should lose their stability upon heating the dispersion to The adsorption of diblock copolymers containing different slightly above ambient due to the limited solubility of PVME block length ratios of polyvinylmethyl ether ( PVME ) and in water (2). We will soon report on this thermal flocculation poly ( vinyloxy-4-butyric acid ) ( PVOBA ) from aqueous solubehavior. The potentials of these block copolymers were tions onto a-Fe 2 O 3 ( hematite ) was compared with the adsorpevaluated with a-Fe 2 O 3 as a model system because a-Fe 2 O 3 tion of the PVOBA and PVME homopolymers. The effects on is a stable, water-insoluble oxide with suitable value of its the colloidal stabilization in relation to the chemical composiisoelectric point. tion of the polymers were evaluated. The pH was between 7.5
The block copolymers and also the PVOBA homopolymer and 9 in all experiments. The PVME homopolymer hardly adsorbs on a-Fe 2 O 3 , while an adsorbed amount of 0.31 mg /m 2 was proved to be effective stabilizers for a-Fe 2 O 3 in aqueous found for PVOBA, which behaves as a polyelectrolyte at the solutions. Dispersions stabilized with some of these poly-pH values used. The adsorbed amount of the block copolymers mers appeared to flocculate upon the addition of salt. Obviincreases with increasing PVME chain length up to 0.95 mg / ously, different mechanisms of colloidal stabilization are m 2 . The high affinity character in the adsorption isotherm is active, depending on the chemical composition of the block lost above a certain length of the PVME chain in the block copolymers.
copolymer. The adsorbed layer thickness increases with the
The colloidal stability of a dispersion depends on the total PVME chain length, while at the same time the chain density particle interaction which is the sum of several individual on the surface decreases. Adsorption of PVOBA causes a high contributions. The universal van der Waals forces always cause absolute value of the zeta potential (ÉzÉ) of the a-Fe 2 O 3 particles. The value of ÉzÉ drops with an increasing length of the an attraction between chemically identical particles in a liquid PVME chain. The stability of a-Fe 2 O 3 dispersions stabilized (3). With a-Fe 2 O 3 there is also an attraction due to its weak with block copolymers with a short PVME chain is mainly due permanent magnetism (4). This attraction depends heavily on to electrical double-layer repulsion. Full steric stabilization is the particle size and is negligible for small particles. obtained with sufficiently long PVME chains in the block copol-Basically, there are two possibilities for introducing repulymer, making these dispersions insensitive to the salt concentrasive forces in addition. Particles can be charged by adsorption. ᭧ 1997 Academic Press tion of ionic species, leading to electrical double-layer repul-Key Words: poly(vinyl ether) block copolymers; a-Fe 2 O 3 ; adsion (5). The charged surface causes an electrical double sorption; colloidal stabilization. layer in the solution with a thickness (1/k) which depends on the salt concentration. Such dispersions usually flocculate at high salt concentrations due to a decrease of 1/k. The