Effect of DMF on the Rheological Properties of Telechelic Polyelectrolyte Hydrogels

The influence of addition of dimethylformamide (DMF) on the linear and nonlinear rheological properties of a reversible hydrogel composed of triblock copolymers of ionized poly(acrylic acid) (PAA) end-capped with short polystyrene (PS) blocks is reported. Steady state, oscillatory shear, and stress relaxation experiments were carried out to characterize the rheological behavior of a 0.6 wt % PS-PAA-PS physical gel in water-DMF binary solvent mixtures. The rheological properties of the hydrogel were shown to depend significantly on DMF content. The main effect of addition of DMF is the overall rearrangement of the structure from a concentrated suspension of (percolating or closely packed) swollen finite polyelectrolyte clusters (microgels) to a more homogeneous, kinetically, controlled 3D transient network. The structural rearrangement is induced mainly by changes in the PS mobility and exchange dynamics as well as by PAA conformational changes.