Topologically patterning of polyvinyl alcohol
microstructures for vertical-/hybrid-aligned nematic
liquid-crystal gratings doped with polyhedral
oligomeric silsesquioxane nanoparticles
Can Yang (杨 璨), Sensen Li (李森森), Wensong Li (李文松), Haijie Zuo (左海杰),
Lujian Chen (陈鹭剑)*, Baoping Zhang (张保平), and Zhiping Cai (蔡志平)
Department of Electronic Engineering, School of Information Science and Engineering,
Xiamen University, Xiamen 361005, China
*Corresponding author: lujianchen@xmu.edu.cn
Received March 30, 2015; accepted May 27, 2015; posted online June 26, 2015
We demonstrate a soft lithography approach for fabrication of a topographically patterned polyvinyl alcohol
(PVA) liquid-crystal (LC) alignment layer. This specific approach employs modified micromolding in capillaries
for negative replication of the PVA microstructures on indium tin oxide (ITO) substrates from patterned poly
(dimethylsiloxane) molds in a single step, leading to planar alignment on the desired regions. By doping with
polyhedral oligomeric silsesquioxane nanoparticles, which can induce vertical alignment on bare ITO surfaces,
periodic LC phase gratings based on an alternating vertical-aligned/hybrid-aligned nematic geometry are pre-
sented as an application, and a theoretical model was used to simulate and examine the experimental results.
OCIS codes: 160.3710, 160.4670, 160.5470, 050.1950.
doi: 10.3788/COL201513.081603.
Recently, topological patterns with surface relief micro-
structures have attracted intense attention due to their
robust capability for precise manipulation of the focal
conic domains (FCDs) in smectic liquid-crystal (LC)
phases
[1]
. By controlling the dimensions and symmetry of
the microstructures, the growth and arrangement of FCD
defects can be directed. Many micro-fabrication tech-
niques have been employed to fabricate these topological
patterns. Quite a few of the most exciting works fall into
an area known as soft lithography
[2,3]
, which is inexpensive,
convenient, and is of growing importance for a multitude
of applications such as microfluidics, cell biology, lab-on-
a-chip, and flexible electronics. Among them, micromold-
ing in capillary (MIMIC) is remarkable for its fidelity in
transferring the patterns from the mold to the polymeric
structures by capillary filling the channels
[4]
. It can
directionally constrain the growth of materials and pat-
tern free-standing microstructures from a broad range of
materials
[5]
.
Soft lithography is such an appealing route that can
also be used to pattern LC alignment layers, which are
crucial to obtain LC devices with a periodic refractive
index change
[6,7]
. Aside from rapid prototyping of both mi-
croscale and nanoscale structures, it can be used to yield
patterned LC alignment layers of various materials and
control the surface chemistry of them. Moreover, it is appli-
cable to patterning the LC alignment layers with complex
microstructures on curved or flexible substrates in a
compelling way that cannot be addressed effectively with
other sophisticated techniques, such as holographic re-
cording
[8]
, micro-rubbing
[9]
, mask photopolymerization
[10]
,
and atomic force microscope (AFM) patterning
[11]
.
For example, soft imprint lithography is used for exact
duplication of rubbed polyimide substrates to form align-
ment layers
[12]
. It can also produce polymer films with
checkerboard patterns of square wells, which vertically
align rod-shaped LCs in both nematic and smectic
phases
[13]
. Using microcontact printing method, Chae et al.
formed durable, ultrathin LC alignment layers that ensure
a fast response time, thermal stability, and compartmen-
talized multidomain alignment
[14]
. It was also demon-
strated that r eplica molding was employed to fabricate a
flexible LC device
[15]
. In this Letter, we propose a modified
MIMIC approach for the fabrication of a topography
patterned LC alignment layer using polyvinyl alcohol
(PVA). To illustrate the present method, several PVA gra-
tings with different periods were fabricated. Combining
rubbing PVA stripes with nanoparticle-induced vertical
alignment, which was previously implemented by doping
polyhedral oligomeric silsesquioxane (POSS) into LCs
[16]
,
we obtained LC phase gratings based on an alternating
vertical-aligned (VA)/hybrid-aligned nematic (HAN)
geometry. The fabrication process and the characterization
of the LC cells are described, and the experimental results
are analyzed using a model of the LC phase grating.
The poly(dimethylsiloxane) (PDMS) elastomeric mold
was prepared by cast molding. First, the prepolymer of
PDMS, a mixture of the elastomer base and curing agent
(Dow Corning, Sylgard 184) at a weight ratio of 10∶1, was
poured over the silicon master with different topographic
patterns. It was necessary to place the mixture under a
laboratory vacuum for 10 min to remove the air bubbles
from the mixture. Subsequently, the mixture was cured at
60°C for over 6 h and peeled off.
COL 13(8), 081603(2015) CHINESE OPTICS LETTERS August 10, 2015
1671-7694/2015/081603(5) 081603-1 © 2015 Chinese Optics Letters