Synthesis and nanoimprinting of high refractive index and highly transparent polythioethers based on thiol-ene click chemistry

Article abstract of DOI:10.1002/pola.29181

This work demonstrates the UV nanoimprinting lithography (UV-NIL) of high refractive index and highly transparent polythioethers based on thiol-ene click chemistry. Herein, 9,9-bis(3-mercaptopropylphenylether)fluorene (BMPF) is designed as a new thiol mon

Full text of DOI:10.1002/pola.29181

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Synthesis and Nanoimprinting of High Refractive Index and Highly
Transparent Polythioethers Based on Thiol-Ene Click Chemistry
1
Kazuhiro Nakabayashi,1 Shigeki Sobu,1 Yuji Kosuge,2 Hideharu Mori
1Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan
2Fuji Chemicals Industrial Co., Ltd., 1-3-12 Azabudai, Minato-ku, Tokyo 106-0041, Japan
Correspondence to: H. Mori (E-mail: h.mori@yz.yamagata-u.ac.jp)
Received 11 June 2018; Accepted 28 June 2018
DOI: 10.1002/pola.29181
ABSTRACT: This work demonstrates the UV nanoimprinting lithog-
raphy (UV-NIL) of high refractive index and highly transparent
polythioethers based on thiol-ene click chemistry. Herein, 9,9-bis
(3-mercaptopropylphenylether)fluorene (BMPF) is designed as a
new thiol monomer with a high refractive index, high transpar-
ency, and good processability for UV-NIL. Colorless polythioethers
are synthesized from BMPF and ene monomers under mild thiol-
ene click reaction conditions. Excellent transmittance (96%) of
400 nm light is observed in all the polymer films and high refrac-
tive index values of 1.5972–1.6382 are attained. UV-NIL using
thiol-ene photopolymerization affords polymer nanoimprinting
patterns with various features on the order of 100–500 nm without
any fractures. To the best of our knowledge, this is the first report
on UV-NIL of high refractive index and highly transparent poly-
mers. Through proper monomer and polymer design, novel poly-
thioethers with suitable glass transition temperature (Tg) values
are developed with high refractive index, high transparency, and
good UV-NIL processability. Furthermore, UV-NIL based on thiol-
ene click chemistry is accomplished at the nanoscale. © 2018
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018
KEYWORDS: polythioether; thiol-ene click reaction; UV nanoim-
printing lithography; photo-radical polymerization; high refrac-
tive index; high transparency
temperature (Tg) at around 150 ꢀC.11,12 However, it is quite
difficult to design a polymer with a Tg suitable for injection
molding while maintaining a high refractive index and high
transparency. Therefore, there is a compelling need to develop
a method that enables nano- and microfabrication of high
refractive index and highly transparent polymers without
sacrificing performance.
INTRODUCTION Polymers with high refractive index and high
transparency are attractive for the development of high-
performance components in advanced display devices, various
lenses, optical waveguides, and diffractive gratings.1–3 The
introduction of substituents with a high molar refraction and
a small molar volume, such as aromatic rings, halogen atoms
except fluorine, sulfur atoms, and metal atoms, efficiently
increases the refractive index of the polymers.4–6 To date, a
variety of high refractive index and highly transparent poly-
mers have been developed by the efficient incorporation of
the above components, particularly sulfur atoms and aromatic
rings; several examples have accomplished a high refractive
index value of over 1.7.7–10 Recently, the processability of high
refractive index polymers has been paid much attentions as
well as the performance (e.g., high refractive index, high trans-
parency, and high thermal stability) because nano- and micro-
fabrication of high refractive index polymers is absolutely
necessary for the miniaturization of the opto-integrated
assembly process and devices. Injection molding is commonly
used for the fabrication of high refractive index and highly
transparent polymers; however, it is not suitable for nano-
and microfabrication. Furthermore, polymers processable by
injection molding need a relatively high glass transition
UV nanoimprint lithography (UV-NIL) has garnered increasing
attention as a next-generation technique for polymer fabrica-
tion due to its facile, cost-effective, and high-throughput pro-
duction process.13–17 A typical procedure for UV-NIL is as
follows: a photopolymerizable monomer mixture is cast onto
a substrate, and the mixture is then covered with a patterned
mold. The photopolymerization proceeds under UV irradiation
to yield polymers fabricated in the nano- and microscale after
peeling off the patterned mold. The selection of an efficient
photopolymerization system is key for successful UV-NIL.
The thiol-ene click chemistry can be one of promising candi-
dates for UV-NIL. The thiol-ene click reaction is conducted
mildly in the presence of organocatalyst or under UV-induced
photo-radical polymerization condition.18–21 UV-NIL using
thiol-ene photopolymerization between pentaerythritol
Additional supporting information may be found in the online version of this article.
© 2018 Wiley Periodicals, Inc.
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contributes to high transparency by reducing the interchain
packing of the polymers.31–33 Furthermore, the aliphatic seg-
ments can provide flexibility in the polymer backbone, afford-
ing good UV-NIL processability with a suitable Tg and
preventing fractures in the nanoimprinting patterns upon
removing the mold. Polymerizations based on the thiol-ene
click reaction using BMPF were investigated with several ene
monomers: 1,4-bis(acryloyloxy)butane (BAB), divinylsulfone
(DVS), and bis[4-(acryloyloxy)thiophenyl]sulfide (BATS). Each
polymerization was successfully carried out with both an
organocatalytic Michael reaction and a photo-radical polymer-
ization to yield the desired colorless polymers (P1-P3). All
three polymers exhibited excellent transparency as high as
96% at 400 nm due to the 9,9-diarylsubstituted fluorene-
based structure. Furthermore, P3 with the highest sulfur con-
tent achieved the highest refractive index (1.6382). UV-NIL of
P1-P3 by utilizing thiol-ene photopolymerization was also
investigated. The P1-P3 nanoimprinted patterns with various
features on the order of 100–500 nm were successfully fabri-
cated without any fractures.
FIGURE 1 Schematic of UV-NIL of P1-P3 by the thio-ene click
EXPERIMENTAL
chemistry.
[Color
figure
can
be
viewed
at
Materials
wileyonlinelibrary.com]
9,9-Bis(3-bromopropylphenylether)fluorene,34 9,9-bis(3-mer-
captopropylphenylether)fluorene (BMPF),34 bis[4-(hydroxy)
thiophenyl]sulfide,35 and bis[4-(acryloyloxy)thiophenyl]sulfide
(BATS)36 were synthesized by reference to previous litera-
tures. All reagents and solvents were used as received unless
otherwise stated.
tetrakis(3-mercaptobutylate) and several commercially avail-
able multifunctional enes was first reported by Carter and
coworkers, which yielded the corresponding polymer network
fabricated with feature size on the order of 100 nm.22 UV-NIL
using silicone-containing monomers, which is primarily used
in resist applications, has been also investigated, affording
defined features from sub-100 nm to several microns.23–26
Recently, thiol-ene click chemistry has been investigated for
the development of high refractive index materials, because
sulfur atoms can be efficiently incorporated into polymer
backbones. Zhao and coworkers reported the thiol-ene photo-
polymerization of commercially available pentaerythritol tet-
raacrylate and pentaerythritol tetrathioglycolic to give a
colorless thiol-ene network material with a refractive index of
1.556.27 In addition to the incorporation of sulfur atoms, novel
materials have been synthesized by using thiol and/or ene
monomers with silicon, zinc, tin, zirconium, and germanium
atoms, affording materials with refractive index values of
1.563–1.744.28–30 These results demonstrate that thiol-ene
click chemistry is an efficient system for developing high
refractive index materials. Thiol-ene click chemistry could also
be applicable in the UV-NIL of high refractive index and highly
transparent polymers; however, to the best of our knowledge,
this has never been explored.
Synthesis of 9,9-bis(3-bromopropylphenylether)fluorene
The 2-butanone solution (45.0 mL) of 9,9-bis(4-hydroxyphe-
nyl)fluorene (4.21 g, 12.0 mmol), K2CO3 (4.98 g, 36.0 mmol),
and 1,3-dibromopropane (7.3 mL, 72.0 mmol) was stirred at
ꢀ
80 C for 12 h under the nitrogen atmosphere. After the reac-
tion, the reaction mixture was then diluted with CH2Cl2 and
washed with brine, and the organic layer was dried over
MgSO4. After the removal of solvents, the residue was purified
by column chromatography (CH2Cl2: hexane = 1: 1) to yield a
white solid (4.44 g, 66%). 1H NMR (CDCl3, δ, ppm): 7.75 (d,
1H), 7.35 (m, 1H), 7.11 (t, 1H), 7.09 (t, 1H), 6.75 (t, 1H), 6.72
(t, 1H), 4.06 (t, 2H), 3.83 (t, 2H), 2.00 (quint, 2H). Note that
the purified product contained a small amount of 9,9-bis
[4-(2-propenyloxy)phenyl]fluorene as a byproduct. However,
this purified product was used for the next reaction without
further purification because 9,9-bis[4-(2-propenyloxy)phenyl]
fluorene did not have any effect to the next reaction.
Synthesis of 9,9-bis(3-mercaptopropylphenylether)
fluorene (BMPF)
Herein, our aim is to demonstrate UV-NIL of high refractive
index and highly transparent polymers based on thiol-ene
click chemistry (Fig. 1). A new 9,9-diarylsubstituted fluorene-
based thiol monomer, 9,9-bis(3-mercaptopropylphenylether)
fluorene (BMPF), was designed and synthesized for this pur-
pose. A 9,9-diarylsubstituted fluorene skeleton can enhance
the refractive index of a polymer because of its aromatic con-
tent, and the orthogonal structure at the 9-position of fluorene
The DMF solution (37.5 mL) of 9,9-bis(3-bromopropylpheny-
lether)fluorene (4.44 g, 7.53 mmol) and thiourea (2.28 g,
ꢀ
30.0 mmol) was stirred at 80 C for 12 h under the nitrogen
atmosphere. After cooling to room temperature, a 1 M NaOH
aqueous solution (75.0 mL) was added, and the resulting mix-
ture was stirred for 1 h. Then the mixture was acidified with
a 1 M HCl aqueous solution to pH = 3. The mixture was
2
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TABLE 1 Synthesis of P1 by Michael reaction condition.
Yield (%)b
Color
a
a
Run
Solvent
Temp. (ꢀC)
Time (h)
Mn
Mw/Mn
1
2
3
4
5
6
7
8
a
DMAc
THF
RT
RT
RT
RT
RT
60
60
60
6
6
–
–
54
55
80
81
82
78
70
91
White
White
White
White
Yellow
White
White
Yellow
2000
3100
5700
9400
3900
4600
9900
1.08
1.38
1.28
1.56
2.03
1.29
1.44
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
4
6
24
4
6
24
b
Measured by SEC using polystyrene standards in chloroform.
Yields of methanol-insoluble parts.
diluted with CH2Cl2 and washed with brine, and the organic
layer was dried over MgSO4. After the removal of solvents, the
residue was purified by column chromatography (CH2Cl2: hex-
ane = 2: 1) to yield a white solid (1.65 g, 44%). 1H NMR
(CDCl3, δ, ppm): 7.75 (d, 1H), 7.35 (m, 1H), 7.11 (t, 1H), 7.09
(t, 1H), 6.75 (t, 1H), 6.72 (t, 1H), 4.00 (t, 2H), 2.70 (q, 2H),
2.03 (quint, 2H), 1.37 (t, 1H). 13C NMR (CDCl3, δ, ppm): 157.5,
151.7, 139.9, 138.2, 129.2, 127.7, 127.3, 126.0, 120.1, 114.0,
65.4, 64.1, 33.3, 21.3. Anal. calcd for C31H30O2S2: C 74.66, H
6.06, S 12.86; found: C 74.65, H 6.05, S 13.07.
(br, 2H), 2.76 (t, 2H), 2.68 (t, 2H),2.58 (t, 2H), 2.01 (br, 2H),
1.67 (br, 2H). Mn = 5700 (Mw/Mn = 1.28). Tg = 30 ꢀC.
Synthesis of P1 by photo-radical polymerization in
solution condition
The dry chloroform solution (2.0 mL) of BMPF (0.25 g,
0.50 mmol), BAB (0.10 g, 0.50 mmol), and 2,2-dimethoxy-
2-phenylacetophenone (DMPhAc) (0.01 g, 3 wt% for total
amount of BMPF and BAB) was stirred for 15 min under the
exposure of UV light (λ = 365 nm, 10 mW/cm, cumulative
radiation = 60 mJ/cm2). After the reaction, the reaction mix-
ture was poured into methanol to yield a white precipitate.
The obtained polymer was collected with filtration, and then
dried in vacuo at 80 ꢀC (0.35 g, 96%).
Synthesis of bis[4-(acryloyloxy)thiophenyl]sulfide (BATS)
To the dry THF solution (8.6 mL) of bis[4-(hydroxy)thiophe-
nyl] sulfide (0.68 g, 2.00 mmol) and dimethylaniline (0.97 g,
8.00 mmol), the dry THF solution (2.4 mL) of acryloyl chloride
(0.49 mL, 6 mmol) was added dropwise at 0 ꢀC under the
nitrogen atmosphere. The reaction mixture was stirred at
room temperature overnight, and the solvent was then
removed under reduced pressure. The residue was washed
with NaHCO3 aqueous solution and brine, and the organic
layer was dried over MgSO4. After the removal of solvents, the
residue was purified by column chromatography (ethyl ace-
tate: hexane = 2: 5) to yield a white solid (0.64 g, 68%). 1H
NMR (CDCl3, δ, ppm): 7.33 (m, 4H), 7.25 (m, 4H), 6.39 (dd,
2H), 6.09 (q, 2H), 5.84 (dd, 2H), 4.33 (t, 4H), 3.18 (t, 4H). 13C
NMR (CDCl3, δ, ppm): 165.9, 134.5, 133.8, 131.6, 131.4, 130.2,
128.0, 82.9, 32.1. Anal. calcd for C22H22O4S3: C 59.17, H 4.97,
S 21.54; found: C 59.02, H 4.78, S 21.50.
Synthesis of P1 by photo-radical polymerization in bulk
condition
The mixture of BMPF (0.25 g, 0.50 mmol), BAB (0.10 g,
0.50 mmol), and DMPhAc (0.01 g, 3 wt%) was stirred at room
temperature until becoming homogeneous. The mixture was
then drop-casted onto a glass substrate and UV light was
exposed for 1 min to yield transparent P1 film.
UV nanoimprint lithography of P1
Nanoimprinting was carried out using a UV nanoimprint kit
(Toyo Gosei Co. Ltd.) including a UV lamp (Eishin Kagaku
Co. Ltd. UV mini-light) and a master micro-pattern quartz
mold (Toppan Printing Co. Ltd.). A homogeneous mixture of
BMPF (0.25 g, 0.50 mmol), BAB (0.10 g, 0.50 mmol), and
DMPhAc (0.01 g, 3 wt%) was drop-cast onto the silicon sub-
strate and the quartz mold was set onto the mixture. After
exposure to UV light for 1 min, the mold was detached.
Synthesis of P1 by Michael reaction condition
The typical procedure is as follows (Run 4 in Table 1); To the
dry chloroform solution (2.0 mL) of BMPF (0.25 g, 0.50 mmol)
and 1,4-bis(acryloyloxy)butane (0.10 g, 0.50 mmol), triethyla-
mine (TEA) (0.1 mL) was added dropwise at room tempera-
ture under the nitrogen atmosphere. Then the reaction was
continued at room temperature for 6 h. After the reaction, the
reaction mixture was poured into methanol to yield a white
precipitate. The obtained polymer was collected with filtra-
Characterization
1
The H and 13C NMR spectra were recorded with a JEOL JNM-
ECX400. The Elemental analysis was carried out on a Perkin-
Elmer 2400 II CHNS/O analyzer. The UV–vis spectra were
recorded on a JASCO V-630BIO UV–vis spectrophotometer.
The number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) were estimated by size exclusion
chromatography (SEC) using a HLC-8320 system. The column
set was as follows; a guard column (TSK guard column HHR-H)
1
ꢀ
tion, and then dried in vacuo at 80 C (0.29 g, 81%). H NMR
(CDCl3, δ, ppm): 7.73 (br, 1H), 7.35 (br,1H), 7.33 (br, 1H),
7.31 (br, 1H), 7.09 (br,1H), 6.73 (br,1H), 4.08 (br, 2H), 3.98
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SCHEME 1 Synthesis of P1-P3.
and three consecutive columns (TSKgel GMHHR-M, TSKgel
GMHHR-M, and TSKgel superH-RC) eluted with chloroform at a
flow rate of 1.0 mL/min. Polystyrene standards were
employed for calibration. Thermal gravimetric (TG) analysis
was performed on a SEIKO TG/DTA6200 at a heating rate of
analysis to confirm that the desired monomers were synthe-
sized (Fig. S1 and S2).
Synthesis and characterization of polymers
The polymerizations of BMPF and ene monomers (BAB, DVS,
and BATS) based on the thiol-ene click reaction were investi-
gated by both an organocatalytic Michael reaction and a
photo-radical polymerization (Scheme 1). First, the polymeri-
zation between BMPF and BAB was carried out by Michael
reaction in the presence of triethylamine (Table 1), which
enables us to confirm the reactivities of the thiol and ene
monomers. As a result of screening reaction conditions (sol-
vent, reaction temperature, and reaction time), colorless P1
that did not dissolve in common organic solvents was
obtained using DMAc as a solvent. The reaction did not pro-
ceed efficiently in THF, as seen by the Mn and yield of the
obtained product. Using chloroform, yellow P1 with Mn
~10,000 was obtained in a relatively high yield (Table 1, Run
5 and 8). The more moderate reaction conditions in chloro-
form (i.e., a shorter reaction time) afforded colorless P1 with
Mn = 3100–5700 in relatively high yields. The obtained color-
less P1 showed high solubility in common organic solvents
such as chloroform, dichloromethane, toluene, THF, and DMAc
(Table S1). The structure of P1 was characterized by 1H and
13C NMR analysis (Fig. 2). The peaks of protons corresponding
to methylene groups were clearly observed at 2.76 and
2.68 ppm in the 1H NMR spectrum of P1 (Fig. 2a). The charac-
teristic peaks of carbons corresponding to the carbonyl group
(171.9 ppm) and C-9 in the fluorene moiety (64.1 ppm) were
also clearly observed (Fig. 2b). These data indicate that the
thiol-ene click reaction was successfully carried out under the
ꢀ
10 C/min under a nitrogen atmosphere. Differential scanning
calorimetry (DSC) analysis was performed on
a SEIKO
DSC6200 under nitrogen atmosphere. The measurements
were conducted the samples heated to 300 ꢀC followed by
cooling to −50 ꢀC, and then heating again up to 300 ꢀC at a
cooling and heating rate of 10 ꢀC/min. The glass transition
temperature (Tg) was determined as the midpoint tempera-
ture of the heat capacity change during the second heating
step. Scanning electron microscopy (SEM) measurements
were performed on a Hitachi SU8000 microscope at accelerat-
ing voltages of 2.0 kV. Refractive-indx mesurement was per-
formed on a ATAGO digital Abbe refractometer DR-A1.
RESULTS AND DISCUSSION
Synthesis and characterization of monomers
A new 9,9-diphenylsubstituted fluorene-based thiol monomer
BMPF was synthesized by a two-step reaction (Scheme S1).
The nucleophilic substitution reaction between 9,9-bis
(4-hydroxyphenyl)fluorene and 1,3-dibromopropane to yield
9,9-bis(3-bromopropylphenylether)fluorene. Then the treat-
ment of 9,9-bis(3-bromopropylphenylether)fluorene with thio-
urea and the following hydrolysis were carried out to yield
BMPF. A new ene monomer BATS was synthesized by a two-
step reaction from 4,40-thiobisbenzenethiol. BMPF and BATS
were characterized by 1H NMR, 13C NMR, and elemental
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the bulk mixture of BMPF, DVS, and DMPhAc, which can be
considered a solvent-free green process. In contrast, a DMAc
solution was used for the direct preparation of P3 films
because BATS is a solid, yielding the colorless and transparent
P3 film (Fig. 3). Based on these results, BMPF has the ability
to polymerize via thiol-ene click chemistry under three differ-
ent conditions (an organocatalytic Michael reaction, photo-
radical polymerization in solution, and solvent-free photo-
radical polymerization) to afford the novel colorless poly-
thioethers P1-P3. This good photopolymerizability of BMPF
can result in efficient UV-NIL processing (see details below).
TABLE 2 Synthesis of P1-P3 by photo-radical polymerization in
solution condition.a
b
b
Run
Solvent
Mn
Mw/Mn
Yield (%)c
Color
d
d
P1
P2
P3
a
CHCl3
CHCl3
DMAc
–
–
96
69
90
White
White
White
4300
3200
1.12
1.79
The polymerization was carried out under the exposure of UV light
(λ = 365 nm) for 15 min.
Measured by SEC using polystyrene standards in chloroform.
Yields of methanol-insoluble parts.
The sample did not dissolve in chloroform.
b
c
d
The thermal properties of P1-P3 were investigated by TG and
DSC analyses. P1-P3 synthesized by the Michael reaction con-
ditions were used for the analyses. As can be seen in the TG
profiles (Fig. 4), P1-P3 exhibited a 5% weight loss tempera-
ture above 330 ꢀC under a nitrogen atmosphere. This
observed thermal stability is sufficient for the optical device
fabrication process. In the DSC analysis, the glass transition
temperature of P1-P3 was observed at 30, 95, and 23 ꢀC,
respectively, which is relatively low due to their flexible ali-
phatic polymer backbones (Fig. 5).
organocatalytic Michael reaction conditions to yield the
desired P1.
Prior to the photo-radical polymerization under the solvent-
free condition, we conducted the photo-radical polymerization
under the typical solution conditions to check the photopoly-
merizability of the monomers. Photo-radical polymerization of
BMPF and BAB was investigated in chloroform for 15 min
under exposure to UV light (λ = 365 nm) in the presence of
2,2-dimethoxy-2-phenylacetophenone (DMPhAc) as a photo-
radical polymerization initiator. The obtained colorless prod-
uct did not dissolve even in chloroform and was obtained
quantitatively, implying that P1 with a high molecular weight
and/or some cross-linking was formed (Table 2). Owing to a
recent concern for a sustainable economic manufacturing sys-
tem, a solvent-free green reaction that could be used in an
environmentally friendly and economic manufacturing system
was investigated. The bulk mixture obtained from solid BMPF,
liquid BAB, and solid DMPhAc was drop-cast onto a glass sub-
strate, and solvent-free photo-radical polymerization was then
carried out under UV light in a quite short reaction time
(1 min) to yield a colorless and transparent P1 film (Fig. 3).
The solvent-free condition may accelerate the polymerization
compared to the solution condition, which could lead to the
formation of the cross-linked structure and/or large molecular
weight in some degree even in short polymerization time
(1 min). The fact that the monomers and the formed polymer
have no light absorption around 365 nm contribute to the effi-
cient progress of the photo-radical polymerization.
Optical properties
The optical property is one of the most important characteris-
tics for high refractive index polymers. The transparency of
the P1-P3 films was investigated by UV–vis transmission. The
P1-P3 films (film thickness 20–30 μm) were prepared by
drop-casting each chloroform solution onto glass substrates.
Following the synthesis of P1, polymerizations of BMPF and
other ene monomers (DVS and BATS) were investigated under
the Michael reaction conditions (Scheme 1 and Table S2). The
optimal reaction between BMPF and DVS occurred in chloro-
form at room temperature for 6 h to yield colorless P2 with
Mn = 6900 in 81% yield. P3 was synthesized from BMPF and
BATS in DMAc to yield colorless P3 with a high molecular
weight (Mn = 10000–48400). The structures of P2 and P3
1
were also confirmed by H and 13C NMR analysis (Fig. S3 and
S4). The synthesis of P2 and P3 by photo-radical polymeriza-
tion under solution conditions was also carried out to yield
colorless P2 and P3 in 69 and 90% yield, respectively
(Table 2). The colorless and transparent P2 film (Fig. 3) was
successfully prepared by photo-radical polymerization from
FIGURE 2 1H (a) and 13C (b) NMR spectra of P1 in CDCl3.
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FIGURE 3 Pictures of P1-P3 films prepared by photo-radical
polymerization. [Color figure can be viewed at
wileyonlinelibrary.com]
FIGURE 5 DSC profiles of P1-P3 under a nitrogen atmosphere.
[Color figure can be viewed at wileyonlinelibrary.com]
As can be seen in Figure 6, the cut-off wavelengths of the
P1-P3 films were ~280 nm, and all films exhibited excellent
transparency of 96% at 400 nm. This observed transparency
of P1-P3 is one of the highest values among high refractive
index polymers. For example, the cut-off wavelength is
~400 nm in most sulfur-containing polyimides that have been
commonly studied as high refractive index materials.37–39 The
flexible aliphatic polymer backbone and the orthogonal struc-
ture at the 9-position of fluorene may prevent interchain
packing, contributing to the excellent transparency of P1-P3.
Furthermore, it should be noted that the excellent transpar-
ency was maintained even after harsh treatment (thermal
treatment at 200 ꢀC for 6 h under ambient atmosphere) due
to the high thermal and chemical stability of P1-P3 (Fig. S5).
The refractive index of P1-P3 was then investigated with an
Abbe refractometer (Table 3). P1-P3 exhibited a refractive
index of 1.5972, 1.6266, and 1.6382, respectively. The sulfur
content in the polymer repeating units in P1-P3 was calcu-
lated to be 8.99, 14.87, and 16.44 wt%, respectively. The
observed refractive index increases with an increase in the
sulfur content; P3 with the highest sulfur content achieved
the highest refractive index of over 1.63. The high number of
aromatic units might also contribute to the enhanced refrac-
tive index of P3.
UV nanoimprint lithography
In order to miniaturize opto-integrated assembly processes
and devices, it is quite important to investigate the UV-NIL
processability of high refractive index and highly transparent
polymers. Additionally, the manipulation of Tg is crucial,
because applicable Tg range of the polymers are practically
dependent on the process.40–42 For example, low temperature
NIL, including UV-NIL and room temperature NIL, happens
under the temperature of below or near to Tg of the poly-
mer.42 A homogeneous mixture of BMPF, ene monomers, and
DMPhAc was drop-cast onto a silicon substrate, and then cov-
ered by
a patterned quartz mold. The photo-radical
TABLE 3 Optical properties of P1-P3.
Sulfur
content
Run (wt%)
Cut-off
wavelength
(nm)
Transmittance
(%)a
Refractive
index
P1
P2
P3
a
8.99
14.87
16.44
317
340
335
96
96
96
1.5972
1.6266
1.6382
FIGURE 4 TG curves of P1-P3 under a nitrogen atmosphere.
[Color figure can be viewed at wileyonlinelibrary.com]
Transmittance at 400 nm.
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polymerization proceeded under exposure to UV light for
1 min, and the mold was then peeled from the substrate. For
the preparation of the P1- and P2-based nanoimprinting pat-
terns, the solvent-free green process was employed, whereas
a DMAc solution was used for P3-based pattern formation.
Figure 7 exhibits the SEM images of the obtained nanoim-
printing patterns of P1-P3. The P1-P3 nanoimprinting pat-
terns with various features on the order of 100–500 nm were
successfully fabricated in just 1 min, which is due to the good
photopolymerizability of BMPF. Fabrication of the defined
nanoimprint patterns indicated that P1–P3 provided a clean
mold release without fracture or deformation of the embossed
structures. Upon releasing the mold, the rigidity of the poly-
mer can be a critical issue for the fabrication of the defined
nanoimprinting patterns. If a polymer is rigid, a fracture of
the patterns might occur. The flexible polymer backbone, as
seen by the relatively low Tg (23–95 ꢀC), contributed to the
clean mold release in P1-P3. These results demonstrate that
the defined P1-P3 nanoimprinting patterns on a 100–500 nm
scale were obtained due to proper monomer and polymer
design that enhanced processability for UV-NIL.
FIGURE 6 UV–vis transmittance spectra of P1-P3 thin films.
[Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 7 SEM images of UV-NIL patterns of P1-P3 (P1: a and b, P2: c, P3: d).
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CONCLUSIONS
16 S. D. Moon, N. S. Lee, S. I. Kang, J. Micromech. Microeng.
2003, 13, 98.
UV-NIL of high refractive index and highly transparent poly-
mers has been successfully realized based on thiol-ene click
chemistry. A novel thiol monomer, BMPF, was designed, that
could provide high refractive index, high transparency, and
good UV-NIL processability. Colorless polythioethers (P1-P3)
were synthesized from BMPF and several ene monomers
under mild thiol-ene click reaction conditions. The P1-P3 thin
films exhibited excellent transmittance of 96% at 400 nm due
to the flexible polymer backbone and the orthogonal structure
at the 9-position of fluorene. The refractive indexes of P1-P3
were high values in the range of 1.5972–1.6382 due to the
incorporation of sulfur atoms and a fluorene-based aromatic
structure. The refractive index increased with an increase in
the sulfur content; P3 with the highest sulfur content
(16.44 wt%) achieved the highest refractive index of 1.6382.
Furthermore, UV-NIL using thiol-ene click chemistry was
developed; the defined P1-P3 nanoimprinting patterns with
various features on the order of 100–500 nm were readily
fabricated without any fractures, which might be a result of
the relatively low Tg due to the flexible polymer backbone.
Consequently, high refractive index and highly transparent
polymers with good UV-NIL processability were developed by
proper monomer and polymer design. This result opens a
new possibility of thiol-ene click chemistry for the develop-
ment of optical materials and their nanoscale fabrication.
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