Introduction Hydrophobically modi®ed water-soluble polymers have attracted much interest in recent years [1±14]. These polymers are widely used in industries for paint formu- lation and paper coating. Such polymers are commonly referred to as associative thickeners. The three most important types of commercial associative polymers are 1. Hydrophobically modi®ed ethylene oxide urethane (HEUR) block copolymer. 2. Hydrophobically modi®ed hydroxyethyl cellulose (HMHEC). 3. Hydrophobically modi®ed alkali-soluble emulsion (HASE) polymer. Hydrophobically modi®ed water-soluble polymers un- dergo self-association in aqueous solution by virtue of their hydrophobic groups. The association can be either intramolecular or intermolecular or both, depending on the microstructure of the polymer and the size of the hydrophobes. At low concentration, these associative polymers form a micellelike structure, and the size gradually increase with the increasing polymer concen- tration. The solution properties of these polymers have been studied using a wide range of techniques, including viscosity [15±18], static and dynamic ¯uorescence [19, 20], static and dynamic laser light scattering (LLS) and microcalorimetry [21, 22]. While much research has largely been focused on the HEUR and HMHEC [5, 23±27] systems, relatively few studies have been carried out with HASE polymers. HASE polymers can be classi®ed as hydrophobically modi®ed anionic polyelectrolytes. They are manufac- tured as low-pH latexes, and contain acid groups along the polymer backbone. When neutralised in aqueous solution with a suitable base, the latex particles are ionised as a result of an acid±base reaction. The partially neutralised polymer becomes water-soluble, and hence the polymer chain expands due to the mutual repulsion of the negative charges along the backbones. When the HASE polymer is in a solution form, and at a concentration greater than a critical value, the hydro- Colloid Polym Sci 277:1172±1178 (1999) Ó Springer-Verlag 1999 ORIGINAL CONTRIBUTION M. Siddiq K.C. Tam R.D. Jenkins Dissolution behaviour of model alkali-soluble emulsion polymers: effects of molecular weights and ionic strength Received: 15 February 1999 Accepted in revised form: 13 July 1999 M. Siddiq á K.C. Tam (&) School of Mechanical and Production Engineering Nanyang Technological University Nanyang Avenue, Singapore 639798 Singapore R.D. Jenkins Union Carbide Asia Paci®c Inc. Technical Center, 16 Science Park Drive The Pasteur, Singapore 118227, Singapore Abstract The dissolution behaviour of model alkali-soluble polymer emulsions with dierent molecular weights was studied using conduct- ometric and potentiometric titration and laser light scattering techniques. The behaviour of the copolymer was studied as a function of the degree of neutralisation, a = [NaOH]/ [COOH]. The polymer latex swells with increasing a values up to a = 0.5±0.7, after which the dissoci- ation of polymer chains commences. At a = 1.0, all the polymer latexes dissociate into individual polymer chains. By combining the results of static and dynamic laser light scat- tering, we observed that the polymers have a compact conformation at a = 0. This compact conformation changes to a random coil at around a = 0.5, which then becomes a fully extended coiled conformation at a = 1.0, when all the COOH poly- mer groups are hydrolysed. The dis- solution of low-molecular-weight polymers is faster than that of high- molecular-weight polymers. Key words Dissolution á Molecular weights á Ionic strength á Neutralisation á Light scattering