Effect of supercritical CO2 in modified polystyrene 3D latex arrays

The effect of supercritical CO2 (scCO(2)) in 3D latex arrays formed by monodispersed particles of polystyrene (PS), PS cross-linked with divinylbenzene (PS-DVB), and PS block copolymers with 2-hydroxyethyl methacrylate (PS-HEMA), methacrylic acid (PS-MA), acrylic acid (PS-AA), itaconic acid (PS-IA), and a mixture of methacrylic and itaconic acid (PS-IA-MA) has been studied. Sorption of CO2 into the polymer particles leads to a decrease in the glass transition temperature of the polymer and the swelling of the particles and induces their coalescence. 3D-latex arrays of the former compositions were treated in scCO(2) at temperatures and pressures ranging from 40 to 80 C and from 85 to 197 bar, respectively. The effect of CO2 on the polymeric template was assessed by scanning electron microscopy and N-2 adsorption analysis. Bare PS and PS-HEMA particles sintered readily in scCO(2) at 40 C and 85 bar. On the other hand, particles containing carboxylic acid groups on their surface (PS-MA, PS-AA, PS-IA, and PS-IA-MA) were, at the same temperature and pressure, more resistant to the CO2 treatment. For a given polymer composition, the sorption of CO2 inside the polymer particles, the swelling, and the degree of coalescence depend on the pressure, temperature, and time of the CO2 treatment. Analysis of the pore size distributions from the N-2 adsorption data has allowed us to quantify the degree of coalescence of the particles in the matrix. By careful control of the experimental variables, the coalescence of the 3D latex array could be finely tuned using CO2.