We study thermoplastic elastomers of triblock copolymers, of the form polystyrene-polyisoprene-polystyrene (SIS), where the microphase-separated PS blocks act as physical cross-links for the PI elastic network. Two compositions are examined: one with micellar cubic and the other with hexagonal cylindrical morphology of PS. Remarkably, the long-time stress relaxation is very similar; it reveals a continuous crossover between the typical response of a classical rubber at low temperatures and that of an entangled melt at high temperatures, above the Tg of polystyrene micelles. However, at a temperature T* well below the glass transition Tg(PS), the stress relaxation experiences a crossover between the classical rubber power law and a much faster relaxation above T*, which follows a stretched-exponential law for the extension modulus E(t) ∼ b exp[-(t/τ)0.2]. Time-temperature superposition is possible at all temperatures below Tg(PS), but the classical WLF equation could not be made to describe the shift factors. These results are interpreted in terms of transient (breakable under stress) cross-links and a local stress relief when PS chains are pulled out of the glassy micelles, with the characteristic time of PS release at each temperature below Tg(PS) determining the mechanical response at a given strain rate.
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