A Bulk Dielectric Polymer Film with Intrinsic Ultralow Dielectric Constant and Outstanding Comprehensive Properties

Article abstract of DOI:10.1021/acs.chemmater.5b01798

A bulk dielectric polymer film with an intrinsic ultralow k value of 1.52 at 10 kHz has been successfully synthesized based on a novel polyimide FPTTPI. More importantly, such outstanding dielectric properties remain stable up to 280 °C. The excellent ultralow dielectric properties are mainly because of the larger free volume (subnanoscale), which intrinsically exists in the amorphous region of polymeric materials. Meanwhile, FPTTPI also shows excellent thermal stability and mechanical properties, with a glass-transition temperature (T_g) of 280 °C, 5 wt % loss temperature of 530 °C, and a residual of 63% at 800 °C under N₂. It was soluble in common solvents, which made it possible to undergo simple spin-on or efficient, low-cost, and continuous roll-to-roll processes.

Full text of DOI:10.1021/acs.chemmater.5b01798

Article
pubs.acs.org/cm
A Bulk Dielectric Polymer Film with Intrinsic Ultralow Dielectric
Constant and Outstanding Comprehensive Properties
†
,§,∇
†,∇
†
†
,†
†
‡
Yiwu Liu,
Chao Qian, Lunjun Qu, Yunan Wu, Yi Zhang,* Xinhui Wu, Bing Zou,
†
‡
†
†
†
†
Wenxin Chen, Zhiquan Chen, Zhenguo Chi, Siwei Liu, Xudong Chen, and Jiarui Xu
†PCFM Lab and GD HPPC Lab, Guangdong Engineering Technology Research Center for High-Performance Organic and Polymer
Photoelectric Functional Films, State Key Laboratory of Optoelectronic Material and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-sen University, Guangzhou, Guangdong 510275, People’s Republic of China
‡
Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan, Hubei 430072, People’s Republic
of China
§
Key Laboratory of New Materials and Technology for Packaging, Hunan University of Technology, 88 Taishan Road, Zhuzhou,
People’s Republic of China
*
S Supporting Information
ABSTRACT: A bulk dielectric polymer film with an intrinsic ultralow k value of 1.52 at
0 kHz has been successfully synthesized based on a novel polyimide FPTTPI. More
1
importantly, such outstanding dielectric properties remain stable up to 280 °C. The
excellent ultralow dielectric properties are mainly because of the larger free volume
(
subnanoscale), which intrinsically exists in the amorphous region of polymeric
materials. Meanwhile, FPTTPI also shows excellent thermal stability and mechanical
properties, with a glass-transition temperature (T ) of 280 °C, 5 wt % loss temperature
g
of 530 °C, and a residual of 63% at 800 °C under N . It was soluble in common solvents,
2
which made it possible to undergo simple spin-on or efficient, low-cost, and continuous
roll-to-roll processes.
23−27
INTRODUCTION
the free volume by rearranging the material structure.
■
However, it seems that no true dielectric generational
extendibility to the ultralow-k region can be achieved without
embracing the concept of porosity, either for organosilicates or
With the development of ultralarge-scale integration (ULSI) to
high speed and high integration in the semiconductor industry,
and with the continuing miniaturization in the dimensions of
electronic devices utilized in ULSI circuits, an urgent need
exists for high-performance low-k and ultralow-k dielectric
28−32
organic polymers.
can be less than 1.5,
The k-value of these porous materials
but the method itself is complicated,
33−39
1
−4
difficult to control, and expensive. Moreover, the pore
structure, the size, and the distribution would greatly affect
the homogeneity of the materials, which makes this technique
difficult for large-area applications. In addition, the porosity
tends to dramatically reduce the mechanical strength and
increase the permeability of the materials, thereby making them
too fragile for practical uses. Therefore, the switch to high-
performance nonporous ultralow-k polymer insulators con-
tinues to be a formidable challenge to chemists, physicists,
materials scientists, and integration engineers. Furthermore, as
far as we know, homogeneous polymers with an intrinsic k-value
of <2.0 have not ever been reported in the literature.
materials (low-k: k ≤ 2.5; ultralow-k: k ≤ 2.0) has arisen.
Such dielectrics materials would reduce the capacitance
between the metal interconnects, the resistance-capacitance
delay, the line-to-line crosstalk noise, and the power
5
−7
dissipation;
these materials also have important application
prospects in the fields of interlayer dielectric, semiconductor
packaging (chips modules, etc.), and high-frequency, low-loss
boards etc. So far, research of low-dielectric materials as an
alternative to the workhorse dielectrics silicon dioxide (k =
3
.9−4.3) are continually being pursued today, which mainly
8−13
including organosilicates and organic polymers.
Compared with inorganic dielectric materials, organic
polymer materials often have a lower dielectric constant,
because of the lower materials density and lower individual
bond polarizability. Moreover, they show distinct advantages, in
Polyimides have been widely used as materials for electronic
packaging and electrical insulation in microelectronics indus-
tries, because of their excellent thermal, mechanical, and
40−42
14−18
dielectric properties.
It is one of the most promising
terms of easy chemical and geometric structural design.
polymer candidates for the next generation of high-perform-
Thus, they have attracted much interest. Generally, by
decreasing the dipole strength or the number of dipoles or a
combination of both, the dielectric constant of full dense
polymer materials can be lowered to 2.2−2.6.
common way is fluorination of dielectric materials or increasing
Received: May 13, 2015
Revised: September 8, 2015
5
,19−22
The most
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.chemmater.5b01798
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
ance interlayer dielectrics. However, the k-value of the
traditional polyimides, such as commercial DuPont Kapton
polyimide films (see Figure 1), is typically 3.1−3.5, which
elemental analyzer. Infrared spectra were recorded on a Bruker
TENSOR 27 Fourier-transform infrared (FT-IR) spectrometer. Gel
permeation chromatography (GPC) was performed with a Waters
Model 717 plus autosampler and a Waters Model 1515 isocratic
HPLC pump. Two Waters columns, connected in series, were used
with DMF as the eluent and were calibrated with narrow
polydispersity polystyrene standards. The intrinsic viscosity of the
−1
FPTTPI in DMF (0.5 g L ) was measured with an Ubbelohde
viscometer. Thermogravimetric analyses (TGA) were performed with
a TA Instruments Model Q50 thermal analyzer under N /air with a
2
heating rate of 20 °C/min from 30 °C to 1000 °C; the samples were
heated under flowing nitrogen/air (40 mL/min). Differential scanning
calorimetry (DSC) curves were obtained with a Netzsch Model DSC
2
04 thermal analyzer at a heating rate of 25 °C/min from 30 °C to 400
°
C under flowing nitrogen. The glass-transition temperature (T ) was
g
read at the midpoint of the transition in the heat capacity and was
taken from the second heating trace after rapid cooling from 400 °C at
a cooling rate of 40 °C/min. Thermal mechanical analysis (TMA) was
used to study the thermal coefficient of expansion (CTE) of the
FPTTPI film with a heating rate of 10 °C/min from 50 °C to 350 °C.
A tensile test was performed on samples cut from a sheet 35−50 μm
thick, and the test was performed using a SANS Model CTN6103
instrument according to Standard GB/T16421-1996. The specimen
size was 10 mm × 100 mm, and the jaw separation was 50 mm. The
jaw speed was first set to 2 mm/min. When elongation reached 1 mm,
the jaw speed was changed to 20 mm/min. The morphology of the
surface of FPTTPI was studied using a field-emission scanning
electron microscopy (FE-SEM) system (Hitachi, Model s4800, Japan)
and an atomic force microscopy (AFM) system (Bruker, Model
Multimode 8, Germany). The dielectric constant was measured at
frequencies between 100 Hz and 1 MHz at 20 °C and 50% relative
humidity, using a Solartron SI 1260 impedance/gain phase analyzer in
conjunction with two copper electrodes (10.2 mm × 10.2 mm). The
amplitude of the ac electric field was ∼1 V/cm. The samples were thin
films with a size of 12 mm × 12 mm. Silver paste was coated onto both
surfaces of the PI film to ensure excellent contact between the
electrodes and the PI film. Positron lifetime measurements were
performed as follows: two identical samples with dimensions of 1.5
Figure 1. Chemical structure of the DuPont Kapton polyimide films,
FPTTDA, and FPTTPI.
makes it difficult to meet the requirement of ultralow k-values
43
of <2.0 for the technology nodes below 130 nm. Hence,
research on polyimides with ultralow dielectric constants is of
great significance.
In this contribution, we present a new structural polymer
(
(
(
FPTTPI), which contains both a rigid polyimide main chain
PI MC) and a large rigid nonplanar conjugated side chain
LRNPC SC). It was synthesized by polycondensation between
a novel diamine (FPTTDA) and a dianhydride (6FDA). Their
chemical structures are shown in Figure 1. The solution-cast
FPTTPI film shows an intrinsic k value of 1.52 with a dielectric
−3
loss on the order of 10 at 10 kHz. More importantly, the
dielectric properties remain stable up to 280 °C. The excellent
ultralow dielectric properties are mainly due to the larger free
volume (subnanoscale) that intrinsically exists in the
amorphous region of polymeric materials. Meanwhile, the
FPTTPI shows excellent thermal stability and mechanical
6
22
mm × 10 mm × 10 mm were sandwiched with a 1.1 × 10 Bq Na
2
2
positron source; the Na nucleus emits a 1.28 MeV γ-ray
simultaneously (within a few picoseconds) with the positron; the
positron lifetime is determined from the time delay between the
emission of the birth gamma (1.28 MeV) and one of the 0.511 MeV
annihilation photons; lifetime measurements were conducted using a
fast−fast coincidence system with a time resolution of ∼220 ps and a
channel width of 12.6 ps. We analyzed the lifetime spectra using the
data processing program PATFIT. Before analyzing each spectrum, we
subtracted the positron source components (392 ps/16.40%, 2.05 ns/
0.70%). The variance of the fits was ∼1.1. The film density was
measured by a density balance (Mirage SD-200 L, Japan) with an
accuracy of 0.1 mg.
properties, with a glass-transition temperature (T ) of 280 °C, 5
g
wt % loss temperature of 530 °C, and a residual of 63% at 800
°
C under N . In addition, it is soluble in common solvents,
which made it possible to undergo simple spin-on or efficient,
2
low-cost, and continuous roll-to-roll processes.
EXPERIMENTAL SECTION
■
Materials. 4-Bromobenzophenone, 4-bromobenzyl bromide,
triethyl phosphate, 4-(trifluoromethyl)- phenylboronic acid, 4-amino-
phenylboronic acid hydrochloride, tetrakis(triphenylphosphine)palla-
dium (Pd[P(C H ) ] ), potassium tert-butanolate, Aliquat 336
Synthesis of Phenyl(4′-(trifluoromethyl)biphenyl-4-yl)
Methanone (FPTC). 4-Bromobenzophenone (10.4444 g, 0.04 mol)
and 4-(trifluoromethyl)phenyl-boronic acid (8.5469 g, 0.045 mol)
were dissolved in THF (400 mL) and mixed in a 500 mL flask. Then,
6
5 3 4
(
tricaprylylmethylammonium chloride), cesium fluoride (CsF), 1-
fluoro-4-nitrobenzene, palladium 10% on carbon (10% Pd/C), and
hydrazine monohydrate (NH NH ·H O), were purchased from Alfa
2 M aqueous K
were added. The mixture was stirred for 45 min under argon at room
temperature. Tetrakis(triphenylphosphine) palladium (Pd(Pph ) )
3 4
CO solution (120 mL) and 10 drops of Aliquat 336
2
2
2
2
3
Aesar and were used as received. 4,4′-(Hexafluoro-isopropylidene)-
diphthalic anhydride (6-FDA) was purchased from Alfa Aesar and was
heated at 140 °C under vacuum for 12 h prior to use. Analytical-grade
dimethylformamide (DMF) was purified by distillation under an inert
nitrogen atmosphere. Tetrahydrofuran (THF) was distilled from
sodium/benzophenone. All other solvents and reagents were
purchased as analytical grade from the Guangzhou Dongzheng
Company and were used without further purification.
(catalytic amount) was subsequently added as a catalyst, and the
reaction mixture was stirred at 75 °C for 24 h. After cooling to room
temperature, the product was concentrated and purified by silica-gel
column chromatography using dichloromethane/n-hexane (v/v = 1/
1
1). The yield of the product was ∼92%. mp: 160 °C; H NMR (400
MHz, DMSO-d ): δ 8.00 (d, J = 8.2 Hz, 2H), 7.95 (d, J = 8.3 Hz, 2H),
6
Instrumentation. All NMR spectra were recorded on a Bruker
Model AVANCE AV 400 spectrometer. Samples were composed of a
solution of 5−15 mg of each compound in 0.5 mL of deuterated
dimethyl sulfoxide (DMSO) using tetramethylsilane (TMS) as the
internal reference. Mass spectra were measured on a Thermo EI mass
spectrometer (DSQ II). Elemental analysis was performed on a CHNS
7.88 (d, J = 8.2 Hz, 4H), 7.79 (d, J = 7.2 Hz, 2H), 7.71 (t, J = 7.4 Hz,
1H), 7.60 (t, J = 7.6 Hz, 2H); 13C NMR (100 MHz, DMSO-d ): δ
6
195.25, 142.89, 142.37, 136.936, 136.63, 132.71, 130.37, 129.52,
2
3
128.57 (q, J = 31.8 Hz), 128.56, 127.82, 127.20, 125.88 (q, J = 3.3
Hz), 124.20 (q, 1J = 270.4 Hz); IR (KBr): 1647 cm (CO
−1
−1
−1
stretching), 1177 cm (C−F stretching), 1100−700 cm (Ar−H
B
DOI: 10.1021/acs.chemmater.5b01798
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
stretching). UV/vis: λmax 282 nm; HRMS (m/z): [M]+ calcd for
for C H F N O , 733.2183; found, 733.2189. Analysis for
45
30
3
3
4
C H F O, 326.0913; found, 326.0907. Analysis for C H F O:
C H F N O : calcd: C, 73.66; H, 4.12; N, 5.73; found: C, 73.79;
45 30 3 3 4
20
13
3
20 13
3
calcd, C, 73.61; H, 4.02; found: C, 73.14; H, 4.09.
H, 4.23; N, 5.66.
1
1
Synthesis of (E)-4-(2-(4-bromophenyl)-1-phenyl-vinyl)-4′-
Synthesis of (E)-N -(4-aminophenyl)-N -(4′-(2-phenyl-2-(4′-
(trifluoromethyl)biphenyl-4-yl)vinyl) -biphenyl-4-yl)benzene-
1,4-diamine (FPTTDA). A mixture of FPTTDN (14.6746 g, 0.02
mol), 10% Pd/C catalyst (0.5 g), hydrazine monohydrate (16 mL),
and ethanol (300 mL) was placed in a three-necked flask and heated at
80 °C for 24 h. The mixture was then filtered to remove the Pd/C
catalyst. After the mixture cooled to room temperature, the
precipitated crystals were isolated by filtration, recrystallized from
(
trifluoromethyl)biphenyl (FPTBr). 4-Bromo-benzyl bromide
(
12.4965 g, 0.05 mol) and triethyl phosphite (24.924 g, 0.15 mol)
were mixed in a 500 mL flask. The mixture was stirred for 24 h under
argon at 150 °C. After the mixture was cooled to room temperature,
FPTC (13.0524 g, 0.04 mol) and THF (400 mL) were added. Then,
potassium tert-butanolate (8.4158 g, 0.075 mol) was added, and the
reaction mixture was stirred at room temperature for 12 h. After
reaction, the product was concentrated and purified by silica-gel
ethanol twice, ground into powder, and dried under vacuum. Yield:
89%. mp: 228 °C; H NMR (400 MHz, DMSO-d
1
column chromatography, using n-hexane. The yield of the product was
6
): δ 7.88 (t, J = 13.8
1
∼
72%. mp: 149 °C; H NMR (400 MHz, DMSO-d ): δ 7.91 (d, J =
Hz, 2H), 7.82 (t, J = 8.6 Hz, 4H), 7.75 (dd, J = 13.2, 8.5 Hz, 4H), 7.62
(d, J = 8.2 Hz, 2H), 7.51−7.12 (m, 26H), 7.05 (t, J = 1.5 Hz, 2H), 6.83
6
8
.3 Hz, 2H), 7.81 (d, J = 8.3 Hz, 2H), 7.74 (d, J = 8.3 Hz, 2H), 7.48−
.40 (m, 5H), 7.35 (d, J = 8.5 Hz, 2H), 7.22−7.16 (m, 3H), 6.97 (d, J
(dd, J = 8.1 Hz, 8H), 6.63 (d, J = 7.0 Hz, 4H), 6.56 (dd, J = 8.5, 3.6
7
=
1
3
8.5 Hz, 2H); 13C NMR (100 MHz, DMSO-d ): δ 143.36, 142.06,
Hz, 8H), 4.98 (s, 8H); C NMR (100 MHz, DMSO-d ): δ 149.69,
6
6
1
1
1
49.35, 146.28, 146.14, 145.66, 144.96, 144.36, 144.03, 143.05, 143.04,
40.73, 140.42, 139.09, 138.40, 138.09, 137.86, 136.75, 136.25, 136.05,
1
41.84, 139.18, 137.62, 135.97, 131.07, 130.79, 129.52, 128.95, 128.09
2
3
(
q, J = 32.1 Hz), 127.75, 127.60, 127.15, 126.86, 126.77, 125.61 (q, J
1
−1
34.91, 130.35, 130.20, 129.85, 129.62, 129.54, 129.03, 128.84, 128.57
=
3.4 Hz), 124.23 (q, J = 275 Hz), 119.96; IR (KBr): 3045 cm (
2
−1
−1
(q, J = 23.6 Hz), 128.38, 128.30, 128.23, 128.05, 127.94, 127.82,
C−H stretching), 1482 cm (CC stretching), 1164 cm (C−F
3
3
−1
−1
1
3
1
1
27.71, 127.46, 127.08, 126.60, 126.21 (q, J = 3.8 Hz), 126.18 (q, J =
.8 Hz), 125.66, 125.36, 125.05 (q, J = 310.0 Hz), 117.24, 117.04,
stretching), 1100−700 cm (Ar−H stretching), 698 cm (C−Br
1
+
stretching). UV/vis: λ
323 nm; HRMS (m/z): [M] calcd for
max
−
1
−1
15.30; IR (KBr): 3371 cm (N−H stretching), 1617 cm (δ N−H),
C H BrF , 478.0538; found, 478.0533. Analysis for C H BrF :
27
18
3
27 18
3
−1
−1
264 cm (C−N stretching), 1100−700 cm (Ar−H stretching).
calcd, C, 67.65; H, 3.79; found: C, 67.99; H, 3.95.
+
UV/vis: λmax 322 nm; HRMS (m/z): [M] calcd for C H F N ,
Synthesis of (E)-4′-(2-phenyl-2-(4′-(trifluoromethyl) biphen-
yl-4-yl)vinyl)biphenyl-4-amine (FPTPA). FPTBr (9.5866 g, 0.02
mol) and 4-aminophenylboronic acid hydrochloride (3.4682 g, 0.02
mol) were dissolved in THF (100 mL) and mixed in a 250 mL flask.
Then, 2 M aqueous K CO solution (30 mL) and 10 drops of Aliquat
45 34
3
3
673.2699; found, 673.2707. Analysis for C45
H, 5.09; N, 6.24; found: C, 80.26; H, 5.12; N, 6.18.
H F N : calcd, C, 80.22;
34 3 3
Synthesis and Preparation of Polyimide Film (FPTTPI). To a
solution of FPTTDA (1.3475 g, 2 mmol) in purified DMF (15.8 mL)
in a 50 mL flask, 6FDA (0.8885 g, 2 mmol) was added in one portion.
Thus, the solid content of the solution was ∼15 wt %. The mixture was
stirred at room temperature under argon for ∼12 h to produce a
viscous poly(amic acid) (PAA) solution. The weight-average molecular
2
3
3
36 were added. The mixture was stirred for 45 min under argon at
room temperature. Tetrakis(triphenylphosphine) palladium (Pd-
Pph ) ) (catalytic amount) was subsequently added as a catalyst,
(
3
4
and the reaction mixture was stirred at 75 °C for 24 h. After cooling to
room temperature, the product was concentrated and purified by silica
gel column chromatography using dichloromethane/n-hexane (v/v =
weight (M ) values and the polydispersity (PDI) of the resultant PAA
w
4
(FPTTPAA) estimated from GPC were 29.7 × 10 and 2.23,
respectively, measured in DMF at a concentration of 4 mg/mL at 50
°C. The PAA solution was subsequently uniformly coated onto a clean
and dry glass (film thickness could be controlled), and thermal
imidization was performed in a vacuum drying oven to produce the
FPTTPI film. The polyimide film was subsequently removed from the
1
1
/1). The yield of the product was ∼85%. mp: 188 °C; H NMR (400
MHz, DMSO-d ): δ 7.97 (d, J = 7.5 Hz, 2H), 7.82 (d, J = 7.5 Hz, 4H),
6
7
.33 (dd, J = 14.3, 7.5 Hz, 11H), 7.13−7.04 (m, 3H), 6.60 (d, J = 8.4
13
Hz, 2H), 5.16 (s, 2H); C NMR (100 MHz, DMSO-d ): δ 148.93,
6
1
1
1
2
43.99, 143.04, 140.97, 140.81, 139.63, 137.92, 134.57, 131.17, 130.24,
1
2
glass substrate after the oven had cooled to room temperature. H
NMR (400 MHz, DMSO-d ): δ 8.11 (d, J = 7.0 Hz, 2H), 7.96 (d, J =
28.85, 128.51, 128.40 (q, J = 31.6 Hz), 127.97, 127.92, 127.80,
3
1
27.56, 127.37, 127.03, 126.24 (q, J = 3.7 Hz), 125.13, 124.84 (q, J =
6
−1
1
6.9 Hz, 3H), 7.86−7.54 (m, 10H), 7.49−7.33 (m, 10H), 7.30−7.04
70.2 Hz), 114.67; IR (KBr): 3375 cm (N−H stretching), 1617
−1
−1
−1
−1
(m, 11H); IR (film): 1774 and 1719 cm (CO stretching), 1100−
cm (δ N−H), 1284 cm (C−N stretching), 1165 cm (C−F
−1
−1
7
00 cm (Ar−H stretching).
stretching), 1100−700 cm (Ar−H stretching); UV/vis: λ 352 nm;
max
+
HRMS (m/z): [M] calcd for C H F N, 491.1855; found, 491.1848.
33
24 3
Analysis for C H F N: calcd: C, 80.63; H, 4.92; N, 2.85; found: C,
33
24 3
RESULTS AND DISCUSSION
■
8
1.08; H, 4.99; N, 2.82.
Synthesis of (E)-N,N-bis(4-nitrophenyl)-4′-(2-phenyl-2-(4′-
Synthesis and Characterization of FPTTDA. The
(trifluoromethyl)biphenyl-4-yl)vinyl) biphenyl-4-amine
FPTTDN). FPTPA (9.831 g, 0.02 mol), 1-fluoro-4-nitrobenzene
FPTTDA diamine was synthesized via the Wittig−Horner
reaction and Suzuki coupling reaction, as shown in Scheme 1.
The chemical structures of all the intermediate products and
the target diamine were confirmed by H NMR, C NMR
H−H COSY, C−H QC, and C−H BC) spectra, mass spectra,
FT-IR, and elemental analysis; the results are shown in Figures
S1−S5 in the Supporting Information. The thermal properties
of FPTTDA were studied by DSC and TGA. These results
revealed that the diamine exhibited good thermal stability. The
(
(
5.644 g, 0.04 mol), and cesium fluoride (6.0764 g, 0.04 mol) were
mixed in a 500 mL flask, and DMSO (150 mL) was added. The
mixture was stirred for 24 h under argon at 150 °C. After reaction, the
product was concentrated and purified by silica-gel column
chromatography using dichloromethane/n-hexane (v/v = 1/1). The
1
13
(
1
yield of the product was ∼76%. mp: 210 °C; H NMR (400 MHz,
DMSO-d ): δ 8.23−8.15 (m, 8H), 7.97 (d, J = 8.2 Hz, 2H), 7.91 (d, J
6
=
8.2 Hz, 2H), 7.82 (dd, J = 11.8, 6.1 Hz, 6H), 7.79−7.70 (m, 6H),
7
1
1
1
1
1
1
1
.53 (dd, J = 8.4, 6.8 Hz, 4H), 7.48−7.30 (m, 12H), 7.29−7.17 (m,
5
% and 10% weight-loss temperatures (T ) of FPTTDA in
nitrogen were 404 and 417 °C, respectively.
d
8H), 7.13 (d, J = 8.5 Hz, 2H); 13C NMR (100 MHz, DMSO-d ): δ
6
51.96, 144.26, 143.98, 143.90, 142.88, 142.83, 142.67, 142.00, 141.78,
40.72, 140.22, 138.15, 138.07, 138.05, 137.73, 137.69, 136.77, 136.70,
31.15, 130.50, 130.46, 130.18, 129.63, 128.91, 128.81, 128.55 (q, J =
9.2 Hz), 128.33, 128.24, 128.16, 128.07, 127.95, 127.80, 127.75,
27.69, 127.48, 126.61, 126.54, 126.27 (q, J = 5 Hz), 126.05, 126.05,
Synthesis and Characterization of FPTTPI. The FPTTPI
polyimide was prepared in a conventional two-step procedure
by the reaction of equal molar amounts of diamine FPTTDA
with commercially available aromatic tetracarboxylic dianhy-
dride (6FDA) in dimethylformamide (DMF) to form the
precursor poly(amic acid) (FPTTPAA), followed by thermal
cyclodehydration to obtain the novel polyimide FPTTPI, as
2
3
1
−1
−1
−1
24.82 (q, J = 269.9 Hz), 123.15; IR (KBr): 1582 cm , 1495 cm
−1
(
−NO stretching), 1165 cm (C−F stretching), 1100−700 cm
2
+
(Ar−H stretching); UV/vis: λ 372 nm; HRMS (m/z): [M] calcd
max
C
DOI: 10.1021/acs.chemmater.5b01798
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Scheme 1. Synthesis Route of the Diamine FPTTDA
Meanwhile, the characteristic absorption peaks of the imide
group at 1774 cm (asymmetrical stretching of carbonyl) and
−1
1
1
719 cm− (symmetrical stretching of carbonyl), and the C−N
−1
bond at ∼1264 cm were clearly shown in the FT-IR spectra
of the polyimide, which indicated the successful reaction
between FPTTDA and the dianhydride 6FDA and the
complete imidization reaction of the FPTTPAA. In addition,
the desired formation of the polyimide was further confirmed
by 1H NMR, as shown in Figure S7 in the Supporting
Information.
The molecular weight of the FPTTPI was determined by
GPC, and the qualitative solubility behavior of the correspond-
ing polyimide is summarized in Table 1. The inherent viscosity
Table 1. GPC Results and the Solubility of FPTTPI
a
GPC Results
parameter
value
Mw × 105
PDI
8.96
3.50
Solvent Data
b
solvent
solubility
NMP
+ +
+
DMSO
DMAc
DMF
+
+
m-cresol
+
a
With respect to polystyrene standards, with DMF as the eluent. Flow
b
rate, 1 mL/min; test temperature, 50 °C. Legend: (+ +) soluble at
room temperature; (+) partially soluble at room temperature, soluble
on heating (90 °C). The solubility was determined using 10 mg of PI
in 1 mL of solvent.
shown in Scheme 2. The chemical structure of FPTTPI was
confirmed by FT-IR spectra, as shown in Figure S6 in the
Supporting Information. Compared with the spectrum o1f
FPTTDA, the characteristic absorption at 3371 and 1617 cm−
(N−H stretching) in the spectrum of FPTTPI disappeared.
−1
−1
of FPTTPI in DMF (0.5 g L ) is 0.31 g dL , as measured by
an Ubbelohde viscometer at 30 °C. The M and M of the
n
w
Scheme 2. Preparation of the FPTTPI Based on FPTTDA
and 6FDA by a Conventional Two-Step Procedure
5
5
polyimide was 2.56 × 10 and 8.96 × 10 , respectively, and thus
a polydispersity index of 3.50 was observed, as determined by
GPC. The polyimide was soluble in many common polar
solvents such as N-methylpyrrolidone (NMP), dimethylaceta-
mide (DMAc), dimethyl sulfoxide (DMSO), DMF, and m-
cresol. The enhanced solubility can be attributed to the
introduction of the rigid large nonplanar conjugated side chain
into the polymer. Such structures may increase the steric
hindrance effect, thus reducing the interactions between the
polymer macrochains. The excellent solubility makes the
polymer a potential candidate for practical applications in
microelectronics by simple spin-on or efficient, low-cost and
continuous roll-to-roll processes.
The polyimide could be processed to a flexible and tough
film via solution casting or spin-on processes. Its thermal
properties were investigated by TGA and DSC (see Table S1 in
the Supporting Information), and the results of the TGA
revealed that the polyimide exhibited good thermal stabilities in
the atmospheres of N2 and air. The 10% weight-loss
temperatures (T % and T %) were 530 °C in N and 512
d5
d10
2
°
C in air, respectively. The amount of carbonized residue (char
yield) in N was 63% at 800 °C. In addition, the polyimide film
2
exhibited a high T of 280 °C and showed no clear endothermic
g
melting peaks up to the decomposition temperatures in the
DSC thermograms. These results are in agreement with the
WAXD results (see Figure S8 in the Supporting Information),
which demonstrated the amorphous nature and isotropic
properties of the material. Moreover, the polyimide film
D
DOI: 10.1021/acs.chemmater.5b01798
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
showed excellent mechanical properties (see Table S1); the
tensile strength and tensile modulus of the as-cast film were
9
2.1 MPa and 2.5 GPa, respectively. The thermal coefficient of
expansion (CTE) of the FPTTPI film was measured by a
thermomechanical analyzer (TMA Q400), which is 60 μm/(m
°
C) within the range of 100−200 °C (see Figure S9 in the
Supporting Information). The high T , the excellent thermal
g
stability, and the good mechanical properties of the soluble PI
could be due to the aromatic rigid nonplanar structure and low
polarity of the diamine monomer FPTTDA and are expected to
meet the requirements of heat resistance and high processing
and application temperatures in the microelectronics industry.
Dielectric Properties of the FPTTPI Film. The dielectric
properties of the polyimide film were measured at the
frequencies between 100 Hz and 1 MHz at 20 °C and 50%
relative humidity using a Solartron SI 1260 impedance/gain
phase analyzer (Solartron Group Ltd., U.K.). A commercial
insulating material, DuPont Kapton polyimide film, was also
measured for comparison. The measurement was carried out as
follows. First, a capacitance measurement was carried out; then
the dielectric constant of the FPTTPI and Kapton film was
calculated using eq 1:
Figure 3. Morphology of the FPTTPI film: (a) image of the flexible
FPTTPI film produced by a solution casting process; (b) a typical
AFM image of the FPTTPI film surface.
⎛
⎞
ε′ = C ⎜ ⎟
l
by a solution casting process. The film surface was smooth and
ε ⎝ A⎠
0
(1)
compact, with a surface roughness (R ) of 2.88 Å (see Figure
q
3
b), which was confirmed by scanning electron microscopy
Here, ε′ is the dielectric constant of the material between the
(
SEM) and atomic force microscopy (AFM). The polyimide
plates, C the capacitance of the material (in Farads), ε the
0
−12
−1
film contained no nanoporous structures, which prompted us to
consider the fact of larger free volume (subnanoscale) that
intrinsically exists in the amorphous region of polymeric
materials. The rigid nonplanar conjugated side chain of the
polymer may lead to structural (static or dynamic) disorder and
contribute to the local free volume, which may play an
important role in the ultralow-k properties of the polymer.
Positron annihilation lifetime spectroscopy (PALS) is a
useful method for studying atomic- and subnanometer-sized
electric constant (ε ≈ 8.854 × 10 F m ), l the thickness of
the film (in meters), and A the area of overlap of the two plates
in square meters); the results are shown in Figure 2. The
0
(
44−46
holes in materials,
and is often used to characterize the
free volume of polymer materials. In liquids and porous solids,
a fraction of the positrons injected from a radioactive source
form positronium, which can annihilate from the para state (p-
Ps, singlet spin state) or the ortho state (o-Ps, triplet spin state),
with a relative formation probability of 1:3. p-Ps decays mainly
via self-annihilation by emitting two γ-rays with a lifetime of
∼
125 ps, whereas the self-annihilation lifetime of o-Ps, which
emits three γ-rays, is as long as 142 ns. The lifetime of o-Ps
confined in local free volumes will be reduced to a typical value
of 1−5 ns, because of collisions of Ps with molecules. The o-Ps
will pick off one electron from the surrounding molecules and
annihilate it by emitting two γ-rays, which are called pick-off
annihilation. The pick-off annihilation lifetime of o-Ps is highly
sensitive to the size of the free volume.
Figure 2. Dielectric properties of the FPTTPI film and a DuPont
Kapton polyimide film; inset shows a plot of the dielectric constant of
FPTTPI versus temperature at 10 kHz.
FPTTPI provided good electrical properties, with a dielectric
constant of 1.52 and a dissipation factor of ∼0.008 at 10 kHz,
which is much lower than that of the commercial Kapton film
The positron lifetime spectra measured for FPTTPI and
Kapton films are shown in Figure 4. From a detailed analysis of
45
the lifetime spectra using the PATFIT routine (see Table 2),
we found two exponential decay components (τ and τ ) for
(
dielectric constant of 3.49) and the most of the reported
1
2
ultralow-k nanoporous organosilicate dielectric films (with k
the Kapton film and three exponential decay components (τ1,
τ , and τ ) for the FPTTPI film. The shortest lifetime τ was
0.29 ns for Kapton and 0.24 ns for FPTTPI, which is due to the
33
values in the range of 1.66−1.71) to date. Moreover, these
outstanding dielectric properties remain stable up to 280 °C
2
3
1
(
see inset in Figure 2), which is near the T of this polymer.
p-Ps lifetime and the free annihilation lifetime of the positrons.
g
Morphology of the FPTTPI Film and Mechanisms
The second lifetime component τ is attributed to positron
2
Studies. The exciting results caused us to examine the
morphology of the polymer film. Figure 3a showed a rolled
sample demonstrated the film flexibility, which was produced
annihilation in the amorphous region, which is 0.41 ns for
Kapton and 0.50 ns for FPTTPI. The long-lifetime component
τ3 is attributed to pick-off annihilation of o-Ps at holes in the
E
DOI: 10.1021/acs.chemmater.5b01798
Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
characteristic of the material formed only through a simple
solution casting process, and they are stable up to 280 °C. Its
excellent collective properties, such as high thermal stability and
mechanical properties, as well as good solubility, make it a
promising candidate for the ULSI industry. It is also our belief
that such a design strategy is beneficial for lowering the k value
and simultaneously maintaining the overall properties of
polymers, and it also can be extended to other novel high-
performance polymer systems.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.5b01798.
■
*
S
NMR results of all the intermediate products, FPTTDA
and FPTTPI; WAXD patterns of the FPTTPI film;
mechanical and thermal proterites of FPTTPI; exper-
imental setup for the dielectric constant measurement
Figure 4. Positron lifetime spectra measured for the FPTTPI film and
a DuPont Kapton polyimide film.
Table 2. Analyzed Data for the Positron Lifetime in the
DuPont Kapton and FPTTPI Films
(PDF)
Kapton
1.4406
FPTTPI
1.3221
AUTHOR INFORMATION
Corresponding Author
*E-mail: ceszy@mail.sysu.edu.cn.
Author Contributions
These authors contributed equally to the preparation of this
work.
Notes
■
density (g/cm3)
exponential decay (ns)
τ1
0.29
0.41
0.24
0.50
2.47
τ2
∇
τ3
intensity (%)
I1
I2
I3
34.4
65.6
46.1
41.8
12.1
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support by the National 973 Program of China
(Nos. 2014CB643605 and 2011CB606100), the National
Natural Science Foundation of China (Nos. 51373204,
1173214, 51233008, J1103305), the Doctoral Fund of the
Ministry of Education of China (No. 20120171130001), and
the Science and Technology Project of Guangdong Province
amorphous phase and can be used to estimate the size of local
free volumes in amorphous phases by eq 2:
4
6,47
5
−
1
⎡
= 0.5 ns⎢1 −
⎣
⎤
⎥
⎦
r
1
2π
⎛ 2πr ⎞
τ
+
sin
⎜
⎟
o‐Ps
⎝
⎠
r + Δr
r + Δr
(2)
(
No. 2015B090915003) are gratefully acknowledged.
where the prefactor of 0.5 ns is the spin-averaged Ps
annihilation lifetime, which is also observed in densely packed
molecular crystals; a value of Δr = 0.166 nm is obtained by
fitting eq 2 to the observed porous materials and was
determined by Eldrup and Jean.
Compared with Kapton, FPTTPI shows a long-lifetime
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