291883 Isotope Effect On Critical Micelle Concentration of Mixed Pluronic Block Copolymers in a Protic Ionic Liquid

Monday, October 29, 2012
Hall B (Convention Center )
Leo DeRita1, Dongcui Li1, Carlos R. López-Barrón2 and Norman J. Wagner2, (1)Chemical & Biomolecular Engineering, University of Delaware, Newark, DE, (2)Chemical Engineering, University of Delaware, Newark, DE

Pluronics are triblock copolymers formed by a polyoxypropylene block (PO) and two polyoxyethylene blocks (EO), (EO)x-(PO)y-(EO)x. Due to their ability to self-assemble into micelles when dissolved in protic solvents, Pluronics represent excellent candidates 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]. Self-assembly of amphiphilic molecules such as Pluronic polymers in a RTIL media can potentially create desired structural and rheological characteristics, thus they are of particular interest.

Previous studies on the mixed Pluronic polymers in a protic ionic liquid, ethyl ammonium nitrate (EAN), have shown very interesting microstructures including spherical, wormlike micelles [6], gels and liquid crystals [7] in the semi- and concentrated regimes. In this study, we aim to systematically study the self-assembled structures at very low compositions in order to complete the very rich phase diagram of mixed Pluronics. Low composition mixtures L121/F127 and L121/P123 (just above the Pluronics’ critical micelle concentrations (CMC)) are potential candidates for spontaneous formation of thermodynamically stable vesicles which are useful in drug delivery applications. This hypothesis is based on the fact that  L121 ((EO)5-(PO)68-(EO)5 )), P123 ((EO)20-(PO)68-(EO)20 )), and F127 ((EO)106-(PO)68-(EO)106)) have the same center PPO block length () and hence there is no entropic penalty to be paid for the PPO block to mix in the vesicle bilayer interiors. Additionally, the difference in PEO lengths between the polymers will allow the minimizations of the bilayer curvature by dynamic migration of the polymers between the interior and exterior of the vesicles. Several experimental probes (i.e., small-angle neutron scattering, rheology, conductivity, surface tension, isothermal titration calorimetry and microscopy) are used to determine the structural, thermodynamic and rheological changes associated to the micelle and vesicle formation, aggregation, and gelation. As part of this characterization, here we present the micelle formation study in both hydrogenated EAN (h-EAN) and deuterated EAN (d-EAN). Of potential concern is the impact of deuteration of some of the hydrogen in EAN in future SANS characterizations. It has been demonstrated that 1) the CMC of Pluronics in h-EAN and d-EAN are validated via multiple techniques; 2) the H-D isotope substitution significantly affects the critical micelle concentration of the Pluronics-EAN mixtures due to the stronger deuterium-bonding comapared to hydrogen-bonding [8].

[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

[8] López-Barrón, C.R., Wagner, N.J., Soft Matter, 2011,7, 10856-10863.

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