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Macromolecular Self-assembly

(Christopher Barrett, Adi Eisenberg, Musa Kamal, Bruce Lennox, Milan Maric, and Linda Reven)

With the transition to new technologies in fields such as micro electronics or biomedicine, microstructural control of materials is essential, yet the required structures are often too complex to be usefully produced by laying down one element at a time. Self-assembly (the notion that physical or chemical driving forces produce the necessary material microstructure without the need for any external intervention) is emerging as a very important tool in the manipulation and production of nano structured materials, including macromolecular or biomolecular materials. Research at MIAM in self-assembly has two main components: the use of polymer and block-copolymers as architectural units in the design of complex structures, and the control, manipulation, and functionalization of surfaces.

Polymers afford the design from first principles of novel morphologies by modifying the supporting medium, the polymer, or colloid forming conditions leading to tubules, compartmented spheres, lamellar and columnar phases, or pin cushions, just to name a few. Scattering techniques (static light scattering, dynamic light scattering, small angle neutron and X-ray scattering) as well as conventional electron microscopy allow the determination of the self-assembled structure, and other thermodynamic properties including the order of the transition underlying the self-assembly and possible reversibility.[E.A. Lysenko, A. Eisenberg, “Formation of multilayer polyelectrolyte complexes by using block ionomer micelles as nucleating particles”. J. Phys. Chem. B 108, 12352 (2004)]. Particular applications include the use of block copolymers as compatibilizers in polymer blends, and as templates for ordered structures, including materials suitable for catalysis or membranes. In the latter case, the goal is the design of novel materials for super-absorbent ion-exchange resins, for controlled drug delivery and a first-principles design of a nano reactor [M. Maric and C.W. Macosko, “Block copolymer compatibilizers for polystyrene/poly(dimethylsiloxane) blends”. J. Poly. Sci. B- Pol. Phys. 40, 346 (2002)]. Given that these nano-scale assemblies provide microphase-separated domains of precisely controlled size, nano-reactors can, for example, synthesize semi-conducting quantum dots, as well as serve as catalytically active sites within the assembly architecture. Some of the more novel applications of polymer self-assembly are in the bio-pharmaceutical field. They are actively being developed to sequester toxins before they are absorbed from the intestinal tract into the bloodstream, and drug delivery vehicles which allow the site-specific delivery of lipophilic drugs [R. Savic, A. Eisenberg, et al. “Micellar nanocontainers distribute to defined cytoplasmic organelles”. Science 300, 615 (2003)].

Beyond equilibrium studies, research is also under way to elucidate the kinetics of morphological instabilities mediating self-assembly, including for example transitions from rods to vesicles, from spheres to rods, or among phases of different symmetries in block coplymers.[ A.A. Choucair, A.Eisenberg, et al. “Kinetics of fusion of polystyrene-b-poly(acrylic acid) vesicles in solution”.Langmuir 19, 1001 (2003)]. Non-equilibrium studies are also useful in terms of material processing, with research being focused on several aspect of solidification. They include the kinetics of crystallization of polymer melts under pressure, shear, and non-isothermal conditions, and the development of morphology in thin films [L. Feng and M.R. Kamal, “Spherulitic crystallization behavior of linear low-density polyethylene”. Pol. Eng. Sci. 45, 74 (2005)]. Self-assembly is also greatly influenced by processing flows, so that parallel efforts address film blowing, injection molding, general interfacial tension driven effects, and the general effects of shear flows in microstructure evolution of the self-assembled phases.

The second area of activity concerns the study of structure/property relationships of classes of molecules which form surfaces and interfaces, their self-assembly, and their functionaliztion for both Nanotechnology and Biology applications. The goal of this research is to design, prepare, and characterize novel molecular systems, to relate the properties and performance of these structures and simple devices to the structure of the molecules, and to elucidate the molecular origins of the optical and mechanical behavior of polymer surfaces, interfaces, and thin films. Of course, the understanding of structure/property relationships will enable the rational design of molecular systems in order to optimize the performance of a given molecular based device. These studies include not only molecular design and synthesis (for example, of novel surfactants or lipids), but also kinetic studies (enzymology of functionalized surfaces), surface chemical techniques (Langmuir films, electrochemistry, scanning probe microscopy), spectroscopy (NMR, IR) and polymer chemistry.

Specific applications are in a number of areas summarized in what follows. Research is conducted on the synthesis of stabilized gold and platinum nanoparticles with target applications in drug delivery, or biorecognition sensors schemes in which the particles are either used as anchoring elements, or as metal surfaces for resonance studies [M.K. Corbierre, R.B. Lennox et al. “Polymer-stabilized gold nanoparticles and their incorporation into polymer matrices”. J. Am. Chem. Soc. 123, 10411 (2001)]. Novel lipids are also being synthesized for direct design of functionalized surfaces, with attendant lipid dynamics studies, lipid reactivity and enzyme activity. These novel lipids include two-headed lipids, or the preparation of two-dimensional lithography masks [M.V. Meli and R.B. Lennox. “Preparation of nanoscale Au islands in patterned arrays”. Langmuir 19, 9097 (2003)]. Photochemistry is another area of application in which polymers with photo-switchable groups allow one to address thin films or micro-structures of the materials for reversible changes in optical, geometric, or mechanical properties. Changes in optical properties can be used to store digital or holographic information in thin films, or even to draw waveguides to channel light in optical circuits. Similarly, structural or surface properties can be switched on or off with these materials, with investigation focused on reversible control of surface wetting, or bio-activity such as protein adsorption or cell growth [R.H. El Halabieh, C.J. Barrett, et al. “Using light to control physical properties of polymers and surfaces with azobenzene chromophores”. Pure and Appl. Chem. 76, 1445 (2004)]. Finally, we mention research focusing on polymers with charged groups (or polyelectrolytes) that allow one to directly build multi-layer structures by the repeated and sequential dipping of charged substrates (such as treated glass, silicon, metals, or plastic) into a dilute solution of either positively or negatively charged polymer chains. An advantage of this method of self-assembly for nano-device fabrication is that it is all aqueous, making it naturally compatible with many systems of biological interest, such as molecular recognition, sensing, or drug delivery [S.E. Burke and C.J. Barrett, “Controlling the physicochemical properties of weak polyelectrolyte multilayer films through acid/base equilibria”. Pure and Appl. Chem. 76, 1387 (2004)].

Characterization tools are essential in the determination of the structure and properties of the self-assembled structures, be it in bulk or on surfaces. It is well known that characterization of these highly heterogeneous or amorphous solid phases can be difficult or impossible to achieve by conventional methods such as X-ray diffraction. Solid state NMR spectroscopy is playing an increasingly important role in this field, which current studies addressing both organic monolayers and polyelectrolyte films [M. McCormick, C.J. Barrett, L. Reven, et al. Smith RN, Graf R, et al. “NMR studies of the effect of adsorbed water on polyelectrolyte multilayer films in the solid state”. Macromolecules 36, 3616 (2003)] . Solid-state NMR techniques are used in conjunction with a battery of more established techniques (transmission electron microscopy, various vibrational or light spectroscopies, or atomic force microscopy) to interrogate the structure and properties of these self-assembled surfaces.