| Vesicular structures are promising materials for encapsulation applications, and vesicles composed from block copolymers are especially attractive in this regard because of their cost, purity, tunability and stability. However, vesicle formation generally requires specialized procedures. This dissertation demonstrates that 1,2-butylene oxide/ethylene oxide di-block copolymers spontaneously form μm-sized, multi-lamellar vesicles (“onions”) over a broad range of copolymer concentrations upon simple mixing with water. Slow copolymer hydration without agitation produces giant vesicles with diameters greater than 0.1 mm.; This behavior is consistent with a geometric argument that asserts vesicle formation is preferred only when the proper ratio of hydrophobe volume to the product of head group area and hydrophobe length is achieved. The relatively large volume to length ratio of the butylene oxide hydrophobe is critical to vesicle formation, but formation also depends on the ethylene oxide head group length. For BO block lengths 15 units or less, BO/EO copolymers with EO blocks between 5 and 15 units long spontaneously form large MLVs upon simple mixing with water. EO lengths greater than 15 result in small vesicles or no vesicles at all. We also found that EO block length polydispersity strongly affects structure formation. Substitution of an SO3− group for the EO block produced small vesicles, further evidence that the BO geometry is key. For BO/EO copolymers with BO block lengths of 15 to 25 units, ultra-sonication is required for vesicle formation, and the resulting structures have diameters of about 70 nm.; BO/EO MLVs formed by hand-shaking have shelf lives of over 2 years and survive thermal cycling, mild sonication and moderate shear. In contrast, extrusion of the same dispersion through a porous membrane reduces vesicle sizes, although final structure diameter is independent of shear rate and the pore size used for extrusion. Vesicle formation is reversed by thermal cycling. Ultra-sonication also reduces structure diameter.; Copolymer composition was analyzed using nuclear magnetic resonance (NMR), gel permeation chromatography (GPC) and mass spectrometry (MS). Vesicle capacity for encapsulating both hydrophobic and hydrophilic materials was explored. Phase diagrams were prepared of the copolymer, a model hydrophobe and water. The release rate of an encapsulated hydrophile was also studied. Structures were characterized using plane-polarized light microscopy, dynamic light scattering and cryogenic scanning electron microscopy. |
