Tailoring Electromechanical Properties of Natural Rubber Vitrimers by Cross-Linkers

The growing demand for smart polymeric transducers such as dielectric elastomer actuators and energy harvesters has urged the use of sustainable and recyclable elastomeric materials. Vitrimer chemistry has shed light on future reprocessable and recyclable thermosets and elastomers. In this work, epoxidized natural rubber (ENR) vitrimers were prepared using diacid or triacid cross-linkers and formed covalently cross-linking networks via thermally triggered reversible beta-hydroxy ester bonds. The cross-linked ENR elastomers exhibited Arrhenius-type viscoelastic behavior with a complete stress relaxation between 140 and 160 degrees C, that is, vitrimer characteristics, which were highly dependent on the cross-linking temperature. The mechanical and dielectric properties of the ENR vitrimers can be tuned by varying the molecular structure and concentration of the cross-linkers. Among the diacid and triacid cross-linkers, Pripol 1017 fatty polyacid (P1017) and 3,3'-dithiopropionic acid (DTPA) had similar effects on the cross-linking density and mechanical properties of the ENR vitrimers. The highest tensile strength of 8.70 +/- 1.9 or 15.6 +/- 2.6 MPa was obtained at 6 mol % of P1017 or DTPA, respectively. While for diamide-based diacid cross-linker (DME), 8 mol % was needed to reach the highest tensile strength of 13.1 +/- 2.7 MPa for the elastomer. The three ENR vitrimers showed increased relative permittivity epsilon' = 5 similar to 7 at 1 kHz while maintaining low dielectric losses compared to traditional dicumyl peroxide-cured ENR, with epsilon' = 3.57 at 1 kHz. With the optimized acidic cross-linker concentrations of P1017 at 6 mol 96, DTPA at 6 mol %, and DME at 8 mol 96, the ENR vitrimers exhibited improved actuation capabilities at lower electrical fields. Utilizing dynamic cross-linkers to tune the electromechanical properties of dielectric elastomers and the reversibly cross-linked polymer networks will open new opportunities for smart and sustainable dielectric elastomer devices.