Symmetric blends of complementary diblock copolymers: Multiorder parameter approach and Monte Carlo simulations

Symmetric diblock copolymer blends A_fB_1-f/A_1-f/B_f (0 ≤ f ≤ 0.5) are theoretically discussed in terms of a multiorder parameter approach and numerically investigated by Monte Carlo simulations. Theoretically, our main result is that below f ≅ 0.3, but still in the microphase separation region given by 0.21 ≤ f ≤ 0.5, the concentration profiles of the long and short A-blocks as well as the long and short B-blocks are out of phase. Monte Carlo simulations were used to investigate the nature of the phase transition, micro versus macro, as a function off. Using the canonical ensemble, the microphase separation temperature (MIST) was determined. The macrophase separation temperature (MAST) was studied with the semi-grand-canonical ensemble combined with the histogram extrapolation technique. The phase diagram differs considerably from the theoretical predictions due to the stretching/polarization of the molecules already far above the transition temperature, thus stabilizing the macroscopically homogeneous state. The out-of-phase behavior between the long and short blocks near the critical value f ≅ 0.21, separating the micro- and macrophase separation regimes, was confirmed by the simulations.