Nano-strengthening by employing nanoparticles is necessary for high-efficiency strengthening of elastomers, which has already been validated by numerous researches and industrial applications, but the underlying mechanism is still an open challenge. In this work, we mainly focus our attention on studying the variation of the tensile strength of nanofilled elastomers by gradually increasing the filler content, within a low loading range. Interestingly, the percolation phenomenon is observed in the relationship between the tensile strength and the filler loading, which shares some similarities with the percolation phenomenon occurring in rubber toughened plastics. That is, as the loading of nanofillers (carbon black, zinc oxide) increases, the tensile strength of rubber nanocomposites (SBR, EPDM) increases slowly at first, then increases abruptly and finally levels off. Meanwhile, the bigger the particle size, the higher the filler content at the percolation point, and the lower the corresponding tensile strength of rubber nanocomposites. The concept of a critical particle-particle distance (CPD) is proposed to explain the observed percolation phenomenon. It is suggested that rubber strengthening through nanoparticles is attributed to the formation of stretched straight polymer chains between neighbor particles, induced by the slippage of adsorbed polymer chains on the filler surface during tension. Meanwhile, the factors to govern this CPD and the critical minimum particle size (CMPS) figured out in this work are both discussed and analyzed in detail. Within the framework of this percolation phenomenon, this paper also clearly answers two important and intriguing issues: (1) why is it necessary and essential to strengthen elastomers through nanofillers; (2) why does it need enough loading of nanofillers to effectively strengthen elastomers. Moreover, on the basis of the percolation phenomenon, we give out some guidance for reinforcement design of rubbery materials: the interfacial interactions between rubber and fillers cannot be complete chemical bonding, and partial physical absorption of macromolecular chains on the filler surface is necessary, otherwise the formation of stretched straight chains would be seriously hindered. There should exist such an optimum crosslinking density for a certain filler reinforced rubber system, and as well an optimum filler loading for rubber strengthening. Additionally, the different percolation behaviors of Young's modulus, the tensile strength and the electrical conductivity are compared and analyzed in our work. Lastly, molecular simulation indicates that it is not possible to strengthen glassy or hard polymer matrices by incorporating spherical nanoparticles. In general, by providing substantial experimental data and detailed analyses, this work is believed to promote the fundamental understanding of rubber reinforcement, as well provide better guidance for the design of high-performance and multi-functional rubber nanocomposites.

By employing an idealized model of a polymer network and filler, we have investigated the stress-strain behavior by tuning the filler loading and polymer-filler interaction in a broad range. The simulated results indicate that there actually exists an optimal filler volume fraction (between 23% and 32%) for elastomer reinforcement with attractive polymer-filler interaction. To realize this reinforcement, the rubber-filler interaction should be slightly stronger than the rubber-rubber interaction, while excessive chemical couplings are harmful to mechanical properties. Meanwhile, our simulated results qualitatively reproduce the experimental data of Bokobza. By introducing enough chemical coupling between the rubber and the filler, an upturn in the modulus at large deformation is observed in the Mooney-Rivlin plot, attributed to the limited chain extensibility at large deformation. Particularly, the filler dispersion state in the polymer networks is also characterized in detail. It is the first demonstration via simulation that the reinforcement mechanism stems from the nanoparticle-induced chain alignment and orientation, as well as the limited extensibility of chain bridges formed between neighboring nanoparticles at large deformation. The former is influenced by the filler amount, filler size and filler-rubber interaction, and the latter becomes more obvious by strengthening the physical and chemical interactions between the rubber and the filler. Remarkably, the reason for no obvious reinforcing effect in filled glassy or semi-crystalline matrices is also demonstrated. It is expected that this preliminary study of nanoparticle-induced mechanical reinforcement will provide a solid basis for further insightful investigation of polymer reinforcement.

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In this study, the conductive silicone rubber composites filled with nickel-coated graphite (NCG) have been prepared, and their morphology structure, electrical conductivity, electromagnetic interference shielding efficiency (EMI SE), and mechanical properties have been investigated with reference to the NCG filler loading. The mechanical strength of NCG particle was poor that it can be easily ground into smaller particle during the mixing process if the shear force during mixing is large enough. The electrical conductivity of the composites existed an obvious threshold value with the variation of the loading amount of the conductive filler. EMI SE of the composites increases with the decrease of the volume electrical resistivity. The Payne effect can be used to characterize the intensity of the three-dimensional conductive network structure in silicone rubber matrix, and the difference of storage modulus in the low and high shear strain has good linear correlation with the electrical conductivity. So, the electrical conductivity and EMI SE can be estimated by means of the difference of storage modulus obtained from rubber process analysis test.

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The effect of the number of layers, various defects and functional groups on solubility parameter of graphene was studied through molecular dynamics (MD) simulation. We predigested three-component Hansen solubility parameters (δD, δP, δH) to two-component solubility parameters (δvdW, δele), and the two-component solubility parameters of graphene functionalized by different groups, such as hydroxyl, carboxyl, amino, methyl and epoxy with different grafting ratios were obtained. Further, the graphene/styrene butadiene rubber (SBR) composites were constructed to investigate the effect of functional groups and grafting ratios on components compatibility. It was found that the defects and functional groups had strong impact on solubility parameter of graphene, whereas the number of layers had a negligible effect. Two-component solubility parameters were proven to be able to predict compatibility of graphene and SBR. Additionally, the common effect of multifunctional groups on solubility parameter was also investigated. The combination of multifunctional groups with a proper content can obtain lower solubility parameter than a single group. The present study is expected to provide significant insight into the relationship between solubility parameter of graphene and compatibility of graphene/SBR composites.