Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest for applications such as sophisticated optical objects, sensors and actuators. To exploit the full potential of these nanoscale materials we need precise alignment of LC molecules on a substrate and the ability to image LC director field with nanometer scale resolution, especially for complex geometries and topologies. However, existing non-glassy, low molecular weight nematic liquid crystals (NLCs) have a tendency to reorient during fast freeing making surface alignment difficult. Further, the high cost of liquid crystal monomers (LCMs) and polymers (LCPs) has limited the implementation of LCMs and LCPs in practical applications.
Here we show, for the first time, the direct, real-space mapping of the nematic director at the nanoscale on complex topographies by using a specially designed photocrosslinkable LCM with strong dipole-dipole interactions, resulting in a stable nematic phase. The stable nematic LCM can suppress spurious defects in the bulk, leading to uniform LC alignment on topographical surfaces, while traditional LCMs often fail to align uniformly on the same surfaces. More importantly, the optical signature of LCMs can be faithfully “locked” upon polymerization, allowing us to directly visualize LC director field and defect structures in the third dimension using scanning electron microscopy (SEM).
The strong anchoring ability on various surfaces and preservation of LC director field during polymerization are the key outcomes of our innovative molecular design. In turn, it allows us to systematically study LC defect structures under various boundary conditions. Moreover, our LC monomer can be produced at a cost of roughly 10% of that of a commercially available small molecule LC, 5CB, but with equal flexibility to be aligned on various boundary conditions.
Armed with the newfound ability, we investigate more complex director field structures, including point defects and line defects created by colloids with homeotropic anchoring suspended in NLC, as well as various metastable states with 100 nm accuracy. While optical techniques give ambiguous interpretations of the full three-dimensional LC director, our SEM images provide detailed and direct measurement of the director field, so accurate that we can use SEM image to calculate the elastic constants and bending energy of LCs along the defect line as we move from the bulk towards the boundary.
Functional Liquid Crystal Monomers (LCMs) have been used for decades in conjunction with Liquid Crystal Polymers (LCPs) as actuators and sensors, but the challenge of controlling and maintaining LC alignment on surfaces has remained an inherent obstacle. Existing materials typically have high phase transition temperatures, and the low viscosity at these temperatures limits the ability to control the director field during polymerization.
LC structures and compositions that incorporate strong dipole-dipole interactions lead to a highly stable nematic phase with strong anchoring strength. Upon photo-cross-linking, the orientation of the mesogens can be faithfully locked without reorientation. This has the advantage of allowing for direct visualization of the LC director field and defect structures with 100 nm resolution.
Top panel shows schematic illustrations of the chemical structures of various liquid crystal monomers, and random intermolecular dipole-dipole interactions between nitro and carbonyl groups. Different colors represent different functional groups: red for nitro; blue for carbonyl.
Bottom panel shows SEM images of silica colloids suspended in crosslinked liquid crystal polymers where either a point defect (A) or a line defect (B) is formed to screen the charge of the colloid. Insets: Polarized optical microscopy (POM) images of point (A) and line (B) defects circumscribing silica colloids. The director field of the LC is represented by the red dotted line and the red crosses show the position of defects. Scale bars: 2 µm
- Large nematic window of 40K to > 100K
- Excellent surface anchoring characteristics.
- Optical signatures remain unchanged in the liquid crystal polymers allowing for direct visualization by SEM with 100 nm resolution.
- Lower cost compared to 5CB, a well-established liquid crystal small molecule, and processability is consistent with current LC manufacturing processes.
- Optical film, coating or encapsulant with high positive birefringence
- Actuators, sensors, or artificial muscle
- Optical wave guides or polarizers
- Photovoltaic substrates
Stage of Development:
Early stage research
Xia, Y.; Serra, F.; Kamien, R. D.; Stebe, K. J. and Yang, S.*, "Direct mapping of local director field of nematic liquid crystals at the nanoscale", Proc. Nat. Acad. Sci. USA, 2015, 112 (50), 15291–15296. DOI
Docket # 15-7434