Small Angle Neutron Scattering Study of Complex Coacervate Micelles and Hydrogels Formed from Ionic Diblock and Triblock Copolymers

A complex coacervate is a fluid phase that results from the electrostatic interactions between two oppositely charged macromolecules. The nature of the coacervate core structure of hydrogels and micelles formed from complexation between pairs of diblock or triblock copolymers containing oppositely charged end-blocks as a function of polymer and salt concentration was investigated. Both ABA triblock copolymers of poly[(allyl glycidyl ether)-b-(ethylene oxide)-b-(allyl glycidyl ether)] and analogous poly[(allyl glycidyl ether)-b-(ethylene oxide)] diblock copolymers, which were synthesized to be nearly one-half of the symmetrical triblock copolymers, were studied. The poly(allyl glycidyl ether) blocks were functionalized with either guanidinium or sulfonate groups via postpolymerization modification. Mixing of oppositely charged block copolymers resulted in the formation of nanometer-scale coacervate domains. Small angle neutron scattering (SANS) experiments were used to investigate the size and spacing of the coacervate domains. The SANS patterns were fit using a previously vetted, detailed model consisting of polydisperse coreshell micelles with a randomly distributed sphere or body-centered cubic (BCC) structure factor. For increasing polymer concentration, the size of the coacervate domains remained constant while the spatial extent of the poly(ethylene oxide) (PEO) corona decreased. However, increasing salt concentration resulted in a decrease in both the coacervate domain size and the corona size due to a combination of the electrostatic interactions being screened and the shrinkage of the neutral PEO blocks. Additionally, for the triblock copolymers that formed BCC ordered domains, the water content in the coacervate domains was calculated to increase from approximately 16.8% to 27.5% as the polymer concentration decreased from 20 to 15 wt %.