Structure–Property Relationships of Elastomeric Vinylogous Urethane Thermosets and Their Application as Closed-Loop Recyclable Strain Sensors

Developing closed-loop recyclable thermosets and understanding their structure-property relationships are essential steps in advancing a circular materials economy. Here, we present a vinylogous urethane (<b>VU</b>) thermoset with closed-loop recyclability, synthesized through the reaction of polytetrahydrofuran bisacetoacetate (<b>aPTHF</b>) and tris(2-aminoethyl)amine (<b>TREN</b>). These <b>VU</b> polymers exhibit high elasticity, with only a 3-9% residual strain observed after cyclic tensile testing at a maximum strain of 100%, depending on the molecular weight of <b>aPTHF</b> and network cross-link density. The two structural parameters also allow modulation of the mechanical and stress-relaxation properties of <b>VU</b> elastomers. To investigate the hydrolysis of the <b>VU</b> linkages within the hydrophobic <b>aPTHF</b> matrix, we employed a heterogeneous system using a biphasic mixture of HCl and CDCl₃. Our findings show that the hydrophobic <b>VU</b>s remain stable in pure water but can be dissociated under acidic conditions, with the dissociation rate accelerated at higher temperatures and/or in the presence of higher HCl concentrations. These detailed investigations indicate the potential of <b>VU</b> elastomers as sustainable substrates for wearable sensors. We therefore conduct a case study of synthesizing a strain sensor through the incorporation of multiwalled carbon nanotubes (<b>MCN</b>s) into the <b>VU</b> elastomer matrix. The sensor can robustly detect various movements. Moreover, acidic treatment of both the neat polymer and the sensor composite using a HCl and diethyl ether solvent mixture allows for the excellent recovery of <b>aPTHF</b> (>90%) and <b>TREN</b> (86%), without discernible damage to the <b>MCN</b>s reclaimed from the latter.