Concentration-Dependent Mechanical Behavior of Physically Assembled Triblock Copolymer Gels

Abstract

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.