Solution rheology of hydrophobically modified associative polymers: Solvent quality and hydrophobic interactions

The Theological behavior of a model hydrophobically modified alkali soluble emulsion (HASE) polymer, comprised of a random copolymer backbone of methacrylic acid and ethylacrylate and pendant hydrophobic macromonomers, is examined in cosolvents of water and propylene glycol (PG) of different proportions. We find the solvent solubility parameter to have a direct impact on both the satteady and dynamic behavior of the polymer solutions. In particular, scaling of the relative viscosity (η_rel) and the elastic modulus (G′) at a fixed frequency with the solvent solubility parameter (δ_s) reveals the presence of two distinct regimes with different dependences on δ_ss. In "water-rich" solvents, both η_rel and G′ show a strong dependence on δ_s in contrast to "PG-rich" solvents, in which there is slight or no dependence on β_s. The concentration dependences of both η_rel and G′ are also found to be different in water-rich solvents from that in PG-rich solvents. In water-rich solvents, η_rel and G′ reveal power-law dependences with exponents of 2.5 and 3.2, respectively, compared to exponents of 1.4 and 2.3 in PG-rich solvents. The different behavior in PG-rich solvents is ascribed to the presence of minimal hydrophobic associations, with the polymer behavior analogous to that of unmodified polymers without hydrophobes. This hypothesis is supported by the similarity in η_rel scaling with concentration observed for both the HASE polymer in PG-rich solvents and a similar polymer without the hydrophobes in both solvents. The lack of hydrophobic interactions in the PG-rich solvents may be attributed to the observed decrease in polymer coil dimension together with a lower tendency of the hydrophobes to form micelles in less polar media.