Synthesis and characterization of highly transparent and hydrophobic fluorinated polyimides derived from perfluorodecylthio substituted diamine monomers

Article abstract of DOI:10.1002/pola.27461

Two new perfluorodecylthio substituted aromatic diamines, namely 2,4-diamino-1-(1H,1H,2H,2H-perfluorodecathio) benzene (DAPFB) and 2,2′-Bis((1H,1H,2H,2H-perfluorodecyl) thio)[1,1′-biphenyl]4,4′-diamine (BPFBD) were synthesized and polycondensed with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) to produce two new perfluorinated polyimides (PI2 and PI4). The chemical structures of these polyimides were confirmed by Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. Two other polyimides (PI1 and PI3) were also synthesized from 6FDA and analogous perfluorodecylthio unsubstituted diamines to investigate the incorporation effect of perfluorodecylthio group on various physical and chemical properties of the synthesized PIs. Compared with PI1 and PI3, PI2 and PI4 exhibited improved solubility, optical transparency, and hydrophobicity, lower moisture absorption, dielectric constant, and thermo-mechanical stabilities owing to the presence of the perfluorodecylthio side group in the polymer chain. Even though thermo-mechanical properties of PI2 and PI4 (T_d⁵: 413 and 404 °C, T_g: 220 and 209 °C, tensile strength of 101 and 76 MPa, tensile modulus of 1.7 and 1.5 GPa and elongation at break of 8 and 10%, respectively) were reduced in comparison to PI1 and PI3 but still were good enough for most of the practical applications. Most importantly, the presence of the perfluorodecylthio side group in BPFBD considerably reduced the dielectric constant of PI4 to 2.71 which was quite low as aromatic polyimide.

Full text of DOI:10.1002/pola.27461

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ARTICLE
Synthesis and Characterization of Highly Transparent and Hydrophobic
Fluorinated Polyimides Derived from Perfluorodecylthio Substituted
Diamine Monomers
Pradip Kumar Tapaswi, Myeon-Cheon Choi, Saravanan Nagappan, Chang-Sik Ha
Department of Polymer Science and Engineering, Pusan National University, Busan, 609–735, Republic of Korea
Correspondence to: C.-S. Ha (E-mail: csha@pnu.edu)
Received 13 September 2014; accepted 28 October 2014; published online 24 November 2014
DOI: 10.1002/pola.27461
ABSTRACT: Two new perfluorodecylthio substituted aromatic
diamines, namely 2,4-diamino-1-(1H,1H,2H,2H-perfluorodeca-
thio)benzene (DAPFB) and 2,2’-Bis((1H,1H,2H,2H-perfluorode-
cyl)thio)[1,1’-biphenyl]4,4’-diamine (BPFBD) were synthesized
and polycondensed with 4,40-(hexafluoroisopropylidene)diph-
thalic anhydride (6FDA) to produce two new perfluorinated pol-
yimides (PI2 and PI4). The chemical structures of these
polyimides were confirmed by Fourier transform infrared
(FTIR) and nuclear magnetic resonance (NMR) spectroscopy
and elemental analysis. Two other polyimides (PI1 and PI3)
were also synthesized from 6FDA and analogous perfluorode-
cylthio unsubstituted diamines to investigate the incorporation
effect of perfluorodecylthio group on various physical and
chemical properties of the synthesized PIs. Compared with PI1
and PI3, PI2 and PI4 exhibited improved solubility, optical
transparency, and hydrophobicity, lower moisture absorption,
dielectric constant, and thermo-mechanical stabilities owing to
the presence of the perfluorodecylthio side group in the poly-
mer chain. Even though thermo-mechanical properties of PI2
and PI4 (Td5: 413 and 404 ꢀC, Tg: 220 and 209 ꢀC, tensile
strength of 101 and 76 MPa, tensile modulus of 1.7 and 1.5
GPa and elongation at break of 8 and 10%, respectively) were
reduced in comparison to PI1 and PI3 but still were good
enough for most of the practical applications. Most impor-
tantly, the presence of the perfluorodecylthio side group in
BPFBD considerably reduced the dielectric constant of PI4 to
C
V
2.71 which was quite low as aromatic polyimide. 2014 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53,
479–488
KEYWORDS: dielectric properties; fluoropolymers; high perform-
ance polymers; high temperature materials; polyimides;
synthesis; transparency
perfluoroalkyl groups and pendent trifluoromethyl groups
have also been recognized as one of the most promising
method to synthesize processable and colorless PIs.17–29 The
high electronegativity along with low molar polarization of
the fluorinated groups could ultimately lead to the synthesis
of organo-soluble and optically transparent aromatic PIs hav-
ing low dielectric constants and low water absorptions.
INTRODUCTION Polyimides (PIs) are basically composed of
alternating electron donating diamines and electron accepting
dianhydride moieties. The presences of these alternating
donor and acceptor moieties in the main chain induce severe
intra- and interchain electronic interaction through charge
transfer complex (CTC) formation which increases the inter-
chain packing density and thereby increases the
thermomechanical-dimensional stabilities and chemical resist-
ance.1,2 But these charge transfer interactions also contribute
to the poor solution processability, deep reddish-brown colo-
ration, and high dielectric constants of conventional aromatic
PIs.3–5 Various structural modifications such as bulky lateral
substituents,6–8 flexible linkages,9–11 noncoplanar biphenyl
moieties12,13 as well as unsymmetrical structures14–16 have
been made to decrease the coloration and enhance the solu-
bility of conventional aromatic PIs by decreasing the CTC for-
mation. On the other hand, introducing fluorinated
substituents such as hexafluoroisopropylidene linkages,
In this study, two new aromatic diamines with perfluorodecylthio
side group 2,4-diamino-1-(1H,1H,2H,2H-perfluorodecathio)ben-
zene (DAPFB) and 2,2’-Bis((1H,1H,2H,2H-perfluorodecyl)thio)
[1,1’-biphenyl]4,4’-diamine (BPFBD)) were synthesized and poly-
condensed with 4,40-(hexafluoroisopropylidene)diphthalic anhy-
dride (6FDA) to produce two new perfluorinated PIs. Two other
PIs were also synthesized by the condensation of 6FDA and two
analogous diamines, namely 2,4-diamino-1-fluorobenzene
(DAFB) and 4,4’-diamino-2,2’-diiodobiphenyl (DAIB) to investi-
gate the incorporation effect of perfluorodecylthio side group on
Additional Supporting Information may be found in the online version of this article.
C
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The filtrate after washing successively with water and 10%
NaHCO3 solution was evaporated under vacuum. The crude
product was purified further by flash chromatography using
6% EtOAc solution in hexane as eluent and silica gel as sta-
tionary phase. Yield (isolated): 65%. Off white solid, Mp 95–
96 ꢀC (EtOAc: hexane 5 1:5), 1H NMR (400 MHz, CDCl3): d
7.14 (d, J 5 8.4 Hz, 1H, Ph-H), 6.06–6.02 (m, 2H, Ph-H), 4.28
(s, 2H, –NH2), 3.66 (s, 2H, –NH2), 2.81–2.76 (m, 2H, –SCH2),
2.38–2.20 (m, 2H, –CH2CF2); 13C NMR (100 MHz, CDCl3): d
145.2, 144.6, 144.4, 127.6, 126.1, 122.0, 29.9 (m), and 23.4;
19F NMR (500 MHz, CDCl3): 280.87 (t, J 5 10.5 Hz, 3F),
2113.95 (m, 2F), 2121.79 to 2121.97 (m, 6F), 2122.78 (s,
2F), 2123.33 (s, 2F), and 2126.18 (s, 2F) (Supporting Infor-
mation Fig. S3); Anal. calcd. for C16H7F17N2O4S: C, 29.74; H,
1.09; N, 4.33. Found: C, 29.59; H, 1.17; N, 4.40; FTIR (KBr,
cm21): 3085, 1588, 1520, 1470, 1369, 1366, 1206, and 1149
(Supporting Information Fig. S4).
SCHEME 1 Synthesis of DAPFB.
the solubility, optical transparency, thermomechanical stability,
dielectric constant, water absorption, and surface properties of
the synthesized PIs.
EXPERIMENTAL
Materials
1H,1H,2H,2H-perfluorodecanethiol (97%), 2,4-dinitro-1-fluo-
robenzene (ꢁ99%) and 1-Iodo-3-nitrobenzene (99%) were
purchased from Aldrich and were used as received. 4,40-
(Hexafluoroisopropylidene)diphthalic
anhydride
(6FDA,
Aldrich, 99%) was dried at 120 ꢀC for 1 day under vacuum
before use. All the other chemicals used in this study were
supplied from Aldrich and were used as received.
Synthesis of 2,4-Diamino-1-fluorobenzene (DAFB)
2,4-Dinitro-1-fluorobenzene (1 mmol) was dispersed under
N2 atmosphere in 5 mL of glacial acetic acid (AcOH) and
0.5 mL of water in a 20 mL of round bottomed flask. With
vigorous stirring, 670 mg of Fe powder (100 mesh) was
added slowly in a small portiꢀon over a period of 10 min
keeping the temperature at 25 C. The reaction mixture after
stirring for an additional 6 h was diluted with 15 mL of
EtOAc and filtered through Cellite. The filtrate after washing
successively with water and 10% NaHCO3 solution was
evaporated under vacuum. The crude product was purified
further by chromatography using 4% EtOAc solution in hex-
ane as eluent and neutral alumina as stationary phase. Yield
(isolated): (82%). Light yellow liquid, 1H NMR (400 MHz,
CDCl3): d 6.74 (dd, J 5 8.8 and 8.0 Hz, 1H, Ph-H), 6.09 (dd,
J 5 7.6 and 2.8 Hz, 1H, Ph-H), 5.99-5.95 (m, 1H, Ph-H), 3.60
(s, 2H, -NH2), 3.41 (s, 2H, -NH2) (Supporting Information Fig.
S5); Anal. calcd. for C6H7FN2: C, 57.13; H, 5.59; N, 22.21.
found: C, 56.99; H, 5.67; N, 22.27; FTIR (neat, cm21): 3379,
3304, 3201, 1601, 1492, 1259, 814, and 698.
Synthesis of 2,4-Dinitro-1-(1H,1H,2H,2H-
perfluorodecathio)benzene (DNPFB)
The synthetic route of DNPFB is outlined in Scheme 1.
1H,1H,2H,2H-perfluorodecanethiol (PFDT) (1 mmol) and
Et3N (1.1 mmol) were dissolved in 15 mL of 33% dimethyl-
formamide (DMF) solution in ethyl acetate (EtOAc) under N2
atmosphere. 2,4-Dinitro-1-fluorobenzene (0.8 mmol) dis-
solved in minimum volume of DMF was added drop wise
into to the PFDT solution under vigorous stirring at 25 ꢀC.
The stirring was continued for another four hours which
results a light yellow precipitate to appear. The precipitate
was filtered and the filtrate was further concentrated by
rotary evaporator which results more precipitate to appear.
Both of these two precipitates were washed with minimum
volume of hexane and dried in vacuum at 80 ꢀC for 12 h.
Yield (isolated): 90%. Light yellowish solid, Mp 140–141 ꢀC
1
(EtOAc : hexane 5 1:5), H NMR (400 MHz, CDCl3): d 9.11 (d,
4,4’-Diamino-2,2’-Diiodobiphenyl (DAIB)
J 5 2.8 Hz, 1H, Ph-H), 8.44 (dd, J 5 9.2 and 2.4Hz, 1H, Ph-H),
7.53 (d, J 5 9.2 Hz, 1H, Ph-H), 3.30 (m, 2H, –SCH2), 2.57–
2.51 (m, 2H, –CH2CF2) (Supporting Information Fig. S1); 13C
NMR (100 MHz, CDCl3): d 150.0, 149.1, 138.4, 106.5, 104.3,
100.8, 31.8 (m) and 25.9 (Supporting Information Fig. S2);
Anal. calcd. for C16H7F17N2O4S: C, 29.74; H, 1.09; N, 4.33.
Found: C, 29.59; H, 1.17; N, 4.40; FTIR (KBr, cm21): 3085,
1588, 1520, 1470, 1369, 1366, 1206, and 1149 (Supporting
Information Fig. S3).
The compound was prepared from 1-iodo-3-nitrobenzene
(Scheme 2) following a reported procedure of the synthesis
of 4,4’-diamino-2,2’-dibromobiphenyl.30 21.20
g (530.00
mmol) of sodium hydroxide was placed in a 250 mL, three-
neck, round-bottom flask equipped with a mechanical stirrer,
a condenser, and a stopper and 60 mL of ethanol. The reac-
tion mixture was heated at 70 ꢀC for 0.5 h to dissolve most
of sodium hydroxide. After adding 12.38 g (49.50 mmol) of
Synthesis of 2,4-Diamino-1-(1H,1H,2H,2H-
perfluorodecathio)benzene (DAPFB)
DNPFB (1 mmol) was dispersed under N2 atmosphere in
5 mL of glacial acetic acid and 0.5 mL of water in a 20 mL
of round bottomed flask. With vigorous stirring, 670 mg of
Fe powder (100 mesh) was added slowly in a small portion
over a period of 10 min keeping the temperature at 25 ꢀC.
The reaction mixture after stirring for an additional 6 h was
diluted with 15 mL of EtOAc and filtered through Cellite.
SCHEME 2 Synthesis of BPFBD.
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1-Iodo-3-nitrobenzene dissolved in 80 mL of ethylene glycol,
the reaction mixture was heated at reflux for 3 h. The hot
reaction mixture was poured slowly into 500 mL of ice to
form precipitate. The deep brown precipitate was collected
by filtration and washed several times with cold water. The
compound was dried in vacuum at 60 ꢀC for 24 h to yield
3,3’-diiodoazoxybenzene (3). The crude product was used in
the next procedure without further purification. Yield: 6.75 g
(60.0%).
7.12 (dd, J 5 8.5 and 2.5 Hz, 2H, Ph-H), 6.68 (d, J 5 8.5 Hz,
2H, Ph-H), 4.35 (s, 4H, –NH2), 2.95 (t, J 5 8.3 Hz, 4H, –SCH2),
2.39–2.28 (m, 4H, –CH2CF2); 13C NMR (125 MHz, CDCl3): d
147.1, 135.2, 130.6, 122.6, 117.0, 116.1, 31.8 (m), and 25.3;
19F NMR (500 MHz, CDCl3): 280.87 (t, J 5 10.5 Hz, 3F),
2113.95 (m, 2F), 2121.79 to 2121.97 (m, 6F), 2122.78 (s,
2F), 2123.33 (s, 2F), and 2126.18 (s, 2F) (Supporting Infor-
mation Fig. S8); Anal. calcd. for C32H18F34N2S2: C, 33.70; H,
1.59; N, 2.46. Found: C, 33.56; H, 1.65; N, 2.54; FTIR (KBr,
cm21): 3475, 3373, 1605, 1475, 1203, 1146, 814, and 662
(Supporting Information Fig. S7).
6.0 g (13.3 mmol) of compound (3) was placed in a 250 mL,
three-neck, round-bottom flask equipped with a mechanical
stirrer, a condenser, a thermometer, 60 mL of tetrahydrofu-
ran and 80 mL of glacial acetic acid. 8.0 g of Zn dust (<10
lm) was added slowly over a period of 0.5 h maintaining
the reaction temperature below 40 ꢀC. Once the addition of
zinc dust was over, 12 mL of 85% phosphoric acid was
added slowly maintaining the reaction temperature below 55
ꢀC. After stirring for 30 min, the reaction mixture was fil-
tered to remove zinc dust and the filtrate was poured into
150 mL of cold water. The aqueous phase was extracted
twice with 50 mL of dichloromethane (DCM). The combined
organic phase was concentrated to 50 mL and added drop-
The General Procedure for the Preparation of the
Perfluorinated Pls
To a solution of diamine (1.0 mmol) in CaH2 dried N-methyl-
2-pyrrolidone (NMP) (15 wt % of solid content), 6FDA (1
mmol) was added in one portion (Scheme 3). The reaction
mixture was stirred for 7 days (24 h for DAFB and DAIB) to
form poly(amic acid) (PAA) precursor. An extra NMP (3 mL)
was then poured into the reaction flask and in situ chemical
cyclodehydration was carried out by adding an equimolar
mixture of acetic anhydride (1 mL) and pyridine (0.5 mL)
into the PAA solution with stirring at room temperature for
12 hꢀunder a nitrogen flow. The solution was then heated at
120 C for 6 h to perform a complete imidization (for DAFB
and DAIB, this step was skipped). The reaction mixture was
then allowed to cool to room temperature, before being
poured into large excess of methanol. The resulted precipi-
tate was filtered, washed thoroughly with methanol and hot
water, and dried at 100 ꢀC under vacuum to yield PIs. The
PI films were prepared with the casting solutions of 15 wt
% of the polyimides in NMP solutions. The PI solutions were
spin-coated onto 3 inch diameter fused silica (amorphous
SiO2) substrates and dried at 80 ꢀC for 8 h, 150 ꢀC for 2 h,
ꢀ
wise to 30 mL of conc. hydrochloric acid at 0 C under stir-
ring. The stirring was continued for 1 h to allow off white
precipitate to appear. The precipitated salt was filtered,
washed several times with DCM, and dissolved in 100 mL of
water. The aqueous solution was made alkaline by 10%
aqueous sodium hydroxide solution and organic content was
extracted with DCM. The DCM extract was evaporated to
dryness to give off white solid. The benzidine monomer was
further purified by recrystalization from 50% EtOAc solution
in hexane. The product yield was 2.4 g (42.0%). Off white
31
1
ꢀ
ꢀ
solid, Mp 170–171 C (Lit. Mp 169–170 C), H NMR (400
MHz, DMSO-d6): d 7.11 (d, J 5 2.0 Hz, 2H), 6.76 (d, J 5 8.4
Hz, 2H), 6.58 (dd, J 5 8.4 and 2.4 Hz, 2H), 5.20 (s, 4H, –NH2)
(Supporting Information Fig. S6); FTIR (KBr, cm21): 3413,
3304, 3201, 1621, 1594, 1471, 1279, 842, 684, and 575
(Supporting Information Fig. S7).
ꢀ
ꢀ
200 C for 2h and 250 C for 1 h under nitrogen flow. After
drying, the glass plates were immersed in water to facilitate
removal of the flexible, free-standing polyimide films.
Characterization
1H, 13C, and 19F NMR spectra were recorded on a Varian
Unity Plus NMR spectrometer (both 400 and 500 MHz). Ele-
mental analysis was conducted with an elemental analyzer
(EA, Vario EL, Elemental Analysen Systeme). The attenuated
total reflection-Fourier transform infrared (ATR-FTIR, JASCO
FT/IR-4100) spectra were obtained with 32 scans per spec-
trum at a 2 cm21 resolution. A robust single reflection acces-
sory (JASCO ATR Pr0450-S) with a germanium IRE (n 5 4.0,
incidence angle 5 45ꢀ) was used for the ATR measurements.
Average molecular weight (Mw) and polydispersity index
(PDI) of the soluble PIs were estimated by gel permeation
2,2’-Bis((1H,1H,2H,2H-perfluorodecyl)thio)
[1,1’-Biphenyl]4,4’-diamine (BPFBD)]
CuI (380 mg, 2 mmol) and K2CO3 (2.76 g, 20 mmol) were
added to a glass vial equipped with a mechanical stirrer
(Scheme 2). The vial was evacuated and backfilled with N2
by three times. DAIB (2.18 g, 5 mmol), PFDT (5.28 g, 11.00
mmol), 2-propanol (40 mL), and ethylene glycol (10 mL)
were added at room temperature and the tube was heated
to 80 ꢀC and stirred for 24 h. The use of both 2-propanol
and ethylene glycol was helpful to result good yield with
controllable viscosity. The reaction mixture was then allowed
to cool to room temperature, before diluting with DCM
(100 mL). The DCM extract obtained after filtration was con-
centrated in vacuo. The crude product was purified further
by flash chromatography using 5% EtOAc solution in hexane
as eluent and silica gel as stationary phase. Yield (isolated):
chromatography (GPC) using
a Waters 515 differential
refractometer with Waters 410 HPLC Pump and two Styrogel
HR 5E columns in DMF (0.1 mg L21) solvent at 42 ꢀC, cali-
brated with polystyrene standards. To investigate the ther-
mal stability of the PIs, thermogravimetric analyses (TGA)
were performed under N2 using a TGA Q50 Q Series thermal
21
ꢀ
ꢀ
3.14 g (55%). White solid, Mp 72 C (EtOAc : hexane 5 1:5),
analyzer at a heating rate of 10 C min from room temper-
ature to 700 ꢀC. Measurements of glass transition
1H NMR (500 MHz, CDCl3): d 7.36 (d, J 5 2.0 Hz, 2H, Ph-H),
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SCHEME 3 Synthesis of fluorinated polyimides.
RESULTS AND DISCUSSION
temperatures were performed using a DSC Q 100 TA instru-
21
ꢀ
ment at a heating rate of 10 C min under N2 atmosphere.
The transparencies of the PI films were measured from
ultraviolet-visible spectra recorded from one accumulation
on a OPTIZEN 3220UV spectrometer optimized with a spec-
tral width of 200–800 nm, a resolution of 0.5 nm, and a
scanning rate of 200 nm min21; the thickness of each film
was ꢂ20 mm. Wide-angle X-ray diffraction (WXAD) measure-
ments of the pulverized samples were conducted at room
temperature in the reflection mode using a Rigaku diffractom-
eter (Model Rigaku Miniflex). The Cu Ka radiation (k 5 1.54
Å) source was operated at 50 kV and 40 mA. The 2h scan
data were collected at 0.01ꢀ intervals over the range 1.5ꢀ–40ꢀ
and at a scan speed of 0.58ꢀ (2h) min21. Tensile properties
were determined from stress–strain curves obtained with
Kyungsang Universal Testing Measurement (UTM) Materials
Testing System (Model KSU.05) at a strain rate of 10 mm
min21 at room temperature. Measurements were performed
with film specimens (50 mm in gauge length, 10 mm wide
and 0.1 mm thick). Each reported value is the average of five
different measurements. The dielectric constant (e) was
obtained at 1 MHz using an impedance-gain phase analyzer
(HP4194A) and the formula e 5 C d/Ae0, where C is the
observed capacitance, d is the film thickness, A is the area,
and e0 is the free permittivity. The thickness of each film was
1.0 6 0.05 mm. Plasma treatment of the indium-tin oxide (ITO)
plates were done using Plasma cleaner; Harrick Plasma PDC-
32A. Each reported value is the average of three different
measurements. Water absorptions (WA) were determined by
weighing the changes of PI films (3.0 3 1.0 3 0.005 cm3)
Monomer Synthesis and Characterization
The syntheses of the diamine monomers DAPFB and BPFBD
are depicted in Schemes 1 and 2, respectively. As shown in
Scheme 1, the perfluorinated diamine monomer DAPFB was
obtained through a two-step synthetic route. The first step
involved in the synthesis of DNPFB (1) by S-arylation of
PFDT with 2,4-dinitro-1-fluorobenzene using triethylamine
(Et3N) as a base. DNPFB was reduced in the second step by
Fe/AcOH to obtain DAPFB (2). The same procedure was also
followed to reduce 2,4-dinitro-1-fluorobenzene to 2,4-
diamino-1-fluorobenzene (DAFB). A three step synthetic pro-
cedure was adapted for the synthesis of BPFBD (Scheme 2).
Treatment of 1-iodo-3-nitrobenzene with sodium hydroxide
in a mixture of ethanol and ethylene glycol gave the azoxy
compound (3) in 60% yields. The reaction of (3) with zinc
dust in acetic acid and phosphoric acid led to the intermedi-
ary formation of hydrazobenzene that instantly underwent
the benzidine rearrangement reaction by concentrated
hydrochloric acid to yield DAIB (4) in 42% yields. The dia-
mine monomer BPFBD was obtained after the CuI catalyzed
cross-coupling reaction of DAIB with PFDT in 55% isolated
yields.
The structures of the synthesized diamine monomers and of
the intermediates were confirmed by FTIR, 1H NMR, 13C
NMR, and 19F NMR spectroscopy, and elemental analyses.
Supporting Information Figure S4 shows the FTIR spectra of
DNPFB and DAPFB. DNPFB gave characteristic bands at
1540–1342 cm21 (NO2 asymmetric and symmetric stretch-
ing). After reduction, the characteristic absorptions of the
nitro group disappeared in DAPFB, and the amino group
exhibited N–H stretching bands in the region of 3474–
3331 cm21. Both DNPFB and DAPFB show very sharp
stretching band at 1204–1149 cm21 for C–F bonds. The
appearance of a very strong band at 1204–1143 cm21 in the
FTIR spectrum of BPFBD confirmed the successful cross-
coupling reaction between 4,4’-diamino-2,2’-diiodobiphenyl
and PFDT (Supporting Information Fig. S7). The successful
modification can also be realized from the appearance of
ꢀ
before and after immersion in water at 25 C for 24 h. Each
reported value is the average of five different measurements.
The static contact angle (SCA) was measured using a drop
€
shape analysis system (DSA 100, Kruss GmbH, Germany) with
a 500 lL syringe and a needle, 0.5 mm in diameter, and
38 mm in length. The liquid flow used for the SCA measure-
ments was 6 lL at a flow rate of 600 rpm. Each reported
value is the average of three different measurements. The
melting points (uncorrected) were measured with a BUCHI
Melting Point B-545 apparatus.
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FIGURE 1 NMR spectra of DAPFB in CDCl3 (a: 1H NMR, b: 13C NMR).
C–H stretching vibration for –CH2 group at 2927–
2852 cm21. Figure 1 shows the 1H and 13C NMR spectra of
DAPFB in which all the peaks were clearly assigned. The 1H
NMR spectrum shows two singlets at 4.28 and 3.66 ppm
corresponding to the two –NH2 groups in DAPFB. Two multi-
plates appeared at the most up-field region: the multiplate at
2.29 ppm (higher multiplicity) can be assigned as –CH2CF2
protons while the protons in –SCH2 group resonate at 2.29
ppm (lower multiplicity). The down field protons can be
assigned as aromatic protons. 13C-NMR spectrum exhibits
seven characteristic peaks; five coming for six aromatic car-
bons while the two methylene carbons constitute the
remaining two signals. The clear multiplate centred at 29.8
ppm can be assigned as the methylene carbon of –CH2CF2
group. The peaks of eight carbons of perfluorooctyl group
Polyimides Synthesis and Characterization
Four fluorinated PIs were prepared by two-step polyconden-
sation method involving ring-opening polyaddition of 6FDA
with DAPFB, DAFB, DAIB, and BPFBD, leading to the appro-
priate poly(amic-acid) (PAA) intermediate and subsequent
cyclodehydration (Scheme 3). In general, the cyclodehydra-
tion step (imidization step) can be achieved either by ther-
mal treatment of PAA solution (thermal imidization) or by
the addition of dehydrating agents, such as a mixture of ace-
tic anhydride and pyridine to the PAA solution. Thermal
imidization method often results low molecular weight PIs
due to the possible chain breaking at very high imidization
temperatures. Chemical imidization has an advantage over
thermal imidization in this regard as it operates at relatively
lower temperatures (more often at room temperature). On
the other hand, chemical imidization is not suitable for insol-
uble PIs. Therefore, if a clear solution and not a suspension
results during the polymerization reaction, chemical polyi-
midization using a dehydrating agent is better than the ther-
mal imidization. Accordingly, in the present work, chemical
imidization using acetic anhydride and pyridine as dehydrat-
ing agent has been utilized to prepare the soluble fluorinated
PIs. The imidization proceeded homogeneously throughout
the course of reaction and afforded clear, low-viscous poly-
mer solutions. The obtained fluorinated PIs precipitated in
off white (PI1 and PI3) and white (PI2 and PI4), powder-like
form after the reaction mixture being poured into large
excess of MeOH. The steric and electronic characteristics of
the perfluorinated groups can reduce the nucleophilicity of
the fluorinated diamines and thereby reducing their
disappeared due to the carbon–fluorine coupling effect.18 19
F
NMR signals from the long perfluorinated alkyl chain were
clearly observed as a triplet peak at 280.87 ppm corre-
sponding to methyl fluorines (CF32(CF2)72) and multiple
peaks from 2113 to 2126 ppm attributed to methylene flu-
orines (CF32(CF2)72) (Supporting Information Fig. S3). Sim-
ilar trends were observed in the 1H, 13C NMR (Fig. 2) and
19F NMR (Supporting Information Fig. S8) spectra of BPFBD.
NH2 protons resonate at 4.35 ppm while –SCH2 and –CH2CF2
protons appeared at 2.95 and 2.34 ppm, respectively. Eight
peaks were obtained in the 13C-NMR spectrum of BPFBD of
which 6 for 12 aromatic carbons represented two carbons
each. The peaks at 31.8 and 25.3 ppm can be assigned as
the methylene carbon of –CH2CF2 and –SCH2 groups,
respectively.
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FIGURE 2 NMR spectra of BPFBD in CDCl3 (a: 1H NMR, b: 13C NMR).
reactivity towards electrophilic dianhydrides. The presence
of large number of highly electronegative fluorine atoms in
the perfluorodecylthio side group reduced the reactivity of
DAPFB and BPFBD towards 6FDA and that necessitates the
longer reaction time and higher imidization temperature for
DAPFB and BPFBD based PIs (PI2 and PI4).
(symmetrical carbonyl stretching vibrations), 1367–1358 (C–
N stretch), and 1101–1091 and 723–716 (imide ring defor-
mation), together with some strong absorption bands in the
region of 1256–1137 cm21 due to the C–F stretching (Fig.
3). The absence of amide and carboxyl bands indicates a vir-
tually complete conversion of the PAAs precursor into PIs.
The chemical structures of PIs were confirmed by means of
elemental analysis, ATR FTIR, 1H NMR, and 19F NMR spec-
troscopy. All the PIs exhibited characteristic imide group
absorptions bands in the region of 1788–1785 cm21 (asym-
metrical carbonyl stretching vibrations), 1732–1719 cm21
The repeat unit structures of PI2 and PI4 were also well-
supported by the 1H NMR spectra, as shown in Figure 4. 1H
FIGURE 3 ATR FTIR spectra of (a) PI4 (b) PI3 (c) PI2 (d) PI1.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
FIGURE 4 1H NMR spectra of (a) PI2 and (b) PI4 in CDCl3.
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TABLE 1 Elemental Analysis and GPC Molar Mass of PIs
Elemental Analysis (%)
GPC Molar Mass
Formula
Polyimide Code
PI1
(Molecular Weight)
C
H
N
Mw 31024
PDI
1.8
(C25H9F7N2O4)n
(534.34)n
Calcd.
Found
Calcd.
Found
Calcd.
Found
Calcd.
Found
56.19
56.78
42.27
41.39
44.05
43.47
39.55
42.15
1.70
1.54
1.32
1.57
1.55
1.31
1.30
1.23
5.24
5.18
2.82
3.04
3.31
3.45
1.81
1.64
5.38
PI2
PI3
PI4
(C35H13F23N2O4S)n
(994.52)n
3.01
4.19
1.89
2.1
1.6
2.3
(C31H12F6I2N2O4)n
(844.24)n
(C51H20F40N2O4S2)n
(1548.78)n
NMR chemical shifts of PI2 and PI4 were assigned according
to the labeling presented in Figure 4. The assignments of all
the protons are in complete agreement with the proposed PI
structures. The most down field protons are of 6FDA (8.2–
7.9 ppm) while the remaining aromatic protons are of dia-
mines (7.8–7.6 for DAPFB and 7.6–7.2 ppm for BPFBD). The
aliphatic protons resonate at 3.24 and 2.46 ppm in PI2 while
at 3.11 and 2.33 ppm in PI4. The spectra confirm the fully
imidized structure, as indicated by the absence of PAA pro-
ton peaks (between 10 and 14 ppm) and clear proton
assignment in accordance with the repeat unit structure. The
1H NMR spectra of PI1 and PI3 (Supporting Information
Figs. S9 and S10, respectively) were similar to that of PI2
and PI4. The incorporation of hexafluoroisopropylidene
tone), and nonpolar (toluene, xylene, chloroform (CHCl3), tet-
rahydrofuran (THF)) solvents were used to dissolve these
PIs at room temperature (25 ꢀC) or at 60 ꢀC. The PIs were
considered to be soluble in a solvent if a solution of 5% (w/
w) concentration could be prepared. All the four PIs studied
are soluble in all of these common organic solvents except in
toluene and in xylene (Table 2). The excellent solubility of
these PIs certainly due to the presence of bulky, asymmetric,
fluorinated hexafluoroisopropylidene group on the 6FDA
moieties that disrupts the conjugation of the aromatic rings
and thereby hinder the charge transfer interactions between
neighboring molecules. PI2 and PI4 were even soluble in
highly nonpolar toluene and xylene. The higher solubility of
PI2 and PI4 is considered due mainly to the differences in
chain packing caused by the aliphatic perfluorodecylthio side
group. The presence of hydrophobic, bulky long alkyl chain
perfluorodecylthio side group resulted in reduction of the
interchain interaction which leads to less efficient chain
packing. The resulting relatively loose packing of these PIs
allows solvent molecules to penetrate through the polymeric
chains better and thereby improve the solubility.
group
(C(CF3)2)
and
perfluorodecylthio
group
(S(CH2)2(CF2)7CF3) in PI2 and PI4 can be realized by the 19F
NMR spectra of PI2 and PI4 (Supporting Information Figs.
S11 and S12, respectively). The integration ratio of peaks a
(assigned as –CF3 of C(CF3)2 group) and b (assigned as –CF3
of S(CH2)2(CF2)7CF3 group) in Supporting Information Fig-
ures S11 and S12 confirmed the presence of one and two
perfluorodecylthio group(s) per unit repeat structure of PI2
and PI4, respectively. The elemental analysis values of the
polymers generally agreed well with the calculated values
for the proposed structures. The observed values were in
good agreement with the calculated ones. The results of the
elemental analyses and GPC analyses of the polyimides are
listed in Table 1. The molecular weight values, Mw, are in the
range of 53,800–18,900 Da, and polydispersity index (Mw/
Mn) in the domain of 2.3–1.6, a range expected for condensa-
tion polymers. The Mw values of PI2 and PI4 were lower
than PI1 and PI3. The presence of large number of highly
electronegative fluorine atoms in the perfluorodecylthio side
group reduced the reactivity of DAPFB and BPFBD towards
6FDA and thereby results polyimides with lower molecular
weights.
Crystallinity
Supporting Information Figure S13 shows WAXD patterns of
the fluorinated PIs. For all these PIs, the reflection patterns
are featureless and very broad that appeared in the region
2h 5 8ꢀ–23ꢀ (centred at 2h 5 16ꢀ), indicating that they are
TABLE 2 Solubility Data of the Polyimides Synthesized in this
Work
Solventsa
PI
NMP DMAc DMF DMSO Toluene Xylene CHCl3 THF Acetone
PI1 11 11
PI2 11 11
PI3 11 11
PI4 11 11
11 11
11 11
11 11
6
1
11 11 1
11 11 6
11 11 1
11 11 6
11
6
11
11
11
Solubility
1
1
11
In order to examine the solubility of the PIs, both polar (N-
methyl-2-pyrrolidinone (NMP), dimethylacetamide (DMAc),
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ace-
a
Solubility: (11) soluble at room temperature; (1) soluble upon heat-
ing at 60 ꢀC; (6) partially soluble or swells.
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yellow to deep brown, depending on their structures, due to
the formation of intra- or intermolecular CTC. Figure 5
exhibits the UV–vis absorption spectra of PI films (ꢂ20 lm
thick). The data are summarized in Table 3. All of these PIs
are actually based on 6FDA which usually show better opti-
cal transparency than most of the other commercially avail-
able dianhydrides due to the presence of bulky –CF3 group
in 6FDA that is quite effective in hindering the CTC forma-
tion between polymer chains through steric hindrance.33 The
spectral shapes of the four PIs vary considerably depending
on the nature of diamines. The cutoff wavelengths (k0) of the
films are in the range of 382–408 nm. Transmittances of the
PI films measured at 450 nm are 74% for PI1, 85% for PI2,
80% for PI3, and 89% for PI4. The polyimide film prepared
from BPFBD and 6FDA (PI4) was completely colorless. The
much improved optical transmittance of PI2 and PI4 com-
pare to PI1 and PI3 may be due to the presence of bulky,
electron withdrawing perfluorodecylthio side group in
DAPFB and BPFBD which effectively decreases the CTC for-
mation between polymer chains through steric hindrance
and the inductive effect (by decreasing the electron-donating
property of diamine moieties). The interchain cohesive force
becomes weaker due to lower polarizability of the C–F bond
of the perfluorinated side groups in DAPFB and BPFBD and
this also may have contributed towards higher film transpar-
ency of PI2 and PI4. The decrease in intermolecular CTC for-
mation in PI2 and PI4 is also evidenced from their much
higher solubility in toluene and xylene.
FIGURE 5 Optical transmission spectra of the PI films. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
all amorphous. This is due to the diffraction of a poor inter-
molecular packing combined with amorphous halo.32 The
maxima of the broad peaks shifted toward lower 2h values
(higher d-spacing) with the introduction of perfluorode-
cylthio side group in the polyimide network (PI1, 2h 5 16.7ꢀ,
d-space 5 5.3 Å; PI2, 2h 5 15.8ꢀ, d-space 5 5.7 Å; PI3,
2h 5 16.6ꢀ, d-space 5 5.4 Å, PI4, 2h 5 15.1ꢀ, d-space 5 5.9 Å).
Introduction of the hydrophobic, bulky, long alkyl chain ali-
phatic perfluorodecylthio side groups in the polymer back-
bone increase the disorder of the polymer chains, resulting
in decreasing of the interchain interactions and thereby
reducing the chain packaging efficiency to hinder the poly-
mer crystalization. The d-spacing is usually considered to
represent the distance between segments of different chains
and related to the free volume of a polymer. Normally, as the
d-spacing value increases, the free volume also increases and
so as the solubility of polymer. PI2 and PI4 with higher d-
spacing value showed higher solubility even in highly nonpo-
lar solvents such as toluene and xylene.
Thermal Properties
The thermal deformation and decomposition behavior of
the PIs were evaluated by DSC and TGA measurements
under nitrogen. The results are presented in Table 3. Sup-
porting Information Figure S14 compares the DSC curves
of the synthesized fluorinated PIs. The Tg values of the
polymers taken from the second heating curves obtained
in DSC plots were found to be in the range of 209–294
ꢀC. The Tgs of PI2 and PI4 from DAPFB and BPFBD (220
and 209 ꢀC, respectively) were much lower than PI1 and
PI3 (273 and 294 ꢀC, respectively). All the four PIs
reported in this work were synthesized from the same
dianhydride (6FDA) and, therefore, we can assume that
the differences between Tg values are originating from the
diamine component. It appears that the fluorinated side
groups in DAPFB and BPFBD have some influence on the
Optical Properties
Optical transparency of the PI films is considered to be a
crucial factor for various optical applications. Generally, aro-
matic PI films exhibit intense colors ranging from light
TABLE 3 Thermal, Mechanical Properties, and Dielectric Constants of API Films
PIs
TS (MPa)
EB (%)
TM (GPa)
e
WA (%)
hW (ꢀ)
k0 (nm)
Td5 (ꢀC)
Td10 (ꢀC)
Tg (ꢀC)
Rw700 (%)
PI1
PI2
PI3
PI4
120 6 6
101 6 5
101 6 8
76 6 9
6 6 1
8 6 1
5 6 1
10 6 1
2.6 6 0.4
1.7 6 0.3
2.1 6 0.5
1.5 6 0.4
3.10
2.86
2.98
2.71
1.31
0.40
1.14
0.32
91.97 6 2.5
97.37 6 1.5
93.73 6 1.5
107.03 6 3.0
408
388
396
382
467
413
438
404
494
420
459
409
273
220
294
209
51
34
44
36
TS: tensile strength, EB: elongation at break, TM: tensile modulus, e: dielectric constant, WA: water absorption, hW: water contact angle, k0: cutoff
wavelength, Tg: glass transition temperature; Td5, Td10: temperatures at 5 and 10% weight loss, respectively; Rw700: residual weight ratio at 700 ꢀC in
nitrogen.
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The first decomposition in perfluorinated PI2 and PI4 can be
due to the loss of perfluorodecylthio side groups.
Mechanical and Dielectric Properties
All of the PIs afforded transparent, free standing and tough
films. The mechanical properties of the fully aromatic fluori-
nated PIs were investigated using the stress–strain curves
(Supporting Information Fig. S15) obtained from UTM and
the average values of five different measurements for each
PIs are listed in Table 3. All of these four PIs exhibited
appreciable mechanical stiffness, tensile strength varying
from 76 to 120 MPa, elongation at break from 5 to 10%,
and tensile modulus in the range 1.5–2.6 GPa indicating that
they are strong and tough polymeric materials. Apart from
lower inter-chain packing density, the relatively lower molec-
ular weight (Mw) of PI4 may have contributed towards its
lower mechanical stiffness.
FIGURE 6 TGA curves of the (a) PI1 (b) PI3 (c) PI4 and (d) PI2.
The dielectric constants of PI films, reported in Table 3 were
in the range of 2.71–3.10 measured at 1 MHz. PI2 and PI4
exhibited lower dielectric constants than PI1 and PI3. The
lowest dielectric constant was obtained for PI4 (2.71). The
lower dielectric constants of the PI2 and PI4 are mainly
attributed to the higher fluorine content in these two polyi-
mides. The strong electronegativity of fluorine atoms would
result in very low C–F polarizability while the high free vol-
umes originated from the bulky perfluorodecylthio side
groups ensured less efficient chain packing and thereby
decreasing the dielectric constants.
lower Tgs of PI2 and PI4. The incorporation of bulky, elec-
tron withdrawing, hydrophobic perfluorodecylthio side
group lowers cohesive energy and reduces the formation
of inter-chain CTC and thereby the interchain packing den-
sity also weaken considerably that shifts the glass transi-
tions to lower temperatures. An additional decrease in
interchain packing density might also have occurred in PI4
due to the noncoplanar structure of the two ortho-
substituted phenyl rings of BPFBD (2,20-disubstituted
biphenyl structure) and this explain the lowest glass tran-
sition temperature of PI4.
Water Absorption
As shown in Figure 6 (TGA), all PIs showed good thermal
stability, such as 5% weight loss temperatures (Td5) in the
range of 404–467 ꢀC and 10% weight-loss temperaturꢀes
Table 3 shows the water absorption of the PIs. All the four
PIs exhibited very low water absorptions (WA%) which
ranges from 0.32 to 1.31%. All these PIs are actually based
on 6FDA which is known to produce PIs with low moisture
absorption due to the hydrophobic nature of the –CF3 group.
PI2 and PI4 exhibited lower moisture absorption of 0.40 and
0.32%, respectively. The lower water absorption of the PI2
and PI4 might be due to the higher hydrophobicity derived
from the presence of perfluorodecylthio side groups in their
polymer backbones.
ꢀ
(Td10) in the range of 409–494 C. Weight residues at 700 C
ranging from 34 to 51% are observed in nitrogen. PI2 and
PI4 with perfluorodecylthio side group diamines (DAPFB and
BPFBD) exhibited lower Td5 and Td10 compare to PI1 and PI3.
This result is basically consistent with the glass transition
results. Unlike the single step decomposition of PI1 and PI3,
thermal decomposition in PI2 and PI4 occurred in two steps.
FIGURE 7 Water contact angle on the polymer films PI1–PI4. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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5 Y. Kim, J.-H. Chang, Macromol. Res. 2013, 21, 228–233.
Static Contact Angles
Static contact angles of water dripped onto the surface of
the polyimide films were tested to investigate the hydropho-
bic character of PIs film surfaces and the measured contact
angle values were in the range of 91.97ꢀ–107.03ꢀ. The water
contact angles (hW) of the PI films are listed in Table 3. Fig-
ure 7 depicts the profiles of a droplet on the PI surface. PI2
and PI4 displayed higher hW values, 97.37ꢀ and 107.03ꢀ,
respectively, therefore better hydrophobic characteristics,
compared to PI1 and PI3 (91.97ꢀ and 93.73ꢀ, respectively).
The results indicate that the presence of hydrophobic tri-
fluoromethyl group of 6FDA and perfluorodecylthio group of
DAPFB/BPFBD in the polyimide backbone of PI2 and PI4
could efficiently lead to the migration of the fluorocarbon
chain segments to the film surface and thereby fluorine
enrichment at the surface, thus reducing the surface tension
and making the surface more hydrophobic.34
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CONCLUSIONS
The efficient synthesis of two new perfluorodecylthio substi-
tuted aromatic diamines DAPFB and BPFBD was reported in
this work. Both DAPFB and BPFBD were reactive enough to
yield two new perfluorinated aromatic polyimides (PI2 and
PI4) by the polycondensation with 6FDA. A comparative
investigation between the perfluorinated PIs (PI2 and PI4)
and nonperfluorinated PIs (PI1 and PI3) revealed that PI2
and PI4 exhibited higher solubility and optical transparency,
but lower thermal and mechanical properties due to the
presence of bulky, electron withdrawing perfluorodecylthio
side group which lowers cohesive energy and reduces CTC
formation and interchain packing density. The much higher
concentration of hydrophobic fluorinated substituents in PI2
and PI4 increases their hydrophobicity and decreases mois-
ture absorption and dielectric constant. The eminent combi-
nation of several properties guaranteed PI2 and PI4 as
potential candidates for high-performance microelectronic
applications.
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ACKNOWLEDGMENTS
5858–5860.
The work was supported by the National Research Foundation
of Korea (NRF). Grant funded by the Ministry of Science, ICT,
Future planning, Korea, Pioneer Research Center Program (No.
2010-0019308/2010-0019482), and the Brain Korea 21 Plus
Program (21A2013800002).
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