Structural changes in precipitated silica induced by external forces

Abstract

The morphology of pure precipitated silica, silica filled in polydimethylsiloxane rubber, and silica filled in styrene butadiene rubber was studied by means of small-angle X-ray scattering experiments. The silica at a length scale of a few nanometers consists of primary particles, which form aggregates, and clusters with aggregates as basic units. It is evidenced that the aggregate branching, represented by the mass fractal dimension, and the aggregate diameter are different if pure silica and silica in rubber are compared. Contrary, the size of the primary particles and their surface are not influenced. It is demonstrated that the change in the aggregate morphology is due to the external mechanical forces appearing during the mixing process. This is achieved by model experiments using a pistil and a mortar and a composite with different silica fractions. By that means, a systematic change in the morphology with grinding time is observed. Then, the experiments on the composite demonstrate that the major contributions to the mass fractal dimensions are due to the external mechanical forces. In order to test reproducibility and universal validity in the case of precipitated silicas, independent experiments on one silica and further silicas are performed. Several important conclusions are obtained from the study. First, it is shown that a comparison of different pure silica samples without knowing their history may be difficult or questionable. Second, it becomes evident that it is not sufficient to provide only a description of the materials, rather than the details of the sample treatment have to be reported. Therefore, solely the characterization of the morphology of the pure silica is not sufficient to be compared to the mechanical properties of the composites.

Correlation of mass fractal dimension and cluster size of silica in styrene butadiene rubber composites

“…Recently, it was shown by TEM, e.g., Ref. 32, and by X-ray 33,34 experiments that the shape of silica clusters can be changed by external forces. In particular, the size of the objects can be changed, e.g., Refs.…”

“…32-36, and may be accompanied by a decrease of the mean number of primary particles in the clusters. 32,34 By that means the external forces compact and break the clusters. Ehrburger and Lahaye 36 measured the pressure P transmitted by silica samples during low speed uniaxial compaction as a function of the volume fraction ⌽.…”

Small-angle scattering from multiphase fractals

“…(11) and (13) [and correspondingly Eqs. (12) and (14) in the fractal ranges]. As a consequence, it governs the form of the scattering intensity.…”

“…SAS data (X-ray or neutron) often show a succession of power-law regimes whose scattering exponents are taking arbitrarily values from −4 to −1 [11][12][13][14][15][16][17]. On a double logarithmic scale, the scattering intensity looks like connected straight-line segments.…”

Recent advances in structural and dynamical properties of simplified industrial nanocomposites

“…In the vast majority of cases, work focused on the microstructure (i.e., dispersion) of model colloids, usually spherical and if possible monodisperse nanoparticles in size [18,30]. While they are much more used in industrial applications due to their optimized properties, industrial nanofillers are not particularly well suited to extract more detail than just some average fractal dimension of aggregates from scattered intensities [21,[31][32][33], due to their disordered nature. This is why qualitative observations made by TEM are mostly reported in the literature [34,35].…”