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.