Adsorption
isotherms and the molecular weight fractionation of polyvinyl alcohols (PVA) with carboxylate groups upon adsorption on BaTiO, were determined. One of the polyvinyl alcohols with carboxylate groups is a block copolymer, while the others have carboxylate groups randomly distributed along the polymer chain. The copolymers were characterised by infrared spectroscopy (IR) and size exclusion chromatography (SEC). Lactone formation was found in one of the PVAs while the others contain only free carboxylate groups. The carboxylate content varies from 0.7 to 3.4 mol% and the ester content from almost zero to 22 mol%. An interesting detail is the difference in the wavelength at which the carboxylate groups absorb in the IR spectra. This difference is probably related to the distribution of the carboxylate groups in the chain. The molecular weights, as determined by SEC relative to polyethylene oxide, vary from 33000 to 110000 for the copolymers studied. An adsorbed amount of 1.5 mg mm2 for the block copolymer was found. The random copolymers all have adsorbed amounts in the plateau region close to 0.41 mg mm2 in spite of the differences in chemical composition.
There is, however, a significant difference in the shape of the adsorption isotherms at low equilibrium concentration and also in the molecular weight fractionation upon adsorption,
The random copolymer with the highest molecular weight in the adsorbed fraction is the only one suitable for making stable BaTiO, dispersions.
The adsorbed amount of the block copolymer increases above a certain equilibrium concentration without further exchange of short adsorbed molecules for longer ones. This behaviour can be explained by assuming an increasing stretching of the adsorbed chains in order to allow new chains to adsorb.