Latex particle self-assembly and particle microchemical symmetry: PS HEMA latex particles are intrinsic dipoles

Poly[styrene-co-(2-hydroxyethyl methacrylate)] copolymer latex (PS/HEMA) self-assembles easily, forming both macrocrystals and colloidal crystals. Two types of microchemical data were obtained for these particles: high-resolution scanning electron microscopy using a field-emission gun (FESEM) in the BEI (backscattered electron imaging) and SEI (secondary electron imaging) modes as well as energy-loss spectroscopy imaging in the transmission electron microscope (ESI-TEM). BEI and SEI particle pictures for the same macrocrystal domain are not coincident, showing that the symmetry of the chemical domains distribution in the particle differs from the overall morphological symmetry. The ESI images show that the distribution of groups bearing electrical charges within the particles (sulfate residues and potassium ions) is also asymmetrical. The PS/HEMA particles are thus intrinsic electrical dipoles (or multipoles), regularly aligned within the macrocrystals. Particle polarity was previously acknowledged in the literature (but only as a response to an external electrical field), and we propose that this is a decisive factor for the ease of PS/HEMA latex particle self-arraying.