Theoretical Study of Phase Behaviors of Symmetric Linear B(1)A(1)B(2)A(2)B(3) Pentablock Copolymer.

The nanostructures that are self-assembled from block copolymer systems have attracted interest. Generally, it is believed that the dominating stable spherical phase is body-centered cubic (BCC) in linear AB-type block copolymer systems. The question of how to obtain spherical phases with other arrangements, such as the face-centered cubic (FCC) phase, has become a very interesting scientific problem. In this work, the phase behaviors of a symmetric linear B(1)A(1)B(2)A(2)B(3) (f(A1) = f(A2), f(B1) = f(B3)) pentablock copolymer are studied using the self-consistent field theory (SCFT), from which the influence of the relative length of the bridging B(2)-block on the formation of ordered nanostructures is revealed. By calculating the free energy of the candidate ordered phases, we determine that the stability regime of the BCC phase can be replaced by the FCC phase completely by tuning the length ratio of the middle bridging B(2)-block, demonstrating the key role of B(2)-block in stabilizing the spherical packing phase. More interestingly, the unusual phase transitions between the BCC and FCC spherical phases, i.e., BCC → FCC → BCC → FCC → BCC, are observed as the length of the bridging B(2)-block increases. Even though the topology of the phase diagrams is less affected, the phase windows of the several ordered nanostructures are dramatically changed. Specifically, the changing of the bridging B(2)-block can significantly adjust the asymmetrical phase regime of the Fddd network phase.