Cut Growth Patterns in Double Networks of a Natural Rubber Vulcanizate

The effect of aging at 90 deg. C and 110 deg. C under nitrogen, with and without applied strain, on black-filled natural rubber, with an inefficient sulfur cure was investigated. Samples aged under strain became ``double networks'' and retained a residual extension ratio. Cut growth behavior of the double networks was complex. The perpendicular specimens (cut parallel to the applied strain during aging), had cracks that either were ``super-blunted'' or exhibited ``forward'' crack splitting. Parallel specimens were weaker than perpendicular and single network ones, and exhibited a critical cut size, for which a small increase in cut size resulted in an abrupt decrease in strength. For cuts smaller than the critical size, crack geometries were ``super-blunting'', while specimens with larger cuts typically showed simple lateral propagation, without longitudinal cracking or crack splitting. There was some evidence that parallel double network specimens had greater ability to strain-crystallize. However, decreased cut growth resistance of these suggests that limited chain extensibility is controlling.