"Electrospinning Fibers via Complex Coacervation"
Electrospun fibers are high-surface-area materials widely used in applications ranging from batteries to wound dressings. Typically, an electrospinning precursor solution is prepared by dissolving a high-molecular-weight polymer in an organic solvent to form a sufficiently entangled solution. Our approach bypasses the requirement for entanglements and completely avoids toxic chemicals by focusing on using an aqueous complex coacervates solution. Coacervates are a dense, polymer-rich liquid phase resulting from the associative electrostatic complexation of oppositely charged macroions.
We were the first to demonstrate that liquid complex coacervates could be successfully electrospun into polyelectrolyte complex (PEC) fibers. A canonical coacervate system was formed with poly(4-styrene sulfonic acid, sodium salt) and poly(diallyldimethylammonium chloride). Characterization of the binodal phase behavior demonstrated the thermodynamic linkage of the polymer and salt concentrations (CP and CS): greater CP indirectly controlled by decreasing CS. Our results showed that electrospun fibers had smaller and more uniform diameters with increasing applied voltage, separation distance, and CP. The resulting fibers were ultra-stable to heat and organic solvents because of the strong electrostatic attraction between polymers. Coacervates has the potentials to be developed into an environmentally benign electrospinning precursor platform.
Having demonstrated coacervates electrospinnability, we hypothesized that the associative interactions that drive coacervation can also enable electrospinning. Therefore, we synthesized a set of backbone-matched methacroloyl polymers of different chain lengths and formed coacervates. Amazingly, all the coacervates were successfully electrospun into continuously fibers, including an oligomeric Nanion/Ncation 6/9 coacervate system where no physical chain entanglements were possible. After correlating the spinnability of coacervates with their rheological behavior, we found out that spinnable coacervates had prolonged relaxation behaviors due to interpolymeric electrostatic interactions. The ability to electrospin oligomeric coacervates has significant impacts on decoupling polymer chain length from electrospinnability thus, enabling fibers formation from chemical species that were previously considered non-spinnable.
Knowing the long history of applications where complex coacervates were used for encapsulation, we also investigated the ability to electrospin cargo-loaded PEC fibers via coacervation. We used a family of six fluorescent dyes with systematic structural difference as model drugs. All dyes preferentially partitioned into the coacervates phase, allowing the subsequent electrospinning of highly-loaded fibers. Dyes that were electrostatically attracted to PSS to undergo π-π interactions partitioned more favorably in the coacervate phase, slowed the release from within PEC fibers when exposed to aqueous solutions, as well as exhibited enhanced uptake by fibers. These findings have the potentials to use the PEC fibers in applications related to biomedicine, energy, and separations, where controlling the uptake and release of cargo into sponge-like mats is needed.
In summary, this dissertation demonstrated the first electrospinning of aqueous complex coacervates solution into PEC fiber mats, identified that the spinning mechanism was electrostatic interactions as an alternative of entanglements, and studied the associated dye encapsulation and release properties of the mats to enable their use across a range of applications.