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A graphic from Dadmun's ChemSusChem publication depicting general PET depolymerization.

Dadmun Group Explores the Future of Plastic Recycling with Polymer Chemistry

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Professor Mark Dadmun and his research group recently published papers in both ChemSusChem and Nature Communications. These papers describe the group’s ongoing efforts to tackle more efficient plastic recycling through polymer chemistry.

Plastics, which are made of polymers, have been increasingly woven into modern life since the 1950’s. Single-use plastics originally gained popularity with consumers through a combination of convenience and affordability. However, the durability of plastics, a key feature that contributed to their success, has led to concerns over their persistence in the waste stream.  

Since the 1980’s, plastic recycling has been explored as a possible solution. Unfortunately, early recycling methods were ineffective with plastics, and modern recycling rates remain low at only around 9% globally.

Plastic recycling has been a long-term area of research for Dadmun, whose lab broadly focuses on investigating how the specific molecular structures of a polymer impact its properties. Understanding how to control these structures is critical to effectively recycling polymer materials, but when Dadmun sought to apply his earlier work to recycling, he was met with roadblocks.

“In the early 2000’s, the general sentiment was that we were never going to recycle polymers,” said Dadmun. “It’s only been in the last 10 years that perspectives have changed and the idea that we can and will develop new materials or new processes for plastics recycling has been supported.”

Dadmun’s group has used this shift to apply their research to the problems of modern chemical recycling. Chemical recycling directly manipulates the molecular structure of plastic polymers, breaking them down into their component parts and allowing them to be used to create new materials.

“Polymers are long-chain molecules, hundreds to thousands of individual units bonded together, and it’s that really big structure that gives them many of their properties,” said Dadmun. “Unzipping that long chain of molecules is straightforward but expensive, and this is the problem polymer scientists have been trying to solve.”

One approach is to break down all the individual units of the polymer and try to link them back together. However, this method can be inefficient as it requires the breaking and reforming of thousands of chemical bonds. Dadmun’s recent work investigates the possibility of breaking polymers down into larger groupings of units to be repolymerized, a process that should be more time and energy efficient.

The first of Dadmun’s recent publications focuses on understanding how a polymer breaks down into individual units. Shelby Watson-Sanders, a recent PhD graduate and member of Dadmun’s research group, co-authored the publication that describes this work.

“My interest in plastic waste initially sparked from a paper that addressed the release of estrogen chemicals from plastic when heated in the microwave. This revelation made me aware that plastic is ubiquitous and could pose health risks,” said Watson-Sanders. “In this work, we observed that our consumer waste plastic flakes broke apart into a powder and remained undissolved in solution until later in the reaction.”

Watson-Sanders went on to discover that some parts of the plastic broke down more easily, while others remained intact. These results suggested a possible explanation for the creation of micro and nanoplastics and led to the development of the second paper, published in Nature Communications. Watson-Sanders noted that the results of this work and the subsequent second publication prompted her to evolve her dissertation into an exploration of recycling plastic waste and the potential consequences of neglecting consumer waste.

In addition to Watson-Sanders, the team working on this research included undergraduate student Kendra Day. At this year’s Department of Chemistry Undergraduate Research Symposium, Day described this research in her award-winning presentation. She graduated this spring and will begin graduate studies at Cornell University in the fall. In the future, Dadmun and his team hope this ongoing work will contribute to more effective methods for polymer recycling. This could, in turn, contribute to increased plastic recycling and decreased plastic waste in the environment, as well as the development of new materials.

Polymer

UT Leads World in Polymer Science

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From your clothing to the fiber-optic cables bringing you high-speed internet, polymers are everywhere, with applications in nearly all fields of science and industry. Polymer science plays a crucial role in providing solutions to global needs including food, clean and abundant water, air, energy, and health.

Researchers at the University of Tennessee, Knoxville, in fields including chemistry, physics, chemical engineering, biosystems engineering, and forestry are investigating polymers through a variety of fundamental scientific problems with real-world impact—from designing and creating new advanced materials to improving industrial processes to creating sustainable biofuels.

As an indication of the significance of their work, UT has been ranked the top global university for polymer science in U.S. News and World Report’s Best Global Universities. The ranking is based on research performance from 2015 through 2019 as well as citations from publications through April 29, 2021.

Polymer science research within UT’s College of Arts and Sciences includes work being conducted by the research group of UT-ORNL Governor’s Chair for Polymer Science Alexei Sokolov. The team is advancing fundamental understanding and design of novel polymeric materials for various current and future technologies—from gas separations and carbon capture to 3D printing.

Sokolov’s group also works on polymer electrolytes for use in new generations of solid-state batteries and other energy storage technologies.

“We are working on polymers with dynamic bonds that are recyclable and have self-healing ability,” said Sokolov. “These polymers might replace current plastics and drastically reduce pollution.”

A segment of a bulky polymer chain, polynorbornene, showing elements of carbon (in black), oxygen (in red), silicon (in green), and hydrogen (in white). This unique polymer is one of several developed by Associate Professor Brian Long’s research group to study advanced gas separation membranes.

An illustration of a unique polymer chain, polynorbornene, that was developed by Associate Professor Brian Long’s research group to study advanced gas separation membranes.

Another area of focus, led by the research group of Associate Professor of Chemistry Brian Long, is creating new synthetic materials to separate greenhouse gases such as carbon dioxide from nonharmful gases in a more energy-efficient and cost-effective manner. This research has shown tremendous promise, with implications for reducing industrial greenhouse gas emissions.

“Think about what your body is touching right now—your clothing, your chair, your phone or computer. What are you touching that’s not a polymer or that doesn’t contain polymers? Polymers have provided solutions to almost every societal need in modern human history—even the DNA, RNA, and proteins in our body are polymers,” said Long.

Researchers at UT are even tackling one of the most pressing global needs today—how to minimize or eliminate waste plastics in the environment. For example, research efforts led by Professor Mark Dadmun and Assistant Professor Johnathan Brantley seek to develop new chemical methods to aid recycling of waste plastics, improve the properties of new products and materials made from mixed plastic waste streams, and create a circular plastics economy.

Commenting on the announcement of the ranking, Vice Chancellor for Research Deborah Crawford said, “Our researchers deserve this recognition for their work advancing our understanding of polymers and how they can contribute to making life and lives better. At UT, our commitment is to contribute to the creation of a more just, prosperous, and sustainable future through world-class research and scholarship. Our polymer scientists and engineers are doing just that!”

About the ranking

The polymer science ranking is determined by 10 indicators, including the impact of citations and research publications. Impact is calculated based on data from the Clarivate Web of Science, a web-based research platform. The Web of Science is a web-based research platform that covers more than 21,100 of the most influential and authoritative scholarly journals worldwide in the sciences, social sciences, and arts and humanities.

Brantley Group Published in JACS

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The Brantley group recently published a paper in JACS entitled “Electroediting of Soft Polymer Backbones” Alan Fried, Breana Wilson, and Nick Galan contributed to the research, under the supervision of Johnathan Brantley.

The paper discusses new methodology for degradation and functionalization of olefin-containing polymers through electrochemistry. This method can be carried out in both homogeneous and heterogeneous systems, in addition to using mild conditions and being experimentally simple.

The work was completed in memory of Alan Fried.

Long Group Published in Chem. Eur. J.

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The Long Group published their research “Mechanochemical Formation, Solution Rearrangements, and Catalytic Behavior of a Polymorphic Ca/K Allyl Complex” in Chemistry—A European Journal. Authors Brian Long and Alicia Doerr, graduate student, collaborated with Vanderbilt University and the University of Rochester. 

Without solvents present, the often far‐from‐equilibrium environment in a mechanochemically driven synthesis can generate high‐energy, non‐stoichiometric products not observed from the same ratio of reagents used in solution. Ball milling 2 equiv. K[A´] (A´ = [1,3‐(SiMe3)2C3 H3]– ) with CaI2  yields a non‐stoichiometric calciate, K[CaA´3], which initially forms a structure (1) likely containing a mixture of pi‐ and sigma‐bound allyl ligands. Dissolved in arenes, the compound rearranges over the course of several days to a structure (2) with only  η3‐bound allyl ligands, and that can be crystallized as a coordination polymer. If dissolved in alkanes, however, the rearrangement of 1 to 2 occurs within minutes. The structures of 1 and 2 have been modeled with DFT calculations, and 2 initiates the anionic polymerization of methyl methacrylate and isoprene; for the latter, under the mildest conditions yet reported for a heavy Group 2 species (one‐atm pressure and room temperature).

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Sokolov Group Published in Energy Storage Mater. and ACS Macro Lett.

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The Sokolov Group recently published their work “Anomalously high elastic modulus of a poly(ethylene oxide)-based composite electrolyte” in Energy Storage Materials.

The practical use of lithium metal anodes in solid-state batteries requires a polymer membrane with high lithium-ion conductivity, thermal/electrochemical stability, and mechanical strength. The primary challenge is to effectively decouple the ionic conductivity and mechanical strength of the polymer electrolytes.

They report a remarkably facile single step synthetic strategy based on in-situ crosslinking of poly(ethylene oxide) (xPEO) in the presence of a woven glass fiber (GF). Such a simple method yields composite polymer electrolytes (CPE) of anomalously high elastic modulus up to 2.5 GPa over a broad temperature range (20 °C – 245 °C) that has never been previously documented.

An unsupervised machine learning algorithm, K-mean clustering analysis, was implemented on the hyperspectral Raman mapping at the xPEO/GF interface. Using such a unique means, we show for the first time that the promoted mechanical strength originates from xPEO and GF interactions through dynamic hydrogen and ionic bonding. High ionic conductivity is achieved by the addition plasticizer (e.g. tetraglyme), where trifluoromethanesulfonate anions are tethered to the xPEO matrix and Li+ cations are favorably transported through coordination with the plasticizer.

Further, stringent galvanostatic cycling tests indicates the CPE can be stably cycled for >3000 h in a Li-metal symmetric cell at a moderate temperature (nearly 1500 Coulombs/cm2 Li equivalents), outperforming most of the PEO-based electrolytes. The GF reinforced CPE reported here has multifunctional uses, such as solid electrolytes for all solid-state batteries and membranes for redox-flow batteries.

Although the focus of this study is on lithium-based batteries, the results are equally promising for other alkali metal based batteries such as sodium and potassium.

The Sokolov Group also had their work “Turning Rubber into a Glass: Mechanical Reinforcement by Microphase Separation” published in ACS Macro Letters.

Supramolecular associations provide a promising route to functional materials with properties such as self-healing, easy recyclability or extraordinary mechanical strength and toughness. The latter benefit especially from the transient character of the formed network, which enables dissipation of energy as well as regeneration of the internal structures. However, recent investigations revealed intrinsic limitations in the achievable mechanical enhancement.

This manuscript presents studies of a set of telechelic polymers with hydrogen-bonding chain ends exhibiting an extraordinarily high, almost glass-like, rubbery plateau. This is ascribed to the segregation of the associative ends into clusters and formation of an interfacial layer surrounding these clusters. An approach adopted from the field of polymer nanocomposites provides a quantitative description of the data and reveals the strongly altered mechanical properties of the polymer in the interfacial layer. These results demonstrate how employing phase separating dynamic bonds can lead to the creation of high-performance materials.

Dadmun Published in Chemical Reviews

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Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive “designer” solvents exhibiting a host of tunable physicochemical properties.

Mark Dadmun, professor and ORNL-UT joint faculty member, contributed to this collaborative piece “Deep Eutectic Solvents: A Review of Fundamentals and Applications” in Chemical Reviews.

A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure–property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments.

This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Zhao Receives Excellence in Research Award

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Each year, Dean Theresa Lee and members of her cabinet, with help from department heads, recognize faculty in the College of Arts and Sciences for their excellence in teaching, research and creative activity, and lifetime achievements.

Due to the ongoing pandemic, however, we were unable to host the annual awards banquet in-person. Each faculty member received a plaque and congratulations from the dean. We posted a video to the college YouTube channel here, which features each faculty award winner.

We seek to recognize faculty members who excel in scholarship and creative activity while also being fully engaged in the other responsibilities of faculty jobs, primarily teaching and service. To this end, the college honors faculty in three stages of their research careers – early, mid, and senior – with awards for excellence in research or creative achievement, as well as honoring a faculty with an award for Distinguished Research Career at UT.

Bin ZhaoBin Zhao is the Paul and Wilma Ziegler Professor in the Department of Chemistry, received a senior career excellence in research award. He has made significant contributions to the field of macromolecular brush materials, from precise synthesis to fundamental understanding and potential applications of surface brushes, polymer brush-grafted particles (hairy particles), and brush polymers. He is widely recognized, nationally and internationally, as one of the leading figures in this field. His work on stimuli-responsive polymers has also received wide attention. Zhao is a dedicated research mentor who seeks to use research opportunities to cultivate scientific reasoning and spirit in his graduate and undergraduate students.

“I am very excited to receive this great honor and recognition from our college” Zhao said. “I look forward to continuing contributing to the research mission of our university in the years ahead.”

Brantley Group Published in ACS Macro Letters

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The Brantley Group published their work “General Access to Allene-Containing Polymers Using the Skattebøl Rearrangement” in ACS Macro Letters. Primary author is Nick Galan, graduate student in the Brantley Group.

Postsynthetic modification is a powerful strategy for tuning soft materials. While methods for side-chain functionalization abound, modifications of backbone structural elements can be difficult to achieve. This challenge arises, in part, from a lack of intrinsically reactive motifs that can be installed in the main chain of a polymer. Incorporating established synthetic handles into polymer architectures is paramount for overcoming this limitation.

Allenes are salient examples of moieties that could be leveraged in a wide range of postsynthetic modifications; however, the synthesis of a polyallene has proven elusive. Using the metathesis polymer of norbornene as a model architecture, the Brantley Group have established the Skattebøl rearrangement as a facile route to polyallenes. Polymers with varying allene content (20–95%) were readily prepared in excellent yields (89–94%). These materials possess unique optical properties and can be engaged through further postsynthetic modifications. As such, polyallenes could serve as valuable platforms for developing functional soft materials.

Nicholas Galan
Nick Galan

“Our results suggest that installing allenes within soft materials could open underexplored chemical space for polymer design and modification,” Galan said. “As such, we expect that our work will serve as a general platform for developing new types of tunable or stimulus-responsive materials.”

Brantley Group Published in Macromolecules

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The Brantley Group published their work “Exploring Combinatorial Approaches to Polymer Diversification” in Macromolecules. 

Diversity-oriented strategies can facilitate the rapid exploration of chemical space during small-molecule synthesis, but similar approaches are underutilized for macromolecular substrates. Expanding the repertoire of soft material manipulations to accommodate iterative diversifications could enable the design of bespoke polymers with a range of novel structures and properties.

“To explore this concept, we chose to leverage the efficiency of Suzuki–Miyaura cross-coupling to rapidly access an array of functionalized polystyrene surrogates from a readily accessible polystyrene-p-pinacol boronic ester,” Brain Jacobs, postdoc, said. “A variety of C(sp2) electrophiles efficiently coupled with our model polymer (51–99% functionalization) in moderate-to-good yields (44–96%). “

Optimized coupling reactions afforded products with minimal changes in overall dispersity (as determined by gel permeation chromatography), which suggested that the desired coupling occurred with good fidelity. “Select products were subjected to further modifications (e.g., Wittig olefination, reduction, imine condensation) to showcase the diverse array of reactivities that can be accessed using our strategy.” Jacobs said. 

Two Journal Covers from the Sokolov Group

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The Sokolov Group’s primary focus of research on dynamics of soft materials, including dynamics of biological macromolecules, and nano-composite materials.

Their recent publication “Correlation between the temperature evolution of the interfacial region and the growing dynamic cooperativity length scale” was featured on cover of The Journal of Chemical Physics. They presented an analyses that revealed a clear correlation between the temperature dependence of the characteristic relaxation time, ln(τα(T)/τ0), and the interfacial layer thickness, Lint(T), in nanocomposite materials.

The group also published their work “Perspectives for Polymer Electrolytes: A View from Fundamentals of Ionic Conductivity” which was on the cover of Macromolecules. This research analyzes fundamental mechanisms controlling ionic conductivity and suggests design of novel polymer electrolytes with enhanced conductivity.