Structural control over soft matter on molecular through microscopic lengthscales is a vital tool to optimizing properties for applications ranging from energy (solar and thermal) to biomaterials. For example, while molecular structure affects the electronic properties of semiconducting polymers, the crystal and grain structure greatly affect bulk conductivity, and nanometer lengthscale pattern of internal interfaces is vital to charge separation and recombination in photovoltaic and light emission effects. Similarly, biological materials gain functionality from structures ranging from monomeric sequence through chain shape through self-assembly. We work to both understand the effects of structure on properties and gain pattern control in these inherently multidimensional problems. We are particularly interested in materials for energy applications such as photovoltaics, fuel cells, and thermoelectrics.
Why control Structure?
Nanopatterning is the science of controlling the structure and behavior of matter at an intermediate scale between the molecule and the macroscopic world, that is the nanoscale (1-100 nm). Over the last several years, this field has garnered much attention as a primary step towards the fabrication of nanodevices. Many studies have demonstrated a sophisticated level of control over the self-assembling, coil-type polymer systems to produce nanometer scale patterns for lithography and secondary synthesis steps. Intricate block copolymer patterns may be formed and controlled by relying on the competition between unfavorable mixing and stretching of the dissimilar blocks.
Thermodynamics of Functional Polymer Self-Assembly
Nanoscale control and patterning of functional block copolymers presents a new challenge due to non-idealities in molecular conformation and mixing interactions that are present in these materials. In many cases, the functionality stretches the chain into a rod-like shape. Typical rod-like polymers include helical proteins and semiconducting polymers with rigid π-conjugated backbones. For example, rod-coil block copolymers with an amino acid-based rod blocks have been suggested as models for membrane structural proteins or DNA gels and for use as artificial membranes. There is also great interest in rod-coil block copolymers for use in organic electronics, where controlling the active layer morphology and interfacial structure in multicomponent devices on the 10 nm length scale of an exciton diffusion length is critical to optimizing photovoltaic device performance. As the stiffness of one of the blocks is significantly increased, however, a new class of intermolecular interactions is introduced including liquid crystallinity. A number of stunning and intriguing phases have been observed in rod-coil systems and the Segalman group has developed the first weakly segregated model system with which to probe the thermodynamics of self-assembly of these important materials.