pH-Tunable rheological properties of a telechelic cationic polyelectrolyte reversible hydrogel

Steady shear properties and linear and nonlinear viscoelastic behaviors of a poly(methyl methacrylate)-poly(dimethyl amino ethyl methacrylate)-poly(methyl methacrylate) polymer, (PMMA-PDMAEMA-PMMA), telechelic polymers in salt-free aqueous solution have been investigated as a function of concentration and pH. Above a critical concentration, a transient physical network is formed through an association mechanism between hydrophobic end groups, leading to a gel-like behavior. The gel-like polymer solutions were shown to exhibit a peculiar flow behavior, associated with time fluctuation of the transient first normal stress difference, attributed to orientation effects of the stiff charged polymer chains. The viscoelastic behavior was shown to be governed by two pH dependent time scales: a short time scale controlled by the lifetime of the hydrophobic associative junctions and a long time scale corresponding to the network relaxation time. All rheological results show strong evidence that Coulomb interactions, which control both macromolecular chain rigidity and inter-chain interactions, lead to specific pH-tunable properties of great potential interest.