Raman imaging of surface and sub-surface graphene oxide in fiber reinforced polymer nanocomposites

Visualization of dispersion is a challenging and unresolved issue for high aspect ratio nanofillers, such as surface-modified graphene oxide (mGO) and carbon nanotubes, in fiber reinforced polymer (FRP) composites. Conventional metrologies to study dispersion (location, spacing, size, etc.) have proven difficult and impractical when the fibers are present, particularly at the industrially-relevant, low mass fractions of nanofillers. Here, Raman spectroscopy and imaging are exploited to investigate the dispersion of mGO in unsaturated polyester (UP) resin composites both with and without glass microfiber reinforcement. Through comparative TEM and Raman maps on the same sample area, we validate a detailed Raman methodology to detect mGO agglomerate positions, even at low loadings of < 0.1 % by mass, where the sensitivity of the mGO Raman signal is dependent on incident laser wavelength. More importantly, we expand this to FRP composites, where by a proper choice of incident wavelength to avoid glass fiber fluorescence, Raman imaging is able to identify surface (2D) and sub-surface (3D) mGO microstructures near microfibers in the inter- and intralaminar regions. This measurement technique will find significant use in the FRP composite community as a readily available method to qualitatively correlate dispersion properties to processing techniques and/or mechanical performance.