Tuning Arm Architecture Leads to Unusual Phase Behaviors in a (BAB)5 Star Copolymer Melt

The self-assembly behavior of (B(1)AB(2))(5) star copolymers, composed of five asymmetric BAB-triblock arms joined at the end of B-2-blocks, has been investigated using the self-consistent field theory. The special architecture enables a few different sophisticated mechanisms such as the conformational asymmetry from the star topology, the effect of combinatorial entropy from the multiple arms that enhances the formation of bridging configurations for the core B2-blocks, the local segregation between the two different B-blocks, and the solubilization effect of the short B-2-block in the majority A-domain, each of which has been individually demonstrated to play an important role in impacting the self-assembly behavior of block copolymers before. As a result, the combination of these mechanisms leads to many unusual phase behaviors of (B(1)AB(2))(5) with tunable asymmetry tau =f(B1)/(f(B1) + f(B2)) between the two B-blocks, where f(B1) and f(B2) are the volume fractions of B-1- and B-2-blocks, respectively. For example, reentrant phase transitions between the BCC and FCC spherical phases are observed with minority A-domains, whereas the width of the overall spherical phase region at the opposite side of the phase diagram exhibits two maxima as tau increases. The expansion of the spherical phase region at the first maximum is induced by the reduced effective volume fraction due to the solubilization effect and thus is solely occupied by the BCC phase. While the expansion at the second maximum originates from the formation of enlarged "core shell" domains due to the effect of local segregation, leading to the formation of complex Frank Kasper spherical phases. In addition, no stable gyroid phase composed of A-network is observed in the phase diagram of tau = 4/5, while the gyroid phase region in the opposite side of the phase diagram is expanded significantly. The absence of the gyroid phase is a very rare phenomenon for block copolymers and here may result from the combined effect of different sophisticated mechanisms.