Highly Fluorinated Trifluorovinyl Aryl Ether Monomers and Perfluorocyclobutane Aromatic Ether Polymers for Optical Waveguide Applications

Article abstract of DOI:10.1021/ma034467g

A series of novel trifunctional and tetrafunctional trifluorovinyl aryl ether monomers and perfluorocyclobutane (PCFB) aromatic ether polymers have been synthesized and characterized. These PFCB polymers exhibit desirable properties for low loss optical w

Full text of DOI:10.1021/ma034467g

Macromolecules 2003, 36, 8001-8007
8001
Highly Fluorinated Trifluorovinyl Aryl Ether Monomers and
Perfluorocyclobutane Aromatic Ether Polymers for Optical Waveguide
Applications
Sh a r on Won g, Hon g Ma , a n d Alex K.-Y. J en *
Department of Materials Science and Engineering, Box 352120, University of Washington,
Seattle, Washington 98195-2120
Rick Ba r to a n d Cu r tis W. F r a n k
Department of Chemical Engineering, Stanford University, Stanford, California 94305
Received April 12, 2003; Revised Manuscript Received August 18, 2003
ABSTRACT: A series of novel trifunctional and tetrafunctional trifluorovinyl aryl ether monomers and
perfluorocyclobutane (PCFB) aromatic ether polymers have been synthesized and characterized. These
PFCB polymers exhibit desirable properties for low loss optical waveguide applications. Monomers used
in this study were prepared from the key intermediate, 4-bromophenyl trifluorovinyl ether. Through
thermal dimerization of the aromatic trifluorovinyl ether moieties, these monomers can be either melt or
solution polymerized to form PFCB-containing prepolymers with good processability for the fabrication
of optical waveguides. After a post-thermal cross-linking process, the resulting thermosets possess low
optical loss (0.30 dB/cm at 1310 nm with 1% of DR-1 doping), high thermal stability (Tg: 85-350 °C),
good solvent resistance, and low surface roughness.
In tr od u ction
aryl ether monomers (Figure 1) and the resulting PFCB
aromatic ether polymers. The trifunctional monomer
consists of triazine as the center core to form a rigid
structure with three trifluorovinyl ether groups. The
tetrafunctional monomers are highly fluorinated with
two siloxane linkages that contain four trifluorovinyl
ether groups. Highly fluorinated monomers are desir-
able for minimizing the absorption optical loss at the
telecommunication operating wavelengths (1.3 and 1.55
µm) because the heavier fluorine atoms shift the over-
tone absorption signal to a longer wavelength. The
siloxane-containing structural units are introduced into
PFCB polymers because of their chemical, thermal, and
oxidative resistance and flexibility at low temperatures.
High-speed information optics, once so prohibitively
expensive that only long-haul communications was
feasible, is being applied to ever-shorter distances,
penetrating from metro-area to access and intra-
computer interconnect networks. However, the pace of
developing higher bandwidth and more cost-effective
solutions to sustain the rapid growth of bandwidth
presents huge challenges and opportunities for materi-
als and optical scientists and engineers. Among the
candidate material systems, high expectations have
been placed on polymers as the materials choice for
highly integrated optical components and planar light-
wave circuits. State-of-the-art optical polymers are
particularly attractive in integrated optical waveguide
devices because they offer rapid processability, cost-
effectiveness, high yields, high performance such as
lower optical loss and smaller birefringence compared
to those of silica, power-efficient thermal actuation due
to larger thermooptic coefficient than in silica, and
compactness owing to a large refractive index con-
trast.1,2 Although the work of integrated polymer com-
ponents is going on globally in the areas of switches,
modulators, lasers, amplifiers, and sensors, the ideal
optical polymers are limited to a few kinds of polymers
such as polyacrylates, polyimides, polycarbonates, and
cyclobutenes.
Perfluorocyclobutane (PFCB) aryl ether polymers
were first prepared in Dow Chemical through the
radical-mediated thermal cyclopolymerization of tri-
fluorovinyl ethers. These PFCB polymers do represent
a unique class of optical polymers with a combination
of excellent processability and high performance such
as low dielectric constant, low moisture absorption, good
thermal and thermal oxidative stability, low birefrin-
gence, and optical transparency.3-7 In this paper, we
report the synthesis and characterization of a series of
novel trifunctional and tetrafunctional trifluorovinyl
Exp er im en ta l Section
All chemicals were purchased from Aldrich, SynQuest, or
Lancaster Synthesis Inc. and used as received unless otherwise
specified. Tetrahydrofuran (THF) and ether were distilled
under nitrogen from sodium with benzophenone as the indica-
tor. Pyridine was distilled over calcium hydride. The inter-
mediate 4-bromophenyl trifluorovinyl ether was synthesized
by modifying the method that was reported previously in the
literature.8
1H NMR spectra (200 MHz) were taken on a Bruker-200
FT NMR spectrometer, and 19F NMR spectra were recorded
on a Bruker AF 300 spectrometer. Elemental analysis was
taken at QTI (Whitehouse, NJ ). ESI-MS spectra were obtained
on a Bruker Daltonics Esquire ion trap mass spectrometer.
Thermal analyses were performed on a TA Instruments 2010
differental scanning calorimeter (DSC) at a scan rate of 10
°C/min, and a Hi-Res TGA 2950 thermogravimetric analyzer
(TGA) at a scan rate of 10 °C/min, under a nitrogen atmo-
sphere. Gel permeation chromatography (GPC) was run
through a Waters 515 HPLC pump in conjunction with Waters
Styragel 7.8 × 300 mm column and a Waters 410 differential
refractometer with THF as the solvent. All imaging was
conducted in the tapping mode on a digital multimode Nano-
scope III scanning force microscope, with 512 × 512 data
acquisitions at a scan speed of 1.4 Hz in air at room temper-
10.1021/ma034467g CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/27/2003

8002 Wong et al.
Macromolecules, Vol. 36, No. 21, 2003
F igu r e 1. Highly fluorinated trifluorovinyl aromatic ether monomers.
ature. Oxide-sharpened silicon nitride tips with integrated
cantilevers of the nominal spring constant of 0.38 N/m were
used. The photothermal deflection spectroscopy (PDS) experi-
mental test bed of this study used a 1 kW Hg (Xe) dc short
arc lamp as the broadband illumination source at 12.6 Hz
CF2, Fc), 46.72 (3F, dd, trans-CFdCF2, Fb), 53.53 (3F, dd, CFd
CF2, Fa), (J ab ) 109.9 Hz, J ac ) 55.0 Hz, J bc ) 113.8 Hz). Anal.
Calcd for C27H12F9N3O3: C, 54.28; H, 2.02; N, 7.03. Found: C,
54.10; H, 1.95, N, 6.97. ESI-MS (m/z): calcd, 597.1; found,
597.0.
1
chopping frequency, with a /8 m dual-grating monochromator.
1,10-[Bis(t r id eca flu or o-1,1,2,2-t et r a h yd r oct yl)[b is(4-
tr iflu or ovin yloxy)p h en ylsiloxyl]]p er flu or o-1H,1H,10H,-
10H-d eca n e (3). To a 100 mL three-neck round-bottom flask
were added 7.59 g (30 mmol) of 4-bromophenyl trifluorovinyl
ether and 45 mL of dry ether. The mixture was placed under
N2 and cooled to -78 °C, and t-BuLi (17.6 mL, 30 mmol, 1.7
M in pentane) was added dropwise via a syringe and allowed
to stir for 1 h. Tridecafluoro-1,1,2,2-tetrahydroctyltrichlorosi-
lane (4.41 mL, 15 mmol) was added dropwise via a syringe to
the mixture and allowed to stir for 24 h. The mixture was
filtered followed by solvent removal. HOCH2(CF2)8CH2OH
(1.73 g, 3.75 mmol) was added to the mixture, along with
pyridine (1.22 mL, 15 mmol) and THF (20 mL). The mixture
was allowed to stir for 30 min at room temperature, after
which the solvent was removed and the crude product was
separated quickly twice over neutral aluminum oxide eluting
with hexane to afford a clear liquid (6.20 g, 87%). 1H NMR
(200 MHz, CDCl3, TMS): δ 1.57 (4H, t, J ) 10.0 Hz), 2.15 (4H,
m), 4.10 (4H, t, J ) 13.3 Hz), 7.14 (8H, d, J ) 7.82 Hz), 7.47
(8H, d, J ) 8.79 Hz). 19F NMR (300 MHz, CDCl3, C6F6): δ
The probe beam was a 10 mW CW He-Ne laser, attenuated
to ∼1.5 mW.
Tr is[(4-tr iflu or ovin yloxy)p h en yl]tr ia zin e (2). To a well-
stirred solution of 4-bromophenyl trifluorovinyl ether (5.31 g,
21.0 mmol) in dry ether (30 mL) at -78 °C was added tert-
butyllithium (12.4 mL, 21.0 mmol, 1.7 M in pentane) dropwise
under a nitrogen atmosphere. The mixture was stirred at -78
°C for 1.0 h followed by the addition of cyanuric chloride (1.29
g, 7.0 mmol). The reaction mixture was warmed slowly to room
temperature and stirred for 1 h and then quenched with water.
The mixture was extracted with hexane for three times. The
organic layers were collected, washed with water, and dried
over Na2SO4. The solvent was then removed by rotary evapo-
ration under reduced pressure, and the crude product was
purified through a packed neutral aluminum oxide column
eluting with hexane and hexane/methylene chloride (20/1) to
1
afford a white solid (1.46 g, 35%). H NMR (200 MHz, CDCl3,
TMS): δ 7.19 (6H, d, J ) 9.20 Hz), 8.67 (6H, d, J ) 9.20 Hz).
19F NMR (300 MHz, CDCl3, C6F6): δ 38.08 (3F, dd, cis-CFd

Macromolecules, Vol. 36, No. 21, 2003
Trifluorovinyl Aryl Ethers and PFCB Ethers 8003
-85.20 (22F, m), 38.28 (4F, dd, cis-CFdCF2, Fc), 46.49 (4F,
dd, trans-CFdCF2, Fb), 49.45 (16F, m), 53.40 (4F, dd, CFd
38.08 (4F, dd, cis-CFdCF2, Fc), 46.36 (4F, dd, trans-CFdCF2,
Fb), 49.80 (6F, s), 53.34 (4F, dd, CFdCF2, Fa), 56.72 (4F, m),
(J ab ) 91.6 Hz, J ac ) 61.0 Hz, J bc ) 109.9 Hz). Anal. Calcd for
CF2, Fa), 56.76 (4F, m), (J ab ) 97.6 Hz, J ac ) 61.0 Hz, J bc
)
109.9 Hz). Anal. Calcd for C58H28F54O6Si2: C, 36.61; H, 1.48.
Found: C, 36.47; H, 1.40. ESI-MS (m/z): calcd, 1902.1; found,
1902.1.
C63H32F44O6Si2: C, 42.58; H, 1.82. Found: C, 42.50; H, 1.76.
ESI-MS (m/z): calcd, 1776.1; found, 1776.2.
Solu tion P olym er iza tion of 3. Monomer 3 (0.300 g, 0.16
mmol, 30 wt %) dissolved in mesitylene (0.700 g, 5.82 mmol)
was heated at 150 °C in a 15 mL two-neck round-bottom flask
under nitrogen for 30 h. The molecular weight was monitored
using GPC.
Solu tion Cop olym er iza tion of 2 a n d 3. Monomers (0.300
g, 30 wt %) were weighed out by molar ratio 3:2 of monomer
2 and 3, dissolved in mesitylene (0.700 g, 5.82 mmol), and
heated at 150 °C in a 15 mL two-neck round-bottom flask
under nitrogen for 19 h. The molecular weight was monitored
using GPC.
Su bstr a te P r ep a r a tion . The glass substrates were cleaned
by ultrasonicating in acetone for 5 min, methanol for 5 min,
and chloroform for 5 min. Fused silica substrates for optical
loss measurements were prepared through the Piranha clean-
ing procedure. The substrates were soaked in a solution of
sulfuric acid (H2SO4):hydrogen peroxide (H2O2) (2:1) for 15 min,
rinsed with distilled water, followed by soaking in a solution
of distilled water:hydrofluoric acid (HF) (10:1) for 10 s, and
then rinsed with distilled water. The HF solution was neutral-
ized with a sodium carbonate solution for disposal. The
substrates were then dried in a vacuum oven for 24 h at 85
°C.
F ilm P r ep a r a tion . Films were prepared by spin-coating
the filtered (through a 0.2 µm Teflon filter) solution of the
prepolymer onto the substrate and soft-baked at 65 °C for 10
min. The residual solvent was evaporated in a vacuum oven
overnight at 85 °C. The final curing was performed at 200 °C
and held for 30 min for the first sample. The second sample
was heated at 200 and 225 °C, each with a 30 min interval.
The third sample was heated to 200, 225, and 250 °C, each
with a 30 min interval. The fourth sample was heated to 200,
225, 250, and 300 °C, each with a 30 min interval. Each sample
was allowed to cool and then reheated so that one sample could
be removed from the micromanipulator to give transparent
films.
1,10-B i s [m e t h y l[b i s (4-t r i flu o r o v i n y lo x y )p h e n y l-
siloxyl]]p er flu or o-1H,1H,10H,10H-d eca n e (4). To a 100 mL
three-neck round-bottom flask were added 7.59 g (30 mmol)
of 4-bromophenyl trifluorovinyl ether and 45 mL of dry ether.
The mixture was placed under N2 and cooled to -78 °C, and
t-BuLi (17.6 mL, 30 mmol, 1.7 M in pentane) was added
dropwise via a syringe and allowed to stir for 1 h. Trichlo-
romethylsilane (4.41 mL, 15 mmol) was added dropwise via a
syringe to the mixture and allowed to stir for 24 h. The mixture
was filtered followed by solvent removal. HOCH2(CF2)8CH2-
OH (1.73 g, 3.75 mmol) was added to the mixture, along with
pyridine (1.22 mL, 15 mmol) and THF (20 mL). The mixture
was allowed to stir for 30 min at room temperature, after
which the solvent was removed and the crude product was
separated quickly twice over neutral aluminum oxide eluting
with hexane to afford a clear liquid (3.36 g, 72%). 1H NMR
(200 MHz, CDCl3, TMS): δ 1.55 (6H, s), 4.01 (4H, t, J ) 13.3
Hz), 6.97 (8H, d, J ) 8.78 Hz), 7.54 (8H, d, J ) 8.78 Hz). 19F
NMR (300 MHz, CDCl3, C6F6): δ 38.0 (4F, dd, cis-CFdCF2,
Fc), 46.14 (4F, dd, trans-CFdCF2, Fb), 49.73 (16F, m), 53.40
(4F, dd, CFdCF2, Fa), (J ab ) 97.6 Hz, J ac ) 48.8 Hz, J bc ) 115.9
Hz). Anal. Calcd for
C44H26F28O6Si2: C, 42.66; H, 2.12.
Found: C, 42.55; H, 2.04. ESI-MS (m/z): calcd, 1238.1; found,
1238.2.
1,4-[B is (t r id e c a flu o r o -1,1,2,2-t e t r a h y d r o c t y l)[b is -
(4-t r iflu or ovin yloxy)p h en ylsiloxyl]]t et r a flu or oh yd r o-
qu in on e (5). To a 100 mL three-neck round-bottom flask were
added 7.59 g (30 mmol) of 4-bromophenyl trifluorovinyl ether
and 45 mL of dry ether. The mixture was placed under N2 and
cooled to -78 °C, and t-BuLi (17.6 mL, 30 mmol, 1.7 M in
pentane) was added dropwise via a syringe and allowed to stir
for 1 h. Tridecafluoro-1,1,2,2-tetrahydroctyltrichlorosilane (4.41
mL, 15 mmol) was added dropwise via a syringe to the mixture
and allowed to stir for 24 h. The mixture was filtered followed
by solvent removal. Tetrafluorohydroquinone (0.68 g, 3.75
mmol) was added to the mixture, along with pyridine (1.22
mL, 15 mmol) and THF (20 mL). The mixture was allowed to
stir for 30 min at room temperature, after which the solvent
was removed and the crude product was separated quickly
twice over neutral aluminum oxide eluting with hexane to
afford a clear liquid (5.42 g, 89%). 1H NMR (200 MHz, CDCl3,
TMS): δ 1.56 (4H, t, J ) 10.0 Hz), 2.10 (4H, m), 7.15 (8H, d,
J ) 8.79 Hz), 7.48 (8H, d, J ) 8.79 Hz). 19F NMR (300 MHz,
CDCl3, C6F6): δ -85.11 (22F, m), 38.06 (4F, dd, cis-CFdCF2,
Fc), 46.34 (4F, dd, trans-CFdCF2, Fb), 49.82 (4F, s), 53.32 (4F,
dd, CFdCF2, Fa), 56.47 (4F, m), (J ab ) 91.5 Hz, J ac ) 61.0 Hz,
J bc ) 109.9 Hz). Anal. Calcd for C54H24F42O6Si2: C, 39.96; H,
1.49. Found: C, 39.85; H, 1.40. ESI-MS (m/z): calcd, 1622.0;
found, 1622.1.
1,9-[Bis(tr id eca flu or o-1,1,2,2-tetr a h yd r octyl)[bis(4-tr i-
flu or ovin yloxy)p h en ylsiloxyl]]h exa flu or obisp h en ol (6).
To a 100 mL three-neck round-bottom flask were added 7.59
g (30 mmol) of 4-bromophenyl trifluorovinyl ether and 45 mL
of dry ether. The mixture was placed under N2 and cooled to
-78 °C, and t-BuLi (17.6 mL, 30 mmol, 1.7 M in pentane) was
added dropwise via a syringe and allowed to stir for 1 h.
Tridecafluoro-1,1,2,2-tetrahydroctyl trichlorosilane (4.41 mL,
15 mmol) was added dropwise via syringe to the mixture and
allowed to stir for 24 h. The mixture was filtered followed by
solvent removal. Hexafluorobisphenol (1.26 g, 3.75 mmol) was
added to the mixture, along with pyridine (1.22 mL, 15 mmol)
and THF (20 mL). The mixture was allowed to stir for 30 min
at room temperature, after which the solvent was removed and
the crude product was separated quickly over neutral alumi-
num oxide eluting with hexane to afford a clear liquid (5.24 g,
79%). 1H NMR (200 MHz, CDCl3, TMS): δ 1.40 (4H, t, J )
10.0 Hz), 2.10 (4H, m), 6.98 (4H, d, J ) 8.79 Hz), 7.06 (4H, d,
J ) 8.78 Hz), 7.14 (8H, d, J ) 8.78 Hz), 7.47 (8H, d, J ) 8.78
Hz). 19F NMR (300 MHz, CDCl3, C6F6): δ -85.20 (22F, m),
Resu lts a n d Discu ssion
Syn th esis a n d P olym er iza tion of High ly F lu or i-
n a t ed Mon om er s. Suitable optical materials for
waveguide applications must exhibit low optical loss,
implying both improved processing to avoid light scat-
tering and low absorption in the near-infrared (NIR),
especially at 1310 and 1550 nm, the transmission
wavelengths used in telecommunications. Low absorp-
tion can be achieved by replacing hydrogen atoms with
heavier atoms (halogens), shifting the high-absorption
signal toward longer wavelengths. Compared to the
commercially used monomer 1 (Tetramer and Oak-
wood), a series of novel and highly fluorinated aromatic
trifluorovinyl ether monomers such as the trifunctional
one, 2, and the tetrafunctional ones, 3-6, have been
recently developed (Figure 1).
Monomers 2-6 were synthesized using the 4-bro-
mophenyl trifluorovinyl ether as the key intermediate
(Figure 2). The synthesis of monomer 2 is through a one-
pot, two-step reaction process. The whole process in-
volved the formation of the organolithium derivative of
the 4-bromophenyl trifluorovinyl ether using t-BuLi,9
followed by the in-situ addition of cyanuric chloride to
form 2 via the nucleophilic substitution. The use of
cyanuric chloride allowed for the synthesis of a more
compact and rigid monomer compared to monomer 1.
The triazine group further decreases the optical loss at
1310 and 1550 nm by decreasing the C-H content in
the resulting polymer.

8004 Wong et al.
Macromolecules, Vol. 36, No. 21, 2003
F igu r e 2. Synthesis of trifunctional monomer 2 and tetrafunctional monomers 3-6.
The general synthetic procedures for monomers 3-6
are also shown in Figure 2. Similar to the synthesis of
monomer 2, the first step involved is the preparation of
a reactive carbanion through the lithiation of 4-bro-
mophenyl trifluorovinyl ether at -78 °C with t-BuLi in
ether. Tridecafluoro-1,1,2,2-tetrahydroctyltrichlorosi-
lane or trichloromethylsilane was then added in situ to
form a disubstituted silyl chloride. The last step was to
condense the disubstituted silyl chloride with either a
fluorinated alkyl diol or a fluorinated aromatic diphenol
to form the monomers containing four trifluorovinyl
aromatic ether groups. The organolithium derivative of
the 4-bromophenyl trifluorovinyl ether intermediate
appears to be stable at a low temperature. This deriva-
tive then reacts readily with an electrophile to create a
variety of trifluorovinyl ether compounds. The fluori-
nated alkyl chains incorporated allow for an increase
in the flexibility of the monomer, and the high fluorine
content helps to reduce moisture absorption, dielectric
constant, and optical loss.
Polymerization of the monomers was performed both
in melt and in solution. Because of an increased double-
bond strain and a lower π-bond energy of fluoroolefins,
the cyclodimerization of aromatic trifluorovinyl ether
functionality is favored to form fluorinated C-C single
bonds.10 Differential scanning calorimetry (DSC) was
used to determine and monitor the [2π + 2π] cy-
clodimerization of the monomers in bulk. After the
polymerization of aromatic trifluorovinyl ether mono-
mers, an equal mixture of cis- and trans-1,2-disubsti-
tuted PFCBs is obtained in the polymer. This is due to
the mechanism of stepwise head-to-head cycloaddition
to form the more stable diradical intermediate followed
by rapid ring closure.3b,c Solution polymerizations in
mesitylene at 150 °C for monomer 3 and a mixture of 2
and 3 in a 3:2 molar ratio were performed to prepare
film samples (Figure 3). After several hours, the mono-
mers began cross-linking, causing the molecular weight
to increase (Table 1). The concentration of monomer or
comonomers used was 30 wt % in mesitylene. The prog-
ress of monomer to polymer conversion was monitored
by GPC. The desired degree of polymerization was to
obtain a material with molecular weight near 10 000.
P r op er ties of P F CB Th er m oset P olym er s. DSC
and thermogravimetric analysis (TGA) were used to
measure the thermal properties of the materials. As
expected, the more flexible polymers exhibited a lower
glass transition temperature (Tg) and decomposition
temperature (Td) due to the long fluorinated alkyl side
chain and the bridges (Table 2). The methyl end group
for monomer 4 compared to the long fluoroalkyl side
chains in monomers 3 increases both the Tg and Td of
the resulting polymer because it offers less flexibility.
The different bridges also affect the thermal properties
of the monomers. A flexible bridge gives the PFCB
polymer a lower Tg and Td as seen in monomer 3 while
the incorporation of the aromatic rings in monomers 5
and 6 causes the monomer to be more rigid, resulting
in both higher Tg and Td of PFCB polymers.

Macromolecules, Vol. 36, No. 21, 2003
Trifluorovinyl Aryl Ethers and PFCB Ethers 8005
F igu r e 3. Copolymerization of monomers 2 and 3.
Ta ble 1. Molecu la r Weigh t of P F CB P olym er a n d
Cop olym er d u r in g Solu tion P olym er iza tion in Mesitylen e
a t 150 °C Mon itor ed by GP C
Ta ble 3. Th er m a l P r op er ties of Th er m oset P F CB P olym er
a n d Cop olym er F ilm s Cu r ed a t Differ en t Tem p er a tu r es
for 0.5 h u n d er Nitr ogen .
polymerization time
at 150 °C in
prepolymer film from
monomera
curing temp (°C) Tg (°C)b Td (°C)c
monomer
mesitylene (h)
Mw
Mn
Mw/Mn
3
200
225
250
300
200
225
250
300
88
90
93
106
85
108
139
170
276
297
316
329
306
309
339
348
3
30
16
19
4959 2567
7922 3263
9402 3717
1.93
2.43
2.53
2 and 3
(3:2 molar ratio)
2 and 3 (3:2 molar ratio)
Ta ble 2. Th er m a l P r op er ties of Th er m oset P F CB
P olym er s Ach ieved by Bu lk P olym er iza tion for 0.5 h a t
250 °C u n d er Nitr ogen
a
polymer from
monomera
Tg
Td
polymer from
monomera
Tg
Td
Prepared from the polymerization of monomer in mesitylene
at 150 °C under nitrogen and spin-coating the prepolymer solution.
(°C)b
(°C)c
(°C)b
(°C)c
b
By DSC at 10 °C/min under nitrogen. c By TGA at 10 °C/min
under nitrogen.
2
3
4
>350
93
137
422
310
412
5
6
116
115
402
381
15 s. After curing the samples, the thickness of the film
was measured to be ∼4 µm. Data for the thermal
analysis of the films prepared are given in Table 3. The
final temperature of curing also affects the Tg and Td.
The Tg’s become much higher with curing at higher
temperatures because of the telechelic nature of PFCB
polymers. For the films from the monomer 3, the higher
curing temperature gives Tg data similar to that of the
bulk polymerization at the same temperature such as
250 °C. At the lower curing temperatures, the degree
of cross-linking of the polymer is relatively small so that
the two measurements give lower Tg’s than that for the
bulk material. It has been shown that cross-linking can
enhance the mechanical properties of polymers at high
temperatures and increase the Tg.13 However, film
coloration was observed at curing temperatures above
250 °C. The films changed from being colorless to having
a hint of yellow color possibly due to oxidation or
decomposition of the film at higher temperatures.
Solubility of the films cured at different temperatures
was also tested (Table 4). The solvents used were
cyclopentanone, tetrahydrofuran (THF), and dimethyl-
a
b
Bulk polymerization at 250 °C for 0.5 h under nitrogen. By
DSC at 10 °C/min under nitrogen. c By TGA at 10 °C/min under
nitrogen.
Thin films were prepared by spin-coating the solution
onto a glass substrate after filtering the crude prepo-
lymerized product. The samples were heated for 24 h
in a vacuum oven to remove the solvent and then cured
to 200, 225, 250, and 300 °C. For the polymer derived
from monomer 3, the molecular weight obtained was
∼5000 with a final polymer concentration of ∼50 wt %
in mesitylene. The resulting solution was spun onto
glass substrates at a spread rate of 500 rpm for 3 s and
a spin rate of 650 rpm for 15 s. After curing the samples,
the thickness of the film was measured to be ∼1 µm.
For the copolymer derived from 2 and 3 (3:2 molar
ratio), the molecular weight obtained was ∼10 000 with
a final polymer concentration of ∼40 wt % in mesitylene.
The solution was spun onto glass substrates at a spread
rate of 500 rpm for 3 s and a spin rate of 600 rpm for

8006 Wong et al.
Macromolecules, Vol. 36, No. 21, 2003
F igu r e 5. Optical loss spectra of the copolymer from mono-
mers 2 and 3 (3:2 molar ratio) at the final curing temperature
of (A) 200 °C and (B) 225 °C for 0.5 h measured by PDS with
∼1 wt % DR-1 dopant.
Optical loss measurements were obtained using
the photothermal deflection spectroscopy (PDS)15 for
the copolymer derived from the monomers of 2 and 3
(3:2) at the final curing temperatures of 200 and 225
°C. To measure the optical loss, these films were doped
with ∼1 wt % DR-1 for the PDS calibration because
the prepared films of polymer itself were colorless
and transparent. Thus, the intrinsic optical loss of
the polymers should be lower than that obtained from
the spectrum. From the spectra (Figure 5), the optical
losses for the copolymer cured at 200 °C were measured
to be 0.30 dB/cm at 1310 nm and 0.45 B/cm at 1550 nm.
The copolymer cured at 225 °C also gave similar results
with losses of 0.35 dB/cm at 1310 nm and 0.47 dB/cm
at 1550 nm. The refractive index of the copolymer can
be tuned through different ratios of the rigid aromatic
monomer 2 and the highly fluorinated monomer 3.
Although the siloxane linkages in the polymers contain-
ing monomers 3-6 are susceptible to hydrolysis, how-
ever, the high fluorination in these monomers signifi-
cantly decreases the moisture sensitivity of the resulting
polymers.
F igu r e 4. Tapping mode AFM topographs of surfaces of the
copolymer from monomers 2 and 3 (3:2) cured at (A) 225 °C
for 0.5 h, RMS ) 0.53 nm, and at (B) 250 °C for 0.5 h, RMS )
0.52 nm.
formamide (DMF), which are commonly used in device
fabrication. The films cured at 200 °C were found to be
slightly soluble in these solvents while the films cured
at 225 °C and above exhibited good solvent resistance.
The film samples prepared visually appeared to be
uniform and crack-free. Atomic force microscopy (AFM)
was employed to study the morphology and surface
properties of the materials (Figures 4). The morphology
of the spin-coated copolymer film at final curing tem-
peratures of 225 and 250 °C were studied in tapping
mode.12,13 The RMS surface roughnesses were both
small and similar, 0.53 nm at 225 °C and 0.52 nm at
250 °C. The surfaces of the spin-coated films exhibit
uniform fractal morphology, which is characteristic for
glassy polymers.14 AFM images obtained show that the
films are fairly uniform, and no phase separation is
observed.
Ta ble 4. Solu bility of Th er m oset P F CB P olym er s a n d Cop olym er s Cu r ed a t Differ en t Tem p er a tu r es for 0.5 h u n d er
Nitr ogen
polymer film
from monomer
final curing
temp (°C)
color of film
cyclopentanone
THF
DMF
3
200
225
250
300
200
225
250
300
colorless
colorless
slightly yellow
yellow
colorless
colorless
slightly yellow
yellow
soluble
soluble
soluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
soluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
soluble
insoluble
insoluble
insoluble
2 and 3 (3:2)

Macromolecules, Vol. 36, No. 21, 2003
Trifluorovinyl Aryl Ethers and PFCB Ethers 8007
1998, 3281, 2. (b) Eldada, L.; Shacklette, L. W. IEEE J . Select.
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Con clu sion
A series of highly fluorinated trifluorovinyl ether
containing monomers and PFCB polymers with siloxane
linkages were synthesized and characterized. Through
the copolymerization between a monomer with highly
fluorinated and flexible alkyl chains and a more rigid
comonomer, its thermal properties can be fine-tuned by
varying the composition as well as the final curing
temperature of the films. The copolymer exhibited good
adhesion on substrates, and thick, homogeneous, “crack-
free” films with good solvent resistance were attainable.
Optical loss measurements of these films (with 1%
dopant for calibration) using PDS show very low optical
loss (0.30 dB/cm at 1310 nm and 0.45 dB/cm at 1550
nm) which demonstrates their suitability for the fabri-
cation of low optical loss waveguides.
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Sci., Part A: Polym. Chem. 1993, 31, 3465. (b) Smith, D. W.,
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Ack n ow led gm en t . Financial support from the
National Science Foundation (NSF-NIRT and the NSF-
STC Program under Agreement DMR-0120967) and the
Air Force Office of Scientific Research (AFOSR) under
the MURI Center on Polymeric Smart Skins is acknowl-
edged. Alex K.-Y. J en thanks the Boeing-J ohnson
Foundation for its support.
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MA034467G