Ambidextrous Surfactants for Water-Dispersible Polymer Powders from Dispersion Polymerization in Supercritical CO₂

Abstract

A new concept is introduced in which a surfactant, poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) (Mw = 5500 g/mol PDMS, 900 g/mol PMA), is utilized to stabilize an organic latex in either a nonpolar medium, dense CO2, or water. The latex particles, in this case poly(methyl methacrylate) (PMMA), were synthesized by dispersion polymerization in supercritical carbon dioxide. In CO2, the PDMS block provides steric stabilization while the PMA block adsorbs to the particle surface. Upon transfer to water, the PDMS block collapses onto the surface and the PMA block ionizes for pH > 5 to stabilize the latex by electrostatic repulsion, as shown by zeta potential measurements. The surfactant is “ambidextrous” in that it provides stabilization in either CO2 or water, by different mechanisms in each medium. Smaller more uniform particles were produced in CO2 with a mixture of the commercially available surfactant, PDMS-g-pyrrolidonecarboxylic acid (PDMS-g-PCA) (Mw = 8500 g/mol, ∼2 PCA groups) and PDMS-b-PMA.