Fabrication and characterization of imprinted photonic crystalline polymer matrices via multiple UV polymerizations

Article abstract of DOI:10.1039/c2jm35151j

A photonic polymer network was imprinted using multiple UV-induced polymerizations in the presence of cholesteric liquid crystals (CLCs). Here, the imprinted polymer matrices provided a chiral environment due to the chirality of the polymer networks. The fabricated cell exhibited Bragg reflections even in the absence of anisotropic materials. Moreover, a phototunable liquid crystal cell was fabricated with photosensitive liquid crystalline ethyl 4-[4-(11-acryloyloxy undecyloxy) phenyl azobenzoyl-oxyl] benzoate. The tuning properties of the cells were examined as a function of the irradiation time and concentration of the photosensitive material. Both the tuning rate and dark relaxation of E-Z photoisomerization of the embedded LCs in the polymer network were also examined.

Full text of DOI:10.1039/c2jm35151j

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PAPER
Fabrication and characterization of imprinted photonic crystalline polymer
matrices via multiple UV polymerizations†
a
a
Chih-Chieh Chien, Jui-Hsiang Liu and A. V. Emelyanenkob
*
Received 2nd August 2012, Accepted 12th September 2012
DOI: 10.1039/c2jm35151j
A photonic polymer network was imprinted using multiple UV-induced polymerizations in the
presence of cholesteric liquid crystals (CLCs). Here, the imprinted polymer matrices provided a chiral
environment due to the chirality of the polymer networks. The fabricated cell exhibited Bragg
reflections even in the absence of anisotropic materials. Moreover, a phototunable liquid crystal cell
was fabricated with photosensitive liquid crystalline ethyl 4-[4-(11-acryloyloxy undecyloxy) phenyl
azobenzoyl-oxyl] benzoate. The tuning properties of the cells were examined as a function of the
irradiation time and concentration of the photosensitive material. Both the tuning rate and dark
relaxation of E–Z photoisomerization of the embedded LCs in the polymer network were also
examined.
The l0 of the CLC can be altered by adjusting the refractive
Introduction
index n or the helical pitch P (eqn (1)) under the action of various
external forces, such as heat, light, electrical fields, or mechanical
stress.4–11 Cholesteric liquid crystalline materials have been
extensively studied and many applications, such as reflective
liquid crystal displays, optical memory and optical sensors, for
these materials have been found.12–14
Cholesteric liquid crystals (CLCs) are known to exhibit selective
reflection due to their periodic helical structures. The maximum
reflection of unpolarized light from a CLC is 50% because only
circularly polarized light with one handedness is reflected. For
example, for a right-handed CLC, right-handed circularly
polarized incident light is reflected and left-handed polarized
incident light is transmitted.1,2 The central wavelength (l0) of the
reflection can be calculated using the following equation:3
An increasingly common trend for fabricating tunable CLC
devices is the use of light-induced CLC materials. These photo-
tunable CLC devices are demonstrated using photoresponsive
chiral dopants or the CLC mixture with chromophores that go
through photochemical transitions.5,15 The helical twisting power
of these materials changes upon exposure to UV light, resulting
in changes in helical pitch; therefore, the CLC reflection band can
be adjusted using UV irradiation. Among the chromophore
materials, azobenzene derivatives are the most widely used in
CLC media due to their thermal stability, easy processability and
photo-reversibility.16–18
l0 ¼ navg P,
(1)
where P is the pitch of the helical structure and navg is the
average refractive index of the cholesteric phase defined as
navg ¼ (ns + no)/2, where ns and no are the extraordinary and
ordinary reflective indices, respectively. CLCs are formed by
introducing chiral elements into liquid crystalline molecules or by
doping small amounts of chiral dopants into nematic liquid
crystalline hosts. The chiral environment induces slight distor-
tions in the molecular alignment, yielding a helical microstruc-
ture. The P of a CLC helix is defined as a unit length for a 360ꢀ
molecular rotation related to the concentration of the chiral
dopant and the helical twisting power (HTP) of the chiral
compound.2
One approach for the fabrication of phototunable CLC
devices is based on the use of azobenzene-based nematic LC
(azo-NLC) mixtures that undergo photoisomerization.15,19
Another approach is the introduction of a chiral azobenzene
derivative into a nematic LC (NLC). During E–Z photo-
isomerization, the configuration of azobenzene changes, result-
ing in changes in the twisting power or optical characteristics. A
chiral azobenzene derivative dissolved in a host NLC can act as a
chiral dopant to induce a CLC phase and to control the helical
pitch.20–22
aDepartment of Chemical Engineering, National Cheng Kung University,
Tainan, Taiwan 70101. E-mail: jhliu@mail.ncku.edu.tw; Fax: +886-6-
2384590; Tel: +886-6-2757575 ext.62646
bDepartment of Physics, Moscow State University, Moscow 119991,
Russia. E-mail: emel@polly.phys.msu.ru
In a series of investigations of phototunable devices on CLCs,
the synthesis of highly ordered liquid crystalline polymers and
chiral polymers was discussed due to their optical properties and
their applications in LC cells and recording films.15,23 These
polymers are formed by the UV curing of CLC mixtures
† Electronic supplementary information (ESI) available: Synthesis
process and characterization of 4,40-bis(6-(acryloyloxy)-hexyloxy)
biphenyl (BAHB) and ethyl 4-[4-(11-acryloyloxy undecyloxy) phenyl
azobenzoyl-oxyl] benzoate (A11AEt). See DOI: 10.1039/c2jm35151j
22446 | J. Mater. Chem., 2012, 22, 22446–22451
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containing small concentrations of the LC monomer and initi-
ator. Theoretically, an increase in monomer content would create
a denser polymeric network. However, an increase in monomer
may lead to the loss of the anisotropic properties of the original
CLC mixture. In this paper, an improved imprinting method was
used, and the memory effects of the helical structure of the
polymer network were improved through multiple UV-induced
polymerizations.24 When this technique was used, the imprinted
polymer films themselves exhibited Bragg reflection properties
without any added liquid crystals or chiral compounds. More-
over, this is the first paper to demonstrate the tuning of the
reflective wavelength of the imprinted polymer template by filling
Fig. 2 Schematic representation of the fabrication of photonic crystals
via multiple UV-induced polymerizations.
the cell with
photoirradiation.
a
non-chiral azobenzene derivative via
Polymeric network preparation
The sample cells were fabricated following typical procedures.
Two ITO (Indium tin oxide) glasses coated with polyvinyl
alcohol (Mw ¼ 20 000) were parallel rubbed after washing. The
two glass plates were then separated by 12 mm spacers and sealed
with epoxy resin on two sides of the cell. The cell was filled with a
homogeneous mixture containing achiral nematic LC HSG-
22200 (48.3%), chiral dopant CB15 (38.3%), bifunctional
monomer BAHB (13.3%) and photoinitiator Irgacure-184 (0.1%)
by capillarity. To improve the memory effect, a new approach for
Experimental
Materials
Nematic LC HSG-22200 was purchased from Fusol Material Co.,
LTD. The LC exhibits a nematic phase over a wide range of
temperatures, up to 114.5 ꢀC. The chiral dopant CB15 was
purchased from Merck and used at a concentration of 38.3%; it
induces a left-handed helical structure in the host nematic LC. The
photoinitiator Irgacure-184 was obtained from Ciba Specialty
Chemicals.25 The bifunctional monomer BAHB (4,40-bis-
(6-(acryloyloxy)-hexyloxy)biphenyl) was synthesized following
the processes described previously.26 The azobenzene chromo-
phore A11AEt (ethyl 4-[4-(11-acryloyloxyundecyloxy) phenyl
azobenzoyl-oxyl] benzoate) was synthesized according to a
previouslyreported method.27–29 The chemical structuresof CB15,
Irgacure-184, BAHB and A11AEt are shown in Fig. 1. Details of
the synthesis of BAHB and A11AEt are given in the ESI.†
preparing
a high-density imprinted polymer template via
multiple UV-induced polymerizations was used.24 The cell was
fabricated through a circle mask with a diameter of 6 mm (Fig. 2)
and irradiated with 254 nm UV light several times. After 1 hour
of photopolymerization, the cell was kept in the dark for 1 day.
During this period, unreacted monomers from the surrounding
region diffused into the UV-irradiated area. The photo-
polymerization/diffusion process was repeated five times. After
the photopolymerization was complete, the LC mixture and
unpolymerized monomer were extracted by immersing the
sample cell in acetone and CHCl3 for 2 days. The sample cell was
Analysis apparatus
ꢀ
then dried in a vacuum at 40 C. A schematic representation is
Chemicals used in this investigation were all confirmed using a
spectrophotometer, although the spectra are not shown. 1H
NMR spectra were analyzed on a Bruker 200 MHz FT-NMR.
The FT-IR spectra were recorded with a Jasco 410. Scanning
electron microscope (SEM) microphotographs were measured
with a JEOL HR-FESEM JSM-6700F (Osaka, Japan) instru-
ment. UV spectroscopy measurements were carried out with a
Jasco V-550 UV-VIS spectrophotometer. The birefringence of
the materials was analyzed using polarized optical microscopy
(POM).
shown in Fig. 3a–c illustrating the fabrication procedure.
UV-tunable color cell fabrication
After the liquid crystals were removed from the sample cell, the
empty cell was refilled with an achiral mixture of nematic LC
HSG-22200 and azobenzene chromophore A11AEt in a range of
concentrations from 0.1 to 6% (Fig. 3(d)). The phototuning was
Fig. 1 Chemical structures of (a) chiral dopant CB15, (b) photoinitiator
Irgacure-184, (c) bifunctional monomer BAHB and (d) photosensitive
azobenzene compound A11AEt.
Fig. 3 Illustration of the formation of the photonic crystalline polymer
matrices with refilled materials.
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performed with UV light (365 nm) at an intensity of 0.6 mW. The
reflection spectra of the fabricated sample cells were then esti-
mated using an optical probe.24
444 nm due to the n–p* transition was also observed. This
absorption was much weaker than that due to the p–p* transi-
tion because the n–p* transition corresponds to a ‘‘forbidden’’
electronic transition with a low probability of occurrence. As
shown in Fig. 5(b), after 45 hours in the dark, the absorption
returned to its original level. This result suggests that A11AEt
gradually changed from Z to E thermally, without any light
exposure. Theoretically, the photo-induced configurational
isomerization from Z to E is expected to alter the helical pitch of
cholesteric liquid crystals. Of course, the average refractive
indices of anisotropic materials can be different due to variations
in molecular arrangements. Depending on orientation, configu-
rational isomerization may increase or decrease the average
refractive index of anisotropic materials. Accordingly, adding
A11AEt into the photonic cells is expected to reveal the photo-
tunable phenomena.
Results and discussion
Multiple UV-induced polymerizations
The spectra presented in Fig. 4 shows the selective Bragg
reflections of the LC cell before and after the photo-
polymerization and after washing out the LC mixture. Different
photopolymerization/diffusion times led to different perfor-
mances of the imprinted polymer network. Fig. 4a and b show
the spectra obtained when the polymer cell was irradiated one
time and five times, respectively. A slight blue shift was observed
after UV polymerization due to crosslinking-induced volume
shrinkage.1,24,30 After the LC mixture and unreacted monomers
were washed out, the reflection band of the cell irradiated five
times shows a much higher response than the cell irradiated one
time. This difference is ascribed to the higher degree of poly-
merization. During the multiple photopolymerization/diffusion
procedures, the density of the polymer structure in the UV-
irradiated area increased due to the diffusion of the monomer
when the cell was kept in the dark. This result suggests that the
highly cross-linked polymer networks prepared via multiple
photopolymerizations of the monomer BAHB had well-estab-
lished helical structures. After washing, even though the system
did not contain any chiral compounds, the cell exhibited a clear
Bragg reflection. This result is further evidence of the existence of
imprinted helical polymer matrices. The Bragg reflection indi-
cates the presence of photonic crystalline structures.
To study the chiral efficiencies of the imprinted polymer
matrices, the imprinted polymer cell was filled with an achiral
mixture containing 95% nematic LC HSG-22200 and 5% azo-
benzene chromophore A11AEt. As shown in Fig. 6, a reflection
band at 450 nm was observed. Theoretically, a red shift and a
broadened reflection band should be observed due to higher
refractive index of the cell filled with liquid crystalline material
compared to that of the empty imprinted cell (the refractive index
of air is 1) based on eqn (1). However, as shown in Fig. 6, a
narrow band with no significant shift was observed after the
refilling of the cell with the azobenzene/LC mixture. This result
suggests that the filled liquid crystal molecules arranged in a
specific order, decreasing the imprinted birefringence (Dn) of the
imprinted matrices. However, the average value of the refractive
The reflection spectra of LC cells in Fig. 4 were measured
under normal lab conditions. The non-zero base line is due to the
refraction of surrounding light.
UV-tunable selective reflection light
Azobenzene derivatives are well-known photochromic materials
that exhibit light-induced E–Z photochemical isomerization.31,32
As shown in Fig. 5(a), UV irradiation caused a configurational
change of A11AEt from E to Z, leading to a decrease in UV
absorption to approximately 362 nm and an increase in
UV absorption to approximately 444 nm. A11AEt showed a
strong absorption band at 362 nm in the E-form. This absorption
is attributed to the p–p* transition. A weak absorption band at
Fig. 5 Dependence of reflection spectra of the fabricated cell on (a) UV-
irradiation time and (b) recovering in the dark.
Fig. 4 Reflection spectra of the LC cells with (a) one-time and (b) five-
time photopolymerization/diffusion processes before and after photo-
polymerization and after removing the LC mixture.
Fig. 6 Reflection spectra of (a) the imprinted empty cell, (b) the cell after
being refilled with the LC mixture (95% HSG22200 + 5% A11AEt) and
the LC cell after UV irradiation for (c) 1 h, (d) 2 h, (e) 3 h and (f) 4 h.
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Fig. 7 POM textures of (a and d) the empty cell and (b and e) the LC cells before and (e and f) after photoirradiation with 365 nm light. Transmittance
POM texture: (a)–(c); reflectance POM texture: (d)–(f).
index was kept, maintaining the reflection central wavelength. A
decrease in the birefringence narrows the band width of the
reflection light.
To clarify the effect of chirality on the reflection band, the
phototuning of an azobenzene mixed cholesteric liquid crystal
(azo-CLC) containing N-LC HSG-22200 (54%), CB15 (41%) and
A11AEt (5%) was studied. When exposed to 365 nm UV light,
the reflection band decreased in intensity but did not undergo
any obvious change, as shown in Fig. 9. The decrease in reflection
is due to the destruction of the helical structure caused by the
E–Z photoisomerization of the azobenzene. In the case of the
The E- and Z-form isomers have different physical and
chemical properties due to the different spatial arrangements of
their aromatic moieties; the E-form is nearly planar, while the Z-
form has a three-dimensional (3D) conformation.33 To elucidate
the photo-induced changes in the Bragg reflection of the
photosensitive azobenzene, the cell was exposed to UV light
(0.6 mW at 365 nm) for several hours.
Initially, the refilled imprinted cell had a selective reflection
band at 450 nm. After UV irradiation at 365 nm for 1 hour, the
reflection band changed from 450 nm to 525 nm. Further UV
light exposure (for a total of 4 hours) drove the red-shift reflec-
tion wavelength to 560 nm, as shown in Fig. 6. The textures of
both the empty film and the LC film before and after photo-
irradiation with 365 nm light were investigated using both
transmittance and reflectance polarized optical microscopy
(POM). Fig. 7 shows the color-tuning properties of the imprinted
films. The typical liquid crystalline POM textures were observed,
as were the birefringences of the imprinted matrices.
Mechanisms of the phototunable behaviors
Fig. 8 Schematic representation of the pitch tuning of the fabricated
photonic cells via photo-induced isomerization of the azo compound.
Fig. 8 presents a schematic representation of the pitch tuning of
the imprinted cells containing photochromic A11AEt. Based on
eqn (1), change of both the pitch (P) and the refractive index
(navg) causes variations in the reflection central wavelength. After
UV irradiation at 365 nm, the molecular structure of azobenzene
A11AEt changed from the linear E-form to the 3D Z-form. The
3D Z-form may disturb the arrangement order of the liquid
crystal molecules, leading to an increase in the refractive index of
the imprinted polymer network. Theoretically, the molecular
arrangement inside the polymer matrices has a specific birefrin-
gence value that may increase or decrease the birefringence of the
imprinted matrices. Similar phenomena in CLC cells were
reported by Bunning et al.23 A red-shift in the reflection wave-
length of a CLC film based on the photosensitive chiral dopant
QL-76 upon UV irradiation due to a UV-induced change in the
HTP value was reported.
Fig. 9 Reflection spectra of the azo-CLC mixture containing HSG2200,
CB15 and A11AEt (a) before irradiation and after (b) 1 h, (c) 3 h UV
irradiation and (d) recovery in the dark for 12 h.
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phototuning of the achiral azo-LC mixture in the polymer
matrices, shown in Fig. 6, the central wavelength was maintained
within 20 nm. This result suggests that the additional chirality
improved the efficiency of the imprinted matrices. The reflection
wavelength of the azo-CLC mixture did not shift during UV
irradiation. This result is ascribed to the high concentration of
chiral dopant, which decreases the configurational effect of the
E–Z transition. In the case of the phototuning of the achiral azo-
LC mixture in polymer matrices (Fig. 6), the chirality was due
only to the imprinted polymer matrices. The molecular
arrangement in the polymer matrices is more easily affected by
the E–Z transition, leading to a clear red-shift in the reflection
central wavelength. From Bragg theory, the band width can be
expressed by (Dl ¼ Dnp). After UV irradiation, the randomness
of LC molecules was increased due to the E–Z transition leading
to the decrease of Dn. For an isotropic material, Dn ¼ 0.
Accordingly, in Fig. 9, some peak narrowing upon irradiation
was observed.
Fig. 11 (a) SEM of polymer matrices and (b) a real sample cell fabri-
cated using the multiple UV-polymerizations.
The system reported here is distinguished from other reported
work in that the chiral environment in this work comes from
imprinted polymer matrices without any added chiral dopant. In
this study, the polymer density was much higher due to the
multiple UV-induced polymerizations. As shown in Fig. 6, the
helical pitch was well established. Filling the cell with solvent
increased the refractive index of the polymer matrices, leading to
red shifts in the reflection band.
Theoretically, the photoisomerization between the E- and
Z- forms is a reversible photoreaction. The photostationary state
spontaneously returns to the initial state through back-isomeri-
zation from Z to E. However, in our case, spontaneous Z–E
photoisomerization was not observed during the long-term dark
relaxation. A possible explanation is that the Z-form isomer was
trapped in the ‘‘confined space’’. Due to the high density of the
polymer network, the dark relaxation from the 3D Z-form
azobenzene was restricted.
Effect of the concentration of A11AEt on phototuning
Several azo-NLC mixtures with various concentrations of
A11AEt (0.1–6%) were filled through capillarity into the
imprinted polymer cell. Upon UV irradiation at 365 nm, the
reflection wavelength of the 0.1% cell changed from 450 to
472 nm after UV irradiation. For those cells containing 0.5, 1, 3,
5, 5.5 and 6% of photoisomerizable A11AEt, the reflection
wavelength shifted from 450 nm to 507, 519, 543, 560, 563 and
564 nm, respectively. According to the explanations in the
mechanism section, the red-shift of the reflection wavelength is
based on the photoisomerization of azobenzene from E to Z.
Theoretically, 3D structured molecules usually reveal higher free
volumes, resulting in variations in the birefringences of their
molecular arrangements. As shown in Fig. 10, an increase in the
azo-NLC concentration caused an increase in the red-shift of
the reflection wavelength. At lower concentrations of azo-NLC,
the shift in wavelength is proportional to the concentration. With
increases in the concentration of azo-NLC, the shift reaches a
steady-state. This result suggests that, by controlling the azo-
NLC concentration, the reflection band can be continuously
adjusted and the color of the thin film is tunable.
In order to provide evidence of the imprinting of the polymer
network, a cross section of a sample cell was analyzed. Fig. 11(a)
shows the SEM image of the cross-section of the fabricated
sample cell. Fig. 11(b) shows the real image of the imprinted
liquid crystal cell. Abbreviation of National Cheng Kung
University ‘‘NCKU’’ was imprinted on a glass cell via multiple-
UV-irradiation through a mask. Fig. 11(a) shows the existence of
the porous structure of the fabricated polymer matrix after
removing of LC. Furthermore, Fig. 11(b) shows the real image of
the reflection color of the LC cell. The results suggest that with a
mask, colorful patterns with various colors are available by using
this method.
Conclusions
By using multiple UV-induced polymerizations, a helical poly-
mer matrix was imprinted in the presence of a cholesteric liquid
crystal (CLC). This polymer template was found to exhibit a
Bragg reflection (ꢁ450 nm), even after the CLC mixture was
washed out, due to the memory effects of the high density
polymer network. Refilling the imprinted cell with photosensitive
azo-NLC material resulted in phototuning through UV irradia-
tion. The original reflection wavelength underwent a red-shift to
560 nm after 4 hours of UV irradiation (5 wt% A11AEt). The
reflection shifts increased with increasing concentrations of azo-
NLC. Comparatively, the CLC mixture decreased in reflection
and a slight red-shift in wavelength was observed due to UV
irradiation. However, when the imprinted polymer was refilled
with azo-NLC, a clear red-shift and a fixed reflection were
Fig. 10 Dependence of the reflected central wavelength of the fabricated
LC cell on the concentration of A11AEt.
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Acknowledgements
The authors would like to thank the National Science Council
(NSC) of the Republic of China (Taiwan) for financially sup-
porting this research under Contract no. NSC 99-2923- E-006
-004 -MY3 and NSC 98-2221-E-006 -007 -MY3.
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