A Simulation Study of Enhancing the Compatibility in Block Copolymer AB/Homopolymer C Blends by Solvents

The phase behavior of diblock copolymer AB/homopolymer C blends in solution was studied by simulated annealing method. The study was focused on the compatibilization between the homopolymer and the copolymer with repulsive interactions due to the addition of solvent. We investigated the amount of solvent (characterized by the concentration of the polymer segments Phi), the volume fraction f(c) of the homopolymer C, the ratio of homopolymer chain length to the copolymer chain length X, the repulsion between C and A or B segments, epsilon(AC) and epsilon(BC), and the attraction between solvent and C on the phase behavior of the blending system. We built phase diagrams in the space of F and fc at different X. Studies have shown that when X is less than 0.5, and the values of epsilon(AC) and epsilon(BC) are not very large, the addition of solvent strongly selective to A and C segments can improve the compatibility of the system. In solution, lamellae, gyroids, cylinders in layers, core-shell cylinders and cylindrical structures are formed. Moreover, the structure of cylinders in layers has not been observed in the neat diblock copolymer system. C-segments are distributed at the interfaces between A/B domains at high F region, while they are inside the A domain at low F region. Short homopolymer chains are easily compatible with diblock copolymers. Macroscopic phase separation has taken place in long homopolymer chains even with a small amount of homopolymer added. Moreover, increasing the values of epsilon(AC) and epsilon(BC) will reduce the compatibility of the system. The transformation between different phases and that from microphase separation to macrophase separation are the results of the competition between the energy and entropy of the system. The macrophase separation in the solution system is different from that in the melt system. Some C-segments are at the interface of AB domains to reduce the contact between A and B segments in the solution system, while all C-segments are separated from the AB domains in the melts.