Bicontinuous Microemulsion Region in Blends of A+B Homopolymers with an AB Diblock Copolymer

Field-theoretic simulations are applied to the disordered phase of ternary blends involving A- and B-type homopolymers with small amounts of AB diblock copolymer. As the Flory–Huggins interaction parameter, χ, increases from zero, structureless blends develop A- and B-rich domains that merge to form two interpenetrating clusters that span the entire volume, a morphology known as a bicontinuous microemulsion (BμE). Here, we calculate the percolation transition, where ordinary disordered blends transform to BμE. Next, we test whether this boundary can be determined directly from the structure function, S ( k ), measured by scattering experiments using the Teubner–Strey (T-S) analysis developed for lyotropic liquid crystals. The traditional functional form used in the analysis to fit S ( k ) proves to be inaccurate, but this is readily resolved by a simple modification. Although the modified T-S analysis is sensitive to the BμE morphology, it is only able to provide a crude estimate of the BμE boundary.