Crosslinking of Epoxidized Natural Rubber with Borax for Self-Healing and Self-Repairing Properties: pH Dependence.

Epoxidized natural rubber (ENR) crosslinked using borax, which exhibits self-healing and self-repairing properties, is successfully developed. The crosslink formation of ENR by using borax under neutral and alkaline conditions is investigated. Fourier transform infrared spectroscopy (FTIR) shows that the borate-ester bond is formed in ENR prepared under both neutral and alkaline conditions, whereas boron nuclear magnetic resonance (11 B-NMR) results exhibit that the ENR prepared under alkaline conditions more actively forms crosslink networks with borax. Moreover, the crosslink density and gel content increase significantly with the presence of borax in alkaline conditions. The crosslink density and gel content of ENR with 10 phr borax are higher by 155% and 36%, respectively, than those of neat ENR. Furthermore, the formation of the crosslinking ENR by borax enhances self-healing and self-repairing properties. The healing efficiency significantly increases from 1.09% to 85.90%, when ENR is developed under alkaline conditions with 30 phr borax. These results represent the first successful demonstration of the efficient use of borax as a crosslinker in ENR, which exhibits its promising self-healing and self-repairing properties under atmospheric conditions without the need for external stimuli. The ENR prepared in this work holds great promise for various self-healing rubber applications.

Highly stretchable and robust transparent conductive polymer composites for multifunctional healthcare monitoring.

Soft, stretchable, conductive thin films have propelled to the forefront of applications in stretchable sensors for on-skin health monitoring. Stretchable conductive films require high conformability, stretchability, and mechanical/chemical stability when integrated into the skin. Here, we present a highly stretchable, conductive, and transparent natural rubber/silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film. Overcoating the PEDOT:PSS layer results in outstanding mechanical robustness and chemical stability by suppressing the mechanical and chemical degradation of the nanowire networks. Moreover, the introduction of the organic surface modifier enhances the bonding strength between the natural rubber substrate and AgNW at the interface. The highly conformable composite films are integrated into multifunctional on-skin sensors for monitoring various human motions and biological signals with low-power consumption. We believe that the highly stretchable, robust, and conformable natural rubber/AgNW/PEDOT:PSS composite film can offer new opportunities for next-generation wearable sensors for body motion and physiological monitoring.