ELECTRICAL AND DIELECTRIC PROPERTIES OF RUBBER

Designing Supertough and Ultrastretchable Liquid Metal-Embedded Natural Rubber Composites for Soft-Matter Engineering

Functional elastomers with incredible toughness and stretchability are indispensable for applications in soft robotics and wearable electronics. Furthermore, coupled with excellent electrical and thermal properties, these materials are at the forefront of recent efforts toward widespread use in cutting-edge electronics and devices. Herein, we introduce a highly deformable eutectic-GaIn liquid metal alloy-embedded natural rubber (NR) architecture employing, for the first time, industrially viable solid-state mixing and vulcanization. Standard methods of rubber processing and vulcanization allow us to fragment and disperse liquid metals into submicron-sized droplets in cross-linked NR without compromising the elastic properties of the base matrix. In addition to substantial boosts in mechanical (strain at failure of up to ∼650%) and elastic (negligible hysteresis loss) performances, the tearing energy of the composite was enhanced up to 6 times, and a fourfold reduction in the crack growth rate was achieved over a control vulcanizate. Moreover, we demonstrate improved thermal conductivity and dielectric properties for the resulting composites. Therefore, this work provides a facile and scalable pathway to develop liquid metal-embedded soft elastomeric composites that could be instrumental toward potential applications in soft-matter engineering.

Sulfur-Modified Carbon Nanotubes for the Development of Advanced Elastomeric Materials

The outstanding properties of carbon nanotubes (CNTs) present some limitations when introduced into rubber matrices, especially when these nano-particles are applied in high-performance tire tread compounds. Their tendency to agglomerate into bundles due to van der Waals interactions, the strong influence of CNT on the vulcanization process, and the adsorptive nature of filler-rubber interactions contribute to increase the energy dissipation phenomena on rubber-CNT compounds. Consequently, their expected performance in terms of rolling resistance is limited. To overcome these three important issues, the CNT have been surface-modified with oxygen-bearing groups and sulfur, resulting in an improvement in the key properties of these rubber compounds for their use in tire tread applications. A deep characterization of these new materials using functionalized CNT as filler was carried out by using a combination of mechanical, equilibrium swelling and low-field NMR experiments. The outcome of this research revealed that the formation of covalent bonds between the rubber matrix and the nano-particles by the introduction of sulfur at the CNT surface has positive effects on the viscoelastic behavior and the network structure of the rubber compounds, by a decrease of both the loss factor at 60 °C (rolling resistance) and the non-elastic defects, while increasing the crosslink density of the new compounds.

Poisson ratio mismatch drives low-strain reinforcement in elastomeric nanocomposites

Introduction of nanoparticulate additives can dramatically impact elastomer mechanical response, with large enhancements in modulus, toughness, and strength. Despite the societal importance of these effects, their mechanistic origin remains unsettled. Here, using a combination of theory and molecular dynamics simulation, we show that low-strain extensional reinforcement of elastomers is driven by a nanoparticulate-jamming-induced suppression in the composite Poisson ratio. This suppression forces an increase in rubber volume with extensional deformation, effectively converting a portion of the rubber's bulk modulus into an extensional modulus. A theory describing this effect is shown to interrelate the Poisson ratio and modulus across a matrix of simulated elastomeric nanocomposites of varying loading and nanoparticle structure. This model provides a design rule for structured nanoparticulates that maximizes elastomer mechanical response via suppression of the composite Poisson ratio. It also positions elastomeric nanocomposites as having a qualitatively different character than Poisson-ratio-matched plastic nanocomposites, where this mechanism is absent.

POLYMER ELECTROLYTE BLENDS OF MONO-CARBOXYLIC ACID–MODIFIED EPOXIDIZED NATURAL RUBBER AND POLY(ETHYLENEOXIDE)

Natural rubber with 50% of isoprene units epoxidized (ENR50) was treated with excess of mono-carboxylic acids to completely ring open the epoxide groups. The reaction was carried out by heating ENR50 dissolved in toluene with each of acetic and benzoic acid separately at 105 °C. The ring-opening reaction has produced hydroxyl –OH and ester –O-COR groups, leading to an increase in Tg. The products were characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry. By the solution casting method, each of the modified ENR50s was blended with poly(ethylene oxide) (PEO) in various ratios in toluene, and 2% lithium perchlorate (LiCIO4) was added as the dopant. Results show that the modified ENR and PEO formed incompatible blends. The PEO spherulite growth rate in the blends increased with PEO content. The electrical conductivity was found to increase with the weight fraction of PEO in the blend. At the ratio of 25/75, the acetic acid–modified ENR50/PEO blend exhibits a conductivity value of 3.1 × 10−8 S cm−1. The benzoic acid–modified ENR50/PEO achieved conductivity of 5.8 × 10−7 S cm−1 at the ratio of 30/70. These blends form conducting polymer electrolytes with potential application in batteries.