pH-responsive disruption of 'liquid marbles' prepared from water and poly(6-(acrylamido) hexanoic acid)-grafted silica particles

A chain transfer agent was immobilized onto the surface of 11-mu m diameter silica particles (CPD-SiO2) for use in reversible addition-fragmentation chain transfer (RAFT)-controlled radical polymerization. pH-responsive poly(6-(acrylamido) hexanoic acid) (PAaH)-grafted silica particles (PAaH-SiO2) were prepared via RAFT-controlled radical polymerization using CPD-SiO2. Immobilization of the PAaH chains onto the surface of silica particles was confirmed by thermogravimetric analysis, attenuated total reflection-Fourier transfer infrared and scanning electron microscopy measurements. The solubility of PAaH in water is strongly dependent on the pH of the solution. PAaH-SiO2 was flocculated at pH 3 because of the hydrophobic interaction of the grafted PAaH chains with protonated carboxyl pendant groups. In contrast, PAaH-SiO2 was dispersed at pH 10 because of electrostatic repulsion between the grafted PAaH chains with pendant carboxylate ions. Millimeter-sized 'liquid marbles' can be prepared using the pH-responsive PAaH-SiO2 particles. The 'liquid marble' can be transferred intact onto the surface of a neutral or acidic water pool and exhibit long-term stability. When the pH of the water pool becomes alkaline, the 'liquid marble' immediately bursts on the surface of the water pool. Polymer Journal (2011) 43, 778-784; doi: 10.1038/pj.2011.55; published online 22 June 2011