(309a) Self-Assembly of Pluronic Block Copolymers and Vesicle Formation in a Protic Ionic Liquid

Pluronics are triblock copolymers formed by a polyoxypropylene block (PO) and two polyoxyethylene blocks (EO). They self-assemble into micelles when dissolved in protic solvents and these microstructures are excellent candidates for applications such as nano-carriers for drug delivery applications [1] and as templates for nano-structured materials [2-3]. Room temperature ionic liquids (RTIL) are salts with melting point (m.p.) below room temperature. Classified as green solvents due to their negligible vapor pressure, RTILs are good alternatives to volatile organic compounds in synthesis, catalysis, extraction and separation [4-5]. Of interest here is a fundamental, molecular-level understanding of the self-assembly of amphiphilic molecules, such as Pluronic polymers, in a RTIL.  This can facilitate the rational design of desired structures and rheological characteristics for specific applications

Previous our studies of Pluronic polymers in a protic ionic liquid, ethyl ammonium nitrate (EAN) have elucidated self-assembled microstructures at intermediate concentrations, including spherical, wormlike micelles [6], gels and liquid crystals [7]. In this study, we systematically investigate the self-assembled structures in a model system, L121/EAN mixture at lower concentrations . These compositions are potential candidates for spontaneous formation of thermodynamically stable vesicles which are useful in drug delivery applications. A broad combination of experimental probes (i.e., ultra-small, small-angle neutron scattering, small-angle x-ray scattering, dynamic light scattering, rheology, conductivity, and fluorescence microscopy) are used to determine the structural, thermodynamic and rheological changes associated to the vesicular structures. The onset unilamellar vesicle formation is found upon increasing the concentration of Pluronic polymer L121 or increasing the temperature, defining a critical vesicle concentration (CVC) or temperature (CVT). Further increases in concentration or temperature leads to the continuous growth of multilamellar vesicles. Such unilamellar-multilamellar vesicle structural transition is shown to be thermoreversible by applying heating-cooling cycles on the sample, and in-situ SANS-temperature study. To date, this is the first experimental report on the spontaneous formation of thermoreversible vesicles in Pluronic-ionic liquid systems.

[1] Batrakova and Kavanov, J. Controlled Release 2008, 130, 98–106

[2] Hess et al., Chem. Mater. 2009, 21, 2125–2129

[3] Sakai and Alexandridis, Nanotechnology 16 (2005) S344–S35

[4] Evans et al., J.Coll. Int. Sci. 1982, 88, 89

[5] Greaves et al., Langmuir 2007, 23, 402

[6] Lopez-Barron, C.R., Li, D., DeRita L., Wagner, N., Macromolecules, in preparation

[7] Lopez-Barron, C.R., Wagner, N., et al., Phys. Rev. Lett. 2012, 108, 258301