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Mechanical responses after the uniaxial deformation of graded styrene–butadiene rubber (SBR) with a gradient in the crosslink points in the thickness direction were investigated as compared with those of homogenously vulcanized SBR samples. The elongational residual strain of a graded sample was found to depend on the part with a high crosslink density. Therefore, it showed good rubber elasticity. After stress removal, moreover, the graded sample showed a marked warpage. This suggested that shrinking stress acted on the surface with a high crosslink density, which would avoid a crack growth on the surface. The sample shape was then recovered to be flat very slowly, indicating that the shrinking stress worked for a long time. This unique rubber elasticity, i.e., slow strain recovery with an excellent strain recovery, makes graded rubber highly significant.

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An intensive literature search shows that research in the field of self-healing rubbers is still in its infancy. By analyzing the various reviews and reports available, most of the results prove to be complicated; however, a few studies show promising self-healing properties of new elastomers. Most of these materials were prepared by relatively sophisticated chemical syntheses. Many of the studies on self-healing materials also deal with commercial rubbers, but the mechanical performance of these self-healing systems is very poor for practical application, perhaps because self-healing systems are usually prepared with an uncured or pseudo-crosslinked matrix structure. The poor mechanical properties are related to the highly viscous but inelastic nature of the uncured or only partially cured rubber compound. Importantly, most of the studies have been conducted on rubber systems without reinforcing fillers. For filler-reinforced rubber systems, the question is whether the working principle of reversible bonds is applicable. This literature review attempted to compile the current promising self-healing systems, describe their underlying chemical mechanisms, and discuss the self-healing concept from a thermodynamic perspective. In addition, this review is focused on the critical discussion of the principle and origin of self-healing behavior and finally draws conclusions on the applications and opportunities for further developments in this field.

Cryogenic transmission electron Microscopy (cryo-TEM) and rheological characterization were conducted in order to understand structural development of vanadium pentoxide gels during processing. Sols were prepared by ion exchange from sodium metavanadate solutions. Cryo-TEM revealed that fine threads about 1.5nm wide initially form and grow into ribbons approximately 25nm wide and at least 1000nm long. The threads appear to self assemble into the ribbons. During this structural development, the dynamic viscosity increased. Upon steady shearing of the sols, the system exhibited thixotropy, i.e. the viscosity decreased with time under constant shear stress and subsequently rheopexy, the viscosity increased with time. Comparison of the structure before and after shearing indicated that during the rheological experiments aggregation of small particles or fragments was occurring.