OPTICAL TRANSMISSION IN SUSPENSIONS OF FILM-FORMING LATEX AT HIGH VOLUME FRACTION

Abstract

Optical transmission spectra of thin layers of standard commercial acrylic and styrene-butadiene latex suspensions show the existence of sharp transmittance minima over a narrow range of high latex volume fractions . The observed stop bands are attributed to diffraction effects arising from the evolution of moderately long-range order in the latex. Experiments show that the film-forming latexes self-assemble readily and rapidly into robust ordered arrays. The latex suspensions have fairly low photonic strength and dynamical diffraction accounts well for the observed variation of stop-band wavelength with latex diameter indicating that the latexes have ordered in either an FCC or BCC lattice. Dynamical diffraction theory is developed for non-absorbing crystals of arbitrary thickness to provide a description of stop-band depths and widths. A scattering factor for the latex spheres can be estimated using Rayleigh-Gans theory, but the quantitative description of stop-band profiles given by the combination of dynamical diffraction and Rayleigh-Gans yields relatively poor agreement with experiment. Possible reasons for this lack of agreement include diffuse scattering and absorption by the latex suspension and overestimation of the scattering factor for the latex spheres by the Rayleigh-Gans approximation.