Triblock copolymers in a selective solvent: Dilute and semi‐dilute solutions

Triblock copolymers polystyrene‐polyisoprene‐polystyrene were studied in dilute and semi‐dilute solutions in a selective solvent, i.e. a non‐solvent for polystyrene and a good solvent for polyisoprene. The mass of the copolymer is equal to 1.6×10⁵g/mol and it contains 70% of polyisoprene. At a concentration C equal to 1.6×10⁻³ g/cm³ triblock aggregation occurs. Experiments performed on dilute solutions¹ plead in favor of loose and polydisperse aggregates rather than spherical micelles: 1) the form factor of the aggregates shows a smooth decrease at qR<1; 2) internal modes are observed by quasi‐elastic light scattering; 3) specific viscosity measurements show that the internal concentration of unimers and aggregates are identical; 4) the unimer concentration and the aggregation number depend on the copolymer concentration. In semi‐dilute solutions² (C>3×10⁻² g/cm³), dynamical properties are strongly affected by the temperature. Viscosity and longest relaxation time increase by a factor 10⁵ as the temperature decreases from 50°C to 5°C, whereas the plateau modulus G remains independent of the temperature and proportional to the concentration. Small angle neutron scattering experiments reveal a cubic structure having a unit cell dimension d independent of the temperature and varying as C^−1/3 leading to G∼1/d³. Quasi‐elastic light scattering experiments performed on this system show a wide relaxation function extending over 7 decades of time. These results argue for a lattice made of polystyrene nodes linked together by the polyisoprene middle blocks, the dynamics being governed by the glass transition of the nodes.