(538d) Gelation Behavior of Thermoreversible Diels–Alder Networks and Their Application for Reversible Adhesives

Thermoreversible networks using the Diels–Alder (DA) chemistry can switch between a network state and a segment state. When they are introduced into a thermoset polymer such as epoxies, the network state provides advantages of thermoset polymers, for example, strong mechanical properties and excellent resistance against chemicals and environmental factors. At an elevated temperature (~120°C), however, it turns into a segment state through the retro-DA reaction, which permits self-healing, recycle, and 3D printing processes. In this talk, gelation behavior of DA epoxies and their application for reversible adhesives will be presented. Study of network formation from precursors is an important fundamental topic, but it also has significance in the application aspect. The current 3D printing approaches for thermoset polymers require a long post-curing step (e.g., 6 – 48 hours) to reach the full mechanical strength. A long post-curing step is challenging because it increases the overall processing time and, moreover, it may change the final dimensions of a printed part from the original settings. Accelerating the formation of the DA network can decrease the processing time and reduce the needs of a post-curing step. The gelation behavior of DA networks was studied by rheometry with respect to precursors of various architecture, functionality, and molecular weight. A precursor containing high functionality was prepared and examined for its impact on gelation behavior. A small fraction (~5 wt%) of highly functional microspheres brought in an efficient acceleration of the formation of DA networks. Next, DA epoxy samples were examined for the debonding-on-demand adhesive. Adhesive strength of DA epoxy samples was measured by lap shear testing as a function of temperature. Its adhesive strength maintained at the same level from room temperature to 120°C but dropped greatly (~20 times) above 120°C because of a transition by the retro-DA reaction. DA epoxy composite samples containing plasmonic nanoparticles were examined to present debonding under light exposure. This photothermal conversion can offer instant debonding exclusively at target area. Adhesive strength of re-attached samples was also measured and compared to those from pristine samples.