SYMMETRICAL BLENDS OF COMPLEMENTARY DIBLOCK COPOLYMERS - MULTIORDER PARAMETER APPROACH AND MONTE-CARLO SIMULATIONS

Symmetric diblock copolymer blends A(f)B(1-f)/A(1-f)B(f) (0 less than or equal to f less than or equal to 0.5) are theoretically discussed i n terms of a multiorder parameter approach and numerically investigate d by Monte Carlo simulations. Theoretically, our main result is that b elow f congruent to 0.3, but still in the microphase separation region given by 0.21 less than or equal to f less than or equal to 0.5, the concentration profiles of the long and short A-blocks as well as the l ong and short B-blocks are out of phase. Monte Carlo simulations were used to investigate the nature of the phase transition, micro versus m acro, as a function off. Using the canonical ensemble, the microphase separation temperature (MIST) was determined. The macrophase separatio n temperature (MAST) was studied with the semi-grand-canonical ensembl e combined with the histogram extrapolation technique. The phase diagr am differs considerably from the theoretical predictions due to the st retching/polarization of the molecules already far above the transitio n temperature, thus stabilizing the macroscopically homogeneous state. The out-of-phase behavior between the long and short blocks near the critical value f congruent to 0.21, separating the micro- and macropha se separation regimes, was confirmed by the simulations.