Effect of Encapsulated Species on Block Copolymer Micelle Self-Assembly



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Due to their amphiphilic nature, block copolymers spontaneously form assemblies known as micelles in selective solvents. Minimization of the solution free energy drives the micellization process, with contributions from the polymer chains, surrounding solvent, and any additive molecules. Through manipulation of the polymer characteristics, choice of solvent, and addition of guest molecules, the formation of the nano-scale assemblies can be controlled and tuned for many applications, such as cosmetic products, nanoreactors, oil recovery, and targeted drug delivery. In this thesis, the impacts of encapsulated species on key micelle structural parameters, such as aggregation number and size, and dynamic processes, such as unimer exchange, were investigated. In the first study, micelles containing poly(ethylene oxide)-b-polycaprolactone [PEO-PCL] diblock copolymers in water/THF mixtures were investigated. The presence of THF altered the interfacial tension between the micelle core and surrounding solvent, and the hydrophobic cores swelled similarly for two micelle series with similar block ratio yet differing molecular weight. Differences in the dependencies of the core and micelle radii on the solvent composition originated from differing trends in aggregation number for the two series. The exchange of unimers between micelles, within the PEO-PCL system in water/THF mixtures, was explored using time-resolved SANS, in which little intermixing was observed over many hours, even at elevated temperatures. The relaxation function, quantified from the time-dependent neutron scattering intensity, exhibited a linear trend on a logarithmic time, consistent with prior studies in which core chain coupling limited the expulsion rate of chains from micelle cores. In this scenario, the expulsion of a single chain from the micelle core required rearrangement of multiple other core chains. In addition, micelles containing PEO-PCL in water were loaded with commercial chemotherapy drugs and their precursors, in order to investigate the effect of cargo size and polarity on micelle structural parameters. The guest molecule size impacted core loading to a greater extent than polarity, and only the smallest, most polar drug resulted in significant structural changes. Commercially available poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) [PEO-PPO-PEO] triblock copolymers were micellized in water, to explore the effects of temperature, polymer concentration, and the presence of hydrophobic molecules on micelle structural parameters. At low polymer concentrations, no micellization occurred at room temperature, but increasing both polymer concentration and temperature resulted in significant increases in micelle size. The effect of hydrochlorothiazide (HCT), a commercial diuretic, on the micelle structural parameters was explored. For micelles near both the critical micelle concentration and temperature, the presence of HCT induced micellization. At higher polymer concentrations, drug addition increased chain aggregation and micelle size. Beyond the critical micelle temperature, little structural impact occurred with addition of HCT, thus drug encapsulation was most impactful near the unimer-micelle phase boundary.



Block copolymers, Micelles, Unimer, Block copolymer micelles, Self-assembly, Drug delivery, SANS, TR-SANS, Interfacial tension, Co-solvent, Hydrophobic drug, Unimer exchange, Biocompatible polymer, Pluronic, Drug loaded micelles, Encapsulated species


Portions of this document appear in: Cooksey, Tyler J., Avantika Singh, Kim Mai Le, Shu Wang, Elizabeth G. Kelley, Lilin He, Sameer Vajjala Kesava et al. "Tuning Biocompatible Block Copolymer Micelles by Varying Solvent Composition: Core/Corona Structure and Solvent Uptake." Macromolecules 50, no. 11 (2017): 4322-4334.