Comparative investigations on the effects of pendent trifluoromethyl group to the properties of the polyimides containing diphenyl-substituted cyclopentyl Cardo-structure

Article abstract of DOI:10.1016/j.jfluchem.2014.04.016

Two series of Cardo-type polyimides (PIs), fluorinated or nonfluorinated, were prepared from condensation of the fluorinated Cardo-type diamine 1,1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclopentane (BATFPCP) or the analogous nonfluorinated Cardo-type diamine 1,1-bis[4-(4-aminophenoxy)phenyl] cyclopentane (BAPCP) with four aromatic dianhydrides PMDA, ODPA, BTDA and BPDA, respectively. Both fluorinated and nonfluorinated PIs with Cardo-structure in polymer backbone generally retained good thermal properties comparable to aromatic PIs, with glass transition temperature (T_g) of 227-270 °C and 234-340°C, 5% weight loss temperature in nitrogen (air) of 471-483°C (448-474°C) and 472-482°C (464-492°C), respectively. The fluorinated and nonfluorinated PIs with Cardo-structure in the polymer backbone showed tensile strengths of 90.1-101.7 MPa and 78.6-99.6 MPa, tensile modulus of 1.44-1.83 GPa and 1.25-1.50 GPa, respectively, also comparable to those of the traditional aromatic PIs. In comparison with nonfluorinated series, the fluorinated PIs possessed better solubility, higher optical transparency and lower water absorption. The fluorinated PIs were readily soluble in a variety of organic solvents, exhibited high optical transparency and light color with the cut off wavelength at 373-424 nm and transmittance at 500 nm generally higher than 80%, and the water absorptions were measured as low as 0.18-0.93%. Additionally, the fluorinated series showed higher hydrophobicity by water contact angles and water absorption measurements and lower dielectric constant by dynamic dielectric measurements.

Full text of DOI:10.1016/j.jfluchem.2014.04.016

Accepted Manuscript
Title: Comparative Investigations on the Effects of Pendent
Trifluoromethyl Group to the Properties of the Polyimides
Containing Diphenyl-substituted Cyclopentyl Cardo-structure
Author: Xuemei Wang Feng Liu Jiancheng Lai Zhiqiang Fu
Xiaozeng You
PII:
S0022-1139(14)00118-3
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jfluchem.2014.04.016
FLUOR 8312
To appear in:
FLUOR
Received date:
Revised date:
Accepted date:
1-3-2014
28-4-2014
30-4-2014
Please cite this article as: X. Wang, F. Liu, J. Lai, Z. Fu, X. You, Comparative
Investigations on the Effects of Pendent Trifluoromethyl Group to the Properties of
the Polyimides Containing Diphenyl-substituted Cyclopentyl Cardo-structure, Journal
of Fluorine Chemistry (2014), http://dx.doi.org/10.1016/j.jfluchem.2014.04.016
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Highlights
A fluorinated Cardo-type diamine was prepared.
The electron-withdrawing, hydrophobic and low-polarizable pendent CF3 groups were
introduced to the PIs.
The CF3 groups led to desired reduction in CTC, coloration, water absorption and dielectric
constant of PIs containing Cardo-structure.
1
Page 1 of 31

Comparative Investigations on the Effects of Pendent Trifluoromethyl Group
to the Properties of the Polyimides Containing Diphenyl-substituted
Cyclopentyl Cardo-structure
Xuemei Wang a, Feng Liu a*, Jiancheng Lai b, Zhiqiang Fu a, Xiaozeng You b*
a. Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Honggutan New
District, Nanchang 330031, PR China
b. State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical
Engineering, Nanjing National Laboratory of Microstructure, Nanjing University, Nanjing
210093, PR China
*. Correspondence authors:
Feng Liu: liuf@ncu.edu.cn, tel (fax): (+) 86 791 83969984
Xiaozeng You: youxz@nju.edu.cn, tel (fax): (+) 86 25 83592969
2
Page 2 of 31

Abstract: Two series of Cardo-type polyimides (PIs), fluorinated or nonfluorinated, were
prepared from condensation of the fluorinated Cardo-type diamine 1,
1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclopentane (BATFPCP) or the
analogous nonfluorinated Cardo-type diamine
1,1-bis[4-(4-aminophenoxy)phenyl]cyclopentane (BAPCP) with four aromatic dianhydrides
PMDA, ODPA, BTDA and BPDA, respectively. Both fluorinated and nonfluorinated PIs with
Cardo-structure in polymer backbone generally retained good thermal properties comparable
to aromatic PIs, with glass transition temperature (Tg) of 227-270oC and 234-340oC, 5%
weight loss temperature in nitrogen (air) of 471-483oC (448-474oC) and 472-482oC
(464-492oC), respectively. The fluorinated and nonfluorinated PIs with Cardo-structure in the
polymer backbone showed tensile strengths of 90.1-101.7MPa and 78.6-99.6MPa, tensile
modulus of 1.44-1.83GPa and 1.25-1.50GPa, respectively, also comparable to those of the
traditional aromatic PIs. In comparison with nonfluorinated series, the fluorinated PIs
possessed better solubility, higher optical transparency and lower water absorption. The
fluorinated PIs were readily soluble in a variety of organic solvents, exhibited high optical
transparency and light color with the cutoff wavelength at 373-424nm and transmittance at
500nm generally higher than 80%, and the water absorptions were measured as low as
0.18-0.93%. Additionally, the fluorinated series showed higher hydrophobicity by water
contact angles and water absorption measurements and lower dielectric constant by dynamic
dielectric measurements.
Key Words: polyimide, Cardo-structure, trifluoromethyl, structure-properties relationship
3
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1. Introduction
Aromatic polyimides (PIs) have earned a reputation as high-performance materials in the
aerospace and microelectronics industries in the form of films, moldings and reinforced
composites, owing to their excellent thermal stability, good chemical and radiation resistance,
mechanical and electrical properties [1,2]. However, aromatic PIs show poor processing
performance such as high melting temperature and limited solubility in most organic solvents
as a result of the rigid backbones which, together with highly conjugated aromatic structures
and intermolecular charge transfer complex (CTC), further lead to intense coloration, reduced
optical transparence and high dielectric constant, hence restricting their applications in
optoelectronic and microelectronic fields such as flexible display and printed circuit board
[3-5]. Polymer structure modification becomes necessary for such difficulties to be overcome.
It has been reported that the enhancement of the practical properties like the tractability,
optical and dielectric properties of PIs could be effectively achieved by introduction of
flexible linkage, bulky substituents, and structurally unsymmetrical segments into the
polymer backbone [6-8]. The main concepts behind these approaches lie in the reduction of
conjugated aromatic characteristics and intermolecular interactions such as chain packing,
charge transfer complex and electronic polarization interactions. Nevertheless, the
improvements of these properties are usually obtained accompanied by the sacrifice of the
inherent thermal and mechanical properties [9].
Of the strategies of polymer structure modification of PIs, the introduction of
Cardo-structure into the polymer backbone stands out as it could upgrade the tractability of
PIs by expanding the polymer interchain distance and reducing the polymer chain packing
while essentially maintaining the thermal and mechanical properties of the PIs [10-13]. From
the viewpoint of synthetic methodology, the Cardo-type structure could be incorporated into
polymer backbone in terms of diamine or dianhydride monomers that are readily attained by
reacting Cardo-type diol with the anhydride or amine precursor such as trimellitic anhydride,
chlorophatic anhydride, or p-chloronitrobenzene with ester or ether linkage [14-17]. We have
recently used this synthetic method to develop a series of alicyclic-functionalized PIs
derivated from natural-(D)-camphor, which demonstrated better optical and dielectric
properties than analogous aromatic PIs [18-20]. However, compared with the aromatic
analogues, these alicyclic-functionalized PIs showed a reduction in thermal properties, which
would hamper their applications where high-temperature fabrication processes are needed. As
4
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a continuation, this piece of work was initiated to develop functional PIs, aiming at
comprehensively evaluating the effect of the Cardo-structure and fluorinated functionality to
the properties the polymers, with an emphasis on improving the tractability, optical and
dielectric properties and simultaneously maintaining the inherent thermal and mechanical
properties of PIs [21]. It was justifiably expected that the introduction of the bulky
Cardo-structure into polymer backbone should play an important role in retaining the thermal
properties and leading to the enhanced solubility as well. However, it has been also worried
that while the Cardo-structure helps improve the solubility of PIs by increasing the free
volume, it may possibly result in an increase of moisture absorption that in turn increases the
dielectric constant of PIs [22]. Herein we synthesized a diphenyl-substituted cyclopentyl
Cardo-type diamine monomer with two pendent trifluoromethyl (CF3) groups. We envisaged
that the CF3 groups would offset the negative effects of Cardo-structure in that it inhibits the
moisture absorption due to its hydrophobic nature and further decreases the dielectric
constant due to its low polarizability and the reduced moisture absorption [23-25].
Experimentally,
the
fluorinated
Cardo-type
diamine
monomer
1,1-bis[4-(4-amino-2-trifluoromethylphenoxy)phenyl]cyclopentane (BATFPCP) and its
nonfluorinated counterpart 1,1-bis[4-(4-aminophenoxy)phenyl]cyclopentane (BAPCP) were
prepared starting from cyclopentanone through three steps, as shown in Scheme 1. The
diphenol compound 1,1-bis(4-hydroxyphenyl)cyclopentane (BHCP) was synthesized from
the reaction of cyclopentanone with excessive amount of phenol in the presence of hydrogen
chloride and acetic acid as catalyst and 3-mercaptopropionic acid as co-catalyst. Aromatic
nucleophilic substitution reaction of BHCP with 1-chloro-4-nitro-2-(trifluoromethyl)benzene
or p-chloronitrobenzene in the presence of potassium carbonate afforded the intermediate
dinitro
compound
1,1-bis[4-(4-nitro-2-trifluoromethylphenoxy)phenyl]cyclopentane
(BNTFPCP) or 1,1-bis[4-(4-nitrophenoxy)phenyl]cyclopentane (BNPCP). The following
catalytic hydrogenation of the dinitro compounds of BNTFPCP or BNPCP by hydrazine
monohydrate under Pd/C catalyst afforded the target diamine BATFPCP or BAPCP.
5
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O
OH
HCl AcOH
F3C
HO
OH
Cl
NO2
Cl
NO2
BHCP
DMAc
K2CO3
DMF
K2CO3
CF3
F3C
O
NO2
O2N
O
O
NO2
O2N
O
BNPCP
BNTFPCP
Pd/C
H2NNH2
H2NNH2
Pd/C
CF3
F3C
H2N
O
O
NH2
H2N
O
O
NH2
BATFPCP
BAPCP
Scheme 1. The general synthetic route of BATFPCP and BAPCP
A series of fluorinated and analogous non-fluorinated PIs containing Cardo-structure
were prepared by condensing BATFPCP or BAPCP with four aromatic dianhydrides PMDA,
ODPA, BTDA and BPDA, respectively, and the synthesized PIs were abbreviated as
FCPIa~FCPId or CPIa~CPId correspondingly. The synthesis of the above PIs was shown in
Scheme 2. All the PIs were subjected to the solubility, thermal, mechanical, optical, water
absorption, and dielectric properties measurements for comparative structure-properties
relationship studies.
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CF3
F3C
O
H2N
O
O
NH2
H2N
NH2
O
or
BAPCP
BATFPCP
O
C
O
Thermal or Chemical
imidization
C
Ar
O
O
C
O
C
O
dianhydride
fluorinated group
O
C
O
O
C
O
C
CF3
O
F3C
C
N
Ar
N
or
O
O
N
Ar
O
N
C
O
C
O
C
O
C
O
n
n
cardo-structure
cardo-structure
CPI series
FCPI series
O
C
O
For dianhydride
structure:
Ar=
,
,
,
(BPDA, d)
(BTDA, c)
(ODPA, b)
(PMDA, a)
Scheme 2. The synthesis of fluorinated and nonfluorinated PIs containing Cardo-structure (FCPI series and
CPI series)
2. Results and discussion
2.1. Monomer synthesis
The structures of the intermediate and target products in each step were confirmed by IR
1
13
spectra, H NMR and C NMR spectroscopy. As shown in Supporting Information (Figure
S1), the nitro groups of compound BNTFPCP give two characteristic bands at 1535 and 1341
cm-1 corresponding to asymmetric and symmetric stretching of -NO2. After the reduction, the
characteristic absorptions of the nitro groups disappeared and the amino groups show a pair
of N-H stretching bands at 3420 and 3356 cm-1. Similarly, the absorption bands at 1508 and
1341 cm-1 attributed to asymmetric and symmetric stretching of the NO2 groups in the dinitro
compound BNPCP disappeared after the reduction, and the characteristic bands of the amino
1
groups at 3433 and 3356 cm-1 appeared as corresponding to N-H stretching. In the H NMR
spectra in Fig. 1, the signals of aromatic and cyclopentyl protons of BNTFPCP appeared in
the range of 6.94-8.57 and 1.74-2.34 ppm, and the signals of corresponding protons of
BATFPCP were upfield shifted to 6.74-7.21 and 1.66-2.26 ppm. The proton H5 of BNTFPCP
resonated at the farthest downfield due to the inductive effect (-I effect) of
7
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electron-withdrawing –NO2 and –CF3 groups, and the H4 and H7 ortho-oriented to the
electron-donating ether (C-O) group shifted to the upfield as the result of +C effect. Similarly,
the protons H5 and H6 of BATFPCP shifted to the upfield due to the electron-donating
property of the amino group.
8
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9
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Fig. 1. The 1H NMR spectra of the intermediate products and the diamine monomers.
In 13C NMR spectra in Supporting Information (Figure S2), both BNTFPCP and
BATFPCP show a total of 14 main signals. BNTFPCP shows three quartets because of the
13
heteronuclear C-19F coupling. The large quartet centered at about 122 ppm is attributed to
the –CF3 attached carbon. The single C-F (C14) couple constant in this case is 272 Hz. The
CF3-attached carbon (C9) also shows a clear quartet centered with a smaller coupling constant
of 32.5 Hz due to C-F coupling. Also, the resonance of the C10 carbon (ortho to the CF3 group)
was split by the three fluorine atoms three-bond coupling. The close quartet has an even
smaller coupling constant (5 Hz) because the interaction operated over more bonds. Similar
2
3
splitting patterns (1JC-F = 272 Hz, JC-F = 30Hz and JC-F = 5Hz) are also found in the 13C
NMR spectrum of BATFPCP. The spectra absorptions of BNPCP and BAPCP are similar to
those of the reported cyclododecane-containing cardo-structure diamine [15], and slightly
different chemical shifts of some peaks from the reported values could be attributed to their
different Cardo-structures. These results clearly confirm that the diamines prepared herein are
consistent with the proposed structure.
2.2 Polymer synthesis
The inherent viscosity of CPIa-d and FCPIa-d series were measured as 1.02, 0.94, 1.07,
0.98 dL/g and 0.92, 0.75, 0.82, 0.87 dL/g, respectively, which show that both BAPCP and
BATFPCP display adequate condensation reactivity with the applied aromatic dianhydrides.
10
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The characteristic absorption at 1383-1374 cm-1 attributed to C-N stretching and the
absorption bands at about 1720 cm-1 and 1775 cm-1 attributed to asymmetrical and
symmetrical stretching of imide carbonyl were observed in both fluorinated and
nonfluorinated PIs with Cardo-structure, and the fluorinated series show the absorption at
about 1260 cm-1 corresponding to characteristic C-F stretching of the pendent trifluoromethyl
groups. Similar to those of the monomers, the +C effect of ether oxygen atoms linked with
the two phenyl rings and –I effect and C-F coupling of the attached CF3 groups could be
observed in the 1H NMR and 13C NMR spectra of the resulting PIs. The representative IR and
NMR spectra of FCPIb were shown in Supporting Information (Figure S3).
2.3 Solubility and morphological structure of the polymers
The solubility behaviors of the chemically imidized PIs were investigated in various
kinds of solvents by dissolving 100mg of powdery polymer samples in 2mL of the solvent
either at room temperature or at elevated temperature, and the results are listed in Table 1.
Generally, the PIs containing diphenyl-substituted cyclopentyl Cardo-structure show better
solubility than traditional aromatic PIs [26, 27]. For instance, the introduction of the
Cardo-structure made PIs soluble in the solvents other than polar solvents DMAc and NMP.
It was found that the solubility of these PIs with Cardo-structure shows a correlation to the
polymer backbone structures and pendant groups as well. The fluorinated polymers FCPIb
and FCPIc showed excellent solubility in polar solvents such as NMP, DMF, DMAc and
DMSO, and they were also soluble in low-boiling-point solvents such as CH2Cl2 and THF at
room temperature or when heated, whereas, FCPIa and FCPId showed slightly lower
solubility. That could be attributed to the flexbile molecular structure of ODPA and BTDA
corresponding to the dianhydride moiety of FCPIb and FCPIc and the comparably more rigid
structure of PMDA and BPDA corresponding to the dianhydride moiety of FCPIa and FCPId
[28]. It is also seen that the fluorinated PIs showed better solubility in tested solvents than the
analogous non-fluorinated PIs. That could be attributed to the presence of the bulky CF3
group which increased the disorder in the chains and inhibited the dense chain packing, thus
reducing the interchain interactions and enhancing the solubility of the PIs.
The morphological structure of the chemically imidized powdery PIs was analyzed by
wide angle X-ray diffraction (WAXD) and illustrated in Supporting Information (Figure S4).
It was observed that the synthesized fluorinated and nonfluorinated PIs both present a broad,
low intensity peaks around 2θ=15-20°, inferring that these polymers are completely
11
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amorphous [29, 30]. That is reasonable because all the structures have the packing-disruptive
diphenyl-substituted cyclopentyl Cardo-structure. It seemed that the nonfluorinated series
display a wider reflection hump than the fluorinated series. According to Bragg’s equation
(λ=2dsinθ) [31, 32], one can calculate the mean intermolecular distance (d) from the peak
maximum. The bigger calculated d values of fluorinated Cardo-type PIs (5.3879-5.7810 Å)
compared with those of nonfluorinated Cardo-type PIs (4.9894-5.4672 Å) indicates a loose
chain packing density of the fluorinated series. This result could be attributed to the steric
hindrance and low polarizability of the pendent bulky CF3 group that increases the free
volume and accordingly reduces the polymer interchain interaction. These characteristics also
endow desired properties with fluorinated series than nonfluorinated series such as lighter
color in addition to higher solubility in solvents [33, 34].
12
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Table 1.
The solubility, d-spacing and optical properties of the PIs
Solubility a
WAXD b
Optical Properties c
Codes
NMP
++
++
++
++
--
DMAc
+h
DMF
+h
++
++
+h
--
DMSO
--
m-Cresol
THF
--
CH2Cl2
2θ(°)
d(Å)
λ0(nm) T500(%) δ(μm)
FCPIa
FCPIb
FCPIc
FCPId
CPIa
--
--
++
+h
--
16.439
16.036
16.210
15.314
17.762
16.199
16.382
16.589
5.3879
5.5222
5.4635
5.7810
4.9894
5.4672
5.4064
5.3394
405
373
424
398
422
373
441
398
80.46
82.68
77.44
82.94
56.06
76.81
50.27
80.80
39
27
37
40
45
34
45
51
++
++
+h
++
++
+h
--
++
++
++
--
++
+h
--
--
--
--
CPIb
++
++
+h
++
+h
++
+h
--
+h
+h
--
++
++
++
+h
--
++
--
CPIc
CPId
--
--
--
a Qualitative solubility measured with 100mg of the polymer in 2mL of solvent. ++, soluble at room temperature; +h, soluble when heated; --, insoluble even heated.
b The d value is typical for the average chain-chain distance calculated from the prominent peak in WAXD pattern by Brag equation. c λ0 refers to the absorption edge
from the UV-vis spectra of the polymer films, T500 is the transmittance at 500nm in the UV-vis spectra, and δ is the measured thickness of the film samples.
13
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2.4 Optical Properties of the polymers
Thin films of thermally imidized PIs were measured for optical transparency with UV-vis
spectroscopy. Fig.2 shows the UV-vis transmittance spectra of the films, and the cut-off
wavelength (λ0) values and the percentage of transmittance at 500 nm from these spectra are
listed in Table 2. It could be seen that the fluorinated series exhibited a λ0 in the range of
373-424 nm, and the transparency at 500 nm (T500) was higher than 77%. Apparently, the
fluorinated series revealed lower λ0 and higher optical transparency than their respective
analogous nonfluorinated series [35]. The λ0 could also be used to elucidate the color
intensities of the PI films. It is clear that the values of λ0 decrease in the order of FCPIc >
FCPIa > FCPId > FCPIb, and the color of these films changed in turn from dark yellow to
colorless [36, 37]. The fact that fluorinated series showed lighter color than nonfluorinated
series could be explained as attributed to the reduction of the intermolecular CTC between
alternating electron-donating diamine moieties and electron-accepting dianhydride moieties
[38]. The steric hindrance of pendent CF3 groups enlarge the polymer interchain distances
and the electron-withdrawing effect of attached CF3 group decreases the electron-donating
property of the diamine moieties [39], which together lead to the reduced interchain CTC
formation of the fluorinated PIs.
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Fig. 2. Comparison of UV-visible spectra between FCPI series and CPI series.
2.5 Thermal Properties of the polymers
The thermal behaviors of the films of thermally imidized PIs were evaluated by dynamic
mechanical analysis (DMA) and thermogravimetric analysis (TGA). The DMA curves of the
PIs are shown in Fig.3 and the thermal analysis data from the DMA curves are summarized in
Table 3. It can be seen that the PIs with Cardo-structure, either fluorinated or nonfluorinated,
showed comparable thermal property as represented by the Tg values of FCPI series and CPI
series in the range of 227-270oC and 234-300oC, respectively. It confirms that the
introduction of Cardo-structure is one modification measure that could effectively retain the
inherent thermal property of PIs. Apparently, as is reported, the Tg values of the fluorinated
series were slightly lower than those of the unfluorinated series [40]. This would
understandably be the result of the decreased electronic interchain interactions and loose
dense packing caused by the low-polarizable bulky pendent CF3 groups. The Tg values of
these PIs show a dependence on the dianhydride component structure, and as expected,
decrease with increasing flexibility of the polymer backbones in the dianhydride monomer
order of PMDA > BPDA > BTDA > ODPA [41]. FCPIb obtained from ODPA, for instance,
shows the lowest Tg in fluorinated series because of the presence of a flexible ether linkage
between every two phthalimide units. In the same manner, FCPIa derived from PMDA,
exhibits the highest Tg because of the rigidity of the polymer backbone.
Thermal decomposition of the PIs was evaluated by TGA in air or in nitrogen
atmosphere at a heating rate of 10oC/min, and the typical TGA curves as represented by
15
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FCPId and CPId are reproduced in Fig.4. The decomposition temperature at 5% and 10%
weight loss (Td5 and Td10) determined from the original TGA thermograms are listed in Table
3. The fluorinated series show good thermal stability and have no notable weight loss below
470oC. The values of Td5 of FCPI series are recorded in the range of 471-483oC in nitrogen
and those of CPI series in the range of 472-482oC. The FCPI series left more than 51% char
yield at 800oC in nitrogen except FCPIa [42]. The TGA data indicates that these PIs with
Cardo-structure, both fluorinated and nonfluorinated, possess fairly high thermal stability
even with the introduction of the bulky CF3 pendent groups [43].
These experimental results infer that the PIs containing the diphenyl-substituted
cyclopentyl Card-structure in the polymer backbone, with or without fluorinated group, enjoy
great thermal stability to withstand the harsh environments of many high-tech applications.
The in-plane coefficients of thermal expansion (CTE) of the PIs were summarized in Table 3.
The fluorinated PIs possess CTE values of 36.1-51.2 ppm/oC during the temperature scope of
25-300oC. The thermal stability of the fluorinated PI films as illustrated by CTE shows good
consistence with the rigidity of the polymer chain. For example, FCPIa obtained from PMDA,
possesses the lowest CTE owing to the rigid structure of pyromellitic imide rings.
Fig. 3. Tan delta curves of the PIs by DMA technique.
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Fig. 4. TGA curves of FCPId and CPId.
2.6 Mechanical properties of the polymers
The films of thermally imidized PIs were subjected to tensile tests, and the results are
summarized in Table 2. The fluorinated series show tensile strength at break of
90.1-101.7MPa, elongation at break of 12.7-14.2%, and initial modulus of 1.44-1.83GPa. The
tensile properties of FCPI series are comparable with those of the CPI series. FCPIa based on
the aromatic dianhydride PMDA, exhibits low elongation at breakage of 12.7% due to the
rigid polymer backbone. FCPIb possesses the best mechanical properties with the elongation
at break of 14.1% and tensile strength of 101.7MPa, which could be attributed to the presence
of flexible ether linkage between two aromatic rings in the dianhydride moiety segment of
ODPA [44]. Besides increased solubility and reduced color intensity, the pendent
trifluoromethyl groups endow FCPI series with retained mechanical properties as compared
with the non-fluorinated CPI series [45]. The difference between the tensile properties of the
FCPI and CPI series seems slight and both series behave as strong and tough polymeric
materials.
17
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Table 2.
The thermal, mechanical, and surface, moisture uptake properties of the PIs.
Thermal properties
Mechanical properties d
γs
Water
absorption
(%) g
Char
Yield
(%)b
CTE
(ppm/
oC) c
(mN/
Codes
θ (o) e
Tg
Td5 (oC)
Td10 (oC)
TM
EB
TS
m) f
(oC) a
(GPa)
(%)
(MPa)
In N2
In air
In N2
In air
FCPIa
FCPIb
FCPIc
FCPId
CPIa
270
227
238
248
300
234
250
256
471
483
474
482
481
482
472
479
448
455
470
474
466
492
464
470
501
510
506
509
504
504
507
509
477
499
502
507
504
512
498
512
41
51
51
54
34
48
39
50
36.1
44.1
51.2
42.1
32.3
40.3
45.0
36.2
1.83
1.55
1.44
1.49
1.25
1.33
1.28
1.50
12.7
14.1
13.8
14.2
12.0
12.6
10.4
14.6
90.1
101.7
100.4
95.0
78.6
98.6
92.2
99.6
81.6
80.9
77.1
84.5
74.1
78.2
75.9
74.5
34.21
34.78
36.42
32.62
38.57
36.03
37.96
38.82
0.46
0.20
0.93
0.58
0.89
0.32
1.02
0.73
CPIb
CPIc
CPId
a Glass transition temperature (Tg) was measured by DMA at a heating rate of 10oC/min. b Residual weight percentage at 800°C in N2. c The in-plane coefficients of
thermal expansion. d TS: tensile strength; EB: elongation at break; TM: tensile modulus. e Equilibrium contact angle was measured at ambient temperature and double
distilled water as solvent for a time period of 120 s depending on the stability of the drop, and the data was the average value of ten experiments. f Surface energy is
obtained indirectly from the water contact angle by Owen equation. g Water absorption (%) = (W-W0/W0) × 100 %; where W is the weight of polymer sample after
immersed in water for 96 h at room temperature and W0 the weight of polymer sample after being dried in vacuum at 100oC for 8 h..
18
Page 18 of 31

2.7 Contact angle, surface free energy and water absorption of the polymers
The contact angles (θ) of the thermally imidized PI films against water are listed in Table
3. Fig.5 depicted the profiles of a droplet on the FCPI and CPI series surface. Obviously, all
the fluorinated FCPI series show larger contact angles than pure Cardo-type CPI series, and
the θ of the two series are in the range of 77.1-84.5° and 70.1-78.2°, respectively. The results
indicate that the presence of pendent trifluoromethyl group could strikingly increase the
hydrophobicity of the Cardo-type PIs. With measured contact angles, surface energy can be
calculated by the following equation: 1+cosθ=2(γs / γl)1/2exp[-β (γl-γs)]2 [46-48], where, β is a
constant with a value of 0.0001247 (m2/mJ)2, which is determined from contact angle data for
low energy solids; γs and γl are the surface energy of the solid and the surface energy of the
testing liquid, respectively. The calculated results of surface free energy in Table 2 show that
the γs values of the fluorinated series present a corresponding decrease compared with those
of pure Cardo-type PIs. Large contact angle and low surface free energy of the fluorinated
series could mainly originate from the hydrophobic trifluoromethyl groups, which lead to the
migration of the fluorocarbon chain segments to the film surface and fluorine enrichment at
the surface, thus reducing the surface tension and making the surface more hydrophobic.
Additionally, the hydrophobic trifluoromethyl groups inhibited the water absorption of the
polymers [49]. As noted in Table 2, fluorinated series exhibit relatively low water uptakes.
The values of moisture uptake for CPI series are in the range of 0.32-1.02%, whereas for
FCPI series the values are shown to be smaller, ranging in 0.20-0.93%.
Fig. 5. Representative images of contact angles of the PI films.
19
Page 19 of 31

2.8 Dielectric properties of the polymers
Dielectric properties of the thermally imidized PI films (FCPIb and CPIb) were evaluated
on the basis of dielectric constant (ε'), dielectric loss (ε'') and their variation with frequency
and temperature. Fig.6(a) shows the frequency dependence of the dielectric constant of the
two samples at three selected temperatures (-25oC, 0oC, 25oC). It could be seen that the value
of ε' of both polymers decrease gradually with increasing frequency [50]. The ε' depends
upon the ability of the polarizable units in the polymer structure to orient fast enough to keep
up with the oscillations of an alternating electric field. When the frequency increases, the
orientational polarization would gradually lose pace with the changing frequency, and so the
dielectric constant ε' begins to decrease. Fig.6(b) shows the temperature dependence of ε' at
three different frequencies (1Hz, 1KHz, 1MHz). A gradual increase in ε' of FCPIb was
observed with the increase of temperature at fixed frequency, and this effect was more
prominent at lower frequencies. The ε' variation with temperature of CPIb after 300K (25oC)
is the same as that of FCPIb. It is explained that as the temperature increases, the polymer
chain become more mobile and the dipoles become freer to respond to the applied field, and
thus the dielectric constant increases as the polarization increases with increasing temperature
[51]. However, the ε' variation with increasing temperature of CPIb appears complicated
below 25oC with an initial increase followed by a decrease, and the mechanisms behind this
experimental observation are still under investigation.
20
Page 20 of 31

Fig. 6. (a). Dielectric constant (ε') vs. frequency of FCPIb and CPIb at selected temperatures.
(b). Dielectric constant (ε') vs. temperature of FCPIb and CPIb at selected frequencies.
A general comparison of ε' and ε'' between FCPIb and CPIb at 25oC is presented in Table
3. Compared with traditional aromatic (PMDA-ODA) PI represented by Kapton whose
dielectric constant (ε') was measured as about 3.68 at 1MHz at room temperature [52], the PIs
with Cardo-structure represented by FCPIb and CPIb showed lower dielectric constant and
thus displayed better potential for microelectronic application as the signal propagation
velocity and fidelity are inversely related to the dielectric constant of the insulating material
[22]. The dielectric constant of FCPIb was in the range of 3.02-3.12, lower than those of CPIb
ranging in 3.31-3.35. The presence of pendent trifluoromethyl group in the diamine moiety
segments contributes to the decrease the dielectric constant of fluorinated PI by three aspects.
One is the reduced polarizability of polymer by strong electron-withdrawing inductive effect
of trifluoromethyl group; secondary, the pendent fluorinated groups disturb the polymer chain
packing and so lead to an increased free volume; additionally, the high hydrophobicity of
trifluoromethyl group accordingly reduces the moisture content of fluorinated PI [53].
Meanwhile, low values of the dielectric loss (ε'') of the PIs containg Cardo-structure, both
fluorinated and nonfluorinated, are indicative of minimal conversion of electrical energy into
21
Page 21 of 31

heat in the dielectric material, and also advantageous for lowering the electrical signals loss
in the dielectric medium.
Table 3
Comparison of ε' and ε'' of FCPIb and CPIb at selected frequencies and 25oC.
Dielectric constants (ε')
Dielectric loss (ε'')
1Hz 1KHz
Polymer
1Hz 100Hz 1KHz 1MHz
FCPIb
CPIb
3.12 3.07
3.35 3.34
3.06
3.33
3.02
3.31
0.0238 0.0096
0.0083 0.0035
2.9 General properties comparison between polymers in this work and those previously
reported
Three previously reported structurally-similar PIs Ref-A, Ref-B and Ref-C, which were
prepared from ODPA condensed with cyclohexyl-containing Crado-type diamine
1,1-bis[4-(3,4-aminophenoxyl)phenyl]cyclohexane,
1,1-bis[4-(4-amono-2-trifluoromethylphenoxy)phenyl]cyclohexane,
and
1,1-bis[4-(2-trifluoromethyl-4-amonophynoxy)phenyl]-4-tert-butylcyclohexane, respectively,
have been used for properties comparison with the PIs in this work. Two PI films CPIb and
FCPIb in this work have been used for comparison so that all the compared PIs were derived
from the same dianhydride ODPA. Table 4 lists the general properties of the pure Cado-type
PIs of CPIb and Ref-A, and the fluorinated Cado-type PIs FCPIb, Ref-B and Ref-C. It can be
observed from the solubility behavior data and the UV-vis cut-off wavelength (λ0) values that
CPIb possesses better solubility and higher transparence than Ref-A, and that FCPIb displays
comparable solubility and optical properties with the analogous fluorinated films Ref-B and
Ref-C. Particularly, cyclopentyl Cardo-type PIs CPIb and FCPIb show an advantage over the
analogous cyclohexyl Cardo-type PIs Ref-A and Ref-B on two important properties Tg and
elongation at break (EB), which may originate from the polymer backbone structural
variation between the five-membered ring of cyclopentyl moiety and the six-membered ring
of cyclohexyl moiety. Although having the comparable thermal properties with FCPIb, Ref-C
demonstrates inferior general mechanical properties possibly due to the presence of the extra
tert-butyl group attached to the cyclohexyl ring. As can be seen from the measured dielectric
constant (ε'), the cyclopentyl ring and cyclohexyl ring structure as well show to be more
effective in bringing down the ε' value than the tert-butylcyclohexyl ring does. It indicates
that the FCPIs prepared in this work are more suitable for microelectronic application due to
the better combined practical properties.
22
Page 22 of 31

Table 4
General properties comparison of CPIb and FCPIb with the reported structurally-similar PIs
solubility
Optical properties
Thermal properties
Td10 (oC)
Char
oC ) In N2 In air Yield
Mechanical properties
TM EB TS
(GPa) (%) (MPa)
Dielectric constant (ε')
code
λ0
δ
Tg
NMP DMAc DMF DMSO THF
1KHz
1MHz
(nm)
(μm)
(
++
++
++
++
++
++
--
++
++
++
++
--
++
++
++
+h
--
++
++
+h
+h
--
++
++
++
373
34
234 504
512
536
499
519
494
48
54
51
45
42
1.33 12.6 98.6
3.33
3.43
3.06
3.30
4.07
3.31
3.24
3.02
3.08
4.03
CPIb
Ref-Aa
FCPIb
Ref-Bb
Ref-Cc
393
373
377
372
69
27
40
77
230 532
227 510
214 523
231 515
1.72
12
105
1.55 14.1 101.7
2.1
1.1
9
12
95
86
CF3
F3C
O
O
O
O
O
O
O
O
N
N
O
O
Ref-B:
N
N
Ref-A:
Ref-C:
O
O
O
O
n
n
O
CF3
F3C
O
O
O
O
N
N
O
O
n
H3C
C
CH3
CH3
a. All the original data have been obtained from Reference 14 and 15. b. All the original data have been obtained from Reference 15. c. All the original data have been
obtained from Reference 21.
23
Page 23 of 31

3. Conclusion
This paper presented two series of PIs (FCPI series and CPI series) that were derived
from the corresponding fluorinated and nonfluorinated diamines containing
diphenyl-substituted cyclopentyl Cardo-structure (BATFPCP and BAPCP). It is demonstrated
that the introduction of Cardo-structure into the PI’s polymer backbone leads to better
solubility, higher optical transparence and lower dielectric constant while retaining the
thermal and mechanical properties generally comparable to those of aromatic PIs.
Comparative studies reveal that the pendent CF3 groups attached to the diamine moiety are
shown to be effective to further inhibit the water absorption and reduce the dielectric constant
of the PIs, owing to the synergistic effect of the decreased electron-donating properties of
diamine moiety, increased steric hindrance, reduced polarizability and enhanced
hydrophobicity. The eminent combination of the practical properties makes the resulting
fluorinated PIs with cyclopentyl Cardo-structure (FCPI series) in this work potential
competitive candidates for fabricating microelectronic devices.
Acknowledgements
The authors are thankful to the NSFC (Grant Nos. 51263014, 21271099 and 50803026),
the Major State Basic Research Development Program (Grant No. 91022031) and Jiangxi
provincial Education Department (Grant No. GJJ13113) for their financial support of this
work.
4. Experimental
4.1 Materials
Cyclopentanone, 3-mercaptopropionic acid, 1-chloro-4-nitro-2-(trifluoromethyl)benzene,
p-chloronitrobenzene, anhydrous potassium carbonate, 10% palladium on activated carbon
(Pd/C), hydrazine monohydrate were of analytical purity grade and used as received.
Pyromellitic dianhydride (PMDA), 3, 3', 4, 4'-oxydiphthalic anhydride (ODPA), 3, 3', 4,
4'-benzophenonetetracarboxylic dianhydride (BTDA), 3, 3', 4, 4'-biphenyltetracarboxylic
dianhydride (BPDA) were recrystallized from acetic anhydride and dried in a vacuum oven at
150oC overnight prior to use. N, N-dimethylformamide (DMF), N, N-dimethylacetamide
(DMAc) were purified by distillation under reduced pressure over phosphorus pentoxide and
stored over 4Å molecular sieves. Other commercially available materials were used without
further purification.
24
Page 24 of 31

4.2 Measurements
IR spectra were recorded on an Agilent Fourier Transform Infrared spectrometer FTS-40.
1H NMR and 13C NMR spectra were measured on Bruker DRX 400 spectrometers with
CDCl3 or DMSO-d6 as the solvent and tetramethylsilane as the internal reference. The
melting points were measured with a microscopic melting point meter SGW-X4. Elemental
analyses were carried out on a Perkin-Elmer 2400 C, H, N analyzer. The inherent viscosities
of the poly (amic acid) were measured with an Ubbelohde viscometer at 30oC at a
concentration of 0.5 g/dL using DMAc as the solvent. Contact angle measurements were
carried out by using Drop Master 300 Contact AM system (Kyowa Interface Science Co.,
Saitama, Japan). Wide-angle X-ray diffraction (WAXD) measurement were performed at
25oC on a Bede XRD Di system using graphite-monochromatized Cu-Kα radiation
(λ=0.15405nm). Ultraviolet visible (UV-vis) spectra of the polymer films were recorded on a
Shimadzu UV-visible spectrophotometer UV-2450. Thermogravimetric analysis (TGA) were
conducted with a Perkin-Elmer TGA-2 in flowing nitrogen or in air at a heating rate of
10oC/min. Dynamic mechanical analysis was conducted with DMA Q800 V20.22 Build 41
using tensile mode at the frequency of 1Hz. The stress-strain behavior of the PI film samples
were tested by an Instron universal tester model 1122 (GB/T1040.1-2006) at room
temperature with a stretching rate of 2.5mm/min, and the data was the average value of five
experiments except for the maximum and the minimum value. Coefficients of thermal
expansion (CTEs) of the films were detected in the optical dilatometer DIL-806 dilatometer
(Baehr-Thermoanalyse GmbH, Germany) with the heating rate of 10K/min and measurement
precision of 0.1µm. Dielectric spectroscopy measurements of the PI films with the
temperature variation in the range of 150-520 K and the frequency variation in the range of
1-107 Hz have been performed using a Novocontrol Dielectric Spectrometer (GmbH
Germany), CONCEPT 40.
4.3 The synthesis of the fluorinated and nonfluorinated Cardo-type diamine monomers
4.3.1 The synthesis of BHCP
A
mixture of cyclopentanone (8.41g, 0.1mol), phenol (37g, 0.4mol),
3-mercaptopropionic acid (1.03mL), 90ml hydrochloric acid/ acetic acid (volume ratio 2:1)
was placed in a 500mL three-necked flask equipped with a mechanical stirrer and reacted at
room temperature for 24 h. The purple precipitate was washed with hot water and dissolved
in 2mol/L aqueous sodium hydroxide solution, acidized with concentrated hydrochloride acid,
25
Page 25 of 31

filtrated and washed with water, dried at 60oC under vacuum and recrystallized from toluene
twice to afford pink crystals in 73.6 % yield (m. p.: 153-155oC). IR (KBr, cm-1): 3253 (O-H
stretching); 3033(Ar-H stretching); 2969, 2866 (C-H Stretching). 1H NMR (DMSO, δ, ppm):
9.08 (s, 2H, H5), 7.05 (d, J = 8.6 Hz, 4H, H3), 6.61 (d, J = 8.6 Hz, 4H, H4), 2.14 (s, 4H, H2),
1.57 (s, 4H, H1).13C NMR (DMSO, δ, ppm): 156.27 (C7), 139.72 (C4), 128.69 (C5), 115.12
(C6), 54.28 (C3), 38.63(C2), 22.84 (C1). Elemental analysis (%): calculated: C, 80.28; H, 7.13;
found: C, 80.26; H, 7.16.
4.3.2 The synthesis of BNTFPCP
To
a
three-necked round bottom flask of BHCP (2.54g, 0.01mol) and
1-chloro-4-nitro-2-(trifluoromethyl)benzene (4.74g, 0.021mol) completely dissolved in DMF
was added potassium carbonate (4.14g, 0.03mol). The reaction was stirred at 100oC for 8 h,
then cooled to room temperature and poured into water. The precipitate was washed with
ethanol, recrystallized from DMF/water (volume ratio1:1) and dried in vacuum at 60oC to
afford white crystals in 87.1% yield (m. p.: 116-118oC). IR (KBr, cm-1): 3097(Ar-H
stretching); 2981, 2866 (C-H Stretching); 1535, 1341(-NO2 stretching); 1263, 1160 (C-F and
C-O stretching). 1H NMR (CDCl3, δ, ppm): 8.57 (d, J = 2.7 Hz, 2H, H5), 8.29 (dd, J = 9.2, 2.7
Hz, 2H, H6), 7.40-7.35 (m, 4H, H3), 7.06-7.00 (m, 4H, H4), 6.94 (d, J = 9.2 Hz, 2H, H7), 2.34
(t, J = 6.8 Hz, 4H, H2), 1.82-1.74 (m, 4H, H1).13C NMR (CDCl3, δ, ppm): 161.08 (C8), 151.92
3
(C7), 146.49(C11), 141.68 (C4), 129.76 (C5), 129.03 (C12), 123.91 (C10, quartet, JC-F = 5
Hz),123.88 (C14, quartet, 1JC-F = 272 Hz), 120.36 (C9, quartet, 2JC-F = 32.5 Hz), 120.09 (C6),
116.97 (C13), 56.28 (C3), 38.88 (C2), 22.91 (C1). Elemental analysis (%): calculated: C, 58.87;
H, 3.51; N, 4.43; found: C, 58.85; H, 3.51; N, 4.46.
4.3.3 The synthesis of BATFPCP
To a mixture of BNTFPCP (5g, 0.008mol), 10% Pd/C (0.3g), and 300mL ethanol in a
three-necked flask was added hydrazine monohydrate (25mL) dropwise over a period of 0.5 h
at 60oC. The reaction mixture was refluxing for 8 h. Filtration while hot to remove the
catalyst and concentration using a rotation evaporator gave white solid, which was
recrystallized from ethanol to afford BATFPCP with 86% yield (m. p.: 187-188oC). IR (KBr,
cm-1): 3420, 3356 (N-H stretching); 3051 (Ar-H stretching); 2969, 2866 (C-H Stretching);
1
1261, 1224, 1160, 1125 (C-F and C-O stretching). H NMR (CDCl3, δ, ppm): 7.21-7.15 (m,
4H, H3), 6.93 (d, J = 2.8 Hz, 2H, H5), 6.84-6.80 (m, 6H, H4,7), 6.74 (dd, J = 8.7, 2.7 Hz, 1H,
H6), 3.71 (s, 4H, H8), 2.26-2.18 (m, 4H, H2), 1.75-1.66 (m, 4H, H1).13C NMR (CDCl3, δ,
26
Page 26 of 31

ppm): 156.87 (C7), 146.56 (C8), 143.46 (C11), 142.16 (C4), 128.12 (C5), 124.17 (C14, quartet,
1JC-F = 272 Hz), 121.85 (C9, quartet, 2JC-F = 30 Hz), 119.33 (C12), 117.34 (C13), 112.91 (C6),
123.0 (C10, quartet, 3JC-F = 5 Hz), 54.75 (C3), 38.93 (C2), 22.96 (C1). Elemental analysis (%):
calculated: C, 65.03; H, 4.58; N, 4.89; found: C, 65.04; H, 4.55; N, 4.91%.
4.3.3.4 The synthesis of BNPCP
To the mixture of BHCP (5.08g, 0.02mol) and p-chloronitrobenzene (6.62g, 0.042mol) in
DMAc in a three-neck round bottom flask was added potassium carbonate ( 8.28g, 0.06mol).
The reaction was stirred at 150oC for 8 h, then cooled to room temperature and poured into
water. The precipitate was washed with ethanol, recrystallized from DMF/water (volume
ratio1:1) and dried in vacuum at 60oC for 8 h to afford brown needle-like crystals in 81%
yield (m. p.: 167-169oC). IR (KBr, cm-1): 3097 (Ar-H stretching); 2969, 2866 (C-H
1
Stretching); 1508, 1341 (-NO2 stretching); 1250, 1172 (C-O stretching). H NMR (CDCl3, δ,
ppm): 8.20 (d, J = 7.6 Hz, 4H, H6), 7.34 (d, J = 7.3 Hz, 4H,H3), 7.10-6.95 (m, 8H, H4, 5), 2.33
(s, 4H, H2), 1.77 (s, 4H, H1).13C NMR (CDCl3, δ, ppm): 163.37 (C8), 153.55 (C7), 146.78
(C11), 142.62 (C4), 128.81 (C5), 126.94 (C10), 120.09 (C9), 117.09 (C6), 55.79 (C3), 38.94 (C2),
22.96 (C1). Elemental analysis (%): calculated: C, 70.15; H, 4.87; N, 5.64; found: C, 70.17; H,
4.84; N, 5.63.
4.3.5 The synthesis of BAPCP
To a mixture of BNPCP (5g, 0.01mol), 10% Pd/C (0.3g), and 300mL ethanol in a
three-necked flask was added hydrazine monohydrate (25mL) dropwise over a period of 0.5 h
at 60oC. The reaction mixture was kept refluxing for 8 h. Filtration while hot to remove the
catalyst and concentration using a rotation evaporator afforded pale yellow solid, which was
recrystallized from ethanol to afford BAPCP with 85% yield (m. p.: 143-144oC). IR (KBr,
cm-1): 3433, 3356 (N-H stretching); 3059 (Ar-H stretching); 2969, 2878 (C-H Stretching);
1238, 1160 (C-O stretching). 1H NMR (CDCl3, δ, ppm): 7.15 (s, 4H, H3), 6.82 (d, J = 22.8 Hz,
8H, H4, 5), 6.66 (s, 4H, H6), 3.57 (s, 4H, H7), 2.23 (s, 4H, H2), 1.69 (s, 4H, H1).13C NMR
(CDCl3, δ, ppm): 156.57 (C7), 148.76 (C8), 142.72 (C11), 142.53 (C4), 127.99 (C5), 121.10
(C9), 115.62 (C6), 115.22 (C10), 54.66 (C3), 38.93 (C2), 22.98 (C1). Elemental analysis (%):
calculated: C, 79.79; H, 6.46; N, 6.42; found: C, 79.79; H, 6.47; N, 6.40.
4.4 The synthesis of PIs
The synthesis of FCPIa is used as an example to illustrate the general process of
27
Page 27 of 31

preparing the PIs.
4.4.1 Thermal cyclodehydration
Diamine BATFPCP (1g, 0.0017mol) was completely dissolved in 12.5mL dried DMAc
in a 50mL flask, and the equimolar dianhydride PMDA (0.3810g, 0.0017mol) was added in
one portion. The mixture was stirred at room temperature for 12 h to form a viscous poly
(amic acid) (PAA) solution and then poured onto a glass plate, placed in an oven at 80oC for 5
h to remove the solvent. The semidried PAA film was further thermally imidized by
sequential heating at 100oC for 1 h, 150oC for 1 h, 200oC for 1 h, 280oC for 2 h. A flexible
polyimide film was stripped off from the glass surface by being soaked in boiled water.
4.4.2 Chemical imidization
PAA solution was prepared using the same synthetic method as that for the thermal
imidization. Then a mixture of acetic anhydride and triethylamine (volume ratio 2:1) was
added and reacted at 120oC for 4 h. The resulting solution was slowly poured into
methanol/water (volume ratio 1:1) while stirring and the precipitate was washed thoroughly
with methanol and hot water, and then dried in vacuum oven at 100oC for 12 h.
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