Effect of Grafting on Rheology and Structure of a Simplified Industrial Nanocomposite Silica/SBR

Abstract

An un-cross-linked SBR-system filled with precipitated silica nanoparticles of radius ≈10 nm by mixing is studied as a function of the fraction of graftable matrix chains (140 kg mol–1) varying from 0% to 100%, for a low (ΦSi = 8.5 vol %) and high (16.7 vol %) silica volume fraction. The linear rheology in shear shows a strong impact of the grafting on the terminal flow regime, and a shift to longer relaxation times with increasing grafting. Simultaneously, the plateau modulus stays approximately constant for the low ΦSi, suggesting a link to the silica content. The microstructure of the silica is characterized by using a combination of transmission electron microscopy and small-angle X-ray scattering data. We apply a quantitative model of interacting aggregates, and determine the average aggregation number (decreasing from 160 to 30 with grafting), aggregate size (50 to 30 nm), and compacity (55% to 35%). While the linear rheology seems to be dominated by the matrix composition, both the mixing rheology and the structure display a saturation with increasing grafting fraction. A closer analysis of this effect indicates that a critical amount of grafting is needed to trigger structural evolution. To summarize, a quantitative study of complex nanocomposites with several features of industrial systems demonstrates that the grafting density can be used as a fine-tuning parameter of rheology and microstructure.

Depercolation of aggregates upon polymer grafting in simplified industrial nanocomposites studied with dielectric spectroscopy

“…Details are given in ref [36,37]. The polymer matrix is made of styrenebutadiene (SB) random copolymers, with two types of chains of molecular mass 140 kg.mol log-normal law with R 0 = 8.55 nm, σ = 27%.…”

“…The polymer matrix is made of styrenebutadiene (SB) random copolymers, with two types of chains of molecular mass 140 kg.mol log-normal law with R 0 = 8.55 nm, σ = 27%. [36] Nanocomposite formulation is the same as before [36,37] and is detailed in the SI. Real silica volume fractions in nanocomposites have been measured by thermogravimetric analysis (Q500, TA Instruments) and lie in the range Φ si = 8.4%v -21.1%v (i.e., between 21 and 63 phr).…”

“…In this case, shape parameters (γ α and δ α ) were imposed to the same values as in the pure matrix (see SI). To account properly for its relaxation strength in the nanocomposite, we have considered the filler effect on the local field according to ref [43], using a simple model for two-phase heterogeneous materials with 2.5 and 5.5 for the dielectric permittivity of SB and silica 36,37 , respectively.…”

“…[36][37][38][39] Here simplified refers to the absence of any crosslinker and related catalysts. We have built a global model describing structural data where primary silica beads (typical size 17 nm) are organized within small aggregates (typical size 80 nm, aggregation number about 50, giving an internal volume fraction, i.e., compacity, of the order of 35%), which themselves fill large-scale fractal branches (typical width 150 nm).…”

“…The key question of this second article on dynamics is to understand the relationship between structure and dynamical processes by now varying principally the amount of graftable SB chains (%D3) in order to control the filler structure [37]. Indeed, local dynamics as observed by NMR has been correlated with rheology [32] but the question of the relative importance of large-scale structure with respect to local connections (bridges) through immobilized layers remains open.…”