Stress relaxation in transient networks of symmetric triblock styrene-isoprene-styrene copolymer

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 T_g of polystyrene micelles. However, at a temperature T* well below the glass transition T_g(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 T_g(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 T_g(PS) determining the mechanical response at a given strain rate.