold corresponds to a unique nonlinear rheological signature. Thus, in dynamic strain sweep tests, the nonlinearities are dependent on the applied angular frequency, potentially detecting the emergence of a weakly connected network that is being disrupted by the flow. This is valid for both the third relative higher harmonic from Fourier-transform rheology, I3/1, as well as the third relative viscous, v3/1, Chebyshev coefficient. The angular frequency dependency comprised non-quadratic scaling in I3/1 with the applied strain amplitude and a sign change in v3/1. The development of the nonlinear signatures was monitored up to concentrations in the conductor region to reveal the influence of a more robust percolated network.
Highlights
Since the early days of polymeric materials, filled systems remain essential for improving and/or inducing new functional properties otherwise unavailable in plastics
We present an industrially relevant melt mixed system where a novel 3D hierarchical reduced graphene oxide (HrGO) filler has been successfully dispersed in isotactic polypropylene
Pronounced increase in viscosity at low shear rates is apparent at concentrations φ > 2.5 wt%, with the power law model, η 1⁄4 Kγn−1, where K and n are the consistency and flow behavior indices, appropriately describing the shear thinning behavior for γ > 10 1/s. This is considered to be indicative of a yield stress behavior, as strong attractive forces between nanofillers are known to lead to the formation of structures that could support a yield stress (Dealy and F. 1999)
Summary
Since the early days of polymeric materials, filled systems remain essential for improving and/or inducing new functional properties otherwise unavailable in plastics. We show that the electrical percolation threshold corresponds to a unique nonlinear signature whose characteristics develop with increasing concentration which could potentially provide a clear distinction between the different network evolutionary mechanisms and the influence of shear thereon. The nonlinear material response as expressed by the third relative higher harmonic, I3/1, from dynamic strain sweep tests is presented in Fig. 5 for representative filler concentrations.
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