ChaRacterization of Carbon-Filled Polymers by Specular Reflectance

Carbon-filled polymers, due to their opacity, are normally difficult to characterize by IR spectroscopy. We describe a method for obtaining spectra based on a specular reflectance measurement at normal incidence using the IR microscope. Peaks in the specular reflectance spectrum have a derivative-like shape due to refractive index dispersion. After application of the Kramers-Kronig transformation, an absorption index spectrum is obtained; this has a more conventional appearance and is thus much easier to interpret. The technique was found to be applicable so long as certain criteria were met, such as optical flatness and no contributing reflectance effects from the subsurface. Various polymeric materials were examined which contained carbon in the range 2 to 25% w/w. This approach was found unsuitable for carbon-fiber/polymer composites which show predominantly diffuse reflectance due to scattering at the surface features.