Cross-linked cationic diblock copolymer worms are superflocculants for micrometer-sized silica particles

Abstract

A series of linear cationic diblock copolymer nanoparticles are prepared by polymerization-induced self-assembly (PISA) <i>via</i> reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) using a binary mixture of non-ionic and cationic macromolecular RAFT agents, namely poly(ethylene oxide) (PEO₁₁₃, <i>M</i>_n = 4400 g mol^-1; <i>M</i>_w/<i>M</i>_n = 1.08) and poly([2-(methacryloyloxy)ethyl]trimethylammonium chloride) (PQDMA₁₂₅, <i>M</i>_n = 31 800 g mol^-1, <i>M</i>_w/<i>M</i>_n = 1.19). A detailed phase diagram was constructed to determine the maximum amount of PQDMA₁₂₅ stabilizer block that could be incorporated while still allowing access to a pure worm copolymer morphology. Aqueous electrophoresis studies indicated that zeta potentials of +35 mV could be achieved for such cationic worms over a wide pH range. Core cross-linked worms were prepared <i>via</i> statistical copolymerization of glycidyl methacrylate (GlyMA) with HPMA using a slightly modified PISA formulation, followed by reacting the epoxy groups of the GlyMA residues located within the worm cores with 3-aminopropyl triethoxysilane (APTES), and concomitant hydrolysis/condensation of the pendent silanol groups with the secondary alcohol on the HPMA residues. TEM and DLS studies confirmed that such core cross-linked cationic worms remained colloidally stable when challenged with either excess methanol or a cationic surfactant. These cross-linked cationic worms are shown to be much more effective bridging flocculants for 1.0 μm silica particles at pH 9 than the corresponding linear cationic worms (and also various commercial high molecular weight water-soluble polymers.). Laser diffraction studies indicated silica aggregates of around 25-28 μm diameter when using the former worms but only 3-5 μm diameter when employing the latter worms. Moreover, SEM studies confirmed that the cross-linked worms remained intact after their adsorption onto the silica particles, whereas the much more delicate linear worms underwent fragmentation under the same conditions. Similar results were obtained with 4 μm silica particles.