Epps Research Group Highlighted Projects
Tapered block polymers are block polymers with interfacial regions that taper from one polymer block to another polymer block (in a well-defined fashion over a well-defined region of the copolymer). The incorporation of a tapered region between the blocks offers an opportunity to manipulate block polymer segregation strength independent of molecular weight and chemical constituents, which allows the design of materials with improved mechanical properties while retaining the desired phase separated structures in the vicinity of the order-disorder transition temperature.
Therefore, tapered block polymers 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.
Lithium batteries are considered promising candidates for next generation energy storage devices. In order to improve battery performance and safety, significant effort has been put towards 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 polymers 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. Additionally, the lithium salt dissolved in the conducting domain modifies the interactions between different blocks which potentially alters the domain sizes and/or morphologies of the resulting material.
Renewable polymers are needed to help reduce global dependence on petrochemicals. Many renewable polymers are also biodegradable, biocompatible, and beneficial for a variety of applications, such as compostable cups and cutlery, elastomeric shoe soles and car tires, sturdy machine parts and electronics casings, and compatibilizing agents. Lignin and fatty acids are two of the most abundant renewable waste streams that can contribute to the collection of renewable (biobased) polymers already available. Lignin is a byproduct from pulp and paper mill manufacturing and typically is burned for energy. Fatty acids can come from waste cooking oils and sometimes are fed to livestock or converted to biofuels. Alternatively, these renewable resources can provide lignin model compounds (LMCs) shown structurally in the figure and n-alkyl (“fatty”) alcohols of varying aliphatic chain-length and degree of saturation. We can incorporate such LMCs and fatty alcohols into materials through functionalization and subsequently polymerization. The functional handles and structural diversity in these biobased monomers provide means for adjusting properties to our needs. Thus, we are interested in de novo design of practical lignin- and fatty acid-based polymers, such as polycarbonate resins and block copolymer thermoplastic elastomers, to create next-generation plastics.
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.
Polymeric nanomaterial assemblies have several attractive features including tunability, control over the size and structure of the assemblies, and enhanced stability. Furthermore, the chemical versatility of polymers enables the incorporation of various stimuli-responsive moieties. Our group is interested in exploiting these valuable properties to develop polymer assemblies for novel shape-changing nanomaterials, which could be utilized in a broad range of applications.
We are currently focusing on biomedical aspects such as nucleic acid and small molecule drug delivery. Controlled release of these therapeutic molecules is widely recognized as one of the most significant challenges hindering clinical success. To this end, our lab has designed and synthesized novel photo-responsive block copolymers to bind and encapsulate nucleic acids and mediate efficient release upon application of the photo-stimulus. We are continuing to investigate the nanocarrier self-assembly process and developing methods to spatiotemporally control the release of the cargo. From a precision medicine standpoint, we are also working on the development of bio-responsive amphiphilic micelles for theranostic applications.