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Reversible Gelation of Polyethyleneimine Solutions Using CO2

Christopher L. Kitchens1, Susnata Samanta2, Ejae John2, Pamela Pollet2, Kris Griffith3, Kent Richman3, Robert P. Apkarian4, Charles Liotta2, and Charles A. Eckert5. (1) Department of Chemical and Biomolecular Engineering, Clemson University, 127 Earle Hall, Clemson, SC 29634-0909, (2) School of Chemistry and Biochemistry, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100, (3) American Pacific Corporation, Las Vegas, NV 89109, (4) Emory University, Integrated Microscopy & Microanalytical Facility, Atlanta, GA 30322, (5) Chemical and Biomolecular Engineering, Georgia Tech, 311 Ferst Drive NW, Atlanta, GA 30332-0100

We have demonstrated the ability to form reversible gels, synthesized by the addition of carbon dioxide to liquids with amine functionality. There is a large potential impact of these gels, including controlled delivery of pharmaceuticals or cleaning agents, structural network for materials synthesis, and CO2 capture. For these applications, detailed information on the structure, properties, and dynamics of the gels is necessary. Recently, we have taken advantage of the reversible reaction of CO2 with primary and secondary amines, to provide a reversible non-ionic to ionic transition with applications in reversible ionic liquids1 and switchable surfactants2. For this system we are applying this reversible chemistry to gel formation. The reaction of amines with CO2 is well documented in the literature and in most cases, results in the formation of carbamate salts. In a similar fashion, bubbling CO2 in liquid polyethyleneimine (PEI) results in the formation of a solid salt. However, with the presence of a fluid solvent, a stable gel can be formed.

The method of gelation itself is remarkable by which a gel is formed by simply bubbling gaseous CO2 in a liquid solution of 5% 20% PEI (1200 MW) in an alcohol, such as 1-octanol. The gels are synthesized and stable at room temperature. Applying nitrogen to the gel causes a slow evolution of the CO2 and eventual complete reversal of the gel, resulting in the original liquid. The gel decomposition by CO2 evolution can be escalated by applying heat, after which the gel can be reformed by bubbling CO2 through the liquid. The chemistry is known for the reaction of to form carbamates and gels3,4, what is unknown is the mechanism of self-assembly and 3-dimensional structure that occurs and results in the gel state. There is a structure-activity relationship which allows the gel formation of which we hope to understand and thus design systems that can undergo the gel state by reaction with CO2. The carbamate functionality has ionic character, which provides a balance between the hydrophilic and hydrophobic species and is critical in the self-assembly into a reversible cross-linked structure and thus forming a gel.

We have investigated the underlying physical structure and reversibility using a variety of analytical methods including; elemental analysis, IR, Cryo HRSEM (Cryogenic High Resolution Scanning Electron Microscopy), and SANS (Small Angle Neutron Scattering). From the elemental and 15N NMR analysis, we have confirmed the reaction of CO2 with the amines to form carbamates. The observed physical properties, including increased viscosity, reduced vapor pressure, and liquid immobilization, demonstrate gel formation on the macroscale.

On the microscale, the reversible nature of the gels inhibits their studies under high vacuum setups, as required for many characterization techniques. Cryo electron microscopy techniques have been employed to study the structure and morphology of hydrogels, where the hydrogels are cryogenically immobilized enabling imaging under high vacuum environments. From the Cryo HRSEM analysis, the gels appear to be uniform in structure initially, however gel ripening is observed over time resulting in polymorphism and observed needle-like structures and cylindrical networks. SANS is an extremely beneficial technique for studying the structure and morphology of hydrogels. The benefits of using SANS for this study over alternative methods include: non-destructive analysis, ambient sampling environment, contrast variation between the PEI-CO2 gel structure and immobilized fluid, and accessible length scales of 1 400nm. The SANS results demonstrate the reversible nature of the gels and the presence of elliptical particles at low PEI concentrations and a transition to cylinders with elliptical cross sections at high PEI concentrations.

Reversible gels are promising materials with current applications, as well as, future applications ranging from drug delivery to optical/electronic applications. Understanding the structure and physical properties will allow us to fully explore the potential of these unique materials.

1. Jessop, P. G.; Heldebrant, D. J.; Li, X. W.; Eckert, C. A.; Liotta, C. L., Green chemistry - Reversible nonpolar-to-polar solvent. Nature 2005, 436, (7054), 1102-1102.

2. Liu, Y.; Jessop, P. G.; Eckert, C. A.; Liotta, C. L., Switchable Surfactants. Submitted to Science 2006.

3. Carretti, E.; Dei, L.; Baglioni, P.; Weiss, R. G., Synthesis and characterization of gels from polyallylamine and carbon dioxide as gellant. J. Am. Chem. Soc. 2003, 125, (17), 5121-5129.

4. George, M.; Weiss, R. G., Chemically reversible organogels via "latent" gelators. Aliphatic amines with carbon dioxide and their ammonium carbamates. Langmuir 2002, 18, (19), 7124-7135.