Hyperbranched fluorinated polyimides with tunable refractive indices for optical waveguide applications

Article abstract of DOI:10.1016/j.polymer.2009.10.005

A series of halogenated hyperbranched polyimides (HBPIs) with tunable refractive indices (RIs) has been designed and prepared for use as novel optical waveguide materials. The hyperbranched structure of these polymers leads to multifarious and controllable chemical structures. The degree of branching in these HBPIs was estimated using ¹H NMR spectroscopy. The halogen-terminated HBPIs were found to exhibit high glass transition temperature, good thermal stability, excellent transparency, low birefringence and low optical loss in the wavelength windows for telecommunication. Due to the high polarizability of the C-Cl bond at the terminal groups, RIs were precisely controlled by adjusting the C-Cl bond content without additional optical losses. Finally, several waveguide devices were fabricated by the reactive ion etching (RIE) technique and were found to exhibit good optical propagation at a wavelength of 1.55 μm.

Full text of DOI:10.1016/j.polymer.2009.10.005

Polymer 51 (2010) 694–701
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Hyperbranched fluorinated polyimides with tunable refractive indices
for optical waveguide applications
*
Hong Gao, Changqing Yan, Shaowei Guan, Zhenhua Jiang
Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, P. R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of halogenated hyperbranched polyimides (HBPIs) with tunable refractive indices (RIs) has been
designed and prepared for use as novel optical waveguide materials. The hyperbranched structure of
these polymers leads to multifarious and controllable chemical structures. The degree of branching in
these HBPIs was estimated using 1H NMR spectroscopy. The halogen-terminated HBPIs were found to
exhibit high glass transition temperature, good thermal stability, excellent transparency, low birefrin-
gence and low optical loss in the wavelength windows for telecommunication. Due to the high polar-
izability of the C–Cl bond at the terminal groups, RIs were precisely controlled by adjusting the C–Cl bond
content without additional optical losses. Finally, several waveguide devices were fabricated by the
reactive ion etching (RIE) technique and were found to exhibit good optical propagation at a wavelength
Received 27 February 2009
Received in revised form
29 September 2009
Accepted 5 October 2009
Available online 21 October 2009
Keywords:
Hyperbranched polyimide
Tunable refractive indices
Optical waveguide
of 1.55 mm.
Ó 2009 Published by Elsevier Ltd.
1. Introduction
Although RIs of polyimides increase with increasing content of
non-fluorinated dianhydride, a higher content of non-fluorinated
dianhydride is undesirable because it often leads to higher optical
loss with the increment of C–H bonds. In addition, LFPIs deposited
on a silicon or silica substrate usually exhibit a large in-plane/out-
of-plane birefringence, a result of both chain orientation on the film
plane and the residual stress in the polyimide films originated from
the CTE mismatch between the films and substrates [12]. Current
efforts to develop optical waveguide polyimides are aimed at the
design and synthesis of novel fluorinated polyimides with excellent
optical properties. Hyperbranched structures have played an
increasingly important role in reducing the birefringence [13,14]
and allow facilitated tailoring of RIs [15].
Polymer optical waveguides have attracted considerable atten-
tions for their possible application as light transmission compo-
nents in optical communication systems [1–3]. For advanced
integrated optical waveguide applications such as optical attenua-
tors, optical interconnects, splitters and arrayed waveguide grat-
ings (AWG), the polymer optical materials should have excellent
properties in terms of low optical losses, low birefringence [4,5],
and high Tg. Waveguides devices with complex structures are often
expected to meet more severe requirements, such as control of RIs,
good interfacial adhesion, and similar coefficients of thermal
expansion (CTE) [6].
In order to satisfy these requirements, various types of optical
polymers have been arranged and developed. Among these poly-
mers, the fluorinated polyimides are a class of promising wave-
guide materials due to their high thermal stability, high optical
transparency and low optical losses at the wavelength windows for
telecommunication (the central wavelengths are 0.85, 1.31, and
In recent years, the hyperbranched polyimides (HBPIs) have been
widely examined in many potential applications, such as gas sepa-
ration and photo-patterning. Fang et al. first reported the preparation
of HBPIs which were derived from a triamine (tris(4-aminophenyl)-
amine) and several dianhydrides, and investigated their physical and
gas transport properties as well [16,17]. Yin et al. synthesized another
triamine(1,3,5-tris(4-aminophenoxy)benzene)forthepreparationof
HBPIs [18]. However, these monomers without fluorine do not fit for
the application of HBPI as optical waveguide materials due to the high
C–H content and low optical transparency. In our previous work,
1.55 mm).[7–9] In case of linear fluorinated polyimides (LFPIs),
copolymerization has been employed to control RIs, as demon-
strated in the 6FDA/PMDA-TFDB copolymer system whose RIs are
controllable over a wide range by varying the 6FDA content [10,11].
a
fluorinated triamine (1,3,5-Tris(2-trifluoromethyl-4-amino-
phenoxy)benzene) was synthesized and polymerized into the HBPIs,
and it was considered as a promising candidate for optical waveguide
materials [19].
* Corresponding author. Tel./fax: þ86 431 85168886.
E-mail address: jiangzhenhua@jlu.edu.cn (Z. Jiang).
0032-3861/$ – see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.polymer.2009.10.005

H. Gao et al. / Polymer 51 (2010) 694–701
695
Scheme 1. Synthesis of HBPIs: (a) m/n ¼ 1:1, a dianhydride solution was added to a TFAPOB solution, 40 ꢀC, 24 h, (b) chemical imidization, m-xylene 170 ꢀC, 5 h; (c) m/n ¼ 2:1,
a TFAPOB solution added to a dianhydride solution, 40 ꢀC, 24 h, (d) chemical imidization, triethylamine, acetic anhydride, 40 ꢀC, 8 h; (e) 3,5-ditrifuoromethylaniline (2CF3) and 3,5-
dichloroaniline (2Cl) with different ratio added to the solution of anhydride-terminated PAA, 40 ꢀC for 8 h.
In this paper, a series of HBPIs with different halogenated
terminal groups were prepared and characterized as novel optical
waveguide materials, and their physical and optical properties of
HBPI were examined. By copolymerizing HBPIs with different
halogen-containing terminal groups, the RIs were precisely
controlled without additional optical loss. These HBPIs with similar
chemical structures were promising candidates for both the core
and cladding of waveguides. Furthermore, several waveguide
devices were fabricated from the HBPIs by the RIE technique, and
their optical properties were investigated.
Oxydiphthalic anhydride (ODPA) and 3,30-4,4’-Benzophenonete-
tracarboxylic dianhydride (BTDA) were purified by recrystallization
from acetic anhydride before use. 3,5-bis(trifluoromethyl)aniline
and 3,5-Dichloroaniline were purchased from Aldrich Chemical Co.
and used without further purification. N,N-dimethylacetamide
(DMAc) was refluxed in presence of CaH2 and distilled under
vacuum before use.
2.2. Characterization
Differential scanning calorimetry (DSC) was performed on a Met-
tler Toledo DSC821e at a heating rate of 20 ꢀC/min under a nitrogen
atmosphere. The thermogravimetric analysis (TGA) was performed
using a Perkin Elmer TGA-7 thermal analyzer system at the heating
rate of 10 ꢀC/min under a nitrogen atmosphere. Fourier transformed
2. Experimental section
2.1. Materials
The synthesis of 1,3,5-Tris(2-trifluoromethyl-4-aminophe-
noxy)benzene (TFAPOB) had been reported elsewhere [19].
4,40-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,40-
Fig. 1. IR spectra of ODPA-based HBPIs: (a) amino-terminated AM-OD, (b) anhydride-
Fig. 2. 1H NMR spectra of model compounds and 6FDA-based HBPI, (a) model 1, (b)
terminated AD-OD, (c) halogen-terminated HG-OD (2CF3100%).
model 2, (c) model 3, (d) AM-6F, and (e) AD-6F.

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H. Gao et al. / Polymer 51 (2010) 694–701
Scheme 2. Synthesis of the model compounds: (a) m/n ¼ 1/1, 40 ꢀC, 5 h; (b) chemical imidization, m-xylene, 170 ꢀC, 5 h; (c) m/n ¼ 1/2, 40 ꢀC, 5 h; (d) m/n ¼ 1/3, 40 ꢀC, 12 h; and (e)
chemical imidization, triethylamine, acetic anhydride, 40 ꢀC, 12 h.
FT-IRspectra(KBr)weremeasuredusingaNicoletImpact410infrared
spectrometer. Gel permeation chromatograms (GPC) were obtained
by a Waters 410 instrument with tetrahydrofuran (THF) as the eluent
Woollam M-2000UI spectroscopic ellipsometer. In-plane/out-of-
plane birefringences (at 650 nm) of the polymer films was deter-
mined from the coupling angles of TE (transverse electric) or TM
(transverse magnetic) optical guided modes with a gadolinium
gallium garnet (GGG) prism at 650 nm. UV–visible absorption spectra
were recorded on a UV2501-PC spectrophotometer. Scanning elec-
tron micrographs (SEM) were observed on a SHIMADZU SSX-550
microscope. Atomic force microscopy (AFM) of film surfaces were
1
and polystyrene as the standard. The H and 19F NMR spectra were
recorded at500 and 470.5 MHz using aBruker 510 NMR spectrometer
with tetramethylsilane and CFCl3 as the references (0 ppm) respec-
tively. Near-infrared spectra (Near-IR) were measured on a Varian
Kera 500 spectrometer. RIs of polymer films were measured by a J.A.

H. Gao et al. / Polymer 51 (2010) 694–701
697
Table 1
to room temperature, the solution was precipitated from ice water.
The crude product was dried in vacuo and purified by column
chromatography with silica gel and a mixture of ethanol/cyclo-
hexane (1/1, by volume) as the eluant to give a yellow powder.
Physical properties of the HBPIs.
x(mol%)a Mn
Mw/Mn Tg(ꢀC)c Td(ꢀC)d DBe
b
Polymers
AD-OD
AM-OD
HG-OD (2CF3 100%)
HG-OD (2CF3 50%)
HG-OD (2CF3 0%)
AD-BT
–
–
1
0.5
0
–
–
1
0.5
0
–
–
1
0.5
0
13,000 2.17
27,000 1.96
19,000 2.02
27,000 3.05
25,000 2.71
10,000 1.96
15,000 2.50
22,000 2.07
15,000 2.24
26,000 2.55
6, 000 1.92
26,000 1.96
16,000 2.55
33,000 2.24
28,000 3.52
220
240
217
212
202
230
241
242
227
220
232
243
242
234
226
516
528
530
526
502
526
532
515
512
508
506
523
514
515
526
0.93
0.60
EIMS m/e: 733. 1H NMR (500 MHz, DMSO-d6, ppm)
d: 8.02 (1H),
7.95–7.86 (4H, Ar–H of phthalic anhydride residues), 7.76 (1H), 7.37
(1H), 6.98 (2H), 6.90 (2H), 6.79 (2H), 6.24 (2H), 6.15 (1H), 5.51
(4H, NH2).
0.90
0.66
AM-BT
HG-BT (2CF3 100%)
HG-BT (2CF3 50%)
HG-BT (2CF3 0%)
AD-6F [19]
2.3.2. Synthesis of model compound 2
The aforementioned procedure for the synthesis of model
compound 1 was followed except that 0.148 g (1 mmol) of phthalic
anhydride was used, then the final reaction solution was poured
into ice water. The crude product was dried in vacuo and was also
purified by column chromatography, a mixture of ethanol/cyclo-
hexane (1/2) was used as the eluant.
0.87
0.62
AM-6F [19]
HG-6F[19] (2CF3 100%)
HG-6F (2CF3 50%)
HG-6F (2CF3 0%)
a
EIMS m/e: 863. 1H NMR (500 MHz, DMSO-d6, ppm)
d: 8.02 (2H),
7.97–7.85 (8H, Ar–H of phthalic anhydride residues), 7.78 (2H), 7.36
(2H), 7.05 (1H), 6.91 (1H), 6.82 (1H), 6.56 (1H), 6.46 (2H), 5.55 (2H,
NH2).
Molar ration of (3,5-di-trifluoromethyl)aniline to the total terminated group.
Number average molecular weight determined by GPC in THF vs polystyrene.
Glass transition temperature measured by DSC with a heating rate of 10 ꢀC/min
b
c
in nitrogen.
d
Onset temperature for 5% weight loss measured by TGA with a heating rate of
10 ꢀC/min in nitrogen.
e
Determined by 1H NMR analysis.
2.3.3. Synthesis of model compound 3
A mixture of 0.302 g (0.5 mmol) of TFAPOB, 0.222 g (1.5 mmol)
of phthalic anhydride, and 10 mL of DMAc was magnetically stirred
under a nitrogen flow in a oven dried 100-mL, three-necked flask at
40 ꢀC for 12 h. Subsequently, a mixture of 1 g of triethylamine and
3 g of acetic anhydride was added. The solution was kept at 40 ꢀC
for 12 h and then condensed under reduced pressure to give
a yellow precipitate. The crude product was collected by filtration
and recrystallized with N-methylpyrrolidinone (NMP).
observed with a Digital Instrument, Nanoscope IIIa. AFM tapping
mode images were taken at room temperature in air with the micro-
fabricated rectangle crystal silicon cantilevers (Nanosensor). Topo-
graphic imageswere obtainedataresonancefrequencyofca. 365 kHz
for the probe oscillation.
2.3. Prepareation of model compounds
EIMS m/e: 993. 1H NMR (500 MHz, DMSO-d6, ppm)
d: 8.02 (3H),
7.97–7.87 (12H, Ar–H of phthalic anhydride residues), 7.79 (3H),
7.37 (3H), 6.81 (3H).
2.3.1. Synthesis of model compound 1
TFAPOB (0.302 g, 0.5 mmol) was dissolved in 10 mL of DMAc in
an oven dried, three-necked flask with stirring under a nitrogen
atmosphere. A solution of 0.074 g (0.5 mmol) of phthalic anhydride
in 5 mL of DMAc was added dropwise through a syringe over 0.5 h.
After the addition, the reaction was further conducted for 5 h at
40 ꢀC. Then, 30 mL of m-xylene was added, and the mixture was
heated to 170 ꢀC with a Dean–Stark apparatus for 5 h. After cooling
2.4. Polymer synthesis
2.4.1. Synthesis of amino-terminated HBPIs
The general polymerization method for amino-terminated
HBPIs was reported elsewhere [13]. A typical procedure for the
polyaddition of TFAPOB with ODPA is described as follows. TFAPOB
(0.604 g, 1 mmol) was dissolved in DMAc (10 cc) in a oven dried,
three-necked flask under a nitrogen flow. A solution of ODPA dia-
nhydride (0.302 g, 1 mmol) in DMAc (10 cc) was added dropwise to
the mixture through a syringe over 1 h under magnetic stirring at
40 ꢀC. The reaction was further conducted for 24 h. Then, m-xylene
(10 cc) was added, and the reaction mixture was heated to 170 ꢀC
for 5 h with a Dean–Stark trap. After cooling to room temperature,
the mixture was precipitated from ethanol (200 cc). The polymers
were collected by filtration and dried under vacuum at 80 ꢀC for
24 h. In this study, the amino-terminated HBPIs obtained from
ODPA, BTDA and 6FDA dianhydrides will be referred to as AM-OD,
AM-BT and AM-6F, respectively.
2.4.2. Synthesis of anhydride-terminated HBPIs
The general polymerization procedure is outlined in Scheme 1.
A typical procedure for the polyaddition of TFAPOB with ODPA is
introduced as follows. To a solution of 2 mmol (0.62 g) dianhydrde
(ODPA) in dried DMAc (10 cc) in a 50 mL flask, 1 mmol (0.604 g) of
TFAPOB dissolved in 5 mL of DMAc was added dropwise through
a syringe over 1 h. The mixture was stirred at 40 ꢀC for 24 h to
afford anhydride-terminated poly(amic acid) (PAA) solution. The
PAA was subsequently converted to PI through a chemical imid-
ization process. The chemical imidization was carried out via the
addition of 1 g of triethylamine and 3 g of acetic anhydride into the
PAA solution at 40 ꢀC for 8 h. The resulting homogeneous PI
Fig. 3. 19F NMR spectra of the HG-6F series.

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H. Gao et al. / Polymer 51 (2010) 694–701
for 1 h and 250 ꢀC for 1 h. By adjusting the concentration of PI
solutions, the film thickness was controlled in the range of 2–4 m.
Table 2
Solubility of the HBPIs.
m
Polymers
Solventsa
NMP DMAc DMSO THF CHCl3 Actone Ethanol
2.6. Waveguide fabrication
AD-OD/AM-OD
HG-OD series
AD-BT/AM-BT
HG-BT series
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þ
–
–
–
–
–
–
–
–
–
–
–
–
–
b
b
þþ
þ
þ
Three layered rib-type optical waveguides were fabricated by
spin coating PI solution for the under-cladding and core onto an
fues silicon substrate. Next, the aluminum layer was sputtered
onto the core layer and patterned by photolithography. Then
a core waveguide was produced using oxygen RIE. After that, the
mask was removed and the core ridges were formed. Finally, the
core ridges were covered with PMMA layer with lower refractive
index.
–
b
b
þ
þþ
þ
–
AD–6F/AM-6F[19] þþ
þþ
þþ
þ
HG-6F series
þþ
þþ
þ
Qualitative solubility was determined with as 10 mg of polymer in 1 mL of solvent.
þþ: soluble at room temperature; þ: soluble on heating at 100 ꢀC; – insoluble.
a
NMP, N-methyl-2-pyrrolidone; DMAc, N,N-dimethylacetamide; DMSO,
dimethyl sulfoxide; THF, tetrahydrofuran.
b
Special samples: The solubility of HG-OD(2CF30%) and HG-BT(2CF30%) were ‘‘þ’’
in DMSO and is ‘‘–’’ in CHCl3.
3. Results and discussion
3.1. Polymer synthesis
solution was poured into ethanol to give a white precipitate, which
was collected by filtration, washed thoroughly with ethanol and
dried under vacuum at 80 ꢀC for 24 h. The anhydride-terminated
HBPIs obtained from ODPA, BTDA and 6FDA will be referred to as
AD-OD, AD-BT and AD-6F, respectively.
HBPIs were first prepared from A2 þ B3-type monomers by Fang
[16]. In this work, the same method was employed to synthesize
a series of halogen-terminated HBPIs for the fabrication of wave-
guide devices. The three-step synthetic procedure is shown in
Scheme 1. Three dianhydrides (6FDA, ODPA, BTDA) and triamine
(TFAPOB) were condensed with different monomer addition orders
and molar ratios to form different hyperbranched PAA precursors.
Subsequently, the halogen-terminated monomers (3,5-bis(trifuoro-
methyl)aniline and 3,5-dichloroaniline) were added into the anhy-
dride-terminated precursors for end-capping, then all the PAAs
precursors were chemically imidized to give the corresponding
HBPIs. The structures of the amino-, anhydride-, and halogen-
terminated HBPIs were confirmed by FT-IR and 1H NMR spectros-
copy. Fig.1 shows the IR spectra of AM-OD, AD-OD and HG-OD (2CF3,
100%) HBPIs. The absorption peaks at 1792 cmꢁ1 (C]O asymmet-
rical stretching), 1738 cmꢁ1 (C]O symmetrical stretching) and
1373 cmꢁ1 (C–N stretching) are the characteristic absorption bands
for PI, and no characteristic band of PAA (1680 cmꢁ1) is detected. As
for the amino-terminated HBPI (a), the band at 3438 cmꢁ1 is
assigned to the stretching absorption of N–H at the terminal amino
groups. The peak at 1853 cmꢁ1 (C]O stretching) indicates the
existence of terminal anhydride groups for the anhydride-termi-
nated HBPIs (b), while, No such two absorption bands are found in
the halogen-terminated HBPIs (c), and this phenomenon demon-
strates that halogen-termination was successfully achieved.
The structural perfection of hyperbranched polymers has been
generally characterized by the degree of branching (DB), which was
defined by Frechet as follows:[20]
2.4.3. Synthesis of halogen-terminated HBPIs
The 3,5-bis(trifuoromethyl)aniline (x mmol) (x ¼ 0, 0.5, 1) and
3,5-dichloroaniline (1-x) were both added into an equimolar
amount of PAA precursors for AD-OD that was prepared by the
same procedures. The reaction mixtures were stirred at 40 ꢀC for
8 h. Then, a mixture of triethylamine (1 g) and acetic anhydride
(3 g) was added, and was stirred at 40 ꢀC for another 8 h. After
cooling to room temperature, the mixture was precipitated from
ethanol (200 cc). The polymers were collected by filtration and
dried under vacuum at 80 ꢀC for 24 h. The halogen-terminated
HBPIs obtained from ODPA were signed as HG-OD (2CF3 100%)
(x ¼ 1), HG-OD (2CF3 50%) (x ¼ 0.5), and HG-OD (2CF3 0%) (x ¼ 0).
HG-BT and HG-6F series were also prepared using the same
procedure. The polymer of x ¼ 0.25 and 0.75 were prepared for the
series of HG-OD to estimate the controllability of refractive index.
The polymerization procedures were also outlined in Scheme 1.
2.5. Film preparation for optical measurement
The HBPIs thus synthesized were dissolved in cyclohexanone at
a concentration of 10 wt%. The solution was filtered with a 1 mm
Teflon membrane syringe filter. The filtered solution was spin-
coated onto a silicon wafer and fused silica substrate at a spin rate
of 1500 rpm for 1 min. followed by curing at 140 ꢀC for 4 h, 200 ꢀC
DB [ ðD DTÞ=ðD D T DLÞ
(1)
where D, T, and L refer to the numbers of dendritic, terminal, and
linear units in the polymer, respectively. Experimentally, DB is
usually estimated from 1H NMR or 13C NMR spectroscopy by
comparing the integrals of peaks assignable to respective units.
Fig. 2 shows 1H NMR spectra of the amino-terminated and anhy-
dride-terminated HBPIs derived from 6FDA. For the amino-termi-
nated AM-6F, the peaks in the range of 8.16–7.75 ppm are
assignable to the hydrogens of 6FDA residues, while the peaks in
the range 7.56–6.18 ppm were corresponds to the hydrogens in the
phenyl rings of TFAPOB residue, which contain the information on
the degree of branching. The peak at 5.48 ppm is attributable to the
free amino hydrogens in the terminal and linear units. To clarify the
detailed assignment of peaks in the range of 7.56–6.18 ppm, three
kinds of model compounds were synthesized from phthalic anhy-
dride and TFAPOB as shown in Scheme 2. The TFAPOB residues in
model compounds 1, 2, and 3 are basically same as the counterparts
Fig. 4. UV-vis spectra of the halogen-terminated HBPIs.

H. Gao et al. / Polymer 51 (2010) 694–701
699
Fig. 6. Relationship between the RIs and the content of 3,5-bis(trifluoromethyl)aniline
(x ¼ molar ratio of 3,5-bis(trifluoromethyl)aniline to the total terminal groups).
Fig. 5. Near-IR spectra of HG-OD (A: HG-OD (2CF3 100%); B: HG-OD (2CF3 50%), and C:
HG-OD (2CF3 0%)).
the amino-terminated HBPI was estimated as 62% according to eq.
(1). The 1H NMR spectra of AM-OD and AM-BT display peaks with
the chemical shifts similar to those of AM-6F, and the DB values
were calculated as 60% and 66%, respectively. The DB values higher
than 0.5, which are for a statistic growth [21], could be caused by
the low conversion of the amino groups. As shown in Table 1, the DB
values for the amino-terminated HBPIs are dependent on their
in terminal, linear, and dendritic units of the HBPI, respectively. For
comparison, their 1H NMR spectra are also shown in Fig. 2(a–c). The
peaks in the range of 7.56–6.18 ppm observed for the amino-
terminated AM-6F can be compared with those of model
compounds 1, 2 and 3 as well as that for anhydride-terminated AD-
6F. Model 1 demonstrates eight distinct peaks at 8.02 (H18), 7.76
(H17), 7.37 (H16), 6.98 (H15), 6.90 (H14), 6.79 (H13), 6.24 (H12) and
6.15 ppm (H11). Model 2 also exhibits eight distinct peaks but with
different chemical shifts at 8.02 (H28), 7.78 (H27), 7.36 (H26), 7.05
(H25), 6.91 (H24), 6.82 (H23), 6.56 (H22) and 6.46 ppm (H21).
Model 3 exhibits four distinct peaks at 8.02 (H38), 7.79 (H37), 7.47
(H36), 6.81 (H31). The amino-terminated AM-6F demonstrates the
peaks in the range of 7.62–7.78 ppm which includes the over-
lapping signals from the terminal H17, linear H27, and dendritic
H37. In addition, the peaks in 7.20–7.50 ppm include the over-
lapping signals from the terminal H16, linear proton H26, and
dendritic proton H36. The peak around 6.78 ppm is related to both
the terminal H14, H15 and the linear H24, H25. Furthermore, the
peak at 6.73 ppm corresponds to the terminal H13, linear H23, and
dendritic H31. Therefore, the margin of the integration of latter two
peaks can afford the resonance of dendritic proton H31. The peaks
around 6.52 ppm are assignable only to the linear H21 and H22,
while the peak at 6.14 ppm is independently ascribed to the
terminal H11 and H12. The integral of the terminal units is very
close to that of the dendritic units, which indicates that the DB of
molecular weights.
A lower molecular weight means lower
conversions of the amino groups, which correspond to a higher DB
values.
For the anhydride-terminated AD-6F, supposing the amino
groups of TFAPOB monomer are reacted (i.e., TFAPOB residues are
completely ‘‘dendritic’’), the HBPI should have the ‘‘completely
branched’’ structure (DB ¼ 1). However, it should be noted that the
anhydride-terminated HBPIs are less perfect in structure than
common dendrimers due to the random polymerization process.
In Fig. 2(e), a resonance in 6.2–6.4 ppm is detectable, which indi-
cates that the polymers also have linear and terminal units, and the
estimated DB value is 87%. The same phenomena were also
observed in the NMR spectra of anhydride-terminated AD-OD and
AD-BT, in which the estimated BD values were 93% and 90%,
respectively. As for the anhydride-terminated HBPIs, large amount
of semidendritic units may exist, however, the estimated DB values
could be still higher than the actual values; for halogen-terminated
HBPIs, the amino groups of the linear units are capped by the end
groups, and so the linear and terminal units are not detectable in
the NMR analysis. Therefore, the DB value is not sufficiently effec-
tive to express the structural perfection of HBPI in this case.
In the case of halogen-terminated HBPIs, 1H NMR spectra are
unable to confirm the structure of the terminal halogen groups in
detail, and different halogen-terminated structures are characterized
by 19F NMR (Fig. 3). The peaks observed at ꢁ61.5 ppm and ꢁ63.7 ppm
are attributable to the –CF3 groups of 6FDA and triamine, respectively
and the peak at ꢁ62 ppm is assignable to the –CF3 associated with the
terminal monomer of 3,5-ditrifuoromethylaniline. No fluorine signal
relating to the 3,5-dichloroaniline-terminated HBPIs was detected. It
is notable that the fluorine contents in the HBPIs can be easily
controlled by varying the feed ratio of the terminal monomer. This
property is advantageous for the resulting HBPIs in waveguide
applications because the facile structural control and reproducibility
of optical properties such as refractive index are critical for the design
and fabrication of waveguide devices.
Table 3
Optical properties of the halogen-terminated HBPIs.
Polymers
Refractive indexa Birefringenceb
1.31 m 1.55 m Refractive index at 650 nm nTE–nTM
m
m
TE mode
(nxy
TM mode
(nz)
)
HG-OD (2CF3 100%)
HG-OD (2CF3 50%)
HG-OD (2CF3 0%)
HG-BT (2CF3 100%)
HG-BT (2CF3 50%)
HG-BT (2CF3 0%)
HG-6F [19] (2CF3 100%) 1.5160 1.5140 1.5542
HG-6F (2CF3 50%)
HG-6F (2CF3 0%)
1.5807 1.5727 1.6252
1.5970 1.5930 1.6293
1.6109 1.6090 1.6326
1.5863 1.5842 1.6362
1.5883 1.5846 1.6381
1.5944 1.5851 1.6360
1.6154
1.6199
1.6253
1.6247
1.6325
1.6330
1.5521
1.5538
1.5542
0.0098
0.0094
0.0073
0.0086
0.0056
0.0060
0.0021
0.0026
0.0043
1.5234 1.5206 1.5564
1.5309 1.5254 1.5585
Table 1 illustrates the physical properties of the HBPIs. The
molecular weights (Mn) and the polydispersities were estimated as
6,000–33,000 and 1.92–3.52, respectively. All the HBPIs exhibit good
thermal properties with the 5% weight loss at temperatures above
500 ꢀC. The DSC thermograms revealed the glass transition
a
The refractive indices of the polymer films were obtained using a spectroscopic
ellipsometer.
b
The birefringence of the polymer films were measured by the method of
coupling angles of TE or TM optical modes with a prism at 650 nm.

700
H. Gao et al. / Polymer 51 (2010) 694–701
Fig. 7. SEM micrographs of the waveguide devices: straight waveguide (A) and flexural waveguide (B).
temperatures (Tg) of the HBPIs ranged from 200 to 243 ꢀC. Tg’s of the
halogen-terminated HBPIs are lowered due to the chemical substi-
tution at the terminal group, which can be explained by the reduction
in the end group polarity of the amino- and anhydride-terminated
HBPIs. All the HBPIs display excellent solubilities in common organic
solvents, such as DMAc, NMP and cyclohexanone (Table 2). The
solubility of the halogen-terminated HBPIs is better than that of the
anhydride-terminated HBPIs, which is attributed to the enhanced
polarity of the terminal groups [22]. All the HBPIs readily form tough,
flexible, and optically transparent films by casting or spin coating
techniques, properties essential in optical applications.
formation of intermolecular charge-transfer-complexes (CTC)
between the electron-donating triamine and the electron-acceptor
dianhydride. Bulky, electron-withdrawing –CF3 groups attached to
the polymer backbones may induce this effect. On the other hand, it
has been assumed that the intermolecular packing in hyper-
branched polymer chains is incompact, which prevents CTC
formation and may enhance the optical transparency in the visible
region.
Low optical loss in the near-IR region is one of the most
important requirements for waveguide materials. In our previous
paper, we have reported that the end-capping by –CF3 is an effec-
tive route to improve the optical transparency of HBPIs in the near-
IR region for telecommunication region [19]. In this study, the
absorptions in the halogen-terminated HBPIs were also researched
by near-IR absorption spectra Fig. 5. Absorption peaks are
normalized by dividing with their film thickness. The first combi-
3.2. Optical properties
As shown in Fig. 4, the UV-vis spectra of the HG-6F series exhibit
cut-off wavelengths around 370 nm and the transparency is around
85% in the range of 380–800 nm. This excellent optical trans-
parency of the fluorinated HG-6F is attributed to the reduced
nation bands 2
n
þ
d
and the weak combination bands 3
n
þ
d of the
C–H stretching and bending, appear in the region of 1.25–1.4
mm
Fig. 8. AFM images of HBPI: (a) AFM 3D micrograph of the pattern, (b) surface roughness of the waveguide beam measured by AFM, (c) surface roughness of the silica layer of the
substrate measured by AFM.

H. Gao et al. / Polymer 51 (2010) 694–701
701
waveguide. In the 3D micrograph of the pattern, smooth surfaces
are obtained for both of the top and sidewall of the core layer
waveguide. Based on the AFM image, the film thickness was esti-
mated as ca. 1 mm, and the surface roughness is less than 5 nm. The
transmission spectra of the waveguide device were measured by
a tunable laser at the center wavelength at 1550 nm and a spectrum
analyzer. (Fig. 9) Light from a tunable semiconductor laser was
coupled into the input-waveguide through a single-mode optical
fiber. The near-field mode pattern at the output-channels was
observed with a near-IR vidicon after magnification by an objective
lens. The transmission spectra and near-field patterns demonstrate
that the fluorinated HBPIs are a promising candidate for optical
waveguide applications with low optical loss at the wavelengths for
telecommunication.
4. Conclusions
A series of HBPIs with amino-, anhydride- and halogen-termi-
nated end groups were prepared successfully. All the halogen-
terminated HBPIs had excellent physical properties, including high
glass transition temperatures, good thermal stabilities, and ease of
film processing. By comparing NMR spectra of HBPIs with those of
model compounds, DBs values of the amino- and anhydride-termi-
nated HBPIs were found to be higher than 60% and 90%, respectively.
Furthermore, the halogen-terminated HBPIs exhibited excellent
optical properties. It was shownthathigh –CF3 content could increase
the film transparency, and the hyperbranched isotropic structures
could reduce the birefringences. By substituting the large numbers of
end groups with fluorinated and chlorinated monomers, the poly-
mers may express precisely tunable RIs and the low optical loss at the
same time. Consequently, the waveguide devices were successfully
fabricated by RIE using the HBPIs, and thesewere found to exhibit low
Fig. 9. Near-field pattern of a output-waveguide with TE polarized light at 1550 nm.
and 1.00–1.20
terminated polymers exhibit similar attenuation at the telecom-
munication wavelengths (1.31 and 1.55 m) due to their similar
mm respectively. In addition, the –Cl and –CF3-
m
harmonic (overtone) vibrational absorption [23–26]. These results
show that the –Cl and –CF3 shift the hydrogen–carbon overtone
absorption to longer wavelength and reduced the optical loss at the
telecommunication wavelengths. In addition, both Cl- and CF3-
terminaed polymers exhibit similar attenuation at telecommuni-
cation wavelengths due to the small second harmonic (overtone)
vibrational absorption, which can avoid further increase in optical
loss which bring by introducing C–H bonds.
RIs of the halogen-terminated HBPIs were measured by spectro-
optical propagation losses at 1.55 mm.
scopic ellipsometerat 1.31 and 1.55 mm. TheRI valuesarein therange of
1.5160–1.5309, 1.5807–1.6109 and 1.5863–1.5944 for HG-6F, HG-OD,
and HG-BT series, respectively (Table 3). The HG-6F series with higher
fluorine contents shows relatively lower RIs, because C–F bonds are
much less polarizable than C–H bonds. HG-6F series showed relatively
lower RIs because of their higher fluorine content. In fact, a good linear
relationship is observed between RIs and –CF3 contents as shown in
Fig. 6. Taking the HG-OD series for example, RIs are increased from
1.5807 to 1.6109 as the feed ratio of 3,5-bis(trifluoromethyl)aniline is
increased from 0 to 100%. This linear relationship represented good
control of the RI by varying the feed ratio of terminal monomers.
Compared with conventional copolymerization methods, the modifi-
cation of the end groups by different halogen-containing monomers in
HBPIs allows straightforward control of the RI values.
Acknowledgment
The present work was supported by the National Natural
Science Foundation of China (50773025).
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The in-plane/out-of-plane birefringences (
halogen-terminated HBPIs at 650 nm are summarized in Table 3.
The n values of the HG-6F series are as low as 0.003, which are
Dn) measured for the
D
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can be attributed to the hyperbranched structures which effectively
prevent dense molecular packing and prevent in-plane orientation
of PI chains. Moreover, the –CF3 groups and –O- linkages in the
triamine monomer also lower the birefringence due to the added
flexibility of the polymer chains.
3.3. Fabrication of polymer waveguide devices
´
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heights of the waveguides are in the range of 1–5
mm. Fig. 8 shows
the atomic force microscope (AFM) image of the polymer
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