Monolayers of mono- and bidisperse spherical polymer particles at the air/water interface and Langmuir-Blodgett layers on solid substrates

Monodisperse spherical polymer particles with anionic and cationic shells were studied for their monolayer formation and compression behaviour on an aqueous subphase as a function of pH and salt (KCl) concentration. In addition, monolayers of monodisperse and bidisperse mixtures of 434 and 214 nm sized anionic particles were studied for their morphology by scanning electron microscopy (SEM). The anionic particles were prepared by soap-free emulsion polymerization of styrene and acrylic acid, and the cationic particles from styrene and 2-acryloxyethyl trimethylammonium chloride. Independent of the chemical nature of the shell, the particles formed monolayers at high salt or acid concentration in the subphase. However, at neutral pH and if no salt was present in the subphase only a part of the spheres formed monolayers, while the residual particles disappeared into the subphase. The origin of this behaviour is discussed in terms of ionization and electrostatic shielding of the polar groups. Compressed monolayers of monodisperse particles consisted of randomly oriented domains of up to 20 particles with small holes in between, the holes not exceeding two particle diameters in size. Films of bidisperse mixtures were highly disordered. If small particles were present in excess, they formed a fairly disordered monolayer and the large particles were situated on top or below this layer. If the number ratio of both sorts of particles approached unity, the texture became disordered and bi- and multilayered aggregates were observed. (C) 1997 Elsevier Science Ltd. All rights reserved.