Self-Assembly Behavior of ABCBA Linear Pentablock Terpolymers in the Melt

The phase behavior of linear ABCBA pentablock terpolymers is examined and compared with corresponding linear ABC triblock terpolymers by using the 3-D self-consistent mean-field theory. In particular, phase diagrams of the melts are constructed and used to discuss how the self-assembled morphologies are influenced by the compositions of the three components and the block number per chain. Since the two free ends of A blocks in the ABCBA copolymers enable the macromolecules to relieve the packing frustration within the structures as well as more types of chain conformation, the ABCBA pentablocks exhibit diverse complex network structures and binary crystalline phases of cylinders and spheres. Compared with most linear diblock and triblock copolymers, for which the gyroid phase occupies a narrow region in the phase space, the ABCBA pentablocks tend to form a variety of continuous networks including diamond, hexagonally perforated lamellae, Fddd network, and gyroid. Moreover, the ABCBA pentablocks exhibit different packing orders of alternating A/C spheres and cylinders than ABC. By varying the length of the B-blocks and the ratio of compositions of A and C, a large number of binary metallic and ionic crystals, such as NaCl, CsCl, ZnS, CaF2, Li3Bi, Nb3Sn and Cu3Au, and alternating A/C cylinders with coordination numbers of A/C equal to 4/4, 6/3, and 4/2, are obtained from the ABCBA pentablock terpolymers. With decreasing symmetric χN value (from 80 to 47.5), the effect interaction parameter between the two neighboring blocks is much smaller as decreasing the incompatibility degree of symmetric χN value so that behavior of the pentablock chains are similar to triblock and diclock chains. Accordingly, the complex networks of ABCBA pentablocks such as diamond, hexagonally perforated lamellae, tend to transfer to gyroid observed in most linear diblock and triblock cases. Moreover, the trend of different packing orders of alternating A/C spheres and cylinders with unequal coordination numbers transfer to those with equal coordination numbers is also observed with decreasing symmetric χN value. On the other hand, when the interaction parameter between the two end blocks is much weaker than those between the neighboring blocks, the copolymers prefer to form morphologies with A/C interfaces, which however is not favored due to the chain topology. Thus, more complex ordered structures can be formed in these frustrated copolymer systems including B-formed cylinders, spheres, and helices on cylinders; lamellae with B-cylinders or B-spheres at the interfaces; segmented types of cylinder or gyroid; core/perforated shell types of cylinder or gyroid; knitting patterns. As our simulated ABCBA pentablock terpolymers have the synthetic advantages, the fascinating self-assembling results displayed in this study enable the ABCBA as one of the most efficient routes to functional materials