Effects of Labile Mesogens on the Morphology of Liquid Crystalline Block Copolymers in Thin Films

The self-assembly of liquid crystals (LCs) in small volumes is mediated by the anchoring conditions of the constituent mesogens at the confining surfaces. In hierarchically ordered systems such as LC block copolymers, the importance of anchoring at external as well as internal interfaces can give rise to intriguing phenomena. Here, we examine the morphology of thin films of liquid crystalline block copolymers (LC BCPs) in the presence of non-bonded, or labile, mesogens and explore the manner in which external anchoring conditions of the LC control the orientation of the block copolymer superstructure. We find that the addition of 4′-(hexyloxy)-4-biphenylcarbonitrile (6OCB) mesogens induces an orientation change of cylinder- or lamellae-forming LC BCPs, from parallel to perpendicular with respect to the film surface. Systematic variations of free mesogen concentration, thermal annealing conditions, film thickness, and the type of free mesogens added were conducted. Our results indicate that labile 6OCB mesogens co-assemble with their backbone-bonded counterparts and adopt homeotropic anchoring at the free surface (air interface). Coupled with homogeneous anchoring of the LC mesophase at the internal block copolymer interface, the labile mesogens induce a perpendicular orientation of the BCP superstructure in thin films. We exploit this behavior as a mechanism to control BCP morphology and demonstrate the formation of perpendicular structures on topographically patterned substrates. Investigations conducted with different mesogens point to the importance of the labile mesogen anchoring at the air interface and steric constraints on co-assembly with backbone-bonded mesogens as key aspects dictating the ability of labile mesogens to control the BCP morphology in thin films. On this basis, we discuss design rules for mesogenic additives that can be used to engineer LC BCP morphology in thin films.