Nano-imprinted polymer films for aligning liquid crystals

 

Alignment of liquid crystals (LCs) is necessary to ensure uniform optical properties of display pixels and to enhance the physical properties of LC devices. Polymer films nano-imprinted with various topographic patterns can be used to align liquid crystals. For example, linear patterning of thiol-ene polymer films yielded uniform alignment, while shallow square wells showed bistable alignment of nematics1, illustrated in Figure 1.

Checkerboard patterns of square wells of width varying from 200 nm to 800 nm align calamitic (rod-like) LCs vertically, horizontally or tilted depending on the depth/width ratio of the wells. The LCs prefer to orient parallel on polymer films that are smooth but when the films are topographically patterned, the increasing elastic energy density as the wells become narrower eventually overcomes the surface anchoring of the polymer and the average orientation of LCs makes a transition from planar to vertical2, as shown in Figure 2. Compared to other alignment methods, such as mechanical rubbing or chemical modification of substrates, this method enables us to control the alignment of LCs accurately in a variety of directions solely by varying the scale and depth of the topographic patterns.

Figure 1 : Bistable alignment of a nematic liquid cell with a patterned surface of shallow square wells. The average director field is along one of the two diagonals of the squares. This cell shows a domain boundary across which the director orientation jumps by 90°.

Figure 2. Polarizing optical microscope image of a nematic liquid crystal cell made with a nano-imprinted polymer film. The director of the uniform dark area, which has the finest square well pattern, is vertical while the director in the other patterned areas is slightly tilted. The size of the scale bar is 20 µm. Insets show the topography of the polymer film and a cartoon of the director field in a deep square well.

References

[1] Y. Yi, M. Nakata, A. Martin, and N. Clark, Applied Physics Letters 90, 163510 (2007).
[2] Y. Yi, G. Lombardo, N. Ashby, R. Barberi, J. E. Maclennan, and N. A. Clark, Physical Review E 79, 041701 (2009).

Text and images contributed by Youngwoo Yi.