Liquid‐Crystalline Polymer and Block Copolymer Domain Alignment Controlled by Free‐Surface Segregation

Abstract

The surface effects and anchoring of liquid crystals (LCs) have long been significant concerns for material chemists and physicists.[1] The surface alignment of LCs by mechanical rubbing [2] is a widely recognized phenomenon and of particular significance in technological applications for display device fabrication.[3] Surface molecular orientations [4] and topographical grooves and undulations [5] of the substrate provide LC alignment effects. Furthermore, in the past two decades, the photoalignment of LCs on photoreactive polymer film surfaces by anisotropic irradiation [6] has become a significant method and an alternative to mechanical rubbing processes. The aligning substrates are not limited to polymer surfaces; various types of surfaces, such as hard inorganic materials [7] and soft bio-related interfaces, [8] can be used for the alignment induction. In addition to low-molecular-mass LCs, polymer LC materials are also aligned by the surface effect.[9] Despite the tremendous amount of accumulated knowledge and the number of potential applications, the surface alignment processes developed to date mostly involve manipulations on the surfaces of solid or condensed phases.Herein, we report on LC alignment alternation, which is attained by a modification of the free surface (air-film interface). The homeotropic surface anchoring effect and layer structuring at the free surface of calamitic LC molecules have been shown experimentally [10] and have been further verified by theoretical simulations.[11] To modify the free surface, the present approach adopts the surface segregation [12,13] of a small amount of a free-surface-active polymer. We demonstrate here that the coverage of the surface with the free-surface-active polymer leads to a homeotropic-to-parallel orientation change of LC mesogens, which further leads to an efficient in-plane photoalignment of microphase separation (MPS) domains of a relevant LC block copolymer by linearly polarized light (LPL).[14] With regard to the MPS alignment control, the important role of a top coat layer has recently been demonstrated by Bates et al. [15] In this case, a polar-to-nonpolar chemical conversion of the top layer is achieved to fulfill the requirement of spin-casting from an aqueous solvent and to provide a neutral (non-preferential) layer for the hydrophobic block copolymer during the annealing. In the present approach, in contrast, no additional coating procedure is required, providing a simple, versatile method for the desired alignment control of MPS domains.First, the alignment behavior of an LC azobenzene (Az) homopolymer (PAz in Figure 1 a; M n = 8.0 10 4 , M w /M n = 1.13, and g-43 8C-SmC-100 8C-SmA-120 8C-iso) was examined. A spincast film of PAz was prepared (thickness: 100-200 nm) from a chloroform solution. After annealing at 130 8C (above the isotropization temperature) for 10 min followed by gradual cooling via a smectic LC phase to room temperature, this film spontaneously formed the out-of-plane (perpendicular) orientation of Az side mesogens, a...

Free-surface molecular command systems for photoalignment of liquid crystalline materials

“…1 , right). This approach was inspired by our recent results showing that the surface coverage on the free surface of photoresponsive azobenzene side chain LC polymer films leads to a homoeotropic to planar orientational change in the mesogenic groups across the entire film 21 22 . This pre-oriented state leads to an effective in-plane photoalignment using LPL irradiation.…”

Photo-triggered large mass transport driven only by a photoresponsive surface skin layer

“…The θ gly values of a mixture of PAz and PCPBz (5:95 by weight) after annealing at 90 °C for 10 min before and after UV irradiation were 99.2 ± 1.6° and 89.4 ± 1.6° at 25 °C, which agree well with those of the pure PAz film (Table 1 ). These results indicate that annealing selectively segregates the PAz component to the free surface 41 – 43 .…”

“…Such a surface is called the command layer 45,52,63 . Recent investigations have revealed that a photoactive command layer can be placed at the free surface in SCLCP films [41][42][43][44][45][46][47] . Similarly, the present work offers another command layer scenario where large www.nature.com/scientificreports/ molecular amplification is reflected in macroscopic surface morphing.…”

“…www.nature.com/scientificreports/ areas. Related to the molecular orientation in SCLCP films, an Az-containing command layer at the free surface or inkjet-printed patterns could control the orientation of the majority of the mesogens of the base film [41][42][43][44][45] . Kawatsuki et al 46,47 also controlled of mesogen orientation from free surfaces using different types of photoreactive units.…”

New strategies and implications for the photoalignment of liquid crystalline polymers

“…Fukuhara et al 127 demonstrated that a PS-b-PAz film with vertically aligned PS MPS cylinders can be converted to a planar orientation by mixing in a small amount of PBMA-b-PAz and then annealing (case f in Figure 4). Coverage of the segregated PBMA-b-PAz at the free surface of PS-bPAz is responsible for this orientational inversion.…”