SOLUTION RHEOLOGY OF A HYDROPHOBICALLY-MODIFIED ALKALI-SOLUBLE ASSOCIATIVE POLYMER

Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to mater ials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volu me. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer c oncentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c, these ass ociations are predominantly intramolecular. At higher polymer concentr ations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoreti cal considerations of associative polymer systems and is reflected in an unusually high concentration dependence of the zero shear viscosity , with eta(0) similar to c(8). Evidence for shear-induced structuring in steady shear, large amplitude oscillatory shear, and parallel super posed steady and dynamic shear is presented. Such structuring is more pronounced at lower polymer concentrations, consistent with the format ion of intermolecular associations at the expense of intramolecular. I n contrast to the simple linear telechelic associative polymers consid ered in a number of previous studies, the network dynamics of the poly mer are no longer represented by a single characteristic time. This de viation from a classical Maxwellian response in oscillatory shear is i nterpreted as a broadening of the relaxation spectrum, arising from th e coexistence of both hydrophobic associations and topological entangl ements. Mechanistically, stress relaxation is better envisaged in term s of ''hindered reptation'' [Liebler et al. (1991)] of the chains, rat her than Rouse-like behavior moderated purely by the hydrophobe diseng agement rate [Annable et al. (1993)]. (C) 1997 The Society of Rheology .