Current research projects include:
A rapidly growing area of materials research involves the surface-grafting of tethered layers that respond to external stimuli, such as pH, temperature, and light. Grafting stimuli-responsive polymer layers to a surface creates materials with the ability to sense and respond to environmental conditions. We use a variety of chemical synthesis techniques to create polymers that can carry positive and negative charges, are amphiphilic, or have polyelectrolyte blocks with tertiary amine pendant groups. The in-situ thickness change of the tethered polymer layer or polymer particle diameter is then measured as a function of environmental pH.
Who work on the project:
- David Chem (Ph.D. candidate)
- Steven Vong (Undergraduate student)
We have two primary research interests related to the development of microsensors: (i) synthesis of polymers with selective bioaffinities and (ii) control of fluid flow in micro (and/or nano) channels via grafting of responsive polymers. (i) We are working to develop polymers that incorporate functional groups that will allow for selective adsorption of specific analytes (e.g., enzymes, proteins, toxins). Potential applications include medical diagnostic platforms, biohazard and environmental remediation sensors, and separation devices. (ii) Grafting of responsive polymer layers that can reversibly expand and contract inside micro- and nano-channels would provide the ability to control fluid transport in microfluidic devices. While our target substrates are polymeric, systems are being built using both polymeric and inorganic substrates and flow in the modified channels will be measured as a function of stimuli input.
Our interest lies in utilizing biomass to produce renewable polymers, also known as bioplastics. The term bioplastics is applied to polymers which are derived from agricultural and bacterial by-products and provide a sustainable alternative to petrochemical derived polymers. Depending on the monomer type and content, polymers can be produced with a wide range of crystallinities, glass transition temperatures, and melting temperatures which can be used in the production of both fibers and films. Bioplastics have a significant advantage over conventional plastics because in addition to providing a sustainable alternative to petroleum-based polymers, the bioplastic polymers we have targeted have the potential to be fully biodegradable.
Surface property characterization and deposition of drug molecules and particulate matter in airways is critical to understanding the delivery and absorption of desired (therapeutics) or undesired (contaminants) materials during inhalation. Inhaled therapeutics can be targeted for local delivery to the lungs or transportation across the endothelium into the circulatory system, but have been found to be inefficient and decreases further in patients with obstructive lung diseases such as asthma or COPD. In direct inhalation therapy as well as other transmucosal delivery methods (nasal and oral uptake via spray, gel, tablets, and lozenges), effective mucoadhesion of the drug formulation is key. By investigating the surface energetics of lung tissues, the wetting and adhesion properties can be modeled to determine the optimal chemical properties necessary in newly developed therapeutics in order to increase absorption. Models of wetting and adhesion interactions between the drug suspension and the lung tissues can be incorporated into computational simulations of the flow and may lead to better prediction of the distribution and absorption of inhalation therapeutics.
The Walters research group has experience in the surface-modification of nano- and micro-meter sized particles with grafted polymer brushes. Nanoparticles (NPs) formed from a polymer or metal cores decorated with a polymer corona can be used as a multi-functional nanomedicine to combine imaging, hyperthermia cellular treatments, transport within the body, and drug delivery in a single carrier vehicle. The polymer corona decorating the core can be used to control particle distribution, deliver medicines in a controlled or site-specific manner, increase biocompatibility and promote cellular uptake.
The growth of new bacteria and antibiotic resistance is a common problem in modern society. The development of new antibacterial agents is essential to overcome the upcoming challenges of new bacteria and antibacterial resistance. To synthesize antibacterial macromolecules through ROMP, we correctly tune cationic charge density and hydrophobicity to combat bacteria. The hydrophobic site of the polymer helps bind with the bacteria’s phospholipid membrane and a positively charged surface target to penetrate negatively changed bacterial cells. These characteristics of our synthesized polymer are achieved either by unique monomer synthesis followed by polymers or surface modification of the synthesized polymers. Research article publication: https://onlinelibrary.wiley.com/doi/10.1002/pol.20190194
Our group develops polyelectrolytes—both weak and strong—for a variety of applications ranging from “smart” stimuli responsive systems to solid electrolytes in electrochemical applications. One particular subclass our group synthesizes are poly(ionic liquid)s (PILs) formed from polymerizable ionic liquid monomers. These PILs can be swelled with free ionic liquids to form ionogels with excellent thermal properties, high ion conductivity, and long-term stability due to the nonvolatility and ionic nature of the ionic liquids. Our group is developing solid and pseudo-solid PIL-based ionogel polyelectrolytes for lithium and sodium ion batteries and for ionic soft actuator systems. In particular, we are exploring ways to optimize the electrochemical and mechanical properties of the polyelectrolytes for application performance.
Who work on the project:
- Dr. Kayla Foley (Post doctoral fellow)
- David Chem (Ph.D. candidate)
- Steven Cockmon (Undergraduate student)
- Alfredo Carrillo (Undergraduate student)
Poly(ionic liquid)s, strong polyelectrolytes formed from polymerizable ionic liquids, can be complexed with paramagnetic transition metal salts (e.g., Co2+, Fe3+) to form magnetically responsive polymers known as magnetic poly(ionic liquid)s (MPIL). These materials show potential interest in several fields including Lewis acid catalysis, in situ magnetic nanoparticle synthesis templates, macromolecular magnets, and magnetic stimuli response. To facilitate expansion of MPIL applications, our group takes a deeper fundamental study of the binding of the paramagnetic species with the PIL polymer and its impact on the magnetic and polymer properties, particularly in multicomponent (e.g., copolymers, blends). We also examine the impact of PIL self-assembled structure on the magnetic properties and stimuli responsive behavior.
Figure X. Magnetic susceptibilities of a magnetic poly(ionic liquid) (MPILs) copolymerized with polyacrylamide as a function transition metal salt concentration. Insert shows the magnetic attraction of the cobalt-based MPILs. The figure is reproduced from Foley et al. (Hyperlink)
Research article publications:
https://pubs.rsc.org/en/content/articlehtml/2023/me/d3me00076a https://asmedigitalcollection.asme.org/FEDSM/proceedings/FEDSM2022/85840/V002T06A006/1147156
People who work on the project:
- Dr. Kayla Foley (Post doctoral fellow)
- Steven Cockmon (Undergraduate student)
- Alfredo Carrillo (Undergraduate student)
The growing demand for bio-based and renewable polymer materials has motivated many valorization studies of cheap but underutilized biodegradable raw materials such as lignin. Our group is interested in modifying lignin by reduced functionalization methods such as esterification, phenolation, and amination to produce a less bulky and more linear lignin polymer. Modified lignin can be used in water treatment applications such as dye and heavy metal ions removal. Moreover, we aim to develop copolymer materials solely from lignin and different types of polymers, including petroleum-based polymers, stimuli-responsive polymers, and poly(ionic) liquid via various polymerization techniques — yielding copolymers that are biodegradable with good and comparable thermal and mechanical properties. The copolymers are suitable for applications ranging from sustainable material for water remediation to biodegradable fillers in thermoplastic production for food, medical packaging, and novel materials for safe energy devices.
Who work on the project:
- David Chem (Ph.D. candidate)
- Steven Vong (Undergraduate student)
The major goal is the development of an adsorbent with numerous adsorption sites, good biocompatibility, and functionalization ability to enhance adsorption-desorption kinetics, with a large surface area suitable for the adsorption of both organic and inorganic pollutants in an aqueous solution. Nanoparticles are synthesized, and specific functionalization modifications are anchored on nanoparticles for advanced separation to capture different pollutants or undesired chemical species that can be safely disposed of or reused in other applications. Also, surface modification via self-assembled monolayers and polymer brushes allows for tailoring surface properties for a wide range of applications in water remediation whilst developing and enhancing grafting strategies to facilitate robust, reproducible, and novel polymer-grafted NP synthesis routes.
Who work on the project:
- Olufemi Ogunjimi (Ph.D. student)
- Benjamin Hopkins (Undergraduate student)
- John Verdaris (Undergraduate student)
Our research group is pioneering a new approach to exhibit the potential of lignin, a valuable byproduct of the paper and pulp industry. Instead of the traditional trial-and-error process, we harness the power of machine learning, specifically a recurrent neural network (RNN), to predict the properties of lignin-based copolymers. By merging machine learning with material development, lignin-based units can be synthesized through innovative modification techniques, which allows the prediction of complex copolymer properties, including those derived from lignin. Through the synthesis of modified lignin units, the development of a PyTorch RNN model, and the exploration of the PolyInfo database, we have paved the way for more efficient copolymer development. Also, our research group employs advanced computational tools such as UCSF Chimera and IQmol for precise molecular editing, visualization, and molecular dynamics simulations to analyze the electrostatic, hydrophobic, and metal-coordinating interactions of poly(ionic liquid) (PILs). The goal of these projects was to investigate the properties of lignin-based copolymers and PILs, ultimately advancing our understanding of these materials to broader polymer development and machine learning integration for environmental and materials science applications.
Among different electromagnetic-based energy harvesting materials and devices, the ferrofluidic induction method is one of the techniques explored by Walter’s research group. This process leverages the pulsation of ferrofluids (iron-based nanofluids) within solenoids to generate electrical voltage. In this project, we develop an analytical model and utilize computational modeling techniques to understand the behavior of ferrofluid pulsation within energy harvesting devices. Furthermore, we examine the heat and mass transfer performance of two-phase closed thermosyphons (TPCTs) and oscillating heat pipes (OHPs). The analytical model is used to predict the electromotive force (EMF) induced in these devices based on the behavior of pulsating ferrofluids and their magnetic characteristics. This model is validated using experimental data, demonstrating that higher pulsation frequencies and stronger magnetic fields lead to increased induced EMF as more ferrofluid nanoparticles align. Computational fluid dynamics (CFD) simulations are then employed to explore the heat transfer capabilities of closed thermosyphons and oscillating heat pipes. Simulations were performed using a three-dimensional finite-volume flow solver (ANSYS Fluent) to ensure both precision and computational efficiency. Essentially, this research work develops comprehensive simulation methods for analyzing and understanding multiphase flow, heat transfer, and energy harvesting in ferrofluidic systems with implications for predictive modeling and various applications such as electronics cooling, waste heat recovery, and renewable energy generation. This project provides practical guidelines for designing efficient ferro-nanofluids and advancing next-generation technologies.
Other projects of interest include:
- micro- and nano-particle technologies:
- sensors
- separations
- janus nanoparticles
- drug delivery
- heat transfer applications
- synthesis and surface modification of bioplastics and magnetic nanoparticles
- pyrolysis oil characterization, aging, and filtration studies