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

Krogstad, D.V.; Lynd, N.A.; Choi, S.H.; Spruell, J.M.; Hawker, C.J.; Kramer, E.J.; Tirrell, M.V.
Macromolecules
2013
46, 1512-1518

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.