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

Abstract

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.

100th Anniversary of Macromolecular Science Viewpoint: Opportunities for Liquid Crystal Polymers in Nanopatterning and Beyond

“…Recent work by Osuji et al presented a different approach to manipulate thin-film morphologies in LC BCPs. 39 The method relies on blending labile mesogens with neat LC BCPs (Figure 3i). The CB-and poly(lactic acid) (PLA)-based side chain LC BCPs used for the study formed parallel lamellae and cylinders as neat materials (i.e., without additives) in thin films.…”

“…On the left side, AFM images show parallel cylinders and lamellae (top and bottom, respectively), and on the right side, AFM images show perpendicular cylinders and lamellae (top and bottom, respectively). Adapted with permission from ref . Copyright 2021 American Chemical Society.…”

“…Recent work by Osuji et al. presented a different approach to manipulate thin-film morphologies in LC BCPs . The method relies on blending labile mesogens with neat LC BCPs (Figure i).…”

Properties and molecular self‐assembly of liquid crystalline hard–hard diblock and hard–soft–hard triblock copolymers influenced by Flory–Huggins interaction parameter and volume fraction of distinct segments

“…Self‐assembly through phase separation in the BCs generally occurs by two major processes called thermal annealing 67–69 and solvent vapor annealing (SVA) 70–72 to enhance the mobility of block copolymer chains, and thus produce a morphology at thermodynamic equilibrium. Among these annealing techniques, the SVA is known to be a simple, rapid, low‐cost, versatile and highly efficient technique for producing highly ordered microdomains at room‐temperature 71,73 .…”

Enhancing ionic conductivity in tablet–bottlebrush block copolymer electrolytes with well-aligned nanostructures via solvent vapor annealing

“…However, most oriented nanostructures studied so far have been of cylindrical morphologies, and the oriented LAM morphology with a higher f value has rarely been reported. 24 Stein and co-workers studied the LAM structure of the diblock copolymer of polystyrene (PS) and a PIL poly(1-(2-acryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)imide). 25 They demonstrated that the ion conductivity of PS-PIL BCPE films was approximately 20 times higher in the perpendicular direction than in the through-plane direction.…”