Domain-boundary structure of styrene-isoprene block copolymer films cast from solutions. III. Preliminary results on spherical microdomains

The microdomain structure of styrene‐isoprene A‐B type block copolymers having a nearly constant fraction of polyisoprene block segments (13 ∼ 22 wt percent) was investigated by small‐angle X‐ray scattering (SAXS) as a function of the molecular weights of the copolymers. The styrene‐rich block copolymers all have spherical microdomains of polyisoprene block segments dispersed in a matrix of polystyrene block segments. The size of the spherical domains increases with increasing molecular weight of the polyisoprene segment with a power of ca. 0.6. The thickness of the domain‐boundary interphase arising from a partial mixing of the incompatible segments at the domain‐boundary interface is also estimated by analyzing a systematic deviation of the SAXS intensity distribution from Porod's law at large scattering angles on the basis of the infinite slit‐height approximation. The results indicate the interfacial thickness to be about 20Å and to be almost independent of the molecular weight of the block copolymers studied. The applicability of the infinite‐slit height approximation in the analysis of SAXS intensity distributions at large scattering angles is also discussed in an approximate fashion.