Associative polymers bearing <i>n</i>-alkyl hydrophobes: Rheological evidence for microgel-like behavior

Abstract

Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable shear thinning and, a characteristic power-law behavior (G′, G″∼ω0.4) under oscillatory shear persisting over a broad range of time scales. Concentration-independent master curves are obtained for the storage modulus, G′, with the level of G′ increasing with hydrophobe size. The similarity in the dynamic response suggests that there exists a qualitative equivalence in microstructure over the range of systems, the only difference being the “softness” or compressibility of the particles. Data from this study are also contrasted with those for a similar HASE polymer bearing a smaller number of alkylaryl hydrophobes [J. Rheol. 41, 427–444 (1997)]. In the latter case, the rheology can be interpreted in terms of hydrophobic associations and chain entanglements occurring in solution. Thus, subtle variations in molecular architecture are shown to cause significant differences in morphology and microstructure for these polymer systems.