SURFACE-MORPHOLOGY OF POLY(BUTYL ACRYLATE) POLY(METHYL METHACRYLATE) CORE-SHELL LATEX BY ATOMIC-FORCE MICROSCOPY

Atomic force microscopy (AFM) has been applied to investigate surface morphology of poly(butyl acrylate)/poly(methyl methacrylate)(PBA/PMMA) core shell latex particles synthesized by a two-stage seeded emulsion polymerization. Imaging has been performed in air on individual surfactant-free particles. With AFM operating in the standard contact mode, the forces applied by the tip are too large and induce artifacts and damage the core shell latexes. In contrast, AFM in the tapping mode scans the surface morphology of these soft particles with no damage and the quality and resolution of the obtained images are quite remarkable. Roughness and height distributions can be measured on PBA/PMMA core shell latexes as well. PBA/PMMA latex particles have been investigated at different polymer weight ratios: 100/0, 90/10, 80/20, 70/30, 50/50. The surface of the PBA seed is very flat and smooth. At a 90/10 weight ratio, the PBA core is shown to be partially covered by PMMA microbeads suggesting an initiation of the second stage polymerization by a nucleation mechanism. Building up on these initial sites, primary particles are then coagulating. At a 80/20 weight ratio, the PMMA microdomains have grown in size and are joining together to form a ''raspberry''-like structure shell. From 70/30 weight ratio, these subparticles are coalescing together completing the shell formation and forming a continuous and uniform PMMA film. In addition the mean roughness R(a) of the core shell latex particles is measured to increase with PMMA coverage and to reach a plateau from around a 70/30 polymer weight ratio indicating that we are going from elastomeric and smooth surfaces toward more stiff and rough ones with PMMA coverage. These data provide strong support for the main contribution of an aggregation mechanism. AFM is used to study the effect of added surfactants on the morphology of core shell latex particles by modifying the interfacial energy between the polymer phases during the emulsion polymerization. According to the employed surfactant, a bulging structure indicating a partial engulfing by PMMA and a ''half moon'' type structure are evident. AFM provides a very direct and relevant tool for following these effects and controlling the latex equilibrium morphology.