Associative polymers bearing n-alkyl hydrophobes: Rheological evidence formicrogel-like behavior

Rheological techniques are used to probe the behavior of hydrophobic alkali -swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueou s alkaline media. The polymers possess a comb-like architecture with a poly electrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes ( similar to 16 per polymer chain) tethered to the backbone via polyether si de chains. The size of the hydrophobes is varied from n-C-8 to n-C-20 in th is study. It is shown that, at such a level of hydrophobic modification, an d at relatively high polymer concentrations, the microstructure in these po lymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and d isintegrate to form a system of close-packed, compressible (''soft'') aggre gates. 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 sh ear rates followed by considerable shear thinning and, a characteristic pow er-law behavior (G', G " similar to omega(0.4)) under oscillatory shear per sisting over a broad range of time scales. Concentration-independent master curves are obtained for the storage modulus, G', with the level of G' incr easing with hydrophobe size. The similarity in the dynamic response suggest s that there exists a qualitative equivalence in microstructure over the ra nge 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 [E nglish et al., J. Rheol. 41, 427-444 (1997)]. In the latter case, the rheol ogy can be interpreted in terms of hydrophobic associations and chain entan glements occurring in solution. Thus, subtle variations in molecular archit ecture are shown to cause significant differences in morphology and microst ructure for these polymer systems. (C) 1999 The Society of Rheology. [S0148 -6055(99)00505-2].