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.

Mechanical properties including the failure behavior of physically assembled gels or physical gels are governed by their network structure. To investigate such behavior, we consider a physical gel system consisting of poly(styrene)-poly(isoprene)-poly (styrene)[PS-PI-PS] in mineral oil. In these gels, the endblock (PS) molecular weights are not significantly different, whereas, the midblock (PI) molecular weight has been varied such that we can access gels with and without midblock entanglement. Small angle X-ray scattering data reveals that the gels are composed of collapsed PS aggregates connected by PI chains. The gelation temperature has been found to be a function of the endblock concentration. Tensile tests display stretch-rate dependent modulus at high strain for the gels with midblock entanglement. Creep failure behavior has also been found to be influenced by the entanglement. Fracture experiments with predefined cracks show that the energy release rate scales linearly with the crack-tip velocity for all gels considered here. In addition, increase of midblock chain length resulted in higher viscous dissipation leading to a higher energy release rate. The results provide an insight into how midblock entanglement can possibly affect the mechanical properties of physically assembled triblock copolymer gels in a midblock selective solvent.

Physically assembled gels have promising applications in many fields because of their tunable mechanical properties. Here, we report the mechanical properties as a function of polymer volume fraction (ϕ) for a physical gel system that consists of poly­(styrene)–poly­(isoprene)–poly­(styrene) [PS–PI–PS] in mineral oil. The PI-block molecular weight is higher than the entanglement molecular weight, which leads to the entanglement of PI-blocks at higher ϕ. The micellar microstructure for all gels results in a similar stress–relaxation mechanism, as captured by the superposition of stress–relaxation results. Tensile testing experiments reveal a stretch-rate dependent mechanical response for the gels with entangled PI-blocks. A combination of cavitation rheology and fracture experiments capture the critical energy-release rate (Γ0) as Γ0 ∼ ϕ2.0. Similarly, the gel modulus (G′) scales with the polymer volume fraction as ϕ1.92. The gel mechanical responses are dictated by the state of midblock that changes with the polymer volume fraction.

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