Effects of Polymer and Salt Concentration on the Structure and Properties of Triblock Copolymer Coacervate Hydrogels

Abstract

Structure–property relationships were established for complex coacervate hydrogels formed from binary aqueous solutions of oppositely charged ABA triblock copolymers. The charged triblock copolymers were synthesized by functionalizing poly[(allyl glycidyl ether)-b-(ethylene oxide)-b-(allyl glycidyl ether)] with either guanidinium or sulfonate functional groups. When aqueous solutions (ca. 5–40 wt %) of these oppositely charged polymers were mixed, the electrostatic interactions of the functionalized blocks led to the association of the oppositely charged end-blocks into phase-separated complex coacervate domains bridged by the uncharged, hydrophilic PEO midblock. The resulting structures were studied by small-angle X-ray scattering (SAXS) and dynamic mechanical spectroscopy. The organization of the coacervate domains was shown to affect substantially the viscoelastic properties of the hydrogels, with the storage modulus increasing significantly as the mixtures transformed from a disordered array of domains to an ordered BCC structure with increasing block copolymer concentration. As the polymer concentration was further increased to 30 wt %, a hexagonal structure appeared, which coincided with a 25% drop in the modulus. Further structural changes, resulting in variations in the viscoelastic response, were also induced through changes in salt concentration. The viscoelastic properties and the physical nature of the cross-links have important implications for the applicability of these gels as injectable drug delivery systems.