Chain exchange in aqueous solutions of block polymer micelles
Date
2014
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Publisher
University of Delaware
Abstract
Amphiphilic block polymers consist of covalently linked hydrophobic and hydrophilic polymer segments. These materials spontaneously assemble into various nanoscale structures in aqueous solution such as spherical micelles, cylindrical micelles, vesicles, and other complex morphologies. Utilizing block polymer micelles as drug delivery vehicles, aqueous nanoreactors, and other dispersant technologies demands the ability to encapsulate hydrophobic molecules within the hydrophobic micelle core, which is typically achieved through organic cosolvent processing methods. While cosolvent processing is known to affect micelle dynamics and chain exchange, processing effects in a highly selective solvent after cosolvent removal are less understood. This work investigates the stability of poly(1,2-butadiene-b-ethylene oxide) (PB-PEO) core-shell spherical micelles under various processing conditions after cosolvent removal. Processing conditions such as cosolvent addition and removal, polymer concentration, temperature, solution agitation, and interfacial contact were found to influence the stability of block polymer micelles in aqueous solutions. Complementary in situ characterization methods, including dynamic light scattering, cryogenic transmission electron microscopy, and small angle neutron scattering, were used to monitor the resulting micelle size evolution and chain exchange following dialysis into water. Despite highly unfavorable PB-water interactions, micelle size relaxation was discovered in aqueous solutions when the micelles were significantly perturbed. Notably, coupled shear and interfacial effects were found to be a key driving force for chain exchange and micelle growth, which parallels findings for other macromolecular solution assemblies such as proteins. Key findings demonstrated that micelle relaxation timescales were sensitive to the agitation method, agitation rate, temperature, molecular weight, and polymer concentration. Comprehending the interplay between various processing parameters is necessary to better control the kinetic pathways for micelle chain exchange and to further develop materials for emerging applications that hinge on the stability of macromolecular assemblies in aqueous solutions.