STATIC STRUCTURE FACTORS OF COMPRESSIBLE POLYMER BLENDS AND DIBLOCK COPOLYMER MELTS

The incompressibility constraint is removed within the random-phase approximation (RPA). We thereby derive a set of three static structure factors for monomer-monomer correlations within a binary polymer blend as a function of the three excluded-volume interactions that are required for compressible blends. The results are compared to the conventional RPA structure factor. It is shown that the incompressible limit of the blend structure factors recovers the conventional RPA scattering functions when the intrachain excluded-volume parameters are equal. Otherwise, the usual empirical definition of the Flory interaction parameter is a function of all three excluded-volume parameters, the composition of the blend, and the scattering wavevector. Introducing compressibility into the RPA is, however, shown not to alter usual RPA predictions about the behavior of the structure factor and correlation length near the spinodal. The compressible RPA structure factors are also compared with those emerging from integral equation approaches. A brief outline is given of the extensions of the compressible RPA theory to block copolymer melts. We discuss the manner in which overall density fluctuations might affect some experiments on melts and blends and theories of microphase separation in block copolymers. © 1990, American Chemical Society. All rights reserved.