Douglas H. Adamson
Associate Professor of Chemistry & Polymer Program Director
Ph.D. University of Southern California
We are a materials synthesis group. Typically these materials are model polymers: well-defined polymers normally synthesized by high vacuum anionic polymerization and well characterized with respect to molecular weight, composition, microstructure and chain architecture. In addition, our studies include the self-assembly of these materials as well as polymer composites with nanofillers.
The advantage of anionic polymerization is the lack of significant termination or chain transfer reactions. They are thus referred to as living systems, or living polymers. Anionic active sites react with monomer and stay reactive, even as the monomer is used up. If more monomer is added, the living polymer reacts with the additional monomer until it is also gone, while maintaining the reactive anion. In this way block copolymers consisting of two or more segments of chemically distinct polymer chains can be synthesized. The living character also allows for other architectures such as stars and brushes, where linking agents react with the anions at the chain ends and tie together separate polymer chains.
Well-defined polymers have applications in a large number of research areas. Our research is commonly done in collaboration with physicists, engineers, and biologists. We are currently involved in a wide range of research areas. Several of these are described below.
We are investigating several systems with the goal of using synthetic, non-peptide polymers to mimic the functions and characteristics of naturally occurring materials. One example is the protein Silicatein a. Silicatein a is a protein found in the sponge Tethya aurantia. It has been shown to catalyze the condensation of silica precursors such as tetraethoxysilane (TEOS) at neutral pH and ambient temperatures. The strategy for mimicking this protein begins with the synthesis of functional block copolymers, namely poly(2-vinylpyridine-b-1,2 butadiene). This polymer is made by sequential high vacuum anionic polymerization and hydroxylated using hydroboration chemistry to give a block copolymer with functional groups that mimic the histidine and serine residues shown to be active in the catalytic function of the protein. Besides the formation of silica, other ceramics based on titania, alumina and zirconia are also being developed.
A further interest of ours is mimicking the general structure of proteins using synthetic polymers. Proteins are composed of largely hydrophobic amino acid chains. These chains fold in such a way as to bury the hydrophobic portion of the chain and present a thin hydrophilic surface that stabilizes the protein in their aqueous environment. Modeling studies have suggested that a specific chain architecture with a long hydrophobic chain ending in short hydrophilic grafts may fold in a way that mimics proteins. As there is much speculation that the hydrophobic environment of enzymes plays a critical role in their catalytic activity, such a molecule would be expected to show exciting catalytic properties.
Also a biomimetic structure, polymersomes are vesicles formed by amphiphilic block copolymers. These vesicles are analogous to ones formed in biological systems from lipids. In that case the vesicles are termed liposomes, in our case they are called polymersomes. In both cases, vesicles are made of amphiphilic molecules that self-assemble to form a bilayer membrane. This membrane assembles itself in such a way as to have the hydrophilic ends facing the water while burying the hydrophilic ends in the center of the membrane. This is the same basic configuration as a cell membrane (although the cell membrane is much more complex with different lipids and transmembrane proteins). An attractive characteristic of our system is that a very small amount of material will encapsulate a large volume of water.
We have developed a solvent injection method and a microfluidics device for forming polymersomes. We use fluorescence microscopy and dynamic light scattering to visualize the polymersomes and to measure their size and size distributions. We have concentrated the vesicles by removing water and find a sharp viscosity increase measured by parallel plate rheology.
We are investigating the use of nanofillers in polymer composites. Exfoliated graphite and graphite oxide are the fillers we are interested in. Graphite oxide is produced in our labs by oxidizing natural flake graphite. This material can then be thermally exfoliated and reduced, or exfoliated in solution by sonication. The thermally exfoliated material has a crumpled morphology with much of the original graphite lattice in place. This can be important in applications were conductivity is required. The sonically exfoliated GO retains its functional groups and has a flat morphology. This can be useful for compatiblization with various matrixes. In addition, we have developed a solvent system for the exfoliation of pristine graphite and are exploring the applications of these sheets as well. Both graphene materials represent a new class of fillers with very high aspect ratios and relatively low costs.
1. Zhang, S. Y.; Adamson, D. H.; Prud’homme, R. K.; Link, A. J., “Photocrosslinking the polystyrene core of block-copolymer nanoparticles” Polymer Chemistry, 2011, 2(3), 665-671.
2. Li, L.; Guo, X. H.; Adamson, D. H.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K., “Flow Improvement of Waxy Oils by Modulating Long-Chain Paraffin Crystallization with Comb Polymers: An Observation by X-ray Diffraction” Industrial & Engineering Chemistry Research, 2011, 50(1), 316-321.
3. Kumar, V.; Adamson, D. H.; Prud’homme, R. K., “Fluorescent Polymeric Nanoparticles: Aggregation and Phase Behavior of Pyrene and Amphotericin B Molecules in Nanoparticle Cores” Small, 2010, 6(24), 2907-2914.
4. Kumar, V.; Hong, S. Y.; Maciag, A. E.; Saavedra, J. E.; Adamson, D. H.; Prud’homme, R. K.; Keefer, K. L.; Shakrapani, H., “Stabilization of the Nitric Oxide Prodrugs and Anticancer Leads, PABA/NO and Double JS-K, through Incorporation into PEG-Rpotected Nanoparticles” Molecular Pharmaceutics, 2010, 7(1), 291-298.
5. Papalia, J. M.; Marencic, A. P.; Adamson, D. H.; Chaikin, P. M.; Register, R. A., “Thin Films of Block Copolymer-Homopolymer Blends with a Continuously Tunable Density of Spherical Microdomains” Macromolecules, 2010, 43, 6946-6949.
6. Prud’homme, R. K.; Ozbas, B.; Aksay, I.; Register, R.; Adamson, D., “Functional Graphene-Rubber Nanocomposites” patent US7,745,528 issued June 29, 2010.
7. Prud’homme, R. K.; Aksay, I. A.; Adamson, D.; Abdala, A., “Thermally Exfoliated Graphite Oxide” patent US7,658,901 issued February 9, 2010.
8. Papalia, J. M.; Adamson, D. H.; Chaikin, P. M.; Register, R. A., “Silicon nanowire polarizers fro far ultraviolet (sub-200 nm) applications: Modeling and fabrication” J. Appl. Phys., 2010, 107, 084305-1 – 084305-6.
9. Budijono, S. J.; Russ, B.; Saad, W.; Adamson, D. H.; Prud’homme, R. K., “Block copolymer surface coverage on nanoparticles” Colloids and Surfaces A: Physicochemical and Engineering Aspects”2010, 360, 105-110.
10. Marencic, A. P.; Adamson, D. H.; Chaikin, P. M.; Register, R. A., “Shear alignment and realignment of sphere-forming and cylinder-forming block-copolymer thin films” Physical Review E, 2010, 81, 011503-1 – 011503-10.
Polymer Program: 860.486.3582: firstname.lastname@example.org