Controlled Degradation Of Epoxy-amine Thermosets Based On Diels- Alder Chemistry

Epoxy thermosets are a popular and widely-used commercial polymer because they have outstanding chemical- and thermal-resistance as well as mechanical and insulating properties due to their irreversible, permanent three-dimensional (3-D) cross-linked network from the curing reaction of the epoxy resin with the hardener. However, most epoxy thermosets are not malleable and insoluble. This makes reprocessing and reuse impossible. Therefore, most epoxy thermosets can only be used once. If they could be re-used multiple times, their service life would be prolonged resulting in decreased carbon emissions and a reduction in organic waste. In this work, we design and fabricate two epoxy-amine thermosets that could be easily recylced and reused based on Diels-Alder (DA) chemistry, including:1, Soluble epoxy thermoset at room temperature. An epoxy-amine thermoset embedded with Diels-Alder (DA) bonds can be transformed into soluble polymers via sonochemistry under mild temperature (ca.20℃). Sonication can effectively induce the position-oriented cleavage of DA bonds (i.e.:retro-DA) of the fully swelled epoxy thermoset in dimethyl sulfoxide (DMSO), leading to the soluble polymers. The yield is up to 85%, and the Mn and PDI of soluble polymer are 48.3 kg/mol and 1.7 respectively. Of importance, such sonochemical process can be regulated on demand via switching on-and-off of the sonication. The obtained soluble polymers can be re-cured to form epoxy-amine thermosets via DA reaction. This sonochemical method might provide an unprecedented and efficient way to the controlled degradation and recycling of the epoxy thermosets containing the dynamic covalent bonds likes DA groups, and realize position-oriented degradation of a thermoset via polymer mechanochemistry.2, Malleable epoxy thermoset at high temperature. A malleable epoxy thermoset that can be forged at 130-150℃ via network topology transformation without adding any additional catalysts or plasticizers. This malleable epoxy thermoset is formed during the curing reaction of a commercially available epoxy resin (diglycidyl ether of bisphenol A, DGEBA) and a purposely designed diamine, namely 2-(6-aminohexyl)-4-(aminomethyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-1,3 (2H)-dione via a [4+2] Diels-Alder reaction. Upon heating at 130-150℃, the three-dimensional (3-D) cross-linked network of the resulting epoxy thermoset is quickly transformed into a one-dimensional (1-D) linear polymer via a retro-Diels-Alder reaction (r-DA). The 3-D cross-linked network could be regenerated via DA reaction at 75℃ for at least 24 h. The recovered thermoset presented a 13% reduction in storage modulus at 50℃ versus the pristine thermoset. This is a novel approach to malleable and recyclable epoxy thermosets.In summary, based on the DA chemistry, two methods are developed for recycling and reusing of the epoxy-based thermosets through reversible 3D/1D topology transformation in solution by force or in bulk by heating. In both protocols, the epoxy thermosets degrade in a controlled manner without using additional catalysts or additives, and no small molecular degradation product was formed.