Self-assembly of latex particles into proton-conductive membranes

We present results from our investigation of the concept and practice of using surface-charged latex nanoparticles as building blocks for conductive membranes. Nanoparticles were synthesized in water by free-radical copolymerization of two hydrophobic monomers, butyl acrylate (BA) and methyl methacrylate (MMA), a cross-linker, N,N'-methylenebis(acrylamide) (BIS), and a charged monomer, sulfonate styrene sodium salt (NaSS). The resultant nanospheres were characterized with static and dynamic laser light scattering. Thin films were cast from dispersions of particles, followed by incubation at similar to 110 degrees C to yield free-standing membranes. Conductivity, water uptake, and ion exchange capacity were measured. The membranes possess higher conductivities than both amorphous films cast from sulfonated poly(BA-MMA-styrene) ionomers and poly(BA-BIS-MMA-NaSS) gel films. TEM images provide visual evidence of particle and film structure and suggest the existence of continuous hydrophilic channels formed naturally through close-packing of surface-charged nanospheres. Neutron scattering confirms the particulate structure of the membranes.