Sandra L. Burkett: Research Interests
The common theme in my past, present, and future research is the manipulation of interactions and interfaces between inorganic and organic components in order to control the assembly of hybrid systems and materials. Biological mineralization processes such as the formation of teeth, bones, and shells provide examples of and inspiration for how organic molecules, macromolecules, and molecular aggregates can influence the formation of inorganic and composite materials. The high degree of spatial and compositional control of mineral deposition within biomineralized systems is achieved through the use of organic molecules and matrices in several roles. One of the hallmarks of biomineralization is compartmentalization, which facilitates regulation of precursor concentrations for temporal and phase control, and which provides spatial delimitation of the region to be mineralized. Chemical complementarity and specific intermolecular interactions between the organic matrix and the soluble inorganic building blocks further serve to control where and when the mineral is formed; these organic–inorganic interactions can induce formation of mineral phases under conditions of temperature, pressure, pH, or precursor concentration that do not give rise to the particular phase in a purely inorganic system. An understanding of the nature of the interactions at the interface between the organic and inorganic components and the design of new synthetic methods that make use of these interactions are of considerable interest for the development of environmentally benign methods for industrial processing of inorganic materials. Interactions at the interface between the organic and inorganic species present within biomineralized systems can also lead to elaborate architectures at the sub-micron scale that far exceed the level of complexity that can be achieved currently in the chemical laboratory. The ability to design and manipulate the structures of organic and inorganic materials at the sub-micron length scale is one of the key goals of the current boom in nanotechnology research, which has implications for fields such as medicine and the semiconductor/computer industry. In my research, I explore the tandem themes of chemistry in confined spaces and interfacial control of nanostructures for a diverse range of organic–inorganic hybrid systems. My work is largely focused on synthesis, but I also use solution- and solid-state NMR techniques to probe the nature and consequences of interfacial interactions. I an interested both in “bio-inspired” synthesis of hybrid materials via mineralization of organic matrices, and in the “inverse” process of controlling organic macromolecular structure by or within inorganic matrices.
My primary current research focus is the construction of organic–inorganic hybrid materials by controlled growth of polymer brushes within the intergallery spaces of synthetic clay-like layered materials. Akin to the mechanical enhancements imparted to biogenic minerals by the integration of organic macromolecules in very low concentrations, the presence of only a few weight percent of clay can enhance the mechanical strength, thermal stability, and barrier properties of a polymer if the individual sheets of the clay (which have nanometer thickness and approximately micron lateral dimensions) are well dispersed in the polymer matrix. However, control of molecular-level integration of the inorganic and organic components to form an intercalated structure (in which the polymer chains penetrate the interlayer space of the clay but registry between the layers is maintained) or an exfoliated structure (in which the individual clay layers are separated and dispersed randomly in the polymer matrix) is strongly dependent on the nature of the interfacial interactions and is difficult to achieve. The key challenge in the preparation of polymer–clay nanocomposites is the incompatibility of hydrophilic clay platelets with the typically nonpolar polymers of interest. My approach is to prepare tethered polymer chains, or brushes, via a “grafting from” approach, in which reactive organic moieties that are covalently bonded to the inorganic lamellae of synthetic zirconium phosphate (Zr(O3POH)2 or Zr(O3PO– R+)2) and zirconium phosphonate clays (Zr(O3PR)2) provide initiation sites for the growth of individual polymer chains that are end-tethered to the surface (with no non-tethered polymer formed).