SMALL-ANGLE X-RAY-SCATTERING FROM BLOCK POLYMERS IN THE DISORDERED STATE .3. VALIDITY OF THE PSEUDOBINARY APPROXIMATION FOR BLOCK POLYMER-SOLUTIONS

We investigated the validity of the pseudobinary approximation proposed in our earlier work (Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules 1983, 16, 1093) for scattering from semidilute-to-concentrated solutions of block copolymers in the disordered (or single) phase. The validity, which has never been checked carefully, was investigated by comparing scattering functions numerically calculated on the basis of the pseudobinary approximation and that of the full mean-field equation reported by Benmouna and Benoit (Benmouna, M.; Benoit, H. J. Polym. Sci., Polym. Phys. Ed. 1983, 21, 1227). The criteria for the pseudobinary approximation to be valid were found to be (i) the optical theta-condition and (ii) non-selectivity of solvent for the block polymer chains being satisfied. An enormous simplification of the scattering equation brought by this approximation makes it possible to further elaborate the scattering theory to incorporate the nonclassical effects as reported recently (Olvera de la Cruz, M. J. Chem. Phys. 1989, 90, 1995; Fredrickson, G.; Leibler, L. Macromolecules 1989, 22, 1238).