Effect of Loop and Dangling End Defects on Strain Stiffening in Double Network Elastomers

We investigate the impact of loop and dangling end defects on the strain stiffening behavior of regularly-crosslinked polymer networks using a double network strategy. Regularly-crosslinked networks are synthesized by crosslinking tetrafunctional poly(n-butyl acrylate) star polymers using a thiol-bromine click reaction. In a first series of samples, the polymer concentration is varied during synthesis to introduce loop defects. In a second series of samples, the polymer concentration is held constant, but some of the difunctional crosslinker is replaced with a monofunctional thiol to introduce dangling ends. We find that both types of defects result in softer networks with delayed strain stiffening. As in previous work, the moduli of the double networks are higher than expected from the moduli of the precursor networks from which they are prepared. Comparison of the loop and dangling end samples suggests that loops in the initial samples may become elastically-effective once entangled with the second network. This work provides new insights into how the interactions of the first and second networks affect the mechanical properties of double network elastomers, and the extent to which double network can (and cannot) be used to understand the behavior of the underlying first networks.