Optical properties of perfluorocyclobutane aryl ether polymers for polymer photonic devices

Article abstract of DOI:10.1021/ma035161c

New thermoplastic PFCB aryl ether monomers, 1,5-bis(trifluorovinyloxy) naphthalene (TFVON) and 1,4-bis(trifluorovinyloxy)benzene (TFVOB), were synthesized, and homopolymers, PTFVON and PTFVOB, and a copolymer, TFVON-co-TFVOB, were successfully prepared fr

Full text of DOI:10.1021/ma035161c

5724
Macromolecules 2004, 37, 5724-5731
Optical Properties of Perfluorocyclobutane Aryl Ether Polymers for
Polymer Photonic Devices
J ieu n Gh im , Deu g-Sa n g Lee, Bu Gon Sh in , Doojin Va k , Don g Kee Yi,†
Mi-J eon g Kim , Hw a -Su b Sh im , J a n g-J oo Kim ,‡ a n d Don g-Yu Kim *
Center for Frontier Materials, Department of Materials Science and Engineering, Gwangju Institute of
Science and Technology (GIST), 1 Oryong-dong, Puk-gu, Gwangju 500-712, Republic of Korea
Received August 8, 2003; Revised Manuscript Received December 18, 2003
ABSTRACT: New thermoplastic PFCB aryl ether monomers, 1,5-bis(trifluorovinyloxy)naphthalene
(TFVON) and 1,4-bis(trifluorovinyloxy)benzene (TFVOB), were synthesized, and homopolymers, PTFVON
and PTFVOB, and a copolymer, TFVON-co-TFVOB, were successfully prepared from them by 2π + 2π
cyclopolymerization. The C-H vibrational absorption of the monomer was investigated by IR and near-
IR spectrum analysis. The optical properties of the polymers in the form of plastic optical fibers and thin
films were examined. The absorptions corresponding to fundamental and overtones of C-H vibrations in
PFCB monomers were greatly reduced due to the presence of the fluorine substituents, which could lead
to low attenuation loss of the resulting polymers in the near-IR region. Polymerization conditions such
as temperature and time significantly affected the optical loss values of the polymers as well as their
molecular weights and thermal properties. We prepared plastic optical fibers of the PFCB polymers, and
optical windows were observed at 820, 910, 1025, 1260, and 1530 nm. The attenuation loss of PTFVON
was typically 0.083 dB/cm at 910 nm, the lowest optical window, 0.190 dB/cm at 1300 nm, and 0.227
dB/cm at 1550 nm. Copolymerization further reduced optical loss, and optical loss of the copolymer was
about 0.03-0.1 dB/cm lower than that of the homopolymer. A birefringence of the spin-coated films of
the PFCB polymers was also investigated and found to be exceptionally low. A birefringence of less than
0.001 was found in all the polymers. The lowest birefringence of 0.0005 was measured for PTFVON films.
In tr od u ction
Among these polymers, PFCB-containing polymers
possess many advantages over other fluorinated poly-
mers such as their ease of processing and excellent
thermal and mechanical properties. Monomers of per-
fluorocyclobutane (PFCB) poly(aryl ether)s are easily
synthesized in two or three steps and can be polymer-
ized in the neat or in solution without the need for a
catalyst or initiator by 2π + 2π step-growth cyclopoly-
merization, resulting in conversions in excess of 99%.21,22
Through the insertion of various functional groups via
Suzuki coupling, lithium-halogen exchange reactions,
or other methods between trifluorovinyl aryl ether
(TFVE) linkages,23-25 PFCB polymers could lead to an
increased processability, durability, chemical resistance,
high thermal stability, and optical properties, thus
resulting in various optical materials for use as both
core and cladding materials for waveguides, coatings,
light-emitting diodes, liquid crystal polymers, and in-
terlayer dielectrics.26-30
Much attention has been paid to photonic applications
of PFCB polymers. Nevertheless, our knowledge of
detailed optical properties such as optical loss at various
wavelengths and the birefringence of the PFCB poly-
mers is, to some extent, limited.20,22
In this study, we reported on the preparation of new
PFCB polymers and two of their most important optical
properties for photonic components: optical attenuation
at various wavelengths and optical birefringence (∆ )
nTE - nTM). A new naphthalene-based PFCB monomer,
1,5-bis(trifluorovinyloxy)naphthalene (TFVON), was syn-
thesized and polymerized.31 The naphthalene ring pro-
vides a rigid structure to the polymer and could yield a
moderately high Tg polymer comparable to the well-
known biphenyl-based PFCB polymer.22 However, com-
pared to 1,4-disubstituted biphenyl units, 1,5-disubsti-
tuted naphthalene could show better birefringence
In photonic telecommunication devices, plastic optical
fibers (POF) and polymer waveguides are of great
interest due to their ease in processing, controllability
of optical properties, and their low cost.1-7 However,
most hydrocarbon-based polymers show a large trans-
mission loss in the visible and near-IR region due to the
vibrational overtone absorption of C-H bonds. There-
fore, fluorinated polymers have been investigated be-
cause they show a low optical loss.1-6 From the point of
view of low optical loss, perfluorinated polymers would
be expected to show the best transmission behaviors.
Teflon AF of Dupont and Cytop of Asahi Glass are
typical examples of commercialized amorphous fluo-
ropolymers that have good optical properties and show
low transmission loss values.7 However, the high cost
of preparation of these polymers inhibits their use in
potential applications to photonic devices. In addition,
the lack of controllability of optical and other properties
has also prevented the widespread use of these materi-
als. Therefore, fluorinated and deuterated poly(methyl
methacrylate) (PMMA)6,8 and polystyrene (PS)9 have
been investigated for use in POF and waveguides, and
fluorinated polyimides,10-12 fluorinated poly(aryl
ether)s,13-15 highly fluorinated acrylates,1,16 and per-
fluorocyclobutane (PFCB) aryl ether polymers17-20 have
been studied for use as waveguides.
* To whom correspondence should be addressed: Tel +82-62-
970-2319, FAX +82-62-970-2304; e-mail kimdy@kjist.ac.kr.
† Current address: Institute of Bioengineering and Nanotech-
nology, 51 Science Park Road, #01-01/10 the Aries, Singapore
Science Park II, Singapore 117586.
‡ Current address: School of Materials Science and Engineering,
College of Engineering, Seoul National University, San 56-1,
Shinrim-Dong, Kwanak-Gu, Seoul, 151-744 Korea.
10.1021/ma035161c CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/25/2004

Macromolecules, Vol. 37, No. 15, 2004
Perfluorocyclobutane Aryl Ether Polymers 5725
Fa), -123.60 (1F, dd, trans-CFdCF2, Fb), -131.36 (1F, dd, CFd
CF2, Fc) (J ab ) 72.17, J ac ) 43.6, J bc ) 82.62). FTIR (KBr):
1838 cm-1 (m, CFdCF2).
1,4-Bis(2-br om otetr a flu or oeth yloxy)ben zen e. This com-
pound was prepared according to the synthetic method of 1,5-
bis(2-bromotetrafluoroethyloxy)naphthalene using hydroquino-
ne (27.5 g, 0.25 mol) with sodium hydride (12 g, 0.5 mol) in
DMSO (400 mL). The yield was 43%. 1H NMR (300 MHz,
CDCl3): δ ) 7.27 (4H). 19F NMR (282.65 MHz, CDCl3): δ )
-83.548 (t, J ) 3.11 Hz), -65.80 (t, J ) 3.11 Hz).
characteristics because the molecular axis is perpen-
dicular to the main backbone direction. We also pre-
pared 1,4-bis(trifluorovinyloxy)benzene (TFVOB) and a
corresponding polymer that has a higher fluorine con-
tent than PTFVON. Effects of copolymerization of these
two monomers on optical properties were also investi-
gated.
For the studies of optical attenuation, infrared (IR)
and near-IR spectra of the monomers were analyzed,
POFs were prepared using these PFCB polymers, and
their optical loss values as a function of various polym-
erization conditions were investigated. From the pre-
pared POFs, we were able to obtain full transmission
loss spectra of these polymers in the near-IR region.
1,4-Bis(tr iflu or ovin yloxy)ben zen e (TF VOB). This com-
pound was prepared according to the synthetic method of 1,5-
bis(trifluorovinyloxy)naphthalene using 1,4-bis(2-bromotet-
rafluoroethyloxy)benzene in acetonitrile. The residual crude
product was purified by column chromatography with hexane
to obtain a colorless liquid. The yield was 54%. The synthetic
method for TFVOB was also described in the patent disclosure
from Dow Chemical Corp.32 1H NMR (300 MHz, CDCl3): δ )
7.27 (4H). 19F NMR (282.65 MHz, CDCl3): δ ) -121.5 (1F,
dd, cis-CFdCF2, Fa), -127.4 (1F, dd, trans-CFdCF2, Fb),
-135.2 (1F, dd, CFdCF2, Fc) (J ab ) 72.2, J ac ) 43.6, J bc ) 82.6).
FTIR (KBr): 1833 cm-1 (m, CFdCF2).
P olym er iza tion of P F CB Mon om er s. PFCB monomers,
distilled and degassed, were placed in a glass tube, which was
sealed under a high-vacuum condition. The monomers in the
sealed glass tube were bulk polymerized under various condi-
tions, as shown in Table 1. The glass tube was then removed,
leaving a transparent plastic rod, which was dried to remove
unpolymerized monomers in a vacuum oven below the glass
transition temperature. All the polymerization was carried out
at 180 °C for 18 h unless otherwise mentioned. The PFCB ring
structure in the polymers was typically characterized by
multiple peaks of 19F NMR in the range of -127 to -133 ppm
and by FT-IR. When the monomers were polymerized, the CFd
CF2 stretching vibration peak at 1833 cm-1 disappeared, and
new PFCB ring peaks appeared at 965 cm-1. Characterizations
of the polymers such as molecular weights and thermal and
optical properties are summarized in Table 2.
Nea r -IR Sp ectr u m Mea su r em en t. Quantitative analysis
of fundamental C-H stretching vibrations in the region of
2600-3400 cm-1 was carried out using a KBr liquid cell with
0.5 mm spacer. Overtone bands and combination bands of
C-H vibrations were measured using a quartz cell with a path
length of 0.2 cm. IR and near-IR spectra of benzene and 1,4-
difluorobenzene (2FB) were also measured as reference spectra
for the quantitative comparison.
Refr a ctive In d ex a n d Bir efr in gen ce Mea su r em en t.
Prepared PFCB polymers were dissolved in cyclohexanone at
a concentration of 40 wt %. The solution was filtered with a
0.2 µm Teflon membrane syringe filter. The filtered solution
was spin-coated on the silicon wafer substrate at a spin rate
of 1000 rpm for 2 min. After coating the films were baked at
180 °C for 1 h. By adjusting concentration of the polymer
solution, the film thickness was controlled. The thickness of
the obtained films was about 6-8 µm. The refractive indices,
nTE and nTM of the polymer films, were measured with a Sairon
SPA-3000 prism coupler at 1550 nm.
Exp er im en ta l Section
Ma ter ia ls. Hydroquinone, 1,5-dihydroxynaphthalene, so-
dium hydride, granular zinc, and 1,2-dibromotetrafluoroethane
were purchased from Aldrich Chemical Co. and used without
further purification. Dimethyl sulfoxide, acetonitrile, and
diethyl ether were purchased from Oriental Chemical Indus-
tries, dried over calcium hydride, and distillated before use.
Granular zinc was activated with 0.1 M hydrochloric acid,
washed with ethanol and ether, and dried at 140 °C under
reduced pressure for 10 h.
Ch a r a cter iza tion of Mon om er s a n d P olym er s. 1H NMR
and 19F NMR spectra were obtained with a J EOL J NM-LA
1
300 WB FT-NMR in chloroform-d. The chemical shifts of H
NMR were referenced to tetramethylsilane (TMS) at 0 ppm,
and those of 19F NMR were referenced to fluorotrichlo-
romethane (CFCl3) at 0 ppm. IR and near-IR spectra were
measured by a Perkin-Elmer IR 2000 series and Hitachi
U-3501, respectively. Gel permeation chromatography (GPC)
data were collected from a Waters model at 40 °C equipped
with four Waters columns. Tetrahydrofuran was used as the
eluent at a flow rate of 1.0 µL/ min, and polystyrene was used
as the standard. The eluent was monitored with a refractive
index detector. Thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) measurements were
performed with a TA Instrument 2100 series under a nitrogen
atmosphere at a heating rate of 10 °C/min.
1,5-Bis(2-br om otetr a flu or oeth yloxy)n a p h th a len e. To a
1 L, three-neck flask equipped with two addition funnels and
a mechanical stirrer were added DMSO (400 mL) and sodium
hydride (12 g, 0.50 mol). DMSO (400 mL) solution dissolved
with 1,5-dihydroxynaphthalene (40 g, 0.25 mol) was added to
a sodium hydride solution slowly through an addition funnel.
The solution was stirred at room temperature for 1 h. After
evaporation of hydrogen, 1,2-dibromotetrafluoroethane (130 g,
0.50 mol) was added dropwise over 1 h through an addition
funnel to the reaction mixture, the temperature of which did
not exceed 30 °C using a water-ice bath. The solution was
stirred for 12 h at room temperature and then heated for 10 h
at 50 °C. The reaction mixture was diluted with water and
extracted with diethyl ether, and the organic phase was
washed three times with water and dried over MgSO4. The
reaction mixture was purified by silica gel chromatography
using hexane to obtain the colorless liquid. The yield was 56%.
1H NMR (300 MHz, CDCl3): δ ) 8.1 (2H, d, J ) 4.8 Hz), 7.58
(2H, t, J ) 4.8 Hz, 5.1 Hz), 7.49 (2H, d, J ) 5.1 Hz). 19F NMR
(282.65 MHz, CDCl3): δ ) -83.55 (t, J ) 3.11 Hz), -68.06 (t,
J ) 3.11 Hz).
P la stic Op tica l F iber F or m a tion . PFCB monomers dis-
tillated and degassed to remove impurity were placed in a glass
tube with the diameter of 10 mm and the length of 100 mm,
which was sealed in a high-vacuum condition. The monomers
in the sealed glass tube were polymerized under the conditions
summarized in Table 1. After polymerization, the plastic rod
was carefully removed from the glass tube and placed in the
middle of a coiled heater and drawn at 170-180 °C, leading
to plastic optical fibers with a typical diameter of 1 mm.
Op tica l Loss Mea su r em en t. The optical attenuation
spectrum of a POF was measured by the cut-back method. The
light from a tungsten lamp passed through a monochromator
(Acton Research Co. sp-150) controlled by a computer. The
emitted light from the monochromator was focused to the end
surface of the PFCB fibers, connected to optical coupler. The
transmitted light from the fiber was focused to the photo-
detector. After fiber cut, optical attenuation of the fiber was
1,5-Bis(tr iflu or ovin yloxy)n a p h th a len e (TF VON). 1,5-
Bis(2-bromotetrafluoroethyloxy)naphthalene (40 g, 0.077 mol)
was added via an addition funnel to a stirred solution of
activated Zn (12.7 g, 0.193 mol) and dried acetonitrile (400
mL) at 80 °C under nitrogen for 29 h. After completion of the
reaction, acetonirile was evaporated. The residual crude
product was purified by column chromatography with hexane
to obtain a white solid. Melting point ) 91 °C. The yield was
65%. 1H NMR (300 MHz, CDCl3): δ ) 8.1 (2H, d, J ) 4.8 Hz),
7.58 (2H, dd, J ) 4.8 Hz, 5.1 Hz), 7.49 (2H, d, J ) 5.1 Hz). 19
NMR (282.65 MHz, CDCl3): δ ) -116.9 (1F, dd, cis-CFdCF2,
F

5726 Ghim et al.
Macromolecules, Vol. 37, No. 15, 2004
Sch em e 1. P r ep a r a tion of Tr iflu or ovin yl Ar yl Eth er Mon om er s by F lu or oa lk yla tion a n d Deh a logen a tion
Rea ction
Sch em e 2. Hom o- a n d Cop olym er iza tion of Tr iflu or ovin yl Ar yl Eth er Mon om er s
Ta ble 1. Molecu la r Weigh ts a n d Th er m a l a n d Op tica l P r op er ties of th e P TF VON Dep en d in g on Va r iou s P olym er iza tion
Con d ition
a
a
PTFVON
temp (°C)
time (h)
Mn
Mw
PD
Tgb (°C)
Tdc (°C)
optical lossd (dB/cm)
PTFVON-1
PTFVON-2
PTFVON-3
PTFVON-4
PTFVON-5
160
160
180
180
200
7
12
12
18
24
2 450
4 584
10 878
21 000
45 718
2 657
6 134
24 543
47 300
121 727
1.10
1.34
2.25
2.25
2.66
98
100
136
138
152
233
375
388
394
407
0.576
0.417
0.264
0.240
0.336
a
b
d
In THF vs polystyrene. 10 °C/min in N2. c Temperature of 5.0% weight loss by TGA at the rate of 10 °C/min in N2. Measured at
1550 nm.
measured again. Optical loss of the fiber was determined by
difference of optical attenuation.
The polymers were obtained by thermal bulk polymer-
ization via 2π + 2π cyclodimerization, as shown in
Scheme 2. Polymerization conditions such as tempera-
ture and reaction time were very important determi-
nants of thermal properties as well as the optical
properties of the synthesized PFCB polymers. These
aspects are discussed in a later section of this report.
Table 1 shows the reaction conditions used for the
polymerization of poly[1,5-bis(trifluorovinyloxy)naph-
thalene] (PTFVON). Trifluorovinyl aryl ethers readily
dimerize through 2π + 2π cyclodimerization at elevated
temperatures, leading to stable perfluorocyclobutane
rings.33 The temperature required for the dimerization
depends on the electronic structure of the aryl groups,
typically in the range 150-250 °C.24,25,34,35 The DSC
thermogram of TFVON showed a melting endotherm
at 91 °C and a thermal polymerization exotherm at Tonset
) 153 °C, as shown in Figure 1. Therefore, the thermal
bulk polymerization of TFVON was carried out at a
Resu lts a n d Discu ssion
Mon om er Syn th esis a n d Cyclop olym er iza tion .
PFCB monomers, TFVON and TFVOB, were synthe-
sized in high purity in two steps: fluoroalkylation with
1,2-dibromotetrafluoroethane and dehalogenation via
zinc, as shown in Scheme 1. In the case of fluoroalky-
lation, we found that, instead of a commomly used base
such as potassium hydroxide (KOH), the use of sodium
hydride (NaH) was more convenient and significantly
reduced the reaction time. DMSO and DMF were used
as solvents, and their purity was found to be important
for achieving a high yield of fluoroalkylated products.
The dehalogenation reaction followed a procedure previ-
ously reported by Babb and Smith.21,24 Monomers were
purified by silica gel column chromatography and then
vacuum-distilled to remove moisture and impurities.

Macromolecules, Vol. 37, No. 15, 2004
Perfluorocyclobutane Aryl Ether Polymers 5727
F igu r e 1. DSC thermograms of TFVON and PTFVON.
temperature above 160 °C. The synthesized polymers,
PTFVON-1 and PTFVON-2, had low Mn below 5000 and
a relatively low Tg. When the polymerization tempera-
ture was raised to 180 °C, higher molecular weight
polymers were obtained (PTFVON-3, -4). The Mn of
20 000 was measured in the case of PTFVON-4 when
the polymerization time was increased. The Tg and Td
of the polymers increased as the polymerization time
and temperature were increased, and the Tg and Td of
PTFVON-4 were measured at 138 and 394 °C, respec-
tively. PTFVON-5 showed better thermal and mechan-
ical properties than the others, but severe yellowing was
observed. Most organic polymers are subject to yellow-
ing upon thermal aging due to thermal oxidation leading
to the formation of conjugated molecular structures.
Yellowing can be minimized when the polymerization
was performed under inert atmosphere or under vacuum.
Therefore, we polymerized all the monomers in sealed
tubes under vacuum. Nevertheless, this yellowing phe-
nomenon was difficult to avoid in our experiments
especially when the polymerization was carried out at
a higher temperature or when a longer polymerization
time was used.
As discussed above, the naphthalene-based PFCB
polymer, PTFVON, was successfully prepared and
showed high Tg and Td, as expected. However, the
number of remaining C-H bonds was still quite large.
Therefore, we also synthesized a phenylene-based PFCB
polymer, PTFVOB, which has a lower fraction of C-H
bonds. Because PTFVON, prepared at 180 °C for 18 h,
had a high molecular weight and low yellowing, 1,4-
bis(trifluorovinyloxy)benzene (TFVOB) was polymerized
under the same conditions. A DSC thermogram of
TFVOB also showed a similar polymerization exotherm
(Tonset ) 160 °C) to that of TFVON. The PTFVOB had
a relatively low Tg of 41 °C due to the smaller aromatic
group compared to the naphthalene of PTFVON.
The molecular weights and thermal and optical
properties of the polymers are summarized in Table 2.
Copolymerization generally induces a more amorphous
morphology via effectively introducing the irregularity;
therefore, TFVON and TFVOB were also copolymerized
in 1:1 ratio (wt %) under the same conditions. The
F igu r e 2. Refractive indices and birefringence of PFCB
polymer films at 1550 nm.
copolymer showed a Tg of 91 °C, which is an intermedi-
ate value between the Tgs of PTFVON and PTFVOB,
indicating the random copolymerization of two mono-
mers. The molecular weights (M
n) of the polymers were
typically 20 000. The prepared PFCB polymers showed
good solubility in common organic solvents such as THF,
chloroform, and toluene. Excellent optical quality films
could be prepared by spin-coating.
Optical properties such as refractive index and bire-
fringence of the polymer films were measured, as shown
in Figure 2. Refractive indices of the polymers were in
the range 1.44-1.50, and PTFVOB had a low refractive
index of 1.4446 due to its high fluorine content. PT-
FVON showed a higher refractive index due to the
larger aromatic content, and the copolymer exhibited
an average value between the two homopolymers. The
birefringence of the spin-coated films of the PFCB
polymers was investigated and found to be exceptionally
low. Α ∆n of less than 0.001 was measured in all the
polymers. The lowest birefringence of 0.0005 was mea-
sured for PTFVON films. This may be due to the
amorphous polymer structure resulting from the ran-
dom arrangement of cis- and trans-1,2-disubstituted
PFCB rings.33 In addition, these birefringence values
were much lower than previously reported data for
biphenyl-based PFCB and triphenylethane-based PFCB
polymers.19,20,35 Birefringence is, in general, caused by
stress during the film preparation. In particular, the
high rate of spinning during the spin-coating process
causes the polymer chains to extend in a direction
parallel to the film plane. Therefore, in such cases, the
refractive index in the TE mode is larger than that of
the TM mode. This higher refractive index in the TE
mode can be canceled by the incorporation of a struc-
tural unit that has high polarizability in the direction
perpendicular to the polymer backbones. Because of
this, the 1,5 linkage of naphthalene in the PTFVON
Ta ble 2. Molecu la r Weigh ts a n d Th er m a l a n d Op tica l P r op er ties of th e P F CB P olym er s a n d Cop olym er
a
a
b
e
e
PFCB
PTFVON
PTFVOB
TFVON-co-TFVOB
Mn
Mw
Tg (°C)
Tdc (°C)
hydrogen contentd
nTE
nTM
∆ne
21 000
16 200
18 000
47 300
22 100
39 500
138
41
394
397
340
19.4
16.6
18
1.4999
1.4455
1.4795
1.4994
1.4446
1.4788
0.0005
0.0009
0.0007
91
a
b
In THF vs polystyrene. By DSC at the rate of 10 °C/min in N2. c Temperature of 5.0% weight loss by TGA at the rate of 10 °C/min
in N2. % of the number of C-H bond per the total number of bonds in the repeating unit. e Measurement at 1550 nm.
d

5728 Ghim et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 4. FT-IR spectra of the PFCB monomer, TFVOB, and
reference compounds in the region of C-H fundamental
stretching vibration.
even though the effect was more evident in the case of
the phenylene-based TFVOB and PTFVOB. PFCB poly-
mers prepared in this study contain a high density of
fluorine substituents on the aromatic conjugated sys-
tems through ether linkage; therefore, a strong induc-
tive electron-withdrawing effect would be applied to the
aromatic rings, leading to an decrease of bond length
and an increase of the vibrational frequencies of C-H
bonds.36,37 Electron-withdrawing linkages other than
fluorine, such as sulfone, also exhibit similar effects as
recently reported.11 Even though those relationships
between electronic effects and the vibrational frequen-
cies are quite well-known, to our knowledge, the inten-
sity of the C-H bond in these cases is not clearly
understood to date. As shown in Figure 3, the reduction
in C-H vibrational intensity was quite remarkable in
these compounds; therefore, low vibrational absorption
loss at near-IR region may be expected.
To measure the intensity of vibrational absorption of
C-H bonds in these monomers and polymers, funda-
mental C-H stretching vibrations of the PFCB mono-
mers in the region of 3000-3400 cm-1 were measured
using a KBr liquid cell with a 0.5 mm spacer. The IR
spectra of benzene and 1,4-difluorobenzene (2FB) were
also measured as reference spectra. The monomer
TFVON was not measured because it was a solid at
room temperature. Figure 4 shows the FTIR spectra for
the TFVOB and reference compounds with the same
path length in the region of 2600-3400 cm-1. Compared
to benzene, it is clear that the C-H intensity of TFVOB
was dramatically reduced. However, an exact quanti-
fication was difficult because the peaks were overlapped
with many other peaks, largely consisting of higher
harmonics of lower frequency vibrations.
The overtone and combination bands of C-H vibra-
tions of the TFVOB monomer and reference compounds
were also measured using a quartz cell with a path
length of 0.2 cm in the near-IR region, as shown in
Figure 5. The first overtone bands (2υ) of the aromatic
C-H fundamental vibrations (1υ), the first combination
bands (2υ + δ) of the C-H stretching and bending, and
the second overtone bands (3υ) appeared in the region
of 2000-1550, 1300-1500, and 1100-1200 nm, respec-
tively. The near-IR spectrum would be still difficult to
analyze due to a number of overtone and combination
bands of various stretching vibrations, for example C-O
bonds, C-C bonds, and CdC bonds.38 The absorption
peaks in the region of the first overtones and combina-
tion bands were very complicated, but the complexity
of the second overtone bands was reduced compared to
F igu r e 3. FT-IR spectra of TFVOB and PTFVOB.
polymer backbones is believed to show a very low
birefringence. This is consistent with the relatively
higher birefringence (∆n ) 0.005) of biphenyl-based
PFCB polymers,35 where the polarizability of biphenyl
groups would be the largest in the direction parallel to
the extended polymer backbones.
To confirm this hypothesis, we measured s- and
p-polarized UV-vis spectroscopy spectra as a function
of tilt angle of incident beam to investigate the align-
ments of naphthalene moiety in the spin-coated film.
However, unfortunately we could not confirm the naph-
thalene alignment due to their very low degree of
orientation (birefringence of 10-4 in our case). Some
other materials tested having the birefringence of 10-3
in our laboratory could show detectable differences in
the polarized spectra and orientation tendency; however,
the materials with 10-4 birefringence did not reveal
clear difference between TE and TM modes. For optical
waveguide devices, low birefringence is a key require-
ment,1 and from that sense, PFCB polymers represent
good candidates for polymer waveguide materials with
low birefringence.
IR a n d Nea r -IR Sp ectr u m An a lysis. Figure 3
shows the FTIR spectra of TFVOB and PTFVOB, which
reveal two types of important information: one is the
confirmation of the polymerization and the other is the
estimation of the intrinsic optical loss by C-H vibra-
tional absorption. When the monomers were polymer-
ized, the CFdCF2 stretching vibration peak at 1833
cm-1 disappeared, and a new PFCB ring peaks appeared
at 965 cm-1. Thus, the polymerization could be con-
firmed by the difference in the intensity of the two
peaks.22 In addition, the C-H stretching vibration peaks
of TFVOB and PTFVOB in 3000-3400 cm-1 contained
information concerning optical loss at visible and near-
IR region. If the intensity of the C-H fundamental
stretching vibration is strong, the overtone band strength
will be increased, and as a result, a large optical loss in
the visible and near-IR region would be expected.
As shown in Figure 3, the C-H fundamental stretch-
ing vibrations for both TFVOB and PTFVOB were
observed to be very low compared to the other peaks,
despite the presence of the four C-H bonds on each
aromatic ring. This can be attributed to the strong
electronic effects of fluorine atoms substituted near the
aromatic ring. The naphthalene-based TFVON and
PTFVON also exhibited low C-H vibrational intensity,

Macromolecules, Vol. 37, No. 15, 2004
Perfluorocyclobutane Aryl Ether Polymers 5729
F igu r e 5. Comparison of C-H vibrational intensity of PFCB
monomers with 1,4-difluorobenzene (2FB) and benzene in the
near-infrared region.
F igu r e 6. Optical loss of PTFVON at 1550 nm polymerized
under different conditions.
mer and reported a value of 0.25 dB/cm at 1515-1565
nm.19 Smith et al. also reported that the attenuation
loss was consistent with the reported value or could be
lower than previously reported values.20,39 However,
optical loss spectra or optical windows of PFCB poly-
mers in the near-IR region have not yet been reported.
As discussed earlier, we investigated C-H overtone
bands of PFCB monomers, which showed negligible
absorption in the optical sources. However, the path
length of 0.2 cm in the quartz cell was too short to
determine the exact optical loss and optical windows in
the near-IR region. Therfore, we fabricated PFCB
polymers as plastic optical fibers having a sufficient
path length and carried out measurements of optical
loss and windows in the region of 700-1600 nm.
A number of optical loss factors exist, such as the
intrinsic loss of C-H vibrations, the main intrinsic loss
factor in the near-IR region, impurities, and an extrinsic
loss factor due to structure imperfections. In addition,
PFCB polymers showed a slight yellowing, when po-
lymerized at temperatures above 160 °C, and the
yellowing was more severe for long polymerization times
and temperatures. Because yellowing affects optical loss
in the short wavelength,1,10 we examined this aspect as
a function of polymerization conditions. A randomly
distributed copolymerization would also lead to a more
amorphous structure than a homopolymer. Because of
this, a PFCB amorphous copolymer, TFVON-co-TFVOB,
was prepared, and the optical loss was measured.
Figure 6 shows optical loss changes as a function of
various polymerization conditions as summarized in
Table 1. When polymerized at 160 °C for 7 h, PTFVON-
1, although transparent, was brittle and generated a
number of voids during the drawing process, probably
due to unpolymerized monomers. The optical loss of
PTFVON-1 was 0.6 dB/cm at 1550 nm. As the polym-
erization time increased, the molecular weights and the
temperature at which a 5% weight loss of PTFVON-2
occurred were about doubled, and the optical loss of
PTFVON-2 was reduced by nearly 50%. In the cases of
PTFVON-3 and PTFVON-4, the optical loss was steadily
reduced, and PTFVON-4 had a low loss of 0.227 dB/cm,
which was in the same range as those reported for
PFCB polymers.19,20,39 The reduction in the optical loss
for the PTFVON-1-4 can be attributed to an increase
in the molecular weight and corresponding improvement
of thermal and mechanical stability during fiber draw-
ing. However, when polymerized at 200 °C for 24 h the
optical loss abruptly increased. As discussed earlier, it
is likely that the increase in optical loss mainly resulted
the lower overtones and combination bands. Because of
this, the effects of fluorination on the C-H vibrational
overtones were more clearly observed in the second
overtone band region. The hydrogen content of TFVOB,
2FB, and benzene, defined as percent of the number of
C-H bond per the total number of bonds in the
repeating unit,13 was 16.6, 33.3, and 50%, respectively.
When the hydrogen content was compared with the
intensity of peak absorbance at the second C-H over-
tone bands, we found that the intensity of C-H overtone
absorption of fluorinated materials was largely reduced.
The hydrogen content of TFVOB was reduced to one-
third of benzene; however, the intensity at peak absor-
bance was reduced to about one-fourth of benzene. This
additional reduction of vibrational C-H overtone in-
tensity is believed to result from the fluorine substitu-
tion because even for the 2FB, the similar intensity
reduction was observed. This result may indicate that
the partially fluorinated polymers may show much
lower optical loss than expected if properly substituted.
Detailed studies on this phenomenon including quan-
tum mechanical calculation are under way.
In addition, the peaks for the fluorinated materials
were shifted about 15 nm to shorter wavelength com-
pared to those of benzene. This is likely due to the
δ-electron withdrawing effect of fluorine, which leads
to a decrease in C-H bond length and an increase in
the C-H frequencies.36,37 Thus, the proper substitution
of an aryl group could be used to tailor the C-H
absorption so as to be either shifted or retained. This
frequency shift is important from two aspects. First, it
can significantly affect optical loss at a fixed optical
source wavelength.17 Second, the absorption frequency
shift also determines the position of the low loss optical
windows. Especially for POF applications, the optical
window position should be matched with the optical
source for the longer transmission. In the spectra shown
in Figure 5, the wavelengths of the optical telecom
source at 1300, 1310, and 1550 nm seem to be quite far
from the vibrational frequencies, and absorption at
those wavelengths looks negligible.19,39 However, be-
cause the measurement was performed with a relatively
short path length, it was difficult to estimate the optical
loss quantitatively in these wavelengths from the
results, reported here.
Op tica l Loss of P F CB P olym er s. Only limited
information is available in the literature regarding the
optical loss of PFCB polymers. Petermann et al. studied
waveguides using a triphenylethane-based PFCB poly-

5730 Ghim et al.
Macromolecules, Vol. 37, No. 15, 2004
Ta ble 3. Op tica l Loss of th e P F CB P olym er s a t Low Loss Win d ow a n d Op tica l Sou r ce
optical loss (dB/cm)
optical loss window (nm)
optical source (nm)
1310
polymer
820
910
1025
1260
1530
1300
1550
PTFVON-4
TFVON-co-TFVOB
0.092
0.082
0.083
0.080
0.097
0.087
0.120
0.092
0.225
0.200
0.190
0.156
0.218
0.181
0.227
0.200
cladding. At the present time it is difficult to evaluate
the extent to which the extrinsic factors contribute to
the loss, but the attenuation loss could be greatly
lowered by the optimization of the polymerization and
processing.
Con clu sion
We successfully synthesized new PFCB monomers
and corresponding homo- and copolymers from them
through 2π + 2π cyclopolymerization. Thermally stable
polymers with relatively high Tg could be obtained. For
naphthalene-based polymers PTFVON, Tg and Td of
PTFVON were measured at about 140 and 400 °C,
respectively. Phenylene-based low-Tg PFCB polymer
PTFVOB and a random copolymer of the two monomer
were also successfully obtained. The birefringence of the
spin-coated films of the PFCB polymers was found to
be lower than 0.001.
The optical absorption properties of the PFCB poly-
mers were investigated via IR and near-IR spectra and
as well as their optical loss as plastic optical fibers. The
findings show that the C-H absorption intensity of
PFCB monomers was dramatically reduced in the IR
and near-IR region. This could lead to a significant
reduction in optical attenuation loss of the final poly-
mers. When the polymerization was carried out, the
optical loss varied from 0.6 to 0.2 dB/cm depending on
the polymerization conditions. In addition, the optical
loss of the copolymer was about 0.003-0.01 dB/cm lower
than that of the homopolymer. As a result, the optical
loss values were strongly dependent on the polymeri-
zation conditions.
We first prepared plastic optical fibers of the PFCB
polymers, and optical windows were observed at 820,
910, 1025, 1260, and 1530 nm. The attenuation loss of
PTFVON was typically 0.083 dB/cm at 910 nm, the
lowest optical window, 0.19 dB/cm at 1300 nm, and
0.227 dB/cm at 1550 nm. However, these loss values
could be greatly improved by optimizing the polymeri-
zation and processing. Along with low birefringence,
good processability, thermal properties, and low optical
loss, PFCB polymers represent promising candidates for
use in polymer photonic devices.
F igu r e 7. Optical loss spectrum of PTFVON (s) and TFVON-
co-TFVOB (- - -) in the region of 700-1600 nm.
from severe yellowing by thermal aging during the
polymerization or by thermal degradations during fiber
drawing at a high temperature due to the higher Tg of
PTFVON-5.1,10,40
Figure 7 shows optical attenuation spectra of the
PFCB polymers. The second overtone bands 3υ and the
third overtone bands 4υ were in the regions 1050-1250
and 815-910 nm, and the first combination bands 2υ
+ δ and the weak second combination bands 3υ + δ were
in the regions 1250-1530 and 910-1050 nm, respec-
tively. Both homo- and copolymer exhibited similar
attenuation spectra. As expected, the attenuation be-
came smaller as the wavelength became smaller be-
cause intensities of the higher vibrational overtones
become weaker. However, below 800 nm, the attenua-
tion increased again with decreasing wavelength. The
optical loss factors that were dominant in the short
wavelength region are known to be Rayleigh scattering
and electronic transitions. In our case, yellowing would
be another important loss factor in the short wavelength
regions.
Optical windows were observed at 820, 910, 1025,
1260, and 1530 nm. Table 3 shows the loss values of
the PFCB polymers at the optical windows and optical
sources. The attenuation loss of PTFVON-4 was 0.083
dB/cm at 910 nm, the lowest optical window, 0.190 dB/
cm at 1300 nm, and 0.227 dB/cm at 1550 nm. The loss
of the copolymer, TFVON-co-TFVOB, would be expected
to show a lower loss due to increased fluorine contents
and more amorphous morphology. The loss of TFVON-
co-TFVOB was about 0.080 dB/cm at 910 nm of the
lowest optical window, 0.156 dB/cm at 1300 nm, and
0.20 dB/cm at 1550 nm. The optical loss for the
copolymer was about 0.003-0.01 dB/cm lower than
PTFVON-4. The major factors contributing to optical
loss are thought to be intrinsic loss due to higher
harmonics of aromatic C-H vibrations and absorption
by yellowing at a short wavelength region. However,
several extrinsic loss factors should also contribute
significantly to the total attenuation of the fiber because
the optical fiber formation process we employed was not
optimized. These factors could include monomer purity,
microvoids in fibers, and especially the lack of existing
Ack n ow led gm en t. The authors especially thank
Dr. J ae-Wook Kang and Sung Ho Beak for the mea-
surement of the RI of PFCB polymer films. This work
was supported by the Technology Development of
Plastic Optical Fiber, the NGNTD program from MOCIE
(Ministry of Commerce, Industry, and Energy).
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