An ultrahigh performance cross-linked polybenzoxazole via thermal conversion from poly(benzoxazine amic acid) based on smart o -benzoxazine chemistry

Article abstract of DOI:10.1021/ma502297m

A main-chain-type polybenzoxazine with amic acid and benzoxazine groups as repeating units, generally termed as poly(benzoxazine amic acid) (poly(BZaa)), has been synthesized. Different from traditional main-chain polybenzoxazines, poly(BZaa) can undergo polymerization and imidization reactions to form cross-linked polyimide (cPI) after certain thermal treatment. cPI can further undergo a typical cyclization to give cross-linked polybenzoxazole (cPBO) after an additional thermal treatment. A model reaction is designed based on the reaction of 3-phenyl-2,4-dihydro-2H-benzo[e][1,3]oxazin-9-amine with a phthalic anhydride. Fourier transform infrared (FTIR) and proton nuclear magnetic resonance (¹H NMR) spectroscopies are used to confirm the structure of model compound. The ring-opening polymerization behavior of benzoxazine monomers and main-chain polybenzoxazine is studied by differential scanning calorimetry (DSC). Thermal properties of the cross-linked polymers are also studied by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA).

Full text of DOI:10.1021/ma502297m

Article
pubs.acs.org/Macromolecules
An Ultrahigh Performance Cross-Linked Polybenzoxazole via
Thermal Conversion from Poly(benzoxazine amic acid) Based on
Smart o‑Benzoxazine Chemistry
Kan Zhang,†,‡ Jia Liu,‡ and Hatsuo Ishida*,‡
†School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
‡Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
ABSTRACT: A main-chain-type polybenzoxazine with amic acid and
benzoxazine groups as repeating units, generally termed as poly-
(benzoxazine amic acid) (poly(BZaa)), has been synthesized. Different
from traditional main-chain polybenzoxazines, poly(BZaa) can undergo
polymerization and imidization reactions to form cross-linked polyimide
(cPI) after certain thermal treatment. cPI can further undergo a typical
cyclization to give cross-linked polybenzoxazole (cPBO) after an
additional thermal treatment. A model reaction is designed based on
the reaction of 3-phenyl-2,4-dihydro-2H-benzo[e][1,3]oxazin-9-amine
with a phthalic anhydride. Fourier transform infrared (FTIR) and proton
nuclear magnetic resonance (1H NMR) spectroscopies are used to confirm the structure of model compound. The ring-opening
polymerization behavior of benzoxazine monomers and main-chain polybenzoxazine is studied by differential scanning
calorimetry (DSC). Thermal properties of the cross-linked polymers are also studied by dynamic mechanical analysis (DMA)
and thermogravimetric analysis (TGA).
rate of polymerization. Furthermore, no volatiles are released,
and nearly zero shrinkage is achieved upon polymerization.
Recently, we developed a new class of cross-linked PBOs via
thermal conversion from ortho-amide functional benzoxazines
as shown in Scheme 1.14 The cross-linked PBOs show further
advantages including excellent flexibility in the molecular
design, easy processability, and cost effectiveness. Among this
novel class of cross-linkable PBOs, the polybenzoxazole based
on main-chain type shows the best balance of thermal stability,
mechanical properties, and flame resistance. However, the
synthesis of main-chain-type benzoxazine polymers through
Mannich base polycondensation results in few shortcomings,
such as poor solubility of the products due to molecular rigidity,
leading to a low molecular weight and broad polydispersity. In
our previous work, we prepared benzoxazine with primary
amine using protection and deprotection methods15 and then
used the primary amine-containing benzoxazines to prepare a
main-chain-type poly(benzoxazine amide).16 In addition, Lin et
al. also prepared primary-amine-containing benzoxazines by the
steric hindrance control method. On the basis of the primary
amine-containing benzoxazines, they prepared poly-
(benzoxazine imide) thermosets. The experimental results
show that incorporating benzoxazine into polyimides increased
the Tg, tensile modules, flame retardancy, dimensional stability,
and contact angle without losing too much in thermal
stability.17
INTRODUCTION
■
Aromatic polymer films have attracted much attention in the
microelectronic and aerospace applications due to their
excellent thermal, electronic, and mechanical properties.1
Among these polymers, polybenzoxazoles (PBOs) have
superior thermal stabilities compared to any other rigid
polymers, such as polyimides and polybenzimidzoles.1,2
However, the very high rigidity of PBOs also leads to
difficulties in synthesis and fabrication. One possible approach
to prepare PBO film is the use of a strong acid, such as
poly(phosphoric acid) (PPA), which can dissolve PBOs.3
However, the use of such strong acid is environmentally
unfriendly and unfavorable in a view of manufacturing. The
residual acid can also lead to aging problems as well under
humid and warm application conditions. Thus, the fabrication
of PBOs without the use of strong acid as solvent is strongly
desired. Another way is to develop a PBO precursor that is
soluble in organic solvent.4,5
Polybenzoxazine (PBz) has attracted much attention due to
its excellent mechanical and thermal properties with good
handling capability for material processing and composite
manufacturing.6−13 Benzoxazines can be polymerized through
the cationic ring-opening of oxazine ring with or without an
added initiator and/or catalyst. Besides, their extraordinarily
rich molecular design flexibility allows designing various
molecular structures with desired properties. Moreover, they
release no reaction byproduct during the ring-opening
polymerization reactions. No strong acid or alkaline catalysts
are required for the synthesis of monomers. However, some
acids, such as carboxylic acids and phenols, will accelerate the
Received: November 13, 2014
Revised: December 4, 2014
Published: December 11, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of Polybenzoxazole via Thermal Conversion from ortho-Amide Functional Benzoxazine
linear viscoelastic limit. Thermogravimetric analysis (TGA) was
performed by a TA Instruments Q500 TGA using nitrogen as a
purge gas (40 mL/min) at a heating ramp rate of 10 °C/min. Micro-
Raman scattering measurements were done by a Horiba Jobin Yvon
LabRam HR800 spectrometer connected with a charge coupled
detector and two grating systems (600 and 1800 lines/min) at room
temperature. The optical power of the He−Ne laser used was 17 W at
632.8 nm. The system was calibrated using the silicon line at 520 cm−1.
Preparation of 4,5,6,7-Tetrachloro-2-(2-hydroxyphenyl)-
isoindoline-1,3-dione (1). Tetrachlorophthalic anhydride (13.75 g,
0.048 mol), o-aminophenol (5.77 g, 0.0523 mol), and 100 mL of acetic
acid were mixed in a 250 mL round flask equipped with a reflux
condenser. The mixture was stirred and refluxed at 120 °C for 6 h.
Then the reaction mixture was cooled to room temperature. The
precipitate was filtered and washed with 300 mL of methanol. Removal
of solvent by evaporation afforded a yellow crystal (yield ca. 89%). 1H
NMR (300 MHz, DMSO-d6), ppm: δ = 6.87−7.36 (4H, Ar), 9.93
(OH). IR spectra (KBr, cm−1) = 3414 (O−H stretching), 1782, 1717
(imide I), 1304 (imide II, C−N stretching), 741 (CO bending).
Preparation of 4,5,6,7-Tetrachloro-2-(3-phenyl-3,4-dihydro-
2H-benzo[e][1,3]oxazin-8-yl)isoindoline-1,3-dione (2). 80 mL of
xylenes, aniline (3.72 g, 0.04 mol), 1 (15.18 g, 0.04 mol), and
paraformaldehyde (2.41 g, 0.08 mol) were mixed in a 250 mL round
flask equipped with a reflux condenser. The mixture was heated and
refluxed at 120 °C for 12 h. Then the reaction solution was cooled to
room temperature and precipitated into 150 mL of methanol. A yellow
powder product was obtained by evaporation the solvent under
From all these findings, we were encouraged to find a smart
approach to prepare the ultrahigh performance cross-linked
PBOs based on the main-chain-type benzoxazine. We define
the words “ultrahigh performance polymers” in thermal
properties as those polymers that possess 5 wt % weight
reduction temperature, Td5, above 500 °C in this paper. In this
article, we prepared ortho-amine-functional benzoxazines by
protection of the amino group in o-aminophenol using
tetrachlorophthalimide (TCP), carried out the benzoxazine
synthesis, and then deprotected the TCP-protected benzox-
azine using hydrazine hydrate. The yield of the ortho-amine-
functional benzoxazines is much higher compared with our
previous work of para-amine-functional benzoxazines by the
TCP-protected method. Besides, we also prepared a main-
chain-type poly(benzoxazine amic acid) (Poly(BZaa)) based on
the ortho-amine-bifunctional benzoxazine with 3,3′,4,4′-benzo-
phenonetetracarboxylic dianhydride (BTDA). Since we already
know that the ortho-amide-functional benzoxazines can be
polymerized and further thermal treatment to give polybenzox-
azoles based on our previous work,14 and Mathias and others
reported the possible thermal conversion of ortho-hydroxypo-
lyimides into polybenzoxazoles with releasing carbon dioxide at
temperature around 400 °C,18−21 we reasonably predict this
main-chain-type poly(BZaa) should undergo an very interesting
thermal behavior. The detailed synthetic strategy and the
thermal behavior of benzoxazine amic acid will be discussed in
this article.
1
vacuum (yield ca. 95%). H NMR (300 MHz, DMSO-d6), ppm: δ =
4.73 (s, Ar−CH2−N, oxazine), 5.44 (s, O−CH2−N, oxazine), 6.82−
7.31 (8H, Ar). IR spectra (KBr), cm−1: 1786, 1730 (imide I), 1484
(stretching of trisubstituted benzene ring), 1390 (imide II), 1220 (C−
O−C asymmetric stretching), 1161 (C−N−C asymmetric stretching),
932 (out-of-plane C−H).
EXPERIMENTAL SECTION
■
Materials. o-Aminophenol (>98%), tetrachlorophthalic anhydride
(99%), paraformaldehyde (99%), 3,3′,4,4′-benzophenonetetracarbox-
ylic dianhydride (BTDA), and phthalic anhydride were used as
received from Sigma-Aldrich. 4,4′-Diaminodiphenylmethane (DDM,
98%) and aniline were purchased from Aldrich. Chloroform,
dimethylacetamide, hexane, xylenes, methanol, and sodium sulfate
were purchased from Fisher Scientific and used as received.
Preparation of 3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-8-amine (3) by Deprotection of 2. TCP-protected
monofunctional benzoxazine (2) (4.91 g, 0.01 mol) and 60 mL of
chloroform were mixed in a 250 mL round flask. The mixture was
stirred in an ice bath for 15 min. Then the addition of hydrazine
monohydrate (1.5 g, 0.03 mol) was added to the suspension. The
mixture was stirred for 1 h at room temperature, followed by filtration
of the solid residues. The filtrate was washed by 0.5 N aqueous NaOH
solution for one time and then washed with water for 3 times. After
that, the solution was dried over sodium sulfate anhydrous. The
solvent was removed by evaporation using a rotary evaporator under
reduced pressure. Finally, a brown color semisolid material was
Characterization. Proton nuclear magnetic resonance (1H NMR)
spectra were recorded on a Varian Oxford AS300 (300 MHz) using
chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) as solvent.
1
The average number of transients for H MNR measurement was 64.
When quantitative information was desired, the contact time of 10 s
was utilized. Fourier transform infrared (FTIR) spectra were measured
by a Bomem Michelson MB100 FTIR spectrometer, which was
connected with a dry air purge unit and a deuterated triglycine sulfate
(DTGS) detector. Sixty-four scans were coadded to obtain a spectrum
at a resolution of 4 cm−1. All powdered samples were finely ground
with KBr powder and pressed into a disk, and the spectrum was taken
as the transmission mode. Differential scanning calorimeter (DSC)
measurements were conducted with a TA Instruments DSC Model
2920 using nitrogen as a purge gas (60 mL/min) at a heating rate of
10 °C/min. All samples were sealed using hermetic aluminum pans
and covered with lids. Dynamic mechanical analysis (DMA) was
performed on a TA Instruments Model Q800 DMA applying
controlled strain tension mode with amplitude of 10 μm and a ramp
rate of 3 °C/min. Strain sweep was first performed to determine the
1
obtained (yield: 82%). H NMR (300 MHz, CDCl3), ppm: δ = 3.72
(s, NH2), 4.62 (s, Ar−CH2−N, oxazine), 5.42 (s, O−CH2−N,
oxazine), 6.46−7.31 (8H, Ar). IR spectra (KBr), cm−1: 3356 (N−H
stretching), 1497 (stretching of trisubsitituted benzene ring), 1222
(C−O−C asymmetric stretching), 1149 (C−N−C asymmetric
stretching), 930 (out-of-plane C−H).
Preparation of 2,2′-(3,3′(4,4′-Methylenebis(2,1-phenylene))-
bis(3,4-dihydro-2H-benzo[e][1,3]oxazin-8,3-diyl))bis(4,5,6,7-
tetrachloroisoindoline-1,3-dione) (4). 50 mL of xylenes, DDM
(1.05 g, 5.3 mmol), 1 (4 g, 0.011 mol), and paraformaldehyde (0.75 g,
0.025 mol) were mixed in a 250 mL round flask equipped with a reflux
condenser. The mixture was stirred and refluxed at 120 °C for 12 h.
Then the reaction solution was cooled to room temperature and
precipitated into 250 mL of methanol. A yellow powder product was
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Scheme 2. Preparation of Amine-Fuctional Benzoxazine Monomers Using TCP-Protected o-Aminophenol
obtained by evaporation the solvent under vacuum (yield ca. 86%). 1H
NMR (300 MHz, DMSO-d6), ppm: δ = 3.71 (s, CH2), 4.65 (s, CH2,
oxazine), 5.37 (s, CH2, oxazine), 6.39−7.28 (14H, Ar). IR spectra
(KBr), cm−1: 1786, 1730 (imide I), 1513 (stretching of trisubstituted
benzene ring), 1390 (imide II), 1218 (C−O−C asymmetric
stretching), 1129 (C−N−C asymmetric stretching), 925 (out-of-
plane C−H).
RESULTS AND DISCUSSION
■
Preparation of ortho-Amine-Functional Benzoxazines.
The pathway adopted for obtaining amine-functional benzox-
azines started from amine-protected phenols for preparation of
benzoxazine monomers and then deprotected to liberate the
amine. Tetrachlorophthalimide (TCP)-protected aminophenol
(1) was synthesized and used for preparation of ortho-amine-
monofunctional (3) and difunctional (5) benzoxazines at the
first stage, as shown in Scheme 2. TCP-protected monofuc-
tional benzoxazine (2) was synthesized from the Mannich
condensation of 1, paraformaldehyde, and aniline. Then
primary amine-monofunctional benzoxazine monomer (3)
can be achieved by deprotection of 2. o-TCP-protected
bifunctional benzoxazine (4) was prepared from 1, paraformal-
dehyde, and DDM, followed by deprotection to give 5. In this
case for preparing difunctional monomer 4, a considerable
amount of gel formed, which was observed during the early
stage of the synthesis, all disappeared overnight. A transparent
solution was subsequently formed, leading to a high yield. On
the contrary, in our previous work, a p-TCP-protected
bifunctional benzoxazine showed a significant amount of gel
formation, and the insoluble gel did not disappear even after a
long reaction time was employed, resulting in a poor yield.15
One possibility for the high yield of 4 is that the intramolecular
7-membered hydrogen bond between the phenolic OH group
and one of the carbonyl group, albeit weak interaction,
decreases the polarity of 1, which improves the solubility of 1
in aprotic solvent. At the same time, the intramolecular
hydrogen bond also accelerates the ionization of phenolic OH
group during the initial stage for forming benzoxazine ring. The
deprotection of TCP to amine was easily achieved by using
chloroform solution of hydrazine monohydride in a reasonable
high yield.
Preparation of 3,3′-(4,4′-Methylenebis(2,1-phenylene))bis-
(3,4-dihydro-2H-benzo[e][1,3]oxazin-8-amine) (5) by Depro-
tection of 4. Into a 50 mL round flask was added TCP-protected
difunctional benzoxazine (4) (1 g, 2.152 mmol) and 15 mL of
chloroform. The mixture was stirred in an ice bath for 15 min. Then
the addition of hydrazine monohydrate (0.645 g, 12.915 mmol) was
added to the suspension. The reaction solution was stirred for 1 h at
room temperature, followed by filtration of the solid residues. The
filtrate was washed by 0.5 N aqueous NaOH solution for one time and
then washed with water for 3 times. After that, the solution was dried
over sodium sulfate anhydrous. The solvent was removed by
evaporation using a rotary evaporator under reduced pressure. Finally,
1
a pinkish powder was obtained (yield 80%). H NMR (300 MHz,
CDCl3), ppm: δ = 3.69 (s, NH2), 3.81 (s, CH2), 4.56 (s, Ar−CH2−N,
oxazine), 5.36 (s, O−CH2−N, oxazine), 6.37−7.26 (14H, Ar). FTIR
spectra (KBr), cm−1: 3307 (N−H stretching), 1502 (stretching of
trisubstitued benzene ring), 1230 (C−O−C asymmetric stretching),
1153 (C−N−C asymmetric stretching), 928 (out-of-plane C−H).
Preparation of Benzoxazine Amic Acid Model Compound
(6). 20 mL of DMAc, 3 (1 g, 4.4 mmol), and phthalic anhydride (0.66
g, 4.4 mmol) were mixed into a 50 mL round flask. The reaction
solution was stirred for 4 h in an ice bath. Then the mixture was
poured into water. The precipitate was filtered and dried in a vacuum
oven at 50 °C to give an orange crystal (yield ca. 83%). 1H NMR (300
MHz, CDCl3), ppm: δ = 4.64 (s, Ar−CH2−N, oxazine), 5.40 (s, O−
CH2−N, oxazine), 6.68−8.30 (12H, Ar), 10.30 (s, NH). IR spectra
(KBr), cm−1: 3412 (carboxylic acid), 1721 (carbonyl absorption),
1681 (amide I), 1497 (stretching of trisubstituted benzene ring), 1257
(C−O−C asymmetric stretching), 1144 (C−N−C asymmetric
stretching), 925 (out-of-plane C−H).
Preparation of Poly(benzoxazine amic acid) (Poly(BZaa)),
Cross-Linked Polyimide (cPI), and Cross-Linked Polybenzox-
azole (cPBO). In a 100 mL round flask were added 30 mL of DMAc
(g), 5 (2.6 g, 5 mmol), and BTDA (1.61 g, 5 mmol). The mixture was
stirred for 12 h at room temperature to obtain poly(BZaa). Then, the
viscous poly(BZaa) solution was cast over a glass plate by a slicker,
dried overnight at 80 °C, and then heated at 100 °C (1 h), 200 °C (1
h), and 300 °C (1 h) to obtain cPI. The cPI can be further treated at
400 °C for 1 h and then obtain cPBO.
The structures of TCP-protected benzoxazines were
confirmed by 1H NMR and FTIR analyses. As shown in Figure
1, the characteristic resonances attributed to the benzoxazine
structure, Ar−CH2−N−, and −O−CH2−N− for 2 and 4 are
observed at 4.73 and 5.44 ppm and 4.66 and 5.37 ppm,
respectively. Also, the 1H NMR spectra confirm the presence of
methylene group (−CH2−) at 3.71 ppm for 4.
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Figure 1. 1H NMR of TCP-protected phenol (1), TCP-protected
monobenzoxazine (2), and TCP-protected bisbenzoxazine (4).
Figure 3. FTIR spectra of 3 and 5.
The structure of monoamine and diamine-functional
benzoxazine monomers 3 and 5, respectively, were also
confirmed by 1H NMR and FTIR analyses. As shown in Figure
2, the characteristic resonances attributed to the benzoxazine
Preparation and the Polymerization, Thermal Imid-
ization, and Thermal Cyclization Behavior of Benzox-
azine Amic Acid Model Compound (6). Model reaction
between phthalic anhydride and 3 was performed to obtain
benzoxazine amic acid compound 6 (Scheme 3). Figure 4
Scheme 3. Preparation and Thermal Treatment of 6
1
Figure 2. H NMR of amine-functional monobenzoxazine (3) and
amine-functional bisbenzoxazine (5).
structure, Ar−CH2−N−, and −O−CH2−N− for 3 and 5 are
observed at 4.62 and 5.42 ppm and 4.56 and 5.37 ppm,
respectively. The resonance appeared due to the −NH2 protons
at 3.72 ppm for monofunctional benzoxazine 3 and 3.69 ppm
for bifunctional benzoxazines 5. Also, the H NMR spectra
confirm the presence of methylene group (−CH2−) at 3.81
ppm for 5.
Figure 3 shows the FTIR spectra of benzoxazine 3 and 5.
The bands from the C−O−C antisymmetric stretching mode
are at 1222 cm−1 for the monofunctional and 1230 cm−1 for the
bifunctional benzoxazines. Besides, the characteristic out-of-
plane absorption modes of benzene with an attached oxazine
ring are observed at 930 cm−1 for monofunctional benzoxazine
and 928 cm−1 in the case of difunctional benzoxazine.22,23 The
characteristics stretching bands of the primary amine appear at
3483, 3373, and 3320 cm−1 for the monofunctional
benzoxazine and at 3456, 3357, and 3205 cm−1 for the
bifunctional benzoxazine.
1
shows the H NMR spectrum of 6. The disappearance of the
1
amine signal (3.72 ppm) and the appearance of an amide signal
(10.30 ppm) support the reaction of amino and anhydride.
However, a small peak at 4.39 ppm was found in Figure 4. It is
known that acid can catalyze the ring-opening reaction of
benzoxazine; thus, the small peak is likely due to the methylene
from the small amount of ring-opened structure. FT-IR was
used to further confirm the structure of 6. In Figure 5, a broad
OH absorption of carboxylic acid absorption at around 3412
cm−1, a carbonyl absorption at 1721 cm−1, and the amide I
mode at 1681 cm−1 support the structure of amic acid.
Complex and broad bands centered around 3400 cm−1 are
characteristic NH and OH bands that are strongly hydrogen
bonded intramolecularly, further supporting the formation of
amic acid.
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asymmetric stretching mode) and 925 cm−1 (the out-of-plane
bending vibration of C−H) disappeared at 250 °C,
corresponding to the ring-opening of benzoxazine. Meanwhile,
new characteristic absorptions of imide at 1785 and 1385 are
observed due to the imidization of amic acid. The characteristic
peaks at 1785 cm−1 are the typical peaks for imide, which is
attributed to the imide C−C(O)−C in-phase stretching.24
The peak at 1385 cm−1 is the axial C−N stretching.25 However,
these characteristic peaks for imide gradually decrease when
heated from 300 to 400 °C, and meanwhile two peaks at 1615
cm−1 (CN stretching) and 1026 cm−1 (−O−C stretching)
gradually increase. This behavior is consistent with the
benzoxazole ring formation after the benzoxazine ring-opening
and further imidization.26,27
In order to further support the imidization and benzoxazole
formation of 6, the thermal stability of 6 has been studied by
thermogravimentric analysis (TGA) as shown in Figure 7 after
1
Figure 4. H NMR of benzoxazine amic acid (6).
Figure 7. Thermogravimetric analysis of 6 after thermal polymer-
ization at 200 °C for 1 h.
thermal treatment at 200 °C. It is well-established that the
polymerization of benzoxazines occurs through oxazine ring-
opening without producing any byproduct and the thermal
degradation of polybenzoxazines starts over 300 °C.28
However, 6 shows a weight reduction at very low temperature.
As shown from the derivative weight loss curves, the maximum
weight loss rate is around 300 °C, which is due to the
imidization before 300 °C and the cyclization to form
benzoxazole after 300 °C. Thus, both the results of FTIR
and TGA are practically identical to those reported for
polybenzoxazole synthesized through the traditional approach
of the linear poly(o-hydroxyimide) method, indicating that
thermal imidization and benzoxazole formation have been
completed.
Figure 5. FTIR spectrum of 6.
In order to qualitatively study the structural evolution of 6
during heating, FTIR analyses were carried out and the spectra
are displayed in Figure 6. As shown in Figure 6, the
characteristic absorption bands at 1236 cm−1 (C−O−C
Preparation of Poly(BZaa), cPI, and cPBO. The
preparation of poly(BZaa) was performed by reacting 5 and
BTDA with a molar ratio of 1:1 (Scheme 4). The mixture was
reacted at room temperature to form poly(BZaa), followed by a
thermal treatment to imidize and cross-link benzoxazine to
form cPI, and the further thermal treatment to form cPBO.
DSC Thermograms of Benzoxazines. It is well-known
that the ring-opening polymerization reaction of 1,3-benzxox-
azine can be monitored by DSC with an exothermic peak
around 250 °C.28 Figure 8 shows the DSC thermograms of the
TCP-protected benzoxazine monomers 2 and 4. Monomer 2
exhibits an endotherm at 194 °C due to melting and a typical
DSC benzoxazine exotherm maximum at 222 °C. However, the
melting endotherm and polymerization exotherm maximum for
Figure 6. FTIR spectra of 6 after various thermal treatments.
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Scheme 4. Preparation of Poly(BZaa), CPI, and CPBO
Figure 9. DSC thermograms of primary amine-functional benzoxazine
monomers 3 and 5.
Figure 10 shows the DSC thermograms of benzoxazine amic
acid model compound 6 and main-chain-type poly(BZaa). The
Figure 10. DSC thermograms of benzoxazine amic acid model
compound 6 and main-chain-type poly(BZaa).
broad solvent-related endothermic peaks were observed for
both 6 and poly(BZaa). To minimize oxazine ring-opening by
the acid catalysis, we avoided removing the solvent completely,
which requires heating the sample. An exothermic peak with
maxima at 210 and 223 °C can be observed for 6 and
poly(BZaa), respectively. The exothermic peaks show broader
shape compared with traditional benzoxazines, which is due to
the imidization taking place during the polymerization
behavior. It is well-known that imidization is an endothermic
behavior with the release of water, and thus, the observed
apparent exothermic peak is the sum of exothermic and
endothermic events. However, we could not calculate the
enthalpy for the ring-opening since the solvent evaporation and
imidization changed the weight of sample during the DSC
measurement.
DSC Analysis of Poly(BZaa) as a Function of Heat
Treatment. The thermally activated cross-linking behavior of
the main-chain-type poly(benzoxazine amic acid) (PBzaa) by
ring-opening was also studied by DSC. Figure 11 shows the
DSC thermograms of poly(BZaa) after thermal treatment at
150, 200, 250, and 300 °C for 1 h at each temperature. It was
found that the exothermic peak decreased after each treatment
Figure 8. DSC thermograms of TCP-protected benzoxazine
monomers 2 and 4.
difunctional monomer 4 are shifted to as high as 217 and 248
°C, respectively. Different from TCP-protected benzoxazine
monomers, the primary amine-functional benzoxazine shows
multiple thermal behaviors in the form of several exotherms as
shown in Figure 9. Two exothermic peaks were observed with
the maxima at 149, 183 °C and 151, 204 °C for monomers 3
and 5, respectively, which is in part due to the primary amine
group catalyzing the ring-opening of benzoxazine.29,30
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temperature of the E″ curve is around 308 °C for cPI, which
means even the value of Tg for cPI in this study is similar to that
of polybenzoxazoles via thermal conversion from ortho-amide-
functional benzoxazines in our previous study.14 However, the
cPBO shows no Tg before 400 °C as shown in Figure 12
(bottom), which means that this material can be applied at a
very high temperature. We did not examine the behavior at
higher temperatures to find the Tg because the degradation of
polybenzoxazine segments takes place after 400 °C. Based on
the TGA thermogram of cPBO shown in a later section, the
degradation starts mostly around 500 °C. However, the
scanning rate of the DMA (3 °C/min) is slower than the
TGA experiment (10 °C/min). Considering this, we could not
confidently examine the Tg above 400 °C.
Of particular interest is the remarkable constancy of the E′
value of 5 GPa throughout the temperature range examined up
to 400 °C. While there are many polymers that show relatively
high Tgs, one seldom find materials with nearly constant E′
throughout the temperature range examined. This is a
particularly attractive property for an application with a wide
temperature variation during the application in addition to a
constant, elevated temperature.
Figure 11. DSC thermograms of poly(BZaa) after various thermal
treatments.
temperature and practically disappeared at 250 °C, thereby
showing the completion of the ring-opening polymerization
reaction. After further thermal treatment at 300 °C, the product
of poly(BZaa) showed a Tg as high as 265 °C.
Thermal Properties of Cross-Linked cPI and cPBO. The
results of dynamic mechanical analysis (DMA) of the
thermosets are as shown in Figure 12 for the storage modulus
(E′), loss modulus (E″), and tan δ. In this work, the peak of tan
δ is used to mark the glass transition temperature (Tg). As
shown in Figure 12 (top), the Tg determined as the peak
The thermal stability of cPI and cPBO has been studied by
thermogravimetric analysis (TGA), and the result is shown in
Figure 13. In order to further confirm the imidization and
Figure 13. Thermogravimetric analysis of poly(BZaa) after thermal
polymerization at 200 °C for 1 h, cPI, and cPBO.
benzoxazole formation of benzoxazine amic acid, we also
studied the thermal stability of poly(BZaa) after the thermal
treatment at 200 °C for 1 h. In comparison, this poly(BZaa)
was further treated at 300 °C for 1 h, and its TGA thermogram
is designed as cPI in the same figure. Additionally, the cPI was
further treated at 400 °C for 1 h and designed as cPBO. The
TGA thermograms of these cPI and cPBO are also shown in
Figure 13. Poly(BZaa) shows a bimodal degradation profile in
which the maximum weight-loss rate is at 330 °C, which
corresponds to the typical imidization temperature. This lower
temperature profile appears to be heavily overlapped multiple
phenomena. cPI also shows a bimodal degradation profile in
which the maximum weight-loss rate of the first step occurs at
400 °C, which indicates the thermal cyclization to give
polybenzoxazole.
The thermal property data of cPI and cPBO are summarized
in Table 1. The 5% weight loss temperatures (Td5) are as high
as 410 and 511 °C for cPI and cPBO, respectively, as a
consequence of the imide and oxazole formation. Besides, cPI
Figure 12. Dynamic mechanical spectra of cPI (top) and cPBO
(bottom).
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dx.doi.org/10.1021/ma502297m | Macromolecules 2014, 47, 8674−8681

Macromolecules
Article
Table 1. Thermal Properties of CPI and CPBO
ACKNOWLEDGMENTS
K. Zhang gratefully acknowledges the partial financial support
of Chinese Scholarship Council (CSC) Program.
■
Tg (DSC)
(°C)
Tg (DMA)
(°C)
T5
(°C)
T10
(°C)
sample
Yc (wt %)
cPI
265
308
410
511
515
567
70
71
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■
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CONCLUSIONS
■
A new approach has been achieved to synthesize both cross-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: hxi3@cwru.edu (H.I.).
■
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/ma502297m | Macromolecules 2014, 47, 8674−8681