Unexpected Rheological Behavior of Concentrated Poly(methacrylic acid) Aqueous Solutions

Concentrated aqueous solutions of poly(methacrylic acid) (PMAA) display a sharp increase of their viscosity over time under shear. This phenomenon known as antithixotropy is studied as a function of both polymer concentration and ionization degree. Both shear and oscillatory shear experiments are used to characterize the rheological behavior of PMAA solutions, with a peculiar attention to the semidilute regime. In the linear regime, nonionized PMAA solutions tend to behave like associative polymers with a highest exponent for the power law describing the concentration dependence of the viscosity. The rheological behavior of such solutions may be controlled by the balance between intra- and intermolecular interactions involving both hydrophobic and hydrogen-bonding interactions. Under shear, PMAA solutions display a classical Newtonian behavior in the low shear rate region followed by a sharp increase in the viscosity for higher shear rates when the PMAA concentration is high enough. This shear induced gel formation is characterized by a critical shear rate which sharply decreases as the PMAA concentration increases. However, the critical shear stress associated with the gel formation is independent of the concentration and only related to the ionization degree. The shear flow may form connective links between polymer aggregates and induce the formation of a transient network.