Investigation of failure behavior of a thermoplastic elastomer gel

Abstract

Gels are increasingly being used in many applications, and it is important to understand how these gels fail subjected to mechanical deformation. Here, we investigate the failure behavior of a thermoplastic elastomer gel (TPEG) consisting of poly(styrene)–poly(isoprene)–poly(styrene) in mineral oil, in tensile mode, under constant stress, and in fracture tests, where the fracture initiates from a predefined crack. In these gels, the poly(styrene) endblocks associate to form spherical aggregates, as captured using SAXS. Shear-rheology experiments indicate that the poly(isoprene) midblocks connecting these aggregates are loosely entangled. The relaxation behavior of these gels has been captured by time–temperature superposition of frequency sweep data and stress-relaxation experiments. The relaxation process in these gels involves endblock pullout from the aggregates and subsequent relaxation of the chains. An unfavorable enthalpic interaction between the endblock and mineral oil results in a significantly large relaxation time. These gels display rate dependent mechanical properties, likely due to the midblock entanglements. Fracture and creep failure tests provide insights into the gel failure mechanism. Creep experiments indicate that these gels fail by a thermally activated process. Fracture experiments capture the energy release rate as a function of crack-tip velocity. The critical energy release rate is estimated by incorporating the friction force the polystyrene chains are subjected to, as those are pulled out of aggregates, and the enthalpic cost to overcome unfavorable interaction between poly(styrene) and mineral oil. Our results provide further insights to the failure behavior of the self-assembled TPEGs.