Macroradical enables electrical conduction in epoxy thermoset

Abstract

A single material combining the unique properties of stable radical moieties with the versatility of the epoxy polymer is presented. Electrical conduction for any material is mostly achieved using metals, metal-organics, or organic conductors with extended π-orbital frameworks. However, in this combined material, intrinsic electronic conduction is enabled into the non-conjugated and amorphous epoxy thermoset. Furthermore, using the classical epoxy-amine curing ensures that only a highly crosslinked network with a rigid topology is formed. This design is a departure from other macroradicals with flexible backbones. The hole mobility of the neat macroradical epoxy thermoset is quantified to be ~3.1 × 10−6 cm2 V−1 s−1, already in the regime of traditional semiconductors. This electronic conduction can only be a result of radical-toradical hopping because the thermoset still retains active radicals after polymerization with a short alkyl amine when measured by electron spin resonance (EPR) spectroscopy. In the scientific literature, the synthesis of macroradicals using the direct approach is scarce. Herein, to obtain the novel radical epoxy monomer, a very mature yet simple amino-epoxide chemistry is employed to prepare a diglycidylamine based on 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl using the direct approach. EPR spectroscopy also reveals that the radical center remained stable even after epoxidation of the precursor. Consequently, this effort offers a change in thinking on how electronic conduction in open shell polymers operates especially on a rigid backbone and paves for a reassessment of the traditional epoxy polymer as enduring materials for frontier applications.