Random multiblock copolymer-homopolymer blends: Effect of sequence distribution and intramolecular repulsion

Recently, a mean-field analysis of linear random A-B multiblock copolymer melts was introduced, taking the sequence distribution explicitly into account. Here, we extend this approach to mixtures with homopolymers P(C). Within the simple Flory-Huggins description, miscibility in random copolymer-homopolymer mixtures is often attributed to unfavorable A-B interactions described by a positive Flory-Huggins parameter χAB (intramolecular repulsion). The introduction of the sequence distribution sets an upper limit to this effect. For sufficiently large values of χAB the mixture phase separates again, very much like the pure copolymer melt. Likewise, miscibility improves in the direction of the alternating sequence distribution. Our analysis rationalizes frequently observed sequence distribution effects in random copolymer blends.

Symmetric Blends of Complementary Diblock Copolymers: Multiorder Parameter Approach and Monte Carlo Simulations

Symmetric diblock copolymer blends AfB1-f/A1-fBf (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 of f. 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.