TRIBLOCK COPOLYMERS IN A SELECTIVE SOLVENT - DILUTE AND SEMIDILUTE 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 1.6x10(5) g/mol and it contains 70% of p olyisoprene. At a concentration C equal to 1.6x10(-3) g/cm(3) triblock aggregation occurs. Experiments performed on dilute solutions(1) plea d 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 scatte ring; 3) specific viscosity measurements show that the internal concen tration of unimers and aggregates are identical; 4) the unimer concent ration and the aggregation number depend on the copolymer concentratio n. In semi-dilute solutions(2) (C>3x10(-2) g/cm(3)), dynamical propert ies are strongly affected by the temperature. Viscosity and longest re laxation time increase by a factor 10(5) as the temperature decreases from 50 degrees C to 5 degrees C, whereas the plateau modulus G remain s 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 vary ing as C--1/3 leading to G similar to 1/d(3). Quasi-elastic light scat tering experiments performed on this system show a wide relaxation fun ction extending over 7 decades of time. These results argue for a latt ice made of polystyrene nodes linked together by the polyisoprene midd le blocks, the dynamics being governed by the glass transition of the nodes.