Polymeric materials can generally be divided into two categories: (a) thermoplastics, materials such as plastic bottles, grocery bags, and nylon, that soften with heat and can be remolded and recycled; and (b) thermosets, those such as tires, epoxy adhesives, and resins, that have a permanent shape and are very difficult to reprocess. Thermosets thus have limited potential for reuse, ultimately having a negative impact on the environment through resource consumption and accumulation of large amounts of waste material. At the molecular level, the difference between these two classes of materials arises from the fact that thermoplastics consist of discrete polymer chains that can move and flow when heated, whereas thermosets are built from a crosslinked molecular network that is permanently bonded together. To bridge the gap between these materials, recent efforts have focused on making their fixed networks reversible -- that is, enabling them to de-crosslink under certain conditions followed by reformation of the network, allowing thermosets to be remolded and reprocessed. These systems represent a new class of materials known as dynamic polymer networks. While dynamic polymer networks have been the focus of considerable research during the last two decades, many fundamental questions about them remain, detracting from their practical utility and restricting their applications beyond the research lab. This research will leverage the unique characteristics of a novel chemical reaction aimed at de-crosslinking thermosets in order to gain generalized insight into the fundamental principles of dynamic polymer networks. Successful realization of the PI’s research goals will yield a wide variety of reprocessable thermosets and link specific molecular-level characteristics with macroscopic properties, helping to shape future designs and applications of these advanced materials in areas as varied as sustainable manufacturing, advanced medicine, and next-generation batteries. Students participating in this research will be trained in an interdisciplinary manner involving synthetic chemistry, polymer characterization, and a variety of techniques for property determination.
PART 2: TECHNICAL SUMMARY
Dynamic polymer networks are covalently crosslinked materials that combine the desirable properties of thermosets with the reprocessability of thermoplastics. Their crosslinks undergo reversible exchange reactions that are activated under specific conditions, continuously reorganizing the network. Recently the PI demonstrated a new type of these materials based on previously unknown exchange reactions of guanidines. This proposal seeks to leverage the unique capabilities of dynamic guanidine-based polymer networks to answer fundamental questions in the field of dynamic materials and expand the window of achievable properties of these systems. Specific objectives of this proposal are to: (1) use the versatility of the guanidine functionality to both modify its reactivity and introduce noncovalent bonding partners, establishing the link between fundamental molecular-scale properties and macroscopic rheological behavior; and (2) incorporate guanidine crosslinks into a variety of precisely altered polymer structures to elucidate the role of specific characteristics of the “background†polymer in determining the behavior of dynamic networks. These studies will be accomplished by student researchers using a combination of synthetic techniques, polymer characterization, rheological analysis, and dielectric spectroscopy. .
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
