Influence of Nanoparticles on the Coupling Between Optical Dipoles in Epoxy-Silica Nanocomposites During Network Formation

Abstract

High-performance refractometry and infrared spectroscopy are combined in order to elucidate the gelation process and the glass transition during the network formation of epoxies and epoxy-based nanocomposites. Whereas infrared spectroscopy yields the chemical conversion due to the opening of oxirane rings during the covalent network formation, high-performance refractometry is extremely sensitive to the accompanying changes of the arrangement of the molecular network. In accordance with the Lorentz-Lorenz relationship, the evolution of the refractive index seems to reflect that of the mass density during polymerization of the epoxy-based systems within the limits of a few percent. The slight deviations from the Lorentz-Lorenz relationship, which occur during the gelation of the epoxy-based systems, are attributed to long-ranged dipole-dipole interactions, which respond at optical frequencies. This point of view is supported by the fact that chemically inert silica nanoparticles embedded in the pure epoxy matrix as disturbances for these dipole-dipole interactions are able to diminish or even to suppress totally this excess contribution of the refractive index.

KEYWORDS:

• Covalent network formation
• Dipole-dipole interactions
• Epoxy
• Gelation
• Refractive index

ACKNOWLEDGMENT

This work was financially supported by the University of Luxembourg through the project “Static and Dynamic Properties of Nanocomposites”. MP would like to thank the National Research Fund of Luxembourg for the AFR Ph.D. grant and, in addition, the European Commission for the Marie Curie cofounded AFR Postdoc grant.

Notes

^a Estimated n_D using a linear mixing rule for Nanopox® A410 and DGEBA. The term “silica particle” should be understood as the pure silica core and its silane shell.

^a Absolute increase of volume percentage estimated to be below 1% during the curing.

^b According to the model of Bansal and Ardell [34].

^c Gained by linear extrapolation of n_D towards α = 0% in Fig. 6a.

One of a Collection of papers honoring Wulff Possart, the recipient in February 2012 of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M.