Can linear block copolymers stabilize perpendicular lamellae in linear-cyclic block copolymer blend films?

Block copolymer (BCP) thin films with lamellar nanostructures oriented perpendicular to the substrate surface are a potential alternative to photolithography for patterning nanoscale features. Molecular cyclization has been shown to reduce BCP feature size and improve thin film stability, but orientation control also is needed for lithography. Here, we use dissipative particle dynamics (DPD) simulations to study the relative stability of perpendicular (vs. parallel or mixed) lamellae of cyclic and linear BCPs. To mimic experiments, BCP chains are confined between a substrate (2D lattice of DPD beads), and a “gas” (bath of DPD beads). With non-selective surface interactions, both linear and cyclic BCPs form perpendicular lamellae as expected. To test relative stability, we increased the substrate surface preference for one of the blocks until lamellae oriented parallel to the substrate and found that perpendicular lamellae are more stable for linear BCPs than for cyclic BCPs. Moreover, adding 10% linear chains enhances perpendicular lamellae stability in majority-cyclic BCP blends without changing feature size. We seek to understand the thermodynamics behind these differences based on near-surface chain orientations and free energy calculations.