Synthesis and characterization of highly soluble and oxygen permeable new polyimides based on twisted biphenyl dianhydride and spirobifluorene diamine

Article abstract of DOI:10.1021/ma047433x

The novel spirobifluorene diamine monomer, 2,7-bis-amino-2′,7′- di-tert-butyl-9,9′-spirobifluorene, was obtained. The new organosoluble polyimides were prepared from spirobifluorene diamine containing bulky tert-butyl group and conventional dianhydrides as well as 2,2- ′bis(4″-tert-butylphenyl)4,4′,5,5′- biphenyltetracarboxylic dianhydride (BBBPAn) and 2,2′-bis(4″- trimethylsilylphenyl)-4,4′,5,5′biphenyltetracarboxylic dianhydride (BTSBPAn), which are composed of a noncoplanar twisted biphenyl unit containing bulky p-tert-butylphenyl groups and/or p-trimethylsilylphenyl groups by high-temperature one step polymerization. The structures of polymers were confirmed by various spectroscopic techniques. The weight-average molecular weights and polydispersities of resulting polymers were in the ranges 48 300-183 900 and 2.10-3.26, respectively. The bulky and twisted noncoplanar structural feature confers an enhanced solubility of the polyimides because of a decrease in the degree of molecular packing and crystallinity while imparting a significant increase in both T^g and thermal stability by restricting segmental mobility. The new polyimides exhibit the high oxygen permeabilities (P(O₂) = 18-121 barrer) and O₂/N₂ gas separation properties (P(O₂)/P(N₂) = 2.2-9).

Full text of DOI:10.1021/ma047433x

7950
Macromolecules 2005, 38, 7950-7956
Synthesis and Characterization of Highly Soluble and Oxygen
Permeable New Polyimides Based on Twisted Biphenyl Dianhydride
and Spirobifluorene Diamine
Yun-Hi Kim, Hyung-Sun Kim, and Soon-Ki Kwon*
Department of Polymer Science & Engineering and Engineering Research Institute,
Gyeongsang National University, Chinju, 660-701, Korea
Received December 13, 2004; Revised Manuscript Received July 14, 2005
ABSTRACT: The novel spirobifluorene diamine monomer, 2,7-bis-amino-2′,7′-di-tert-butyl-9,9′-spirobif-
luorene, was obtained. The new organosoluble polyimides were prepared from spirobifluorene diamine
containing bulky tert-butyl group and conventional dianhydrides as well as 2,2′-bis(4′′-tert-butylphenyl)-
4,4′,5,5′-biphenyltetracarboxylic dianhydride (BBBPAn) and 2,2′-bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-
biphenyltetracarboxylic dianhydride (BTSBPAn), which are composed of a noncoplanar twisted biphenyl
unit containing bulky p-tert-butylphenyl groups and/or p-trimethylsilylphenyl groups by high-temperature
one step polymerization. The structures of polymers were confirmed by various spectroscopic techniques.
The weight-average molecular weights and polydispersities of resulting polymers were in the ranges
48 300-183 900 and 2.10-3.26, respectively. The bulky and twisted noncoplanar structural feature confers
an enhanced solubility of the polyimides because of a decrease in the degree of molecular packing and
crystallinity while imparting a significant increase in both Tg and thermal stability by restricting segmental
mobility. The new polyimides exhibit the high oxygen permeabilities (P(O2) ) 18-121 barrer) and O2/N2
gas separation properties (P(O2)/P(N2) ) 2.2-9).
Introduction
which is periodically twisted with an angle of 90° at each
spiro-center. This structural feature would restrict the
close packing of the polymer chains and reduce the
probability of interchain interactions, resulting in high
polymer solubility.16-22
Recently, we reported new 3,3′,4,4′-biphenyltetracar-
boxylic dianhydride (s-BPDA)23-24 based polyimides
containing bulky p-tert-butylphenyl groups and/or p-
trimethylsilylphenyl groups, which had good solubility,
high permeability, and permselectivity with good ther-
mal stability and mechanical properties.25
In this article, we report the synthesis and charac-
terization of new polyimides derived from spirobifluo-
rene diamine containing bulky tert-butyl groups and
conventional commercially available dianhydrides as
well as 3,3′,4,4′-biphenyltetracarboxylic dianhydride
(s-BPDA) containing bulky p-tert-butylphenyl groups
and/or p-trimethylsilylphenyl groups. And oxygen per-
meability (P(O2)) and permselectivity of oxygen to
nitrogen (P(O2)/P(N2)) are also studied.
The reputation of aromatic polyimides as polymeric
material with outstanding thermal, mechanical and
electrical properties, as well as chemical resistance led
to the rise of polyimides in applications ranging from
films for electronic industry, high-temperature matrices
for advanced composites, high-temperature adhesives,
to gas separation membranes. Almost all of the poly-
imides studied and used for these applications contain
flexible linkages in the diamine and/or the dianhydride
unit, resulting in a flexible chain conformation.1,2 In
comparison, synthesis and investigations on extended
rod or rodlike aromatic polyimides are rather limited,
although such polyimides have (for example) potential
as materials for thermally stable, high modulus high
strength fibers, as materials with low thermal expan-
sion coefficient, or as materials for separation mem-
branes.
In general, rodlike polymers, due to strong enthalphic
interactions and the minimal increase in conformational
entrophy associated with their dissolution or melting
are basically intractable or only processable under
extreme conditions. To overcome these difficulties,
structural modifications of the polymer backbone, such
as bulky lateral substituents, flexible alkyl side chains,
and kinked comonomers, have been utilized to modify
the polymer properties, either by lowering the interac-
tion or by reducing the stiffness of the polymer
backbone.3-15
Experimental Section
Materials. N,N-Dimethylacetamide was dried over CaH2
and distilled under nitrogen prior to use. m-Cresol was dried
over CaCl2, then over 4 Å Linde type molecular sieves, distilled
under reduced pressure and stored under nitrogen in the dark.
Biphenyl, Mg, 9-fluorenone, potassium phthtalimide, copper
iodide, fluorene, hydrazine monohydrate, 3,3′,4,4′-benzophe-
none tetracarboxylic dianhydride (BTDA), and 4,4′-(hexa-
fluoroisopropylidiene)tetracarboxylic dianhydride (FDA) were
used without additional purification. Other chemicals were
purchased from Aldrich Chemical Co. or Fluka and were used
as received.
Measurements. A Genesis II FT-IR spectrometer was used
to record IR spectra. 1H NMR and 13C NMR spectra were
recorded with a DRX 500 MHz NMR spectrometer, and
chemical shifts are reported in ppm units with tetramethyl-
silane as internal standard. Thermogravimetric analysis (TGA)
was performed under nitrogen on a TA instruments 2050
It has been reported that the spirobifluorene monomer
consists of two identical fluorene moieties connected
through a common tetracoordinate carbon atom. In the
spiro segment, the rings of the connected bifluorene
entities are orthogonally arranged. The resulting poly-
imides would be expected to have a polymer backbone,
* To whom all correspondence should be addressed.
10.1021/ma047433x CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/18/2005

Macromolecules, Vol. 38, No. 19, 2005
Oxygen Permeable New Polyimides 7951
Thermogravimetric analyzer. The sample was heated using a
20 °C/min heating rate from 50 to 800 °C. Differential scanning
calorimetry (DSC) was conducted under nitrogen with TA
instruments 2100 differential scanning calorimeter. The sample
was heated at 20 °C/min from 30 °C to 400 °C. Inherent
viscosities were measured with a Cannon Fenske viscometer
in DMAc solution (0.5 g/dL, 25 °C). Tensile properties were
determined from stress-strain curves obtained with a uni-
versal testing machine (UTM) (LR-10K, Lloyd Instrument
Ltd.) with a load cell of 10 kgf. A gauge of 2 cm and a strain
rate of 0.5 cm/min were used for this study. Measurements
were performed at room temperature with film specimens (1
cm wide, 5 cm long, and 30-80 µm thick). Gas permeabilities
for the polymer membrane with about 30-60 µm of thickness
were measured with a conventional permeability apparatus,
which consists of upstream and downstream parts separated
by a membrane.26-28 The upstream part was maintained to a
constant pressure of 3 kgf/cm2 of either pure O2 or N2 gas
during the experimental period, and the downstream part was
opened to the atmosphere. A bubble gas flow meter was
employed to measure the steady-state permeation flux from
which the gas permeability was calculated.
chloroform as an eluent). Yield: 6 g (50%). Mp: 333 °C. FT-
IR (KBr pellet, cm-1): 3048 (aromatic C-H), 2961 (aliphatic
C-H), 1724 (CdO stretching), 1383 (C-N stretching), 1H NMR
(500 MHz, CDCl3): δ 8.05 (s, 2H), 7.85-7.87 (m, 4H, Ar-H),
7.69-7.73 (m, 6H, Ar-H), 7.52 (d, 2H, Ar-H), 7.42 (d, 2H,
Ar-H), 6.82-6.86 (d of d, 4H, Ar-H), 1.24 (s, 18H, tert-butyl)
2,7-Bis(diamino-2′,7′-di-tert-butyl-9,9′-spirobifluo-
rene) (DADBSBF). A 5 g (6.7 mmol) sample of 2′,7′-di-tert-
butyl-2,7-bis(N-phthalimido)-9,9′-spirobifluorene and 1.68 g
(0.034 mmol) of hydrazine monohydrate was dissolved in 40
mL of THF. The mixture was refluxed for 5 h. After the
solution was cooled to room temperature, the crude product
was filtered. The product was recrystallized in absolute ethanol
and ethyl ether (1:1). Yield: 2.5 g (80%). Mp; 276 °C. FT-IR
(KBr pellet, cm-1): 3353 (NH2), 3032 (aromatic C-H), 2957
(aliphatic C-H), 1362 (tert-butyl CH3), 1H NMR (500 MHz,
CDCl3): δ 7.71 (s, 2H, Ar-H), 7.51 (s, 2H, Ar-H), 7.40 (d, 2H,
Ar-H), 6.76 (d, 2H, Ar-H), 6.64 (d, 2H, Ar-H), 6.03 (d, 2H,
Ar-H), 3.44 (s, 4H, NH2), 1.21 (s, 18H, tert-butyl).
Polymerization. Polymerization procedures were as fol-
lows. The obtained diamine (1 mmol) was dissolved in 25 mL
of m-cresol. After the diamine was dissolved completely, 1
mmol of corresponding dianhydride was added. The mixture
was stirred at room temperature for 2 h. The temperature of
the mixture was gradually elevated to 200 °C and then stirred
for 5 h. An excess m-cresol was added to the mixture, and the
mixture was cooled. The obtained polymer was precipitated
in methanol. The precipitation procedure was carried out
several times. Yield: 96-98%
Preparation of Dense Film. A 5-7 wt % solution of
polymer in chloroform was prepared and filtered through a
0.2 µm syringe filter to remove the non dissolved materials
and dust particles. The solution was then poured into a casting
ring on a leveled clean glass plate. The casting ring was
covered with a piece of filter paper. The casting process took
about 8 h at room temperature. The casting films were dried
in an oven at 40 °C for 6 h without vacuum and for another 6
h with vacuum, and the resulting films samples were dried at
80 °C for 6 h and then at 100 °C for 10 h. Dense films were
obtained by annealing at 150 °C in a vacuum oven for an
additional 24 h before the gas permeability measurement.
Monomer Synthesis. 4,4′-Di-tert-butylbiphenyl (DBBP).
A 1L three-neck round flask was equipped with dropping
funnel, magnetic stirring bar, drying tube, and thermometer.
Biphenyl (50 g, 0.32 mol) and 2,6-di-tert-butyl-4-methyl phenol
(93 g, 0.422 mol) were dissolved in nitromethane. The reaction
mixture was stirred at 15 °C for 30 min. After AlCl3 (74.3 g,
0.557 mol) was dissolved in nitromethane, it was dropped into
the reaction mixture. After the reaction mixture was poured
into the iced water, the crude product was extracted with
hexane. The solvent was evaporated, and product was recrys-
tallized from ethanol; yield after recrystallization: 69 g (80%).
Mp: 128-130 °C. FT-IR (KBr pellet, cm-1): 3028 (aromatic
1
C-H), 2955 cm-1 (aliphatic C-H), 1361 (tert-butyl CH3), H
NMR (500 MHz, CDCl3): δ 7.48-7.58 (dd. 8H. Ar-H), 1.41
(s, 18H, tert-butyl).
2-Bromo-4,4′-di-tert-butylbiphenyl (BDBBP). 4,4′-Di-
tert-butylbiphenyl (40 g, 0.177 mol) was dissolved in methylene
dichloride. After FeCl3 (0.4 g, 2.5 mmol) was added to the
solution at 5-8 °C, then Br2 (33.9 g, 2.1 mol) was added to
the mixture. The reaction was terminated by addition of 1 M
aqueous NaOH after the mixture was stirred for 3 h. The
solvent was evaporated, and the residue was recrystallized
from ethanol; yield after recrystallization: 49 g (90%). Mp:
87-89 °C. FT-IR (KBr pellet, cm-1): 3032 (aromatic C-H),
2998 (aliphatic C-H), 1361 (tert-butyl CH3), 1350 (aromatic
C-Br). 1H NMR (500 MHz, CDCl3): δ 7.29-7.70 (m, 7H, Ar-
H), 1.39-1.41 (d, 18H, tert-butyl).
2,7-Dibromo-2′,7′-di-tert-butyl-9,9′-spirobifluorene
(DBBSBF): A 1.5 g (61.7 mmol) sample of magnesium and
16 g (46.3 mmol) of 2-bromo-4,4′-di-tert-butylbiphenyl were
reacted to give 2-bromo-4,4′-di-tert-butylbiphenyl-2-magnesium
bromide in diethyl ether. After refluxing for 2 h, the ether
solution of 10.74 g (32.7 mmol) of 2,7-dibromofluorenone was
added to the mixture. The reaction mixture was refluxed for
12 h. After acetic acid was added to the reaction mixture, the
reaction was terminated and solvent was evaporated. After
the solid residue was dissolved in acetic acid, 1-2 drops of
HCl was added and the mixture was heated to refluxed. The
crude product was filtered and washed with hexane and
ethanol. Yield: 7 g (40%). Mp: 281-282 °C. FT-IR (KBr pellet,
cm-1): 3048 (aromatic C-H), 2958 (aliphatic C-H), 1361 (tert-
butyl CH3), 1350 (aromatic C-Br). 1H NMR (500 MHz,
CDCl3): δ 7.74 (s, 2H, Ar-H), 7.71 (s, 2H, Ar-H), 7.52-7.53
(d, 2H, Ar-H), 7.44-7.47 (d, 2H, Ar-H), 6.89 (d, 2H, Ar-H),
6.68 (d, 2H, Ar-H), 1.22 (s, 18H, tert-butyl)
Results and Discussion
The novel spirobifluorene diamine monomer 2,7-bis-
amino-2′,7′-di-tert-butyl-9,9′-spirobifluorene was ob-
tained by hydrolysis of 2′,7′-di-tert-butyl-2,7-bis(4′′-N-
phthalimido)-9,9′-spirobifluorene (as shown in Scheme
1). The precursor, 2,7-dibromo-2′,7′-di-tert-butyl-9,9′-
spirobifluorene, was prepared by reacting 2,7-dibromof-
luorenone with the Grignard reagent prepared from
2-bromo-4,4′-di-tert-butylbiphenyl followed by dehydra-
tive ring closure of the resulting carbinol in acetic acid.
The aromatic nucleophilic substitution of 2,7-dibromo-
2′,7′-di-tert-butyl-9,9′-spirobifluorene with phthalimide
gave 2′,7′-di-tert-butyl-2,7-bis(N-phthalimido)-9,9′-spiro-
bifluorene, which on subsequently hydrolysis using
hydrazine afforded the desired monomer diamine. The
yields in each steps were high. The structures of the
obtained compounds in each steps were verified by
spectroscopic techniques. The structure of obtained
1
monomer, DADBSBF, was also confirmed by H NMR
1
and IR and elemental analysis. In the H NMR, the
proton peaks of tert-butyl, amine, and aromatic ring
hydrogens appeared at 1.21, 3.44, and 6.03-7.71 ppm,
respectively. In the FT-IR spectrum, instead of the
carbonyl group of BPISBF, characteristic peaks of the
2′,7′-Di-tert-butyl-2,7-bis(N-phthalimido)-9,9′-spirobi-
fluorene (BPISBF). A mixture of 10 g (0.017 mol) of 2′,7′-
dibromo-2,7-di-tert-butyl-9,9′-spirobifluorene, 9.5 g (0.051 mol)
of potassium phthalimide, and 14.66 g (0.077 mol) of CuI, and
60 mL of DMF was stirred in. The mixture was heated to
refluxed for 12 h. After the reaction mixture was cooled to room
temperature, it was extracted with chloroform. The product
was purified by column chromatography (silicagel. With
amine group appeared at 3353 and 3470 cm-1
.
For the synthesis of polyimides, one-step polymeri-
zation was employed (Scheme 2). The one-step polym-
erization provides some advantages over the conven-

7952 Kim et al.
Macromolecules, Vol. 38, No. 19, 2005
Scheme 1
tional two-step method. Polyimides in bulk are more
easily produced; furthermore, this method is useful for
less reactive diamines and dianhydrides, which cannot
form high molecular weight polyamic acids by two step
method. However, the disadvantage of the one step
method is that for relatively insoluble polyimides, high
molecular weight polymers cannot be obtained because
of premature precipitation. We tried the one-step method
for polymerization on the basis of the hypothesis that
the new polyimides resulting from the spirobifluorene
diamine would be soluble in common organic solvents.
The spirobifluorene diamine and a series of dianhy-
drides were reacted in m-cresol in the presence of a
catalytic amount of isoquinoline. The polyimides were
isolated in quantitative yields by precipitating into
methanol and drying under vacuum. The results of the
polymerization are summarized in Table 1. The inherent
viscosities of polyimides were 0.48-0.59 dL/g. The gel
permeation chromatography (GPC) measurements dem-
onstrated that polymers exhibited Mw and polydisper-
sities (Mw/Mn) in the ranges 48 300-183 900 and 2.10-
3.26, respectively. Regardless of dianhydrides, the
obtained polyimides showed similar inherent viscosities.
From the results, it can be supposed that the polymer-
ization for polyimides depends on the nature of amine
rather than the nature of dianhydride. The elemental
analysis values agreed quite well with calculated values
for the proposed structures of polyimides. The structures
of the synthesized polyimides were characterized by IR
1
and NMR spectroscopies. Figure 1 showed the IR, H
NMR and 13C NMR spectra of polyimide 3 and poly-
imide 4, respectively. The IR spectra of polyimide 3 and
polyimide 4 showed the characteristic imide-ring ab-
sorptions near 1786 (asym CdO str), 1729 (sym CdO
str), 1364 (C-N str), and 741 (imide ring deformation)
cm-1 for polyimide 3 and 1781 (asym CdO str), 1727
(sym CdO str), 1359 (C-N str), and 743 (imide ring
deformation) cm-1 for polyimide 4, respectively. The
proton peaks are assigned for the obtained polyimide 3
and polyimide 4, respectively. In the 13C NMR, the
resonances corresponding to the carbonyl carbons of the
imide ring appeared in the relatively downfield region
(δ 166).
The solubility of polyimides was studied in various
solvents (Table 2). All the polyimides exhibited good
solubility in polar aprotic solvents such as NMP, DMF,
and DMAc, phenolic solvents such as m-cresol, and as
well as chlorinated solvents such as chloroform and
dichlorobenzene. The high solubility of all polyimides
may be attributed to bulky spirobifluorene with tert-
butyl group, though the substituted noncoplanar twisted
biphenyl moiety may have considerablely contributed
to the solubility of polymer 3 and polymer 4. The
crystallinities of the polyimides were evaluated by wide-
angle X-ray diffraction experiment. Table 3 exhibits the
results of X-ray diffraction date. The ratios of half-height

Macromolecules, Vol. 38, No. 19, 2005
Oxygen Permeable New Polyimides 7953
Scheme 2
Table 1. Molecular Weight of the Polyimides
polymer code
Mne(GPC)
Mwe(GPC)
PDI (Mw/Mn)
inh viscf (dL/g)
polyimide 1 (BADBSBF-BTDAa)
polyimide 2 (BADBSBF-FDAb)
polyimide 3 (BADBSBF-BBBPAnc)
polyimide 4 (BADBSBF-BTSBPAnd)
18 300
83 900
32 200
55 700
48 300
183 900
67 600
2.64
2.19
2.10
3.26
0.48
0.59
0.56
0.59
182 000
a 3,3′,4,4′-Benzophenone tetracarboxylic dianhydride. b 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride. c 2,2′-Bis(4′′-tert-butylphen-
yl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride. d 2,2′-Bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride. e Relative
to polystyrene. f Inherent viscosity measured at a concentration of 0.5 g/dL in DMAc at 25 °C.
Table 2. Solubility of Polyimidesa
solvent
polymer code
m-cresol
NMP
DMF
DMAc
DMSO
1,2-dichlorobenzene
chloroform
THF
acetone
polyimide 1
polyimide 2
polyimide 3
polyimide 4
++
++
++
++
++
++
++
++
+-
++
+-
++
++
++
++
++
+-
+-
- -
- -
++
++
+-
++
+-
++
++
++
++
++
++
++
- -
- -
- -
- -
a Key: ++, soluble; +-, partially soluble; - -, insoluble.
width to diffraction angle (∆2θ/2θ) for the polyimides
are all larger than 0.13, which is the value for amor-
phous polyethylene.29 All the new polyimides showed
the amorphous diffraction patterns as a result of the
presence of tert-butyl substituted bulky spirobifluorene
structure. The high solubility of the polyimides also can
be explained by the amorphous character of the poly-
imide.
The thermal properties of the polyimides were inves-
tigated by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) in a nitrogen atmo-
sphere. (Table 3) The incorporation of rigid spirobifluo-
rene units into the polymer backbone led to polyimides
exhibiting a high Tg’s. The Tg’s of polymides were in the
range of 353-398 °C, depending on the structure of the
dianhydride monomer moiety. The high Tg’s of polymer
3 and polymer 4 can be explained by the fact that
noncoplanar twisted biphenyl and steric hindrance of
the bulky substituents restrics the segmental motion.
When the results are compared with the results of the
polyimides derived from noncoplanar twisted biphenyl
containing a bulky p-tert-butylphenyl group or p-tri-
methylsilylphenyl group and conventional aromatic
diamines, the newly obtained polyimides with tert-
butylated spirobifluorene possessed higher Tg’s.23 The
result can be explained by the rigidity of the spirobi-

7954 Kim et al.
Macromolecules, Vol. 38, No. 19, 2005
1
1
Figure 1. (a) FT-IR, H NMR, and 13C NMR spectra of polyimide 3, (b) FT-IR, H NMR, and 13C NMR spectra of polyimide 4.

Macromolecules, Vol. 38, No. 19, 2005
Oxygen Permeable New Polyimides 7955
Table 3. Physical Properties of Polyimides
polymer code
2θ (∆2θ/2θ)
d spacing (Å)
Tge (°C)
Tdf (°C)
char yield (%)g
polyimide 1 (BADBSBF-BTDAa)
polyimide 2 (BADBSBF-FDAb)
polyimide 3 (BADBSBF-BBBPAnc)
polyimide 4 (BADBSBF-BTSBPAnd)
17(0.53)
5.3
353
360
>400
398
525
520
555
550
57
62
65
75
16(0.52)
5.2
17(0.53), 7(0.9)
17(0.59), 6(0.5)
5.2, 12.6
5.3, 14.7
a 3,3′,4,4′-Benzophenone tetracarboxylic dianhydride. b 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride. c 2,2′-Bis(4′′-tert-butylphen-
yl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride. d 2,2′-Bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride. e From
the second trace of DSC measurements conducted with a heating rate of 20 °C/min under a nitrogen atmosphere. f 5% weight loss
temperature in TGA at 20 °C/min heating rate. g Residual yield in TGA at 800 °C under a nitrogen atmosphere.
Table 4. Mechanical Properties of Polyimides
tensile strength
(Mpa)
elongation
tensile
polyimide
at break (%) modulus (Gpa)
polyimide 1
polyimide 2
polyimide 3
polyimide 4
a
a
a
69.6
74.4
68.6
3.8
4.4
14.4
2.4
2.3
2.0
a Polymer film was too brittle to measured.
Table 5. Gas Permeation Properties of Polyimides
permeabilitya
polyimide
O2
N2
selectivity (PO2/PN2)
polyimide 1
polyimide 2
polyimide 3
polyimide 4
18
2
12
12
54
9
28
2.3
4.4
2.2
52.2
121
a Barrer ) 10-10 × cm3(STP) cm/(s cm2 cmHg).
fluorene. In the TGA, all the polyimides have excellent
thermal stability. The 5% weight loss temperatures in
nitrogen were in the range of 520-555 °C. The char
yields of polyimides were in the range of 62-75% in
nitrogen at 800 °C. The results of TGA and DSC
analyses showed an excellent thermal stability of the
polyimides even though the polyimides showed high
solubility.
Figure 2. O2/N2 transport properties of the obtained poly-
imides and reported polyimides.
Except for polymer 1, all the polyimides could be
processed into highly cohesive and good quality films.
These flexible films were subjected to tensile test, and
the results are summarized in Table 4. The polyimide
films had tensile strengths of 69-74 MPa, elongation
at break of 4-14%, and tensile strengths of 2.0-2.4
GPa. It has been established again that the polyimide
4 derived from twisted biphenyl with trimethylsilyl
group has a high elongation at break due to the
flexibility of the trimethylsilyl group.
The oxygen gas-permeation properties of obtained
films of the polyimides were studied (Table 5). The
newly synthesized polyimides had a high oxygen perme-
ability coefficient (P(O2) ) 18-121 barrer) in comparison
with the permselectivity of oxygen to nitrogen (P(O2)/
P(N2) ) 2.2-9). From the results, it is supposed that
the introduction of the bulky rigid spirobifluorene
improved the permeability of polyimide, compared to the
results of polyimides derived from conventional di-
amines and twisted biphenyl with trimethylsilyl group
or tert-butyl group.23 The results of oxygen permeability
and O2/N2 permselectivity are shown in Figure 2 plotted
on Robeson’s 1991 upper bound tradeoff. The perme-
ation results are distributed around the 1991 upper
bound limit. It is believed the highly twisted structure
of the new polyimides allow them to have high free
volume. The result can be proved by d spacings from
the X-ray diffraction data (Figure 3). In the case of
polyimides 3 and 4, two peaks (12.6-14.7, 5.2-5.3 Å)
Figure 3. Wide-angle X-ray diffraction results for the ob-
tained polyimides.
are observed in the spectra. Upon increasing the larger
d spacings in the polyimide, the polymer becomes more
loosely packed, and the fractional free volume increased.
As the results, the newly obtained polyimides showed
the high permeability. Especially, in the case of poly-
imide 4 containing flexible trimethylsilyl group had the
highest oxygen permeability ever reported. It can be
explained by the fact that the highly twisted main chain
with spirobifluorene and twisted biphenyl afford the
high free volume and(or) flexible trimethylsilyl group
has a high solubility toward the oxygen.
Conclusions
The new polyimides containing spirobifluorene moi-
eties in the main chain have been synthesized via the

7956 Kim et al.
Macromolecules, Vol. 38, No. 19, 2005
(9) Wang, C. S.; Leu, T. S. Polymer 2000, 41, 3581.
one step polycondensation of 2,7-bis-amino-2′,7′-di-tert-
butyl-9,9′-spirobifluorene with a bulky tert-butylated
and(or) trimethylsilylated twisted biphenyl dianhydride
as well as conventional dianhydrides. The introduction
of the spiro segment, in which is two fluorene rings are
mutually perpendicular and are connected via a com-
mon tetracoordinated carbon. This structural feature
confers an enhanced solubility of the polyimides because
of a decrease in the degree of molecular packing and
crystallinity while imparting a significant increase in
both Tg and thermal stability by restricting segmental
mobility. From the high solubility, good thermal stabil-
ity and mechanical properties, the new polyimides were
used for oxygen permeable membrane. The new poly-
imides exhibits the highest oxygen permeabilities ever
reported and O2/N2 gas separation properties lying on
or above the upper bound trade off limit.
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Acknowledgment. This work was supported by
Korea Research Foundation Grant (KRF-2000-005-
D00251).
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