BLOCK-COPOLYMER MICROSTRUCTURES IN THE INTERMEDIATE-SEGREGATION REGIME

A detailed examination of the intermediate-segregation regime of diblo ck copolymer melts is presented using the incompressible Gaussian chai n model and self-consistent field theory (SCFT). We find that the comp etition between interfacial tension and chain stretching used to descr ibe behavior in the strong-segregation regime also explains behavior i n this regime. Phase transitions from lamellae (L) to cylinders (C) to spheres (S) occur due to the spontaneous curvature produced as the as ymmetry in the diblock composition increases. Complex phases, gyroid ( G), perforated lamellar (PL), and double diamond (D), have curvatures between those of L and C, and therefore they compete for stability alo ng the L/C boundary. Nevertheless, only C exhibits a region of stabili ty. To explain why, we recognize that interfacial tension prefers the formation of constant mean curvature (CMC) surfaces to reduce interfac ial area, and chain stretching favors domains of uniform thickness so as to avoid packing frustration. While the classical structures, L, C, and S, are successful at doing both simultaneously, the complex phase s are not. Of the complex phases, C is the least frustrated and conseq uently is stable at intermediate decrees of segregation. However, G be comes unstable in the strong-segregation regime because;he relative pe nalty for packing frustration increases with segregation. The PL and D structures are simply too frustrated, and therefore are never stable. (C) 1997 American Institute of Physics.