Impact of an Emergent Hierarchical Filler Network on Nanocomposite Dynamics

Abstract

Aggregated polymer fillers, such as carbon black and silica, at concentrations above the percolation threshold form an emergent structure, the hierarchical filler network, in immiscible systems where dispersion is driven by accumulated strain. It is proposed that the hierarchical filler network is composed of a primary nanoscale network that locally percolates at ∼5 vol % of aggregates, associated with changes in the dynamic spectrum at low strain, and a secondary micrometer-scale network that globally percolates at ∼20 vol % associated with the Payne effect and electrical conductivity. A model is proposed with an elastomer dominated dynamic response described by Einstein–Smallwood behavior at high frequencies and small sizes and a filler network dominated response at low frequencies and large sizes. The nanoscale mesh size correlates with this transition in low strain dynamic response. The micrometer-scale network displays a gel-like dynamic response at very low frequencies and a corresponding gel-like structural scaling regime at large sizes. The hierarchical filler network is described by two crossover frequencies and associated relaxation times, τ* and τcc, and two related structural scaling regimes.

In situ mechanical reinforcement of polymer hydrogels via metal-coordinated crosslink mineralization

“…A monotonic scattering at low q is also observed (Fig. 2e black dashed line), indicating spatial features with a scale that clearly exceeds the 1 µm length-scale limit of USAXS; such large features are absent in TEM images, and thus we attribute this to the macrostructure of the gel as commonly observed in other studies 36 , 37 . In contrast to the In Situ gel, the Mineral-Free gel exhibits substantially weaker scattering in the higher q regime and no discernable scattering below q = 0.1 Å as expected (Supplementary Fig.…”

Structural identification of percolation of nanoparticles

“…Moreover, it is striking to see that percolation is identified by aggregate recognition based on (static) SAXS, whereas it is usually characterized by dynamic methods accessing transport properties, like electrical conductivity, 61 dielectric processes 33 or stress. 58,62 This is due to the use of statistical information on occupation of space on the scale of the finite-sized regions defined by q min .…”

Resolving Segmental Polymer Dynamics in Nanocomposites by Incoherent Neutron Spin–Echo Spectroscopy

“…Moreover, the large-scale distribution of filler in the matrix may be heterogeneous, i.e., dense zones may offer locally more contact and confinement of the polymer than a well-dispersed and homogeneous distribution. To quantify these aspects, the structure of polymer nanocomposites is usually characterized by transmission electron microscopy (TEM), [13][14][15][16] and in particular small-angle X-ray or neutron scattering (SAXS or SANS), 17 combined with appropriate modelling approaches, [18][19][20][21] possibly including simulations. [22][23] The dynamical properties of PNCs may be studied by broadband dielectric spectroscopy (BDS) [24][25][26][27] or NMR.…”