Anomalous Strain Recovery after Stress Removal of Graded Rubber

Abstract

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.

Hysteresis manifestation of graded rubber

“…Subsequently, the no/few crosslinks side shinks. This requires a change in the conformation of the chain segments, which takes a long time, as explained in detail in our preceding paper 17 . Considering this mechanism, the large residual strains were not the equilibrium ones, at least not at 0.017 s −1 .…”

“…This demonstrated that the hysteresis loops were affected by the duration of loading. We previously investigated a graded rubber that exhibited favorable rubber elasticity, that is, a small residual strain, although it took a very long time to recover after the removal of stress 17 . When stretched, chain orientation in the no/few crosslinks side is relaxed (see Figure S5).…”

“…preceding paper. 17 Considering this mechanism, the large residual strains were not the equilibrium ones, at least not at 0.017 s À1 .…”

“…We previously investigated a graded rubber that exhibited favorable rubber elasticity, that is, a small residual strain, although it took a very long time to recover after the removal of stress. 17 When stretched, chain orientation in the no/few crosslinks side is relaxed (see Figure S5). After the removal of stress, the region with the high crosslink density immediately shrinks, causing bending deformation.…”

“…The samples were taken out of the machine after each loading/ unloading process and kept at 25 C. Upon stress removal, the graded rubber immediately exhibited bending, as reported in our previous paper. 17 The actual residual strain was measured when the graded rubber became flat, which took $1 day. The same procedure was also applied to the homogenous rubber.…”