Preparation of pH-responsive poly(methyl methacrylate) nanoparticles with CO2-triggered aggregation

Healthcare industry is one of the major consumers of surface-modified polymer nanoparticles (NPs). If an incorporation of the stimuli-responsive polymers to such NPs is achieved, it can potentially broaden the range of their applications tremendously. In this work, an emulsifying ability of poly(methacrylic acid) (PMAA) and a series of its random methyl methacrylate (MMA) copolymers (PMAA-co-PMMA) prepared by free-radical polymerization was studied. It was found that these random copolymers and PMAA homopolymer could act as the surfactants (at certain pH values) in aqueous emulsion polymerization of MMA, which produces spherical PMMA NPs with carboxylic groups on the particles' surface. With the help of H-1 NMR analysis it was determined that the PMMA NPs synthesized in this manner had 9.6 wt% PMAA (or PMAA-co-PMMA) coating, and due to that, the aqueous dispersions of these NPs exhibited pH-responsiveness. Although all the dispersions prepared were pH-responsive, only PMAA15-co-PMMA85/PMMA latex was found to be CO2-responsive: this sample could undergo CO2-triggered aggregation and N-2-triggered redispersion. This newly developed method of preparation of smart NPs functionalized with carboxylic acid groups is uncomplicated and provides an environmentally friendly way of adjusting the dispersions' properties using gases only, avoiding the necessity of acids and bases addition.