Synthesis and characterization of highly-fluorinated colorless polyimides derived from 4,4′-((perfluoro-[1,1′-biphenyl]-4,4′-diyl)bis(oxy))bis(2,6-dimethylaniline) and aromatic dianhydrides

Article abstract of DOI:10.1016/j.polymer.2015.09.019

The authors report the synthesis and characterization of a polyimide (PI) series derived from 4,4′-((perfluoro-[1,1′-biphenyl]-4,4′-diyl)bis(oxy))bis(2,6-dimethylaniline) (8FBPODMA). The 8FBPODMA monomer containing a perfluorobiphenyl group with two 2,6-d

Full text of DOI:10.1016/j.polymer.2015.09.019

Polymer 76 (2015) 280e286
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of highly-fluorinated colorless
polyimides derived from 4,40-((perfluoro-[1,10-biphenyl]-4,40-diyl)
bis(oxy))bis(2,6-dimethylaniline) and aromatic dianhydrides
Hyeonuk Yeo, Munju Goh, Bon-Cheol Ku, Nam-Ho You*
Carbon Composite Materials Research Center, Institute of Advanced Composites Materials, Korea Institute of Science and Technology, Eunha-ri san 101,
Bondong-eup, Wanju-gun, Jeollabuk-do, 565-905, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
The authors report the synthesis and characterization of a polyimide (PI) series derived from 4,40-
((perfluoro-[1,10-biphenyl]-4,40-diyl)bis(oxy))bis(2,6-dimethylaniline) (8FBPODMA). The 8FBPODMA
monomer containing a perfluorobiphenyl group with two 2,6-dimethylaniline units connected by an
ether bond was synthesized and polycondensed with several aromatic dianhydrides, including 4,40-
biphthalic anhydride (BPDA), 4,40-oxydiphthalic anhydride (ODPA), pyromellitic dianhydride (PMDA),
and 4,40-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), by the traditional two-step imidization
procedure. All PIs shows high optical transparency, higher than 80% at 500 nm for a thickness of ca.
Article history:
Received 3 July 2015
Received in revised form
2 September 2015
Accepted 8 September 2015
Available online 9 September 2015
Keywords:
Polyimide
Refractive index
High fluorine content
Dielectric constant
Polymer optics
10 mm and good thermal properties. The thermal decomposition temperatures (T10%) of the PIs are in the
range of 507e527 ꢀC and the glass transition temperatures (Tg) are in the range of 280e345 ꢀC. In
addition, the PIs have low refractive indices because of their high fluorine contents. In particular,
8FBPODMAe6FDA shows a very low refractive index (1.5389) and a low birefringence (0.0054) at
637 nm.
© 2015 Published by Elsevier Ltd.
1. Introduction
most widely used methods to reduce the dielectric constant while
lowering moisture absorption and improving solubility and optical
Aromatic polyimides (PIs) have been developed for a wide range
of applications, including automobile, aviation, aerospace, and
electronic industries, owing to their excellent chemical resistance,
thermal stability, and electrical properties [1e10]. However, poly-
imides which have strong absorption in the visible region, a deep
reddish color, and a high dielectric constant, are the main obstacles
that limit any optoelectronic and microelectronic applications
[11,12]. This is because PIs have not only conjugated aromatic
structures but also form charge transfer complexes between elec-
tron donor and electron acceptor moieties in the polymer chains
[13e15]. To overcome these problems of PIs, many effort has been
focused on synthesis of Low-k and colorless PIs, without deterio-
ration of the resulting PIs [16e20]. There are several approaches to
prepare colorless PIs, including the introduction of flexible and
unsymmetrical linkages or noncoplanar monomers and bulky or
fluoro-containing substituents [21,22]. Fluorination is one of the
properties [23e26]. However, the aforementioned procedure re-
sults in a decrease of packing density of the polymers that may
reduce thermal properties. Thus, a breakthrough in the tradeoff
between solubility/transparency and positive properties such as
high thermal properties has remained challenging [27]. The intro-
duction of aromatic diamine containing methyl groups at the ortho
position is common and is considered an effective method to
improve optical transparency and thermal properties, not only
because the introduction of methyl groups at the ortho position of
aromatic diamine can induce a twisted structure between CeN
imide bonds which prevents formation of an intermolecular
charge-transfer complex (CTC), but also because methyl groups at
the ortho position of aromatic diamine which can prevent free
rotation of the CeN imide bond, which is effective in increasing the
glass transition temperature [28]. Moreover, incorporation of
fluorine atoms which have low electric polarity and high electro-
negativity can result in low refractive index, dielectric constant,
optical loss, and moisture absorption characteristics for many ap-
plications, such as electronic coating and in electronic devices
[29e31]. In particular, to reduce dielectric properties and optical
* Corresponding author.
E-mail address: polymer@kist.re.kr (N.-H. You).
http://dx.doi.org/10.1016/j.polymer.2015.09.019
0032-3861/© 2015 Published by Elsevier Ltd.

H. Yeo et al. / Polymer 76 (2015) 280e286
281
opaqueness is necessary for application to transparent
2.3. Monomer synthesis
optoelectronics.
In this study, we design a new diamine which contains eight
fluorine atoms and methyl groups at the ortho position. It was
expected that the methyl groups at the ortho position of the di-
amines would enhance glass-transition temperature (Tg) while
improving optical transparency because the methyl groups can
prevent intermolecular packing and free rotation between CeN
imide bonds in the polymer chains. Moreover, the highly fluori-
nated diamine can endow a low refractive index and low-dielectric
constant for the resulting PIs. The structureeproperty relation-
ships, dielectric constant, and thermal properties of the polyimides
are investigated in detail.
2.3.1. 4,40-((Perfluoro-[1,10-biphenyl]-4,40-diyl)bis(oxy))bis(2,6-
dimethylaniline) (8FBPODMA)
Decafluorobiphenyl (7.83 g, 23.4 mmol), 4-amino-3,5-xylenol
(6.76 g, 49.3 mmol), K2CO3 (5.38 g, 38.9 mmol), and DMF (50 ml)
were mixed, and the solution was stirred at 120 ꢀC overnight.
Following concentration of the solution by evaporation, the crude
products were then diluted with methyl tert-butyl ether and
washed with NaHCO3 (aq) and brine. After drying over MgSO4, the
products were concentrated. After purification by flash chroma-
tography on silica gel (12:1 ¼ CHCl3:EtOAc, Rf ¼ 0.35) and recrys-
tallization in ethanol, the compound 8FBPODMA (11.5 g,
20.3 mmol, 86.7%) was obtained as a white powder. 1H NMR
(600 MHz, CDCl3):
d
¼ 6.70 (s, 4H, AreH), 3.48 (s, 4H, eNH2), 2.18 (s,
12H, AreCH3) ppm. 13C NMR (150 MHz, CDCl3):
d
¼ 149.21, 145.59,
2. Experimental section
143.87, 142.63, 140.90, 139.31, 136.58, 123.00, 116.10, 101.85,
17.88 ppm. MS (p-ESI): m/z: calcd for C28H20F8N2O2 þ Hþ: 569.15;
found: 569.10. Elemental analysis: calcd for C28H20F8N2O2: C 59.16,
H 3.55, N 4.93; found: C 59.10, H 3.62, N 4.85.
2.1. Measurements
1H (600 MHz), 13C (150 MHz) NMR spectra were recorded on an
Agilent 600 MHz Premium COMPACT NMR spectrometer. 1H and
13C NMR spectra were obtained by using tetramethylsilane (TMS)
as an internal standard and CDCl3 as a solvent. Elemental analysis
and mass spectrum analysis were performed at the Center for
University-wide Research Facilities at Jeonbuk National University.
The surface functional groups of the PI films were analyzed by
using a Fourier transform-infrared spectroscopy (FT-IR Spectro-
photometer, Nicolet IS10, USA). Thermogravimetric analysis (TGA)
was carried out with a TA 50 (TA Instruments, USA) under nitrogen
gas flow at a heating rate of 10 ꢀC/min. Glass transition tempera-
tures of the PI films were measured by DSC analysis with a Q 50
(TA Instruments, USA) under nitrogen gas flow at a heating rate of
10 ꢀC/min. Dynamic mechanical thermal analyses (DMA) were
2.3.2. 4,40-((Perfluoro-[1,10-biphenyl]-4,40-diyl)bis(oxy))dianiline
(8FBPOA)
Similarly to the synthetic procedure of 8FBPODMA, the 8FBPOA
was synthesized from decafluorobiphenyl (2.00 g, 5.99 mmol), 4-
aminophenol (1.37 g, 12.6 mmol), K2CO3 (1.08 g, 7.81 mmol), and
DMF (10 ml) as a yellow solid (2.25 g, 4.40 mmol, 73.5%). 1H NMR
(600 MHz, CDCl3):
d
¼ 6.90 (d, J ¼ 8.7 Hz, 4H), 6.66 (d, J ¼ 8.7 Hz,
4H), 3.61 (s, 4H) ppm. MS (APIþ): m/z: calcd for
C
24H12F8N2O2 þ Hþ: 513.08; found: 512.99. Elemental analysis:
calcd for C24H12F8N2O2: C 56.26, H 2.36, N 5.47; found: C 56.26, H
2.47, N 5.34.
2.4. Polymer synthesis
evaluated from PI films (30 mm length, 10 mm wide, and ca. 50 mm
thickness) on a DMA (TA Instruments, DMA Q800, USA) at a
heating rate of 3 ꢀC/min with a load frequency of 1 Hz in air. The
UVevisible spectra were recorded on a JASCO V-670 spectrometer.
Inherent viscosities of the PAA precursors in NMP solution (0.5 g/
dL) at 30 ꢀC were measured using a Malvern Y510 viscometer. The
densities of the films were measured by digital readout density
gradient column (Ray-Ran, UK). The out-of-plane (nTM ¼ nxy) and
in-plane (nTE ¼ nz) refractive indices of PI films were measured
with a Metricon PC-2000 prism coupler with a HeeNe laser light
source (wavelength: 637, 1306.5, and 1549.5 nm). The birefrin-
The PI films were fabricated by a conventional two-step method.
First, a NMP solution of the diamine monomer and a dianhydride
was stirred for 1 day under inert atmosphere to obtain viscous
poly(amic acid) (PAA) precursor. Second, the prepared PAA solution
was cast on the several substrates and thermally annealed at 150 ꢀC
for 30 min, 200 ꢀC for 30 min, 250 ꢀC for 1 h. Subsequently, the films
were used, as is, in optical measurements and thermal analysis after
removal from substrates. The monomer, 8FBPODMA, was poly-
merized with 4 kinds of dianhydrides, PMDA, BPDA, ODPA, and
6FDA, and 8FBPOA was reacted only with PDMA. The polyimides,
8FBPODMAePMDA, eBPDA, eODPA, e6FDA, and 8FBPOAePDMA
were named PMDA, BPDA, ODPA, 6FDA, PMDA′, respectively. For
preparation of the PAA solution, the following polymerization
method was used. 8FBPODMA (0.50 g, 0.88 mmol) and NMP
(2.00 ml) were put into a 10 ml flask under an argon atmosphere.
After the monomer was dissolved by stirring, dianhydride, 6FDA
(0.39 g, 0.88 mmol), was added in the flask. Additionally, NMP
(1.47 ml) was added in order to adjust the monomers content to
20 wt%. The mixture was stirred at room temperature for 1 day to
obtain a viscous PAA solution. Other PAA solutions were prepared
by similar procedure to 6FDA and their inherent viscosities were in
the range of 0.26e1.26 (measurement condition: NMP (0.5 g/dL)
solution, 30 ꢀC).
gence (Dn) was calculated to measure the difference between nTE
and nTM. The average refractive index was calculated according to
the equation: nAV ¼ ½ð2nTE þ nTM Þ=3ꢁ1=2. The dielectric constant
(ε) was calculated by the estimated nAV according to Maxwell's
equation: ε ¼ n2.
2
2
2.2. Materials
Decafluorobiphenyl, 4-amino-3,5-xylenol, and 4-aminophenol
were obtained from TCI, and used without further purification.
Aromatic dianhydrides, 4,40-biphthalic anhydride (BPDA), 4,40-
oxydiphthalic anhydride (ODPA), pyromellitic dianhydride (PMDA),
and 4,40-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)
were purchased from TCI, and used after sublimation. Other
commercially available chemicals were used as received. Dime-
thylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) were
purified by a two-column solid-state purification system (Glass-
contour System, Joerg Meyer, Irvine, CA). All reactions were per-
formed under an argon atmosphere.
3. Results and discussion
3.1. Synthesis and characterization of monomer
The novel diamine monomer, 4,40-((perfluoro-[1,10-biphenyl]-
4,40-diyl)bis(oxy))bis(2,6-dimethylaniline) (8FBPODMA), containing

282
H. Yeo et al. / Polymer 76 (2015) 280e286
a perfluorobiphenyl group having two 2,6-dimethylaniline units
connected by an ether bond was synthesized (Scheme 1). In addition,
4,40-((perfluoro-[1,10-biphenyl]-4,40-diyl)bis(oxy))dianiline
(8FBPOA) was prepared for the fabrication of a reference PI that is a
known compound, but the optical properties of its resulting-PI have
been not properly investigated [32]. The monomers were synthe-
sized by the similar reaction of decafluorobiphenyl and aminophenol
derivatives. The structural analysis of the monomers was carried out
by 1H and 13C NMR spectroscopies, mass measurement, and
elemental analysis. In particular, the remarkable differences seen
between the two types of the monomers are due to whether there
are the methyl substituents around the amine or not. We believe this
is the key determinant for inhibition of the molecular packing in the
resulting-PI that promotes the anisotropy of chain arrangement, the
decrease of molecular free-volume, and the generation of the inter-
molecular charge transfer (CT) complexes. In other words, the
additional methyl groups of 8FBPODMA are able to induce the
improvement of optical properties, including low birefringence, low
refractive index, and high transparency of the resulting-PI.
The 1H and 13C NMR spectra of 8FBPODMA measured in the
CDCl3 solution and the all peaks are assigned (Fig. 1). In 1H NMR
spectrum, the three peaks is observed at 6.71, 3.49, 2.19 ppm. The
peak at 6.71 ppm is assignable to aromatic protons and the peak
of the benzylic protons is shown at 2.19 ppm. In addition, the
broad singlet peak of the amine-protons is observed at 3.49 ppm.
In 13C NMR spectrum, the ten signals of aromatic carbons are
observed in the range of 101e150 ppm due to the carbons
bonded to fluorine atoms having split signals. In addition, the
peak of the benzylic carbons is shown at 17.9 ppm. In NMR
studies, all the observed peaks show good correspondence with
the expected structure. In addition, mass measurement and
elemental analysis supported that the desired compound was
synthesized.
Fig. 1. NMR spectra of 8FBPODMA (a) 1H NMR (b) 13C NMR.
3.2. Preparation and characterization of polyimides
Polyimide films were fabricated by the typical two-step thermal
imidization procedure (Scheme 2) [33]. First, the PAA precursors
were synthesized by polymerization with diamine, 8FBPOA or
8FBPODMA, and dianhydride monomers such as 6FDA, PMDA,
BPDA, ODPA in NMP as shown in Scheme 2. Second, the prepared
PAA solutions were casted or spin-coated on silicon wafers or
quartz plates to adjust the film thickness. Finally, the substrates
were annealed by gradual heat treatment at 150 ꢀC for 30 min,
200 ꢀC for 30 min, 250 ꢀC for 1 h to obtain the resulting PI films
under inert atmosphere. The degrees of polymerization were
determined by measuring inherent viscosities of the PAA and their
inherent viscosities were in the range of 0.26e1.26 at 30 ꢀC in 0.5 g/
dL NMP solution (Table 1).
Thermal conversions of PAAs to PIs were tracked by FT-IR
spectroscopy (Fig. 2). All PI films show a similar waveform and
the characteristic absorption of the PIs originated from imide
moieties. The absorption peaks nearby 1780 cmꢂ1, 1720 cmꢂ1, and
1370 cmꢂ1 are assigned to C]O symmetric stretching, C]O
asymmetric stretching, and CeNeC stretching vibration, respec-
tively. These typical peaks of imide groups clearly indicate that the
PAAs are converting to the expecting PIs.
Scheme 1. Synthesis of 8FBPOA and 8FBPODMA.

H. Yeo et al. / Polymer 76 (2015) 280e286
283
Scheme 2. Synthesis of polyimides.
Table 1
(DSC), and Dynamic Mechanical Analysis (DMA). Analysis results
are summarized in Table 1. First, the thermal stabilities and
decomposition behaviors of the PIs were investigated by TGA
(Fig. 3). The PIs synthesized from 8FBPODMA exhibited similar
thermal decomposition behavior. However, the pyrolysis of PMDA′
begins at slightly higher temperatures and the weight loss is more
rapid than for other PIs. All PIs did not exhibit any weight loss up to
400 ꢀC and the 5%-weight-loss temperatures (T5%) of PMDA that has
lowest T5% among the PIs is 478 ꢀC. The T5%s and T10% s of the PIs
synthesized from 8FBPODMA are in the range of 478e484 ꢀC and
507e527 ꢀC, respectively. In addition, their char yields after py-
rolysis at 800 ꢀC are the more than 50%. The PIs derived from
8FBPODMA show thermal stability of a similar degree to Kapton®,
although that was slightly inferior compared to PMDA′. The supe-
rior thermal stability of PMDA′ might be originated from stabili-
zation effect of intermolecular stacking that would be suppressed
in the PIs derived from 8FBPODMA.
Inherent viscosities of PAA and thermal properties of resulting-PIs.
T5% [ꢀC] T10% [ꢀC] Char yieldd [%]
a
b
c
c
Polymer
[
h
]
[dl/g] Tg [ꢀC]
inh
DSC DMA
6FDA
0.72
0.94
1.26
0.26
0.72
286 303
345 363
300 324
280 302
481
478
484
480
559
507
518
522
527
592
50
53
55
53
48
PMDA
BPDA
ODPA
PMDA′
338
e
a
Inherent viscosity of PAAs in 0.5 g/dL NMP solution at 30 ꢀC.
Tg: glass transition temperature.
T5%, T10%: temperatures at 5% and 10% weight loss, respectively.
Remaining weight percentage after TGA analysis at 800 ꢀC under N2
b
c
d
atmosphere.
Next, the glass transition temperatures (Tgs) were measured by
DSC (Fig. 4) and DMA (Fig. 5). The DSC measurements were
Fig. 2. FT-IR spectra of PI films.
3.3. Thermal properties
The thermal properties of the PIs were investigated by Ther-
moGravimetric Analysis (TGA), Differential Scanning Calorimetry
Fig. 3. TGA curves of PI films (in nitrogen atmosphere, 10 ꢀC/min).

284
H. Yeo et al. / Polymer 76 (2015) 280e286
accomplished using a heating rate of 10 ꢀC/min in a nitrogen at-
mosphere and the DMA were recorded at a heating rate of 3 ꢀC/min
with a load frequency of 1 Hz in air. The Tg data are summarized in
Table 1. The Tgs were determined from the peak temperature of the
tan
d plot in DMA measurement and the values are slightly higher
than those estimated by DSC, but the trend shows a good correla-
tion in both. In addition, the Tgs of the PIs derived from 8FBPODMA
were estimated by both of DSC and DMA in the increasing tem-
perature order ODPA < 6FDA < BDPA < PMDA. The Tg values
measured from DSC and DMA are in the range of 286e345 ꢀC and
302e363 ꢀC, respectively. With increased rigidity of dianhydrides,
the resulting PIs have a correspondingly more rigid polymer main-
chain [34]. Interestingly, in comparison with the PIs synthesized
from 8FBPOA, PMDA has high Tg than PMDA′ and the Tg of 6FDA is
higher than that of the PI prepared from 8FBPOA and 6FDA [32].
These improvements in Tg might be due to the ortho-substituted
methyl groups on the diamine monomer reducing the flexibility of
the chain and increase the chain stiffness in the crystalline state.
Fig. 5. DMA curves of PI films (1 Hz, 3 ꢀC/min): (a) storage modulus (E0), (b) loss
modulus (E00), (c) tan
d
.
3.4. Optical properties
All PIs were visually observed as a light yellow except for PMDA′.
In particular, 6FDA has a substantially transparent color. The rela-
tively transparent color of PI films prepared from 8FBPODMA is due
to the reducing effect of intermolecular stacking by ortho-
substituted methyl groups of the diamine. The UVevis spectroscopy
was carried out to investigate the precise optical transparency of the
synthesized PI films (Fig. 6 and Table 2). The thickness of PI films on
quartz plate was adjusted to ca.10 mm by the control of a casting rate
for UVevis measurement. The cutoff wavelength (lcutoff) is in the
increasing wavelength order 6FDA < ODPA < PMDA ¼ PMDA
′ < BDPA. The lcutoff of 6FDA is the shortest wavelength, which re-
sults in the conclusion that 6FDA is the most transparent among the
synthesized PIs. Because BPDA has a more the planar structure than
the others, its lcutoff is shown to be the most red-shifted. In addition,
the transmittances of the PI films measured at 500 nm were in the
order of 94.2% (6FDA), 85.9% (ODPA), 83.1% (BPDA), 81.6% (PMDA),
66.8% (PMDA′). The transmittance values show a similar trend with
the order of lcutoff, except for PMDA′. This result could be explained
by differences in the molecular structure of PIs. While the PI films
synthesized from 8FBPODMA, 6FDA, ODPA, PMDA, and BDPA, have
ortho-substituted methyl groups on the amine that would inhibit the
stacking of polymer chain and increase steric hindrance to interfere
with formation of the CTcomplex, and because PMDA′ does not have
the substituents it would be tinged with yellow in the procedure of
thermal imidization. After all, considering that they are aromatic
Fig. 6. Wavelength dependent transmittance of PI films (film thickness: ca. 10 mm).
Table 2
Optical properties of PI films.
Polymer
Fluorine content [%]
lcutoff a [nm]
Transmittanceb [%]
6FDA
28.28
20.25
18.39
18.04
21.82
310
352
362
339
352
94.2
81.6
83.1
85.9
66.8
PMDA
BPDA
ODPA
PMDA′
a
Cut-off wavelength.
Transmittance at 500 nm and film thickness: ca. 10 mm.
b
polyimides, the PI films derived from 8FBPODMA exhibit excellent
transparency in visible region. In particular, 6FDA achieves consid-
erably high transparency without any bleaching process.
The refractive indices of the PI films were measured by prism
coupling method to investigate the effects of fluorine atoms on the
refractive index, because of the possibility of the PIs having a low
polarizability due to their high fluorine atom content [35]. The in-
plane (nxy), out-of-plane (nz), average refractive indices (nav), in-
plane/out-of-plane birefringences (
infinite wavelength (n∞), and dispersion coefficients of refractive
index (D) of the PI films (thickness < 10 m) are summarized in
Dn), refractive indices at the
m
Table 3. The refractive indices of the PIs are in the range of
1.5389e1.6168 measured at 637 nm and those values are signifi-
cantly lower than that of a conventional PI, such as Kapton [PMDA-
ODA] and Upilex-S [BPDA-PDA] [33]. In addition, the nav of the PIs at
637 nm are in the order of 6FDA < PMDA < ODPA < BPDA < PMDA′,
which tended to match the high order of fluorine content except for
PMDA′. Especially, 6FDA containing the fluorine atoms on the
Fig. 4. DSC curves of PI films (in nitrogen atmosphere, 10 ꢀC/min).

H. Yeo et al. / Polymer 76 (2015) 280e286
285
Table 3
Refractive indices and densities of PI films.
Polymer
Density at 23 ꢀC [g/ml]
Refractive indicesa
Db [ꢃ104]
b
nxy
nz
nav
Dn
n∞
6FDA
1.4225
1.3869
1.3841
1.3824
1.4887
1.5407
1.5818
1.5983
1.5868
1.6314
1.5353
1.5666
1.5917
1.5818
1.5872
1.5389
1.5767
1.5961
1.5851
1.6168
0.0054
0.0152
0.0066
0.0050
0.0442
1.5193
1.5459
1.5554
1.5546
1.5911
0.7912
1.2033
1.6140
1.2546
1.0157
PMDA
BPDA
ODPA
PMDA′
a
Measured at 637 nm, see Section Measurements.
b
n∞ is refractive index at the infinite wavelength and D is dispersion coefficient of refractive index obtained from the fitting by the simplified Cauchy formula
(nl ¼ n∞ þ D
l
ꢂ2).
dianhydride unit exhibited extremely low refractive index and the
nav value is 1.5389. In general, a refractive index decreases in the
film of a low density because a low density causes large free vol-
ume. However, there was no significant difference in densities of
the synthesized PIs compare to those of commercial PIs. These
result means that low refractive indices of the PI films are achieved
by the low polarizability of fluorine atom. Hongham et al. reported
a series of PIs of low refractive indices synthesized from 6FDA and a
highly fluorinated diamine [36]. However, because the resulting-
PAA solutions of Hongham's PIs had an extremely low viscosity
because the polycondensation had been inhibited by steric in-
fluences and the electronic effects of excessively substituted fluo-
rine on diamine monomers, they had no choice but to prepare the
PI films by solid-state chain extension methods. By contrast, 6FDA
is producible using the traditional PI preparation method. In
addition, 6FDA has a similar refractive index despite of a lower
fluorine content compared to Hongham's PIs. In the comparison
between PMDA and PMDA′ synthesized from the same dianhy-
of 1.5193e1.5911 and the order of the values is same as that
measured at specific wavelengths. In addition, all PIs show low D
values because there is a linear relationship between n∞ and D [37].
In particular, 6FDA has the lowest n∞ and D value due to the effect
by large free volume of their highly-fluorinated backbone.
4. Conclusions
We synthesized a novel diamine monomer, 8FBPODMA, con-
taining
a
perfluorobiphenyl group containing two 2,6-
dimethylaniline units connected by an ether bond. A series of PIs
were prepared from 8FBPODMA and the aromatic dianhydrides
PDMA, ODPA, BPDA, and 6FDA by a classical two-step thermal
imidization. All PIs exhibited high thermal stability, i.e. Tg (>280 ꢀC)
and T10% (>500 ꢀC). In comparison with the PIs derived from
8FBPOA, the PIs from 8FBPODMA are more optically transparent
and have lower refractive indices and birefringences due to struc-
tural hindrance of intermolecular packing by their ortho-
substituted methyl groups. In particular, the PI from 8FBPODMA
and 6FDA are almost completely visually transparent (Trans-
mittance: 94.2% at 500 nm) and exhibited a very low refractive
index (1.5389) and birefringence (0.0054) at 637 nm, which origi-
nate from steric hindrances and the atomic effect of a highly-
fluorinated structure. These PIs are good candidates as a substrate
for advanced optoelectronics. In addition, this research provides a
novel strategy from the perspective of molecular design for opto-
electronic applications.
dride, PMDA has lower nav and Dn than those of PMDA′. The ortho-
substituted methyl groups of 8FBPODMA make the resulting PI
have a large molar volume which causes structural suppression of
intermolecular packing. As a result, PMDA exhibits lower nav and
Dn than PMDA′ and the resulting-PIs of 8FBPODMA have relatively
low nav and n compared to other PIs. In particular, the low bire-
D
fringence values in the PI films are observed in the range of
0.0054e0.0152, which have been achieved by low optical anisot-
ropy due to their large free volume due to substituents, such as
ortho-substituted methyl groups and perfluorinated phenylene
units.
Author contributions
The wavelength dispersions of the average refractive indices
measured at 637, 1306.5, and 1549.5 nm of the PI films (Fig. 7). In
addition, the solid lines are fitted with the simplified Cauchy for-
mula. The infinite refractive indices (n∞) of the PIs are in the range
Hyeonuk Yeo and Nam-Ho You contributed equally to this work.
Funding sources
This work was supported by a grant from the Korea Institute of
Science and Technology (KIST) institutional program and a grant
from the Ministry of Trade, Industry & Energy of Korea (2MR3560).
References
[1] D.P. Erhard, F. Richter, C. Bartz, H.W. Schmidt, Fluorinated aromatic poly-
imides as high-performance electret materials, Macromol. Rapid Commun. 36
(2015) 520e527.
[2] Y. Liao, J. Weber, C.F. Faul, Fluorescent microporous polyimides based on
perylene and triazine for highly CO2-selective carbon materials, Macromole-
cules 48 (2015) 2064e2073.
[3] H. Bi, S. Chen, X. Chen, K. Chen, N. Endo, M. Higa, K. Okamoto, L. Wang, Poly
(sulfonated phenylene)-block-polyimide copolymers for fuel cell applications,
Macromol. Rapid Commun. 30 (2009) 1852e1856.
[4] E.M. Maya, A.E. Lozano, J.G. de la Campa, J. de Abajo, Soluble polyamides and
polyimides functionalized with benzo-15-crown-5-pendant groups, Macro-
mol. Rapid Commun. 25 (2004) 592e597.
[5] P.K. Tapaswi, M.C. Choi, S. Nagappan, C.S. Ha, Synthesis and characterization of
highly transparent and hydrophobic fluorinated polyimides derived from
perfluorodecylthio substituted diamine monomers, J. Polym. Sci. A Polym.
Fig. 7. Wavelength dispersion of the refractive indices of PI films.

286
H. Yeo et al. / Polymer 76 (2015) 280e286
Chem. 53 (2015) 479e488.
unsymmetrical diamine containing two trifluoromethyl groups, J. Polym. Sci.
A Polym. Chem. 51 (2013) 4413e4422.
[23] E.R. Francis, Polyamide-acids and Polyimides from Hexafluoropropylidine
Bridged Diamine, U.S. Patent 3,356,648, 1964.
[6] H.-G. Wang, S. Yuan, D.-L. Ma, X.-L. Huang, F.-L. Meng, X.-B. Zhang, Tailored
aromatic carbonyl derivative polyimides for high-power and long-cycle so-
dium-organic batteries, Adv. Energy Mater. 4 (2014) 1301651.
[7] H.J. Yen, C.J. Chen, G.S. Liou, Flexible multi-colored electrochromic and volatile
polymer memory devices derived from starburst triarylamine-based electro-
active polyimide, Adv. Funct. Mater. 23 (2013) 5307e5316.
[8] Y.-G. Ko, W. Kwon, H.-J. Yen, C.-W. Chang, D.M. Kim, K. Kim, S.G. Hahm,
T.J. Lee, G.-S. Liou, M. Ree, Various digital memory behaviors of functional
aromatic polyimides based on electron donor and acceptor substituted tri-
phenylamines, Macromolecules 45 (2012) 3749e3758.
[9] J.-G. Liu, Y. Nakamura, T. Ogura, Y. Shibasaki, S. Ando, M. Ueda, Optically
transparent sulfur-containing polyimide e TiO2 nanocomposite films with
high refractive index and negative pattern formation from poly (amic acid) e
TiO2 nanocomposite film, Chem. Mater. 20 (2007) 273e281.
[10] M.-C. Choi, J. Wakita, C.-S. Ha, S. Ando, Highly transparent and refractive
polyimides with controlled molecular structure by chlorine side groups,
Macromolecules 42 (2009) 5112e5120.
[11] M. Banach, R. Friend, H. Sirringhaus, Influence of the molecular weight on the
thermotropic alignment of thin liquid crystalline polyfluorene copolymer
films, Macromolecules 36 (2003) 2838e2844.
[12] T. Kurosawa, T. Higashihara, M. Ueda, Polyimide memory: a pithy guideline
for future applications, Polym. Chem. 4 (2013) 16e30.
[13] B.R. Bikson, Y.F. Freimanis, Cause of the discolouration of aromatic polyimides,
Polym. Sci. USSR 12 (1970) 81e85.
[14] R.A. Dine-Hart, W.W. Wright, A study of some properties of aromatic imides,
Makromol. Chem. 143 (1971) 189e206.
[15] S. Ando, T. Matsuura, S. Sasaki, Coloration of aromatic polyimides and elec-
tronic properties of their source materials, Polym. J. 29 (1997) 69e76.
[16] Y.-J. Lee, J.-M. Huang, S.-W. Kuo, J.-S. Lu, F.-C. Chang, Polyimide and polyhedral
oligomeric silsesquioxane nanocomposites for low-dielectric applications,
Polymer 46 (2005) 173e181.
[17] Y. Eguchi, E. Unsal, M. Cakmak, Critical phenomenon during drying of semi-
aromatic, transparent and soluble polyimide cast films: real-time observation
of birefringence and other integrated parameters, Macromolecules 46 (2013)
7488e7501.
[18] C.-C. Chang, W.-C. Chen, Synthesis and optical properties of polyimide-silica
hybrid thin films, Chem. Mater. 14 (2002) 4242e4248.
[19] H.-J. Ni, J.-G. Liu, Z.-H. Wang, S.-Y. Yang, A review on colorless and optically
transparent polyimide films: chemistry, process and engineering applications,
J. Ind. Eng. Chem. 28 (2015) 16e27.
[24] R. Reuter, H. Franke, C. Feger, Evaluating polyimides as lightguide materials,
Appl. Opt. 27 (1988) 4565e4571.
[25] S. Ando, T. Matsuura, S. Sasaki, Perfluorinated polyimide synthesis, Macro-
molecules 25 (1992) 5858e5890.
[26] T. Matsuura, S. Ando, S. Sasaki, F. Yamamoto, Polyimides derived from 2,20-
bis(trifluoromethyl)-4,40-diaminobiphenyl. 4. Optical properties of fluorinated
polyimides for optoelectronic components, Macromolecules 27 (1994)
6665e6670.
[27] Y. Guan, D. Wang, G. Song, G. Dang, C. Chen, H. Zhou, X. Zhao, Novel soluble
polyimides derived from 2, 20-bis [4-(5-amino-2-pyridinoxy) phenyl] hexa-
fluoropropane: Preparation, characterization, and optical, dielectric proper-
ties, Polymer 55 (2014) 3634e3641.
[28] Z. Hu, M. Wang, S. Li, X. Liu, J. Wu, Ortho alkyl substituents effect on solubility
and thermal properties of fluorenyl cardo polyimides, Polymer 46 (2005)
5278e5283.
[29] X.D. Li, Z.X. Zhong, J.J. Kim, M.H. Lee, Novel photosensitive fluorinated poly
(arylene ether) having zero birefringence, Macromol. Rapid Commun. 25
(2004) 1090e1094.
[30] L. Tao, H. Yang, J. Liu, L. Fan, S. Yang, Synthesis and characterization of highly
optical transparent and low dielectric constant fluorinated polyimides, Poly-
mer 50 (2009) 6009e6018.
[31] T. Matsuura, Y. Hasuda, S. Nishi, N. Yamada, Polyimide derived from 2, 20-bis
(trifluoromethyl)-4, 40-diaminobiphenyl. 1. Synthesis and characterization of
polyimides prepared with 2, 20-bis (3, 4-dicarboxyphenyl) hexafluoropropane
dianhydride or pyromellitic dianhydride, Macromolecules 24 (1991)
5001e5005.
[32] A.C. Misra, G. Tesoro, G. Hougham, S.M. Pendharkar, Synthesis and properties
of some new fluorine-containing polyimides, Polymer 33 (1992) 1078e1082.
[33] H. Yeo, J. Lee, M. Goh, B.-C. Ku, H. Sohn, M. Ueda, N.-H. You, Synthesis and
characterization of high refractive index polyimides derived from 2, 5-bis (4-
aminophenylenesulfanyl)-3, 4-ethylenedithiothiophene and aromatic dia-
nhydrides, J. Polym. Sci. A Polym. Chem. 53 (2015) 944e950.
[34] D.-J. Liaw, K.-L. Wang, Y.-C. Huang, K.-R. Lee, J.-Y. Lai, C.-S. Ha, Advanced
polyimide materials: syntheses, physical properties and applications, Prog.
Polym. Sci. 37 (2012) 907e974.
[35] M.D. Struble, M.T. Scerba, M. Siegler, T. Lectka, Evidence for a symmetrical
fluoronium ion in solution, Science 340 (2013) 57e60.
[20] M.-C. Choi, Y. Kim, C.-S. Ha, Polymers for flexible displays: from material se-
lection to device applications, Prog. Polym. Sci. 33 (2008) 581e630.
[21] S.D. Kim, S. Lee, J. Heo, S.Y. Kim, I.S. Chung, Soluble polyimides with tri-
fluoromethyl pendent groups, Polymer 54 (2013) 5648e5654.
[36] G. Hougham, G. Tesoro, J. Shaw, Synthesis and properties of highly fluorinated
polyimides, Macromolecules 27 (1994) 3642e3649.
[37] S. Ando, Y. Watanabe, T. Matsuura, Wavelength dependence of refractive
indices of polyimides in visible and near-IR regions, Jpn. J. Appl. Phys. 41
(2002) 5254e5258.
[22] S.D. Kim, S.Y. Kim, I.S. Chung, Soluble and transparent polyimides from