THE ADSORPTION OF POLYVINYL ALCOHOLS WITH CARBOXYLATE GROUPS ON BATIO3

Adsorption isotherms and the molecular weight fractionation of polyvin yl alcohols (PVA) with carboxylate groups upon adsorption on BaTiO3 we re determined. One of the polyvinyl alcohols with carboxylate groupS i s a block copolymer, while the others have carboxylate groups randomly distributed along the polymer chain. The copolymers were characterise d by infrared spectroscopy (IR) and size exclusion chromatography (SEC ). Lactone formation was found in one of the PVAs while the others con tain 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 c arboxylate groups absorb in the IR spectra. This difference is probabl y related to the distribution of the carboxylate groups in the chain. The molecular weights, as determined by SEC relative to polyethylene o xide, vary from 33 000 to 1 10000 for the copolymers studied. An adsor bed amount of 1.5 mg m-2 for the block copolymer was found. The random copolymers all have adsorbed amounts in the plateau region close to 0 .41 mg m-2 in spite of the differences in chemical composition. There is. however, a significant difference in the shape of the adsorption i sotherms at low equilibrium concentration and also in the molecular we ight fractionation upon adsorption. The random copolymer with the high est molecular weight in the adsorbed fraction is the only one suitable for making stable BaTiO3 dispersions. The adsorbed amount of the bloc k copolymer increases above a certain equilibrium concentration withou t further exchange of short adsorbed molecules for longer ones. This b ehaviour can be explained by assuming an increasing stretching of the adsorbed chains in order to allow new chains to adsorb.