Linking failure behavior of physically assembled styrene-isoprene-styrene gels to their network structure

Failure behavior of physically assembled gels is governed by their network architecture. We investigate the failure behavior of physically assembled gels composed of poly(styrene)-poly(isoprene)-poly(styrene) in mineral oil, a midblock selective solvent. The gel network consists of collapsed PS endblock aggregates acting as crosslinks, while PI midblocks bridge those aggregates. The gel architecture, as captured by the small angle x-ray scattering, is tuned by varying the polymer volume fraction and midblock length. Tensile experiments reveal a rate dependent mechanical properties, particularly for the samples with entangled midblocks. Energy release rate (G) for fracture scales linearly with the crack-tip velocity (v) indicating a velocity toughening effect in these gels. The G-v relationship strongly depends on the polymer concentration and chain length. These gels fail as a result of endblocks pullout from aggregate. The pullout process involves an entropic penalty associated with the midblock stretching, friction with other endblocks in aggregates, and an enthalpic penalty associated with pulling the endblocks in the non-favorable solvent. We attempt to incorporate all these factors in estimating G and compare that with the experimental values.