Extremely high thermal resistive poly(p -phenylene benzobisoxazole) with desired shape and form from a newly synthesized soluble precursor

Article abstract of DOI:10.1021/ma3006526

The poly(p-phenylene benzobisoxazole) (PPBO) fiber, known as Zylon, has a very high thermal stability as well as mechanical strength when compared to any other polymers due to its ladder-like rigid structure. However, one of the critical drawbacks of its stiff structure is its insolubility in organic solvents, and only strong acids can be used use for fiber spinning of this polymer. To overcome the poor processability caused by this insolubility in organic solvents, a soluble PPBO precursor was designed and synthesized by the reaction of tert-butyldimethylsilyl (TBS) group-functionalized 4,6-diaminoresoisinol with terephthaloyl chloride for polycondensation. The obtained TBS-functionalized PPBO precursor (TBS-prePBO) shows an excellent solubility in common organic solvents, such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO). Transparent TBS-prePBO films made by the solution-cast method provided PPBO films after thermal treatment at 500°C for 1 h. The structure of the obtained PPBO films was characterized by IR and XRD techniques and found that the films exhibited extremely high thermal stabilities, namely, the synthesized PPBO polymer decomposition temperature reached 670°C in flowing N₂, which is the highest temperature among the organic polymers reported so far.

Full text of DOI:10.1021/ma3006526

Article
pubs.acs.org/Macromolecules
Extremely High Thermal Resistive Poly(p-phenylene
benzobisoxazole) with Desired Shape and Form from a Newly
Synthesized Soluble Precursor
Takahiro Fukumaru,† Tsuyohiko Fujigaya,*,†,‡ and Naotoshi Nakashima*,†,‡,§
†Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka Nishi-ku, Fukuoka 819-0395,
Japan
‡International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka Nishi-ku, Fukuoka
819-0395, Japan
§JST-CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan
S
* Supporting Information
ABSTRACT: The poly(p-phenylene benzobisoxazole)
(PPBO) fiber, known as Zylon, has a very high thermal
stability as well as mechanical strength when compared to any
other polymers due to its ladder-like rigid structure. However,
one of the critical drawbacks of its stiff structure is its
insolubility in organic solvents, and only strong acids can be
used use for fiber spinning of this polymer. To overcome the
poor processability caused by this insolubility in organic
solvents, a soluble PPBO precursor was designed and
synthesized by the reaction of tert-butyldimethylsilyl (TBS)
group-functionalized 4,6-diaminoresoisinol with terephthaloyl
chloride for polycondensation. The obtained TBS-function-
alized PPBO precursor (TBS-prePBO) shows an excellent solubility in common organic solvents, such as N-methylpyrrolidone
(NMP), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO). Transparent TBS-prePBO films made by the
solution-cast method provided PPBO films after thermal treatment at 500 °C for 1 h. The structure of the obtained PPBO films
was characterized by IR and XRD techniques and found that the films exhibited extremely high thermal stabilities, namely, the
synthesized PPBO polymer decomposition temperature reached 670 °C in flowing N2, which is the highest temperature among
the organic polymers reported so far.
°C17) due to the introduction of a single bond linkage in the
INTRODUCTION
Aromatic polymer films have attracted much attention in the
■
main chain structure of the PBOs.
Therefore, in order to realize the fabrication of PBO films
fields of microelectronics and in aerospace and defense
with the ultimate thermal stability, the development of a
applications due to their excellent mechanical, thermal, and
electrical properties and light weight.1 Among these the
processable PPBO is strongly required. Typically, PPBOs are
prepared by condensation polymerization using a strong acid
polymers, polybenzoxazoles (PBOs) have superior thermal
such as poly(phosphoric acid) (PPA) as the solvent and a
stabilities compared to any other heat-resistant polymers, such
condensation reagent26 (Scheme 1)3 since only such an acid
as polyimides (decomposition temperature, ∼ 600 °C)1 and
polybenzimidzoles (PBI: decomposition temperature, ∼620
Scheme 1. Synthetic Scheme of Zylon
°C)2 due to their inert ladder-like rigid structure. Especially,
PPBO3 fibers are known to have the highest thermal stability
(decomposition temperature, 650 °C).4 However, the major
drawback of the rigid structure is the insolubility of the PPBO
in organic solvents as well as the absence of a glass transition
temperature, which hinders the fabrication of PPBO with the
desired shape and form such as films. To overcome the poor
processability, various soluble PBOs have been developed
including fluorinated PBOs,5−11 PBOs containing an aliphatic
main chain,12−15 ester-linked PBOs,16−21 and others.22−25
Received: March 30, 2012
However, unfortunately, improvement of the solubility caused
a lower thermal stability (decomposition temperature, ∼620
Revised: April 28, 2012
Published: May 8, 2012
© 2012 American Chemical Society
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according to previous methods.37 Zylon was kindly provided from
Toyobo.
can be used as the solvent for PPBO. Fabrication of PPBO films
cast from a PPA solution after condensation was reported.27,28
However, the difficulty in the removal of the residual PPA and
deterioration of the mechanical properties have been pointed
out.29
Another strategy for the fabrication of PPBO films is the
fabrication of precursor films followed by conversion to PPBO
films. In this approach, only the PPBO precursor is required to
be soluble in an organic solvent, and in principle, the
polymerization solution is directly used for the fabrication
similar to the fabrication of polyimide films from a polyamic
acid solution. Indeed, poly(hydroxyamide)5,17,30−33 and poly-
(hydroxyimide)34−36 were reported to provide the PBO
structure by a thermal cyclodehydration reaction.
Synthesis of 4,6-Di(tert-butyldimethylsilylamino)-1,3-di(tert-
butyldimethylsiloxy)benzene (TBS-DAR). Under a nitrogen
atmosphere, 4,6-diaminoresorcinol dihydrochloride (0.213 g, 1
mmol) was dissolved in DMF (10 mL), to which tert-butyldime-
thylsilyl chloride (1.5 g, 10 mmol) and triethylamine (2.0 mL, 27
mmol) were added, and then stirred at room temperature for 24 h to
produce a precipitate, which was filtered and then rinsed several times
with water. The obtained solid was dried at 80 °C for 12 h in a vacuum
to give a product as a white powder (0.55 g, yield 93%). 1H NMR (300
MHz, CDCl3, δ ppm): 6.27 (s, 1H, Ar−H), 6.24 (s, 1H, Ar−H), 3.63
(s, 2H, N−H), 1.00−0.95 (m, 36H, (CH3)3), 0.23−0.19 (m, 24H,
Si(CH3)2). IR (ATR, ν, cm−1): 3400 (ν N−H), 2955 (νAr−H), 2860
(ν CH3), 1260 (δ CH3). MALDI MS m/z 596.20 [M]+·, calcd 596.18.
Anal. Calcd for C30H64N2O2Si4: C, 60.34; H, 10.80; N, 4.69. Found: C,
60.35; H, 10.73; N, 4.79.
In this study, we describe the design and synthesis of a
soluble PPBO precursor followed by the fabrication of PPBO
films as shown in Scheme 2. One difficulty in the design and
Synthesis of PPBO Precursor (TBS-prePBO). Under a nitrogen
atmosphere, TBS-DAR (0.597 g, 1.0 mmol) was dissolved in an NMP
solution (2 mL) and then cooled to 0 °C, to which terephthaloyl
chloride (0.203 g, 1.0 mmol) was added, and then the mixture was
stirred at room temperature for 48 h. The solution was poured into
200 mL of methanol to produce a precipitate, which was filtered and
rinsed several times with water. The obtained solid was dried at 80 °C
for 24 h in a vacuum that produced a yellowish powder (0.42 g, yield
82%). The inherent viscosity of the obtained polymer was 0.75 dL g−1
Scheme 2. Synthesis of PPBO via TBS-prePBO
1
(measured at a concentration of 0.5 g dL−1 in NMP at 30 °C). H
NMR (300 MHz, DMSO-d6, δ ppm): 9.70−9.59 (m, 2H, N−H),
8.13−8.10 (s, 4H, Ar−H), 7.79−7.69 (m, 1H Ar−H), 6.64−6.60 (s,
1H, Ar−H), 1.00−0.87 (m, 18H, (CH3)3), 0.26−0.21 (m, 12H,
Si(CH3)2). IR (ATR, ν, cm−1): 3425 (ν N−H), 2955 (ν Ar−H), 2860
(ν CH3), 1660 (ν CO), 1530 (ν N−H) 1365 (δ CH3).
Fabrication of Free-Standing PPBO Films. TBS-prePBO (100
mg) in DMAc (2 mL) filtered through a 0.2 μm PTFE membrane
filter was cast on a glass substrate. The obtained film was dried at 80
°C for 1 h and then at 100 °C for 1 h. The film was peeled off the glass
substrate by immersing the substrate in water. The obtained film was
dried at 80 °C for 4 h under vacuum and subsequently heated in a
vacuum at 200 °C for 1 h, 250 °C for 1 h, 300 °C for 1 h, and finally
350 °C for 1 h. IR (ATR, ν, cm−1): 1625 (CN). Anal. Calcd for
C14H6N2O2 + 0.375H2O: C, 69.78; H, 2.51; N, 11.63. Found: C,
69.78; H, 2.72; N, 11.30.
synthesis of such soluble PPBO precursor is the poor solubility
of the monomer of PPBO, 4,6-diaminoresorcinol (DAR). We
have already reported that the functionalization of the DAR
dramatically improved the solubility and allowed an easy
polycondensation reaction by mixing with terephthaloyl
chloride at room temperature.37 However, the functionalization
with the trimethylsilyl (TMS) group in place of DAR caused
deprotection due to high reactivity toward nucleophiles, such as
moisture water, and provided an insoluble poly(hydroxyamide)
after isolation from the reaction solution probably due to the
strong hydrogen bonding of the phenolic OH groups. We have
chosen the tert-butyldimethylsilyl (TBS) group and used it as a
functional unit of DAR in place of TMS because the TBS group
was reported to be 104 times more stable than the TMS
group.38
Measurements. The inherent viscosity measurements were
1
performed using an Ostwald viscometer. The H NMR spectra were
measured using an AV300 M spectrometer (Bruker Biospin). The FT-
IR spectra were measured using a Spectrum 65 FT-IR (Perkin-Elmer)
spectrometer equipped with an ATR apparatus. The MALDI-MS
spectrograph was obtained using an Autoflex (Bruker). A thermogravi-
metric analysis (TGA) and differential scanning calorimetry (DSC)
measurement were carried out using an EXSTAR TG/DTA 6300 (SII
Nanotechnology) at the heating rate of 10 °C min−1 under flowing N2
(200 mL min−1) and DSC 6220 (SII Nanotechnology) at the rate of
10 °C min−1, respectively. The X-ray diffraction patterns were
collected using an X-ray diffractometer, SmartLab (Rigaku), in the
2θ scan range of 3°−50° using Cu Kα (λ = 0.154 01 nm) radiation
operated at 30 mA and 40 kV. The scanning electron microscope
images of the cross-section morphology of the films were measured at
2.0 kV using a JSM6701F (JEOL). The mechanical properties of the
films were measured at 25 °C using an EZ-S (Shimadzu) at a
displacement rate of 1.0 mm min−1. The size of the specimen was 20
mm, 10 mm, and 20 μm in length, width, and thickness, respectively.
The density of the PPBO film was determined by dividing the weight
of the film with the film volume calculated using the film thickness and
the area.
To the best of our knowledge, this is the first report
describing the fabrication of PPBO films with extremely high
thermal stability (decomposition temperature, 670 °C) without
using a strong acid solvent.
EXPERIMENTAL SECTION
■
Materials. 4,6-Diaminoresorcinol dihydrochloride, terephthaloyl
chloride, and tert-butyldimethylsilyl chloride were purchased from
TCI. The terephthaloyl chloride was purified by recrystallization from
n-hexane, and the other reagents were used as received. Triethylamine
was purchased from Kishida Reagents Chemicals and purified by
distillation over potassium hydroxide. N-Methylpyrrolidone (NMP),
N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc),
dimethyl sulfoxide (DMSO), methanesulfonic acid, methanol, ethanol,
and toluene were purchased from Wako Pure Chemical and used as
received. 4,6-Di(trimethylsilylamino)-1,3-di(trimethylsiloxy)benzene
(TMS-DAR) and poly(hydroxyamide) (prePBO) were synthesized
RESULTS AND DISCUSSION
■
Polymerization and Characterization of PPBO Pre-
cursor (TBS-prePBO). The precursor of poly(p-phenylene
benzobisoxazole) (PPBO) was synthesized by the condensation
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reaction of TBS-functionalized DAR (denoted TBS-DAR, see
Supporting Information, Figure S1) with terephthaloyl chloride
as shown in Scheme 1b. A series of polymerization conditions
were carried out, and the results are summarized in Table 1. It
1). At TBS-DAR = 1.0 M, which is close to the saturated
concentration, the polycondensation at room temperature for
24 and 48 h was found to provide a high molecular weight
polymer with the inherent viscosity of 0.86 and 1.45 dL g−1 in
NMP at 30 °C, respectively (see entries 7 and 8 in Table 1).
The obtained polymer powders were characterized by IR
spectroscopy. As shown in Figure 1a, characteristic peaks of the
amide bonding (1640 and 1550 cm−1, CO stretching mode
and N−H bending mode, respectively) due to the TBS-
prePBO were observed, which clearly indicated the progress of
Table 1. Polymerization Conditions of TBS-prePBO
inherent
concn of TBS- temp time yield
viscosity
a
entry solvent
DAR [M]
[°C]
[h]
[%]
[dL g−1
]
1
2
3
4
5
6
7
8
9
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
DMAc
0.5
0.5
0.5
0.5
0.25
0.5
1.0
1.0
0.5
r.t.
r.t.
r.t.
r.t.
r.t.
80
r.t.
r.t.
r.t.
6
12
24
48
24
24
24
48
48
1
the polycondensation reaction. Figure 1b shows the H NMR
86
72
82
90
70
86
82
94
0.45
0.69
0.75
0.35
0.63
0.86
1.45
0.72
spectrum of the TBS-prePBO. In the polymer, the TBS group
and the aromatic group were determined by 1H NMR
spectroscopy to be TBS:aromatic = 30:6, which indicates that
the hydroxyl groups of the TBS-prePBO were fully protected
by the TBS group. This fact manifests the superior stability of
the TBS group compared to the TMS group as the protecting
group of the prePBO. Successful protection of the hydroxyl
group by the TBS group realized an excellent solubility in the
organic solvents, such as NMP, N,N-dimethylacetamide
(DMAc), and dimethyl sulfoxide (DMSO). Thermal treatment
of the TBS-prePBO was monitored by TGA together with the
IR technique. Figure 2a (solid line) shows the TGA curve of
the TBS-prePBO measured at 10 °C min−1 under flowing N2.
The TGA curve of a nonprotected PPBO precursor (prePBO)
prepared by the condensation of TMS-DAR with terephthaloyl
chloride in NMP at 0 °C for 6 h is also shown in Figure 2a
(dotted line) for comparison. It was found that a greater weight
loss (45.4 wt %) was observed for the TBS-prePBO at around
350−400 °C. Judging from the weight loss of the prePBO in a
similar temperature range (17.3 wt %) that is assignable to the
ring-closing dehydration,17,39,40 the weight loss for the TBS-
prePBO at this temperature range was attributed to the
combination of the deprotection of the TBS (calcd = 45.8 wt
a
Measured at a concentration of polymer = 0.5 g dL−1 in NMP at 30
°C.
was found that no polymer solid was obtained as a precipitate
after the reaction for 6 h in N-methylpyrrolidone (NMP) at
room temperature (entry 1 in Table 1), which is somewhat
different from the polymerization reaction of trimethylsilyl-
protected DAR (TMS-DAR), in which the polymerization for 6
h provided a high molecular weight material.17 However, it was
found that longer reaction time produced polymer solids, and
under such conditions, the viscosities of the solutions increased
with the increasing reaction time (entries 2−4 in Table 1). The
bulky TBS group would be considered to act as a steric
hindrance for the formation of the amide bond. As shown in
Table 1, higher monomer concentrations produced polymers
with higher molecular weights (see entries 3, 5, and 7 in Table
1
Figure 1. (a) FT-IR spectrum of TBS-prePBO. (b) H NMR spectrum of TBS-prePBO.
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Figure 2. (a) TGA curves of prePBO (dotted line) and TBS-prePBO (solid line) measured at the heating rate of 10 °C min−1 under flowing N2. (b)
FT-IR spectra of PPBO prepared from TBS-prePBO (solid line) and Zylon (dotted line).
%) together with the dehydration due to the ring-closing
reaction (calcd weight loss = 7.2 wt %). Indeed, the FT-IR
spectrum of the TBS-prePBO after heating to 500 °C (red line
in Figure 2b (solid line)) shows the characteristic peak at
around 1625 cm−1 to the assignable to the CN vibration
from the benzoxazole ring, supporting the formation of the
PPBO structure. It is notable that the spectrum is in good
agreement with that of the Zylon fibers shown in Figure 2b
(dotted line).
We examined the effect of the annealing time on the thermal
stability of the PPBO. Upon heating to 500 °C, an additional
8.7 wt % weight loss, which corresponded to the 4.8 wt %
weight loss calculated on the basis of the initial TBS-prePBO
weight, was observed after a 15 h heating as shown in Figure 3a,
probably due to the dehydration caused by the ring closure
reaction. Considering the original 45.4 wt % weight loss, the
total weight loss (50.2 wt %) agreed well with the calculated
value of the TBS group and dehydrated water (53.0 wt %).
Figure 3b shows the TGA curves of the PPBO prepared from
the TBS-prePBO annealed at 500 °C at different annealing
times. It is obvious that the onset temperatures of the
decomposition increased with the increasing annealing time,
and the PPBO obtained highest thermal resistivity after a 24 h
annealing. We assumed that the thermal stability increased as a
consequence of the decreasing number of unclosed oxazole
bonds that remained even after the annealing.
ends is dominant for the thermal stability. The IR spectra
measured after annealing at 500 °C for different annealing
times gave almost identical spectra (see Supporting Informa-
tion, Figure S3), suggesting that the concentration of the defect
sites in the PPBO is lower than the detection limit of the IR
method. To examine the presence of Tg, we carried out DSC
study and found that no Tg of the polymer was observed in the
temperature range of 30−350 °C (see Supporting Information,
Figure S4).
Fabrication and Characterization of PPBO Film. The
excellent solubility of the TBS-prePBO allows us to fabricate its
films from the solution-cast method. A DMAc solution of the
TBS-prePBO was cast on a glass substrate, and then the
obtained film was peeled off from the substrate by immersing it
in water as described in the Experimental Section. As shown in
Figure 4a, the obtained free-standing film was yellowish with a
film thickness of 18.1 μm. The film was heated at every 100 °C
for 1 h from 100 to 500 °C under vacuum. The formation of
the PPBO from the TBS-prePBO film upon heating was
monitored by IR at each heating temperature (Figure 4b). The
peak intensity of the amide vibration mode at around 1650
cm−1 was found to decrease after heating at 250 °C. After
heating at 350 °C, the peak at around 1625 cm−1 which is
assignable to the CN vibration of the newly appeared
benzoxazole ring as indicated by the arrow, which is almost
identical to that of the Zylon fibers. The observed spectral
change obtained by the 350 °C heating well agrees with the
TGA result shown in Figure 2a, in which a greater weight loss
by the polymer is recognized.
Of interest, the PPBO prepared from the lower molecular
weight also exhibited identical onset temperatures similar to
those of the highest molecular weight polymer at each
annealing time (see Supporting Information, Figure S2). This
fact suggests that the effect of the defect sites on the main
chain, such as the amide linkage, rather than that at the chain
Figure 4c shows a photograph of the PPBO film prepared
from the TBS-prePBO after heating at 500 °C for 1 h in
vacuum. The film is 11.0 μm thick. The observed 42.7%
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Figure 3. (a) Monitoring the weight reduction upon annealing at 500
°C under flowing N2. (b) TGA curves of PPBO annealed at 500 °C
with given annealing time measured at the heating rate of 10 °C min−1
under flowing N2.
Figure 4. (a) Photograph of the TBS-prePBO film. (b) FT-IR spectra
of TBS-prePBO films obtained after given heating treatments. FT-IR
spectra of TBS-prePBO (black line) and Zylon (red line) are shown
for comparison. (c) Photograph of the PPBO film.
decrease is probably caused by the deprotection of the TBS
group as well as by dehydration. Scanning electron micrograph
(SEM) measurements of the cross section of a PPBO film
showed a densely filled structure without any voids in the film
(see Supporting Information, Figure S5). This is a clear
contrast to that of the PPBO film prepared from a PPBO
precursor protected by the tert-butoxycarbonyl (t-Boc) group,
forming a porous structure generated by the t-Boc decom-
position leading to a low density (1.02 g cm−3).37 The density
range of the PPBO film prepared from the TBS-prePBO was
1.55−1.68 g cm−3, which is comparable to that of the Zylon
fibers (1.54−1.56 g cm−3).4
The mechanical properties of the TBS-prePBO film and the
PPBO film were measured. The tensile strength and tensile
modulus were 50.0 MPa and 1.9 GPa, respectively, for the TBS-
prePBO film and 42.0 MPa and 2.5 GPa, respectively, for the
PPBO film, which were not much different from those of the
PPBO films prepared from a methanesulfonic acid solution
(tensile strength and tensile modulus are 68.0 MPa and 3.6
GPa, respectively41). It is concluded that a PPBO film with
good mechanical properties was fabricated without using any
strong acid as a solvent. However, the mechanical strength of
the PPBO film was much weaker than that of the Zylon fibers.
A lower anisotropic alignment of the chains in the PPBO film
may explain the result. Zylon fibers of PPBO are fabricated
from its lyotropic liquid crystal solution and have a high
anisotropy along the fiber axis direction, resulting in the
extremely high mechanical strength. On the other hand, in the
amorphous film, the polymer chains are randomly entangled
and the stiffness of the main-chain structure is not effectively
reflected in the film. To compare the packing structure of the
polymer chain in the PPBO film together with the Zylon fibers,
X-ray diffraction (XRD) measurements were conducted. The
Zylon fibers showed intense peaks at around 2θ = 16° (d = 0.56
nm) and 26° (d = 0.34 nm), attributed to the “side-by-side”
distance on the (200) plane and “face-to-face” distance on the
(010) plane between two neighboring polymer chains,
respectively (Figure 5a, blue line).42 On the other hand,
PPBO film exhibited a broad weak peak, which clearly indicates
a weaker chain alignment in the PPBO film (Figure 5a, red
line). The intrinsic viscosity measured in methanesulfonic acid
at 30 °C for the PPBO was 5.3 dL g−1, while the PPBO fiber
synthesized using methansulfonic acid as the solvent having the
tensile strength and tensile modulus were 2.6 and 138 GPa,
respectively, is reported to be 14.0 dL g−1.43 The lower
molecular weight of PPBO is expected to affect the mechanical
strength.
In order to confirm the thermal stability of the PPBO films,
the TGA measurements of the PPBO film prepared at the
optimized annealing condition (500 °C for 24 h) were carried
out under flowing N2, and the result is shown in Figure 5b. The
TGA curves of Zylon fibers and the PPBO powder are also
shown for comparison. The PPBO film shows a 5% weight loss
temperature at 671 °C, which is almost identical to that of the
PPBO powder and close to that of the Zylon fibers (690 °C).
The slight weight reduction of the Zylon fibers observed in the
temperature range of 150−650 °C is probably due to the
impurity such as residual solvent or the chemicals used for the
surface treatment since after washing the film with solvent
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Table 2. Comparison of the Thermal Stabilities of PPBO and
Various PBOs
a
T5 and T10 are temperatures at which 5% and 10% weight loss were
Figure 5. (a) X-ray diffraction (XRD) profiles of TBS-prePBO after
heating at 500 °C (red line). XRD patterns of TBS-prePBO (black
line) and Zylon (blue line) are shown for comparison. For the XRD
data upon heating, see Supporting Information, Figure S6. (b) TGA
curves of PPBO film prepared from the TBS-prePBO film annealed at
500 °C for 24 h (10 °C min−1 in flowing N2). The TGA curves of
PPBO and Zylon fiber are also shown as a comparison.
b
recorded by TGA in nitrogen atmosphere, respectively. Determined
by TGA in flowing helium at the rate of 20 °C min−1, and there is no
information about onset, 5% weight loss, and 10% weight loss.
prePBO to PPBO upon heating at 500 °C for 1 h under
vacuum. Transparent free-standing PPBO films were success-
fully fabricated from the TBS-prePBO film. Annealing of the
PPBO film at 500 °C for 24 h provided a film with the thermal
resistivity of 670 °C under nitrogen. To the best of our
knowledge, this is the highest thermal stability among all
organic polymers reported so far. The TBS-prePBO is able to
be provided in various forms, such as PPBO nanofibers and
nanocomposites. The presented study will open a way for the
use of PPBO with the desired shape and form in many
application fields including aerospace areas, high heat-resistance
materials, such as firefighter cloths, microelectronics area, etc.,
in the next generation. It is expected that further improved
monomer design would enable the synthesis of PPBO with
higher molecular weight that has ultimate properties, and such
study is currently under investigation in our laboratory.
(ethanol/hexane = 1/1 v/v), no such evident weight reduction
was observed (see Supporting Information, Figure S7).
Table 2 lists a comparison of the thermal decomposition
temperatures of the PPBO and other processable PBOs
reported in the literature. The PBOs containing an aliphatic
main chain show a dramatically lower thermal stability; namely,
the decomposition temperature is lower by about 200 °C than
that of the PPBO.13 The PBOs containing a fluorinated group
possess a higher thermal stability (∼620 °C)44−46 due to the
strength of the C−F bonding over C−H bonding.47 To date,
aromatic PBOs containing a cardo and triphenylamino
structure show the highest thermal stability among the various
processable PBOs reported so far (630−650 °C).16,21,48 To the
best of our knowledge, our processable PPBO using the soluble
PPBO precursor (TBS-prePBO) has the highest thermal
stability (∼670 °C). It is evident that the PPBO film is a
promising candidate for use as the thermal resistive membranes,
thermal protective coating, and so on.
ASSOCIATED CONTENT
■
CONCLUSION
S
* Supporting Information
■
1
In summary, we synthesized a new stable PPBO monomer, 4,6-
diaminoresolcinol functionalized with the tert-butyldimethylsilyl
group (TBS-DAR). Polymerization of the TBS-DAR with
terephthaloyl chloride gave a soluble PPBO precursor having a
TBS group as the protecting group of the phenolic OH group
(TBS-prePBO). A viscosity of 1.45 dL g−1 in NMP at 30 °C at
a concentration of TBS-prePBO = 0.5 g dL−1 was obtained
after the polymerization for 48 h at room temperature. The IR
and TGA measurements revealed the conversion of the TBS-
FT-IR and H NMR spectrum of PPBO monomer (TBS-
DAR), TGA curves of PPBO from TBS-prePBO with various
molecular weights, IR spectra of the TBS-prePBO after
annealing for given time, DSC curve of TBS-prePBO, X-ray
diffraction profiles of TBS-prePBO after annealing for given
time, and SEM images of a cross section of PPBO film. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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dx.doi.org/10.1021/ma3006526 | Macromolecules 2012, 45, 4247−4253

Macromolecules
Article
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AUTHOR INFORMATION
■
Corresponding Author
*Tel +81-92-802-2840; Fax +81-92-802-2840; e-mail fujigaya-
tcm@mail.cstm.kyushu-u.ac.jp (T.F.), nakashima-tcm@mail.
cstm.kyushu-u.ac.jp (N.N.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Low-Carbon Research
Network (LCnet), Nanotechnology Network Project
(Kyushu-area Nanotechnology Network), Grant-in-aid for
JSPS Research Fellow (No. 231617) (for TF), and Global
COE Program (Science for Future Molecular Systems) funded
by the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
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