The diamond and other non-conventional morphologies in two-scale multiblock AB copolymers

Abstract

We revisit the idea of the existence of the ordered block copolymer phase possessing diamond symmetry Fdm (space group No. 227), which was first put forward within the framework of the strong segregation approach. For this purpose we study the order–disorder and order–order transitions in molten two-scale multiblock copolymers A_mN/2(B_N/2A_N/2)_nB_mN/2via the pseudo-spectral numerical procedure of the self-consistent field theory (SCFT). The phase diagram in the plane (f_C, ), where f_C = n/(m + n) and = χ N(m + n) is the effective energetic Flory–Huggins parameter, is built and some accompanying quantities are analyzed. Near the order–disorder transition line the phase diagram contains the regions where the lamellar, alternating gyroid, diamond and simple cubic phase, respectively, exist. With an increase of the degree of segregation, the diamond phase is replaced by a tetragonal array of cylinders (simple square) phase, which agrees with the preceding results obtained within the Leibler-like weak segregation theory, and with the SCFT calculations for a physically similar melt of linear ABC triblock copolymers with a non-selective middle block. Thus, the diamond morphology in the system under study is shown to exist as an essentially weak or moderately (not strongly) segregated phase. The ways to visualize the patterns of ordering in such morphologies are discussed. A new quantity (topological permeability) to characterize the transport properties in 3D bicontinuous morphologies is introduced and first calculated for real block copolymer ordered morphologies. Some implications of the results obtained for the design of the block copolymer thin films with improved permeability are discussed.