Novel percolation phenomena and mechanism of strengthening elastomers by nanofillers

Abstract

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.

Nanoparticle amount, and not size, determines chain alignment and nonlinear hardening in polymer nanocomposites

“…S11), similar to previous work (40,41), which could also enhance the bridging effect-increase N 3 /N polymer -by increasing the effective bridging region volume relative to the total volume of the composite (42); however, the precise effect of boundary layer thickness on the different composite systems is not clear. Conversion of "slippery" adsorbed (type 1) chains into type 3 chains has been shown, specifically in samples (nearly identical to ours) where limited interaction between the polymer and fillers is present (19,43). As noted in SI Appendix, Fig.…”

“…on either mechanical strain hardening (13)(14)(15)(16) or chain alignment (17)(18)(19)(20), but not both.…”

Correlations between mechanical, structural, and dynamical properties of polymer nanocomposites

“…However, the exact physical mechanisms leading to mechanical reinforcement have proven elusive. Recently, intensive experimental [6][7][8] and computational [9][10][11][12][13][14][15][16][17] efforts have been devoted to answering this fundamental question with the aim to get to better design of reinforced materials. Many parameters play a role: the size and shape of the fillers, the polymer molecular mass and the presence of an entangled network, the strength of the physical and/or chemical interaction between the fillers and the polymers, the filler loading, and dispersion.…”

Influence of nanoparticle size, loading, and shape on the mechanical properties of polymer nanocomposites

“…2 These enhanced properties arise even at small NP loadings and have been exploited at the industrial level for many years already. 3 Nanoparticles dispersed in polymer matrices, also called nanofillers, can significantly influence the rheological, 4 optical, 5 electrical, 6,7 thermal, 8,9 and mechanical 1,6,[10][11][12][13][14][15] properties of the material. Many parameters may play a role in the reinforcement of the polymer matrix: nanoparticle shape and size, loading and dispersion in the polymer matrix, interaction type and strength between the monomers and the nanoparticles, nanoparticle mobility, temperature, entanglement of the polymers, and degree of polymerization.…”