Triblock copolymers in a selective solvent .2. Semidilute solutions

Triblock copolymers polystyrene-polyisoprene-polystyrene are studied in semidilute solutions (1 < C/C* < 8), in a selective solvent, i.e., a nonsolvent for polystyrene and a good solvent for polyisoprene. The mass of the copolymer is equal to 1.6 x 10(5) g/mol and it contains 70% of polyisoprene. Static light scattering, small-angle neutron scattering, viscoelastic, and quasi-elastic light scattering experiments are reported as a function of the concentration and of the temperature. Rheological properties argue in favor of a network made of end-linked copolymers, the cross-link dynamics being governed by the glass transition of the polystyrene terminal blocks. Small-angle neutron scattering experiments reveal that this network is well ordered and has a cubic structure as up to five Bragg peaks are observed. The relaxation function of concentration fluctuations measured by quasi-elastic light scattering is somewhat reminiscent of supercooled liquid and colloids near the glass transition. It is interpreted in terms of polystyrene node cooperative diffusion which becomes anomalous at long times (low q).