Topological effect on the phase behaviour of Triblock Copolymer and Homopolymer Blends

The phase behaviour of binary blends composed of linear symmetric AB-type triblock copolymers and A-homopolymers is examined using the self-consistent field theory, focusing on the occurrence of the Frank-Kasper (FK) phases. Previous studies showed that several FK phases (σ, C14 and C15) could be stabilized by adding A-homopolymers to AB-diblock copolymers. Because diblock copolymers and homologous triblock copolymers exhibit qualitatively similar phase diagrams, it is expected that the addition of A-homopolymers into AB-type triblock copolymers would result in similar phase behaviour. For a given composition, linear symmetric AB-type triblock copolymers can assume two, ABA and BAB, topologies. A naturally arising question is how such a topological difference affects the phase behaviour of the blends. In order to answer this question, two phase diagrams corresponding to BAB/A and ABA/A blends are constructed. A surprisingly large difference between the phase diagrams of ABA/A and BAB/A blends is observed. The small difference between the phase diagrams of neat BAB and ABA melts is amplified by the addition of a homopolymer compatible with their minority block. Further analyses on the density profiles suggest different distributions of A-homopolymers in the BAB/A and ABA/A blends, which is responsible to the very different behaviours of the two systems. The results of our study shed light to the topological effect of block copolymers on the formation of ordered phases in more complex polymeric blends.