Controlling Dynamics in Reprocessable Polymer Networks via Mixed-Mechanism Exchangeable Bonds

Polymers find wide-spread use due to their scalability, cost-effectiveness, and widely tunable mechanical properties. Crosslinked polymer networks have been used for the development of biomaterials, adhesives, and surface coatings. These materials offer high mechanical strength, temperature stability, and solvent resistance. While such features make them ideal for applications ranging from adhesives to building materials, they are generally unable to be recycled or reprocessed. To mitigate this problem, we propose to use dynamic covalent bonds (DCBs) as a tool for reprocesability. The main challenge in using DCBs for crosslinked networks is the uncontrolled flow of the material. To address this, we developed a dual dynamic polymeric system using dissociative and associative bonds to control the onset and rate of flow of dynamic covalent networks.  First, we synthesize the polymer network by Thiol-Michael (TM) addition, which was facilitated by the design of an “associative” monomer containing dissociative end group functionalities. This synthetic approach allows us to (1) incorporate thioester units into the polymer network, which will be used to promote the reprocessing of the material by Thiol-Thioester exchange mechanism (2) control the rate of this exchange reaction by thermally modulating the TM connectivity. Second, we studied the material using spectroscopic and rheological techniques to understand its dynamic action. Utilizing “real-time” FTIR measurements, we determined a scalable relationship between temperature and fractions of free ends upon heating. Mechanical tests using rheological methods show the same effect by observing a decrease in the crosslink density of the material upon temperature trigger.  For our future studies, bond lifetime and relaxation tests will be conducted using rheological techniques to determine the effect of the combinations of these dynamic bonds. With this approach, we will tackle (1) the reprocessability of conventional polymer networks and (2) control the onset and rate of flow of the material by combining multiple dynamic chemistries into one crosslinked network.