Light scattering of PMMA latex particles in benzene: structural effects

Abstract

Intra- and interparticle structural effects were studied in polymethylmethacrylate (PMMA) latex dispersions in a nonpolar solvent with the technique of light scattering. The required transparency of the dispersions was attained by a close matching of the refractive index of PMMA and solvent, for which benzene was chosen. Two latices were studied with divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDM) as crosslinking agents. The latex particles were characterized by electron microscopy in the dry state and by viscosity, sedimentation, light scattering, and laser light scattering spectroscopy in dilute solutions. The molar mass of the particles is in the range of 5 x 109 g mole−1. In benzene they are swollen and a hydrodynamic radius in the range of 200 nm was measured. The light scattering intensity and its angular variation were found to be strongly temperature dependent. This could be explained by the difference in temperature coefficients of the refractive indexes of PMMA and benzene which is sufficient to change their matching appreciably. For the latex containing DVB this phenomenon could be used to characterize the spatial distribution of the DVB inside the particle. At higher concentrations (20–80 g dm−3) light scattering as a function of scattering angle shows the emergence of a diffuse diffraction peak which points to interparticle structural effects. In one case the radial distribution function of particle centers was determined by a Fourier transformation of the structure factor and is discussed in terms of some theories of atomic liquids. The position of the diffraction peaks as a function of concentration is also discussed in terms of an “expanded-lattice” structure. Results suggested that some structure persisted down to concentrations of 2% (w/v) of latex. However, the calculated mean interparticle distances seem much too large to be explained on a basis of interpenetration of peripheral chains of the particles. Attempts were made to interpret the long range interaction by electrostatic repulsions. However, measurements of the electric conductivity of the dispersions do not corroborate this view. Similar structural effects showed up with laser light scattering spectroscopy, where it was found that the diffusion coefficient depended on the scattering angle. On prolonged standing some of the systems showed sharp, bright diffraction peaks in visible light due to the formation of “supramolecular crystals” as reported earlier in the literature.