Associative graft copolymers comprising a polyN-isopropylacrylamide) backbone and end-functionalized polyoxyethylene side chains. Synthesis and aqueous solution properties I.V. Berlinova a, * , I.V. Dimitrov a , N.G. Vladimirov b , V. Samichkov c , Ya. Ivanov c a Institute of Polymers, Bulgarian Academy of Sciences, So®a 1113, Bulgaria b Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA c Central Laboratory of Physico-chemical Mechanics, Bulgarian Academy of Sciences, So®a 1113, Bulgaria Received 8 November 2000; received in revised form 15 January 2001; accepted 21 January 2001 Abstract Novel, water-soluble associative graft copolymers were prepared by radical copolymerization of N-isopropylacrylamide NIPAAm) and polyoxyethylene PEO) macromonomer bearing a terminal per¯uorooctyl, sulfobetaine or trimethylammonium group. The macromonomer contents ranged from 0.04 to 0.2 mol%. Associative properties in dilute and semidilute regime were studied by viscosity, static light scattering and rheology measurements. Below the lower critical solution temperature LCST) the aqueous solution behaviour of the graft copolymers was determined by the balance between intra- and intermolecular side group interactions, which depended on the type of the associative group, the length and degree of grafting of the PEO chains and on external parameters: concentration and temperature. In dilute solution, the polymers containing per¯uorooctyl or sulfobetaine groups form aggregates with molar masses depending on the type and the content of the associative group. In semidilute solution, the graft copolymers revealed a shear thinning behaviour. The copolymer with 0.1 mol% PEO chains bearing per¯uorooctyl groups showed the greatest viscosity enhancement at low shear rate. Above the phase transition temperature the same copolymer formed an elastic gel. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Associative graft copolymers; Polyoxyethylene modi®cation; Thermoassociative behaviour 1. Introduction In recent years studies on associative water-soluble poly- mers have become an attractive ®eld owing to their unique solution properties. Uncharged and ionic polymers bearing groups or segments prone to attractive interactions reveal stimuli sensitivities and their behaviour changes drastically with environmental pH, temperature, solvent composition and ionic strength [1]. They have found application as rheol- ogy modi®ers, additives in enhanced oil recovery or water treatment, for immobilization of enzymes, controlled drug release and microencapsulation [2,3]. The modi®cation of a hydrophilic polymer with a low amount of hydrophobic groups is now a routine procedure for controlling the rheological properties of aqueous ¯uids. The most thoroughly studied associative polymers are the derivatives of polysaccharides [4±6], polyoxyethylene PEO) [7±11] and acrylamides [12±18] bearing alkyl, aralkyl, ¯uoroalkyl groups or steroids. The preference of the hydrophobic groups for intra- or intermolecular inter- actions depends on their nature, content and distribution along the polymer chain. Hydrophobic domains of inter- molecular type appear above a critical aggregation concen- tration, which may be substantially lower than the overlap concentration [18]. At suf®ciently high concentration the attractive interactions between the hydrophobic segments create a transient network, which results in a considerable viscosity enhancement of the solution at low shear rates [8,16,17]. Further enhancement of the thickening performance of water-soluble copolymers is expected from hydrophilic chains possessing inherent hydrophobicity. PolyN-isopro- pylacrylamide) PNIPAAm) is a polymer that reveals remarkable properties when dissolved in water. It exhibits a lower critical solution temperature LCST) ca. 328C [19,20]. The sharp coil-to-globule transition at the LCST has been attributed to an abrupt change of the delicate hydrophilic±hydrophobic balance within a narrow tempera- ture range. There is no aggregation between the PNIPAAm Polymer 42 2001) 5963±5971 0032-3861/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0032-386101)00080-5 www.elsevier.nl/locate/polymer * Corresponding author. Tel.: 1359-2-979-3628; fax: 1359-2-707-523. E-mail address: ivberlin@polymer.bas.bg I.V. Berlinova).