PHASE-BEHAVIOR OF DIBLOCK COPOLYMERS - PRESSURE AND TEMPERATURE-DEPENDENCE STUDIED BY SMALL-ANGLE NEUTRON-SCATTERING

The structure factor of a poly(ethylene-propylene)poly(dimethylsiloxan e) diblock copolymer has been measured by SANS as a function of temper ature and pressure. In contradiction to the random phase approximation the conformational compressibility exhibits a pronounced maximum at t he order-disorder phase transition. The phase boundary shows an unusua l shape: with increasing pressure it first decreases and then increase s. Its origin is an increase in, respectively, the entropic and the en thalpic part of the Flory-Huggins interaction parameter. The Ginzburg parameter describing the limit of the mean-field approximation is not influenced by pressure.