Synthesis and characterization of unsymmetrical benzonitrile-containing polyimides: Viscosity-lowering effect and dielectric properties

Article abstract of DOI:10.1002/pola.26927

A new unsymmetrical diamine, 2-(3-aminophenoxy)-6-(4-aminophenoxy) benzonitrile (3,4-APBN), is synthesized via two consecutive S_NAr reactions and the temperature-dependent reactivity of the fluorides in 2,6-difluorobenzonitrile, whose first S_NAr reaction occurs at 70 °C and second, at 100 °C, allowing timing control of reaction sequence and circumventing the transetherification side reaction. Thus, a series of polyimides (PIs) is prepared from the polymerization of 3,4-APBN with five common dianhydrides (6FDA, DSDA, OPDA, BTDA, and PMDA). For comparison, a second series is also prepared from two symmetrical diamines ([2,6-bis(3-aminophenoxy) benzonitrile (3,3-APBN) and 2,6-bis(4-aminophenoxy)benzonitrile (4,4-APBN)] and 6FDA or PMDA. The processability of the poly(amic acids) (PAAs), for the first series is greatly improved since their solution viscosities are much lower than PAAs based on symmetrical diamines. Besides having high glass-transition temperatures (249-332 °C), and thermal stability [5% weight loss in the range of 505-542 °C (air) and 512-546 °C (nitrogen)], these PIs form tough, transparent and flexible films that have a tensile-strength range of 82.1-121.3 MPa, elongations-at-break of 5.33-9.81%, and tensile moduli of 2.11-2.97 GPa. Their film dielectric constants are 3.08-3.62 at 10 kHz, moderately higher than that (2.92) of analogous PI (CP2) without nitrile groups. Overall, we found that the reduction of structural symmetry in repeat units can improve the polymer processibility as well as increasing their dielectric constants. Copyright

Full text of DOI:10.1002/pola.26927

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Synthesis and Characterization of Unsymmetrical Benzonitrile-Containing
Polyimides: Viscosity-Lowering Effect and Dielectric Properties
David H. Wang,1,2 John K. Riley,1,2 Scott P. Fillery,1,3 Michael F. Durstock,1
Richard A. Vaia,1 Loon-Seng Tan1
1Materials and Manufacturing Directorate, AFRL/RXAS, Air Force Research Laboratory, Wright-Patterson Air Force Base,
Ohio 45433
2UES, Inc., Dayton, Ohio 45432
3National Research Council, Washington, DC 20001
Correspondence to: L.-S. Tan (E-mail: loon.tan@us.af.mil)
Received 14 July 2013; accepted 27 August 2013; published online 24 September 2013
DOI: 10.1002/pola.26927
ABSTRACT: A new unsymmetrical diamine, 2-(3-aminophenoxy)-
6-(4-aminophenoxy)benzonitrile (3,4-APBN), is synthesized via
two consecutive SNAr reactions and the temperature-dependent
reactivity of the fluorides in 2,6-difluorobenzonitrile, whose first
SNAr reaction occurs at 70 ꢀC and second, at 100 ꢀC, allowing
timing control of reaction sequence and circumventing the
transetherification side reaction. Thus, a series of polyimides
(PIs) is prepared from the polymerization of 3,4-APBN with five
common dianhydrides (6FDA, DSDA, OPDA, BTDA, and PMDA).
For comparison, a second series is also prepared from two sym-
metrical diamines ([2,6-bis(3-aminophenoxy)benzonitrile (3,3-
APBN) and 2,6-bis(4-aminophenoxy)benzonitrile (4,4-APBN)]
and 6FDA or PMDA. The processability of the poly(amic acids)
(PAAs), for the first series is greatly improved since their solu-
tion viscosities are much lower than PAAs based on symmetrical
diamines. Besides having high glass-transition temperatures
(249–332 ꢀC), and thermal stability [5% weight loss in the range
of 505–542 ꢀC (air) and 512–546 ꢀC (nitrogen)], these PIs form
tough, transparent and flexible films that have a tensile-strength
range of 82.1–121.3 MPa, elongations-at-break of 5.33–9.81%,
and tensile moduli of 2.11–2.97 GPa. Their film dielectric con-
stants are 3.08–3.62 at 10 kHz, moderately higher than that (2.92)
of analogous PI (CP2) without nitrile groups. Overall, we found
that the reduction of structural symmetry in repeat units can
improve the polymer processibility as well as increasing their
C
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dielectric constants. 2013 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2013, 51, 4998–5011
KEYWORDS: benzonitrile; dielectric properties; polyimides; trans-
etherification; unsymmetrical diamine; viscosity
their solubility and processibility.11–17 For example, the intro-
duction of an unsymmetrical trifluoromethyl group into the
polymer chain has resulted in soluble PIs without compro-
mising the properties via the reduction in solubility-
impeding inter-chain interactions.18–23 Aromatic poly(ether-
imides) derived from an unsymmetrical indane-based dia-
mine have been evaluated for gas permeation properties.24,25
Unsymmetrical diamines containing thiazole ring,26,27 triaryl
imidazole pendant group,28 diphenylpyridyl pendant group,29
naphthalene unit,30 and phthalazinone moiety,31 have been
designed for the syntheses of their respective PIs. In other
cases, the PIs derived from unsymmetrical dianhydrides
have also been prepared for the similar reasons.32,33 How-
ever, most of these unsymmetrical diamines and dianhy-
drides are not practical for large scale production because of
the high cost inevitably incurred from their multistep
INTRODUCTION Polyimides (PI) represent one of the most
important classes of specialty polymers. This is a direct
result of the excellent combination of their physical and
mechanical properties as well as outstanding thermal stabil-
ity. In addition, their monomers are readily available, and are
cheaper than many other high temperature polymers. As a
result, PIs have found many applications in high performance
films and fibers, coatings, and adhesives for a wide range of
applications in, just naming a few, microelectronics, optoelec-
tronics, aerospace structures, nonlinear optical devices, light-
wave guiding materials, and liquid crystal displays.1–6 How-
ever, it is well known that fully imidized, linear PIs possess
extended rigid and planar, aromatic structures are inherently
insoluble in common solvents.7–10 As an approach to circum-
venting this problem, the incorporation of unsymmetrical
repeat units into PIs have been recently used to improve
Additional Supporting Information may be found in the online version of this article.
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syntheses. Apart from enhancing solubility to improve proc-
essibility, the reduction or elimination of structural symme-
try could also have significantly influenced certain polymer
properties that are important for specific applications, and
such effect was rarely called out.
Instrumentation
Transmittance Infrared (IR) spectra were recorded on a Nico-
let Nexus 470 F-T spectrophotometer. Attenuated Total Reflec-
tion IR (ATR-IR) was measured on
a Bruker Alpha-R
spectrometer. Proton and carbon nuclear magnetic resonance
spectra were measured at 300 MHz on a Bruker AVANCE 300
spectrometer. Differential scanning calorimetry (DSC) analysis
was performed in nitrogen (heating rate of 10 ꢀC/min) using a
Perkin-Elmer DSC 7 Thermal Analyzer. Thermogravimetric
analysis (TGA) was conducted in either nitrogen (N2) or air
PIs derived from symmetrical diamines containing benzonitrile
units have been studied for the piezoelectric and other dielec-
tric applications due to the high polarity and thermal stability
of the nitrile groups.34–37 The pendant nitrile groups promote
the adhesion of the polymers to many substrates, possibly
through the polar and hydrogen bonding interactions with the
surface groups that are commonly containing oxygen or OH.38
Polymers with high dielectric constants and low loss are in
growing demand for further miniaturization of microelectronic
systems.39 Kakimoto et al. have showed that attaching benzoni-
trile pendants on certain benzophenone-based PI would signifi-
cantly enhance their dielectric constants.40 Hybrid films based
on nitrile-containing PIs/inorganic particles (pyrite ash, barium
and titanium oxides) have been prepared and their nanoactua-
tion performance has been investigated.41 The piezoelectric
performance of PIs containing one and two nitrile groups per
repeat unit has recently been reported. The PIs containing two
nitrile groups per repeat unit with higher polarity have shown
higher value of remnant polarization than the ones containing
only one CN per repeat unit. However, their values are still low
in comparison with common piezoelectric polymers such as
PVDF.36 Two PIs have been prepared from an unsymmetrical
benzonitrile-containing diamine, but their dielectric properties
were not reported.42
ꢀ
atmosphere at a heating rate of 10 C/min using a TA Hi-Res
TGA 2950 thermogravimetric analyzer. Dynamic mechanical
analysis (DMA) was conducted under a nitrogen atmosphere
with a heating rate of 4 ꢀC/min on a TA Instruments DMA
Q800. UV–vis spectra of polymer thin films and solutions were
measured at 300–800 nm on a Varian Cary-5000 UV–vis-NIR
Spectrophotometer. Intrinsic viscosities were determined with
Cannon-Ubbelohde No. 75 viscometers. Flow times were
recorded in N-methyl-2-pyrrolidone (NMP) solution and poly-
mer conceꢀntrations were approximately 0.5–0.10 g/dL at
30.0 6 0.1 C. Lithium bromide (1.0 wt %) was added to the
solution to suppress polyelectrolyte effect. Viscosity of concen-
trated PAA solution (20 wt % in DMAc) was measured with
LVDV-I Prime C Brookfield Digital Viscometer. Gel permeation
chromatography (GPC) was carried out on an Agilent 1100
Series instrument equipped with refractive index and light
scattering detectors. Tetrahydrofuran (THF) was used as the
eluting solvent. Polystyrene standards were used in molecular
weight determination. Gas chromatography/mass spectros-
copy (GC/MS) was performed using a Varian 1200 Series
instrument. Melting Points were obtained from a Buchi Melt-
In this article, an unsymmetrical benzonitrile-containing dia-
mine was synthesized via a convenient one-pot reaction and
utilized in PI synthesis. As an effort to probe the structural
factors at the molecular level to balance the need for high
dielectric constant, low dielectric loss, and thermal stability
in high performance capacitors for aerospace applications,
two structurally related series of PIs containing benzonitriles
have been synthesized. The effect of the unsymmetrical
structure on the solution viscosity of poly(amic acid) (PAA)
precursors and PIs as well as the dielectric properties of the
PI films and have been studied in conjunction with the ther-
mal, UV–vis and morphological properties of the PIs.
ꢀ
ing Point Apparatus B-545 (at heating rate of 2 C/min). Ten-
sile strength and moduli of the PI films were determined using
a Tinius Olsen H10K-S Benchtop Testing Machine with a cross-
head separation speed of 1 mm/min. Wide Angle X-ray diffrac-
tion (WAXD) experiments were carried out on a Statton box
camera at 53 mm sample to image plate distances in transmis-
sion mode using CuKa generated by a Rigaku Ultrax18 system.
Dielectric Impedance Spectroscopy
Dielectric impedance experiments were performed over
broad-band frequency (1021 to 106 Hz) and temperature
(240 to 150 ꢀC) ranges. Disk-like specimens, formed from
cast thin films, featuring thicknesses of 40 mm, were gold-
sputtered to form electrodes 10 mm in diameter and sand-
wiched between gold-plated brass electrodes, prior to inser-
tion in a Novocontrol ZGS Alpha sample cell, connected to a
Novocontrol Quatro Cryosystem for temperature stabiliza-
tion. Broad temperature cycles were undertaken to eliminate
thermal hysteresis and water contributions.
EXPERIMENTAL
Materials
2,6-Dichlorobenzonitrile (DCBN) and 2,6-difluorobenzonitrile
(DFBN) were obtained from Aldrich Chemical Co. The aro-
matic tetracarboxylic dianhydrides used in this work: pyro-
mellitic dianhydride (PMDA), 3,30,4,40-biphenyltetracarboxylic
dianhydride (BPDA), 4,40-benzophenonetetracarboxylic dia-
nhydride (BTDA), 4,40-oxydiphthalic dianhydride (ODPA),
3,30,4,40-diphenylsulfone tetracarboxylic dianhydride (DSDA)
and 2,20-bis(3,4-dicarboxyphenyl)21,1,1,3,3,3-hexafluoropro-
pane dianhydride (6FDA), were obtained from commercial
sources and purified by sublimation. All other reagents and
solvents were purchased from Aldrich Chemical Co. and used
as received.
Synthesis of Unsymmetrical Diamine via Two-Pot Route
2-Chloro-6-(3-aminophenoxy)benzonitrile (3a)
Into a 500-mL three-necked round-bottomed flask (RFB)
equipped with a magnetic stirrer, a Dean-Stark trap/con-
denser/nitrogen adaptor,
ground-joint glass stopper, 3-aminophenol (5.46 g, 50.0 mmol),
a thermometer/adaptor, and a
potassium carbonate (8.28 g, 60.0 mmol), N-methyl-
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2-pyrrolidinone (NMP, 100 mL) and toluene (50 mL) were
charged. The reaction mixture was heated to reflux under nitro-
gen in an oil bath, and water was removed by azeotropic distil-
glass stopper, 4-aminophenol (2.18 g, 20.0 mmol), potassium
carbonate (2.76 g, 20.0 mmol), NMP (50 mL) and toluene
(10 mL) were charged and water was removed by azeotropic
distillation with toluene by refluxing the mixture in an oil
ꢀ
lation with toluene (100–110 C). After complete dehydration,
ꢀ
the remaining toluene was distilled off until the flask tempera-
bath at 120–130 C with stirring and under nitrogen atmos-
ꢀ
ture reached 120 C. The reaction mixture was allowed to cool
phere. After complete dehydration, the remaining toluene
was distilled off until the flask temperature reached 160 ꢀC.
The reaction mixture was allowed to cool to 130 ꢀC, com-
pound 3a (4.88 g, 20.0 mmol) was added, and the mixture
was stirred again at 160 ꢀC for 24 h under nitrogen atmos-
phere. Then, the mixture was allowed to cool to room tem-
perature and filtered. The filtrate was poured into water and
the precipitated solid was collected by filtration and dried.
The crude product was recrystallized from toluene/heptanes
to 100 ꢀC, 2,6-DCBN (8.60 g, 50.0 mmol) was quickly added
and the mixture stirred at 120 ꢀC for 24 h under nitrogen
atmosphere. The mixture was allowed to cool to room tempera-
ture and filtered. The filtrate was poured into water. The white
solid was extracted with CH2Cl2 and the organic layer was sepa-
rated. It was washed with water three times and dried over
sodium sulfate. After concentrating the extract on a rotavap, the
crude liquid was passed through a short column (basic alumina;
ethyl acetate/hexane 40:60 used as the eluent). Upon stripping
off the solvent from the filtrate, a light yellow liquid was
obtained, which solidified as white crystals.
ꢀ
to afford 3.64 g (57%) of yellow crystals, m.p. 153–156 C.
Anal. Calcd. for C19H15N3O2: C, 71.91%; H, 4.76%; N,
13.24%. Found: C, 72.08; H, 4.75%; N, 13.33%. FTIR (KBr,
cm21): 3400, 3326 (NH2), 2226 (nitrile). Mass spectrum
Yield: 8.9 g (73%), m.p. 68.7–69.1 ꢀC. Anal. Calcd. for
1
C13H9ClN2O: C, 63.81%; H, 3.71%; N, 11.45%. Found: C,
(m/e): 317 (M1). H NMR (DMSO-d6, d in ppm): 5.17 (s, 2H,
63.99%; H, 3.81%; N, 13.46%. FTIR (KBr, cm21): 3468, 3378
(NH2), 2234 (nitrile). Mass spectrum (m/e): 244, 246 (M1).
1H NMR (DMSO-d6, d in ppm): 5.40 (s, 2H, NH2), 6.27–6.29
(d, 1H, Ar-H), 6.33 (s, 1H, Ar-H), 6.48–6.50 (d, 1H, Ar-H),
6.91–6.94 (d, 1H, Ar-H), 7.08–7.12 (t, 1H, Ar-H), 7.40–7.42
(d, 1H, Ar-H), and 7.63–7.67 (t, 1H, Ar-H).
NH2), 5.38 (s, 2H, NH2), 6.25–6.34 (m, 2H, Ar-H), 6.38–6.66
(m, 5H, Ar-H), 6.85–6.91 (m, 2H, Ar-H), 7.01–7.11 (m, 1H,
Ar-H), and 7.36–7.55 (m, 1H, Ar-H); see Figure 1.
Attempted Synthesis of 2-(3-Aminophenoxy)-6-(4-
aminophenoxy)benzonitrile (3,4-APBN; 5) from 2-Chloro-
6-(4-aminophenoxy)benzonitrile (3b)
2-Chloro-6-(4-aminophenoxy)benzonitrile (3b)
Into a 250-mL three-necked round-bottomed flask equipped
with a magnetic stirrer, a Dean-Stark trap/condenser/nitro-
gen adaptor, a thermometer/adaptor, and a ground-joint
glass stopper, 3-aminophenol (2.18 g, 20.0 mmol), potassium
carbonate (2.76 g, 20.0 mmol), NMP (50 mL), and toluene
(10 mL) were charged and water was removed by azeotropic
distillation with toluene by refluxing the mixture in an oil
Into a 500-mL three-necked RBF equipped with magnetic
stir bar, Dean-Stark trap/nitrogen inlet, a thermometer/adap-
tor, and a glass stopper were placed 4-aminophenol (2.73 g,
25.0 mmol), potassium carbonate (4.15 g, 30.0 mmol), 50
mL NMP and 10 mL toluene. Water was removed by azeo-
tropic distillation by refluxing the mixture with stirring at
150 ꢀC. After complete dehydration, the remaining toluene
ꢀ
bath at 150 C with stirring and under nitrogen atmosphere.
ꢀ
was removed by heating to 160 C. The mixture was allowed
After complete dehydration, the remaining toluene was dis-
tilled off until the flask temperature reached 160 ꢀC. The
reaction mixture was allowed to cool to 100 ꢀC, compound
3b (4.88 g, 20.0 mmol) was added, and the mixture was
to cool to 100 ꢀC and 2,6-DCBN (4.30 g, 25.0 mmol) was
added. It was heated at 120 ꢀC for 24 h under nitrogen
atmosphere with 98% conversion as indicated by previous
GC testing. The mixture was allowed to cool to room temper-
ature and filtered. The filtrate was poured into water. The
white precipitate was collected by filtration and air dried. It
was then recrystallized from ethanol/water (40/60) mixed
solvents to afford white crystals.
ꢀ
stirred again at 140 C for 24 h under nitrogen atmosphere.
Then, the mixture was allowed to cool to room temperature
and filtered. The filtrate was poured into water and the pre-
cipitated solid was collected by filtration and dried. The
crude product was recrystallized from toluene/heptꢀanes to
afford 4.35 g (68%) of yellow crystals, m.p. 153–156 C.
Yield: 10.5 g (86%), m.p. 122.2–122.8 ꢀC. Anal. Calcd. for
C
13H9ClN2O: C, 63.81%; H, 3.71%; N, 11.45%. Found: C,
63.74%; H, 3.84%; N, 11.57%. FTIR (KBr, cm21): 3418, 3349
(NH2), 2234 (CBN). Mass spectrum (m/e): 244, 246 (M1).
1H NMR (DMSO-d6, d in ppm): 5.16 (s, 2H, NH2), 6.62–6.64
(d, 2H, Ar-H), 6.69–6.72 (d, 1H, Ar-H), 6.87–6.89 (d, 2H,
Ar-H), 7.29–7.31 (d, 1H, Ar-H), and 7.53–7.58 (t, 1H, Ar-H).
2-Fluoro-6-(3-aminophenoxy)benzonitrile (6a)
Into a 500-mL three-necked flask equipped with a magnetic
stirrer and two adaptors for nitrogen inlet/outlet, 3-
aminophenol (8.74 g, 80.0 mmol), 2,6-DFBN (11.13 g, 80.0
mmol), potassium carbonate (13.24 g, 96.0 mmol) and NMP
(150 mL) were charged. The resulting mixture was stirred
under nitrogen atmosphere at 70 ꢀC for 48 h. The mixture
was allowed to cool to room temperature and poured into
water. The white precipitated solid was collected by filtra-
tion, washed with water and dried. The crude product was
recrystallized from toluene/heptanes (1:ꢀ2) to afford 13.6g
(74.5%) of white crystals, m.p. 79.8–82.6 C.
Attempted Synthesis of 2-(3-Aminophenoxy)-6-(4-
aminophenoxy)benzonitrile (3,4-APBN; 5) from 2-Chloro-
6-(3-aminophenoxy)benzonitrile (3a)
Into a 250-mL three-necked round-bottomed flask equipped
with a magnetic stirrer, a Dean-Stark trap/condenser/nitro-
gen adaptor, a thermometer/adaptor, and a ground-joint
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FIGURE 1 1H NMR spectrum of isomeric mixture of the APBN diamines. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
Anal. Calcd. for C13H9FN2O: C, 68.42%; H, 3.98%; N, 12.27%.
Found: C, 68.48%; H, 4.07%; N, 12.31%. FTIR (KBr, cm21):
3435, 3369 (NH2), 3132, 3077, 2232 (nitrile). Mass spec-
trum (m/e): 228 (M1). 1H NMR (DMSO-d6, d in ppm): 5.39
(s, 2H, NH2), 6.27–6.29 (d, 1H, Ar-H), 6.32 (s, 1H, Ar-H),
6.47–6.49 (d, 1H, Ar-H), 6.77–6.79 (d, 1H, Ar-H), 7.08–7.11
(t, 1H, Ar-H), 7.18–7.22 (t, 1H, Ar-H) and 7.66–7.72(q, 1H,
Ar-H). 13C NMR (DMSO-d6, d in ppm): 92.01, 92.19, 104.56,
106.37, 109.64, 109.83, 110.83, 111.15, 112.86, 130.41,
136.27, 136.38, 150.86, 155.20, 160.32, 161.81, 164.35.
(3.44 g, 31.5 mmol), 2-fluoro-6-(4-aminophenoxy)benzoni-
trile (2.28 g, 10 mmol), potassium carbonate (4.98 g, 36.0
mmol), and 75 mL of anhydrous NMP were charged and the
resulting mixture was stirred at 100 ꢀC for 48 h. The mix-
ture was allowed to cool to room temperature and filtered.
The filtrate was poured into water to precipitate a white
crude product. The white solid was collected by filtration,
washed with water and dried. The crude product was recrys-
tallized from ethanol to afford 8.0 g (84%) of white crystals,
ꢀ
m.p. 160.8–161.8 C.
2-Fluoro-6-(4-aminophenoxy)benzonitrile (6b)
Anal. Calcd for C19H15N3O2: C: 71.91%, H: 4.76%, N: 13.24%
Anal Found: C: 72.12%, H: 4.78%, N: 12.90%. FTIR: 3422,
3400, 3322 (NH2), 2227 (nitrile). Mass spectrum (m/e): 228
Compound 6b was synthesized from 4-aminophenol (8.74 g,
80.0 mmol), 2,6-DFBN (11.13 g, 80.0 mmol), potassium car-
bonate (13.24 g, 96.0 mmol) and NMP (150 mL) according
to the same procedure as 6a to afford 14.6 g (80%) of white
1
(M1). H NMR (DMSO-d6, d in ppm): 5.15 (s, 2H, NH2), 5.35
(s, 2H, NH2), 6.24–6.27 (d, 1H, Ar-H), 6.29–6.30 (t, 1H, Ar-
H), 6.38–6.41 (d, 1H, Ar-H), 6.43–6.45 (d, 1H, Ar-H), 6.50–
6.52 (d, 1H, Ar-H), 6.62–6.64 (d, 2H, Ar-H), 6.88–6.90 (d, 2H,
Ar-H), 7.05–7.09 (t, 1H, Ar-H), and 7.43–7.48 (t, 1H, Ar-H).
13C NMR (DMSO-d6, d in ppm): 92.93, 104.50, 106.39,
108.61, 109.75, 110.79, 113.38, 114.78, 121.31, 130.35,
135.35, 143.65, 146.68, 150.78, 155.60, 160.55, 162.28.
ꢀ
crystals, m.p. 156.3–156.6 C.
Anal. Calcd. for C13H9FN2O: C, 68.42%; H, 3.98%; N, 12.27%.
Found: C, 68.52%; H, 4.11%; N, 12.23%. FTIR (KBr, cm21):
3490, 3396 (NH2), 3100, 3062, 2232 (nitrile). Mass spectrum
1
(m/e): 228 (M1). H NMR (DMSO-d6, d in ppm): 5.18 (s, 2H,
NH2), 6.57–6.59 (d, 1H, Ar-H), 6.61–6.65 (d, 2H, Ar-H), 6.87–
6.91 (d, 2H, Ar-H), 7.09–7.12 (t, 1H, Ar-H), and 7.60–7.65(q, 1H,
Ar-H). 13C NMR (DMSO-d6, d in ppm): 90.77, 90.95, 108.79,
108.98, 111.05, 111.08, 111.56, 114.79, 121.28, 136.30, 136.40,
143.42, 146.94, 161.93, 162.04, 162.08, 164.47.
Synthesis of Unsymmetrical Diamine via One-Pot Route
2-(3-Aminophenoxy)26-(4-aminophenoxy)benzonitrile
(4a, 3,4-APBN), Route 1
Into a 250-mL three-necked flask equipped with a magnetic
stirrer, a nitrogen inlet and outlet, 4-aminophenol (1.39 g,
10.0 mmol), 2,6-DFBN (1.09 g, 10.0 mmol), potassium car-
bonate (1.66 g, 912.0 mmol) and NMP (50 mL) were
charged and the mixture was stirred under nitrogen atmos-
phere at 70 ꢀC for 48 h. After GC-MS spectra indicated the
Synthesis of 2-(3-Aminophenoxy)-6-(4-
aminophenoxy)benzonitrile (4a, 3,4-APBN) from
2-Fluoro-6-(4-aminophenoxy)benzonitrile
Into a 250-mL three-necked flask equipped with nitrogen
inlet, magnetic stir bar, and condenser, 3-aminophenol
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SCHEME 1 Temperature-controlled sequential displacement of O-nitrile-activated halide by a phenolate nucleophile with appropri-
ate basicity.
reaction was completed, 3-aminophenol (1.42 g, 13.0 mmol)
and potassium carbonate (1.66 g, 12.0 mmol) were added
and the mixture was heated at 100 C for 48 h. The mixture
was allowed to cool to room temperature and filtered. The
filtrate was poured into water. The white solid was obtained
by filtration. It was recrystallized from ethanol to afford 1.96
g (61.8%) of white crystals, m.p. 161.7–161.8 ꢀC. The iden-
tity and purity product were confirmed by GC-MS, IR and
NMR spectra.
24 h to afford a viscous PAA (7). The resulting mixture was
diluted with DMAc (4 mL). For the thermally imidized poly-
mer film samples, the PAA/DMAc solution was poured into
glass dishes, followed by vacuum evaporation of DMAc at 50
ꢀC for 12 h.ꢀ The films were furtherꢀheat-treatment at 100
ꢀ
ꢀ
ꢀ
ꢀC/2 h; 150 C/2 h; 175 C/1 h, 200 C/1 h, and 250 C/1 h
under N2 to afford light yellow PI films. For the fiber sam-
ples, a chemical imidization was used instead. Thus, pyridine
and acetic anhydride were added to the PAA. It was agitated
at room temperature for 24 h and poured into ethanol to
afford a fibrous precipitates, which were collected by filtra-
tion. The fibers were soxhlet-extracted with ethanol to
remove the residual DMAc and dried in a vacuum-oven at
150 ꢀC for 24 h. The ATR-IR spectra (films) exhibited
absorptions at 2230–2232 (CN), 1777–1785, and 1714–
1720 cm21 (imide).
2-(3-Aminophenoxy)-6-(4-aminophenoxy)benzonitrile
(4a, 3,4-APBN), Route 2
3,4-APBN was synthesized according to the same procedure
as Route 1 with following modification: 3-aminophenol was
used in the first step and 4-aminophenol was used in the
second step to afford 2.04 g (64.2%) of white crystals after
ꢀ
recrystallization from ethanol, m.p. 161.0–161.8 C. The pure
product was confirmed by GC-MS, IR and NMR spectra.
RESULTS AND DISCUSSION
Monomer Synthesis
2,6-Bis(3-aminophenoxy)benzonitrile (4b, 3,3-APBN) and
2,6-Bis(4-aminophenoxy)benzonitrile (4c, 4,4-APBN)
Syntheses of compounds 4b and 4c were conducted accord-
ing to the procedures described in reference.38
While both 2,6-DCBN (1a) and 2,6-DFBN (1b) have been uti-
lized as co-monomers with bis(phenolate) monomers in the
synthesis of benzonitrile-containing poly(arylene-ether)s, the
polymer-forming conditions are such that both halo-
substituents are generally assumed to have comparable reac-
tivity above certain temperature, leading to various ether-
nitrile polymers.43 During the preparation of the known,
symmetrical diamino monomers namely 3,3-APBN and 4,4-
APBN,38 using the potassium salts of 3-aminophenol (2a) or
4-aminophenol (2b) as a nucleophile [Scheme 1, reactions
General Procedure for PI Synthesis (8a-8i and 9)
Into a 50-mL one-necked, round-bottomed flask equipped
with a magnetic stir-bar, nitrogen inlet, DMAc (10 mL) and
diamine (3.50 mmol) was charged. After diamine was dis-
solved in DMAc, dianhydride (3.50 mmol) was added. The
light yellow solution was agitated at room temperature for
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(i) and (ii)], we found that two halo groups on the same
phenyl ring of either DCBN or DFBN had shown different
but exploitable rates in the aromatic nucleophilic substitu-
tion (SNAr) reaction. Specifically, as observed by GC-MS mon-
itoring of a reaction mixture of 4-DCBN and aminophenol/
K2CO3 (Supporting Information Figure 1), the reactivity of
second halide toward the same nucleophile was apparently
mitigated, and therefore, heating at a higher temperature
was required for the second aromatic displacement reaction
to occur. Thus, this substantial temperature difference (ꢁ20–
40 ꢀC) in the leaving-group activation by the ortho-nitrile
has allowed the isolation of the mono-substituted products,
i.e., 2-chloro-6-(3-aminophenoxy)benzonitrile (3a) and its
fluoro-analog, in good yields. Therefore, this sufficiently
broad difference in reactivity presents an opportunity to
regioselectively synthesize various unsymmetrical diamines,
and in particular, 3,4-APBN. We note that this rationale is
also supported by several reported examples of 2-chloro-ary-
loxy-benzonitriles44 or 2-fluoro-6-aryloxy-benzonitriles45 iso-
lated from the respective reaction of 2,6-dichloro- or
2,6-DFBN with an appropriate aryloxy anion in DMF.
However, in our initial effort to further validate such two-
step synthesis [Scheme 1, (i) and (iv)] of 3,4-APBN (4a), we
used DCBN (1a) as the starting material to react with 3-
aminophenolate (or 4-aminophenolate) at 120 ꢀC and iso-
lated the chloro-aryloxy intermediate 3a (or 3b) before its
use in the second aromatic displacement reaction at 160 ꢀC
with 4-aminophenolate as the second nucleophile. To our
surprise, the isolated mixture, based on 1H NMR peak area
integration (Fig. 1; see also Supporting Information Figure
SI-2 for the peak assignments), was actually comprised of
58% 3,4-APBN (4a), 13% 3,3-APBN (4b), and 29% 4,4-
APBN (4c), which provided a strong evidenꢀce that the trans-
etherification process had occurred at 160 C. Further, when
we conducted a similar experiment in a one-pot fashion that
entailed heating an equimolar mixture of DCBN and 4-ami-
FIGURE 2 NMR spectra of 3,4-APBN: (a) 1H NMR and (b) 13C
ꢀ
nophenol/K2CO3 in NMP at 120 C and monitoring the reac-
NMR both in DMSO-d6.
tion progress by GC-MS, we found that only a single product,
i.e., the 2-chloro-6-(4-aminophenoxy)benzonitrile intermedi-
ate (3b) was detected and ꢁ98% conversion was observed
after 24 h. After the reaction mixture had been allowed to
cool to 100 ꢀC, an equimolar amount of 3-aminophenol/
K2CO3 was added and the reaction temperature was raised
to 140 ꢀC. After 19 h at this temperature, GC-MS detected
the presence of 4,4-APBN and 3,3-APBN in addition to 3,4-
APBN with the elution times of 18.7, 19.1, and 19.4 min, in
that order (Supporting Information Figure SI-1) with 3,4-
APBN being the major component.
4-aminophenoxy (pKa 5 9.99) substituent in 3a could also
act as a leaving group in the presence of 3-aminophenolate
(2b; pKa 5 10.43), and vice versa in the case of 3b via a
dynamic transetherification process, leading eventually to an
equilibrium mixture of all three possible isomers. It is note-
worthy that Mercer et al.46 has reported a successful synthe-
sis of
a model compound, 2-(p-methoxyphenoxy)-6-(p-
cycanophenoxy)benzonitrile (3c in Scheme 2) from a similar
two-step reaction (but in the same-pot) of 2-chloro-6-
fluorobenzonitrile and two different phenolates generated in
situ in the presence of potassium carbonate, namely first
from 4-methoxyphenol (pKa 5 10.27), and then 4-
cycanophenol (pKa 5 7.80). However, it is noted that 2-
chloro-6-fluorobenzonitrile which costs more than 2,6-
dichloro- and 2,6-DFBN was used as the starting material
[see Scheme 1, reactions (v) and (vi)]. On the basis of these
results, we hypothesize that to avoid the occurrence of trans-
etherification in a 2-halo26-aryloxybenzonitile system, the
We attribute the isomeric mixture to the redistribution equi-
librium being driven by the occurrence of transetherification
(i.e. aryloxy-aryloxy exchange reaction) at relatively high
reaction temperatures. The first aromatic displacement reac-
tion occurred quite cleanly at 120 ꢀC to afford the mono-
substituted product, 2-chloro-6-(3-aminophenoxy)benzoni-
trile (3a) or 2-chloro-6-(4-aminophenoxy)benzonitrile (3b).
However, at ꢂ140 ꢀC, it became apparent that the
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SCHEME 2 Alternative routes to the unsymmetrical benzonitrile-ether-diamine from 2,6-DFBN.
second phenolate must be much weaker in its basicity (in
our estimation, by at least 2 pKa units) than the first one,
and the reaction temperature threshold, being tied closely to
the reduced leaving tendency of second halide, should be
and SI-5 for NMR, FTIR spectra, and “ball-stick” molecular
structures of the diamines 4a–c, in that order).38 All the
intermediates and diamines were also confirmed by elemen-
tal analysis, which were in good agreement with the calcu-
lated values.
ꢀ
ꢀ
kept as low as possible (<<150 C, and 100–120 C is opti-
mal from our study).
Synthesis of PIs
Our subsequent tactic for the 3,4-APBN synthesis is further
based on the fact that fluoride is more reactive than chloride
as well as the premise that a much lower reaction tempera-
tures can be used for DFBN (1b) to circumvent the transe-
therification side reaction (Scheme 2). Indeed, the first SNAr
reaction between DFBN and 4-aminophenol (2a) occurred at
The polymerization of the diamines 4 with the dianhydrides
(6) was conducted in DMAc at room temperature for 24 h to
afford PAAs (7), which were subsequently imidized by either
thermal or chemical route to afford the PIs (8) as shown in
Scheme 3. The unsymmetrical diamine (4a) was polymerized
with the dianhydrides includes 6FDA (6a), OPDA (6b), DSDA
(6c), BTDA (6d), and PMDA (6e). Two symmetrical diamines
(4b–c) were also polymerized with 6FDA (6a) and PMDA
(6e) for comparison purposes. The films prepared from ther-
mal imidization are tough and creasable; they were used for
the evaluation of mechanical, electrical, thermal and
ꢀ
70 C to cleanly give the isolable, mono-substituted product,
2-fluoro-6-(4-aminophenoxy)benzonitrile (6a). In a subse-
quent treating 6a with 3-aminophenolate (2b) at 100 ꢀC to
afford pure 3,4-APBN (4a) in good yield. Thus, the reaction
progress of the mixture of 1b, 3-aminophenolate, and 4-
aminophenolate, with the phenolates being introduced at dif-
ferent temperatures, was monitored with the aid of GC-MS
(depicted in Supporting Information Figure SI-3). It is clearly
shown that in these two separate reactions, neither 3,3- nor
4,4-APBN isomers (4b or 4c) was observed. The unsymmet-
1
rical structure of 3,4-APBN was confirmed by the H and 13C
NMR (Fig. 2) and FTIR (Supporting Information Figure SI-4)
spectral analyses as well as the elemental analysis results.
Operationally more convenient, the preparation of 3,4-APBN
was also conducted in a “one-pot” fashion by effecting the
tandem SNAr reactions, via sequentially adding 4-
aminophenol (or 3-aminophenol) to DFBN at 70 ꢀC (in the
presence of K2CO3) and 3-aminophenol (or 4-aminophenol)
at 100 ꢀC. It is critical to keep the reaction ꢃ120 ꢀC, above
which, transetherification would occur, resulting in the for-
mation of three isomers (4a–c). Finally, two symmetrical dia-
mines, namely, 3,3-APBN and 4,4-APBN were prepared by
treating DCBN with 3-aminophenol and 4-aminophenol,
respectively (see Supporting Information Figures SI-2, SI-4,
FIGURE 3 FTIR spectra of the PIs containing the 3,4-di(amino-
phenoxy)benzonitrile moiety.
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FIGURE 5 Viscosity versus stirring speed plots of the PAAs
and PIs derived from 6FDA and the isomeric APBN diamines:
(a) concentrated PAA solution (20 wt %) and (b) concentrated
PI solution (20 wt %). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 4 Inherent viscosity and reduced viscosity plots of the
PAAs derived from 6FDA and the isomeric APBN diamines:
data taken in NMP (a) without 1 wt % LiBr; (b) with 1 wt %
LiBr. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
fluoromethyl groups disrupt the polymer-chain packing and
inter-chain charge-transfer complexation in more stiff PIs. In
fact, the 6FDA-based PIs could even dissolve in chloroform.
On the other hand, the most rigid PMDA-based PIs (8e, g,
and i) are not soluble in any solvents tested. The OPDA-,
DSDA- and BTDA-based PIs (8b, c, and d) showed intermedi-
ate solubility. Therefore, the PI solubility is mainly deter-
mined by structures and geometry of the dianhydrides
(6FDA > OPDA > DSDA > BTDA > PMDA).
morphological properties. The chemical imidization in solu-
tion was performed using a mixture of acetic anhydride and
pyridine as a dehydrating agent/catalyst system. The precipi-
tated, fibrous PI samples were first soxhlet-extracted with
denatured ethanol and oven-dried before they were in the
characterization of their solubility, molecular weights and
solution-viscosity property. Their repeat-unit structures were
verified by ATR-IR spectroscopy, which confirmed the
expected absorptions due to the nitrile around 2230–2232
cm21 and the symmetrical and asymmetrical stretches due
to the imide-carbonyls around 1777–1785 and 1714–1720
cm21, respectively (Fig. 3). The elemental analysis data of
the resulting PIs also agreed well with the calculated values
(Table 1).
Since the 6FDA-based PIs are soluble in THF, their molecular
weights were measured in THF using polystyrene standards.
Polymer 8h exhibited the highest molecular weighs with
average Mn of 66,300 Da. while the molecular weight of poly-
mer 8a is the lowest one with average Mn of 41,400 Da.
Their intrinsic viscosities are in the range of 0.73 and 1.75
dL/g (Table 3).
Solution Properties
The solubilities of the PIs (8) were evaluated in several
organic solvents at 2.0% (w/v) (Table 2). Three 6FDA-based
PIs (8a, f, and h) showed the best solubility in all the sol-
vents except ethanol since the flexible 6FDA units and tri-
The PAA solutions prepared from 3,4-APBN showed much
lower viscosity values than those from 3,3-APBN and 4,4-
APBN. Both the dilute and concentrated solutions of three
6FDA-based PAA were further investigated.
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SCHEME 3 Synthesis of benzonitrile-containing PIs via PAA intermediates. In the inset, the polymer structure for CP2, labeled as
PI 9 in Table 5 is shown. Other members of 6FDA-APBN series are labeled as PIs 8a, 8f, and 8h in Table 1.
Inherent and reduced viscositiesꢀwere measured in 0.5–0.10
g/dL dilute NMP solution at 30 C. All the solutions showed
polyelectrolyte effect without addition of LiBr [Fig. 4(a)]. The
polyelectrolyte effects were suppressed with addition of 1
wt % LiBr [Fig. 4(b)]. However, with or without the presence
of LiBr, the PAA of 3,4-APBN/6FDA invariably exhibited the
lowest viscosity. Its intrinsic viscosity was 0.76 dL/g. The
intrinsic viscosities (with suppressed polyelectrolyte effect)
were much higher the symmetrical analogs: almost double
for 3,3-APBN/6FDA PAA (1.48 dL/g) and triple for 4,4-
APBN/6FDA PAA (2.0 dL/g).
APBN/6FDA PAA was only 250 cP at 30 ꢀC, it increased
almost 9 folds (ꢁ2200 cP) for 3,3-APBN/6FDA PAA and 15-
folds (3700 cP) for 4,4-APBN/6FDA PAA [Fig. 5(a)]. The
ultralow viscosity of 3,4-APBN/6FDA PAA is attributed to the
diamine unsymmetrical structure (Supporting Information
Figure SI-5). For the 3,4-APBN-6FDA polymer, there are three
possible structural arrangements: head-to-head, head-to-tail
and tail-to-tail (Supporting Information Figure SI-6). These
different arrangements make the PAA chains difficult to pack
closely, which dramatically reduces the interaction among
polymer chains thus PAA viscosity. The viscosity of three
6FDA-based PIs at 20 wt % concentration was also meas-
ured using a Brookfield Viscometer as shown in Figure 5(b).
Similar trend was also observed for the chemically imidized
PI samples: 4,4-APBN/6FDA PI (2650 cP at 30 ꢀC) > 3,3-
The viscosities of PAA and chemically imidized PI concen-
trated solutions (20 wt % in DMAc) were measured using a
Brookfield Viscometer (Fig. 5). While the viscosity of 3,4-
TABLE 1 Elemental Analysis of PIs
Theoretical (%)
Elemental Analysis (%)
H
PI No.
8a
Diamine
3,4-APBN, 4a
Dianhydride
6FDA, 7a
C
62.90
H
N
C
N
5.90
2.36
5.79
62.91
70.90
65.38
71.32
69.67
63.26
69.82
63.04
69.55
2.53
3.33
2.89
2.94
2.74
2.67
2.81
2.58
2.79
8b
8c
8d
8e
8f
3,4-APBN, 4a
3,4-APBN, 4a
3,4-APBN, 4a
3,4-APBN, 4a
3,3-APBN, 4b
3,3-APBN, 4b
4,4-APBN, 4c
4,4-APBN, 4c
OPDA, 7b
DSDA, 7c
BTDA, 7d
PMDA, 7e
6FDA, 7a
PMDA, 7e
6FDA, 7a
PMDA, 7e
71.07
65.73
71.64
69.74
62.90
69.74
62.90
69.74
2.90
2.68
2.84
2.62
2.36
2.62
2.36
2.62
7.10
6.57
6.96
8.41
5.79
8.41
5.79
8.41
7.19
6.79
7.18
8.50
5.88
8.58
5.83
8.62
8g
8h
8i
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TABLE 2 Solubility of PIs
PI No.
Ethanol
CH2Cl2
CHCl3
Acetone
THF
DMSO
DMAc
NMP
m-Cresol
8a
8b
8c
8d
8e
8f
2
2
2
2
2
2
2
2
2
1
1
2
2
2
1
2
1
2
1
PS
2
2
2
1
2
1
2
1
2
2
2
2
1
2
1
2
1
2
PS
2
2
1
2
1
2
1
1
1
PS
2
1
2
1
2
1
1
1
PS
2
1
2
1
2
1
1
1
1
2
1
2
1
2
1
1
1
1
2
1
2
1
2
8g
8h
8i
1, Soluble; 2, insoluble; PS, partly soluble.
APBN/6FDA PI (291 cP) > 3,4-APBN/6FDA PI (162 cP). All
the PAA solutions, because of their interchain hydrogen
bonding, are more viscous than the corresponding PIs. The
low viscosity of 3,4-APBN-based PAAs is able to improve the
PI processibility and significantly reduce the amount of used
solvents. Such processing advantage is parallel to that
reported for high-temperature polymer-matrix composite
fabrication as illustrated by the work of Chuang et al.33 that
very low-melt viscosity reactive imide oligomers (5–20 P at
260–280 ꢀC) could be formulated from three different
unsymmetrical dianhydrides with diamines and terminated
with the 4-phenylethynylphthalic anhydride endcapper.47
highest Tgs among all the PIs here, and showed the same
trend as the 6FDA-based PIs, that is, PI 8i containing rigid
ꢀ
4,4-units has the higheꢀst value (332 C), followed by 8e and
then 8g (321 and 294 C).
Table 4 also gives the temperatures of the 5 wt % gravimet-
ric loss in nitrogen and in air. Their 5% weight losses are in
the range of 505–542 ꢀC and 512–546 ꢀC in air and nitro-
ꢀ
gen, respectively. The amount of residue of all PIs at 850 C
in nitrogen atmosphere was higher than 48%, especially, the
PI derived from PMDA and DSDA with the highest char
yields. Obviously, the data from thermal analysis showed
that the resulting PIs had fairly high thermal stability and
the thermo-oxidative stability.
Thermal and Mechanical Properties
The glass transition temperatures (Tgs) were measured on
both DSC and DMA instruments. The PMDA-based PIs are
too rigid for their Tgs to be detected by DSC method (Table
4; see also Table 1 for the diamine/dianhydride composi-
tions for the PI designations below). Therefore, DMA results
were used for the following discussion. The polymer chain
rigidity imposes a direct effect on their Tgs. For three 6FDA-
based PIs, 8f containing totally bent meta,meta or 3,3-units
displays the lowest Tg (249 ꢀC, DSC) while 8h containing
rigid para,para or 4,4-units has the highest value (292 ꢀC).
The Tg (279 ꢀC) of semi-rigid 8a is higher than that of 8f,
but lower than 8h. Three PMDA-based PIs displayed the
These films were subjected to tensile testing. Table 4 shows
the mechanical properties of the PIs, including the tensile
strength, tensile modulus, and elongation at break. The PI
films possess tensile strength of 82–121 MPa, an elongation
at break of 5.3–9.8% and a tensile modulus of 2–3 GPa,
indicative of their being made of strong and tough materials.
The mechanical properties of the PIs 8a and 8e containing
unsymmetrical units are comparable with 8f–i containing
symmetrical units, implying that the unsymmetrical
TABLE 3 Molecular Weights and Viscosity of PIs
Mn 3 1023
Mw 3 1023
PI No.
[g] (dL g21 a
)
(Da)b
(Da)b
PDIb
1.98
8a
8b
8c
8d
8f
0.78
0.98
0.73
1.22
0.99
1.75
41.4
81.5
c
c
c
–
–
–
c
c
c
–
–
–
c
c
c
–
–
–
45.1
66.3
87.2
126
1.93
1.91
8h
a
Intrinsic viscosity ([g]) determined in NMP solutions at 30 ꢀC.
Molecular weights and molecular distributions (PDI) measured by
b
size-exclusion liquid chromatography in THF.
FIGURE 6 Wide-angle X-ray diffraction patterns of PMDA-
c
Insoluble in THF.
based PIs.
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TABLE 4 Thermal and Mechanical Properties of PIs
In Air
In Nitrogen
Modulus
Tensile Strength Elongation
PI No. Tg (DSC, ꢀC)a Tg (DMA, ꢀC)b Td5% (ꢀC) Chard (%) Td5% (ꢀC) Chard (%) (GPa)
(MPa)
(%)
c
c
8a
8b
8c
8d
8e
8f
252
235
254
245
ND
238
ND
284
ND
279
255
281
267
321
249
294
292
332
525
530
505
550
537
519
543
526
542
1.08
3.62
2.66
4.05
0.71
0.26
1.12
0.63
1.42
528
534
512
536
539
528
546
527
537
57.9
57.6
48.5
61.8
61.6
52.3
59.4
52.2
59.6
2.39 6 0.26 103.7 6 5.1
2.68 6 0.37 95.1 6 8.4
2.44 6 0.22 82.1 6 3.4
2.76 6 0.42 88.6 6 9.3
2.11 6 0.19 121.3 6 6.2
2.34 6 0.55 93.7 6 4.4
2.69 6 0.36 115.8 6 5.6
2.47 6 0.25 106.2 6 5.7
2.97 6 0.32 119.2 6 13.5
7.43 6 0.90
8.36 6 1.66
5.33 6 0.97
5.06 6 1.45
6.35 6 0.78
6.29 6 1.57
6.74 6 0.81
9.81 6 0.46
7.13 6 0.67
8g
8h
8i
a
c
Tg measured by DSC at a scanning rate of 10 ꢀC/min in flowing
Temperature at which 5% weight loss occurred on TGA thermogram
nitrogen.
b
obtained with a heating rate of 10 ꢀC/min.
d
Tg measured by DMA at
a
scanning rate of
4
ꢀC/min in flowing
Residual weight (%) at 850 ꢀC in nitrogen.
nitrogen.
structures improve the processing conditions of PAAs and at
same time they preserve the rigidity of PI chains and the
consequential mechanical properties.
Dielectric Properties
Dielectric impedance experiments were performed over
broad band frequency (1021 to 106 Hz) and temperature
(240 to 150 ꢀC) ranges. Dielectric spectra of all the PIs
are shown in Figures 8 and 9. Their permittivity (e0) and
dielectric loss (e00) at 1, 10, and 1 kHz are also listed in
Table 5. For comparison purpose, the dielectric properties
of APB-6FDA (CP2, 9), an analog of 8f containing no nitrile
group, was also measured.49 6FDA-based PIs (8a, 8f, and
8h) showed higher dielectric constants than CP2 (9) due
to the presence of nitriles as high polarity groups on the
polymer backbone. The unsymmetrical polymer 8a exhib-
ited highest value in the whole frequency range among
three 6FDA-based PIs (Fig. 8 and Table 5). Although the
differences are small (3.30 for 8a vs. 3.27 for 8f and 3.17
for 8h), they are noticeable. The high permittivity of 8a
could be attributed to its unsymmetrical repeat units,
which give the neighboring benzonitrile units higher polar-
ity. For the same argument, 3,3-APBN/PMDA polymer (8f)
displayed a higher permittivity than 4,4-APBN/PMDA (8h)
since the latter has more symmetrical structure than the
former. The structures of dianhydrides demonstrate some
influence on the dielectric properties. For 3,4-APBN series,
BTDA PI showed highest dielectric constant (3.62–3.81 at
1–1k Hz) since ketone (benzophenone) groups are easily
polarized in the electric field. The permittivity of PMDA PI
is second due the high polarity of pyromellitimide units.
DSDA PI display a lowest permittivity since the polarity of
two sulfone groups is partially cancelled each other. The
dielectric loss (e00) of CP2 is very low (0.002–0.003). For
the 3,4-APBN PIs, their e00s are higher than CP2, which
probably arises from the polar nitrile groups. 3,4-APBN/
PMDA polymer showed much higher loss at low frequency
(Fig. 9). Since its Tg is over 321 ꢀC and the highest imid-
ization temperature is only 300 ꢀC, it may contain small
amount of solvent and /or PAA residues which contribute
to the higher loss.43
Wide-Angle X-Ray Diffraction (WAXD)
Because of its unsymmetrical nature, 3,4-APBN/PMDA(8e)
PI is expected to be amorphous and this is further confirmed
by its featureless WAXD pattern in Figure 6. WAXD patterns
of both 3,3- and 4,4- and APBN/PMDA (8g and 8i) PI indi-
cated that they are semi-crystalline. 3,3-APBN/PMDA WAXD
result revealed multiple peaks, indicative of high order (crys-
tallinity) in the lateral direction of the polymer backbone.
4,4-APBN/PMDA sample falls in between 3,3- and 3,4-APBN/
PMDA films in terms of relative crystallinity. The long period
diffraction peak at 19 Å for 4,4-APBN/PMDA indicates a bet-
ter ordering along the backbone.
UV–Vis Absorption
It is well known that the PI molecular structures have a direct
effect on the film color and transmittance. Colorless PI films
have been used in LCD, LED and solar battery panel applica-
tions.48 The optical properties of the PI thin films were eval-
uated by UV–vis spectroscopy. The UV–vis spectra of three
APBN/6FDA PIs were measured in both films and DMAc solu-
tions (see Fig. 7 and Supporting Information Figure SI-7,
respectively). As expected, the film and solution of 3,4-APBN/
6FDA are the most transparent since the unsymmetrical struc-
tures disrupts the formation of charge-transfer complexes,
which are considered the major contributor to high color in PI
films, such as Kapton.1 The optical transparency of 3,3-ABPN/
6FDA film and solution is only marginally lower than the
unsymmetrical one due to the bending structure of 3,3-dia-
mine unit. Since 4,4-APBN/6FDA PI exhibits highly symmetri-
cal repeat units, the color of its film and solution is the
darkest. While the 3,4- and 3,3-APBN/6FDA films show about
95 and 93% of transmittance at 400 nm the 4,4-APBN/6FDA
one has only 88% transparency (Supporting Information Table
SI-2). They show similar trend in other wavelengths.
5008
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2013, 51, 4998–5011

FIGURE 7 UV–vis spectra of APBN-6FDA PI thin films (ꢁ3 mm).
Magnified spectral region (400–800 nm) is depicted in the top
spectrum. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE 8 Composite plot of the dielectric constants (top) and
loss (bottom) of 6FDA-based PIs as a function of scanning fre-
quency at ambient conditions.
TABLE 5 Dielectric Properties of PIs
1 Hz
10 Hz
1 kHz
e0
e00
e0
e00
e0
e00
a
PI No.
8a
De10 Hz
10.5
3.39
3.33
3.23
3.81
3.64
3.33
3.53
3.19
3.49
2.92
0.025
0.022
0.026
0.024
0.050
0.02
3.33
3.24
3.17
3.68
3.55
3.28
3.42
3.11
3.25
2.92
0.017
0.012
0.014
0.013
0.025
0.011
0.019
0.021
0.028
0.002
3.30
3.14
3.16
3.62
3.51
3.27
3.39
3.08
3.17
2.92
0.013
0.0077
0.010
0.009
0.018
0.008
0.011
0.016
0.017
0.002
8b
8c
8d
8e
8f
10.0
5.0
13.0
b
–
8.9
–
8g
8h
8i
0.17
0.033
0.051
0.003
7.0
––b
0.7
9
a
Permittivity difference before and after Tg.
b
Not determined because polymer Tg exceeds the upper temperature
limit of the instrument.

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FIGURE 9 Composite plots of the dielectric constants (left) and loss (right) of 3,4-APBN-based PIs as a function of scanning fre-
quency at ambient conditions.
Polarization response (De) is defined as the dielectric con-
stant changes before and after Tgs. While CP2s polarization
response is only 0.7, the 6FDA-APBN-PIs showed a huge
increase to 5.0 and 13.0 due to the presence of benzonitrile
groups on the backbone (Fig. 10 and Table 5). Apparently,
among these structurally isomeric PIs, the repeat-unit asym-
metry has resulted in the largest polarization response.
Together with the same trend observed at room tempera-
ture, it can be speculated that with creative design, the pos-
sibility may exist for such effect on polymer permittivity to
be amplified at relatively high operation temperatures, e.g.
100–150 ꢀC, which are well above room temperature and
below the polymer softening temperatures.
tion side reaction, we demonstrates that an unsymmetrical
diamine could be conveniently prepared by sequentially add-
ing two isomeric (or in a broader scope, structurally differ-
ent) aminophenols at 70 and 100 ꢀC, respectively. Three
synthetic routes have been successfully developed, but the
ease of one-pot route would make the monomer synthesis
more practical and perhaps, commercially viable. We have
prepared a series of new PIs from this unsymmetrical dia-
mine and five dianhydride via thermal and chemical imidiza-
tion of the respective PAAs. These PIs are soluble in
common organic solvents, can form tough and creasable
films, and possess Tg well above 200 ꢀC. More importantly,
as exemplified by the 6FDA-APBN series, when comparing to
the PAA precursors containing symmetrical units, we have
found that the processibility of the unsymmetrical PAAs have
been greatly improved because of an order-of-magnitude
solution-viscosity reduction by the presence of unsymmetri-
cal repeat units, and all three PIs show similar mechanical
properties and thermal stability. Therefore, an element of
asymmetry at the molecular level must have a cumulative
effect on the conformational entropy of the polymer chains
that can enhance not only the solubility, but also reduces the
hydrodynamic volumes (viscosities) of both the PAA precursor
and PI. On the other hand, the repeat-unit asymmetry effect in
enhancing the polymer permittivity is relatively small at ambi-
ent temperature but significantly larger (>3 times) at ca. 250
ꢀC as the polarization response results of the 6FDA-APBN
series have indicated, and provided a starting point toward
designing high temperature polymer capacitors.
CONCLUSIONS
Having found that the differential susceptibility to aromatic
nucleophilic displacement of the fluorides in 2,6-DFBN is
chemically exploitable and a way to avoid transestherifica-
ACKNOWLEDGEMENTS
This work was completed at Air Force Research Laboratory
(AFRL) at Wright Patterson Air Force Base with funding from
Materials and Manufacturing Directorate as well as Air Force
Office of Scientific Research. The authors are grateful to Mar-
lene Houtz (University of Dayton Research Institute) for TGA
and DMA data, Brian Kurish (UES, Inc.) for the assistance of
FIGURE 10 Composite plot of the dielectric constants of 6FDA-
based PIs at 10 Hz and as a function of temperature.
5010
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