Abstract
We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different exponents. In all cases, three power-law regimes, regardless of the MAA content, are observed that can be attributed to the presence of different modes of hydrophobic interaction. However, the transitions shift to lower concentrations as the MAA content increases. With regards to the effects of the macromonomer side-chain concentration, we observe a substantial increase in viscosity at intermediate macromonomer content (1 mol %), possibly due to an increase in the number of intermolecular junctions as the number of hydrophobes per chain increases. This is in contrast to (i) low macromonomer concentration (0.3 mol %) behavior that reveals low viscosity due to weak hydrophobic associations, and (ii) high macromonomer concentration (1.9 mol %) behavior that favors more intramolecular association resulting in lower viscoelastic properties compared to intermediate macromonomer concentrations.
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