Optically transparent sulfur-containing semi-alicyclic polyimide with high refractive index

Article abstract of DOI:10.1246/cl.2010.392

Sulfur-containing semi-alicyclic polyimides (PIs) with high refractive indices up to 1.7272 at 632.8 nm have been prepared by the Michael polyaddition of sulfur-containing bismaleimides (SBMIs) with 4, 4'-thiobis(benzenethiol) (TBT). These PIs also showed high transparency in the visible region, low birefrin-gence (Δn), and a high glass transition temperature.

Full text of DOI:10.1246/cl.2010.392

doi:10.1246/cl.2010.392
392
Published on the web March 13, 2010
Optically Transparent Sulfur-containing Semi-alicyclic Polyimide with High Refractive Index
Yu Nakagawa, Tomohito Ogura, Tomoya Higashihara, and Mitsuru Ueda*
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-H-120 O-okayama, Meguro-ku, Tokyo 152-8552
(Received January 28, 2010; CL-100090; E-mail: mueda@polymer.titech.ac.jp)
O
O
Sulfur-containing semi-alicyclic polyimides (PIs) with high
refractive indices up to 1.7272 at 632.8 nm have been prepared
by the Michael polyaddition of sulfur-containing bismaleimides
(SBMIs) with 4,4¤-thiobis(benzenethiol) (TBT). These PIs also
showed high transparency in the visible region, low birefrin-
gence (¦n), and a high glass transition temperature.
H
N
H
N
O
H2
N
NH2
O
O
DMAc
+
R
R
OH
HO
O
O
O
(CH3CO)2O,
CH3COONa
O
S
N
N
O
(3SDA)
(APTT)
O
R:
S
S
S
R
S
S
SBMI
SBMI-1 from 3SDA
SBMI-2 from APTT
S
High refractive index polymers have been developed in
recent years for optoelectronic applications,1-3 such as encapsu-
lants for organic light-emitting diode devices (OLED),4 charge-
coupled devices (CCDs), and complementary metal oxide
semiconductor (CMOS) image sensors (CISs).5 The general
approach to enhance the refractive index in polymers is the
introduction of substituents with high molar refraction, low
molar volume, or high density, according to the Lorentz-Lorenz
equation.6 Thus, the introduction of heavy halogen atoms,7
sulfur atoms,8-12 and metal atoms13 with high molar refractions
is effective to increase the refractive indices of polymers.
Our previous work on high-n PIs revealed that the
incorporation of sulfide-containing moieties could effectively
increase the refractive indices and decrease the ¦n values of the
PIs.11,14,15 However, the optical transparency of sulfur-contain-
ing PI films in the visible light region is not sufficiently high
and should be improved. To remedy this problem, we recently
reported optically transparent sulfur-containing PI-TiO2 nano-
composite film with a high refractive index and negative pattern
formation from poly(amic acid) (PAA)-TiO2 nanocomposite
film,16 where semi-alicyclic PIs were prepared from alicyclic
dianhydrides, 1,2,3,4-cyclobutanetetracarboxylic dianhydride
(CBDA) and 1,2,4,5-cyclohexanetetracarboxylic dianhydride
(CHDA), and the sulfide-linked aromatic diamines, 4,4¤-thio-
bis[(p-phenylenesulfanyl)aniline] (3SDA) and 2,7-bis(4-amino-
phenylenesulfanyl)thianthrene (APTT), by a two-step polymer-
ization procedure via soluble PAA precursors. Although the PI
derived from CBDA and APTT showed a high refractive index
of 1.7203 at 632.8 nm, it exhibited significantly large ¦n
(0.0220) due to the planar structure of the CBDA moiety. Thus,
it was not adopted to be combined with TiO2 nanoparticles. The
refractive indices of nanocomposites can be approximately
Scheme 1. Synthesis of SBMIs.
O
O
HS
SH
N
O
N
+
O
S
R
SBMI
TBT
m-cresol
pyridine
O
O
S
S
N
N
n
O
O
S
R
SAPI
SAPI-1 from SBMI-1
SAPI-2 from SBMI-2
Scheme 2. Synthesis of SAPIs.
In this study, we report an optically transparent sulfur-
containing semi-alicyclic polyimide (SAPI) with a high refrac-
tive index as an organic matrix. The SAPIs were prepared by
the Michael polyaddition of sulfur-containing bismaleimides
(SBMIs) with 4,4¤-thiobis(benzenethiol) (TBT) because bismale-
imide as a monomer can add more sulfur content in a repeating
unit compared to alicyclic dianhydrides such as CBDA or
CHDA.
As a monomer having a high sulfur content with an alicyclic
ring structure, an SBMI was selected, and two SBMIs were
prepared via two steps from maleic anhydride with 3SDA and
APTT, as shown in Scheme 1.
The structures of SBMIs were characterized on the basis of
1
elemental analysis, as well as FT-IR and H NMR spectroscopy
(Supporting Information20).
estimated by the equation of ncomp = ºpnp + ºorgnorg
,
17,18 where
TBT was chosen as the counter monomer. The Michael
polyaddition is a useful method for preparing polymers with
high molecular weights from dithiols and bismaleimides. The
polymerization of SBMI and TBT was carried out at 100 °C for
SBMI-1 and at room temperature for SBMI-2, respectively, for
several hours in the presence of a catalytic amount of pyridine as
a basic catalyst, as shown in Scheme 2.
The polymerization proceeded in the homogeneous state
and provided SAPIs with number and weight average molecular
weights (Mn, Mw) of (12000, 30000) (SAPI-1), and (23000,
64000) (SAPI-2), respectively. The polymers thus obtained were
n
comp, np, and norg are the refractive indices of the nano-
composite, nanoparticle, and organic matrix, respectively. ºp
and ºorg are the volume fractions of the nanoparticles and
organic matrix, respectively. Thus, it can be concluded from the
equation that, in order to achieve a definite ncomp value with a
definite type of nanoparticle, the higher the value of norg is, the
lower the value of ºp. This is important for the design of high-n
nanocomposites for optical applications because an overload of
the nanoparticles often increases the optical loss and decreases
the processability of the organic matrix.19
Chem. Lett. 2010, 39, 392-393
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

393
Table 1. Optical properties of the SAPI films
Sc
/wt %a
b
c
e
nTE
nTM
¦nd
nav
SAPI-1
SAPI-2
22.82
25.71
1.7124 1.7088 0.00361 1.7112
1.7288 1.7240 0.00478 1.7272
aSulfur content. bIn-plane refractive index. cOut-of-plane
refractive index. dSee Measurement section in Supporting
2
Information.20 eAverage refractive index:
nav = [(2nTE
+
nTM2)/3]1/2
.
are all higher than the nav value of previously reported SAPI
derived from CHDA and APTT (1.6994).16 The ¦n of the PI
films ranges from 0.00361 to 0.00478, which is suitable for
optoelectronic applications.
Figure 1. TGA and DSC curves of SAPIs (solid line: SAPI-1,
dashed line: SAPI-2).
In summary, the SAPIs were successfully prepared by the
Michael polyaddition of SBMIs with TBT, and exhibited high
refractive indices in the range of 1.7112-1.7272 with high
thermal stability (up to 340 °C), high transparency (>400 nm),
and low ¦n in the range of 0.00361-0.00478. These SAPIs can
be good candidates as an organic matrix for sulfur-containing
PI-TiO2 nanocomposite film with a high refractive index.
References and Notes
1
2
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K. Kitamura, K. Okada, N. Fujita, Y. Nagasaka, M. Ueda, Y.
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T. Nakamura, H. Fujii, N. Juni, N. Tsutsumi, Opt. Rev. 2006,
13, 104.
Y.-G. Ju, G. Almuneau, T.-H. Kim, B.-W. Lee, Jpn. J. Appl.
Phys. 2006, 45, 2546.
M. Suwa, H. Niwa, M. Tomikawa, J. Photopolym. Sci.
Technol. 2006, 19, 275.
S. Ando, T. Fujigaya, M. Ueda, Jpn. J. Appl. Phys. 2002, 41,
L105.
T. Matsuda, Y. Funae, M. Yoshida, T. Takaya, J. Macromol.
Sci., Pure Appl. Chem. 1999, A36, 1271.
N.-H. You, Y. Suzuki, D. Yorifuji, S. Ando, M. Ueda,
Macromolecules 2008, 41, 6361.
N.-H. You, Y. Suzuki, T. Higashihara, S. Ando, M. Ueda,
Polymer 2009, 50, 789.
Figure 2. UV-vis spectra of SAPI films (thickness: 13.0 ¯m
(SAPI-1), 10.6 ¯m (SAPI-2).
3
4
5
6
7
8
9
white solids and soluble in chloroform, cyclohexanone, DMF, 1-
methyl-2-pyrrolidinone, and heated £-butyrolactone. A tough
and flexible film was readily obtained by casting a cyclo-
hexanone solution of the polymers onto a fused silica substrate
followed by drying in vacuo.
The structures of SAPIs were characterized by FT-IR and
1H NMR spectroscopy. The IR spectra of the polymers demon-
strated characteristic absorptions of an imide moiety located at
¹1
ca. 1781 and 1716 cm (asymmetric and symmetric stretching
vibrations of the carbonyl group, see Figure S1 in Supporting
1
Information20). The resonances observed by H NMR spectra of
10 N.-H. You, N. Fukuzaki, Y. Suzuki, Y. Nakamura, T.
Higashihara, S. Ando, M. Ueda, J. Polym. Sci., Part A:
Polym. Chem. 2009, 47, 4428.
11 J.-G. Liu, Y. Nakamura, Y. Shibasaki, S. Ando, M. Ueda,
Macromolecules 2007, 40, 4614.
12 C. A. Terraza, J.-G. Liu, Y. Nakamura, Y. Shibasaki, S. Ando,
M. Ueda, J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 1510.
13 T. Sawada, S. Ando, Chem. Mater. 1998, 10, 3368.
14 J.-G. Liu, Y. Nakamura, Y. Shibasaki, S. Ando, M. Ueda,
Polym. J. 2007, 39, 543.
15 J.-G. Liu, Y. Nakamura, Y. Shibasaki, S. Ando, M. Ueda, J.
Polym. Sci., Part A: Polym. Chem. 2007, 45, 5606.
16 J.-G. Liu, Y. Nakamura, T. Ogura, Y. Shibasaki, S. Ando, M.
Ueda, Chem. Mater. 2008, 20, 273.
17 M. Weibel, W. Caseri, U. W. Suter, H. Kiess, E. Wehrli, Polym.
Adv. Technol. 1991, 2, 75.
18 H. Althues, J. Henle, S. Kaskel, Chem. Soc. Rev. 2007, 36,
1454.
19 C. Paquet, P. W. Cyr, E. Kumacheva, I. Manners, Chem.
Commun. 2004, 234.
20 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
SAPIs were all assigned (Supporting Information,20 Figure S2).
Signals at 2.8, 3.4, and 4.6 ppm, assignable to the methine and
methylene (cis and trans) groups of succinimide, respectively,
and those at 7.2 ppm, assinable to the vinylene group were not
observed. Furthermore, the atomic composition of the polymer
was confirmed by elemental analysis.
The thermal properties of SAPI were evaluated by ther-
mogravimetry (TG) and differential scanning calorimetry
(DSC). As shown in Figure 1 (TG and DSC), SAPI-1 and
SAPI-2 exhibit 5% weight loss temperatures at 349 and 348 °C
under nitrogen, and glass transition temperatures (Tgs) at 138
and 162 °C, respectively.
Optical transparency of the PI films is a crucial factor for
their use in optical applications. Figure 2 displays the optical
transmission spectra of SAPIs. Both SAPIs films exhibit very
high transparency (>87%) in the visible region (wavelengths:
400-800 nm).
As shown in Table 1, the refractive index (nav) at 632.8 nm
increases from a minimum of 1.7112 (SAPI-1) to a maximum of
1.7272 (SAPI-2) by increasing the sulfur content. These values
Chem. Lett. 2010, 39, 392-393
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/