Curing-induced filler aggregation in epoxy-amine systems

TL;DR

This study reveals that curing in epoxy-amine systems induces filler aggregation driven by geometric factors, with a new scaling law linking interparticle gaps to aggregation, advancing understanding of composite material properties.

Contribution

It introduces a geometric scaling law that predicts filler aggregation during curing, highlighting the role of interparticle gaps and effective interactions in polymer composites.

Findings

Aggregation increases during curing despite no traditional attractive forces.

A linear scaling with the reduced gap parameter H predicts aggregation behavior.

Curing dynamically generates effective interactions with a spatial range proportional to particle size.

Abstract

Hypothesis: The macroscopic properties of polymer composites are governed by the dispersion and aggregation states of filler particles within a crosslinking matrix. Although curing transforms a liquid precursor into a solid network, its influence on filler aggregation remains insufficiently understood. We hypothesize that curing induces effective attractive interactions between filler particles, leading to aggregation even in non-Brownian systems. Experiment: Fluorescent polystyrene beads were homogeneously dispersed in a bisphenol F epoxy resin. Curing was initiated by adding trimethylhexamethylenediamine, and the evolution of the three-dimensional particle configurations was quantitatively examined using confocal laser fluorescence microscopy before and after completion of curing. Findings: Aggregation was enhanced during curing despite the absence of conventional attractive forces. The aggregation increment cannot be described solely by filler volume fraction but is governed by the mean interparticle gap $H$. Data collapse onto a linear scaling with the reduced gap parameter, identifying a geometric control parameter for curing-induced aggregation. This scaling demonstrates that curing dynamically generates an effective interaction whose spatial range scales with particle size, consistent with previously predicted rigidity-percolation-induced attractions. These findings establish a geometric criterion for predicting final dispersion states in curing polymer composites.