Epps Research Group Highlighted
Projects
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Nanoporous Block Copolymer Templates for Biological and Chemical Separations
The ISM phase portrait (above) not only offers an experimental framework providing further insight into the understanding of the universal principles behind ABC triblock copolymers but also creates a foundation for further development in block copolymer materials for highly selective separation membranes.
Tapered Block Copolymers
Therefore, tapered block copolymers allow for experimental control of the interfacial interactions in order to overcome greater block incompatibilities with increased molecular weight. Recently, our group has successfully synthesized and reported on the self-assembly of tapered block copolymers, using a poly(isoprene-b-styrene) [P(I-S)] based system. Interestingly, for the first time, we show that our normal-tapered poly(isoprene-b-isoprene/styrene-b-styrene) [P(I-IS-S)] and inverse-tapered poly(isoprene-b-styrene/isoprene-b-styrene) [P(I-SI-S)] diblock copolymers self-assemble into double-gyroid network structures. Both synchrotron small-angle X-ray scattering (above) and transmission electron microscopy corroboratively show that the incorporation of tapered regions up to 30% of the total copolymer volume does not extinguish the double-gyroid network. Subsequently, dynamic mechanical analysis (below) indicates that the normal and inverse tapered regions allow us to manipulate the order-disorder transition temperatures (TODT) of our materials while preserving network morphologies that are ideally suited for membrane and ion-conducting applications.
Conducting Polymers for Lithium Battery and Photovoltaic
Applications
Efficiency loss due to
exciton recombination is primarily due to a size-scale discrepancy between the
exciton diffusion length and the distance the exciton must travel between the
site of exciton generation and the interface between electron and
hole-conducting materials. Excitons that do not reach the interface will
recombine, preventing charge carrier separation, which leads to a loss of
efficiency. The inherent nature of block-copolymer self-assembly into nanoscale
structures with domain spacing on the order of exciton diffusion length offers a
potential solution for increasing the number of excitons that reach an interface
before recombining.
Lithium batteries have been considered promising
candidates of next generation energy storage devices. In order to improve
battery performance and safety, a lot of effort has been put into creating new
lithium ion-conducting membranes. These new membranes require high ionic
conductivities to decrease the internal potential losses and adequate mechanical
strength to reduce dendrite formation and prevent short-circuiting between
electrodes. Block copolymers with network structures offer the opportunity to
create 3-dimensional conducting paths for lithium ions to transport between
electrodes and a sturdy matrix to prohibit dendrite formation. Also, the lithium
salt dissolved in the conducting domain changes the interaction parameters
between different blocks which ends up changing the domain sizes and sometimes
the morphologies.
Many applications and devices require controlled distribution of material functionality in multiple dimensions. At the nanometer length scale, attempts to meet this challenge have included template-mediated materials chemistry. Interest in block copolymers has evolved because of their potential use in numerous nanotechnologies including nanotemplating, filtration membranes, and organic optoelectronics (LEDs and photovoltaics). Self-assembly of block copolymers in thin films is a complex phenomenon. A large parameter space, including film thickness, annealing conditions (thermal or solvent), molecular mass, and surface energy, governs the film morphology. Surface energetics and interface interactions also direct morphology orientation.
The behavior of thermally-responsive block copolymers compounds this complexity. When a thermally-responsive block copolymer undergoes a thermal transition resulting in a mass loss, the parameter space expands to include volume fraction shift, thickness decrease, surface energetic shifts of the relative blocks, and a change in substrate and free surface energetics. The resulting phenomenon is impacted by the complexity of multiple and often co-dependent variables. Control in chemically amplified transformations such as in thermal deprotection reactions can prove extremely useful especially when the self-assembly of the block copolymer is affected. Current investigations include controlling the final self-assembled morphology and orientation of thermally-responsive block copolymers using different surface chemistries and fabrication techniques as well as high-throughput methods for rapid characterization and identification of critical parameters.
An important aspect of exploiting high-throughput methods has been the development of novel gradient fabrication devices to efficiently probe the effects of substrate surface energy/chemistry and annealing conditions on block copolymer thin film morphology. These gradient approaches are becoming increasingly important for mapping the phase behavior of new materials for specific applications. In the following example, we used controlled vapor deposition to generate a gradient in substrate surface energy/chemistry and we show how the orientation of a cylinder-forming PS-b-PMMA thin film evolves with changes in substrate surface chemistry from a pure benzyl silane monolayer on silicon (left) to a pure methacryl silane monolayer on silicon (right), with gradient compositions and morphologies shown in between.
We have also designed a solvent resistant microfluidic mixing device that produces discrete gradients in solvent vapor composition and/or concentration to quickly and easily examine the use of solvent mixtures (versus a single solvent) for controlling thin film self-assembly. The image below shows a schematic of our solvent vapor annealing setup with the microfluidic device and its use as a screening tool to locate phase transformations in a poly(styrene-b-isoprene-b-styrene) triblock copolymer as a function of solvent composition and swollen film thickness.
Amphiphilic block copolymers have attracted considerable research attention for their potential usage in several biomedical applications including drug delivery, diagnostics and imaging. These macromolecules are lipid-like in the sense that they will spontaneously self-assemble in aqueous solutions forming various structures including spherical micelles, cylindrical micelles and vesicles. Polymeric self-assembled structures have several benefits over their lipid counterparts, including superior properties for delivery applications, such as that they are highly stable, have a low membrane permeability and a large storage capacity for therapeutic compounds. In addition, polymeric drug delivery systems have been shown to control drug release kinetics and improve circulation times in vivo. 
We have been investigating the effects of solution conditions on block copolymer self-assembly. Using cosolvent mixtures allows control over the size of block copolymer assemblies, as well as the interfacial profile between the core and corona blocks. The use of cosolvent mixtures allows the formation of a wide variety of hybrid nanostructures with enhanced properties useful in the development of biomedical delivery agents. We are interested in taking advantage of these valuable properties and functionalizing polymers with biologically relevant molecules for targeted drug delivery applications. These applications include engineering nano-structures to be sensitive to an external stimulus such that therapeutic drug release can be manipulated based on patient needs.