Monolayer characteristics of monodisperse core-shell latex particles prepared by soap-free emulsion copolymerization

Abstract

The monolayer characteristics of monodisperse core-shell latex particles at the air-water interface and on solid substrates were investigated. Particles were prepared by soap-free emulsion copolymerization of styrene and acrylic acid (1), styrene and 2-hydroxyethylmethacrylate (2), styrene and glycidylmethacrylate (3), and styrene and acryloxyethyl trimethylammonium chloride (4). They consist of a hydrophobic polystyrene core and a hydrophilic shell of the respective poly(meth)acrylate. Diameters lie in the range of 0.2 to 0.5 µm. Particle films at the air-water interface were studied by spreading ethanolic latex dispersions and measuring the π-A curves. For the same sort of particles, π-A curves vary strongly with the pH and the ion concentration of the subphase, and for different particles the π-A curves differ on the same subphase. For example, 1 forms real monolayers only on strongly acidified subphase, while 4 forms monolayers only on fairly concentrated salt solution. 3 with less hydrophilic shell than 2 tends to form 3-D aggregates rather than 2, which forms real monolayers. Particle mono- and multilayers on solid substrates were also studied. Monolayers were prepared using Langmuir-Blodgett (LB) deposition and electrostatic self-assembly (EA). Scanning electron micrographs of LB monolayers indicate a fairly regular, dense packing of the particles. LB deposition of a second layer leads to much less ordered films. EA of particles 1 onto cationic substrates proceeds via fractal growth of two-dimensional particle clusters at the interface. A substrate coverage of about 50% can be reached. Alternating EA of anionic particles 1 and cationic particles 4 leads to irregular 3-D particle clusters on the substrate but not to regular multilayers.