Highly fluorinated and photocrosslinkable liquid prepolymers for flexible optical waveguides

Article abstract of DOI:10.1039/c0jm02604b

We evaluate highly fluorinated liquid prepolymers and their UV-cured films for flexible optical waveguide applications. Two liquid prepolymers, 2,3,5,6,2′,3′,5′,6′-octafluoro-4,4′-bis-[2-{2-[1, 1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,

Full text of DOI:10.1039/c0jm02604b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly fluorinated and photocrosslinkable liquid prepolymers for flexible
optical waveguides†
a
Seung Koo Park, Jong-Moo Lee, Suntak Park, Jin Tae Kim, Min-su Kim, Myung-Hyun Lee
a
a
a
a
b
*
a
*
and Jung Jin Ju
Received 10th August 2010, Accepted 25th October 2010
DOI: 10.1039/c0jm02604b
We evaluate highly fluorinated liquid prepolymers and their UV-cured films for flexible optical
waveguide applications. Two liquid prepolymers, 2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-{2-[1,1-
difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-2,2-difluoro-
ethoxy]-biphenyl (7) and 2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-(2-{2-[1,1-difluoro-2-(2,3,5,6-
tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-
difluoro-ethoxy]-biphenyl (8), are synthesized with decafluorobiphenyl, 2,3,4,5,6-pentafluoro styrene,
and fluorinated triethylene glycol or fluorinated tetraethylene glycol. The prepolymers are
photocurable to give flexible and transparent films. The refractive indices of the films made from the
prepolymers are controlled with their blending composition, 1.417–1.437 at a wavelength of 1.31 mm.
The optical losses of the films prepared from prepolymers 7 and 8 are 0.25 and 0.23 dB cmꢀ1 at
ꢁ
a wavelength of 1.31 mm, respectively. The films are thermostable at temperatures over 400 C. The
ultimate stress, initial modulus, and elongation of the films prepared from prepolymers 7 and 8 are
17.1 MPa, 1.25 GPa, and 1.5% and 12.3 MPa, 0.44 GPa, and 6.9%, respectively. Optical waveguides
fabricated from the prepolymers are flexible without any substrate film and the optical losses at
a bending radius of 1.5 mm are under 0.2 dB. We identify the liquid prepolymers as possible good
candidate materials for flexible optical waveguide applications by studying the optical transmission in
flexible optical waveguides fabricated from them.
actively operated. Therefore, numerous organic materials have
been developed over a long time for passive optical devices.
Most of them are oligomers and polymers containing reactive
moieties crosslinked by UV or thermal treatment.2–9 These
organic materials have been highly fluorinated to lessen the
optical loss at communication bands near a wavelength of 1.31 or
1.55 mm. The overtone absorption of C–H bond stretching
vibrations in hydrocarbon polymers is drastically reduced by
replacing C–H with C–F because the harmonic of the C–F bond
is displaced to longer wavelengths than that of the C–H bond,
which is further away from the communication bands.10 Optical
polymers of organic materials have such a large molecular weight
that a solvent is needed to make a solution for a fabrication
process. A fabrication process requiring a solvent is not suitable
for saving energy and lessening pollutants. Additionally, in
solvent-free film fabrication, it is easier to control the film
thickness and to increase the crosslinking density. In this respect,
a liquid prepolymer which has a crosslinking moiety is more
favorable than a polymer solution. Such a prepolymer is easily
fabricated by using a spin coating method and the fabricated
prepolymer is directly transformed into a film by UV and thermal
curing.
Introduction
Organic materials have been widely used for optical waveguides
in high-speed optical communication systems for almost the last
two decades because of their easy fabrication at lower cost and
high integration for more multiple channels.1 In addition, optical
losses of films fabricated from organic materials are low enough
to transmit optical signals error-free and the thermo-optic coef-
ficient of the films is ten times higher than that of silica, one of
well known optical waveguide materials. Since organic materials
are optically, chemically, and thermally as stable as inorganic
materials and semiconductors when they are used for passively
operated optical devices, they are used commercially for passive
optical devices such as power splitters, variable optical attenua-
tors, and AWG (arrayed waveguide grating) multiplexers.
Inorganic materials and semiconductors are predominantly used
for active optical devices such as optical modulators, wavelength
converters, and optical amplifiers because they have much higher
durability than organic materials when optical devices are
aConvergence Components & Materials Research Laboratory, Electronics
and Telecommunications Research Institute, 138 Gajeongno, Yuseong-gu,
Daejeon, 305-700, S. Korea. E-mail: skpark@etri.re.kr; jjju@etri.re.kr;
Fax: +82 42 860 6248; Tel: +82 42 860 5162
bSchool of Information and Communication Engineering, Sungkyunkwan
University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-
746, S. Korea. E-mail: mhlee@skku.edu; Tel: +82 31 299 4582
Optical transmission systems will replace electrical trans-
mission systems for information transmission over short
distances (within several centimetres) among drive, memory and
display parts etc. in about five years because mass information
transmission with a high speed is needed in information and
communication devices such as mobile phones, computers, and
digital cameras. Flexibility is a crucial property for optical
1
† Electronic supplementary information (ESI) available: IR, H NMR,
19F NMR spectra of compounds 4 and 5, prepolymers 7 and 8, DSC
thermograms of prepolymers 7 and 8, their films, the solubility test
results of the films. See DOI: 10.1039/c0jm02604b
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waveguide materials for optical signal transmission within
mobile devices since optical signals have to be transferred back
and forth inside their extremely narrow spaces.11 However, in the
case of optical devices for optical telecommunications, wave-
guide films do not need flexibility because the waveguides have
always been on a silicon wafer. In order to use organic materials
for optical signal transmission in the information and commu-
nication devices, the films prepared from organic materials have
to show flexibility along with the above-mentioned features of
organic films for optical waveguides in optical telecommunica-
tions. Many researchers have investigated various optoelectronic
integrated circuits including flexible optical waveguides.12–20
Organic materials suitable for flexible optical waveguides have
recently been developed because organic films have flexibility as
an inherent property. However, in this case, an additional
substrate film such as polyethylene terephthalate or polyimide
was used to obtain the flexible optical waveguides because the
core and cladding films were brittle.21–24
product was extracted with ethyl acetate (EA). After EA was
removed by a vacuum evaporator at room temperature, the
orange liquid was applied to a column with EA/hexane (1/5, v/v)
for purification. After the eluent was completely removed by
ꢁ
a vacuum evaporator, the transparent liquid was dried at 35 C
under vacuum.
Compound 4 (10.7 g, 47%). IR nmax(liquid, NaCl)/cmꢀ1
:
3374m (O–H str., hydroxyl); 3038w (]C–H str., vinyl); 2965w
1
(C–H str., methylene); 1291s, 1119s (C–O str., ether). H NMR
dH (CDCl3, 300 MHz): 6.69–6.59 (1H, m, vinyl); 6.10–5.68 (2H,
m, vinyl); 4.53 (2H, t, methylene); 3.88 (2H, t, methylene); 2.81
(1H, s, hydroxyl). 19F NMR dF (CDCl3, 300 MHz): ꢀ78.70 (2F,
m); ꢀ80.95 (2F, m); ꢀ89.08 (4F, m); ꢀ144.45 (2F, m); ꢀ158.17
(2F, m). Anal. calcd. for C14H8F12O4: C, 35.91; H, 1.72. Found:
C, 35.58; H, 1.51; N, 0.13. GC–MS (m/z): calcd. 468.0; found
468.0.
Compound 5 (12.9 g, 45%). IR nmax(liquid, NaCl)/cmꢀ1: 3383m
(O–H str., hydroxyl); 3039w (]C–H str., vinyl); 2966w (C–H str.,
methylene); 1204s, 1091s (C–O str., ether). 1H NMR dH (CDCl3,
300 MHz): 6.69–6.59 (1H, m, vinyl); 6.10–5.67 (2H, m, vinyl); 4.53
(2H, t, methylene); 3.86 (2H, t, methylene); 2.62 (1H, s, hydroxyl).
19F NMR dF (CDCl3, 300 MHz): ꢀ79.29 (2F, m); ꢀ81.74 (2F, m);
ꢀ89.76 (8F, m); ꢀ145.28 (2F, m); ꢀ159.11 (2F, d). Anal. calcd. for
C16H8F16O5: C, 32.89; H, 1.38. Found: C, 32.69; H 1.16; N, 0.14.
GC–MS (m/z): calcd. 584.0; found 584.1.
In our work, we prepared a flexible optical waveguide
without any substrate film. We designed and synthesized two
prepolymers,
difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-
tetrafluoro-ethoxy}-2,2-difluoro-ethoxy]-biphenyl (7) and
2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-{2-[1,1-
2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-(2-{2-[1,1-difluoro-2-
(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-
ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-difluoro-ethoxy]-biphenyl
(8), for a flexible optical waveguide. These prepolymers are
oligomers and liquids and have two UV-crosslinkable moieties
at the end of the molecules. We fabricated a flexible optical
waveguide with the prepolymers. The thermal stability and
optical properties of the films prepared from the prepolymers
were also studied. The mechanical properties of the optical
waveguide films was first investigated. We discussed the
possibility of using the prepolymers as candidate materials for
flexible optical waveguides.
Synthesis of 2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-{2-[1,1-
difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-
tetrafluoro-ethoxy}-2,2-difluoro-ethoxy]-biphenyl (7) and
2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-(2-{2-[1,1-difluoro-2-
(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-
tetrafluoro-ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-difluoro-
ethoxy]-biphenyl (8)
After 3.34 g (10.0 mmol) of compound 6 and 9.37 g (20.0 mmol)
of compound 4 or 11.7 g (20.0 mmol) of compound 5 were
completely dissolved in 15 mL of anhydrous DMAc under
nitrogen, 0.07 g of caesium fluoride (0.1 mol eq. of decafluor-
obiphenyl) and 0.57 g of calcium hydride (3.0 mol eq. of decaf-
luorobiphenyl) were added to the solution ꢁas catalysts. The
reaction was carried out for 2 days at ca. 60 C. After this, the
process was almost the same as for the synthesis of compounds
4 and 5.
Experimental
Materials
2,3,4,5,6-Pentafluoro styrene (1) and anhydrous N,N-dimethyl
acetamide (DMAc) were purchased from Aldrich Chemical Co.,
Inc. Fluorinated triethylene glycol (2) and fluorinated tetra-
ethylene glycol (3) were purchased from Exfluor Research Co.
Decafluorobiphenyl (6) was purchased from Tokyo Chemical
Industry Co., Ltd. All reagents were used as received.
Prepolymer 7 (10.3 g, 84%). IR nmax(liquid, NaCl)/cmꢀ1
:
3038w (]C–H str., vinyl); 2971w (C–H str., methylene); 1297s,
1
1119s (C–O str., ether). H NMR dH (CDCl3, 300 MHz): 6.63–
6.53 (2H, m, vinyl); 6.05–5.65 (4H, m, vinyl); 4.59 (4H, t, meth-
ylene); 4.46 (4H, t, methylene). 19F NMR dF (CDCl3, 300 MHz):
ꢀ78.81 (8F, m); ꢀ89.17 (8F, m); ꢀ138.81 (4F, m); ꢀ144.56 (4F,
m); ꢀ155.90 (4F, d); ꢀ158.21 (4F, m). Anal. calcd. for
C40H14F32O8: C, 39.04; H, 1.15. Found: C, 38.69; H 1.01; N,
0.11. ESI–MS (m/z): calcd. 1230.0; found 1230.1.
Synthesis of 2-{2-[1,1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-
phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-2,2-difluoro-
ethanol (4) and 2-(2-{2-[1, 1-difluoro-2-(2,3,5,6-tetrafluoro-4-
vinyl-phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-1,1,2,2-
tetrafluoro-ethoxy)-2,2-difluoro-ethanol (5)
After 11.4 g (58.5 mmol) of 2,3,4,5,6-pentafluoro styrene and
14.3 g (48.8 mmol) of compound 2 or 20.0 g (48.8 mmol) of
compound 3 were completely dissolved in 22 mL of anhydrous
DMAc under nitrogen, 10.1 g of potassium carbonate was
introduced into the solution with stirring. The reaction was
carried out for 1 day at 80–90 ꢁC. After cooling to room
temperature, the solution was poured into water. The reaction
Prepolymer 8 (13.9 g, 95%). IR nmax(liquid, NaCl)/cmꢀ1
:
3039w (]C–H str., vinyl); 2971w (C–H str., methylene); 1210s,
1
1085s (C–O str., ether). H NMR dH (CDCl3, 300 MHz): 6.69–
6.56 (2H, m, vinyl); 6.10–5.67 (4H, m, vinyl); 4.54 (4H, t, meth-
ylene); 4.43 (4H, t, methylene). 19F NMR dF (CDCl3, 300 MHz):
ꢀ79.29 (8F, m); ꢀ89.54 (16F, m); ꢀ139.26 (4F, m); ꢀ145.01 (4F,
m); ꢀ156.49 (4F, d); ꢀ158.79 (4F, m). Anal. calcd. for
1756 | J. Mater. Chem., 2011, 21, 1755–1761
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ꢁ
C44H14F40O10: C, 36.13; H; 0.96. Found: C, 35.47; H 0.76.; N,
a DuPont DSC 2920 under nitrogen at a heating rate of 10 C
N.D. ESI–MS (m/z): calcd. 1462.0; found 1462.0.
minꢀ1. The thermal behavior of the film was monitored by
thermogravimetry using a TA Instruments TGA Q50 under
nitrogen at a heating rate of 10 ꢁC minꢀ1. The liquid prepolymer
viscosity was measured at 25 ꢁC by an RVDV II Brookfield
viscometer. Scanning electron micrographs were obtained on an
FEI Sirion scanning electron microscope. The film was cut into
7 ꢂ 60 mm strips. The samples were tested with an Instron 4482
at room temperature to determine their mechanical properties.
The extension rate was 5 mm minꢀ1. About 0.1 g of the film was
cut and kept in various solvents (ca. 10 g) with stirring at room
temperature for 120 h for a solubility test. The refractive indexes
were measured using a prism coupler for TE- and TM-polarized
light.
Film preparation
After a 1.5% CGI 124 (a photoinitiator) solution in liquid pho-
tocurable prepolymers 7 and 8 was prepared, it was filtered
through a 0.2 mm Teflon filter. The prepolymer containing the
photoinitiator was spin-coated on a glass plate at ca. 500 rpm for
30 s. The film was UV-cured with a UV lamp with 2 kW power
for ca. 10 min and thermally annealed at 150 ꢁC under nitrogen
for 1 h. The cured prepolymer films on a glass plate were
immersed into water and detached from the plate. The detached
films were dried at 60 ꢁC under vacuum. The film thickness was
around 50–60 mm.
Results and discussion
Measurements
Synthesis and characterization of prepolymers
Infrared (IR) spectra were recorded with a Nicolet 6700 FT-IR
spectrometer. A Bruker 300 MHz NMR spectrometer was used
for 1H and 19F NMR measurements. The elemental analysis was
obtained on a Thermo Scientific FLASH 2000 elemental
analyzer. GC–MS and ESI–MS spectra were obtained on
a Varian CP 3800–1200L mass spectrometer and a Bruker Dal-
tonics Esquire ion trap mass spectrometer, respectively. Thermal
molecular motions of the prepolymers and the related films were
monitored by differential scanning calorimetry (DSC) using
We designed and prepared organic liquid prepolymers for easy
film fabrication on a silicon wafer and for a solvent-free material
system. The prepolymers were designed to have two UV-cross-
linkable moieties at the end of the molecules and to have a suit-
able molecular weight for making films from the liquid
prepolymers and for showing film flexibility without a substrate
film after UV treatment. As shown in Scheme 1, 2-{2-[1,1-
difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-
Scheme 1 Synthesis of prepolymers.
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tetrafluoro-ethoxy}-2,2-difluoro-ethanol (4) and 2-(2-{2-[1,1-
difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-1,1,2,2-
tetrafluoro-ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-difluoro-
ethanol (5) were synthesized from 2,3,4,5,6-pentafluoro styrene
(1), fluorinated triethylene glycol (2), and fluorinated tetra-
ethylene glycol (3). These compounds have a reactive hydroxyl
group at one end of the molecules and a fluorinated styrene
group, which is a crosslinkable moiety, at the other end. In this
reaction, a fluorinated styrene additionally reacted with the
hydroxyl groups of compounds 4 and 5 at the other end as a side
reaction. These side products could be easily separated from
compounds 4 and 5 by application to a column with EA/hexane
cosolvent as an eluent. By using decafluorobiphenyl (6) and
compounds 4 and 5, respectively, 2,3,5,6,20,30,50,60-octafluoro-
4,40-bis-[2-{2-[1,1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-
phenoxy)-ethoxy]-1,1,2,2-tetrafluoro-ethoxy}-2,2-difluoro-
ethoxy]-biphenyl (7) and 2,3,5,6,20,30,50,60-octafluoro-4,40-bis-[2-
(2-{2-[1,1-difluoro-2-(2,3,5,6-tetrafluoro-4-vinyl-phenoxy)-ethoxy]-
1,1,2,2-tetrafluoro-ethoxy}-1,1,2,2-tetrafluoro-ethoxy)-2,2-difluoro-
ethoxy]-biphenyl (8) are synthesized for use as flexible optical
waveguide materials. This reaction could easily occur over some
catalysts such as caesium fluoride and calcium hydride at compar-
atively low temperatures.25 The prepolymers can be easily purified
by application to a column with EA/hexane cosolvent as an eluent.
were 1.4366 and 1.4366, and 1.4169 and 1.4168, respectively. The
refractive index of prepolymer 8 is slightly lower than that of
prepolymer 7 because the fluorine content of prepolymer 8 is
slightly higher than that of prepolymer 7. Fluorine substitution
generally results in a decrease in refractive index. The two pre-
polymers 7 and 8 were well blended due to their chemical
homogeneity. The refractive indices of the films fabricated from
the homogeneous mixtures for TE-polarized light at a wave-
length of 1.31 mm were linearly controlled according to their
compositions (Fig. 1(a)). The availability of refractive index
control is important for the design of waveguide geometry.28
Waveguides were fabricated from prepolymers 7 and 8 on
a silicon wafer for measuring the propagation losses of the pre-
polymer films. Prepolymers 7 and 8 were used for the core
materials, and a commercial material with a refractive index of
1.3852 at a wavelength of 1.31 mm for cladding. The waveguide
geometry of prepolymer 8 is shown in Fig. 1(b). The rib-type
waveguide is 25 mm high and 25 mm wide with an etching depth of
3 mm. In the case of prepolymer 7, the values were 55, 39, and 3
mm, respectively. As shown in Fig. 1(b), the propagation losses of
the films of prepolymers 7 and 8 for TE-polarized light at
1
We know from the results of various analyses (IR, H NMR,
19F NMR, elemental analysis, GC-MS, and LC-MS) that
compounds 4 and 5 and prepolymers 7 and 8 were well prepared
and purified (Figs S1–S4, ESI†). The peak assignment is shown in
1
detail in the Experimental section. According to the H NMR
and DSC results (Figs S3–S5, ESI†), solvents such as DMAc,
ethyl acetate, and hexane were almost removed from the pre-
polymers and there were no molecular or phase transitions in the
prepolymers before thermal crosslinking. Prepolymers 7 and 8
are transparent liquids with viscosities of ca. 310 and 300 cP,
respectively. In order to examine the difference between the
degrees of crosslinking before and after UV treatment, prepol-
ymer 8 and the film prepared from it after UV treatment were
analyzed by IR spectroscopy (Fig S6, ESI†). Prepolymer 8 was
spin-cast on an NaCl window and the coated NaCl window was
ꢁ
UV-cured and annealed at 150 C under nitrogen for 1 h. The
characteristic absorptions at 939 and 3038 cmꢀ1 due to ]C–H
out-of-plane bending and stretching motions of the styrene vinyl
group of prepolymer 8 completely disappeared after UV and
thermal treatments.26,27 This result indicates that the photo-
conversion of the styrene vinyl group of prepolymer 8 was almost
100% after the treatments. Even though the photo-conversion of
UV-curable organic materials is not exactly same as the degree of
polymerization of the organic materials, the photo-conversion is
considered as the degree of polymerization of the materials.26
The UV-induced films are insoluble in organic solvents such as
DMAc, THF, cyclohexanone, acetone, methanol (Table S1,
ESI†). Also, the prepolymers are thermally crosslinked over ca.
200 ꢁC without a photoinitiator (Fig S5, ESI†).
Optical properties
The optical properties of films of prepolymers 7 and 8 were
investigated. The refractive indexes of films of prepolymers 7 and
8 for TE- and TM-polarized light at a wavelength of 1.31 mm
Fig. 1 Refractiveindex variation with weightratios between prepolymers
7 and 8 (a). Optical propagation loss of slab waveguides prepared from
prepolymers 7 and 8 for TE polarization at a wavelength of 1.31 mm (b).
1758 | J. Mater. Chem., 2011, 21, 1755–1761
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a wavelength of 1.31 mm were 0.25 and 0.23 dB cmꢀ1, respec-
tively. The value is quite low, as designed. The propagation loss
was measured by using the cutback method.
Thermal and mechanical properties
The thermal and mechanical properties of waveguide films are
crucial for practical mobile telecommunications applications
because the information transmission devices in which the
waveguides are embedded may be in a thermally and mechan-
ically harsh environment. The thermal stability of the films
prepared from prepolymers 7 and 8 and a mixed prepolymer (50/
50, w/w) film was investigated using TGA. The film thickness was
about 55 mm. Fig. 2 shows that the films are thermally stable up
to ca. 400 ꢁC. The thermal stability of the waveguide films is
almost equal to that of waveguide films prepared from poly
(arylene ether).4,7,29 This high thermostability might be due to the
high fluorine content in the films. The weight loss pattern of the
film prepared from the mixed prepolymer is between those of
the films prepared from prepolymers 7 and 8. The glass transition
temperatures (Tg’s) of the films prepared from prepolymers 7 and
8 were 46.3 and 30.6 ꢁC, respectively (Fig S7, ESI†). This low Tg
is due to a series of aliphatic ether linkages. According to our
preliminary optical transmission experiment, an optical signal
was well transmitted through the waveguide under thermal
cycling test conditions (from ꢀ45 to 85 ꢁC) regardless of the low
Tg of the waveguide films. However, the glass transition
temperatures of the films should be modified for real applica-
tions. The lower the Tg the films have, the more easily the
refractive index of the films is changed with temperature because
thermal expansion of polymers increases greatly over Tg to result
in a large change in the refractive index of polymers.30
Fig. 3 Stress–strain curves of the UV and thermally treated films of
prepolymers 7 and 8.
the film modulus should be below 500 MPa for improving the
folding endurance.22 The film prepared from prepolymer 8 is
more flexible than that from prepolymer 7 as shown in the
difference in stress–strain relationship between the two films.31,32
The greater flexibility of the prepolymer 8 film than the prepol-
ymer 7 film might be due to the higher molecular weight and
greater number of ether linkages of prepolymer 8 than of pre-
polymer 7. It is desirable for a flexible optical waveguide that the
cladding film is more flexible than the core film as in this case
since two more flexible cladding layers envelop the core layer and
protect it from breaking. However, the mechanical properties of
both materials should be further modified for real applications
because films having low stress and modulus can be deformed
easily even with low external force. The ultimate stress and initial
modulus of commercial polyimide films for flexible printed
circuit boards (FPCBs) used for electrical transmission in
information and communication devices are several hundred
MPa and several GPa, respectively.
Fig. 3 shows the mechanical properties of the films prepared
from preopolymers 7 and 8. The mechanical properties of optical
waveguide films are first reported. Four and five strips were taken
for measuring the mechanical properties of prepolymers 7 and 8
films, respectively and average values were found. The ultimate
stress, initial modulus, and elongation of the films were 17.1
MPa, 1.25 GPa, and 1.5% and 12.3 MPa, 0.44 GPa, and 6.9%,
for prepolymers 7 and 8, respectively. It has been reported that
Flexible waveguide properties
A flexible polymer waveguide was fabricated from prepolymers 7
and 8, which were used for core and cladding materials, respec-
tively. After prepolymer 8 containing a photoinitiator was spin-
coated on a 30-nm-thick gold layer on a silicon wafer, UV-cured,
and annealed under nitrogen, prepolymer 7 containing a photo-
initiator was spin-coated and processed in same way. A channel-
type core was formed through a dry etching process, and the size
was ca. 9 mm high and 5 mm wide. Prepolymer 8 containing
a photoinitiator was spin-coated and fabricated again. The total
waveguide thickness was ca. 25 mm. After input and output fibers
were pigtailed at the ends of the waveguide, the waveguide film
was detached from the silicon wafer. The film was easily detached
from the silicon wafer due to the poor adhesion of the gold layer
to the wafer. The waveguide film shows flexibility without any
substrate film. The detached and pigtailed waveguide film is
shown in Fig. 4(a). The waveguide is 70 mm long. The waveguide
film is flexible enough to be mechanically bent down to a radius
of 0.5 mm without any fracture. A single-mode fiber and
a multimode fiber were pigtailed, respectively, for measuring the
Fig. 2 Thermal stability of the UV and thermally annealed films of
prepolymers 7 and 8.
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Fig. 5 Bending loss calculated from variation of the insertion loss with
the bending radius of the flexible waveguide film.
a radius of 1.5 mm, which means the optical loss due to bending
increased to 1.6 dB. These results, as previously discussed in
Fig. 4, mean that the flexible waveguide works well even though
it is severely folded and that prepolymers 7 and 8 are proven to be
good candidate materials for flexible optical waveguides.
Conclusions
We designed and synthesized two liquid prepolymers for flexible
optical waveguides. The optical waveguide films could be easily
prepared from the prepolymers by UV and thermal treatments.
The films were thermostable and flexible as well as optically
transparent, as expected. The film flexibility was identified by the
mechanical test of the films. The refractive index difference
between the films prepared from two prepolymers was suitable
for designing optical waveguide structures and the refractive
indexes were linearly controlled by mixing the two prepolymers.
A flexible waveguide was fabricated using the prepolymers. The
flexible waveguide works well even though it was bent down to
a radius of 1.5 mm. We verified that the two prepolymers
synthesized in this work are well designed and are good candi-
date materials for flexible optical waveguides.
Fig. 4 A fiber-pigtailed flexible waveguide film fabricated in this work
(a). Pictures of the experimental setup used to measure the output beam
profile of the flexible waveguide bent at various radii (b).
optical loss. The beam profile of the bent and tilted flexible
waveguide was observed as shown in Fig. 4(b). The size and
intensity of the beam profiles for the flexible waveguide accord-
ing to the bending radii is almost the same. The modal size of the
beam profile is 7.9 mm in the lateral direction and 4.5 mm in the
vertical direction, which is slightly smaller than the core size.
Even though the flexible waveguide is twisted by 90ꢁ, the modal
size and intensity of the beam profile are not significantly
changed. This means that the optical signal is well transmitted
through the flexible waveguide when it is folded and twisted in
a narrow space such as a mobile phone or digital camera.
Fig. 5 shows quantitatively the variation of the optical loss
with the bending radii. A single-mode fiber was pigtailed to the
input, and a multimode fiber with a core diameter of 50 mm to
the output. The insertion loss of the waveguide is 3.9 dB before
the waveguide film was detached from the silicon wafer. The loss
slightly increased to 4.6 dB after the waveguide film was
detached. This value is for the straight flexible waveguide (radius
¼ N), which is compared with the bending waveguide. The
flexible waveguide film was applied to a slot with a half-circle
having radii from 1 to 30 mm, as shown in Fig. 5, for measuring
the optical loss according to the bending radius. The additional
insertion loss of the detached waveguide was negligible, within
0.17 dB, until the flexible waveguide was bent to a radius of
1.5 mm. The insertion loss significantly increased to 6.2 dB below
Future studies will focus on modifying the chemical structures
and increasing the molecular weights of the prepolymers for
further improving the mechanical properties of waveguides, as
well as on assessing the potential of the prepolymers for practical
applications in flexible electronic devices.
Acknowledgements
This work has been supported by the S/W Computing R&D
program of MKE/KEIT(10035360-2010-01, TAXEL: Visio-
haptic Display and Rendering Engine).
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