Stabilization of aqueous α-Al2O3 suspensions with block copolymers

Water-soluble diblock copolymers (DBCPs) with a polyelectrolyte block are accessible by sequential anionic polymerization of tert-butylmethacrylate (TBMA) and ethylene oxide (EO), followed by polymer analogous conversion of the PTBMA into poly(methyacrylic acid) (PMAA) blocks. These materials are highly efficient dispersants for oxide ceramic powders in aqueous media. A series of block copolymer samples with M_n ranging from 1300 to 38 900, and (EO:MAA) block length ratios from 0.5 to 11.7 were prepared with polydispersities close to 1.2. The influence of overall molecular weight and block length ratio, pH and ionic strength on the stability of aqueous α-Al₂O₃ suspensions was investigated by sedimentation and adsorption experiments, surface plasmon resonance (SPR) and electrokinetic measurements. The copolymers are capable of stabilizing alumina suspensions by a combination of specific adsorption of one block on the particle surface and a shielding effect provided by the nonadsorbing block. In addition, the adsorption of the negatively charged PMAA block on the oppositely charged alumina surface reverses the electrophoretic potential of the oxide particles, a process which is strongly pH dependent. With respect to the powder dispersing efficiency, an optimum was found when the DBCP consisted of a short PMAA anchoring and an approximately tenfold longer PEO stabilizer block with an overall molecular weight of about 5000. Alumina suspensions with the DBCP added were highly dispersed and rather stable against salt addition in a much wider pH window than block-copolymer-free suspensions.