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Liquid Crystals
ISSN: 0267-8292 (Print) 1366-5855 (Online) Journal homepage: http://www.tandfonline.com/loi/tlct20
Study on polyvinylidene fluoride as alignment
layer in twist-nematic liquid crystal display
Yi-Fei Wang, Yu-Qiang Guo, Ya-Xuan Ren, Ming-Zhu Fu, Ji-Liang Zhu & Yu-Bao
Sun
To cite this article: Yi-Fei Wang, Yu-Qiang Guo, Ya-Xuan Ren, Ming-Zhu Fu, Ji-Liang Zhu & YuBao Sun (2017): Study on polyvinylidene fluoride as alignment layer in twist-nematic liquid crystal
display, Liquid Crystals, DOI: 10.1080/02678292.2017.1390791
To link to this article: http://dx.doi.org/10.1080/02678292.2017.1390791
Published online: 24 Oct 2017.
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Date: 25 October 2017, At: 23:59
LIQUID CRYSTALS, 2017
https://doi.org/10.1080/02678292.2017.1390791
Study on polyvinylidene fluoride as alignment layer in twist-nematic liquid
crystal display
Yi-Fei Wanga, Yu-Qiang Guoa,b, Ya-Xuan Rena, Ming-Zhu Fua, Ji-Liang Zhua and Yu-Bao Suna,b,c
ABSTRACT
ARTICLE HISTORY
The material with high dielectric constant can significantly affect the distribution of the electric
field, so this kind of material has great potential in liquid crystal display. In this paper, polyvinylidene fluoride (PVDF) as alignment layer in liquid crystal display was analysed. The optical
property, mechanical property, thermal stability and electrical property of PVDF were measured.
Experiments show that the absorbance of PVDF material is 0.2 (or less) in visible light, which is
better than the conventional alignment material polyimide (PI). The alignment effect can be
generated by mechanical friction and the liquid crystal molecules are ordered or aligned, and
PVDF can maintain good thermal stability as temperature is lower than 400°C. Since the dielectric
constant of PVDF is usually between 6.0 and 8.0, it has significant effect on the distribution of the
electric field in the liquid crystal display, and its dielectric loss is also less than PI. The lower
operating voltage and the faster response time are obtained from the experiment. It can be
confirmed by the experiments that PVDF could be used in liquid crystal display (LCD) as the
alignment layer to improve LCD’s characteristics.
Received 3 July 2017
Accepted 8 October 2017
1.0
Polyvinylidene fluoride;
polyimide; dielectric
constant; operating voltage
PVDF
PI
Transmittance
0.8
0.6
0.4
0.2
0.0
0.0
KEYWORDS
1.0
PVDF
PI
0.8
Transmittance
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a
Department of Applied Physics, Hebei University of Technology, Tianjin, PR China; bSchool of Electronic and Information Engineering,
Hebei University of Technology, Tianjin PR China; cTianjin Key Laboratory of Electronic Materials and Devices, Hebei University of
Technology, Tianjin, PR China
0.6
0.4
0.2
(b)
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0
10
Voltage/V
1. Introduction
Nowadays, liquid crystal displays (LCDs) have become
a kind of common display device with different size in
our daily life, and a number of researches have been
done to optimise the LCDs over the years [1–4]. Many
research groups have demonstrated that nanomaterials
in nematic LCs improved the performance of LCDs [5–
8].The conductive carbon nanotubes were found that it
could be used in TN-LCDs to decrease the driving
CONTACT Yu-Bao Sun
sun_yubao@163.com
© 2017 Informa UK Limited, trading as Taylor & Francis Group
20
30
40
500 510 520 530 540 550
Time/ms
voltage by lowering the ion effect [9]. Moreover, the
incorporation of carbon nanotubes (CNTs) in the
liquid crystal host was confirmed to decrease the optical response time of TN-LCDs [10]. The research field
of alignment layer, to achieve the desired results, has
recently emerged as a promising new area of technological development [11–14]. The alignment layer with
nanosised ferroelectric particles has much larger local
electric fields, which can polarise the liquid crystal
Y.-F. WANG ET AL.
molecules and thus indirectly increase the intermolecular interaction, which will cause the lowering of
switching voltage [15,16]. A super-faster response
time can be achieved due to the single-wall carbon
nanotube and PI composite alignment layer in TNLCDs [17]. Moreover, Hong-Gyu Park et al. [18] produced homogeneously aligned LCs on ZnO using a
sol–gel method and IB irradiation. The results indicate
that ZnO alignment layers deposited by a sol–gel
method have considerable potential in LCD device
applications. Rajratan Basu et al. [19] have argued
that graphene can act as the alignment layers and the
transparent electrodes in an LC cell.
When the dielectric constant of the alignment layer
is increased, the potential drop in the alignment layer is
lowered and the potential drop in the liquid crystal
layer is increased, so that the operating voltage can be
reduced. The potential distribution is computed by
solving the Poisson equation:
Ñ ðε ÑΦÞ ¼ 0
We had used polyimide/polyvinylidene fluoride/
polyaniline (PI/PVDF/PANI) composite as the alignment layer of TN-LCD to lower the operating voltage
[20]. When the thickness of the alignment layer is
constant, the appropriately increase dielectric constant
of the alignment layer lower the operating voltage of
the liquid crystal display. However, the optical property, mechanical property, thermal stability and electrical property of the material are not investigated.
Also, the transparency of the PI/PVDF/PANI composite film is not good because the conductive PANI is
dark green powder, which affects the application of this
composite material in LCD.
In this paper, we use PVDF as the alignment layer to
lower the operating voltage and reduce the response
times. The dielectric constant of PVDF film is about
6.3. The influence of PVDF film as alignment layer in
TN-LCD was studied by simulation and experiment,
the optical property, mechanical property, mechanical
property, thermal property and dielectric property of
PVDF film were studied.
2. Simulation
In order to evaluate the electro-optic characteristics of
the proposed alignment layer material, we made a
series of simulations by the commercial simulation
software TechWiz 1D (Sanayi System Co., Korea).
The parameters of the LC in both simulations and
experiments are shown in Table 1. The rotational
Table 1. Parameter of LCD.
Item
LC
N (Refractive index)
ne
no
1.721
1.517
P/A
ITO
Glass
1.5 + 0.001929i
1.7
1.5
ε
ε∥
26.3
ε⊥
6.8
/
/
4.5
K (pN)
K11 = 13.6
K22 = 6.6
K33 = 9.9
/
/
/
d (μm)
2.8
230.0
0.1
500
ITO: indium tin oxides.
viscosity of LC is 87.0 mPas, and the light wavelength
is 550 nm.
The electro-optical curves of TN-LCDs with different
dielectric constant alignment layers are shown in
Figure 1. The operating voltage is significantly reduced
when the dielectric constant of the alignment layer is
increased. From Figure 1, when PVDF is used as the
alignment layer, which dielectric constant is larger than
6.0, instead of the PI which has a dielectric constant of 3.8,
the operating voltage is remarkably reduced. When the
dielectric constant is larger than 20, the operating voltage
will be reduced more, but the reduction becomes inconspicuous. However, the alignment layer materials with
the dielectric constant between 10 and 20 are scarce.
The response process of the TN-LCDs was simulated and shown in Figure 2; the driven voltage is set as
3 V and 2.5 V respectively. From Figure 2, the rise time
is reduced with the increasing dielectric constant
because of the lower threshold voltage in the cell with
higher dielectric alignment layer as shown in Figure 1,
and the decay time is similar.
3. Experiments
PVDF is a semi crystalline polymer with excellent
piezoelectric and ferroelectric properties. There are
1.0
ε=3.8
ε=4.5
ε=5.5
ε=6.3
ε=10
ε=20
ε=50
ε=100
ε=200
0.8
Transmittance
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2
0.6
0.4
d=0.1
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Voltage/V
Figure 1. (Colour online) Voltage-dependent transmittance
curves of different dielectric constants of alignment layer.
LIQUID CRYSTALS
1.0
1.0
ε=6.3
ε=3.8
d=0.1
ε=6.3
ε=3.8
0.8
Transmittance
Transmittance
0.8
0.6
0.4
d=0.1
0.6
0.4
0.2
0.2
(a)
0.0
3
(b)
0.0
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time/ms
Time/ms
Downloaded by [UAE University] at 23:59 25 October 2017
Figure 2. (Colour online) Time dependent transmittance curves of different dielectric constant of alignment layer at (a) 3 V and (b) 2.5 V.
five crystalline phases of PVDF called α, β, γ, δ and ε.
The β-phase is the polar phase because of its special
structure, in which the H and F are on the opposite
side of the main backbone chain. The special structure
cause non-zero dipole moment in PVDF and lead to
high ferroelectric, piezoelectric and dielectric properties [21–25]. The structural formulas of PI and PVDF
are shown in Figure 3. PI has the low dielectric constant because the symmetrical structure is perpendicular to the polymer’s chain. In PVDF, the asymmetrical
structure shows the dipole moment effect, so it has a
high dielectric constant. Moreover, the resistivity of
PVDF is about 3 × 1012 Ω·cm, which is small lower
than PI [26–28].
In addition, the PVDF material has been widely
used because of its great dielectric property and filmforming property. PVDF films with different thicknesses (5 nm to 1 μm) can be obtained by spin coating
[29]. The results show that the content of β-phase in
PVDF can be improved by annealing and hot pressing
at high temperature, so that the dielectric properties of
PVDF can be improved [30,31].
A certain PVDF powder (Shanghai 3F New Material
Co., Ltd) is added into the N,N dimethylformamide
(DMF, Tianjin Damao Chemical Reagent Co., Ltd), and
PVDF was completely dissolved in DMF after magnetic
stirring and ultrasonic dispersion. Three drops of PVDF–
DMF solution or PI solution on (with the same solid state
content) were dropped on the conductive surface of
indium tin oxides (ITO) glass (2 cm × 2 cm), and the
solution was evenly tiled on the ITO glass by spin coating
process. The rotational speeds of spin coating process are
v1 = 880 r/min (for 9 s) and v2 = 1450 r/min (for 30 s).
These glasses were then placed into the oven and heated
at 90°C for 30 min and then at 180°C for 2 h. 0.1-μm
thickness alignment layers were obtained. After that we
prepared TN-LCD cells through normal fabrication processes, and the cell gap was 3.1 μm which was measured
by UV spectrophotometer.
We prepared and tested LCD cells by series of
instruments of Chengdu Century Branch Instrument
Co., Ltd. The absorbance of PVDF and PI were measured by spectrophotometer (723CRT, Shanghai Youke
Instrument Co., Ltd). The surface of the materials was
characterised by a tapping mode atomic force microscope (Aglient 5500 AFM). Thermo-gravimetric analysis of the PVDF was carried out by synchronous
thermal analyser (SDT-Q600). The dielectric property
was measured by JK2828 LCR Meter.
4. Results and discussion
Figure 4 is the light absorption spectrum of PVDF and
PI solutions in the UV-visible light, and the inner picture on the upper right corner in the figure is the photo
of two solutions in natural light. From the absorbance
graph, the PI solution has a certain light absorption
between 320 and 430 nm, and the absorbance reaches
a maximum of 2.184 at 361 nm. Moreover, it can be
seen that the PI solution shows yellow colour from the
inner picture. This is due to the fact that there are
carboxyl group on the imine ring and the phenyl
group attached to the imine ring in the PI. Although
the absorption ranges of these two groups are in the
Figure 3. (Colour online) The structural formula of PI (left) and PVDF (right).
4
Y.-F. WANG ET AL.
3.0
2.5
100
PVDF
PI
80
Weight / %
Absorbancy
2.0
1.5
1.0
PVDF
60
40
0.5
20
0.0
400
500
600
700
100
800
200
300
ultraviolet region, the red shift of the absorption spectrum occurs when putting them together to form an
aromatic polymer. PI material has a significant absorption peak in the shortwave range of visible light. When
PI is used as the alignment layer, it cannot affect the
display’s colour because of the thin thickness (less than
100 nm). While the absorption of PVDF material is
relatively uniform, it is almost transparent in the natural
light. As a result, PVDF can be fabricate a thicker layer
for the better dielectric and insulation characteristics,
because it is a transparent layer.
Figure 5 shows the AFM plot of the surface of PI
film and PVDF film, respectively. (a) is the morphology of the PI surface, and (b) is the morphology of the
PVDF surface. Although it looks like that there are
more grooves on PI, the grooves on PI are much
shallower than that on PVDF.
Figure 6 shows the thermo-gravimetric curve of the
PVDF. It can be seen from the figure that the thermal
decomposition temperature of PVDF is over 400°C.
When heated to about 500°C, the thermal decomposition
of the material mass fraction is about 70%. Its decomposition temperature is lower than that of conventional
500
600
700
800
Figure 6. (Colour online) TGA thermograph of pure PVDF.
PI, but is far more than the maximum deal temperature
(180°C) during the experiment and production. As a
result, PVDF has good thermal stability.
The electro-optical characteristic curve and switching behaviour of the liquid crystal display was tested
with visible light. Figure 7 shows the electro-optical
1.0
PVDF
PI
0.8
Transmittance
Downloaded by [UAE University] at 23:59 25 October 2017
Figure 4. (Colour online) The absorbance of PI and PVDF.
400
Temperature /
wavelength / nm
0.6
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Voltage/V
Figure 7. (Colour online) Voltage-dependent transmittance for
the different kinds of TN-LCD.
Figure 5. (Colour online) AFM surface topography for (a) PI and (b) PVDF after rubbing process.
LIQUID CRYSTALS
North LCD Engineering R&D Center). Figure 9
shows the response processes of TN-LCD with alignment layer of PVDF and PI under two driven voltages ((a) is 3.0 V and (b) is 2.5 V). The voltage is
applied at t = 0 ms and cut at t = 500 ms. From
Figure 9(a), the rise and decay times (τ r /τ d ) of PVDF
are (6.00 ms/15.91 ms) which is less than that of PI
(6.00 ms/17.59 ms) for 3.0 V; in the decay process,
there has a optic bounce in the two cell, the reason is
the backflow effect in TN-LCD when the driven
voltage is large enough. From Figure 9(b), the rise
time of PVDF is 8.59 ms, and the decay time is
10.55 ms, and the response times of PI is 9.87 ms/
11.56 ms. The response times of PVDF are less than
that of PI because of the stronger electric field
strength and the different pretilt angles (PVDF is
characteristics of these two cells. The increasing dielectric constant of the alignment layer can reduce the
potential drop in the alignment layer, so the horizontal
electric field in liquid crystal layer is increased. As a
result, the operating voltage is reduced. From Figure 7,
the operating voltage (for the transmittance equal to
1.0) of the cell with PVDF as the alignment layer
(1.67 V) is significantly lower than the operating voltage of the cell with PI as the alignment layer (1.87 V),
which is consistent with the simulation results. Figure 8
shows the POM images of the dark and bright states of
the PI and PVDF TN-LC cells, the difference of these
two kinds of alignment layers is very little and has no
effect on contrast ratio.
The switching behaviours of two TN-LCDs were
tested by LCD parameter testers (LCT-5016C. The
Downloaded by [UAE University] at 23:59 25 October 2017
5
P
50µm
A
a
b
c
d
P
50µm
A
Figure 8. (Colour online) The polarised optical microscope (POM) pictures of the (a) (b) PI TN LC cell and (c) (d) PVDF TN LC cell at
dark and bright states, respectively.
1.0
1.0
PVDF
PI
PVDF
PI
0.8
Transmittance
Transmittance
0.8
0.6
0.4
0.6
0.4
0.2
0.2
(a)
0.0
(b)
0.0
0
10
20
30
40
500 510 520 530 540 550
Time/ms
0
10
20
30
40
500 510 520 530 540 550
Time/ms
Figure 9. (Colour online) Transmittance-dependent time for the TN-LCDs with two kinds of alignment layers and two different
driven voltages: (a) 3.0 V and (b) 2.5 V.
6
Y.-F. WANG ET AL.
property and film-forming property. It can be used
to make the thicker film and form uneven and strong
electric field in liquid crystal devices.
Acknowledgment
(a): PI 100 Hz
(b): PI 1k Hz
This work was supported by the National Natural Science
Foundation of China [Grant Numbers 61475042, 11304074
and 11274088] and the National Natural Science Foundation
of Hebei Province [Grant Numbers A2015202320 and
GCC2014048].
Disclosure statement
No potential conflict of interest was reported by the authors.
Downloaded by [UAE University] at 23:59 25 October 2017
(c): PVDF 100 Hz
(d): PVDF 1k Hz
Figure 10. The capacitance and dielectric loss of PI and PVDF.
1.7° and PI is 3.0°). The anchoring energy of PI and
PVDF is in the same scale because of the similar
response time.
Finally, in order to clear the dielectric characteristics
of PVDF, we prepared two cell filled with PI or PVDF.
The capacitance and dielectric loss of PI and PVDF are
tested at two frequency (a) and (c) under 100 Hz, and
(b) and (d) under 1000 Hz, and shown in Figure 10.
The dielectric loss of PVDF is less than that of PI at the
same frequency. The high dielectric constant of PVDF
is due to the trans-conformation in the main chain.
Hydrogen atoms (H) and fluorine atoms (F) are on the
opposite side of the main backbone chain, resulting in
polar polymorph β-phase, as a result, the ion accumulation issues can be negligible. PVDF’s dielectric property is better than PI’s.
5. Conclusion
In this paper, we studied the PVDF’s characteristics
as alignment layer. The experiments show that the
absorbance of PVDF material has a better transparency than PI. PVDF material has good thermal stability, and its thermal weight loss curve remains
stable at the annealing temperature of 180°C, and
its dielectric loss is also less than PI. The operating
voltage of TN-LCD with PVDF is reduced compared
with that of PI, and the response times can be
reduced as well. As a result, PVDF can be used as
the alignment layer of liquid crystal display for
improving
the
electric-optic
performance.
Furthermore, as we expect, PVDF can be used in
liquid crystal lens because of its great dielectric
Funding
This work was supported by the National Natural Science
Foundation of China [11274088,11304074,61475042];
National Natural Science Foundation of Hebei Province
[A2015202320,GCC2014048], the Key Subject Construction
Project of Hebei Province.
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