Synthesis of highly refractive and transparent polyimides derived from 4,4′-thiobis[2,″,6″ -dimethyl-4″ -(p-phenylenesulfanyl)aniline]

Article abstract of DOI:10.1002/pola.23817

New sulfur-containing aromatic diamines with methyl groups at the ortho position of amino groups have been developed to prepare highly refractive and transparent aromatic polyimides (Pls) in the visible region. All aromatic Pls derived from 4,4′-thiobis[2″-methyl-4″-(p-phenylenesulfanyl)aniline (2), 4,4′thiobis[2,″6″-dimethyl-4″-(p-phenylenesulfanyl)aniline (5), and aromatic dianhydride, 4,4′-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (6) were prepared via a two-step polycondensation. All PIs showed good thermal properties, such as 10% weight loss temperature in the range of 497-500 °C and glass transition termperatures above 196°C. In addition, the Pls showed good optical properties, such as optical transparency above 75% at 450 nm with a 10-μm film thickness, high refractive indices ranging from 1.7135 to 1.7301, and small in-plane/out-of-plane birefringences between 0.0066 and 0.0076.

Full text of DOI:10.1002/pola.23817

Synthesis of Highly Refractive and Transparent Polyimides Derived from
4,40-Thiobis[200,600-dimethyl-400-(p-phenylenesulfanyl)aniline]
NAM-HO YOU, TOMOYA HIGASHIHARA, SHINJI ANDO, MITSURU UEDA
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, H-120, O-okayama, Meguro-ku,
Tokyo 152-8552, Japan
Received 2 October 2009; accepted 3 November 2009
DOI: 10.1002/pola.23817
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: New sulfur-containing aromatic diamines with methyl
groups at the ortho position of amino groups have been devel-
oped to prepare highly refractive and transparent aromatic poly-
imides (PIs) in the visible region. All aromatic PIs derived from
4,40-thiobis[200-methyl-400-(p-phenylenesulfanyl)aniline (2), 4,40-
thiobis[2,00600-dimethyl-400-(p-phenylenesulfanyl)aniline (5), and ar-
omatic dianhydride, 4,40-[p-thiobis(phenylenesulfanyl)]diphthalic
anhydride (6) were prepared via a two-step polycondensation. All
PIs showed good thermal properties, such as 10% weight loss
temperature in the range of 497–500 ꢀC and glass transition tem-
peratures above 196 ꢀC. In addition, the PIs showed good optical
properties, such as optical transparency above 75% at 450 nm
with a 10-lm film thickness, high refractive indices ranging from
1.7135 to 1.7301, and small in-plane/out-of-plane birefringences
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between 0.0066 and 0.0076. 2009 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 48: 656–662, 2010
KEYWORDS: high-performance polymers; highly refractive
index; high transparency; low birefringence; polyimides; step-
growth polymerization; transparency
INTRODUCTION In recent years, much attention has been
paid to high-refractive index polymers for their practical
application in advanced optoelectronic fabrications,1–3 such
as encapsulants for organic light-emitting diode devices,4
charge-coupled devices, and complementary metal oxide
semiconductor image sensors.5 According to the Lorentz-Lor-
enz equation, the refractive index of polymers depends on
several factors such as the molar refraction, molar volume,
or density.6,7 Thus, the introduction of heavy halogen atoms,
sulfur atoms, and metal atoms with high molar refractions is
effective to increase the refractive indices of polymers.8,9 Ar-
omatic polyimides (PIs) are one of the most important
classes of advanced polymers, which possess various out-
standing properties, such as excellent thermal and chemical
resistance, low dielectric constants, high mechanical proper-
ties, and inherently high refractive indices. Therefore, much
attention has been devoted to optical device applications
of PIs.
applications. The considerable coloration of aromatic PIs orig-
inates from the formation of intermolecular or intramolecular
charge-transfer complexes (CTCs) between the electron-
donating diamine moiety and the electron-accepting dianhy-
dride moiety.16 To decrease the coloration of conventional PIs,
several approaches, such as the introduction of bulky sub-
stituents with methyl groups at ortho position of diamine,
fluoro-containing substituents, alicyclic moieties, and nonco-
planar or unsymmetrical structures in the polymer main
chains, have been developed for electrooptic device, semicon-
ductor, and optical device applications.17–26 However, the
aforementioned procedures generally result in decrease in the
refractive indices.27 Thus, the breakthrough for a trade-off
between the refractive index and transparency is a challeng-
ing subject. Quite recently, we have developed a new aromatic
PI from 4,40-thiobis[(p-phenylenesulfanyl)aniline] (3SDA) and
4,40-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride con-
taining thioether linkages.28 Although this PI is one of the
highest refractive index aromatic PIs, the optical transparency
in the visible light region is not enough for optical device
application. Therefore, we have focused on the synthesis of
highly transparent aromatic PIs while maintaining high refrac-
tive indices.
Recently, we have developed highly refractive and transparent
PIs derived from aromatic dianhydrides and aromatic dia-
mines containing sulfur atoms with thioether linkages.10–15
All PIs showed high refractive indices, high transparency, and
low birefringence. Although the refractive index of polymers
can be effectively improved by using the aforementioned
methods, optical polymers require several other functions,
such as low in-plane/out-of-plane birefringence (Dn) and high
transparency in the visible light region for optical device
In this study, new aromatic diamines with methyl groups at the
ortho position of amino groups were prepared to develop high-
transparent aromatic PIs in the visible region. Figure 1 shows
the calculated absorption spectra of N-phenylphthalimide
Additional Supporting Information may be found in the online version of this article. Correspondence to: M. Ueda (E-mail: ueda.m.ad@m.titech.ac.jp)
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Synthesis of 4,40-Bis(300-methyl-400-nitrophenylsulfanyl)
diphenyl Sulfide (1)
To a three-necked 100-mL flask was added a mixture of p-
chloronitrobenzene (7.55 g, 0.044 mol), 4,40-thiobisbenzene-
thiol (5.01 g, 0.02 mol), anhydrous K2CO3 (6.91 g, 0.050
mol), and DMF (50 mL). The mixture was heated with stir-
ꢀ
ring at 130 C for 12 h. The solvent was removed by vacuum
distillation. The mixture was poured into water. The precipi-
tate was collected by filtration, washed with water, and dried
in vacuum at room temperature for 24 h. The crude product
was recrystallized from 2-methoxyethanol to afford pale yel-
low crystals, 1. Yield: 7.8 g (75.0%); m.p. 115 ꢀC (DSC peak
temperature).
1H NMR (300 MHz, CDCl3-d6/TMS, ppm): 7.86–7.88 (d,2H),
7.40–7.43 (d, 4H), 7.35–7.38 (d, 4H), 7.13 (s, 2H), 7.06–7.10
(d, 2H), 2.54 (S, 6H). 13C NMR (300 MHz, CDCl3-d6/TMS,
ppm): 136.93, 134.72, 134.57, 132.41, 132.00, 131.97,
131.83, 126.50, 126.47, 125.87, 20.85. Anal. Calcd. for
FIGURE 1 Calculated absorption spectra of N-phenylphthali-
mide and its derivatives having methyl groups at the ortho
positions of CAN imide bond.
C26H20N2O4S3 (520.64): C, 59.98; H, 3.87. Found: C, 60.47; H,
4.09.
Synthesis of 4,40-Thiobis[200-methyl-400-(p-
phenylenesulfanyl)
aniline] (2)
and its derivatives having methyl groups at the ortho position.
As clearly indicated by the optimized torsion angles (x), the
introduction of methyl groups induces highly twisted confor-
mations (x ¼ 42ꢀ ! 66ꢀ ! 73ꢀ) in the phenylimides, which
should prevent intermolecular CTC formation. As seen in the
figure, the calculated absorption peaks originating from p–p*
transitions from the occupied molecular orbitals (MOs) located
at the amine moiety to the unoccupied MOs at the anhydride
moiety (263–279 nm) are significantly reduced, and thus the
optical transmission near the absorption edges (ꢁ300 nm)
should be improved. Based on these results, this article
presents the synthesis and properties of optically transparent
PIs by the polymerization of 4,40-thiobis[200-methyl-400-(p-phe-
nylenesulfanyl)aniline (2) and 4,40-thiobis[2,00600-dimethyl-400-
(p-phenylenesulfanyl)aniline (5) with aromatic dianhydrides
such as 4,40-[p-thiobis(phenylsulfanyl)]diphthalic anhydride
(6). The PIs exhibited relatively high refractive indices in the
range of 1.7135–1.7301 at 633 nm, high thermal stability (10%
weight loss temperatures > 450 ꢀC), low birefringences in the
range of 0.0066–0.0076, and good transmittance higher than
75% at 450 nm. The structural effects of the PIs on the thermal
properties, optical transparency, and refractive indices are
discussed.
A 100-mL three-necked flask was charged with a mixture of
1 (5.21 g, 0.01 mol), ethanol (50 mL), and a catalytic amount
of 10% palladium on activated carbon (0.60 g). The reaction
mixture was refluxed and then hydrazine monohydrate (25
mL) was added. The reaction mixture was refluxed for 12 h.
Then, the hot mixture was filtered to remove the catalyst
and the filtrate was cooled to room temperature. The pre-
cipitated white crystals were filtered out, washed with cold
ethanol, and dried under vacuum at room temperature for
ꢀ
6 h to afford 2. Yield: 3.43 g (74.5%); m.p. 117 C (DSC peak
temperature).
1H NMR (300 MHz, DMSO-d6, ppm): 2.05 (s, 6H), 5.32 (s,
4H), 6.66–6.68 (d, 2H), 6.67–6.95 (d, 4H), 7.05–7.06 (d, 2H),
7.08–7.10 (d, 2H), and 7.13–7.16 (d, 4H). 13C NMR (300
MHz, DMSO-d6/TMS, ppm): 148.6, 140.5, 137.3, 134.4, 131.7,
131.3, 127.0, 122.7, 115.1, 114.4, and 117.6. Anal. Calcd. for
C26H24N2S3 (460.68): C, 67.95; H, 5.25. Found: C, 67.95; H,
5.24.
Synthesis of 4-Bromo-2,6-dimethyl-1-nitrobenzene (3)
To a neat 1-bromo-3,5-dimethylbenzene (20 g, 0.108 mol)
was added dropwise fuming HNO3 (96%, 4.48 mL, 6.8 g, 1
equiv) at ꢂ15 ꢀC, and the mixture was stirred for 6 h at
room temperature. After the reaction, the obtained solution
was poured into water and extracted with methylene chlo-
ride. The solvents were evaporated to give a mixture of
4-bromo-2,6-dimethyl-1-nitrobenzene and 6-bromo-2,4-di-
methyl-1-nitrobenzene. The two isomers were separated by
column chromatography (ethyl acetate/hexane, vol % 1:15)
and recrystallized from ethanol to give 3 as white crystals.
EXPERIMENTAL
Materials
4,40-Thiobisbenzenethiol, 1-bromo-3,5-dimethylbenzene, and
5-chloro-2-nitrotoluene were purchased from TCI, Japan. Ni-
tric acid (fuming, 96%) and cesium carbonate were pur-
chased from Wako, Japan. N,N-Dimethylformamide (DMF)
and N-methyl-2-pyrrolidone dehydrated (NMP) were used as
received. All other chemicals were purchased from TCI and
Wako, Japan. Compound 6 was synthesized according to our
previous works.28
ꢀ
Yield: 3.73 g (15%); m.p. 67 C (DSC peak temperature).
1H NMR (300 MHz, DMSO-d6, ppm): d ¼ 7.28 (s, 2H), 2.28
(s, 6H).
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Synthesis of 4,40-Bis(300,500-dimethyl-400-nitrophenyl-
sulfanyl)diphenyl Sulfide (4)
tion from PAA solutꢀions in an oven for 1 h each step at 80,
ꢀ
120, 150, 200, 250 C and 30 min at 300 C, respectively, as
To a 50-mL round flask was charged with 3 (3.5 g, 15.2
mmol), 4,40-thiobisbenzenethiol (1.73 g, 6.92 mmol), cesium
carbonate (2.27 g, 15.23 mmol), and DꢀMSO (30 mL). The
mixture was heated with stirring at 140 C for 12 h under a
nitrogen atmosphere. Then, the reaction mixture was filtered.
The solvent was evaporated, and the crude solids were puri-
fied by column chromatography using methylene chloride
and recrystallized from 2-methoxyethanol to give 4 as pale
shown in Scheme 2. The film of PI-2 was prepared by a simi-
lar process from PAA solutions.
PI-1
[g]inh: 0.68 g dLꢂ1 (from PAA-1), FTIR (film, cmꢂ1): 1778,
1720 (C¼¼O imide), 1095 (CASAC). ELEM. ANAL. Calcd. for
[C54H36N2O4S6]: C, 66.91%; H, 3.74%; N, 2.89%. Found: C,
66.90%; H, 3.65%; N, 2.58%.
ꢀ
green crystals. Yield: 2.32 g (61.4%); m.p. 120 C (DSC peak
temperature).
PI-2
[g]inh: 0.32 g dLꢂ1 (from PAA-2), FTIR (film, cmꢂ1): 1774,
1724 (C¼¼O imide), 1099 (CASAC). ELEM. ANAL. Calcd. for
[C56H40N2O4S6]: C, 67.44%; H, 4.04%; N, 2.81%. Found: C,
67.04%; H, 3.88%; N, 2.30%.
1H NMR (300 MHz, CDCl3-d6, ppm): d ¼ 2.26 (s, 12H), 7.00
(s, 4H), 7.33 (s, 8H). 13C NMR (CDCl3-d6, ppm): d ¼ 139.4,
135.8, 133.4, 133.2, 132.2, 131.3, 129.9, and 18.0. Anal.
Calcd. for C28H24N2O4S3; (548.70): C, 61.29; H, 4.41. Found:
C, 61.28; H, 4.48.
Measurements
The NMR spectra were recorded on a BRUKER DPX-300S
1
Synthesis of 4,40-Thiobis[2,00600-dimethyl-400-
spectrometer at the resonant frequencies at 300 MHz for H
and at 75 MHz for 13C nuclei using CDCl3 and DMSO-d6 as
the solvents. The 13C DEPT (distortionless enhancement by
polarization transfer) experiment was carried out using
DMSO-d6 as solvent. The FTIR spectra were obtained by a
Horiba FT-120 Fourier transform spectrophotometer. Inher-
ent viscosity was measured with an Ubbelohde-viscometer
with a 0.5 g dLꢂ1 NMP solution at 30 ꢀC. The ultraviolet–
visible (UV–vis) spectra were performed on a Hitachi U-3210
spectrophotometer. The transmittance of PI films was eval-
uated in the wavelengths range of 250–800 nm at 10-lm
film thickness. Elemental analyses were carried out on a
Yanaco MT-6 CHN recorder elemental analysis instrument.
Thermogravimetric analysis (TGA) was estimated by a Seiko
TG/DTA 6300 under nitrogen atmosphere at a heating rate
of 10 ꢀC minꢂ1. Differential scanning calorimetry (DSC) was
estimated by using a Seiko DSC 6300 at a heating rate of
10 ꢀC minꢂ1. Dynamic mechanical thermal analyses (DMA)
were performed on PI film specimens (length: 30 mm, width:
10 mm, and thickness: 30–60 lm) by usingꢂa1 Seiko DMS
(p-phenylenesulfanyl)aniline] (5)
A 250-mL three-necked flask fitted with a magnetic stirrer, a
thermometer, and a dropping funnel was charged with a mix-
ture of 4 (2 g, 36.4 mmol), ethanol (15 mL), and a catalytic
amount of 10% palladium on activated carbon (0.20 g). The
reaction mixture was heated to reflux and then hydrazine
monohydrate (6 mL) diluted with ethanol (15 mL) was
added dropwise over a period of 1.5 h. After the addition
was completed, the reaction system was refluxed for 24 h.
Then, the hot mixture was filtered to remove the catalyst
and the filtrate was cooled to room temperature. The pre-
cipitated white crystals were filtered out, washed with cold
ethanol, and dried under vacuum at 80 ꢀC overnight to
afford 5. Yield: 1.25 g (70.6%); m.p. 73.3 ꢀC (DSC peak
temperature).
FTIR (KBr, cmꢂ1): 1H NMR (300 MHz, DMSO-d6, ppm): d ¼
2.10 (s, 12H), 4.99 (s, 4H), 6.93–6.96 (d, 4H), 7.00 (s, 4H),
7.10–7.14 (d, 4H). 13C NMR (DMSO-d6, ppm): d ¼ 146.3,
140.5, 135.2, 131.7, 131.3, 127.0, 122.3, 114.2, and 18.0.
Anal. Calcd. for C28H28N2S3; (488.73): C, 68.81; H, 5.77.
Found: C, 68.47; H, 5.66.
ꢀ
6300 instrument at a heating rate of 2 C min with a load
frequency of 1 Hz in air. The glass transition temperature
values (Tg) of the PIs were obtained as the peak temperature
of the loss modulus (E00) plot. The in-plane (nTE) and out-of-
plane (nTM) refractive indices of PI films were carried out
using a prism coupler (Metricon, model PC-2000) equipped
with a He–Ne laser (wavelength: 633 nm) and a half-wave-
plate in the light path. The in-plane/out-of-plane birefringen-
ces (Dn) were estimated as a difference between nTE and
Polyimide Synthesis
The PIs were prepared by the general polycondensation pro-
cedure via poly(amic acid) (PAA) precursors, followed by
thermal imidization at elevated temperatures. A typical poly-
merization procedure can be illustrated by the synthesis of
PI-1 as follows. To a solution of 5 (0.4607 g, 1.00 mmol)
in dehydrated NMP (2.0 mL) was added 6 (0.5426 g,
1.00 mmol) under nitrogen atmosphere. An additional NMP
(2.0 mL) was added into the reaction mixture and stirred at
room temperature for 24 h to obtain a viscous PAA solution.
The solid content of reaction system was adjusted to 20 wt
% by weight. The PAA solution was spin-coated on a fused
silica substrate or a silicon wafer, and the thickness of PIs
films was controlled to be ꢁ10 lm for UV–vis and FTIR
measurements. The specimen was adjusted from 30 to 100
lm prepared by casting onto the glass substrate for thermal
properties. The PI films were prepared by thermal imidiza-
nTM, and the average refractive indices were calculated
according to the equation: nAV ¼ [(2nTE þ nTM2)/3]1/2
.
2
RESULTS AND DISCUSSION
Synthesis of Diamines 2 and 5
Diamines 2 and 5 were prepared by a two- and three-step
procedure, respectively, as shown in Scheme 1. Compound 3
was prepared by nitration of 3,5-dimethylbromobenzene
with fuming nitric acid. Then, a mixture of ortho- and para-
substituted nitro compounds was separated by column chro-
matography. Nucleophilic aromatic substitution of 3 with
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ARTICLE
SCHEME 1 Synthetic route for the preparation of diamine (2) and diamine (5).
4,40-thiobisbenzenethiol in the presence of cesium carbonate
produced 4, which was converted to diamine 5 by catalytic
reduction. On the other hand, diamine 2 was prepared by
similar procedure according to the literature.28 The struc-
tures of 2 and 5 were characterized by FTIR and NMR spec-
troscopy. The characteristic nitro absorptions of 1 and 4 at
1504, 1519, 1334, and 1361 cmꢂ1 completely disappeared,
and the absorptions of primary amino and thioether groups
are distinctly observed at 3436–3385 and 1288 cmꢂ1
,
respectively (see Supporting Information).
1
The H and 13C NMR spectra of diamines 2 and 5 are shown
in Figures 2 and 3 with assignments of all peaks, respec-
tively. For example, the signals at 5.31 and 2.05 ppm in
Figure 3(a) are assigned to the amino and methyl groups of
2, respectively. On the other hand, aromatic protons contain-
ing an amino moiety are observed at 7.05–7.10 and 6.67
ppm. In the 13C NMR spectrum [Fig. 2(b)], 11 carbon signals,
FIGURE 2 NMR spectra of diamine (2) (a) 1H NMR; (b) 13C NMR
FIGURE 3 NMR spectra of diamine (5) (a) 1H NMR; (b) 13C NMR
and DEPT-135.
and DEPT-135.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
silica under nitrogen. After thermal conversion from PAAs to
PIs, the characteristic IR absorption peaks of the imide moi-
ety, such as 1774 (as, C¼¼O), 1727 (s, C¼¼O), and 1365 cmꢂ1
(CAN), are observed, as shown in Figure 4. In addition, an
absorption peak of aromatic thioether groups (ArASAAr) is
observed at 1099 cmꢂ1. Furthermore, the elemental analysis
also supported the formation of the expected PIs.
Thermal Properties of PIs
The thermal properties of the PIs, such as 10% weight loss
temperatures (Td10) and glass transition temperatures (Tg),
were evaluated by TGA, DSC, and DMA measurements. The
results are summarized in Table 1. All PIs exhibit good ther-
10
ꢀ
mal stability, such as the Td values at 460–505 C (Fig. 5).
The Tgs of the PIs estimated by DSC (see Supporting Infor-
mation) and DMA (Fig. 6) show similar values. All PIs show
high Tgs in the range of 179–218 in the order of PI-2 > PI-1
> PI-3. This trend is related to the structural effects of the
diamines. The results indicate that the introduction of methyl
groups at the ortho position of amino groups is effective to
increase the Tgs due to the increase in the chain rigidity of
PI-1 and PI-2, that is, the methyl groups significantly restrict
the bond rotation at the CAN imide bond. Figure 6 illus-
trates the variations in the storage modulus (E0), loss modu-
lus (E00), and loss factor (tan d) measured by DMA at a heat-
ing rate of 2 ꢀC minꢂ1 under air. The modulus remains
constant or slightly decreased on heating over a wide tem-
perature range below the Tgs. Above the Tg, the modulus
dramatically decreases, as shown in Figure 6(a). The Tgs
determined from the peak temperatures of E00 were close to
the Tgs estimated from DSC.
SCHEME 2 Synthesis of polyimides.
which accord well with the expected structure, are observed.
Among these peaks, six quaternary carbon signals are
assigned by DEPT-135 measurement. In addition, the struc-
tures of TBPSA (2) and DTBPSA (5) were characterized by
elemental analysis.
Optical Properties
Figure 7 shows the optical transmission spectra of PI films
with PMDA-ODA as the reference PI (ꢁ10 lm thick). All PIs
show much better transparency with the cutoff wavelengths
(kcutoff) at 402–375 nm compared with PMDA-ODA. The
transmittances of PI-1 and PI-2 measured at 425 nm are sig-
nificantly higher than that of PI-3 derived from 6 and 4,40-
thiobis[(p-phenylenesulfanyl)aniline].28 This is attributable to
the ortho methyl groups in the aromatic diamines, which
induce significant steric hindrance around the CAN imide
bond. This is supported by the fact that the dihedral angles
at the CAN bonds in the optimized geometry of the models
are 66ꢀ, 73ꢀ, and 42ꢀ for PI-1, PI-2, and PI-3, respectively, as
Synthesis and Characterization of Polyimides
All PIs were synthesized by a two-step polycondensation of
diamines such as 2 and 5 with 6 via soluble PAA precursors,
followed by thermal imidization at elevated temperatures
(Scheme 2). The inherent viscosities of the PAA solutions are
in the range of 0.32–0.68 g dLꢂ1, as shown in Table 1. Flexi-
ble PI films were prepared by thermal imidization of the cor-
responding PAA films cast onto glass substrates and fused
TABLE 1 Thermal Properties of the Polyimides
Tg (ꢀC)c
a
d
PI
[g]inh of PAA (g dLꢂ1
)
Filmb
DSC
DMA
T10% (ꢀC)
PI-1
PI-2
PI-3e
a
0.68
0.32
Pale yellow
Pale yellow
Pale yellow
197
218
179
194.3
216.7
173.8
474
475
500
Measured at a concentration of 0.5 g dLꢂ1 of NMP solution at 30 ꢀC.
Color at the thickness of ꢁ10 lm.
T
10%, temperatures at 10% weight loss.
6-3SDA (ref. 29).
d
e
b
c
Tg, glass transition temperature.
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ARTICLE
FIGURE 4 FTIR spectra of PI films.
shown in Figure 1. The significantly distorted conformations
in PI-1 and PI-2 effectively reduce the formation of intramo-
lecular and intermolecular CTCs in the main chains.
The in-plane (nTE) and out-of-plane (nTM) refractive indices
of the PI films were measured at 633 nm in the range of
1.7160–1.7505 and 1.7084–1.7437, as listed in Table 2. The
nTE values are slightly higher than those of nTM, which indi-
cates that the PI chains are preferentially oriented in the
film plane.29 The nav of PI films is in the range of 1.7135–
1.7482 in the order of PI-3 > PI-1 > PI-2. The highest nav
observed for PI-3 could have originated from two factors.
First, the introduction of methyl groups into the diamines
reduces the sulfur content, which is the primary factor of
the high refractive indices of sulfur-containing PIs. Second,
the distorted conformation around the CAN imide bond low-
ers the intermolecular packing of PIs, which increases the
molar volumes of PIs.
FIGURE 6 DMA curves of PI-1 and PI-2 (1 Hz, 2 ꢀC minꢂ1): (a)
modulus and (b) tan d.
The relatively low Dn value is related to the highly flexible
and phenyl-sulfanyl-phenyl linkages in the main chains. The
results indicate that aromatic diamines with ortho methyl
groups are effective to improve the optical transparency of
the resulting PIs while maintaining high refractive indices
and low birefringences of the PIs.
All of the PIs show low birefringence (Dn) in the range of
0.0066–0.0076 compared with PMDA-ODA as reference PI.
FIGURE 5 TGA curves of PI films (in nitrogen atmosphere, 10
ꢀC minꢂ1).
FIGURE 7 UV–vis spectra of PI films (film thickness: ꢁ10 lm).
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical Properties of the PI Films
Refractive Indices and Birefringence at 633 nmd
a
b
c
PI
Sc (%)
kcutoff (nm)
d
(lm)
nTE
nTM
nav
Dn
PI-1
19.9
19.3
20.4
0
400
375
402
415
12.9
14.2
9.3
1.7323
1.7160
1.7505
1.7207
1.7256
1.7084
1.7437
1.6427
1.7301
1.7135
1.7482
1.6950
0.0066
0.0076
0.0068
0.0780
PI-2
PI-3e
Ref-PIf
9.2
a
f
Sulfur content.
Cutoff wavelength.
PMDA-ODA with the chemical structure as follows.
b
c
Film thickness for refractive index measurement.
Measured at 632.8 nm, see Measurements section.
6-3SDA (ref. 29).
d
e
CONCLUSIONS
12 You, N.-H.; Fukuzaki, N.; Suzuki, Y.; Nakamura, Y.; Higashi-
hara, T.; Ando, S.; Ueda, M. J Polym Sci Part A: Polym Chem
2009, 47, 4428–4434.
Aromatic diamines containing methyl groups at the ortho
positions of amino groups were newly synthesized to enhance
the optical transparency of the resulting PIs. The PIs derived
from 2 and 5 showed higher transparency compared with 6-
3SDA, while maintaining relatively high refractive indices. At
the same time, the glass transition temperatures of the PI
films were increased and exhibited low birefringences. The
experimental results indicate that the introduction of methyl
groups at the ortho position of aromatic diamines is an effec-
tive way to improve the optical transparency of PIs. These
high optical transparency, high refractive index, and high ther-
mal stability factors make the resulting PIs promising candi-
dates for advanced optical device applications.
13 Liu, J. G.; Nakamura, Y.; Shibasaki, Y.; Ando, S.; Ueda, M.
Macromolecules 2007, 40, 4614–4620.
14 Terraza, C. A.; Liu, J. G.; Nakamura, Y.; Shibasaki, Y.; Ando, S.;
Ueda, M. J Polym Sci Part A: Polym Chem 2008, 46, 1510–1520.
15 You, N.-H.; Nakamura, Y.; Suzuki, Y.; Higashihara, T.; Ando, S.;
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19 Li, F.; Fang, S.; Ge, J. J.; Honigfort, P. S.; Chen, J.-C.; Harris,
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