Viscoelastic Properties of Physical Gels Formed by Associative Telechelic Polyelectrolytes in Aqueous Media

We report on the viscoelastic response of a physical gel formed in aqueous solutions of poly(sodium acrylate) end-capped with short polystyrene (PS) blocks. Above a percolation threshold (c gel = 0.2 wt %) a transient network is formed constituted of PS hydrophobic domains (cross-links) interconnected via fully charged polyelctrolyte chains. Oscillatory shear and step-strain experiments were employed to study the viscoelastic properties. The stretched conformation of the charged middle blocks (bridges) and the high hydrophobicity of the PS ends have fundamental consequences on the various viscoelastic parameters: (1) The linear viscoelastic behavior is only observed at very small strains, indicating that the telechelic polyelectolytes form weak gels. (2) The magnitude of the plateau modulus, in the unentangled regime, is very high due to the stretched conformation of the polyelectrolyte chains that limits the formation of elastically inactive loops. (3) The lifetimes of the physical cross-links are influenced by the high hydrophobicity of the PS ends, compensated largely by the stretching of the polyelectrolyte chains which increases the disengagement probability. The final result is that slow mode relaxation phenomena predominate (relaxation times of the order of 102−103 s).