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Liquid Crystals
ISSN: 0267-8292 (Print) 1366-5855 (Online) Journal homepage: https://www.tandfonline.com/loi/tlct20
Optical diffusers based on uniform nano-sized
polymer balls/nematic liquid crystals composite
films
Le Zhou, Mohsin Hassan Saeed & Lanying Zhang
To cite this article: Le Zhou, Mohsin Hassan Saeed & Lanying Zhang (2019): Optical diffusers
based on uniform nano-sized polymer balls/nematic liquid crystals composite films, Liquid Crystals,
DOI: 10.1080/02678292.2019.1679901
To link to this article: https://doi.org/10.1080/02678292.2019.1679901
Published online: 29 Oct 2019.
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Optical diffusers based on uniform nano-sized polymer balls/nematic liquid
crystals composite films
Le Zhou, Mohsin Hassan Saeed and Lanying Zhang
Department of Materials Science and Engineering, College of Engineering, and Key Laboratory of Polymer Chemistry and Physics of Ministry of
Education, Peking University, Beijing, People’s Republic of China
ABSTRACT
Optical diffusers are promising diffusing materials in the optical devices such as monitors, projec-
tors, fibre optics, light-emitting diode (LED) systems and liquid crystal displays (LCDs). We report
optical diffusers comprising uniformly distributed nano-sized polymer balls/nematic liquid crystals
(LCs) by ultraviolet (UV) click reaction of ene monomer and thiol monomer. By optimising the mass
ratio 1:1 of ene and thiol, of which the average diameter of the corresponding nano-sized polymer
balls is about 900 nm, relatively high optical transmission and haze with 88.99% and 94.49% are
yielded, respectively. Furthermore, by controlling the curing time, the average diameter of nano-
sized polymer balls can be reduced to 810 nm, and the developed film exhibits high transmission
(98.49%) without sacrificing the high haze (91.77%). This paper demonstrates that UV click reaction
is an economical approach to fabricate optical diffusers in a controllable manner.
ARTICLE HISTORY
Received 17 July 2019
Accepted 9 October 2019
KEYWORDS
Optical diffusers; total
transmission; transmission
haze; liquid crystals;
nano-sized polymer balls
1. Introduction
Liquid crystal displays (LCDs) are widely employed
inmobilephones,laptops,tablets, and other optical
devices. As liquid crystals (LCs) cannot emit light,
a backlight source is necessary for processing viable
images, of which optical diffusers play a critical role
in preventing light sources from being seen directly
CONTACT Lanying Zhang zhanglanying@pku.edu.cn
LIQUID CRYSTALS
https://doi.org/10.1080/02678292.2019.1679901
© 2019 Informa UK Limited, trading as Taylor & Francis Group
by viewers in the lighting system. Additionally,
optical diffusers are a key optical component in
the light-emitting diodes (LEDs), which can spread
thepointlightsourceuniformlywithouthigh-
intensity spots.
Generally, there are two different types of optical
diffusers: particle-diffusing type and surface-relief type.
The former type consists of organic, inorganic nanopar-
ticles or core-shell particles embedded in polymer sub-
strates, while the later type employs surface-relief
structures including pyramids, micro-lens, and other
textured surfaces. For the former optical diffusers, the
agglomeration of diffusing particles has led to the low
transmittance of optical diffusers. However, for the later
optical diffusers, the fabrication process is complex, thus
novel high-performance optical diffusers have attracted
considerable attention. G. H. Kim et al. have engineered
an optical diffuser by coating the diffusing material
MBX-8 on a poly (ethylene terephthalate) (PET)/poly-
carbonate (PC)/poly (butylene terephthalate) (PBT)
matrix, the superior thermos-physical property with
improved diffusing ability have been achieved in the
optical diffuser [1]. To avoid warpage of optical diffu-
sers, a series of poly (methyl methacrylate) (PMMA)
based optical diffusers modified by glass fibres have
been fabricated, which show improved diffusing abil-
ities, low water absorption and small shrinkage [2]. To
achieve a uniform size distribution of hemispherical
PMMA droplets on the PET film, they have further
introduced an electrospray coating method, thus trans-
mittance of the fabricated optical diffuser is 50% when
the radius of the PMMA droplets is 10 µm [3].
Additionally, they have studied the effects of the proces-
sing parameters such as field strength and frequency on
the size distribution and roughness of the PMMA dro-
plets [4]. Tzu-Chien Huang et al. have demonstrated
a hybrid extrusion roller embossing process for fast
fabricating an integrated surface-relief and particle-
diffusing diffuser, thus high transmittance (98.0 %)
and high haze (89.5%) are both reached in the integrated
diffuser [5]. By simply mixing silicone polymer and
NaCl aq. solution, micro-scaled water droplets with
various sizes are randomly distributed in silicone poly-
mer, herein, water droplets are regarded as light-
diffusing particles, thus the method has a great potential
in the mass production of optical diffusers [6]. Due to
excellent optical properties of organic-inorganic hybrid
materials, Hu et al. have fabricated multiple micro-
spheres such as polysiloxane@CeO
2
@PMMA micro-
spheres [7–9], boehmite hollow microspheres [10],
ZnO@polysiloxane microspheres [11], nano (ZnO-
CeO
2
) @polysiloxane core-shell microspheres [12],
organosiloxane-coated SiO
2
/CeO
2
microspheres [13]
and 3D flower-like hollow Mg-Al layer double hydro-
xide microspheres [14], then these microspheres are
applied in optical diffusers, which exhibit good diffusion
ability and low incident angle dependence [14].
Recently, by heating the commercial soda-lime-silica
glass, Haider Butt et al. have demonstrated a novel
devitrite-based optical diffuser with large scattering
angle [15]. Similarly, by the thermal treatment of zinc
oxide-containing ceramic glazes, thin nanoscale needle-
like willemite crystals are produced, which lead to light
scattering with high angles of up to 80° [16]. In order to
fabricate an electrically variable optical diffuser, they
combine devitrite and liquid crystals (LCs) to prepare
the device, which could switch the transmitted light
from a horizontal diffusion to a random diffusion [17].
Except for exploring high-performance materials, they
have also developed different production methods such
as femto-second laser [18] and nanosecond pulsed laser
[19] for preparing novel optical diffusers, thus the pro-
duced structures lead to high diffusing efficiency and
a wide viewing angle [18–20]. W. Suthabanditpong et al.
have successfully improved the optical properties of
optical diffuser by utilising hollow silica nanoparticles
with a hierarchical structure as diffusing particles [21].
In our previous works, LCs/polymer composite
films have been found greater advantages over parti-
cle-diffusing type optical diffusers due to the adjusta-
ble polymer microstructures in the composite films
[22–25]. Generally, high haze accompanies with low
transmission, according to Sajad’s work, in the defini-
tion of haze, H λðÞ¼1direct transmissionðλÞ
total transmissionðλÞ
100%, we
only consider the transmission and haze of 560 nm
wavelength light. Although the LCs/polymer compo-
site films have high haze (>90.0%) and their direct
transmissions are low, their total transmissions at 560
nm are over 90% [26]. For LCs/polymer composite
films based optical diffusers, Ma Haipeng et al. have
fabricated an optical diffuser with high transmission
(nearly 90.0%) and transmission haze (88.5%),
wherein the average diameter of LC droplets was
about 3.0 µm that dispersed in the polymer networks
[27]. In our previous study, by thermal curing of
epoxy monomers with thiol and polyamine, optical
diffuser based on a thermally cured polymer-
dispersed liquid crystal (PDLC) was obtained, which
exhibited high transmission (93.0%) and high haze
(95.0%) with the combined morphology of polymer
networks (the average diameter of LC droplets is 1.99
µm) and polymer balls (the average diameter of poly-
mer balls is 2.67 µm) [28]. Additionally, we have also
demonstrated an excellent optical diffuser with ultra-
high transmission (>94.0%) and high transmission
haze (>94.0%), of which the average diameter of the
2L. ZHOU ET AL.
single polymer micro-ball is 3.33 µm [29]. Based on
a transient polymer morphology of polymer balls-
networks in the polymer/LCs composite film, we
have fabricated a switchable optical diffuser that
achieving high transmission (>96.0%), high haze
(>90.0%) and wide viewing angle (from −75° to 75°)
at its offstate, when applied a voltage of 40.0 V, its
viewing angle changes to be from −60° to 60°, which
has widened the applications of optical diffusers [30].
With the development of polymer nanocomposites,
nanoparticles whose diameters are below 40 nm are
added into the polymer matrix to obtain high trans-
mission [31]. According to the theory of Rayleigh
scattering, I
I0¼e3;pxr3
4λ4
np
nm1
ðÞ
hi
,Iand I
0
are the inten-
sities of the transmitted and incident light, respec-
tively. n
p
and n
m
are the refractive indexes of the
particles and polymer matrix, respectively. λis the
wavelength of the incident light, Φ
p
is the volume
fraction of the particles, x is the optical path length.
As seen from the formula, with the increasing of the
size of nanoparticles, the scattered light intensity
enhances [31]. In order to achieve high transmission
and high light scattering intensity in the paper, opti-
cal diffusers based on uniform nano-sized polymer
balls/LCs composite films, which distinguish them-
selves from the conventional particle-diffusing type
and the surface-relief type with the characteristics of
high transmission (>98.0%) when the average dia-
meter of polymer balls is 810 nm, were successfully
prepared. The optical properties including transmis-
sions and hazes of the produced films are assessed
and the effects of thiol PTMP, LC content, curing
time, curing intensity and type of thiols on the prop-
erties of films have been investigated.
2. Experimental
The nematic LC used in the paper is LC20, which is
mixed by the lab (T
NI
= 80 °C, Δn = 0.20, n
o
= 1.50, n
e
=
1.70, Δε =−1.8). 2,4,6-Triallyloxy-1,3,5-triazi
(Abbreviated for TAC, Aldrich, St. Louise, MO, USA)
is used as the UV light initiated free-radical curable
monomer. Photo-polymerisation reaction is initiated
by Iragcure 651 (Ciba Geigy, Jingjiang Hongtai Chem.
Co. Ltd., China). Thiols such as Ethylene glycol di
(3-mercaptopropionate) (Abbreviated for EGMP) with
two functional thiol groups, Trimethylolpropane Tris
(3-mercaptopropionate) (Abbreviated for TTMP) with
three functional thiol groups, and Pentaerythritol Tetra
(3-mercaptopropionate) (Abbreviated for PTMP) with
four functional thiol groups are used as cross-linking
agents. The chemical structures of the monomer TAC,
photo-initiator Iragcure 651, and cross-linking agents
including EGMP, TTMP and PTMP are shown in
Figure 1. All the materials are used as received without
any further purification.
To investigate the influences of the relative contents of
crosslinking agent PTMP and LC20, the preparation con-
ditions including the curing time and the curing intensity,
as well as the structures of the crosslinking agent on the
optical properties of films, five series of films (denoted as
series T1-T5, T6-T10, T11-T15, T16-T20, and T21-T23, as
shown in Tables 1–5) have been prepared by photo-
Figure 1. The chemical structures of TAC, Iragcure 651, EGMP, TTMP and PTMP.
LIQUID CRYSTALS 3
polymerisation with click reaction. Additionally, as the
compositions and curing conditions are the same in sam-
ples T1, T8, T14, T18 and T23, in the part of experimental
and discussion, we use the name T1. Firstly, monomer/
crosslinking agent/LC/initiator/glass beads are mixed
together with a certain proportion, and stirred vigorously
until a homogeneous mixture is obtained. Then, the mix-
ture is uniformly pressed into a film between two layers of
transparent polyethylene terephthalate (PET) films. After
that, all samples are irradiated under a UV lamp (365 nm
35 W Hg lamp, PS 135, UV Flood, Stockholm, Sweden)
with fixed light intensity for a fixed time at around 318.2 K.
The polymer morphologies in the films are
observed by scanning electron microscopy (SEM,
S-4800, Hitachi). The optical diffusers are firstly
dipped into cyclohexane for about 72 h at room
temperature for extracting the LC molecules, and
then dried for 24 h at 353.15 K under vacuum.
Then, the dried samples are sputtered with gold
and subsequently observed by SEM.
The total transmissions and transmission hazes of films
are examined by using a UV-Vis-NIR spectra-photometer
(USA, PE Lambda 950) in the visible light (380–780 nm).
According to the standard of ASTM D-1003, the spectra of
films are obtained by adding an integrating sphere device.
The optical diffusing abilities of films are measured by
a light intensity distribution measurement (HP860 LED
luminous intensity distribution tester) which is designed
by HongPu Optoelectronics Technology Co. Ltd and our
lab. The incident light wavelength is 560.0 nm and the
beam diameter of the light is 2.0 mm. The distance between
the films and the light source is 4.0 cm, while the distance
between the photo-detector and filmsis40.0cm.
Moreover,thesizeofthemeasuredfilm is settled as
5.0 cm × 5.0 cm (length × width). The films and the light
source synchronously rotate from −60.0 degrees to 60.0
degrees with the interval of 1.0 degree, thus the detector
records the corresponding intensities.
3. Results and discussion
3.1 The varied polymer microstructures of films
T1-T5 with the increasing content of the
crosslinking agent thiol
Thiol-ene polymer/LCs composite films have been applied
in switchable windows, imaging technology, and holo-
graphic polymer-dispersed liquid crystals (HPDLCs)
[32,33],whilewehavestudiedtheopticalpropertiesof
thiol-ene polymer/LC composite filmsinthispaper.Asis
known to all, the polymer composition is critical in influ-
encing the polymer morphologies and performance char-
acteristics, thus the compositions of films T1-T5 with
Table 1. The compositions and preparation conditions of films
T1-T5.
Sample ID TAC:PTMP
LC
(wt.%)
651
(wt.%)
Time
(s)
Intensity
(mW/cm
2
)
T1 1:1 50 0.5 800 5.0
T2 1:2 50 0.5 800 5.0
T3 1:3 50 0.5 800 5.0
T4 1:4 50 0.5 800 5.0
T5 1:5 50 0.5 800 5.0
Table 2. The compositions and preparation conditions of films
T6-T10.
Sample ID TAC:PTMP
LC
(wt.%)
651
(wt.%)
Time
(s)
Intensity
(mW/cm
2
)
T6 1:1 40 0.5 800 5.0
T7 1:1 45 0.5 800 5.0
T8 (T1) 1:1 50 0.5 800 5.0
T9 1:1 55 0.5 800 5.0
T10 1:1 60 0.5 800 5.0
Table 3. The compositions and preparation conditions of films
T11-T15.
Sample ID TAC:PTMP
LC
(wt.%)
651
(wt.%)
Time
(s)
Intensity
(mW/cm
2
)
T11 1:1 50 0.5 200 5.0
T12 1:1 50 0.5 400 5.0
T13 1:1 50 0.5 600 5.0
T14 (T1) 1:1 50 0.5 800 5.0
T15 1:1 50 0.5 1000 5.0
Table 4. The compositions and preparation conditions of films
T16-T20.
Sample ID TAC:PTMP
LC
(wt.%)
651
(wt.%)
Time
(s)
Intensity
(mW/cm
2
)
T16 1:1 50 0.5 800 1.0
T17 1:1 50 0.5 800 3.0
T18 (T1) 1:1 50 0.5 800 5.0
T19 1:1 50 0.5 800 7.0
T20 1:1 50 0.5 800 9.0
Table 5. The compositions and preparation conditions of films T21-T23.
Sample ID TAC: EGMP LC(wt.%) 651(wt.%) Time(s) Intensity (mW/cm
2
)
T21 1:1 50 0.5 800 5.0
Sample ID TAC: TTMP LC(wt.%) 651(wt. %) Time(s) Intensity (mW/cm
2
)
T22 1:1 50 0.5 800 5.0
Sample ID TAC: PTMP LC(wt.%) 651(wt. %) Time(s) Intensity (mW/cm
2
)
T23 (T1) 1:1 50 0.5 800 5.0
4L. ZHOU ET AL.
different mass ratios of ene and thiol (PTMP) are demon-
strated in Table 1.
Figure 2 shows SEM images of films T1-T5 pre-
pared by the UV curing of thiol-triazine-induced
separation of polymer and LCs, polymer morpholo-
gies of filmsT1-T5greatlyvaryastherelativeratio
of PTMP increases. Initially, when the mass ratio of
TAC and PTMP is 1:1, uniform nano-sized polymer
balls are formed in the film T1, and the average
diameter of polymer balls is about 900 nm. By
increasing the mass ratio of thiol in curing mono-
mers, polymerisation kinetic decreases, polymer
balls have varied into polymer networks, the poly-
mer networks in the film T2 is the transient state
switching from polymer balls to polymer networks,
the average size of LC droplets is about 18.50 µm.
Continually increasing the thiol content, the curing
kinetic of thiol-ene click reaction and the rate of
phase separation decrease, multiple LC droplets are
dispersed in films T3 and T4, additionally, the aver-
age sizes of LC droplets in films T3 and T4 become
smaller(theaveragesizeinfilms T3 and T4 are 2.10
µm and 1.60 µm, respectively). Further increas-
ing the content of thiol in the film T5 leads to the
absence of polymer networks and polymer balls due
to the incomplete phase separation.
3.2 The optical properties of films T1-T5 with the
increasing content of crosslinking agent thiol
The transmissions of films T1-T5 with different amounts of
PTMP are measured for comparison. In Figure 3(a),trans-
missions of films T1-T5 are dependent on the wavelength
of the incident light. Moreover, transmissions of films T1-
T5 increase with the increasing amount of thiol PTMP.
However, an inverse trend for the transmission hazes of
films T1-T5 is observed with the increase of PTMP con-
centration, as shown in Figure 3(b).Specifically, when the
mass ratio of TAC and PTMP is 1:1, the haze of film is close
to 95.0%, which is above the conventional LCD diffusers
such as particle-diffusing diffuser on the market (transmis-
sion >90.0% and haze >90.0%) [34]. Referring to the diffus-
ing abilities of films T1-T5 (shown in Figure 3(c)), film T1
exhibits an excellent diffusing ability, which is coherent to
haze of film T1. Moreover, films T4 and T5 have weaker
light-diffusing abilities due to their smaller hazes. The
relationship between optical performances (transmission
and haze) of polymer/LCs composite films and the average
diameters of LC droplets that dispersed in polymer net-
worksissummarisedinFigure 3(d). As larger LC droplets
aredispersedinpolymernetworks,thehazeofoptical
diffuser T2 is higher than that in films T3 and T4. On the
condition that the average diameters of LC droplets are
very small, light scattering in films T3 and T4 have greatly
attenuated. While light scattering in film T3 is stronger
than that in film T4, thus transmission of film T4 is higher.
Considering the overall performance of the films, the uni-
form nano-sized polymer balls/LCs composite film T1 is
a better choice, which has combined high transmission,
high haze, and excellent light diffusing ability.
3.3 The varied polymer microstructures of films
T6-T10 with the increasing LC content
Previous studies have revealed that the relative content
of the monomers and LCs has a great effect on the
Figure 2. The polymer morphologies of films T1-T5.
LIQUID CRYSTALS 5
Figure 4. The polymer morphologies of films T6-T10.
Figure 3. (Colour online) Optical properties of films T1-T5: (A) total transmissions of films T1-T5 when the wavelength of light varies
from 380 nm to 780 nm; (B) transmission hazes of films T1-T5 when the wavelength of light varies from 380 nm to 780 nm; (C) light-
diffusing abilities of films T1-T5; (D) the relationship between transmission & haze and the diameter sizes of the LC droplets that
dispersed in the polymer networks when the wavelength of light is 560 nm.
6L. ZHOU ET AL.
microstructures of the polymer matrix [28], thus the
films T6-T10 with different compositions of LC are
attempted and listed in Table 2.
As illustrated in Figure 4, the polymer microstruc-
tures change from polymer networks to polymer
balls with the increasing content of LC from 40.0
wt.% to 60.0 wt.% at the interval of 5.0 wt.% in films
T6-T10. When the LC content is relatively low, LC
droplets with average diameters of 670 nm and 530
nm, respectively, are spherically dispersed in the
filmsT6andT7,asisshowninFigure 4.Asthe
LC content reaches 50 wt.%, due to the lower poly-
merisation kinetic of ene-thiol reaction than that in
films T6 and T7, uniform nano-sized polymer balls
are formed in film T1. Further increasing the LC
content, due to the high viscosity of LCs, the rate
of phase separation decreases, these polymer balls are
connected to each other in films T9 and T10. For
film T9, there is still aggregation of polymer balls,
the average diameter is 1.93 µm, while for film T10
with the highest LC concentration, polymer micro-
structures turn into polymer aggregation without
shape.
3.4 The optical properties of films T6-T10 with the
increasing LC content
The optical properties of films T6-T10 with different
LC content are measured for comparison. As illu-
strated in Figure 5(a), when the LC content is below
60%, transmission of films increases in such manner
T9> T7> T1 > T6. Compared to LCs that dispersed
in film T6, film T7 owns smaller LC droplets, thus
transmission of film T7 is higher than that in film
T6. Because of light scattering of uniform polymer
nanoballs in film T1, transmission of film T9 exceeds
that of film T1. As film T10 occupies the largest
content of LCs, transmission of film T10 decreases
largely due to the coherent scattering of LCs and
polymer balls. Transmission hazes of films are
shown in Figure 5(b),whentheLCcontentis
below 50 wt %, the polymer microstructure is poly-
mer network, with the decreasing average diameter
of nano-sized LC droplets in the film T7, light scat-
tering of film T7 is stronger than that in film T6,
thus transmission haze of film T7 is higher. As the
LC content reaches or exceeds 50 wt %, polymer
Figure 5. (Colour online) Optical properties of films T6-T10: (a) total transmissions of films T6-T10 when the wavelength of light varies
from 380 nm to 780 nm; (b) transmission hazes of films T6-T10 when the wavelength of light varies from 380 nm to 780 nm; (c) light-
diffusing abilities of films T6-T10; (d) the relationship between transmission & haze and the diameter size of the LC droplets that
dispersed in the polymer networks or the polymer balls when the wavelength of light is 560 nm.
LIQUID CRYSTALS 7
network disappears, due to polymer balls and aggre-
gates in films T1, T9 and T10, their transmission
hazes have slightly enhanced overall. Owing to uni-
form polymer nano-balls in the film T1, it has the
highest haze. Although irregular polymer balls exist
in both films T9 and T10, film T10 owns the highest
LC content, light scattering of polymer aggregates
and LCs coherently takes place. Therefore, transmis-
sion haze of film T10 is a little higher than film T9.
Figure 5(c) depicts the light-diffusing abilities of
films T6-T10, which reveals that film T1 has the
best light diffusing ability. The relationship between
optical performances (transmission and haze) of
optical diffuser and the average diameter sizes of
LC droplets that dispersed in polymer networks or
polymer balls is summarised in Figure 5(d).
Considering the overall performance of the films,
uniform nano-sized polymer balls/LC composite
film T1 is a better choice, which has high transmis-
sion, high haze, and excellent light diffusing ability.
3.5 The varied polymer microstructures of films
T11-T15 with the increasing curing time
Except for the compositions of samples, curing process
such as curing time also has a profound effect on the
properties of polymer/LC composite films. To explore
the dependence of polymer morphology under different
UV curing times, the curing intensity is fixed at 5.0
mW/cm
2
and the UV curing times are varied as 200
s-1000 s at the interval of 200 s, which is shown in
Table 3.
Figure 6 describes the influence of curing time on the
polymer microstructures of films T11-T15. When the cur-
ing time is relatively shorter, the rate of phase separation
between LCs and polymer is larger than that of polymer-
isation, thus LC droplets are dispersed in polymer net-
works. Additionally, the average diameter of LC droplets
is 330 nm in the film T11. Increasing the curing time over
400s,theuniquepolymerballsareformedinfilms T12-
T15. Furthermore, the average diameters of polymer balls
in films T12-T15 decrease with the increasing curing time.
Interestingly, polymer balls in films T12 and T13 are non-
spherical, due to the extending curing time, the phase
separation between LCs and polymer tends to be full,
polymer aggregates are divided into uniform polymer
nanoballs. Polymer balls in films T1 and T15 are spherical
and nano-sized, additionally, the average diameters of
polymer balls are 900 nm and 810 nm, respectively.
3.6 The optical properties of films T11-T15 with the
increasing curing time
The curing conditions such as the curing time have
significant influence on the optical properties of opti-
cal diffusers T11-T15. As shown in Figure 7(a),with
polymer networks in the film T11, because of the
difference between LC droplets and polymer sub-
strate, transmission of filmT11islow.Withthe
increasing curing time, firstly, polymer microstruc-
ture in film T12 is polymer aggregates, the interfaces
between polymer and LCs increase, transmission of
film T12 is lower than that in film T11. Continually
extending the curing time, the transmissions of films
enhanceoverall.Withuniformpolymerballsinthe
Figure 6. The polymer morphologies of films T11-T15.
8L. ZHOU ET AL.
films T1 and T15, the average diameter of polymer
nanoballs in the film T15 is less than that in the film
T1, transmission has not attenuated, thus transmis-
sion of film T15 is the highest. Transmission of film
T13 with larger irregular polymer balls is higher than
that of film T1 because of weaker light scattering.
Although there is large difference in the transmis-
sions of films T11-T15, transmission hazes of films
T11-T15 have changed a little, as displayed in Figure
7(b). Corresponding to high transmission hazes in
films T11-T15, all the light-diffusing abilities of films
T11-T15 are excellent (shown in Figure 7(c)).
Comprehensively considering the overall perfor-
mance of the films, as depicted in Figure 7(d),film
T1 has both high transmission and transmission
haze, as well as excellent light diffusing abilities,
which can be applied in the optical diffusers.
Additionally, by extending the curing time to be
1000 s, transmission of film T15 is nearby 100%
without attenuating the transmission haze, which is
a choice for excellent top optical diffusers [26].
3.7 The varied polymer microstructures of films
T16-T20 with the increasing curing light intensity
For the method of polymerisation induced phase
separation in preparing the polymer/LCs composite
films, UV curing intensity has also greatly influenced
the polymer morphologies due to its profound effect
on the UV polymerisation speed of monomers. Thus,
experiments under different curing intensities are per-
formed between 1.0 mW/cm
2–
9.0 mW/cm
2
at the
interval of 2.0 mW/cm
2
, which are demonstrated in
Table 4.
Figure 8 shows the polymer morphologies of films
T16-T20, which reveals a great dependence of UV
intensity on the polymer microstructures of films.
Under the lowest light intensity, owing to the slow
polymerisation rate, the phase separation between
LCsandpolymeriscomplete,LCdropletsare
spherically dispersed in the film T16, of which the
average diameter of LC droplets is 480 nm. With the
increase of light intensity, due to faster polymerisa-
tion rate, polymer aggregates are formed in the film
Figure 7. (Colour online) Optical properties of films T11-T15: (a) total transmissions of films T11-T15 when the wavelength of light
varies from 380 nm to 780 nm; (b) transmission hazes of films T11-T15 when the wavelength of light varies from 380 nm to 780 nm; (c)
light-diffusing abilities of films T11-T15; (d) the relationship between transmission & haze and the diameter sizes of the LC droplets
that dispersed in the polymer networks or polymer balls when the wavelength of light is 560 nm.
LIQUID CRYSTALS 9
Figure 8. The polymer morphologies of films T16-T20.
Figure 9. (Colour online) Optical properties of films T16-T20: (a) total transmissions of films T16-T20 when the wavelength of light
varies from 380 nm to 780 nm; (b) transmission hazes of films T16-T20 when the wavelength of light varies from 380 nm to 780 nm; (c)
light-diffusing abilities of films T16-T20; (d) the relationship between transmission & haze and the diameter sizes of the LC droplets
that dispersed in the polymer networks or polymer balls when the wavelength of light is 560 nm.
10 L. ZHOU ET AL.
T17. Further increasing the curing light intensity
over 5.0 mW/cm
2
, polymer averages are divided
into uniform nano-sized balls in films T1, T19, and
T20, of which the average diameters are 900 nm, 770
nm, and 650 nm, respectively.
3.8 The optical properties of films T16-T20 with the
increasing curing light intensity
Considering the transition of polymer microstruc-
tures from polymer networks to polymer balls, opti-
cal properties of films T16-T20 are varied. As shown
in Figure 9(a), larger LC droplets and smaller LC
droplets are both dispersed in the polymer networks
of film T16, its transmission is the highest. When
extending the curing intensity, polymer networks
turn to be polymer aggregates in the film T17, the
interfaces between polymer and LCs increase, trans-
mission of film T17 is less than that in the film T16.
As the curing intensity is over 5.0 mW/cm
2
,polymer
aggregates are divided into uniform polymer nano-
balls, generally, transmission of polymer nano-balls
/LCs composite film decreases. However, as the aver-
age size of polymer balls is 770 nm in the film T19,
when the wavelength of incident light is over 600
nm, due to the attenuated light scattering, transmis-
sion has enhanced. Considering transmission hazes
of films T16-T20 (shown in Figure 9(b)), with
increasing curing intensity, transmission hazes raise
except for film T20. Owing to the smallest polymer
nano-balls in the film T20, light scattering of film
T20 has weakened as the incident light is mostly
refracted. As for the light-diffusing abilities of films
T16-T20 (depicted in Figure 9(c)), film T1 shows
excellent light diffusing ability. Taking high transmis-
sion and haze into consideration, as shown in Figure
9(d), as the curing intensities are 5.0 mW/cm
2
and
7.0 mW/cm
2
,films T1 and T19 are applicable in the
optical diffusers.
3.9 The varied polymer microstructures of films
T21-T23 with the increasing thiol functionality
Except for the effects of sample compositions and curing
conditions such as curing time and curing intensity on
polymer morphologies, thiol functionality is also influen-
tial on polymerisation in the thiol-ene reaction. Herein,
we have introduced three types of thiol monomers with
different thiol functionalities, which are shown in Table 5.
Comparing the polymer microstructures of films
T21-T23 (shown in Figure 10), with the smaller thiol
functionality in preparing the optical diffusers T21 and
T22, as the polymerisation rates are slower than that in
the film T1, the phase separation is complete, LC dro-
plets are spherically dispersed and their average dia-
meters are 1.77 µm and 710 nm, respectively. With the
most thiol functionality in the film T1, polymerisation
rate is the highest, the complete phase separation is
difficult to reach, the microstructure turns into nano-
sized polymer balls, of which the average diameter of
polymer balls is 900 nm.
3.10 The optical properties of films T21-T23 with
the increasing thiol functionality
As the structure of curing agent thiol has greatly affected
the polymer morphologies of films T21-T23, with the
increasing monomer functionality, transmissions of
films T21-T23 have varied, as shown in Figure 11(a).
Compared to film T21, with smaller LC droplets that
dispersed in the film T22, the interfaces between LC
droplets and polymer substrate increase, transmission of
film T22 is lower than that in the film T21. With the
uniform polymer nano-balls in the film T1, transmission
of film T1 is the lowest. Regarding the transmission hazes
of films T21-T23 (depicted in Figure 11(b)), transmission
hazes increase with the enhancing thiol functionality. As
for the light-diffusing abilities of films T21-T23
(described in Figure 11(c)), film T1 obtains the excellent
light diffusing ability. With both high transmission and
Figure 10. The polymer morphologies of films T21-T23.
LIQUID CRYSTALS 11
transmission haze, film T1 is suitable for applying in the
optical diffusers, as described in Figure 11(d).
4. Conclusion
In summary, a series of optical diffusers based on ene
and thiol system was successfully developed by UV
polymerisation of ene-thiol induced phase separation
method in the paper. By regulating the sample composi-
tions and polymerisation conditions, polymer micro-
structures present two types: polymer networks and
polymer nano-balls. With more thiol content, less LC
content, shorter curing time, weaker curing intensity,
and lower thiol functionality, polymer networks in the
polymer/LCs composite films are preferred. Conversely,
polymer nano-balls are obtained. Consequently, as the
average diameter of polymer balls is in the scale of the
incident light wavelength, high transmission (>98.0%)
of polymer nano-balls/LCs composite film has reached
without declining the transmission haze (>90.0%). In
respect of application area, by adjusting the average
diameter of polymer nano-balls, transmission and haze
of polymer nano-balls/LCs composite films are tunable,
which has enriched the development of optical diffusers.
Additionally, by simply mixing monomers and LCs
together, then triggering the polymerisation of mono-
mers by UV light, this method adjusts to the industrial
production of optical diffusers.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This study was supported by the National Natural Science
Foundation of China (NSFC) (Grant No. 51333001, 51573006,
51573003 and 51602007), the key International Cooperation
Project (Grant No. 51720105002), and NSFC International
Cooperation and Exchanges Projects (Grant No. 51561135014).
Figure 11. (Colour online) Optical properties of films T21-T23: (a) total transmissions of films T21-T23 when the wavelength of light
varies from 380 nm to 780 nm; (b) transmission hazes of films T21-T23 when the wavelength of light varies from 380 nm to 780 nm; (c)
light-diffusing abilities of films T21-T23; (d) the relationship between transmission & haze and the diameter sizes of the LC droplets
that dispersed in the polymer networks or polymer balls when the wavelength of light is 560 nm.
12 L. ZHOU ET AL.
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