Syntheses and characterizations of high refractive index and low birefringence polyimides containing spirobifluorene in the side chain

Article abstract of DOI:10.1016/j.polymer.2017.04.020

Two new diamine monomers having a spirobifluorene structure were synthesized through four steps, and polymerized with three different dianhydrides (PMDA, CBDA and 6FDA) to obtain six polyimides with spirobifluorene structure in the side chain. All the res

Full text of DOI:10.1016/j.polymer.2017.04.020

Polymer 117 (2017) 76e83
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Syntheses and characterizations of high refractive index and low
birefringence polyimides containing spirobifluorene in the side chain
a
,
b
b
c
,
**
, Myong-Hoon Lee b
, *
Pushan Wen , Rui He , Xiang-Dan Li
a
Department of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, Guizhou, 563006, China
Graduate School of Flexible and Printable Electronics, Chonbuk National University, Jeonju, Jeonbuk, 54896, South Korea
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central Minzu University, Wuhan,
b
c
Hubei, 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 January 2017
Received in revised form
Two new diamine monomers having a spirobifluorene structure were synthesized through four steps,
and polymerized with three different dianhydrides (PMDA, CBDA and 6FDA) to obtain six polyimides
with spirobifluorene structure in the side chain. All the resulting polyimides displayed excellent thermal
stability and high glass transition temperature. These polyimides synthesized from CBDA and 6FDA
showed good solubility in typical organic solvents such as NMP, DMF and DMSO. Transparent and flexible
cast films can be obtained from soluble polyimides or poly(amic acid) solution. The transmittance of
polyimide films was higher than 90% in the visible wavelength range (higher than 80% in case of pol-
yimide containing PMDA). The refractive indices of the result polyimides were found to be in the range
from 1.6343 to 1.7007, which is higher than that of normal polymers. In addition, the optical bi-
refringences of these polyimides were extremely low (0.0005e0.0014). The high refractive index and low
birefringence of these polyimides as well as other descent physical and thermal properties would offer
them possible applications in high performance optical materials such as flexible substrates.
31 March 2017
Accepted 6 April 2017
Available online 10 April 2017
Keywords:
Spirobifluorene
Thermal stability
Transparency
Refractive index
Birefringence
©
2017 Elsevier Ltd. All rights reserved.
1
. Introduction
high chemical resistance, low dielectric constant, and high me-
chanical properties [6e8]. In addition, the aromatic PIs often show
inherent high refractive indices because of their highly polarizable
polymer backbones and high aromatic contents. For example, PI
derived from pyromellitic dianhydride (PMDA) and 4,4’-oxydiani-
line (ODA) showed the refractive index of 1.6883 at 633 nm [9].
However, it shows a large birefringence of 0.0841, which degrades
the performance of optical devices. The combination of both high
refractive index and low birefringence are required in advanced
optical applications [10]. Thus, it is quite necessary to redesign the
molecular structures of the conventional PIs so as to fit the re-
quirements of high refractive indices and low birefringence. Ueda's
group [11,12] has developed some PIs with sulphur containing
substituents, of which the PIs possess refractive index higher than
1.7, and the value increase with an increase in the sulphur content
Recently, there has been strong demand to develop polymers
with high thermal stability, high transparency, high refractive index
(
high-n) and low birefringence for various optical applications,
such as coatings for long-period-grating (LPG), refractive index
sensors, various lenses, high performance optical components for
advanced display devices, substrates for organic light emitting di-
odes (OLEDs) and so on [1e4]. According to the Lorentz-Lorenz
equation, introduction of substituents with high molar refraction
and small molar volume can increase the n values of polymers.
Thus, the incorporation of aromatic rings, heteroaromatic rings,
and heavy atoms such as halogen and sulphur is an effective way to
produce high-n polymers [5]. Polyimides (PIs) are good candidates
for optical applications owing to their excellent thermal stability,
g
of the PIs. However, the glass transition temperature (T ) and
transparency of the PIs were somehow sacrificed due to the high
sulphur content.
Polymers containing at least one repeating unit of spiro qua-
ternary carbon in the main chain have attracted considerable in-
terests [13e15]. These polymers were called cardo polymers by
*
Corresponding author.
** Corresponding author.
E-mail addresses: lixiangdan@jbnu.ac.kr (X.-D. Li), mhlee2@jbnu.ac.kr
(
M.-H. Lee).
http://dx.doi.org/10.1016/j.polymer.2017.04.020
032-3861/© 2017 Elsevier Ltd. All rights reserved.
0

P. Wen et al. / Polymer 117 (2017) 76e83
77
Korshak et al. [16], where “cardo” means “hinge” or “loop” in Latin.
,9’-spirobifluorene (SBF) is a famous cardo structure which con-
2.3. Synthesis of monomer
2.3.1. Synthesis of 3,5-dinitrobromobenzene (35DNBB)
3,5-dinitrobromobenzene (35DNBB) was synthesized according
to the reported method [24] except that the crude product was
recrystallized from ethanol to give 3,5-dinitrobromobenzene
9
tains two fluorene units connected at the tetrahedral spiro carbon,
where the two fluorene moieties are perpendicularly intersected
with each other. SBF-containing polymers are intriguing materials
for optical use, since the incorporation of many aromatics into
polymer produces the high refractive index, while the aromatic
group occupies different plane to cause considerable decrease of
optical anisotropy, eventually rendering the low birefringence
ꢀ
(35DNBB) as a slightly yellow solid. Yield: 85%; mp: 76 C (lit.
ꢀ
1
3
75e76 C [24]). H NMR (400 MHz, CDCl , ppm): (d8.93, 1H; d8.64,
2H).
[17e19]. In addition, such polymers exhibit other characteristic
properties, including fine dispersing ability for inorganic fillers,
good solubility in organic solvents, excellent mechanical proper-
ties, good dimensional stability, and so on [20e23].
2.3.2. Synthesis of 2-pinacolborane-9,9’-spirobifluorene (PBSBF)
To a 50 mL two-necked round-bottomed flask, were charged, in
no specific order, 2-bromo-9,9’-spirobifluorene (BSBF) (1.1859 g,
3 mmol), bis(pinacolato)diboron (0.838 g, 3.3 mmol), palladium
acetate (0.0202 g, 0.09 mmol), potassium acetate (0.8834 g,
9 mmol), and 12 mL of DMF. The mixture was degassed by gently
Although a variety of PIs for optical applications have been re-
ported, side chain SBF-containing polyimides have rarely been
introduced. In this work, we synthesized a series of polyimides
containing SBF structures in the side chain. We envisioned that the
polymers with this structural feature can serve as an alternative
candidate for optical materials with not only a high refractive index
originating from both the polyimide backbone and fluorene moiety
but also a low birefringence due to the random orientation of
molecular structure caused by SBF units in the side chain. Herein,
we describe the syntheses of novel polyimides containing 9,9-
spirobifluorene in the side chain along with the evaluation of
their characteristic properties.
bubbling a nitrogen gas for 30 min at room temperature, and then
ꢀ
heated at 80 C under nitrogen protection for 6 h (determined by
TLC analysis). After cooled to room temperature the mixture was
poured into water (80 mL) and extracted with ethyl acetate
(100 mL ꢁ 3). The combined extracts were washed with brine, and
dried over anhydrous sodium sulphate overnight, followed by
removal of the solvent at reduced pressure. The residue was puri-
1
fied by recrystallization in ethyl acetate. Yield: 94%. H NMR
(400 MHz, CDCl
3
, ppm): (
d7.86e7.83, 5H; d7.36, 3H; d7.19, 1H;
d7.11e7.07, 3H; d6.72e6.71, 3H).
2
. Experiments
2.3.3. Synthesis of 2-(3,5-dinitrobenzene)-9,9’-spirobifluorene
(
35DNBSBF)
2.1. Materials
35DNBB (0.741 g, 3 mmol) and PBSBF (1.3271 g, 3 mmol) were
added to a 50 ml two-neck flask with 15 ml of toluene, and the
mixture was bubbled with nitrogen for 30 min. After addition of
All commercially available chemicals were used without
further purification unless otherwise noted. 1,3-Dinitrobenzene
13DNB), N-bromosuccinimide (NBS), bis(pinacolato)diboron
BPDB), potassium acetate, palladium(II) acetate, tetrakis(-
triphenylphosphine)palladium(0), tin(II) chloride dihydrate,
,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and
3 4
Pd(PPh ) (0.104 g, 0.09 mmol), the reaction mixture was further
(
(
bubbled with nitrogen gas for 30 min, and then was added 10 ml of
2
M aqueous K
2
CO
3
solution. The reaction mixture was heated with
ꢀ
stirring at 80 C overnight under nitrogen. After cooled to room
temperature, the mixture was diluted with water (30 mL) and ethyl
acetate (30 mL), filtered through Celite, and the filtrate was
collected. For the filtrate, first, the organic layer and water layer
were separated, and the water layer was extracted with ethyl ac-
etate (30 mL ꢁ 3), next, the entire organic layer was combined
together and washed twice with 50 mL of brine solution. After
drying over sodium sulphate overnight, the solvent was removed at
reduced pressure to afford the crude dark product. After column
1
pyromellitic dianhydride (PMDA) were purchased from Aldrich.
0
4
2
,4 -(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) and
,4-dinitrobromobenzene (24DNBB) were purchased from TCI. 2-
Bromo-9,9’-spirobifluorene (BSBF) was purchased from Fusol
Materials. Concentrated H SO , potassium carbonate, toluene,
2
4
sodium carbonate, ethanol, acetic anhydride, pyridine, N,N-
dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP)
were purchased from Showa Chemical. CBDA, PMDA and 6FDA
were recrystallized from acetic anhydride prior to use. NMP and
chromatography (silica gel, ethyl acetate/hexane), yellow solid was
1
obtained. Yield: 72%. H NMR (400 MHz, CDCl
3
, ppm): (
7.71, 1H; 7.43e7.40, 3H;
d6.77e6.75, 3H).
d8.90, 1H;
DMF were purified by distillation after stirring with CaH
2
d8.59, 2H;
d
8.02, 1H;
d7.93e7.89, 3H;
d
d
overnight.
d7.18e7.14, 3H;
d
7.02, 1H;
2.2. Measurements
2.3.4. Synthesis of 2-(2,4-dinitrobenzene)-9,9’-spirobifluorene
(
24DNBSBF)
1H NMR spectra of intermediates and monomer were recorded
24DNBSBF was obtained as a yellow solid from 24DNBB and
on a Bruker Avance Series DRX-600 NMR spectrometer with tet-
ramethylsilane (TMS) as an internal standard reference in CDCl or
DMSO-d . Fourier transform infrared (FT-IR) spectra were obtained
from KBr pellets or thin films on Si wafer with a Jasco FT-IR-300E
spectrometer. Transmittance of the films was evaluated by a
Scinco Model UV-2100S in a scanning range from 200 to 800 nm.
Thermogravimetric analyses (TGA) were carried out on a TGA 2050
PBSBF following the general procedure for 35DNBSBF. Yield: 62%.
1
3
H NMR (400 MHz, CDCl
3
, ppm): (
7.52e7.50, 1H; 7.41e7.37, 3H;
d6.79e6.72, 4H).
d
8.59, 1H;
d
8.32e8.30, 1H;
6
d7.93e7.83, 4H;
7.19e7.13, 3H;
d
d
d7.33e7.31, 1H;
d
2.3.5. Synthesis of 2-(3,5-diaminobenzene)-9,9’-spirobifluorene
(35DABSBF)
(
1
TA Instruments) under a nitrogen atmosphere at a heating rate of
2 2
SnCl $2H O (1.13 g, 5 mmol) in 3 mL of concentrated HCl was
ꢀ
ꢀ
0
C/min up to 800 C. DSC was performed on a DSC 2010 (TA
added to a mixture of 35DNBSBF (2.4125 g, 5 mmol) and acetic acid
ꢀ
3
Instruments) under a nitrogen flow (60 cm /min) at a heating/
cooling rate of 10 C/min. The refractive indices of the polymer
(20 mL). The mixture was stirred for 6 h at 100 C, and, after cooled
to room temperature, the resulting reaction mixture was neutral-
ꢀ
films were determined by a prism-coupler method on Sairon SPA-
ized with 2 M NaOH to precipitate the solids. The precipitated
solids were collected, washed with water, and dried in oven at 60 C
ꢀ
3
000 model at 623.8 nm.

78
P. Wen et al. / Polymer 117 (2017) 76e83
overnight. The solids were dissolved in CHCl
filtrate was evaporated to give 35DABSBF. Yield: 68%. H NMR
400 MHz, CDCl , ppm): ( 7.85e7.82, 4H; 7.54, 1H; 7.37e7.34, 3H;
6.87, 1H; 6.76e6.71, 3H; 6.15, 2H; 5.90, 1H;
3
and filtered. The
compares the 1H NMR of 35DNBSBF and 35DABSBF, where the
1
typical broad peak of amine appears at 3.35 ppm in the case of
35DABSBF and the aromatic protons of dinitrobenzene ring shift
from 8.90 and 8.59 ppm to 5.90 and 6.15 ppm, respectively, after the
reduction. In addition, there are no peaks of 35DNBSBF remaining
(
d
3
d
d
d
d
7.11e7.07, 3H;
d
d
d
d3.35, 4H).
1
in the H NMR spectrum of 35DABSBF, which proves the reaction
2
.3.6. Synthesis of 2-(2,4-diaminobenzene)-9,9’-spirobifluorene
was carried out successfully. Similarly, the synthesis of 24DABSF
from 24DNBSBF was also confirmed by amine protons at 3.53 ppm
and three aromatic protons in the diaminobenzene ring at 5.98,
6.05 and 7.07 ppm with disappearance of the corresponding peaks
at 8.59, 8.31 and 7.51 ppm, respectively (Fig. 2).
(
24DABSBF)
4 DABSBF was synthesized as yellow solid from 24DNBSBF
2
1
following the general procedure for 35DABSBF. Yield: 66%. H NMR
400 MHz, CDCl3, ppm): 7.87e7.81, 4H; 7.45e7.43, 1H;
6.77e6.72, 4H; 6.06e6.04, 1H;
(
(
d
d
d7.38e7.32, 3H;
d7.11e7.07, 4H;
d
d
Six polyimides containing SBF moiety at the side chain were
synthesized from the combination of two diamine monomers and
three dianhydrides (PMDA, CBDA and 6FDA) by using a general
procedure: Firstly, viscous PAA solution was obtained by the poly-
merization of diamine and dianhydride in dry NMP at room tem-
perature. Then, the PAA was subsequently converted into PI either
by chemical dehydration or stepwise thermal imidization process.
PI-35PM and PI-24PM were synthesized only through a thermal
imidization due to poor solubility (discussed in the next part). The
other four PIs could be obtained either by chemical imidization or
by thermal imidization. Molecular structures and their abbrevia-
tions of resulting polyimides are shown in Scheme 2. All the pol-
yimides were obtained as a tough free-standing film after being
peeled off from the cast film on a glass plate regardless of its
imidization method, suggesting they all have sufficiently high
d5.98, 1H; 3.53, 4H).
d
2.4. Polyimide synthesis
A typical polymerization procedure for the synthesis of poly-
imide was illustrated as follows: dianhydride (1 mmol) was grad-
ually added to a stirred solution of diamine (1 mmol) in dry NMP
(
3 mL). The mixture was stirred at room temperature for 24 h under
nitrogen atmosphere to form a viscous poly(amic acid) (PAA) so-
lution. The PAA was subsequently converted into PI by either a
thermal or a chemical imidization process. Chemical imidization
was carried out by adding acetic anhydride (0.3 mL) and pyridine
(
0.2 mL) into the PAA solution with stirring at room temperature for
ꢀ
12 h, and then, the reaction mixture was heated at 120 C for 6 h,
and subsequently poured into methanol. The grey solid precipitate
molecular weights. The inherent viscosities (
35CB were measured to be in the range of 0.76e0.89 dL g at 30 C
in DMAc.
h
inh) of PI-356F and PI-
ꢀ
ꢂ1
was filtered off, washed with methanol and hot water, and finally
ꢀ
dried in a vacuum oven at 80 C overnight. For the thermal imid-
ization, the above-mentioned PAA solution was spread on a glass
ꢀ
plate, and the solvent was removed at 80 C for 12 h. Imidization
3.2. Solubility
was carried out by thermal cyclodehydration of poly(amic acid) by a
ꢀ
ꢀ
ꢀ
stepwise heating cycle (2 h at 150 C,1 h at 200 C,1 h at 250 C, and
0
Solubility of resulting polyimides toward organic solvents was
evaluated, and listed in Table 1. PI-35CB, PI-356F, PI-24CB and PI-
246F show good solubility in aprotic polar solvents such as
ꢀ
.5 h at 300 C) under vacuum to obtain a self-standing polyimide
film. The polyimides synthesized from 35DABSBF with 6FDA, CBDA
and PMDA were abbreviated as PI-356F, PI-35CB and PI-35PM,
respectively. The polyimides synthesized from 24DABSBF with
FDA, CBDA and PMDA were named as PI-246F, PI-24CB and PI-
4PM, respectively.
DMSO, DMF, DMAc and NMP, as well as in g-butyrolactone (GBA),
but are hardly dissolved in less polar or protonic solvents such as
xylene, chloroform or methanol. The good solubility of polyimides
is generally attributed to the twisted SBF structure which prevented
the strong intra/intermolecular interactions of the polymer chains
[20,21]. On the other hand, PI-35PM and PI-24PM, both synthesized
from PMDA, were insoluble in any solvents. This can be ascribed to
the restricted segmental motion of polymer chain caused by bulky
substituents attached to the more rigid polymer backbone.
6
2
3
. Result and discussion
3
.1. Synthesis and characterization
In order to develop PIs with higher refractive index and lower
birefringence, two new aromatic diamines containing spirobi-
fluorene substituent were designed and synthesized through a
four-step procedure as shown in Scheme 1. Firstly, 3,5-
dinitrobromobenzene (35DNBB) was obtained from bromination
of 1,3-dinitro-benzene (13DNB) by treatment with N-bromosucci-
3.3. Thermal property
Thermal stability of polyimides was evaluated by thermogravi-
metric analysis (TGA) and differential scanning calorimetry (DSC),
and the results are presented in Fig. 3 and Fig. 4, respectively. Glass
nimide (NBS) in concentrated H
ondly, the boronic ester of 9,9’-spirobifluorene was synthesized
from 2-bromo-9,9’-spirobifluorene (BSBF) and bis(pinacolato)
2
SO
4
medium in good yield. Sec-
transition temperature (T
perature (T ), temperature at 5% weight loss (T
10% weight loss (T10), and residual weight at 800 C (R
g
), initial thermal-decomposition tem-
d
5
), temperature at
ꢀ
w
) of all the
diboron in the presence of Pd(OAc)
2
and DMF as a solvent. Then, a
polyimides are summarized in Table 2. All polyimides exhibited
Suzuki coupling reaction was carried out on 2-pinacolborane-9,9’-
spirobifluorene (PBSBF) with 3,5-dinitrobromobenzene (35DNBB)
excellent thermal stability with an onset of degradation tempera-
ꢀ
ture consistently higher than 326 C, and T
5
's were in the range of
ꢀ
or with 2,4-dinitrobromobenzene (24DNBB) using Pd(PPh
3
)
4
in
420e549 C with following order: PI-246F > PI-356F > PI-
24PM > PI-35PM > PI-24CB > PI-35CB. Polyimides synthesized
from 24DABSBF show better thermal stability than those from
35DABSBF. Among the three dianhydrides, polyimides synthesized
combination with CO solution to obtain 2-(3,5-
2
M
K
2
3
dinitrobenzene)-9,9’-spirobifluorene (35DNBSBF) or 2-(2,4-
dinitrobenzene)-9,9’-spirobifluorene (24DNBSBF) in high yields,
respectively. Finally, the dinitro compounds were reduced to di-
amines. Tin(II) chloride dihydrate in acidic condition was chosen as
a catalyst because it can reduce nitro compound to amine in a mild
condition and the post-treatment is easy to operate. The structures
d
from 6FDA were found to have higher T . Those of PI-246F and PI-
ꢀ
356F were 435 and 423 C, respectively. PI-35CB and PI-24CB dis-
played relatively low stability probably due to the poor stability of
cyclobutane structure compare to PMDA and 6FDA. As one of the
most important key parameters for the fabrication of optical device,
1
of obtained compounds in each step were verified by H NMR. Fig. 1

P. Wen et al. / Polymer 117 (2017) 76e83
79
Scheme 1. Syntheses of monomers.
Fig. 1. 1H NMR of 35DNBSBF and 35DABSBF.
glass transition temperatures (T
investigated by DSC, as shown in Fig. 4 and Table 2. Most of poly-
imides showed significantly high T
mainly attributed to the rigid backbone of polyimide and a large
g
) of the polyimides were also
steric hindrance by bulky SBF units. PI-35CB and PI-24CB films
possessed no distinct glass transition up to 400 C, which can
probably be ascribed to the suppressed molecular motions (internal
rotation) in the amorphous regions.
ꢀ
ꢀ
g
's (>295 C), which could be

80
P. Wen et al. / Polymer 117 (2017) 76e83
Fig. 2. 1H NMR of 24DNBSBF and 24DABSBF.
Scheme 2. Syntheses of cardo polyimides.
3.4. Optical property
the same thermal imidization process to avoid the effect of sample
preparation method on the optical properties. Flexible, transparent,
For measuring optical properties, all PI films were prepared by
and strong thin films were obtained from all PIs by casting from

P. Wen et al. / Polymer 117 (2017) 76e83
81
Table 1
Solubility of cardo polyimides.
Sample
DMSO
NMP
DMAc
DMF
m-Cresol
Xylene
Toluene
CH
3
OH
CHCl
3
GBA
THF
PI-35PM
PI-24PM
PI-356F
PI-246F
PI-35CB
PI-24CB
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
þ
þ
þ
þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
þ: soluble. -: insoluble.
Table 2
Thermal properties of cardo polyimides.
ꢀ
ꢀ
ꢀ
ꢀ
Sample
T
g
( C)
T
d
( C)
T
5
( C)
T
10 ( C)
w
R (%)
PI-35PM
PI-24PM
PI-356F
PI-246F
PI-35CB
PI-24CB
300
315
295
311
e
424
431
423
435
374
363
533
549
500
513
420
420
584
584
527
537
457
463
71
74
65
67
67
70
e
addition, high aromatic contents in the SBF units can also provide
high refractive indices. It should be noted that such high refractive
indices were achieved without incorporating any heavy atoms such
as halogen or sulphur which could generate substantial problems
as mentioned previously. The in-plane (nTE) and out-of-plane (nTM
)
refractive indices of the thin films are in the range of 1.6345e1.7011
and 1.6340e1.7000, respectively. nTE values are slightly higher than
those of nTM reflecting the preferential orientation of the main
chains parallel to the film plane. PI-35CB and PI-24CB show rela-
tively low n-values compare to PI-35PM and PI-24PM due to less
aromatic contents. PI-356F and PI-246F show the lowest n-value
mainly because of the high content of fluorine. As shown in Table 3,
Fig. 3. TGA of cardo PIs.
the birefringence (Dn) of polyimides was significantly small in any
case, ranging from 0.0005 to 0.0014. The sufficiently low birefrin-
gence of PI would result mainly from the effect of the SBF structure.
Two fluorene planes of SBF moiety are intersected with each other
ꢀ
at 90 angle and are not folded in one direction, attributing to the
low optical anisotropy of polymer. Especially, PI-356F exhibited the
lowest Dn (0.0005), which can be ascribed to the flexible molecular
structure as well as two bulky trifluoromethyl units, consequently
resulting in an increased chain mobility and a lower chain packing
density. It is noteworthy that the birefringence of PI-35PM and PI-
2
4PM, even though they contain planar PMDA structures, is as
small as 1/60 compared to that of PMDA-ODA PI [9]. It is, therefore,
considered that the unique C -symmetric SBF structure in the PI
2
side chain is responsible for the significantly low birefringence with
high refractive index.
Optical transparency of PI film is a crucial factor for its use in
optical applications. Fig. 5 illustrates the transmittance of PI films
normalized to the thickness of 45
mm between the wavelength
range of 200e800 nm. The films of PI-35CB, PI-356F, PI-24CB and
PI-246F are almost colourless and had good transparency in the
visible region. In particular, the transparency of PI-35CB was as high
as 95% at 400 nm. For PI-35PM and PI-24PM films, both showed
light yellow colour exhibited the transparency of 80% and 53% at
Fig. 4. DSC of cardo PIs.
4
00 nm, and 92% and 80% at 450 nm, respectively. All PIs showed
PAA solutions with a subsequent stepwise thermal imidization
process for optical property measurement. Table 3 summarizes
typical optical properties of polyimides (in film state with thickness
transmittance at 550 nm more than 97% with similar spectral
shapes, which is considerably higher than those of PIs having a
comparable refractive index [11,12]. The high transparency of PIs in
this work is attributed to their amorphous nature owing to the
bulky and rigid SBF groups which effectively prevents the intra-
and/or inter-chromophore interactions and decreases the packing
of 45 mm). As expected, all the polyimides exhibited high average
refractive indices (nav) in the range of 1.6343e1.7007 at 632.8 nm.
In general, the aromatic PIs often show inherently high refractive
indices because of their highly polarizable molecular structures. In

82
P. Wen et al. / Polymer 117 (2017) 76e83
Table 3
Optical properties of cardo polyimides.
Sample
Refractive index
Birefringence
nb
a
n
TE
n
TM
n
av
D
PI-35PM
PI-24PM
PI-356F
PI-246F
PI-35CB
PI-24CB
1.6964
1.7011
1.6345
1.6394
1.6642
1.6731
1.6950
1.7000
1.6340
1.6388
1.6633
1.6722
1.6959
1.7007
1.6343
1.6392
1.6639
1.6728
0.0014
0.0011
0.0005
0.0006
0.0009
0.0009
a
av ¼ [(2nTE þ nTM)/3]1/2
2
2
n
D
.
b
n ¼ (nTE - nTM).
Science Foundation of China (2015JJ3063), and Young Scientist
Exchange Program between China and Korea in 2016. MHL also
thanks to the financial support from the Basic Science Research
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (2013R1A1A2063696).
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