Steric hindrance control synthesis of primary amine-containing benzoxazines and properties of the resulting poly(benzoxazine imide) thermosetting films

Article abstract of DOI:10.1021/ma401756d

Primary amine-containing benzoxazines 3 and 6 were prepared from an unsymmetrical diamine 1, in which the amine group ortho to diethyl substituents, the hindered amine, is not reactive toward aldehyde at room temperature. Based on 6, three poly(benzoxazine imide) (PBzI) thermosets were prepared by low-temperature polymerization, followed by thermal treatment. Experimental data show that incorporating thermosetting benzoxazine into a polyimide increased the T_g, tensile modulus, dimensional stability, flame retardancy, and contact angle without sacrificing too much in thermal stability.

Full text of DOI:10.1021/ma401756d

Article
pubs.acs.org/Macromolecules
Steric Hindrance Control Synthesis of Primary Amine-Containing
Benzoxazines and Properties of the Resulting Poly(benzoxazine
imide) Thermosetting Films
Meng Wei Wang,† Ching Hsuan Lin,*,† and Tzong Yuan Juang‡
†Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan
‡Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan
S
* Supporting Information
ABSTRACT: Primary amine-containing benzoxazines 3 and 6 were prepared
from an unsymmetrical diamine 1, in which the amine group ortho to diethyl
substituents, the hindered amine, is not reactive toward aldehyde at room
temperature. Based on 6, three poly(benzoxazine imide) (PBzI) thermosets were
prepared by low-temperature polymerization, followed by thermal treatment.
Experimental data show that incorporating thermosetting benzoxazine into a
polyimide increased the Tg, tensile modulus, dimensional stability, flame
retardancy, and contact angle without sacrificing too much in thermal stability.
this approach. After curing, thermosets with thermal stability
comparable to classical polybenzoxazines can be achieved. Liu
et al. prepared a main-chain-type polybenzoxazine through the
Diels−Alder reaction of a bisfuranic benzoxazine with
bismaleimide.10 During thermal curing, a retro-Diels−Alder reac-
tion occurred, and simultaneous curing of benzoxazine and
maleimide led to a cross-linked polybenzoxazine with a Tg value
of 242 °C. Ishida et al. prepared main-chain-type polybenzox-
azines by oxidative coupling of propargyl benzoxazines and
ethynyl benzoxazines.11 Thermosets with a char yield as high as
72% can be achieved using this approach.
Since difunctional benzoxazine is prepared from the condensa-
tion of bisphenol/amine/formaldehyde or diamine/phenol/
formaldehyde, it is difficult to precisely prepare benzoxazine
with residual amino groups. Recently, Ishida et al. prepared
benzoxazine with primary amine using protection and
deprotection procedures.12 For example, they reduced the
tetrachlorophthalimide (TCP)-protected p-nitroaniline, carried
out the benzoxazine synthesis, and then deprotected the
TCP-protected benzoxazine using hydrazine hydrate at 20 °C.
They also used TCP-protected aminophenol to prepare
TCP-protected benzoxazine and deprotected the benzoxazine
using hydrazine hydrate. In addition, they used trifluoroacetic
anhydride (TFA)-protected p-aminophenol to prepare TFA-
protected benzoxazines and deprotected the benzoxazine using
sodium boron hydride. The primary amine-containing benzox-
INTRODUCTION
■
Main-chain-type polybenzoxazines are an active concept in the
recent development of benzoxazines. They can be prepared by
Mannich-type polycondensation of diamine, bisphenol, and
formaldehyde.1−5 Other than the Mannich-type polycondensa-
tion, the main-chain-type polybenzoxazines can be prepared
using other approaches. Endo et al. prepared main-chain type
polybenzoxazines based on the click reactions of a bifunctional
alkynyl compound with a benzoxazine core and a bisazide
counterpart.6 Through highly efficient click polycondensation,
the benzoxazine ring was successfully introduced into the
polymer main chain without any side reaction. Yagci et al. pre-
pared a benzoxazine-containing diol from the condensation of
bisphenol A, formaldehyde, and 2-(2-aminoethoxy)ethanol.7
High molecular weight polyetheresters containing benzoxazine
were then prepared by the polycondensation of diacid chloride
and the diol. After curing the polyetheresters, thermosets with
good thermal stability and toughness induced by the soft
etherester can be obtained. The synthetic approach opens new
pathways for preparing curable thermoplastic elastomers. Main-
chain-type polybenzoxazines based on the hydrosilylation
reaction of a diallyllic benzoxazine with 1,1,3,3-tetramethyldisi-
loxane in the presence of a Pt catalyst were developed by the
same group.8 With this approach, it is possible to prepare
benzoxazine-containing polysiloxanes without affecting the
benzoxazine moieties. Yagci et al. also prepared main-chain-
type polybenzoxazine via the Sonogashira coupling reaction
between diiodobisbenzoxazine and diacetylene bisether or
diacetylene bisbenzoxazine.9 Polybenzoxazines with two differ-
ent cross-linkable groups in the main chain can be obtained using
Received: August 22, 2013
Revised: October 20, 2013
Published: November 4, 2013
© 2013 American Chemical Society
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azine was applied to prepared poly(benzoxazine amide).13 By
combining the thermoplastic nature of polyamide and
thermosetting nature of the polybenzoxazine, they created
thermoplastic/thermoset crossover molecules that offer process-
ability, ductility, film formability, chemical resistance, and
dimensional stability.
The literature on the hybrid of polyimide and polybenzoxazine
has rarely been reported.14,15 Takeichi et al. prepared polyimide
and polybenzoxazine alloys.14 The polymer alloy films from
polyimide/benzoxazine are considered to form semi-IPN
consisting of a linear polyimide and polybenzoxazine network,
in which two components are well miscible as confirmed by
transparency and one single Tg. They found that the Tg values
remarkably increased with the content of polyimide. Poly-
benzoxazine enhanced the modulus of polyimide, while
polyimide improved the toughness of polybenzoxazine. Takeichi
et al. also prepared alloys of polybenzoxazine/poly(siloxane
imide) with or without OH groups.15 The hydroxyl poly(siloxane
imide) system formed cross-linked polymer with one Tg while
the poly(siloxane imide) system showed phase separation
and displayed two Tg. To the best of our knowledge, no
poly(benzoxazine imide) (PBzI) has been reported.
recrystallized from methanol. Phthalic anhydride (from Showa),
3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA, from
Acros), 4,4′-oxydiphthalic anhydride (ODPA, from Chriskev), and
4,4′-bisphenol A dianhydride (BPADA, from Chriskev) were recrystal-
lized from acetic anhydride. N,N-Dimethylacetamide (DMAc, from
TEDIA) was purified by distillation under reduced pressure over
calcium hydride (from Acros) and stored over molecular sieves. The
other solvents used were commercial products (HPLC grade) and were
used without any further purification.
Characterization. NMR measurements were performed using a
Varian Inova 600 NMR in DMSO-d6, and the chemical shift was cal-
ibrated by setting the chemical shift of DMSO-d6 as 2.49 ppm. IR spectra
were obtained in the standard wavenumber range of 400−4000 cm−1
using a PerkinElmer RX1 infrared spectrophotometer. Differential
scanning calorimetry (DSC) was performed using a PerkinElmer DSC 7
in a nitrogen atmosphere at a heating rate of 10 °C/min. Thermal
gravimetric analysis (TGA) was performed with a PerkinElmer Pyris1 at
a heating rate of 20 °C/min in a nitrogen or air atmosphere. Dynamic
mechanical analysis (DMA) was performed with a PerkinElmer Pyris
Diamond DMA with a sample size of 5.0 × 1.0 × 0.002 cm. The storage
modulus E′ and tan δ were determined as the sample was subjected to a
temperature scan mode at a programmed heating rate of 5 °C/min at a
frequency of 1 Hz. The test was performed using a tension mode with an
amplitude of 25 μm. Thermomechanical analysis (TMA) was performed
by an SII TMA/SS6100 at a heating rate of 5 °C/min. The coefficient of
thermal expansion (CTE) was measured in the range from 50 to 200 °C.
Flame retardancy of polyimides was performed by a UL-94VTM vertical
thin test. In that test, an 8 in. × 2 in. sample was wrapped around
a 1/2 in. mandrel and then taped on one end. The mandrel was removed,
leaving a cone-shaped sample that was relatively rigid. The two flame
applications were 3 s instead of 10 s. After the first ignition, the flame was
removed and the time for the polymer to self-extinguish (t1) was
recorded. Cotton ignition was noted if polymer dripping occurred
during the test. After cooling, the second ignition was performed on the
same sample, and the self-extinguishing time (t2) and dripping char-
acteristics were recorded. If t1 plus t2 was less than 10 s without any
dripping or the dripping does not ignite the cotton, the polymer was
considered to be a VTM-0 material. If t1 plus t2 was in the range of
10−30 s without any dripping or the dripping does not ignite the cotton,
the polymer was considered to be a VTM-1. The contact angle of the
polymer sample was measured at 25 °C using a FTA100B contact angle
goniometer interfaced with image-capture software by injecting a 5 μL
liquid drop. To obtain reliable contact data, at least three droplets were
dispensed in different regions of the same piece of film, and at least two
pieces of film were used to obtain reliable contact angle data. Thus, at
least six contact angles were averaged for each type of film. The tensile
testing was performed by EZ-SX with sample dimensions of 20 mm ×
5 mm × 50 μm at 25 °C.
Synthesis of 2. 1 (5.0 g, 0.010 mol), 2-hydroxybenzaldehyde
(1.3919 g, 0.011 mol), and DMAc (50 mL) were introduced into a
250 mL glass flask equipped with a condenser, a hydrogen balloon, and a
magnetic stirrer. The mixture was reacted at 25 °C for 12 h. NaBH4
(0.6272 g, 0.017 mol) was added. The reaction mixture was further
stirred at room temperature for 12 h. The mixture was then poured into
water. The precipitate was filtered and dried in a vacuum oven at 105 °C.
A yellow powder (4.8 g, 80% yield) with a melting point of 199 °C
(DSC) and melting enthalpy of 81.7 J/g was obtained. 1H NMR
(DMSO-d6), δ = 0.99 (6H, H2), 1.55 (3H, H20), 2.31 (4H, H3), 4.17
(2H, H25), 4.41 (2H, NH2), 5.96 (1H, NH), 6.41 (2H, H23), 6.77 (1H,
H28), 6.81 (2H, H5), 6.83 (1H, H30), 7.01 (2H, H22), 7.06 (2H, H8, H29),
7.10 (2H, H10, H14), 7.18 (1H, H27), 7.27 (1H, H15), 7.31 (1H, H9), 7.63
(1H, H16), 7.88 (1H, H11), 8.00 (1H, H17), 9.48 (1H, OH). FTIR (KBr):
1191 cm−1 (C−N stretch), 3223 cm−1 (OH stretch), 3394 cm−1 (NH
stretch).
Loustalot et al.16 studied the copolymerization of bismalei-
mide and aromatic diamine with/without dialkyl group ortho to
the amino group. They found the reaction onset temperature of
bismaleimide and aromatic diamine increased with the increase
of steric hindrance created by the alky group. Duffy et al.17,18
studied the effect of steric hindrance on the reaction of silicon-
containing aliphatic amines with phenylglycidyl ether and found
that steric hindrance produced a great change in the reaction rate.
In our previous work,19 we prepared a series of polyimides based
on two unsymmetrical diamines: 1-(4-aminophenyl)-1-(3,5-
dimethyl-4-aminophenyl)-1-(6-oxido-6H-dibenz[c,e][1,2]-
oxaphosphorin-6-yl)ethane (1′) and 1-(4-aminophenyl)-1-(3,5-
diethyl-4-aminophenyl)-1-(6-oxido-6H-dibenz[c,e][1,2]-
oxaphosphorin-6-yl)ethane (1). In the preliminary experiment,
we took advantage of the concept of steric hindrance to precisely
prepare primary amine-containing benzoxazine by the con-
densation of diamine (1′ or 1)/phenol/formaldehyde with a
molar ratio of 1:1:2. We found that a high-purity primary amine-
containing benzoxazine could be prepared from the condensa-
tion of 1/phenol/formaldehyde but failed using the condensa-
tion of 1′/phenol/formaldehyde. This result suggests the steric
hindrance induced by the diethyl groups in 1 is large enough to
prevent the reaction of hindered amino groups with aldehyde.
On the basis of this result, we prepared primary amine-containing
benzoxazines 3 and 6 from the unsymmetric diamine 1, in which
the hindered amino group is stable toward aldehyde. According
to our previous work,19 the hindered amino groups are reactive
toward anhydride, so we prepared three PBzIs based on 6 and
three dianhydrides. After thermal treatment, PBzI thermosets
can be obtained. The detailed synthetic strategy and the effect of
oxazine on the mechanical, flame-retardant, dimensional, contact
angle, and thermal properties of polyimides were discussed.
EXPERIMENTAL SECTION
■
Materials. Unsymmetric diamines 1 and 1′ were prepared according
to our previous work.19 s-Trioxane (from Acros), 2-hydroxybenzalde-
hyde (from Showa), paraformaldehyde (from TCI), and sodium
borohydride (NaBH4, from Acros) were used as received. 5,5′-
Methylenebis(2-hydroxybenzaldehyde) (4) was prepared from
2-hydroxybenzodehyde and s-trioxane in the presence of sulfuric
acid.20 4,4′-Diaminodiphenyl ether (ODA, from Chriskev) was
Synthesis of 3. 2 (5.0 g, 0.008 mol), paraformaldehyde (0.2806 g,
0.009 mol), and chloroform (30 mL) were introduced into a 100 mL
glass flask equipped with a nitrogen inlet, a condenser, and a magnetic
stirrer. The mixture was reacted at 60 °C for 12 h. The mixture was then
poured into n-hexane. The precipitate was filtered and dried in a vacuum
oven at 60 °C. A white powder (3.6 g, 70% yield) with a melting point of
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Synthesis of 6. 5 (5.0 g, 0.004 mol), paraformaldehyde (0.2777 g,
0.009 mol), and chloroform (30 mL) were introduced into a 100 mL
glass flask equipped with a nitrogen inlet, a condenser, and a magnetic
stirrer. The mixture was reacted at 60 °C for 12 h. The mixture was then
184 °C (DSC) and an exothermic peak temperature of 245 °C and
1
exothermic enthalpy of 57.8 J/g was obtained. H NMR (DMSO-d6),
δ = 0.94 (6H, H2), 1.66 (3H, H20), 2.26 (4H, H3), 4.42 (2H, NH2), 4.61
(2H, H25), 5.40 (2H, H32), 6.74 (2H, H5), 6.78 (1H, H30), 6.89 (1H,
H28), 6.93 (2H, H23), 6.95 (1H, H10), 7.00 (1H, H8), 7.12 (2H, H27,
H29), 7.18 (2H, H22), 7.23 (3H, H9, H14, H15), 7.61 (1H, H16), 7.76 (1H,
H11), 7.95 (1H, H17). 13C NMR (DMSO-d6), δ = 13.10 (C2), 23.82
(C3), 24.41 (C20), 48.72 (C25), 52.01 (C19), 78.17 (C32), 115.98 (C23),
116.22 (C30), 118.70 (C8), 120.40 (C28), 120.71 (C12), 121.33 (C13),
122.54 (C18), 123.11 (C17), 123.45 (C10), 125.04 (C11), 125.68 (C26),
126.05 (C5), 127.18 (C27), 127.60 (C29), 127.67 (C9), 128.56 (C4),
130.13 (C22), 130.24 (C15), 131.72 (C14), 131.77 (C6), 133.06 (C16),
136.13 (C21), 141.35 (C1), 146.25 (C24), 150.78 (C7), 153.97 (C31).
FTIR (KBr): 970 cm−1 (N−C−O stretch), 1034 cm−1 (Ar−O−C
symmetric stretch), 1297 cm−1 (Ar−O−C asymmetric stretch),
1370 cm−1 (C−N stretch), 3351, 3448 cm−1 (N−H stretch).
poured into n-hexane. The precipitate was filtered and dried in a vacuum
oven at 60 °C. A white powder 3.4 g (70% yield) with a melting point
of 120 °C (DSC), an exothermic peak temperature of 255 °C, and exo-
1
thermic enthalpy of 147 J/g was obtained. H NMR (DMSO-d6), δ =
0.94 (12H, H2), 1.63 (6H, H20), 2.26 (8H, H3), 3.77 (2H, H33), 4.42
(4H, NH2), 4.52 (4H, H25), 5.33 (4H, H32), 6.69 (2H, H30), 6.74 (4H,
H5), 6.88 (4H, H23), 6.91 (2H, H10), 6.97 (6H, H8, H27, H29), 7.13 (4H,
H22), 7.19 (6H, H9, H14, H15), 7.58 (2H, H16), 7.73 (2H, H11), 7.93 (2H,
H17). 13C NMR (DMSO-d6), δ = 13.12 (C2), 23.82 (C3), 24.38 (C20),
48.85 (C25), 52.01 (C19), 77.96 (C32), 115.86 (C23), 116.16 (C30),
118.71 (C8), 120.69 (C12), 121.10 (C13), 122.56 (C18), 123.04 (C17),
123.44 (C10), 125.00 (C11), 125.69 (C26), 126.00 (C5), 127.02 (C27),
127.61 (C9), 127.84 (C29), 128.62 (C4), 130.15 (C22), 130.21 (C15),
131.51 (C14), 131.73 (C6), 133.03 (C16), 136.56 (C28), 136.14 (C21),
141.34 (C1), 146.26 (C24), 150.76 (C7), 152.16 (C31). FTIR (KBr):
950 cm−1 (N−C−O stretch), 1044 cm−1 (Ar−O−C symmetric stretch),
1228 cm−1 (Ar−O−C asymmetric stretch), 1374 cm−1 (C−N stretch),
3368 and 3474 cm−1 (N−H stretch).
Synthesis of 7. 3 (1.0 g, 1.66 mmol) and DMAc (2.68 g) were
introduced into a 100 mL glass flask equipped with a nitrogen inlet, a
condenser, and a magnetic stirrer. After 3 had dissolved completely,
pathalic anhydride (0.2466 g, 1.66 mmol) was added. The reaction was
carried out at 3−5 °C for 4 h. The mixture was then poured into water.
The precipitate was filtered and dried in a vacuum oven at 60 °C. A light
yellow powder 0.99 g (80% yield) was obtained.
Preparation of PBzIs. Experimental data show that poly-
(benzoxazine amic acid) based on 6 and dianhydrides (BTDA,
ODPA, and BPADA) are viscous, but the film became brittle after
thermal treatment, probably due to too high a cross-linking density.
Therefore, 6/ODA/dianhydride with a molar ratio of 0.1−0.3:0.9−0.7:1
was applied to prepare PBzI. The synthesis of PBzI was exemplified
by the synthesis of O-3 (the composition of polymer code is listed in
Table 1) from the condensation of 6, ODA, and ODPA. To a 100 mL
three-neck round-bottom flask equipped with a magnetic stirrer and a
nitrogen inlet, 6 (0.30 g, 0.25 mmol), ODA (0.1155 g, 0.58 mmol), and
DMAc (2.68 g) were added. After the monomers had dissolved
completely, ODPA (0.2557 g, 0.82 mmol) was added. The reaction was
carried out at 3−5 °C for 12 h to obtain the O-3 precursor. Then, the
viscous poly(amic acid) solution of O-3 precursor was cast on glass by an
automatic film applicator and dried overnight at 60 °C, 100 °C (1 h),
Synthesis of 5. 1 (5.0 g, 0.010 mol), 4 (1.3276 g, 0.005 mol), and
DMAc (50 mL) were introduced into a 250 mL glass flask equipped with
a condenser, a hydrogen balloon, and a magnetic stirrer. The mixture
was reacted at 25 °C for 12 h. NaBH4 (0.4312 g, 0.011 mol) was added.
The reaction mixture was further stirred at room temperature for 12 h.
The mixture was then poured into water. The precipitate was filtered
and dried in a vacuum oven at 105 °C. A yellow powder (4.6 g, 75%
1
yield) was obtained. H NMR (DMSO-d6), δ = 0.99 (12H, H2), 1.55
(6H, H20), 2.31 (8H, H3), 3.70 (2H, H32), 4.17 (4H, H25), 4.41 (4H,
NH2), 5.90 (2H, NH), 6.41−8.00 (34H, Ar−H), 9.30 (2H, OH). FTIR
(KBr): 1198 cm−1 (C−N stretch), 3223 cm−1 (OH stretch), 3384 cm−1
(NH stretch).
Table 1. Sample ID, Composition, and Mechanical Properties of PBzI Thermosets
sample ID
anhydride
6:ODA
film quality
tensile strength (MPa)
elongation at break (%)
initial modulus (GPa)
O-0
ODPA
ODPA
ODPA
ODPA
BTDA
BTDA
BTDA
BTDA
BPADA
BPADA
BPADA
BPADA
0:1
1:9
2:8
3:7
0:1
1:9
2:8
3:7
0:1
1:9
2:8
3:7
flexible
flexible
flexible
flexible
flexible
flexible
flexible
flexible
flexible
flexible
flexible
flexible
118
111
110
103
111
112
110
105
85
8
5
4
3
8
7
4
3
9
6
6
5
1.7
2.6
3.2
3.9
1.7
2.2
2.4
3.3
1.4
1.9
2.2
2.7
O-1
O-2
O-3
BT-0
BT-1
BT-2
BT-3
BPA-0
BPA-1
BPA-2
BPA-3
93
104
113
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Scheme 1. Synthesis of Benzoxazines 3′ and 6′ via Traditional One-Pot Mannich Condensation
Scheme 2. Synthesis of 3 by a Two-Pot Procedure
Figure 1. 1H NMR spectrum of 3 in DMSO-d6.
200 °C (1 h), and 300 °C (1 h), respectively. The other PBzIs were
similarly prepared.
of 3′. Run 1−run 8 failed to prepare 3′ with reasonable purity.
Benzoxazine 3′ prepared in run 9 displayed the best purity, as its 1H
NMR spectrum shown in Figure S1. On the basis of the reaction
condition in run 9, we prepared 6′ by the traditional Mannich
condensation of 1, bisphenol A, and formaldehyde. Figure S2
RESULTS AND DISCUSSION
■
Synthesis of 3′ and 6′ by a One-Pot Procedure.
1
shows the H NMR of 6′ prepared by the reaction condition in
Benzoxazine 3′ was initially synthesized by a traditional one-pot
Mannich condensation of 1, phenol, and formaldehyde (Scheme 1).
Table S1 lists the effect of reaction conditions on the synthesis
run 9, Table S1. Benzoxazine 6′with some impurity was formed but
could not meet the requirement for polycondensation.
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Scheme 3. Synthesis of 6 Prepared by the Two-Pot Procedure
Figure 2. 1H NMR spectra of 6 in DMSO-d6.
1
Synthesis of 3 by a Two-Pot Procedure. In order to
prepare 3 with high purity, we prepared 3 using a two-pot
procedure (Scheme 2).21 In the first pot, 1 reacted with 2-
hydroxybenzaldehyde, yielding an intermediate with imine
linkage. Sodium borohydride was then employed to reduce the
imine linkage, yielding 2. In the second pot, paraformaldehyde
was added to the chloroform solution of 2 to induce ring closure
condensation. Figure S3 shows the 1H NMR spectra of 1 and 2.
From the spectrum of 2, the signal of amino (4.41 ppm) ortho to
the diethyl groups was well preserved, suggesting the stability of
hindered amino toward aldehyde. The signals of phenolic OH
(9.48), CH2NH (4.17), and CH2NH (5.96) support the reaction
of 2-hydroxylbenzaldehyde with the unhindered amino group.
Figure 1 shows the H NMR spectrum of 3. The characteristic
peaks of benzoxazine, O−CH2−N (5.40) and Ph−CH2−N
(4.61) were clearly observed. Figures S4−S6 show the 13C,
1H−1H COSY, and HETERCOR of 3, further supporting the
structure of 3.
Synthesis of 6 by a Two-Pot Procedure. Benzoxazine 6
was prepared by a two-pot procedure similar to 3 except the 2-
hydroxylbenzaldehyde was replaced by 4 (Scheme 3). Figure S7
shows the 1H NMR spectrum of 5. The signal of hindered amino
at 4.41 ppm was well preserved while the signals of phenolic OH
(9.48), CH2NH (4.17), and CH2NH (5.90) support the reaction
of unhindered amino with 4. Figure 2 shows the 1H NMR spec-
trum of 6. The characteristic peaks of benzoxazine, O−CH2−N
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Scheme 4. Synthesis and Thermal Treatment of 7
Figure 3. DSC of thermograms of 3 and 7.
Figure 4. DSC of thermograms of 6, O-3, and O-2 precursors.
(5.33) and Ph−CH2−N (4.52) were clearly observed, supporting
the structure of 6. Figures S8−S10 show the 13C, 1H−1H COSY,
and HETERCOR of 6, further supporting the structure of 6.
Synthesis and Thermal Treatment of 7. Model reaction
between phthalic anhydride and 3 was performed to obtain 7
(Scheme 4). Figure S11 shows the 1H NMR spectra of 3 and 7.
To avoid ring-opening of benzoxazine, 7 was not completely dry,
so solvent peaks were observed in Figure S11. The disappearance
of the amino signal (4.4 ppm) and appearance of an amide
signal (9.2 ppm) support the reaction of amino and anhydride.
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Scheme 5. Synthesis of PBzI Thermosets
Figure 5. Photos of O-series PBzI thermosetting films.
The characteristic peaks (4.6 and 5.4 ppm) of benzoxazine
remained while methylene (4.2 ppm) signal resulting from the
ring-opening of benzoxazine was found. It is known that acid
catalyzed the ring-opening of benzoxazine,22 so 7 with a partial
responsible for the poor solubility. IR spectra were applied to
monitor the imidization of amic acid to imide. Figure S13 shows
IR spectra of 7 after accumulative thermal treatment at each
temperature for 20 min. The carbonyl absorption at 1651 cm−1
and a broad carboxylic acid absorption at around 3387 cm−1 at
room temperature support the structure of amic acid. The
absorption disappeared gradually after thermal treatment. New
characteristic absorptions of imide at 1780 and 1718 cm−1 and
1376 cm−1 for C−N−C appear at temperatures higher than
140 °C, indicating that the imidization of amic acid starts at this
temperature.
1
ring-opened structure was obtained. Figure S12 show the H
NMR spectra of 7 after accumulative thermal treatment at each
temperature for 1 h. The characteristic peaks of benzoxazine at
4.6 and 5.4 ppm reduced gradually after 140 °C and disappeared
completely after 180 °C. The signals marked as (a)−(c) were
signals from ring-opened benzoxazine structure.23 The solubility
of 7 after 260 °C thermal treatment was poor, so its NMR
spectrum was not reliable. The cross-linking and the hydrogen
bonding resulting from the phenolic structure might be
DSC Thermograms of Benzoxazines. Figure 3 shows DSC
thermograms of 3 and 7. Benzoxazine 3 displays an exothermic
8859
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Figure 6. Stress−strain curves of PBzI thermosets.
peak at 245 °C, and the corresponding exothermic enthalpy
is 150 J/g. Benzoxazine 7 shows an exothermic peak with an
enthalpy of 45 J/g. The smaller enthalpy is probably due to the
existence of partial ring-opened structure, as supported by the
signal of ring-opened structure in Figure S11.
Figure 4 shows DSC thermograms of 6, O-3, and O-2
precursors after vacuum drying at 60 °C for 12 h. 6 exhibits an
exothermic peak with an enthalpy of 69 J/g. To observe the
exotherms of O-3 and O-2 precursors in the DSC scan, O-3 and
O-2 precursors were not dried at temperature higher than 60 °C.
The broad endothermic peak of O-3 and O-2 precursors at 100−
180 °C is due to the evaporation of solvent. An exothermic peak
with enthalpy of 28 and 19 J/g was observed for O-3 and O-2
precursors, respectively. Based on model reaction, it is known
that O-3 and O-2 precursors contained a partial ring-opened
structure, which explains the smaller the exothermic enthalpy of
O-3 and O-2 precursors than 6.
Figure 7. DMA thermograms of PBzI thermosetting films.
Synthesis of PBzI. The preparation of PBzI was performed
by reacting 6/ODA/dianhydride with a molar ratio of 0.1−
0.3:0.9−0.7:1 by a two-step procedure (Scheme 5 and Table 1).
The mixture was reacted at 3−5 °C to form poly(benzoxazine
amic acid), followed by a thermal treatment. As suggested by the
model reaction of 7, ring-opening of benzoxazine and thermal
imidization of amic acid occurred after thermal treatment,
leading to PBzI thermosets.
Properties of PBzI Thermosetting Films. Film Quality
and Mechanical Properties. Figure 5 shows the photos of O-
series PBzI thermosetting films. The others are shown in Figures
S14 and S15. As shown in the figures, all the PBzI thermosets are
flexible and bendable. Figure 6 shows the stress−strain curves of
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Table 2. Thermal Properties of PBzI Thermosets
Tg (°C)
a
b
c
d
sample ID
DMA
DSC
TMA
CTE (ppm/°C)
O-0
301
320
348
349
292
316
340
349
233
255
285
305
294
310
325
325
279
300
316
329
218
236
262
284
280
298
305
306
269
291
302
304
214
233
257
273
43
46
32
24
47
48
45
41
59
54
53
48
O-1
O-2
O-3
BT-0
BT-1
BT-2
BT-3
BPA-0
BPA-1
BPA-2
BPA-3
a
Measured by DMA at heating rate of 5 °C/min; Tg was determined
b
from the peak temperature of the tan δ curve. From the second DSC
heating scan at a heating rate of 20 °C/min. Measured by TMA at a
heating rate of 5 °C/min. Coefficient of thermal expansion are
c
d
recorded from 50 to 200 °C.
Table 3. Thermal Stability Data of PBzI Thermosetting Films
a
b
Td5 (°C)
Td10 (°C)
c
sample
N2
air
N2
air
char yield (%)
O-0
546
460
449
449
557
477
464
445
536
503
470
460
588
512
477
459
580
485
488
474
526
503
476
460
588
523
490
487
590
536
505
480
555
538
510
492
627
605
542
515
617
571
566
542
554
553
517
504
59
57
59
58
59
58
60
57
48
51
51
52
O-1
O-2
O-3
BT-0
BT-1
BT-2
BT-3
BPA-0
BPA-1
BPA-2
BPA-3
a
Temperature corresponding to 5% weight loss by thermogravimetry
b
at a heating rate of 20 °C/min. Temperature corresponding to 10%
weight loss by thermogravimetry at a heating rate of 20 °C/min.
c
Residual weight % at 800 °C.
PBzI thermosetting films, and the data of mechanical properties
are listed in Table 1. Incorporating curable benzoxazine reduced
the elongation at break but increased the initial modulus. For
example, elongation at break reduced from 8% for BT-0 to 3% for
BT-3. The tensile strength of the O and BT series was reduced
gradually with the content of 6, while that of the BPA series was
increased gradually with the content of 6. Incorporating 6 not
only can reduce intermolecular interaction because of the bulky
structure of biphenylene phosphinate but also can increase
rigidity due to the cross-linking effect. It is thought that the factor
of reducing intermolecular interaction dominates in the O and
BT series since ODPA and BTDA are rigid dianhydrides. In
contrast, the factor of cross-linking dominates in the BPA series
since BPADA is a flexible dianhydride.
Thermal Properties. The dynamic and thermal mechanical
properties of PBzI thermosets were evaluated by DMA and
TMA, respectively. Figure 7 shows the DMA thermograms of
PBIz thermosetting films, and the data are summarized in Table 2.
The Tg value obtained from the peak temperature of tan δ
increased with the content of 6 (Table 2). For example, the Tg
Figure 8. TMA curves of PBzI thermosetting films.
value increased from 233 °C for BPA-0 to 305 °C for BPA-3. In
addition, the height of tan δ decreased with the content of 6,
demonstrating the power of cross-linking in enhancing the
rigidity. The same trend was observed in DSC measurement, as
listed in Table 2. Figure 8 shows TMA curves of PBIz
thermosetting films. Tg values measured by TMA had the same
trend with those measure by DMA. The coefficient of thermal
expansion (CTE) decreased with the content of 6 (Table 2),
demonstrating the advantage of cross-linking in dimensional
stability. For example, CTE was reduced from 43 ppm/°C for
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Table 4. UL-94 VTM Data of PBzI Thermosets
P
1st burning
time (s)
2nd burning
time (s)
UL-94
grade
sample (wt %)
dripping
O-0
0
4.5
1.4
0.8
0.7
4.3
1.6
1.0
0.8
4.8
2.0
1.2
0.8
2.0
0.5
0.4
0.2
1.7
0.8
0.6
0.3
2.2
1.2
0.8
0.3
no
no
no
no
no
no
no
no
no
no
no
no
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
VTM-0
O-1
1.08
1.83
2.39
0
O-2
O-3
BT-0
BT-1
BT-2
BT-3
BPA-0
BPA-1
BPA-2
BPA-3
1.05
1.80
2.35
0
0.79
1.40
1.88
Figure 10. Contact angle of BT-series PBzI thermosetting films.
thermal stability of PBzI thermosets. As to the flame retardancy,
according to the UL-94VTM vertical thin test, the t1 + t2 of PBzI
thermosets is much smaller than that of neat polyimide and is less
than 4 s for all PBzI thermosetting films (Table 4), so they belong
to the VTM-0 grade. The phosphorus element is thought to be
responsible for the enhanced flame retardancy.
Contact Angle. Figure 10 shows the contact angle of
water for BT-series PBzI thermosets. The photos of other PBzI
thermosets are shown in Figures S17 and 18. The contact angle
increases slightly with the content of 6. Polybenzoxazine has
been reported as a material with a high contact angle,24−26 which
explains the increase of contact angle with the content of 6. In
addition, the hydrophobic diethyl group might be responsible for
the increased contact angle.
Figure 9. TGA thermograms of PBzI thermosetting films in a N2
atmosphere.
CONCLUSIONS
■
O-0 to 24 ppm/°C for O-3. The value of 24 ppm/°C is relatively
low for a polymer without biaxial orientation.
We have successfully prepared a primary amine-containing
benzoxazine 6 from an unsymmetrical diamine 1, in which the
hindered amino group is stable toward aldehyde at room tem-
perature. The advantage of this steric hindrance control synthesis
is that no protection and deprotection procedures are required.
Three series of PBzI thermosetting films based on 6/ODA/
dianhydride were prepared since the hindered amino group is
reactive toward anhydride. Incorporating thermosetting benzox-
azine increased the Tg, dimensional stability, flame retardancy,
Thermal Stability and Flame Retardancy. TGA thermo-
grams of PBzI thermosets in a N2 atmosphere and air atmosphere
are shown in Figure 9 and Figure S16, respectively. The 5 wt %
degradation temperatures decrease with the content of 6.
However, the lowest 5 wt % degradation temperature is still as
high as 445 °C in a nitrogen atmosphere and 460 °C in air atmo-
sphere (Table 3). The data demonstrate the moderate-to-high
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(25) Liao, C.-S.; Wu, J.-S.; Wang, C.-F.; Chang, F.-C. Macromol. Rapid
Commun. 2008, 29 (1), 52−56.
(26) Lin, C. H.; Chang, S. L.; Shen, T. Y.; Shih, Y. S.; Lin, H. T.; Wang,
C. F. Polym. Chem. 2012, 3 (4), 935−945.
and contact angle while maintaining moderate-to-high thermal
stability and mechanical properties.
ASSOCIATED CONTENT
* Supporting Information
Table S1 and Figures S1−S18. This material is available free of
■
S
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: linch@nchu.edu.tw (C.H.L.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the National Science Council of the Republic
of China for financial support.
■
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