Structure and rheological properties of soft-hard nanocomposites: Influence of aggregation and interfacial modification

TL;DR

This paper investigates how silica nanoparticle aggregation and interfacial modifications influence the structure and rheological properties of soft-hard nanocomposites, using SANS and stress-strain measurements to analyze reinforcement mechanisms.

Contribution

It provides a detailed quantitative analysis of silica aggregation and reinforcement in polymer matrices, including effects of pH, volume fraction, and surface grafting, advancing understanding of nanocomposite design.

Findings

Aggregation decreases with polymer grafting, reaching low final aggregation numbers.

Structural parameters like aggregate size and compacity are quantified as functions of pH and volume fraction.

Rheological reinforcement correlates with silica aggregation and structure.

Abstract

A study of the reinforcement effect of a soft polymer matrix by hard nanometric filler particles is presented. In the main part of this article, the structure of the silica filler in the matrix is studied by Small Angle Neutron Scattering (SANS), and stress-strain isotherms are measured to characterize the rheological properties of the composites. Our analysis allows us to quantify the degree of aggregation of the silica in the matrix, which is studied as a function of pH (4-10), silica volume fraction (3-15%) and silica bead size (average radius 78 A and 96 A). Rheological properties of the samples are represented in terms of the strain-dependent reinforcement factor, which highlights the contribution of the filler. Combining the structural information with a quantitative analysis of the reinforcement factor, the aggregate size and compacity (10%-40%) as a function of volume fraction and pH can be deduced. In a second, more explorative study, the grafting of polymer chains on nanosilica beads for future reinforcement applications is followed by SANS. The structure of the silica and the polymer are measured separately by contrast variation, using deuterated material. The aggregation of the silica beads in solution is found to decrease during polymerization, reaching a rather low final aggregation number (less than ten).