Fluctuation effects in blends of A+B homopolymers with AB diblock copolymer

Abstract

Field-theoretic simulations (FTS) are performed on ternary blends of A- and B-type homopolymers of polymerization N_h and symmetric AB diblock copolymer of polymerization N_c. Unlike previous studies, our FTS are conducted in three-dimensional space, with the help of two new semi-grand canonical ensembles. Motivated by the first experiment to discover bicontinuous microemulsion (BME) in the PE-PEP system, we consider molecules of high molecular weight with size ratios of alpha = N_h/N_c = 0.1, 0.2 and 0.4. Our focus is on the A+B coexistence between the two homopolymer-rich phases in the low-copolymer region of the phase diagram. The Scott line, at which the A+B phases mix to form a disordered (DIS) melt with increasing temperature (or decreasing chi), is accurately determined using finite-size scaling techniques. We also examine how the copolymer affects the interface between the A+B phases, reducing the interfacial tension towards zero. Although comparisons with self-consistent field theory (SCFT) illustrate that fluctuations effects are relatively small, fluctuations do nevertheless produce the observed BME that is absent in the SCFT phase diagram. Furthermore, we find evidence of three-phase A+B+BME coexistence, which may have been missed in the original as well as subsequent experiments.