TEMPERATURE AND PRESSURE-DEPENDENCE OF THE ORDER-PARAMETER FLUCTUATIONS, CONFORMATIONAL COMPRESSIBILITY, AND THE PHASE-DIAGRAM OF THE PEP-PDMS DIBLOCK COPOLYMER

The structure factor of a poly(ethylene-propylene)-poly(dimethylsiloxa ne) diblock copolymer has been measured by small-angle neutron scatter ing as a function of temperature and pressure. The conformational comp ressibility exhibits a pronounced maximum at the order-disorder phase transition. The phase boundary shows an unusual shape. With increasing pressure it first decreases and then increases. Its origin is an incr ease, respectively, of the entropic and of the enthalpic part of the F lory-Huggins interaction parameter. The Ginzburg parameter describing the limit of the mean-field approximation is not influenced by pressur e.