Nematic liquid crystal 4-n-pentyl-4’-cyanobiphenyl alignment on micro-grooved surfaces
Date Issued
2006
Date
2006
Author(s)
Chiou, Da-Ren
DOI
zh-TW
Abstract
In this work, the soft embossing method is proposed to fabricate grating structure on polyimide surfaces. This method provides a convenient and rapid way to pattern polymer surfaces. These grating polyimide surfaces are assembled to form liquid crystal cells. It is found that the liquid crystal 4-n-pentyl-4’-cyanobiphenyl (5CB) director uniformly aligns along the groove direction even when the grating width is up to 3 μm. The anchoring energy of these grating polyimide surfaces is higher than that of the typical rubbed surfaces. We also mixed the planar alignment type polyimide prepolymer and homeotropic alignment type one to modify the surface property to successfully adjust the liquid crystal pretilt angle.
The square-wave grating silica surfaces are also fabricated by soft embossing silica sol-gel precursor on glass substrates with an elastomeric polydimethylsiloxane mould. The patterned silica surface could induce the planar alignment of liquid crystal 5CB along the direction of micro-grooves but with no pretilt angle. The pretilt angle of 5CBs is adjusted from 0o to 90o by the further deposition of organosilane self-assembled monolayers with different functional end-groups on the patterned silica surfaces.
Liquid crystal alignment was also investigated on the micro-grooved silica surfaces covered with organosilanes. We fabricated micro-grooved silica surfaces by soft embossing method followed by forming self-assembled monolayers (SAMs) of organosilanes on these micro-grooved silica surfaces. Using the soft embossing method and the broad range of the end-groups of the organosilanes, we could investigate the surface topography and surface polarity effects on the supported liquid crystals independently. By means of the surface polarity effects, we could control the liquid crystal pretilt angles by the modifications of surface chemical properties under fixed surface topography and liquid crystal type. We successfully adjusted the liquid crystal pretilt angle over the whole range from 0o to 90o by tuning the hydrophobicity of the surfaces. The correlations between water contact angle and liquid crystal pretilt angle further confirm that the liquid crystal pretilt angle is indeed only affected by the surface polarity under the same surface topography and liquid crystal type.
Finally, we fabricated patterned chemical functionality surfaces and patterned polyaniline surfaces by micro-contact printing method. With the resulting alternative surface polarity, we can control the alignment of nematic liquid crystals 5CBs on these specific regions. It is found that 5CBs show planar alignment on glass surfaces, n-phenylaminopropyltrimethoxysilane (PMTS) SAMs and polyaniline surfaces according to the polarized optical microscopy, whereas homeotropic alignment on octadecyltrichlorosilane (OTS) SAMs. This is due to the differences of the surface energies or surface polarities between the polar SAMs as well as polyaniline and the OTS SAMs. From this viewpoint, we could make multi-domain alignment of nematic liquid crystals by micro-contact printing method.
Subjects
向列型液晶
微溝槽結構
液晶排列
Nematic liquid crystal
micro-grooved surfaces
liquid crystal alignment
Type
thesis
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