Fluidity and water in nanoscale domains define coacervate hydrogels
Coacervate-based hydrogels, formed in aqueous solution by simple mixing of two oppositely charged ABA block copolyelectrolytes represent a new and versatile approach to the design of bio-inspired gelators. While coacervate-based hydrogels provide high tunability of a range of desirable properties, little is understood about the molecular-level makeup of the nanometer-scale domains. Small angle neutron scattering was employed to quantify the effective polymer density and water content of each domain. Further, electron paramagnetic resonance and Overhauser dynamic nuclear polarization of block-specific spin labels elucidate domain-specific, local, polymer and water dynamics. This unique combination of techniques reveals that the charged A blocks segregate into spherical domains with a radius of 8 nm, and are dispersed in a continuous matrix of water soluble, PEO B blocks. The edges of the spherical A block domains are found to be soft and diffuse, and the B block matrix exhibits higher water and polymer dynamics than the A block domains. The selective measurement of the local water and polymer dynamics shows a viscous and dense, but fluidic environment in the spherical A block domains, thus permitting the designation as a complex coacervate phase. Further, the physical properties of the analogous homopolymers mixed at equal composition to that of the triblock copolyelectrolytes leads to the conclusion that “the whole is greater than the sum of its parts”: nanometer scale complex coacervates only form when the two charged A blocks are covalently linked by a PEO midblock that serves as an intrinsic osmolyte.