Transparent Aromatic Polyimides Derived from Thiophenyl-Substituted Benzidines with High Refractive Index and Small Birefringence

Article abstract of DOI:10.1021/acs.macromol.5b00432

2,2′-Bis(thiophenyl)benzidine (BTPB) and 2,2′-bis(4-chlorothiophenyl)benzidine (BCTPB) were synthesized via a benzidine rearrangement reaction of the corresponding hydrazobenzene derivatives obtained after the reduction of (3-nitrophenyl)(phenyl)sulfane a

Full text of DOI:10.1021/acs.macromol.5b00432

Article
pubs.acs.org/Macromolecules
Transparent Aromatic Polyimides Derived from Thiophenyl-
Substituted Benzidines with High Refractive Index and Small
Birefringence
Pradip Kumar Tapaswi, Myeon-Cheon Choi, Keuk-Min Jeong, Shinji Ando, and Chang-Sik Ha*
†
†
†
‡
,†
†
Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea
‡
Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1-E4-5, Meguro-Ku, Tokyo
152-8552, Japan
*
S Supporting Information
ABSTRACT: 2,2′-Bis(thiophenyl)benzidine (BTPB) and
,2′-bis(4-chlorothiophenyl)benzidine (BCTPB) were synthe-
2
sized via a benzidine rearrangement reaction of the
corresponding hydrazobenzene derivatives obtained after the
reduction of (3-nitrophenyl)(phenyl)sulfane and (4-
chlorophenyl)(3-nitrophenyl)sulfane, respectively. Transpar-
ent polyimides (PIs) with high refractive indices and small
birefringences as well as good thermomechanical stabilities
were synthesized by the conventional two-step thermal
polycondensation of BTPB and BCTPB with five different
dianhydrides in N-methyl-2-pyrrolidone. 3,3′,4,4′-Biphenylte-
tracarboxylic dianhydride (BPDA)/BCTPB PI, a fully aromatic
and non-fluorinated PI, with a highly distorted noncoplanar
conformation of the main chain was transparent and colorless
with a transmittance of ∼83% and >88% at 450 nm and in the visible region, respectively. In addition, all six non-fluorinated
sulfur-containing aromatic PIs exhibited high average refractive indices (1.7112−1.7339), and small birefringences (0.0007−
0
.0022) at 633 nm, because of the high atomic polarizability of sulfur and the rigid bulky molecular structure of BTPB and
BCTPB. The more effective steric effect caused by bulky 4-chlorothiophenyl in BCTPB improved the optical transparency but
decreased the average refractive indices of BCTPB-based PIs compared to BTPB-based PIs.
INTRODUCTION
physical properties, high inherent refractive index, and excellent
mechanical properties. On the other hand, the large
birefringence and deep reddish-yellow coloration of conven-
tional aromatic PIs need to be addressed first before they can
be suitable for optical applications. The intra- and intermo-
lecular charge-transfer (CT) interactions between the electron-
donating diamine and electron-accepting aromatic dianhydride
are considered the main reason behind the deep color of
conventional aromatic PIs. A range of structural modifications,
such as the introduction of bulky substituents with methyl
groups at the ortho position of diamine, fluorine-containing
substituents, alicyclic moieties, and noncoplanar or unsym-
metrical structures in the polymer main chains, have been made
■
A high refractive index (n), small birefringence (Δn), and high
optical transparency along with good thermal-mechanical
stability are the basic requirements of a polymer that can be
used in various optical devices, such as organic light-emitting
1
diodes (OLEDs), microlens components for charge coupled
devices (CCD), and high-performance CMOS image sensors
2
−4
(
CISs), etc. According to the Lorentz−Lorenz equation, the
refractive index of optical polymers can be enhanced
considerably by introducing a substituent with high molar
refraction, low molar volume, high aromatic contents, or high
5
density. Therefore, a range of functional moieties with high
molar refractions, such as aromatic rings, heavy halogen (Cl, Br,
and I) atoms, sulfur atoms, and metal atoms, have been
introduced in polymers to enhance the refractive indices.
Among the various substituents, sulfur with a high atomic
refraction is one of the most effective candidates. Many sulfur-
18
to improve the transparency of conventional PIs. In many
6
−11
cases, however, these modifications result in a decrease in
19
refractive index. Recently, the Ueda group developed a series
of sulfur-containing aromatic PIs suitable for a range of optical
8
−10,20−25
12
applications.
These PIs showed very high refractive
containing polymers, including poly(methacrylates), episul-
1
3
14
fide, polyurethanes, and polyimides, have been used for
15−17
advanced integrated optical applications.
imides (PIs) are among the most successful polymers used in
advanced optical fabrications owing to their good thermal and
Aromatic poly-
Received: March 1, 2015
Revised: April 30, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b00432
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Scheme 1. Synthetic Scheme of Benzidine Monomers (BTPB and BCTPB)
indices in the range 1.74−1.77 and a low birefringence of less
than 0.0093; however, most of the corresponding 10−20 μm
thick PI films were not totally colorless, exhibiting pale to bright
yellowish colors. This might limit their optical applications;
hence, the quest for colorless and high refractive aromatic PIs
still continues.
chlorine atoms in BTPB and in BCTPB is expected to increase
the refractive indices of BTPB- and BCTPB-based PIs.
EXPERIMENTAL SECTION
Materials and Characterization. Thiophenol (97%), 4-chlor-
othiophenol (97%), 1-iodo-3-nitrobenzene (99%), and cyclobutane-
■
A previous study reported that noncoplanar 2,2′,3,3′-
tetrachlorinated benzidine reacts with aromatic and alicyclic
dianhydrides to form colorless PIs with a high refractive index,
1
,2,3,4-tetracarboxylic dianhydride (CBDA, ≥94%) were purchased
from Aldrich and used as received. Pyromellitic dianhydride (PMDA,
97%), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA, Aldrich,
97%), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA,
Aldrich, 99%), and 3,3′,4,4′-benzophenone-3,3′,4,4′-tetracarboxylic
dianhydride (BTDA, Aldrich, 96%) were dried at 120 °C for 1 day
under vacuum prior to use. N-Methyl-2-pyrrolidone (NMP), which
was obtained from the Aldrich Chemical Co., was dried with calcium
26
small birefringence, and good thermal-mechanical stability.
This result proved three things: first, the high atomic
polarizability of chlorine atoms can increase the refractive
index of the polymer; second, even the presence of bulky
chlorine atoms at the 2,2′,3,3′ position of benzidine, the rigid-
rod structure of the polymer main chain remains intact; and
third, the optical transparency of the synthesized PIs can be
improved by the judicious introduction of bulky substituents at
the 2,2′-position of benzidine. Therefore, the introduction of
both sulfur and halogen atoms to aromatic diamine or
dianhydride monomers may lead to the synthesis of high
refractive PIs. On the other hand, to weaken the color of the PI,
it is essential to introduce some bulky substituents in the
diamine or dianhydride monomers to minimize the inter-
molecular CT interaction between the PI chains. Care needs to
be taken regarding the size of the bulky substituent because the
introduction of a very bulky substituent will create a large free
volume in the polyimide backbone; hence, packing will not be
dense enough to induce a high refractive index in the PIs. This
paper reports the synthesis of two new diamine monomers,
hydride (CaH ) and then distilled under reduced pressure. All other
2
chemicals used in this study were supplied by Aldrich and used as
1
13
received. The H and C NMR spectra were recorded on a Varian
Unity Plus-400 NMR spectrometer. Viscosity measurements were
performed using Cannon-Ubbelohde No. 100 and 200 viscometers at
30 °C after dissolving the PAAs in NMP. Elemental analysis was
conducted with an elemental analyzer (EA, Vario EL, Elemental
Analysen Systeme). The attenuated total reflection−Fourier transform
infrared (ATR-FTIR, JASCO FT/IR-4100) spectra were obtained with
−1
3
2 scans per spectrum at a 2 cm resolution. A robust single reflection
accessory (JASCO ATR Pr0450-S) with a germanium IRE (n = 4.0,
incidence angle = 45°) was used for the ATR measurements. To
examine the thermal stability of the PIs, thermogravimetric analysis
(TGA) was performed under nitrogen using a TGA Q50 Q Series
thermal analyzer at a heating rate of 10 °C/min from room
temperature to 800 °C. Measurements of the glass transition
temperature (T ) were taken using a DSC Q 100 TA Instruments at
g
a heating rate of 10 °C/min under a nitrogen atmosphere. The
transparency of the PI films was measured from the ultraviolet−visible
spectra recorded from one accumulation on a OPTIZEN 3220UV
spectrometer optimized with a spectral width of 200−800 nm, a
resolution of 0.5 nm, and a scanning rate of 200 nm/min. The
thickness of each film was ∼20 μm. The excitation and fluorescent-
emission spectra of the PI films were characterized using a Hitachi F-
4500 fluorescence spectrophotometer with a resolution of 0.2 nm and
2
,2′-bis(thiophenyl)benzidine (BTPB) and 2,2′-bis(4-
chlorothiophenyl)benzidine (BCTPB), with a bulky thiophen-
yl/4-chlorothiophenyl side group at the 2,2′-position of the
benzidine system. The synthesis and properties of ten new PIs
of BTPB and BCTPB with five different dianhydrides are also
discussed. The 2,2′-disubstitution (thiophenyl/4-chlorothio-
phenyl) in BTPB and in BCTPB forced the phenyl rings of
the biphenyl system into a non-coplanar rod-like twisted
geometry, which is expected to hinder the close packing of the
BTPB/BCTPB-based PIs chains and reduce both the intra- and
intermolecular CT interactions. The CT interactions are
considered the main reason behind the deep color of
conventional aromatic PIs. Therefore, a decrease in CT
interactions is expected to improve the optical transparency
of the synthesized PI films. Again, the presence of sulfur and
−1
a scanning rate of 240 nm min . The wide-angle X-ray diffraction
WXAD) measurements of the pulverized samples were conducted at
room temperature in the reflection mode using a Rigaku diffractometer
Model Rigaku Miniflex). The Cu Kα radiation (λ = 1.54 Å) source
was operated at 50 kV and 40 mA. The 2θ scan data were collected at
(
(
0
.01° intervals over the range 1.5−50°, at a scan rate of 0.58(2θ)/min.
Dynamic mechanical analysis (DMA) was conducted using a
DMA2980 (TA Instron) instrument. The storage modulus (E) and
tan δ were determined as the sample was subjected to temperature
B
DOI: 10.1021/acs.macromol.5b00432
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scan mode at a programmed heating rate of 10 °C/min and a
frequency of 1 Hz. The coefficient of thermal expansion (CTE) was
measured using a TA Instruments thermal mechanical analyzer
was cooled to 0 °C in an ice bath. Subsequently, 150 mL of an EtOH−
HCl (1:1) solution was slowly added to the diazane solution over an
approximately 2 h period. Once the addition was completed, the ice
bath was removed, and the mixture solution was stirred at room
temperature for 3 h to allow a white solid to precipitate. The
precipitated salt was filtered, washed several times with EtOH, and
dissolved in 200 mL of water. The aqueous solution was made alkaline
by a 10% aqueous sodium hydroxide solution, and the organic content
was extracted with DCM (3 × 75 mL). The DCM extract was
evaporated to dryness to give a white solid. The benzidine monomer
was purified further by recrystallization from a 50% EtOAc solution in
hexane. The product yield was 18.1 g (60.0%); off-white crystalline
(
TMA) with a fixed load of 0.05 N and a heating rate of 10 °C
−1
min . The in-plane (n ) and out-of-plane (nTM) refractive indices of
TE
the PI films were measured using a prism coupler (Metricon, PC-
2
000) at wavelengths of 404.0, 532.0, 632.8, and 829.0 nm at room
temperature. The mean refractive indices (n ) were calculated using
av
the equation nav2 = (2nTE2 + nTM2)/3. The in-plane/out-of-plane
birefringence (Δn) was calculated using the equation Δn = n − nTM.
TE
The thicknesses of the PI films were measured using a sensing-pin type
surface profilometer (DEKTAK-III). The single crystal X-ray
diffraction data were collected on a CCD area detector, Bruker
SMART diffractometer (λ = 0.710 73 Å). The structures were solved
1
solid; mp 152−154 °C (EtOAc:hexane = 1:1). H NMR (400 MHz,
CDCl ): δ 7.33 (dd, J = 8.0 and 1.6 Hz, 4H, Ph−H), 7.25−7.19 (m,
3
27
using the SHELXS program and were refined by full-matrix least-
6H, Ph−H), 6.98 (d, J = 8.0 Hz, 2H, Ph−H), 6.52 (dd, J = 8.4 and 2.4
28
squares on F 2 using the SHELXL program.
Hz, 2H, Ph−H), 6.46 (d, J = 2.4 Hz, 2H, Ph−H), 3.56 (s, 4H, NH )
2
(Figure S4). 13C NMR (100 MHz, CDCl ): δ 146.3, 137.6, 135.8,
Synthesis. Benzidine derivatives containing thiophenyl and 4-
chlorothiophenyl side groups were synthesized through the benzidine
rearrangement reactions of the corresponding hydrazobenzenes
obtained after the reduction of (3-nitrophenyl)(phenyl)sulfane and
3
132.2, 131.9, 131.4, 129.0, 127.0, 116.5, 113.4 (Figure S5). Elemental
analysis: Calculated for C H N S : C, 71.96; H, 5.03; N, 6.99.
2
4
20
2 2
−1
Found: C, 71.79; H, 5.13; N, 7.05. FTIR (KBr, cm ): 3454, 3420,
3331, 1621, 1594, 1464, 1272, 814, and 732.
(
4-chlorophenyl)(3-nitrophenyl)sulfane, respectively, as shown in
(4-Chlorophenyl)(3-nitrophenyl)sulfane (1b). 4-Chlorothiophenol
(21.69 g, 150 mmol), 1-iodo-3-nitrobenzene (37.35 g, 150 mmol),
K CO (41.4 g, 300 mmol), CuI (4.29 g, 22.5 mmol), and PEG-400 (5
2
3
2
3
4
54 mmol), CuI (8.64 g, 45.4 mmol), and PEG-400 (5 mL) were
added to approximately 200 mL of dry N,N-dimethylformamide
DMF) placed in a 500 mL round-bottomed flask in a N atmosphere.
mL) were added to approximately 200 mL of dry DMF taken in a 500
mL round-bottomed flask in a N atmosphere. The reaction mixture
2
(
was heated at 120 °C for 12 h. The contents were then cooled to room
temperature and filtered to give a wine red colored solution. The
filtrate was concentrated by a rotary evaporator and then poured
2
The reaction mixture was heated at 120 °C for 12 h, cooled to room
temperature, and filtered to give a wine red colored solution. The
filtrate was concentrated using a rotary evaporator and then poured
slowly into 400 mL of ice cooled H O to produce a white precipitate.
2
slowly into 400 mL of ice cooled H O and extracted with
The precipitate was collected after filtration and washed repeatedly
2
dichloromethane (DCM) (3 × 100 mL) to give a dark brown liquid
with cold H O. The off-white solid was dried under vacuum at 40 °C
2
after vacuum evaporation. This liquid was used directly for the next
for 12 h. The product was pure enough to use directly in the next step.
1
step without further purification. Yield: 49.9 g (95%); dark brown
Yield: 36.7 g (92%); off-white solid; mp 63−64 °C. H NMR (400
1
liquid. H NMR (400 MHz, CDCl ): δ 8.02−7.97 (m, 3H, Ph−H),
MHz, DMSO-d ): δ 8.11 (bd, J = 8.0 Hz, 1H, Ph−H), 7.98 (t, J = 2.0
3
6
7
.50−7.45 (m, 3H, Ph−H), 7.42−7.37 (m, 3H, Ph−H) (Figure S1).
C NMR (100 MHz, CDCl ): δ 148.7, 143.4, 140.6, 134.2, 133.4,
Hz, 1H, Ph−H), 7.70 (d, J = 8.0 Hz, 1H, Ph−H), 7.65 (t, J = 8.0 Hz,
1
3
13
3
1H, Ph−H), 7.55−7.50 (m, 4H, Ph−H) (Figure S6). C NMR (100
129.9, 129.7, 128.9, 127.5, 123.1, 120.9 (Figure S2). Elemental
MHz, CDCl ): δ 148.7, 139.7, 135.2, 134.5, 134.4, 130.9, 130.0, 129.9,
3
analysis: Calculated for C H NO S: C, 62.32; H, 3.92; N, 6.06.
123.4, 121.3 (Figure S7). Elemental analysis: Calculated for
ClNO S: C, 54.24; H, 3.03; N, 5.27. Found: C, 54.12; H,
12
9
2
−1
Found: C, 62.09; H, 4.03; N, 6.18. FTIR (neat, cm ): 3071, 1526,
348, 732, and 691.
,2′-Bis(thiophenyl)benzidine (BTPB). 46.3 g (200 mmol) of (3-
nitrophenyl)(phenyl)sulfane (1a) in 300 mL of EtOH was poured into
0 mL of a sodium hydroxide solution containing 40 g (1000 mmol)
C
12
H
8
2
−1
1
3.10; N, 5.32. FTIR (KBr, cm ): 1518, 1347, 1087, 821, and 725.
2,2′-Bis(4-chlorothiophenyl)benzidine (BCTPB). 26.5 g (100
mmol) of (4-chlorophenyl)(3-nitrophenyl)sulfane (1b) dispersed in
200 mL of EtOH was poured into 20 mL of a NaOH solution
containing 20 g (500 mmol) of sodium hydroxide. Zinc powder (18 g,
275 mmol) was added slowly to the mixture with stirring, followed by
refluxing for 1 day. The hot reaction mixture was filtered followed by
2
4
of sodium hydroxide. 36 g (550 mmol) of zinc powder was added
slowly to the mixture over 0.5 h with stirring (to control the highly
exothermic reaction), followed by heating under reflux for 1 day. The
hot reaction mixture was filtered then washed with EtOH. The filtrate
washing with EtOH. The filtrate was poured into 500 mL of cold H O
2
was poured into 500 mL of cold H O and extracted with DCM (3 ×
with stirring, and the orange-yellow precipitate formed was filtered.
2
1
00 mL) to give an orange liquid after vacuum evaporation by rotary
The residue was washed several times with H O and dried under
2
evaporator. The diazene product (2a) was converted to the diazane
vacuum at 40 °C for 24 h. Yield 19.6 g (84%). The diazene product
(2b) was converted to the diazane product (3b) by further reduction
product (3a) by further reduction with zinc dust and NH Cl. 5.24 g
4
(
80 mmol) of zinc dust was added slowly with stirring to the as-
with zinc dust and NH Cl. 2.62 g (40 mmol) of zinc dust was added
4
synthesized diazene product (2a) dissolved in 150 mL of acetone
slowly with stirring to 18.7 g (40 mmol) of the diazene product (2b)
under a nitrogen atmosphere followed by the slow addition of 80 mL
dissolved in 100 mL of acetone under a nitrogen atmosphere, followed
of a saturated NH Cl solution. The reaction mixture was then stirred
by the slow addition of 40 mL of saturated NH Cl solution. The
4
4
vigorously for 0.5 h at room temperature so that the reaction mixture
becomes almost colorless (light yellow). The reaction mixture was
made alkaline with a 10% aqueous ammonia solution and extracted
with DCM (3 × 100 mL), giving a light yellow semisolid after solvent
reaction mixture was then stirred vigorously for 1 h at room
temperature until the reaction mixture became almost colorless (light
yellow). The reaction mixture was made alkaline with a 10% aqueous
ammonia solution and extracted with DCM, giving a light yellow solid
1
1
evaporation. Yield: 30.6 g (77%); light yellow semisolid. H NMR
after solvent evaporation. Yield: 17.7 g (94%); light yellow solid. H
(
7
4
−
6
400 MHz, CDCl ): δ 7.32 (dd, J = 6.8 and 1.6 Hz, 4H, Ph−H), 7.31−
NMR (400 MHz, DMSO-d ): δ 8.31 (s, 2H, −NH), 7.81 (d, J = 7.5
3
6
.24 (m, 6H, Ph−H), 7.10 (t, J = 8.0 Hz, 2H, Ph−H), 6.77−6.72 (m,
Hz, 4H, Ph−H), 7.76 (br d, J = 3.0 Hz, 2H, Ph−H), 7.62 (t, J = 6.0
Hz, 2H, Ph−H), 7.48−7.41 (m, 8H, Ph-H) (Figure S8). FTIR (KBr,
H, Ph−H), 6.64 (dd, J = 8.0 and 1.6 Hz, 2H, Ph−H), 5.55 (s, 2H,
−1
−1
NH) (Figure S3). FTIR (KBr, cm ): 3338, 1594, 1478, 746, and
cm ): 3345, 1595, 1472, and 1089.
84.
The benzidine monomer, 2,2′-bis(4-chlorothiophenyl)benzidine
The benzidine monomer, 2,2′-bis(thiophenyl)benzidine (BTPB),
(BCTPB), like BTPB was obtained via a benzidine rearrangement
2
9,30
29,30
was obtained via a benzidine rearrangement reaction
of the
reaction
of the diazane product (3b). The diazane compound of
diazane (hydrazobenzene) product (3a). 30 g (75 mmol) of 3a was
16.9 g (36 mmol) was dissolved in 200 mL of dry EtOH under a N2
dissolved in 350 mL of dry EtOH under a N atmosphere. The mixture
atmosphere. The mixture was cooled to 0 °C using an ice bath, and 80
2
C
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Figure 1. ORTEP diagram of a single crystal of BCTPB (CCDC 1047009).
Scheme 2. Preparation Procedures of PIs
mL of an EtOH−HCl (1:1) solution was added slowly to the diazane
solution for approximately 2 h. Once the addition was complete, the
ice bath was removed, and the mixture solution was stirred at room
temperature for 6 h to allow a white solid to precipitate. The
precipitated salt was filtered, washed several times with EtOH, and
dissolved in 100 mL of water. This aqueous solution was made alkaline
by adding a 10% sodium hydroxide solution, and the organic content
was extracted with DCM. The DCM extract was evaporated to dryness
to give a white solid. The benzidine monomer was purified further by
recrystallization from a 50% EtOAc solution in hexane. The product
days. Crystal data: 0.22 × 0.13 × 0.07 mm, C H Cl N S , light
2
4
18
2
2 2
yellow, f = 469.42; orthorhombic, P2(1)2(1)2(1); a = 9.3298(4) Å, b
w
= 12.8039(6) Å, c = 18.0035(8) Å, α = β = γ = 90°; V = 2150.66(17)
3
Å , Z = 4; T = 200(2) K, R indices [I > 2σ(I)]: R
= 0.0510, wR
= 0.1583; GOF = 1.136.
The torsion angle of the two central nitrogen-connected phenyl rings
=
1
2
0.0947; R indices (all data): R = 0.0918, wR
1 2
(
plane 1: C7−C12; plane 2: C6−C5) was 55.07°, whereas the torsion
angle of two C−S bonds (plane 3: C1−S1; plane 4: C12−S2) was
1
30.24° (Figure 1). The torsion angle between the 1,4-disubstituted
phenyl ring (plane 5: C24−C19 and C13−C14) was 49.03°.
yield was 12.2 g (72.0%); white crystalline solid; mp 168−169 °C
1
(
EtOAc:hexane = 1:1). H NMR (400 MHz, DMSO-d ): δ 7.20 (bs,
RESULTS AND DISCUSSION
6
■
8
H, Ph−H), 6.98 (d, J = 6.4 Hz, 2H, Ph−H), 6.55 (dd, J = 6.4 and 2.0
Monomer Synthesis. The diamine monomers, namely,
BTPB and BCTPB, were synthesized via a benzidine
rearrangement reaction of the hydrazo (diazane) compounds
Hz, 2H, Ph−H), 6.48 (d, J = 2.0 Hz, 2H, Ph−H), 3.63 (s, 4H, NH )
2
(
Figure S9). 13C NMR (100 MHz, DMSO-d ): δ 146.6, 136.7, 134.5,
6
1
1
33.1, 133.0, 132.9, 132.1, 131.9, 131.6, 129.2, 129.1, 129.0, 116.8,
16.7, 113.8, and 113.7 (Figure S10). Elemental analysis: Calculated
(
3a/3b) obtained after the reduction of meta-substituted
nitrobenzene derivatives (1a/1b), as illustrated in Scheme 1.
The meta-substituted nitrobenzene derivatives namely, (3-
nitrophenyl)(phenyl)sulfane (1a) and (4-chlorophenyl)(3-
nitrophenyl)sulfane (1b), were synthesized via the CuI-
catalyzed cross-coupling reaction of 1-iodo-3-nitrobenzene
for C H Cl N S : C, 61.40; H, 3.86; N, 5.97. Found: C, 61.27; H,
2
4
18
2
2 2
−1
3
1
.91; N, 6.05. FTIR (KBr, cm ): 3327, 3078, 3020, 1600, 1470, 1277,
225, 1089, 1011, and 824. A single crystal of BCTPB was obtained
from a mixed solvent solution of BCTPB in EtOH, EtOAc, and DCM
by the slow evaporation of solvents at room temperature for several
D
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Figure 2. Molecular orbital (MO) diagrams of the representative polyimides.
with thiophenol and 4-chlorothiophenol, respectively, using
K CO as a base in dry DMF. Careful optimization showed that
plates and imidized thermally under programmed curing
conditions, where they were dried at 80 °C for 8 h, 150 °C
for 2 h, 200 °C for 2 h, 250 °C for 1 h, and 300 °C for 1 h
under flowing nitrogen. After curing, the glass plate was
immersed in water to facilitate the removal of the flexible, free-
standing PI films. For poly(amic acid)s (PAAs), intrinsic
viscosities at 30 °C were in the range of 0.92−1.26 dL/g (Table
S1). All the PAA solutions (except CBDA/BTPB and CBDA/
BCTPB) were highly viscous, indicating that their molecular
weights were relatively high. This shows that both BTPB and
BCTPB had high reactivity, even in the presence of bulky
thiophenyl/4-chlorothiophenyl substituents at the 2,2′-position.
Normally, diamines with bulky side groups exhibit lower
reactivity in the polycondensation reaction. The observed
higher reactivity of BTPB/BCTPB might be due to electron
donation from the sulfur atom of the thiophenyl side group.
The relatively lower viscosity of CBDA-based PAAs might be
due to the lower reactivity of the alicyclic CBDA. All the PI
films except for CBDA/BTPB and CBDA/BCTPB were
transparent and free-standing. CBDA/BTPB and CBDA/
BCTPB were quite brittle. PI films derived from 6FDA
(6FDA/BTPB and 6FDA/BCTPB) were soluble in both polar
and nonpolar organic solvents such as chloroform (CHCl3),
dichloromethane (CH Cl ), N,N-dimethylacetamide (DMAc),
2
3
better yields could be achieved in the presence of PEG 400,
which might act as both a ligand and mediator. Both BTPB and
BCTPB were synthesized from the meta-substituted nitro-
benzene derivatives (1a/1b) via a three-step reaction, as shown
in Scheme 1. In the first step, 1a and 1b were reduced to the
corresponding diazene compounds (2a and 2b) by reactions
with sodium hydroxide and zinc powder in EtOH. The diazene
compounds (2a and 2b) were then converted to the
corresponding hydrazo compounds (3a and 3b) by zinc and
aqueous ammonium chloride in acetone in excellent yield.
Finally, BTPB and BCTPB were obtained by the benzidine
rearrangement reaction of hydrazo compounds (3a and 3b) by
EtOH-HCl (1:1). To ensure a higher yield of BTPB and
BCTPB, it was necessary to carry out the benzidine
rearrangement reactions immediately after the synthesis of
the relatively less air stable hydrazo compounds (3a and 3b). In
the benzidine rearrangement, various isomers were formed, but
not all could be isolated in high purity. The reactions were
optimized to obtain a higher yield of the benzidine derivatives
(
BTPB/BCTPB) only. Both BTPB and BCTPB were purified
by repeated crystallization from a 50% EtOAc solution in
hexane.
2
2
Polyimides Synthesis and Characterization. Ten new
PIs were prepared by the thermal imidization of their
corresponding precursors, poly(amic acid)s (PAAs). The
PAAs were synthesized by the addition polymerization of
equimolar amounts of either BTPB or BCTPB and five
different dianhydrides (6FDA, CBDA, BPDA, BTDA, and
PMDA) in NMP (Scheme 2). Each dianhydride was added to a
diamine solution, dissolved without heating, and stirred at room
temperature for 2 days. For all the precursor solutions
prepared, the solid contents were 30 wt %. All procedures
were carried out in a glovebox purged with nitrogen. The NMP
solutions of the PAA precursors were spin-coated onto glass
dimethyl sulfoxide (DMSO), and NMP, but all other PI films
were completely insoluble in most of the common organic
solvents (Table S1).
FTIR and NMR Characterization of the PIs. The
chemical structures of the synthesized PIs were confirmed by
ATR FTIR and NMR spectroscopy. Figure S11 presents the
ATR FTIR spectra of the synthesized PIs. The absence of
absorption peaks around 1700 cm− corresponding to the
amide carbonyl stretching of PAAs indicates that the resulting
PI films had been imidized completely. Typical absorption
bands of aromatic PIs appeared in the range of 1782−1774
1
−
1
−1
cm
(CO, asymmetric), 1721−1716 cm
(CO,
E
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Macromolecules
symmetric), 1362−1354 cm (C−N), and 1090−1084 (C−
Article
−
1
1
S
). Figure S12 shows the H NMR spectrum of 6FDA/
aromatic
BCTPB in CDCl . The most downfield protons were assigned
3
to 6FDA (8.04, 7.94, and 7.86 ppm), whereas the remaining
aromatic protons were attributed to BCTPB (7.38 and 7.24
ppm). In the 13C NMR spectrum of 6FDA/BCTPB (Figure
S13), the presence of a very sharp imide carbon peak at 165
ppm and the absence of carboxylic peaks at approximately 200
ppm confirmed the complete imidization.
Molecular Structures. Figure S14 shows the three-
dimensional molecular structures of the model compounds
for the synthesized PIs. The geometries were optimized by
density functional theory (DFT) calculations using a relatively
5,31−35
large basis-set function.
Figure 2 provides more details of
the quantum-chemical calculations, molecular orbital (MO)
diagrams, and calculated electronic transitions of models for the
four PIs at the optimized S geometry (Table S2). Irrespective
0
of the nature of the dianhydrides, the calculated dihedral angles
between the two benzene rings in the diamine moieties (BTPB
and BCTPB) of the synthesized PIs were ≈80° (79°−85),
which are significantly larger than the dihedral angles observed
in the unsubstituted benzidine/biphenyl-tetracarboxylic acid
26
dianhydride-based PIs (≈40°). Consequently, the steric
hindrance caused by the bulky thiophenyl or 4-chlorothio-
phenyl substituents attached to the 2,2′-positions of BPDA or
BCTPB forces the benzene rings of the biphenyl system to be
almost perpendicular, thereby causing significant conforma-
tional distortion of the PI backbones. Interestingly, X-ray
diffraction of a single crystal of BCTPB revealed a dihedral or
torsional angle of 51° between the two benzene rings of the
biphenyl system of BCTPB. Therefore, the significantly larger
dihedral angles in the BCTPB-based PIs originate mainly from
the molecular geometries of the polyimide chains. The
calculated dihedral angles obtained in the BTPB and BCTPB-
based PIs were similar to the dihedral angles observed in the
Figure 3. UV−vis absorption spectra of (a) BTPB-based polyimides
26
and (b) BCTPB-based PIs.
2
,2′-dichlorobenzidine-based PIs. This might be due to the
similar atomic size of the chlorine (99 pm) and sulfur (102 pm)
atoms.
slopes. The absorption edges (λ ) of the films were in the range
E
Crystallinity. Figure S15 shows the wide-angle X-ray
diffraction (XRD) patterns of four representative aromatic PIs
of 321−453 nm. The absorption edges (λ ) of all the PI films
E
derived from CBDA, 6FDA, and BPDA were below 400 nm,
(BPDA/BTPB, BPDA/BCTPB, PMDA/BTPB, and PMDA/
which explains why these films are mostly colorless. The
BCTPB), where it can be seen that the PIs are completely
amorphous. The polymer backbones with bulky thiophenyl/4-
chlorothiophenyl side groups at the 2,2′-position of the
noncoplanar biphenyl structures of BTPB/BCTPB would
increase the disorder of the polymer chains, weaken the
interchain interactions, and reduce the chain packaging
efficiency to hinder polymer crystallization.
absorption edges (λ ) of BPDA/BTPB and BPDA/BCTPB
were 387 and 379 nm, respectively, which are shifted to
significantly shorter wavelengths compared to those of
E
conventional aromatic PIs (λ of PI derived from BPDA and
E
26
p-phenylenediamine was observed at ∼413 nm). The steric
hindrance caused by bulky thiophenyl/4-chlorothiophenyl side
groups of BTPB and BCTPB distorts the main chain
conformations of the PIs significantly. This is evident from
the large dihedral angles of 84°−85° between two benzidine
phenyl groups in BTPB- and BCTPB-based PIs (Figure S14),
which inhibits the conjugation of imide−phenyl and phenyl−
phenyl bonds in the PI chains and weakens the intra- or
intermolecular CT interactions. Therefore, the reduced CT
interactions exhibited by the BTPB- and BCTPB-based PIs
shifted their absorption edges to shorter wavelengths. In
addition, the absorption edges of the BCTPB-based PI films
shifted toward shorter wavelengths compared to the BTPB-
based PIs containing PMDA and BPDA dianhydride. On the
other hand, the dihedral angles between the phenyl rings of the
biphenyl system did not change much in the BTPB- and
BCTPB-based PIs. The introduction of electronegative
chlorines to the skeletal structure of BCTPB reduces the
Optical Properties. The optical transparency of the PI
films in the visible region was considered to be a crucial factor
for a range of optical applications. Generally, aromatic PI films
exhibit intense colors, ranging from light yellow to deep brown,
depending on their structures, due to the formation of intra- or
3
6
intermolecular charge-transfer complexes (CTC). Figure 3
presents the experimental UV−vis absorption spectra of the PI
films (∼20 μm thick). PI films of 6FDA/BTPB, 6FDA/
BCTPB, CBDA/BTPB, CBDA/BCTPB, and BPDA/BCTPB
were transparent and colorless, whereas the PI films of BPDA/
BTPB were transparent with a slightly yellowish tint. All the PI
films derived from PMDA and BTDA were deep yellow in
color. Table 1 lists the absorption edges (λ ) estimated from
E
the point where the absorption curve intersects a bisected line
drawn through the intersection of the extrapolations of the two
F
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Table 1. Absorption Edges (λ ), Optical Transmittance, and Excitation and Emission Peaks (λ , λ ) in the Fluorescence
E
ex
em
Spectra of the Synthesized PIs with Calculated Molecular Orbital Energies (ε) of HOMO and LUMO and Energy Band Gaps
δε) for Monomeric Models of the PIs
(
polyimide
λE (nm)
%T at 400 nm
%T at 450 nm
λex (nm)
λem (nm)
εHOMO (eV)
εLUMO (eV)
Δε (eV)
PMDA/BTPB
PMDA/BCTPB
BPDA/BTPB
BPDA/BCTPB
CBDA/BTPB
CBDA/BCTPB
431
425
387
379
322
321
366
363
453
450
7
14
50
66
76
83
75
76
55
61
81
83
81
85
85
85
28
28
452
451
391
384
339
335
576
574
503
482
413
402
−7.42
−7.64
−7.19
−7.38
−7.48
−7.64
−7.19
−7.37
−7.36
−7.47
−2.31
−2.45
−1.79
−1.85
−0.31
−0.42
−1.72
−1.74
−2.13
−2.14
5.10
5.19
5.40
5.53
7.16
7.22
5.46
5.63
5.23
5.33
6
6
FDA/BTPB
FDA/BCTPB
BTDA/BTPB
BTDA/BCTPB
highest occupied molecular orbital (HOMO) energy level of
BCTPB. Therefore, the energy band gap, the difference
between the MO energies of HOMO and LUMO, for the
BCTPB-based PIs are slightly larger than the BTPB-based PIs
derived from the same dianhydride (Table 1), which shifts the
absorption edges of the BCTPB-based PIs toward shorter
wavelengths. In addition, the more bulky and spatially extended
Figure 5 shows the optical transmission spectra of the PI
films synthesized in this study. The spectral shapes of the PI
4
-chlorothiophenyl side groups in BCTPB increase the free
volumes further compared to the thiophenyl group in BTPB.
Consequently, the intermolecular CT interactions are weak-
ened in the BCTPB-based PIs, which can induce hypsochromic
shifts of the absorption edges of the BCTPB-based PIs.
Figure 4 presents the calculated absorption spectra of the PIs
using the TD-DFT method. The spectra of the PIs containing
Figure 4. Calculated UV−vis absorption spectra of the representative
PIs.
Figure 5. Optical transmission spectra of (a) BTPB-based polyimide
films and (b) BCTPB-based PI films.
6
FDA and BTDA could not be calculated due to the very large
number of constituent atoms. The optical absorption patterns
of CBDA/BTPB and CBDA/BCTPB are well reproduced, and
the hypsochromic shifts observed by BPDA/BCTPB and
PMDA/BCTPB compared to that of BPDA/BTPB and
PMDA/BTPB were partly reproduced, but the experimental
absorption edges for the latter PIs were shifted to significantly
longer wavelengths. The contributions from intermolecular CT
interactions were not included because the TD-DFT
calculations were performed for single model molecules.
Therefore, the mismatch between the calculated and
experimental λE values was caused mainly by an under-
estimation of the CT interactions.
films vary considerably depending on the nature of the
dianhydrides. As shown in Figure 5, the optical transmission
spectra of the PI films derived from the alicyclic dianhydride
CBDA and fluorinated dianhydride 6FDA show high trans-
parency in the visible range (%T at 400 nm are 76% for CBDA/
BTPB, 83% for CBDA/BCTPB, 75% for 6FDA/BTPB, and
76% for 6FDA/BCTPB). The higher optical transparencies of
these PI films can be attributed to the lower electron affinity of
CBDA and the presence of a bulky hexafluoroisopropylidene
moiety in 6FDA, both of which inhibit charge transfer complex
formation considerably. The transparency of the PI film of
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Figure 6. Excitation and fluorescent emission spectra of the PIs: (a) BPDA/BTPB; (b) CBDA/BTPB; (c) PMDA/BTPB; (d)BPDA/BCTPB; (e)
CBDA/BCTPB; (f) PMDA/BCTPB.
BPDA was improved significantly by replacing BTPB with
BCTPB (from 50% of %T for BPDA/BTPB to 66% of %T for
BPDA/BCTPB at 400 nm).
339 nm was red-shifted slightly compared to that of CBDA/
BCTPB PI (335 nm), which can be explained by the smaller
calculated band gap for the CBDA/BTPB (7.16 eV) than that
for CBDA/BCTPB (7.22 eV). Second, PMDA/BTPB and
PMDA/BCTPB showed very weak fluorescence because the
lowest excited states of the PIs derived from the dianhydrides
with localized π conjugation could be attributed to LE(n−π*)
Fluorescent Properties. Fluorescence spectroscopy is used
widely to examine the electronic structures and aggregation
states of polymers owing to its high sensitivity to micro-
3
6
environmental changes in the polymer matrices. Figure 6
shows the one-dimensional excitation and emission spectra of
thin films of six representative PIs. All these PI films showed
apparent fluorescence emission in the visible region between
37,38
or CT(n−π*) states, which emit very weak fluorescence.
Third, BPDA/BTPB and BPDA/BCTPB showed excitation
peaks at 391 and 384 nm and emission peaks at 503 and 482
nm, and the Stokes shifts were 0.85 and 0.75 eV, respectively,
which can be understood as CT interactions inducing efficient
energy transfer from the LE state to the exited CT state.
Therefore, the relatively intense fluorescence was emitted from
the lowest excited states attributable to the CT(π−π*) states.
The calculated band gaps for BPDA/BTPB, BPDA/BCTPB,
PMDA/BTPB, and PMDA/BCTPB were 5.40, 5.53, 5.10, and
5.19 eV, respectively, which coincides well with their respective
Stokes shifts.
402 and 576 nm. Table 1 lists the wavelengths of the maxima of
the excitation and emission spectra of these PIs. First, CBDA/
BTPB and CBDA/BCTPB showed fluorescent emission
originating from their locally excited π−π* (LE(π−π*)) state
at the diamine moieties, which is characterized by the short
excitation wavelengths of 339 and 335 nm as well as by the
Stokes shifts of 0.51 and 0.56 eV, respectively, as estimated
from the difference in the wavelength between the excitation
and emission maxima. The excitation peak of CBDA/BTPB at
H
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Table 2. Refractive Indices (n , nTM, n ), Birefringence (Δn) of the PIs Measured at 633 nm, Abbe Numbers (ν ), Refractive
TE
av
D
Indices at Infinite Wavelength (n ), and Coefficients of Wavelength Dispersion (D) of the PIs
∞
PI
nTE
nTM
nav
Δn
νD
n∞
D (10−3)
6
6
FDA/BTPB
1.6623
1.6489
1.7162
1.7116
1.6900
1.7255
1.7175
1.7342
1.7284
1.6611
1.6482
1.7146
1.7104
1.6878
1.7236
1.7158
1.7334
1.7277
1.6619
1.6487
1.7157
1.7112
1.6893
1.7249
1.7169
1.7339
1.7282
0.0012
0.0007
0.0016
0.0012
0.0022
0.0019
0.0017
0.0008
0.0007
16.47
20.77
17.93
15.37
12.66
15.97
13.11
24.52
31.47
1.6087
1.6087
1.6651
1.6477
1.6148
1.6665
1.6439
1.6948
1.6981
21.36
16.55
21.17
24.40
28.83
24.16
29.06
15.83
12.22
FDA/BCTPB
BTDA/BTPB
BTDA/BCTPB
CBDA/BCTPB
PMDA/BTPB
PMDA/BCTPB
BPDA/BTPB
BPDA/BCTPB
Refractive Indices and Birefringence. Table 2 summa-
the polyimide chains and reduce the refractive index. The n of
av
rizes the in-plane refractive index (n ), out-of-plane refractive
PMDA-containing PIs were lower than the BPDA-containing
PIs due to the lower percentage of aromatic moieties in the
TE
index (nTM), average refractive index (n ), and in-plane/out-of-
av
5
,39
plane birefringence (Δn) of the PIs. The values of n and nTM
polyimide network of the former than the latter.
TE
for the BTPB-based PI films measured at 633 nm are in the
range of 1.6623−1.7342 and 1.6611−1.7334, respectively,
whereas those for the BCTPB-based PI films are in the range
of 1.6489−1.7284 and 1.6482−1.7277, respectively. The facts
that the values of n were slightly higher than those of n for
The calculated refractive indices for PMDA/BTPB, PMDA/
BCTPB, BPDA/BTPB, BPDA/BCTPB, and CBDA/BCTPB at
633 nm were 1.6645, 1.6724, 1.7154, 1.7227, and 1.6210,
respectively, assuming a constant molecular packing coefficient
2
1,35
(K
) of 0.6.
The calculated nav values were all slightly
TE
TM
p
all PI films reflected the preferential orientation of the main
smaller than their experimental navs at 633 nm (Table 2). In
contrast to the experimental average refractive indices, the
calculated refractive indices of the BCTPB-containing PIs are
larger than those of the BTPB-containing PIs. This suggests
that the linear polarizability (α) per its van der Waals volume
chains parallel to the film plane. The n of the BTPB-based PI
av
films measured at 633 nm ranged from 1.6619 to 1.7339,
whereas that for the BCTPB-based PI films ranged from 1.6487
to 1.7282. The n of the PIs depends on the nature of both the
av
(
Vvdw) of BCTPB is larger than that of BTPB, even though the
dianhydride and diamine. The n values of the 6FDA-based PIs
av
effects of molecular packing practically have a more dominant
influence over the experimental nav values. The practical Kp
values of the PIs were estimated from the calculated values of
Vvdw and α (Table 3) and the observed refractive indices of
(
6FDA/BTPB and 6FDA/BCTPB) were much smaller than
those of the non-fluorinated aromatic PIs, obviously due to the
unique fluorine impact contribution (very low polarizability per
atomic volume of fluorine). The PIs derived from CBDA
(
CBDA/BCTPB) showed significantly lower refractive indices
Table 3. Calculated van der Waals Volumes (Vvdw),
than those from the non-fluorinated aromatic PIs, which was
attributed to the aliphatic nature of CBDA. The nav of the
CBDA/BTPB film could not be measured because it was highly
brittle in nature. As expected, all six non-fluorinated sulfur
Molecular Polarizabilities (α), and Estimated Packing
a
Coefficients (K ) of the PIs
p
VvdW (Å3)
α (Å3)
polyimide
α/VvdW
Kp at λ∞
containing aromatic PIs exhibited high n values >1.71 at 633
av
PMDA/BTPB
PMDA/BCTPB
BPDA/BTPB
BPDA/BCTPB
CBDA/BTPB
CBDA/BCTPB
483.0
510.8
554.1
581.9
468.5
496.3
633.5
661.3
573.6
601.4
67.97
72.54
81.84
86.6
0.1407
0.1420
0.1477
0.1488
0.1326
0.1344
0.1350
0.1365
0.1427
0.6740
0.6622
0.6480
0.6395
nm due to the presence of sulfur atoms with very high atomic
refraction and rigid molecular structures of BTPB and BCTPB.
The PIs derived from BPDA (BPDA/BTPB and BPDA/
BCTPB) showed higher refractive indices than the PIs derived
from PMDA and BTDA. The nav of 1.7339 observed for
BPDA/BTPB at 633 nm was the highest of the sulfur-
containing PIs synthesized in this study, whereas the nav of
61.96
66.69
85.51
90.26
81.86
86.64
0.6787
0.6546
0.6372
0.6581
6
6
FDA/BTPB
FDA/BCTPB
1
.7282 observed for BPDA/BCTPB was significantly high for a
BTDA/BTPB
colorless PI. On the other hand, the introduction of chlorine in
BCTPB decreased the nav of BCTPB-based PIs compared to
the analogous BTPB-based PIs. This result can be explained by
the more bulky structures of the BCTPB-containing PIs
increasing their van der Waals volumes significantly compared
to the BTPB containing PIs and inhibiting dense molecular
packing. Consequently, the BCTPB containing PIs inherently
have a larger intermolecular free volume, which is also
consistent with the high optical transparency due to the
significantly weakened intermolecular CT interactions, as
BTDA/BCTPB
0.1441
0.6485
a
2
2
av
The packing coefficients (K ) were calculated as (n − 1)/(n + 2)
p
av
=
(4π/3)K (α /V ), where n , α , and V are the average
p
av
vdW
av
av
vdW
refractive index, the average polarizability, and the van der Waals
2
1
volume of the PIs, respectively.
39
BCTPB and BTPB containing PIs. Kp can be used to be a
measure of the molecular packing density of a PI. When the
dianhydride structure is fixed, the K values estimated for the
p
discussed above. The n s of BPDA/BTPB, BPDA/BCTPB,
BCTPB-containing PIs (e.g., 0.6395 for BPDA/BCTPB) were
significantly smaller than those of the BTPB-containing PIs
(e.g., 0.6480 for BPDA/BTPB), which agrees well with the
tendency of the smaller experimental refractive indices of the
BCTPB-containing PIs. This also shows that the increase in
free volume caused by the substitution of chlorine in BCTPB
av
PMDA/BTPB, and PMDA/BCTPB were larger than those of
BTDA/BTPB and BTDA/BCTPB because of the rod-like
polymer backbone of BPDA and PMDA, which will encourage
dense molecular packing of the polyimide chains. The bent
structure of BTDA will inhibit the dense molecular packing of
I
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was sufficient to loosen the molecular packing and lower the
refractive indices of the BCTPB-based PIs.
The anisotropy in the refractive index, also known as
birefringence”, is an important optical property for applica-
“
tions in lenses and waveguide devices, in which a large
birefringence can cause aberrations or polarization-dependent
losses. All the PIs synthesized in this study showed significantly
small and positive in-plane/out-of-plane birefringence (Δn),
ranging from 0.0022 to 0.0007, despite their rigid molecular
structures and high refractive indices. Conventional PIs with
26
rigid structures generally show large birefringence. The bulky
and highly polarizable thiophenyls at the 2,2′-position of the
non-coplanar biphenylene structure of BTPB and BCTPB
direct almost normal to the biphenyl linkage and endow large
free volumes among the polymer chains and weaken the
intermolecular packing between the polymer chains. These
factors reduce the Δn. BPDA/BCTPB PI can be a promising
candidate for various optical applications owing to the high
transparency, large refractive indices, and very small birefrin-
gence.
The experimental refractive indices were measured at four
different wavelengths: 404, 532, 632.8, and 829 nm. The
refractive index at infinite wavelength (n ) and the coefficient
∞
of wavelength dispersion (D) were calculated using the
simplified Cauchy’s formula:
2
n = n + (D/λ )
λ
∞
Figure 7 shows the wavelength dispersion of the refractive
indices. The Abbe’s number (ν ), which is commonly used to
D
estimate the wavelength dependence of the refractive indices of
optical materials, is given by the equation
Figure 7. Wavelength dispersion of the nav values of (a) BTPB-based
PI films and (b) BCTPB-based PI films.
ν = (n − 1)/(n − n )
D
D
F
C
where n , n , and n are the refractive indices of the material at
D
C
C
the wavelengths of the sodium D (589.3 nm), hydrogen F
486.1 nm), and hydrogen C (656.3 nm) lines, respectively.
Table 2 lists the values of n , ν , and D. Figures 7 and 8 show
(
∞
D
the wavelength dispersion of the experimental and calculated
nav values of the synthesized PIs, respectively. The smaller ν
D
value corresponds to a higher degree of wavelength dispersion,
whereas a large ν value indicates low wavelength dispersion.
D
BPDA/BTPB and BPDA/BCTPB exhibited large ν values of
D
2
4.52 and 31.47, respectively. In general, the refractive index
and Abbe numbers are in a trade-off relation because optical
absorption in the UV−vis region causes an increase in refractive
index at shorter visible wavelengths. In addition, one of the
coauthors reported that BPDA-derived PIs exhibited high
refractive indices with large wavelength dispersion, which
40
41
corresponds to small Abbe numbers, among 11 kinds of PIs.
Therefore, the fact the Abbe numbers for BPDA-derived PIs are
the highest in all the PIs seems to be unusual. One essential
reason may be due to their very good transparency, though the
exact reason is unclear at moment. More systematic
investigation on this point should be further performed in
our future work. The PIs derived from 6FDA and CBDA
Figure 8. Wavelength dispersion of the calculated refractive indices of
the PI films.
trends of the wavelength dispersion of the experimental nav
values could not be reproduced by the wavelength-dependent
calculated nav values, but the DFT calculations correctly
predicted that the nav values of BPDA/BTPB and BPDA/
BCTPB could be >1.74 at the shorter visible wavelengths (ca.
400−550 nm).
Thermal Properties. The thermal decomposition and
deformation behavior of the PIs were evaluated by TGA,
DSC, and DMA measurements under nitrogen. The results are
listed in Table 4. As shown in Figure 9 (TGA), all the PIs
showed good thermal stability, such as 5% weight-loss
(6FDA/BTPB, 6FDA/BCTPB, and CBDA/BCTPB) showed
smaller ν values (16.47, 20.77, and 12.66, respectively) owing
D
to their smaller nav values despite having higher optical
transparency, whereas BTDA/BTPB, BTDA/BCTPB,
PMDA/BTPB, and PMDA/BCTPB exhibited smaller ν values
D
(
17.93, 15.37, 15.97, and 13.11, respectively) owing to their
lower optical transparencies in the UV/vis region despite
having higher refractive indices. As shown in Figure 8, the
J
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a
Table 4. Thermal and Mechanical Properties of the PIs
CBDA/BTPB and CBDA/BCTPB with alicyclic dianhydride
5
10
exhibited lower T and T than the aromatic PIs. All aromatic
d
d
Tg (°C)
PIs showed good thermal stability without significant weight
loss up to 400 °C. PMDA/BTPB and PMDA/BCTPB with
Td5
Td10
(°C)
CTE
(ppm/K)
E′
PIs
DSC DMA (°C)
(GPa)
relatively ordered, linear, and rod-like polymer chains showed
PMDA/BTPB
PMDA/BCTPB
BPDA/BTPB
BPDA/BCTPB
CBDA/BTPB
CBDA/BCTPB
279
282
209
234
278
306
227
243
228
257
245
244
220
239
489
494
483
485
446
453
486
487
468
485
509
512
501
505
461
467
498
507
495
506
10.0
23.9
38.8
39.7
10.74
18.00
5.64
6.14
1.94
2.58
3.54
6.43
2.88
5.35
the highest Td5 and Td
10
.
The glass transition temperatures (T s) were determined by
g
DSC (Figure S16) and DMA (Figure 10). In DMA, T was
g
6
6
FDA/BTPB
214
236
207
221
56.4
60.8
61.4
63.0
FDA/BCTPB
BTDA/BTPB
BTDA/BCTPB
a
T : glass transition temperature; T , Td10: temperature at 5% and
5
g
d
10% weight loss, respectively; CTE: coefficient of thermal expansion;
E′: storage modulus measured at 25 °C.
Figure 10. Temperature dependence of the storage modulus and tan δ
for PIs; filled square and filled circle symbols denote the change of
storage modulus and the change of tan δ, respectively, as a function of
temperature.
determined to be the peak temperature of the tan δ curve. The
DMA thermograms (Figure 10) showed α-relaxations (main
peaks) between 220 and 306 °C, whereas the T s estimated by
g
DSC analysis were in the range 207−282 °C. The T s
g
estimated from DMA were slightly higher (except PMDA/
BTPB) than those estimated from DSC. These differences in
Figure 9. TGA curves of (a) BTPB- and (b) BCTPB-based PIs.
the T values estimated by DMA and DSC may be due to the
g
different frequency responses of the PIs, i.e., f = 0 Hz for DSC
and 2 Hz for DMA. One of the most important key parameters
5
temperatures (T ) in the range 453−494 °C and a 10%
d
1
0
weight-loss temperatures (T ) in the range 461−512 °C. The
is the T of optical device fabrication. All the PIs showed high
d
g
weight residues at 800 °C ranging from 41% to 54% were
observed in nitrogen. The PIs derived from BCTPB exhibited
higher Td5 and Td compared to the BTPB-based PIs
containing the same dianhydride. Decomposition of the
dianhydride segments of the PI chains generally occurs at a
lower temperature than that of the diamine segments, and the
introduction of chlorine with a higher atomic density than
hydrogen reduces the decomposition rate of BCTPB-based PIs.
Tgs, exceeding 200 °C suitable for applications to optoelec-
tronic devices. The PIs derived from BCTPB exhibited higher
Tgs than the analogous BTPB-based PIs, which can be
attributed to the bulkier 4-chlorothiophenyl moiety stiffening
the polymer backbone more effectively than the thiophenyl unit
by introducing a barrier to segmental rotation. The PIs derived
10
from PMDA possessed the highest T s, which are significantly
g
higher than those of the other PIs synthesized in this study.
K
DOI: 10.1021/acs.macromol.5b00432
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
The significantly high T s of the PMDA-based PIs could be
(BCTPB), were synthesized via the benzidine rearrangement
reactions of the corresponding hydrazobenzene derivatives. The
hydrazobenzene derivatives were obtained by the reduction of
(3-nitrophenyl)(phenyl)sulfane and (4-chlorophenyl)(3-
nitrophenyl)sulfane, which were the cross-coupled products
of thiophenol and 4-chlorothiophenol with 1-iodo-3-nitro-
benzene, respectively. Ten new PIs were synthesized via the
conventional two-step polycondensation of BTPB and BCTPB
with five different dianhydrides in NMP. All the PIs except for
CBDA/BTPB and CBDA/BCTPB were self-standing, tough,
and flexible. The PI films except for PMDA/BTPB, PMDA/
BCTPB, BTDA/BTPB, and BTDA/BCTPB showed good
optical transparency in the visible region. In particular, BPDA/
BCTPB, a fully aromatic, non-fluorinated PI, was colorless and
transparent with a transmittance of ∼83% at 450 nm and an
average transmittance of >88% in the visible region. The PIs
underwent 10% weight losses when subjected to thermal
gravimetric analysis between 461 and 512 °C in a nitrogen
atmosphere. All the PIs exhibited high thermal stability with
glass transition temperatures ranging from 207 to 282 °C, as
measured by DSC, and from 220 to 306 °C according to DMA.
The CTE of the PIs films were between 10.00 and 63.00 ppm/
K. Most importantly, the BPDA/BCTPB PI film, a colorless PI
film, exhibited CTE < 40 ppm/K between 50 and 200 °C as
well as good pliability and toughness suitable for applications to
flexible substrates. All six non-fluorinated, sulfur-containing
aromatic PIs exhibited high average refractive indices >1.71 at
633 nm. The PIs synthesized in this study showed significantly
small and positive birefringence ranging from 0.0007 to 0.0022.
g
attributed mainly to the ordered, linear, rigid-rod-like polymer
backbone that increases the intermolecular CT interactions
between the polymer chains. CBDA/BTPB and CBDA/
BCTPB exhibited no T s below 300 °C, which was the
g
measuring limit of the DSC machine. This might be due to the
higher Tg of these two PIs than their decomposition
temperatures.
Figure S17 presents the TMA thermograms of the
representative PIs. Table 4 lists the CTE values of all the
synthesized PIs. The PMDA/BTPB and PMDA/BCTPB films
displayed small CTEs of 10.0 and 23.9 ppm/K, whereas
BPDA/BCTPB and BPDA/BTPB with high refractive indices
and high optical transparencies exhibited CTEs < 40 ppm/K.
The pseudo rigid-rod-like structures of PMDA and BPDA
increased the interchain packing densities of their PIs and
thereby reduced the CTE values. The interchain packing
density in BPDA/BTPB and BPDA/BCTPB decreased to some
extent due to the rotational vibration motion of the two phenyl
rings of BPDA, and their CTEs also increased slightly
compared to the PMDA-based PIs. In turn, the bent structure
of BTDA and bulky hexafluoroisopropylidene group in 6FDA
prevent dense chain packing and result larger CTEs in PIs
42
derived from BTDA and 6FDA. The PIs derived from
BCTPB showed smaller CTEs than analogous BTPB-based PIs.
This can be attributed to the bulkier 4-chlorothiophenyl
moiety, which inhibits dense polymer chain packing. A small
CTE is highly advantageous to apply as a substrate material in
optically transparent and thermally stable PIs for flexible
displays, in which the polymer substrates need to be exposed to
higher temperatures (>200 °C) during the vacuum deposition
processes of inorganic thin films. In this regard, BPDA/BTPB
and BPDA/BCTPB could be promising candidates for various
optical applications owing to the high transparencies, high
refractive indices, low birefringences, and CTEs < 40 ppm/K.
Figure 10 shows the variations in the storage modulus (E′)
and loss factor (tan δ) at various temperatures. Figure 10 shows
that the storage modulus of the PIs remained almost constant
upon heating below their glass transition temperatures, but
In particular, colorless BPDA/BCTPB with n of 1.7282 at 633
av
nm and birefringence of 0.0007 is highly suitable for
applications as flexible films in advanced electronic and optical
devices. The bulky thiophenyl and 4-chlorothiophenyl side
groups at the 2,2′-position of BTPB and BCTPB, respectively,
reduced the interactions between the polymer chains keeping
the rod-like polyimide backbone intact, resulting in PIs with
high thermal stabilities and high transparencies. In addition, PIs
exhibited high average refractive indices and small birefringen-
ces owing to the large atomic polarizability of sulfur and the
rigid bulky molecular structure of BTPB and BCTPB. The
much stronger steric effect caused by bulky 4-chlorothiophenyl
in BCTPB improved the optical transparency but decreased
slightly the average refractive indices of the BCTPB-based PIs
compared to the BTPB-based PIs.
after their T s, the modulus decreased dramatically. The storage
g
modulus (E′) of the aromatic PIs measured at 25 °C were in
the range 18.0 to 3.54 GPa, which reflects the very high
mechanical strength of all synthesized aromatic PIs. PMDA/
BTPB and PMDA/BCTPB registered higher storage moduli
than the other aromatic PIs due to the relatively ordered, linear,
rigid-rod-like polymer backbones, whereas the alicyclic CBDA-
based PIs showed lower storage moduli.
These results suggest that the presence of bulky thiophenyl
and 4-chlorothiophenyl groups at the 2,2′-position of BTPB
and BCTPB, respectively, do not deteriorate the thermal
stability of their PIs significantly. Therefore, 2,2′-bis-
ASSOCIATED CONTENT
■
*
S
Supporting Information
1
Seventeen figures and two tables, including seven H NMR
spectra, five 13C NMR spectra, one ATR FTIR spectrum,
molecular structures, WAXD patterns, and quantum-chemical
calculation, DSC thermograms, and TMA curves. The
Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b00432.
(
thiophenyl)- and 2,2′-bis(4-chlorothiophenyl)-substituted
benzidines (BTPB and BCTPB) in a para-linked polyimide
chain do not change the rod-like structure of the polyimide
backbone considerably but reduce the interactions between the
polymer chains by incorporating bulky thiophenyl and 4-
chlorothiophenyl side groups between the polymer chains. This
can explain the higher thermomechanical stabilities as well as
the higher transparencies of these PIs.
AUTHOR INFORMATION
■
Corresponding Author
*
Fax +82-51-514 4331, Tel +82-51-510-2407, e-mail csha@
pnu.edu (C.-S. Ha).
CONCLUSIONS
Two new diamine monomers, namely 2,2′-bis(thiophenyl)-
benzidine (BTPB) and 2,2′-bis(4-chlorothiophenyl)benzidine
■
Notes
The authors declare no competing financial interest.
L
DOI: 10.1021/acs.macromol.5b00432
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(
31) Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT,
ACKNOWLEDGMENTS
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2
(
004.
32) Slonimskii, G.; Askadskii, A.; Kitaigodorodskii, A. Polym. Sci.
The work was supported by the National Research Foundation
of Korea (NRF). Grant funded by the Ministry of Science, ICT,
and Future Planning, Korea (Acceleration Research Program
No. 2014 R1A2A111054584)); Pioneer Research Center
Program (No. 2010-0019308/2010-001948); the Brain Korea
USSR 1970, A12, 556.
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DOI: 10.1021/acs.macromol.5b00432
Macromolecules XXXX, XXX, XXX−XXX