Synthesis and rheological behavior of new hydrophobically modified hydrogels with tunable properties

New hydrophobically modified hydrogels have been designed in order to obtain a series of gels with identical elastic moduli but variable dissipative properties. The synthesis of these systems has been realized following a three-step procedure: (1) introduction of double bonds onto a poly(acrylic acid) backbone [PAA], (2) hydrophobic modification of the PAA with dodecylamine, and (3) cross-linking of double bonds using dithiol. The characterization of gel precursors shows that hydrophobically modified polymers self-assemble in semidilute solution forming physical gels with temporary hydrophobic clusters. The gelation mechanism induced by reacting pendant double bonds with dithiol was studied by DSC, specific titrations, and rheology. The gelation process was not perturbed by the presence of the hydrophobic groups, and the kinetics follows a first-order dependence on thiol. In the entangled regime, the thiol conversion reaches around 80%, but only about 10% of the thiols effectively promote the formation of chemical cross-links while the other 90% are incorporated into the gel as loops or dangling chains. Once the gels are formed, NMR and SANS clearly demonstrate that hydrophobic side chains continue to form micelles within the network and that these micelles display a much better long-range order than their un-cross-linked precursors in aqueous solutions. All gels, both hydrophilic and hydrophobically modified, display a storage modulus G' which only depends on total polymer concentration and can be described on the basis of the percolation theory (G' similar to epsilon(2.6)) with epsilon, the reduced concentration defined from a fixed concentration at the gel point C-g = 2%. On the other hand, the loss modulus G" increases dramatically relative to the corresponding hydrophilic gel, when hydrophobic groups which formed reversible associations are introduced.