Poly(benzoxazine-co-urethane)s: A new concept for phenolic/urethane copolymers via one-pot method

Article abstract of DOI:10.1016/j.polymer.2010.11.052

Historically, applications for traditional phenolic resin/polyurethane materials are limited due to the inherently weak thermal stability of urethane-phenolic linkage and slow reaction rate. A novel concept has been developed to produce phenolic resin/pol

Full text of DOI:10.1016/j.polymer.2010.11.052

Polymer 52 (2011) 307e317
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Polymer
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Poly(benzoxazine-co-urethane)s: A new concept for phenolic/urethane
copolymers via one-pot method
,
,
b
a,b
Mohamed Baqar a 1, Tarek Agag b , Hatsuo Ishida , Syed Qutubuddin
*
a Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA
b Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Historically, applications for traditional phenolic resin/polyurethane materials are limited due to the
inherently weak thermal stability of urethane-phenolic linkage and slow reaction rate. A novel concept
has been developed to produce phenolic resin/polyurethane copolymers via benzoxazine chemistry.
Received 13 September 2010
Received in revised form
24 November 2010
Accepted 27 November 2010
Available online 4 December 2010
Through one-pot synthesis,
a series of linear poly(benzoxazine-co-urethane) materials has been
synthesized via the reaction of a newly developed dimethylol functional benzoxazine monomer with
4,40-methylene diphenyl diisocyanate and poly(1,4-butyleneadipate). The structure of the copolymers
has been characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic reso-
nance spectroscopy (NMR). The copolymers in the film forms have been further thermally treated for
crosslinking to produce crosslinked poly(benzoxazine-co-urethane) via the ring opening polymerization
of cyclic benzoxazine moieties in the main-chain. The tensile properties of the films have been studied
and compared with those of traditional high performance materials. The thermal properties of the
crosslinked copolymers have also been studied by dynamic mechanical analysis, and thermogravimetric
analysis (TGA).
Keywords:
Hydroxymethyl-benzoxazine
Polyurethane
Copolymers
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
reported approaches are based on copolymerization or blending
with other polymers of high thermal stability such as polyimides
Polyurethanes (PUs) represent one of the most versatile classes
of polymers due to their advantages, which include excellent low
temperature flexibility, superb oil and abrasion resistance, and
extraordinary easy processability [1e3]. PUs are segmented poly-
mers that consist of soft segments derived from polyols and hard
segments derived from isocyanate and chain extenders. The hard
segments offer stiffness to the resulting materials, whereas the soft
segments act as flexible connectors between the hard segments.
PUs also have a wide variety of properties, ranging from elastic to
plastic behavior, which make them suitable for a broad number of
applications, such as coatings and paints, flexible foams, insu-
lations, adhesives, and sealants. Nonetheless, the use of PUs is
associated with some disadvantages, including high water up-take,
poor resistance to polar solvents, and poor thermal stability. They
also undergo dissociation of urethane linkages at elevated
temperatures [4]. As a result, tremendous research efforts have
been reported to improve the thermal stability of PUs. Most of the
[5e8], polyurea [9,10], polyamide [11,12], epoxy [13], and poly-
diacetylene [14].
Polybenzoxazines, as a newly developed class of high perfor-
mance polymers, have attracted much interest of academia and
industries. They are characterized by many useful properties, such
as low moisture absorption, excellent mechanical properties, self-
extinguishing properties, and excellent dimensional stability
[15e18]. They also offer a remarkable flexibility in molecular design
of monomers and, consequently, a versatile performance as poly-
mers [19e21]. Benzoxazine monomers are polymerized by the
thermally activated cationic ring opening polymerization of ben-
zoxazine without any added initiators, catalysts, or by-product
formation [22e26]. The potential application areas of poly-
benzoxazines include coatings, electronic packaging materials,
printed circuit boards, frictional materials, catalysis, and matrices
for composite materials.
Due to the excellent thermal and mechanical properties of
polybenzoxazines, they have been incorporated into PUs aiming
at improving their thermal and mechanical properties. The flow
chart shown in Fig. 1 represents all reported routes for preparing
polybenzoxazine/PU materials. The concept adapted in all the
previous studies is based on blending of either preformed
* Corresponding author. Tel.: þ1 20 40 3326357; fax: þ1 20 40 3350804.
E-mail address: taa16@cwru.edu (T. Agag).
1
On leave from Nasser International University-Libya
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.11.052

308
M. Baqar et al. / Polymer 52 (2011) 307e317
Fig. 1. Previous approaches of polybenzoxazine/PU materials.
benzoxazine monomers or main-chain benzoxazine polymers as
polybenzoxazine precursors, with NCO-terminated PUs followed by
thermal treatment. As a result, the phenolic groups produced upon
ring opening polymerization of benzoxazine react with the NCO
groups of PU prepolymer, leading to the formation of phenolic-
urethane linkage.
terminated PU prepolymer by end-capping of NCO-terminated PU
prepolymer with bisphenol A, which was then mixed with BA-a to
form polybenzoxazine/PU materials [36].
Despite the known slow kinetics of isocyanate reacting with
phenolic compounds and the poor thermal stability of the urethane
linkages produced, most of the aforementioned methods of
synthesizing polybenzoxazine/PU materials used similar concept of
reacting NCO-terminated PU prepolymers with the phenolic groups
of polybenzoxazine [27e36]. It is known that the nature of the
group attached to a urethane linkage controls its thermal stability.
In general, the thermal stability increases in the presence of elec-
tron donating groups and decreases in the presence of electron
withdrawing groups in the following order; aryl-NHCOO-
aryl < alkyl-NHCOO-aryl < aryl-NHCOO-alkyl < alkyl-NHCOO-alkyl
[4,37,38]. Thus, urethane-phenolic linkage produced through the
previous approaches is considered to have lowest thermal stability.
As a result of these disadvantages, phenolic/PU materials have
historically limited applications.
Thus, it is believed that the use of an aliphatic hydroxyl-func-
tional benzoxazine for urethane linkage formation instead of
a phenolic hydroxyl group will lead to polybenzoxazine/PU mate-
rials of fundamentally different structure and properties than the
previously reported approaches. This new approach preserves the
oxazine ring upon main-chain polymer formation, which can then
be used to further crosslink the copolymer to produce poly-
benzoxazine/PU network that contains free phenolic moieties.
Preservation of the phenolic moieties will maintain the well-known
advantages of polybenzoxazines by allowing the formation of intra-
molecular 6-membered ring hydrogen bonding that offers poly-
benzoxazines the unique characteristics [22,23]. In the current
study, a newly developed hydroxymethyl functional benzoxazine
The first study of polybenzoxazine/PU systems was reported by
Takeichi et al. [27,28]. They blended bisphenol A/aniline-type
benzoxazine monomer (BA-a) and phenol/aniline-type monomer
(P-a) with NCO-terminated PU prepolymer. The results of thermal
and mechanical properties indicated that polybenzoxazine/PU
materials with elastomeric properties can be obtained by adding ca.
10e15% of polybenzoxazine, whereas films with greater plasticity
can be produced by increasing the polybenzoxazine content. They
also further improved the tensile modulus by adding organoclay to
produce polybenzoxazine/PU nanocomposites [29]. Wang et al. [30]
prepared interpenetrating polymer networks (IPNs) of poly-
benzoxazine/PU system. Their morphological studies indicated that
the copolymer networks exhibit only physical bonding with slight
phase separation behavior. Rimdusit et al. investigated various
aspects of polybenzoxazine/PU systems. They found that a system
with 10 wt.% PU content exhibited improved flexural strength
without deterioration of the thermal degradation temperature [31].
The char yield was slightly increased by using a polyol of higher
molecular weight [32]. They also studied the effect of various
isocyanates on the properties of a polybenzoxazine/PU system and
the reinforcement of the system by carbon fiber [33]. Yeganeh et al.
studied various polybenzoxazine/PU systems for electrical insu-
lation applications by blending BA-a monomer and main-chain
benzoxazine polymer (BA-ddm) with epoxy-terminated PU pre-
polymer [34,35]. They have also recently prepared phenolic-

M. Baqar et al. / Polymer 52 (2011) 307e317
309
monomer has been used to produce poly(benzoxazine-co-urethane)
s as a main-chain type benzoxazine polymers. The reaction of
aliphatic hydroxymethyl functionalities with isocyanate groups is
expected to produce thermally stable urethane linkages. Applying
this concept of one-pot method will also lead to avoid the draw-
backs associated with the use of NCO-terminated PU prepolymer,
such as poor shelf life due to their high sensitivity towards moisture
and susceptibility to undesirable side reactions. Furthermore, the
unique advantage of the recently developed concept of thermo-
plastic-thermosetting crossover of main-chain type benzoxazine
polymers [39,40] will be highly beneficial in designing varieties of
poly(benzoxazine-co-urethane)s with unique characteristics in
terms of processability and performance. The preparation, charac-
terization, crosslinking and properties of this new class of copoly-
mers are discussed in the current study.
2 mmol) in a 50 mL dry three neck flask in extra dry DMF
(<50 ppm, 20 mL) containing dibutyltindilaurate (0.02 g) under
nitrogen atmosphere. The mixture was heated at 70 ꢀC for 30 min
to afford a yellow solution. The solution was then cooled and
precipitated in cold water, washed by ethanol, and dried in
a vacuum oven at room temperature for 24 h to afford a white
powder, (Yield: 84%).
IR spectra (KBr, cmꢁ1): 1224 (asymmetric stretching of
AreOeC), 1037 (asymmetric stretching of CeOeC), 950 (out-of-
plane vibration, benzene ring to which oxazine is attached).
1H NMR spectra (DMSO-d6, ppm:
d, frequency: 600 MHz): 4.59
(s, CeCH2eNe), 4.98 (s, eCH2eO), 5.38 (s, NeCH2eOe), 9.67 (s,
eNH).
13C NMR spectra (DMSO-d6, ppm,
d): 49.15 (CeCH2eNe), 65.62
(s, eCH2eOH), 79.03 (NeCH2eO).
2. Experimental
2.4. Synthesis of poly(benzoxazine-co-urethane)s
2.1. Materials
The following is an example of the synthesis of poly(benzox-
azine-co-urethane)s with
a 1:0.8:0.2 M ratio of MDI:4HBA-
4,40-Methylenediphenyldiisocyanate (MDI) (>99%), 4,40-dia-
minodiphenylmethane (DDM) (98%), and paraformaldehyde (96%)
were obtained from Acros Organics USA and used as received.
4-Hydroxybenzyl alcohol (4HBA) (98%) was purchased from Sigma
Aldrich. Poly(1,4-butylene adipate) (PBA) with molecular weight of
1000 was purchased from Sigma Aldrich and dried at 80 ꢀC for 10 h
under vacuum just before use. N,N-dimethylformamide (DMF) was
purchased from Fisher and dried over molecular sieves prior to use.
ddm:PBA respectively. Into a 50 mL dried three neck flask was
dissolved 4HBA-ddm (0.594 g, 1.2 mmol), and PBA (0.3 g,
0.3 mmol) in an extra dry DMF (<50 ppm, 35 mL) solvent con-
taining dibutyltindilaurate (0.02 g), under dry nitrogen atmo-
sphere. The mixture was stirred until clear solution was obtained,
followed by adding MDI (0.376 g, 1.5 mmol). The mixture was
then continually stirred at 70 ꢀC for 30 min. The resulting solu-
tion was precipitated in water to afford a slightly yellowish
powder, which was dried under vacuum at room temperature for
24 h.
2.2. Synthesis of methylol-functional benzoxazine monomer
(4HBA-ddm)
IR spectra (KBr, cmꢁ1): 1733 (the C]O stretching of non-
hydrogen bonded carbonyl group of urethane and ester groups),
1540 (the CeN stretching, combined with NeH out-of-plane
bending), 1414 (the stretching of CeNH), 1228 (the asymmetric
stretching of AreOeC),1076 (the asymmetric stretching of CeOeC),
955 (the out-of-plane vibration of benzene ring to which oxazine is
attached).
In a 500 mL flask, 4HBA (19 g, 150 mmol), DDM (15.02 g,
75 mmol), and paraformaldehyde (9.01 g, 300 mmol) were mixed
together and heated at 140 ꢀC in xylenes (175 mL) for 5 h. The
reaction was allowed to cool to room temperature followed by
washing with hexane to afford a yellow product. The product was
then recrystallized in chloroform, (Yield: 80e85%).
1H NMR spectra (DMSO-d6, ppm:
d, frequency: 600 MHz): 4.58
(s, CeCH2eNe), 4.99 (s, eCH2eO), 5.38 (s, NeCH2eOe).
IR spectra (KBr, cmꢁ1): 1228 (asymmetric stretching of
AreOeC), 1076 (asymmetric stretching of CeOeC), 952 (out-of-
plane vibration, benzene ring to which oxazine is attached).
2.5. Crosslinking of poly(benzoxazine-co-urethane)s
1H NMR spectra (DMSO-d6, frequency: 600 MHz, ppm:
d) 4.36
(d, eCH2eO), 4.57 (s, CeCH2eN), 4.99 (t, eOH), 5.36 (s,
NeCH2eOe).
Solutions of the copolymers in DMF were cast onto glass plates
that were pretreated by methylmethoxysilane and dried in an air-
circulating oven at 70 ꢀC for 24 h. The produced films were then
crosslinked at various temperatures in the air-circulating oven. The
temperature profile used to crosslink the samples was as follows:
90 ꢀC/2 h, 120 ꢀC/2 h, 150 ꢀC/2 h, and 180 ꢀC/3 h. The color of the
film changed from yellow to red during the crosslinking process.
13C NMR spectra (DMSO-d6, ppm,
d): 49.23 (CeCH2eNe), 62.61
(s, eCH2eOH), 78.87 (NeCH2eO).
2.3. Synthesis of model compound
The model compound has been synthesized by dissolving
4HBA-ddm (0.5 g, 1 mmol) and phenyl isocyanate (0.238 g,
O
O
NCO
OH
H2N
NH2
HO
N
N
OH
+
2
2
CH2O
4
+
+
HO
n
DMF
70°C
Xylenes
Reflux
O
N
O
O
O
H
N
H
N
HO
N
OH
O
C
O
N
N
O
C
O
Scheme 1. Preparation of dimethylol benzoxazine monomer (4HBA-ddm).
Scheme 2. Preparation of model compound.

310
M. Baqar et al. / Polymer 52 (2011) 307e317
Table 1
Compositions of poly(benzoxazine-co-urethane)s.
Copolymer
code
MDI
(g, mmol)
4HBA-ddm
(g, mmol)
PBA
(g, mmol)
4HBA-ddm
(mol%, wt.%)
I-0
0.25, 1
0.25, 1
0.25, 1
0.25, 1
0.25, 1
0.495, 1
0, 0
50, 66.4
40, 46.8
30, 31.4
20, 18.9
10, 8.6
I-10
I-20
I-30
I-40
0.396, 0.8
0.297, 0.6
0.198, 0.4
0.099, 0.2
0.2, 0.2
0.4, 0.4
0.6, 0.6
0.8, 0.8
Mechanical properties of crosslinked films were evaluated in
uniaxial tension on an Instron 5565 universal tester. Dog bone
shaped specimens were punched from the films using an ASTM
D638 punch tool. An average of four to five specimens were cut and
tested at ambient temperature using 1 mm/min as a strain rate. The
reported modulus values were calculated from the initial slope of
the stress-strain curve at 1% strain.
Fig. 2. 1H NMR spectrum of model compound and benzoxazine monomer.
2.6. Measurements
Both proton (1H) and carbon (13C) NMR spectra were obtained by
Varian Oxford AS600 spectrometer, operating at a proton frequency
of 600 MHz and a corresponding carbon frequency of 150.9 MHz.
Deuterated dimethylsulfoxide (DMSO-d6) was used as a solvent.
Fourier transform infrared spectra (FTIR) were obtained using
a Bomem Michelson MB-100 FT-IR spectrometer, which was
equipped with a deuterated triglycine sulfate (DTGS) detector.
Thirty two scans were recorded at a resolution of 4 cmꢁ1 after
purging with dry air. The spectrum was taken by casting a film from
polymer solution onto a KBr crystal.
Thermal analysis was performed with differential scanning
calorimetry (DSC) using TA Instruments DSC model 2920 to study
the curing behavior and thermal stability. The temperature was
ramped at 10 ꢀC/min and a nitrogen flow rate of 65 mL/min was used
for all tests. All samples were crimped in hermetic aluminum pans
with lids.
3. Results and discussion
3.1. Synthesis of dimethylol-functional benzoxazine monomer
(4HBA-ddm)
4HBA-ddm as a novel benzoxazine monomer has been synthe-
sized following the newly developed method in our laboratory for
the synthesis of difficult benzoxazine monomers [41]. Scheme 1
shows the preparation of 4HBA-ddm. The structure of the mono-
mer has been confirmed by 1H NMR, 13C NMR, and FT-IR spectra. In
the 1H NMR spectrum, the characteristic resonances attributed to
cyclic benzoxazine structure are observed at 4.57 ppm (s,
CeCH2eN) and 5.36 ppm (s, NeCH2eOe), whereas the resonances
of methylol group are observed at 4.36 ppm (d, eCH2eO) and
4.99 ppm (t, eOH). 13C NMR spectra showed the typical resonances
for benzoxazine structure at 49.23 ppm (CeCH2eNe) and
78.87 ppm (NeCH2eO) that further support the formation of
benzoxazine monomer. The FT-IR spectra further show the char-
acteristic absorption bands of benzoxazine structure at 1228 cmꢁ1
due to the stretching of CeOeC and at 952 cmꢁ1 due to the out-of-
plane bending vibration of the benzene ring attached to the oxazine
ring [42,43].
The thermal stability was studied by thermogravimetric analysis
(TGA) with a TA Instruments High Resolution 2950 thermogravi-
metric analyzer using nitrogen as a purging gas. A heating rate of
10 ꢀC/min and nitrogen flow rate of 60 mL was used for all tests.
Dynamic mechanical behavior (DMA) was performed on a TA
Instruments Q800 DMA applying controlled strain tension mode
with amplitude of 10 m
m and a temperature ramp rate of 3 ꢀC/min.
Atomic force microscopy (AFM) experiments were performed by
tapping mode at ambient temperature in air. Phase and height
images of the cross sections were recorded simultaneously using
the Nanoscope IIIa instrument Multi-Mode scanning probe (Digital
3.2. Synthesis of model compound
A model compound has been synthesized from the reaction of
4HBA-ddm and phenyl isocyanate. The reaction was carried out in
dry DMF under nitrogen atmosphere as illustrated in Scheme 2.
This model compound helps to understand the reaction condition
at which the methylol groups (eCH2OH) of the benzoxazine
monomer react with the isocyanate group (eNCO) to form
urethane linkages and to confirm the stability of cyclic benzoxazine
structure under the reaction condition. The structure of the model
Instruments, Santa Barbara, CA). A tip made of silicon (110e140 mm
in length with ca. 327e383 kHz resonant frequency and at
20e80 N/m of force) was used. In these images, the height of any
spot in the phase image together with the corresponded height of
the same spot in the topography image was used to study the
morphology of any specific system under study.
O
O
O
X
O
y HO
OCN
NCO
+
+
z
HO
N
N
OH
O
OH
n
O
PBA-1000
MDI
4HBA-ddm
DMF
70oC
x=y+z
O
O
H
H
N
O
N
O
O
N
N
O
O
O
N
H
O
O
N
H
O
n
O
O
O
Scheme 3. Preparation of main-chain poly(benzoxazine-co-urethane)s.

M. Baqar et al. / Polymer 52 (2011) 307e317
311
955
1733 1540 1325
2275
I-40
258 °C
262 °C
0.5 J/g
1.8 J/g
I-10/10min
I-30
Exo
4HBA-ddm/PBA+MDI
4HBA-ddm/PBA
4HBA-ddm
I-20
I-10
264 °C
262 °C
9 J/g
19 J/g
35 J/g
I-0
248 °C
2500
2000
1500
1000
500
Wave number (cm-1)
0
100
200
300
Fig. 3. FT-IR spectra of I-10 and related compounds.
Temperature (°C)
compound has been confirmed by 1H NMR, 13C NMR, and FT-IR
spectra. 1H NMR spectrum depicted in Fig. 2 shows the character-
istic resonances at 4.59 ppm (s, CeCH2eN) and 5.38 ppm (s,
NeCH2eOe) due to benzoxazine structure, indicating its stability at
this reaction condition. A new resonance was observed at 9.67 ppm
attributed to the eNH of urethane linkage. In the FT-IR spectra,
benzoxazine structure was confirmed by the characteristic
absorption bands at 1224 cmꢁ1 due to the stretching of CeOeC and
the band at 950 cmꢁ1 due to the out-of-plane bending vibration of
the benzene ring that is attached to the oxazine ring. This confirms
the high reactivity of the methylol group in a benzoxazine
Fig. 5. DSC thermograms of crosslinked films at different benzoxazine molar ratio.
monomer towards the isocyanate group to form urethane linkage
in a short time at relatively low reaction temperature.
3.3. Synthesis of main-chain type poly(benzoxazine-co-urethane)s
Main-chain type poly(benzoxazine-co-urethane)s have been
prepared by the reaction of MDI, PBA and different molar ratio of
4HBA-ddm. The proposed structure is depicted in Scheme 3. This
structure does not represent the exact structures of the copolymers,
but rather idealized structure is shown. The compositions of the
synthesized copolymers and their nomenclature are illustrated in
Table 1. The uncrosslinked copolymers will be abbreviated as I# and
the crosslinked forms will be poly(I#). The number (#) is used to
indicate the molar percent of polyol in copolymer.
The consumption of the NCO groups of MDI due to the reaction
with the OH groups of both 4HBA-ddm and PBA to produce
urethane linkages has been followed by FT-IR. Fig. 3 illustrates the
FT-IR spectra of I-10, as an example. FT-IR spectra indicate that the
typical absorption at 2275 cmꢁ1 due to eNCO group disappeared
I-40
259 °C
8 J/g
258 °C
256 °C
I-30
31 J/g
44 J/g
I-20
I-10
Exo
252 °C
180 °C
56 J/g
Exo
262 °C
248 °C
I-0
19 J/g
150 °C
128 J/g
258 °C
224 °C
70 °C
38 J/g
4HBA-ddm
211 J/g
252 °C
56 J/g
124 °C
150
0
50
100
200
250
300
0
50
100
150
200
250
300
Temperature (°C)
Temperature (°C)
Fig. 4. DSC thermograms of uncrosslinked films at different benzoxazine molar ratio.
Fig. 6. DSC thermograms for I-10 at various polymerization temperatures.

312
M. Baqar et al. / Polymer 52 (2011) 307e317
thermal treatment up to 180 ꢀC as depicted in Fig. 5. The residual
1510
1490
1325
1231
955
exothermic peaks represent the remaining unpolymerized ben-
zoxazine in the copolymers. Since the amount of benzoxazine is
varied in the main-chain copolymer, there is a gradual decrease in
the remaining exothermic peaks with decreasing the benzoxazine
content in the copolymers. By comparison of the amount of exo-
therm before and after the crosslinking (Figs. 4 and 5), we can
conclude that the copolymers with low benzoxazine content such
as poly(I-40) and poly(I-30) showed the completion of polymeri-
zation of more than 93%, whereas copolymers with higher ben-
zoxazine content such as poly(I-20), poly(I-10) and poly(I-0)
showed the completion of polymerization of w66e80%.
180 °C
150 °C
120 °C
The crosslinking of I-10 as an example has been studied by DSC
and FT-IR after each thermal treatment cycle. Fig. 6 shows the
DSC thermograms of the films at various thermal treatments. The
DSC thermograms show that the heat of polymerization decreases
from 56 J/g to 19 J/g as the temperature increases from 70 to
180 ꢀC, indicating the presence of residual benzoxazine structure.
Further thermal curing has been avoided to eliminate the possi-
bility of partial degradation of aliphatic segment of PUs copolymer
[1,4,27,28].
90 °C
70 °C
2000
1500
1000
500
Wave number (cm-1)
Fig. 7. FT-IR spectra of I-10 film after polymerizing at different temperature.
To further confirm the crosslinking of the copolymers, FT-IR
spectra of the copolymers have been studied. Fig. 7 shows the FT-IR
spectra of I-10 after various thermal treatments. A gradual decrease
of the characteristic absorption bands of benzoxazines with
thermal treatment was observed at 1510 cmꢁ1 of trisubstituted
benzene, 1325 cmꢁ1 of CH2 of benzoxazine ring, and 1231 cmꢁ1 of
ether linkage. Meanwhile, a new band appeared at 1490 cmꢁ1 of
tetrasubstituted benzene rings, indicating the formation of poly-
benzoxazine and hence the crosslinking of the copolymer [18,25].
However, a residual absorption band at 955 cmꢁ1 due to the out-of-
plane bending vibration of benzene ring is still observed. None-
theless, thermal treatment up to 180 ꢀC has been fixed as last
polymerization cycle to avoid any partial degradation of the
aliphatic component of PU [1,4]. Scheme 4 shows the proposed
structure of the crosslinked copolymer network structure. As
a result of the thermally activated ring opening polymerization of
cyclic benzoxazine in the main-chain, crosslinking is achieved
through the formation of polybenzoxazine as crosslinkers between
copolymer chains.
quickly within 10 min. It was also observed that the C]O stretching
of non-hydrogen bonded carbonyl group of urethane and ester
groups at 1733 cmꢁ1 and the CeN stretching combined with NeH
out-of-plane bending at 1540 cmꢁ1 attributed to the formation of
urethane linkage increased. Also, the band at 955 cmꢁ1 due to the
out-of-plane bending vibration of the benzene ring attached to
oxazine ring is maintained intact, confirming the presence of
benzoxazine structure in the resulting polymer [27,34,38,42,43].
3.4. Polymerization behavior of poly(benzoxazine-co-urethane)s
Differential scanning calorimetry (DSC) has been used to monitor
the crosslinking behavior of the main-chain type poly(benzoxazine-
co-urethane). Fig. 4 shows the DSC thermograms of copolymers
containing different content of cyclic benzoxazine in the main-chain
after drying at 70 ꢀC. The DSC thermograms indicate that as the
content of benzoxazine in copolymers increases, the amount of the
exothermic heat increases. The heat of polymerization of benzox-
azine for crosslinking are 128, 46, 41, 31, and 8 J/g for I-0, I-10, I-20,
I-30 and I-40, respectively. This exothermic heat is attributed to the
crosslinking reaction of poly(benzoxazine-co-urethane)s via ring
opening polymerization of benzoxazine structure.
3.5. Morphology studies
The surface morphologies of main-chain poly(benzoxazine-co-
urethane) films were monitored by AFM in tapping mode and are
displayed in Fig. 8. It was obvious, by comparing the images (a)e(e),
that the higher benzoxazine ratio creates more wrinkled surface
DSC thermograms have been used to monitor the residual
benzoxazine in the main-chain of the crosslinked copolymers after
O
O
H
N
H
N
O
N
O
O
N
O
O
O
O
O
N
H
N
H
O
n
O
O
O
O
O
H
N
H
N
OH
N
HO
N
O
O
O
O
O
N
H
N
H
O
O
n
O
O
O
n
Scheme 4. Proposed idealized, crosslinked structure of the main-chain poly(benzoxazine-co-urethane).

M. Baqar et al. / Polymer 52 (2011) 307e317
313
Fig. 8. AFM morphology of crosslinked films at 180 ꢀC for 3 h: a) poly(I-0), b) poly(I-10), c) poly(I-20), d) poly(I-30), and e) poly(I-40). Left: phase images and right: topographic
images.

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M. Baqar et al. / Polymer 52 (2011) 307e317
Table 2
than the system with lower benzoxazine ratio. This phenomenon
was attributed to the increase in the hard segment content which is
represented by aromatic structure in MDI and 4HBA-ddm in
comparison to the PBA that is responsible for the soft segments in
the moiety. The AFM images exhibit difference in height and phase
in micro-regions around 100 nm, which indicates the presence of
specific microphase-separated morphologies. These regions consist
of hard segment-rich and soft segment-rich domains as in poly(I-
40) and poly(I-30) samples. Besides the small phase micro-regions,
many rougher domains appear in the poly(I-20) and poly(I-10)
samples with the dimension of around 200 nm and more. In the
case of poly(I-0) film as a control, it shows a rough surface due to
the presence of hard segment-rich domains which is attributed to
the high aromatics content in the molecular structure.
Tensile properties of I-20 films after different thermal treatment.
Polymerization
temp.(ꢀC)
120 ꢀC
150 ꢀC
180 ꢀC
Modulus
(MPa)
Tensile
strength (MPa)
Elongation
at break (%)
90
197
533
15
27
32
73
28
16
This behavior is attributed mainly to the decrease of crosslinking
density as the content of benzoxazine decreases. Thus, the more
benzoxazine content in the copolymer, the greater the crosslinking
density, leading to a more rigid structure. The trends in mechanical
behavior are in agreement with reported studies [27,31e36].
Nonetheless, the values of tensile modulus and tensile strength of
crosslinked poly(benzoxazine-co-urethane)s produced through the
concept adapted in this study are higher than that of the reported
polybenzoxazine/PU materials. For example, poly(I-30) with ca.
18 wt.% of polybenzoxazine content showed tensile strength and
tensile modulus of 13.0 and 73.0 MPa. These values are significantly
higher than the reported values for polybenzoxazine/PU materials
containing the same content of polybenzoxazine which showed
tensile strength and tensile modulus of <5 and< 20 MPa, respec-
tively [27,34,35]. For polybenzoxazine/PU materials reported by
Takeichi et al. [27], the films cured at 180 ꢀC had higher elongation at
break than the current concept and all the reported approaches for
3.6. Tensile properties
The tensile properties of the crosslinked poly(benzoxazine-co-
urethane) films have been evaluated. The effect of varying the
thermal treatments of the main-chain poly(benzoxazine-co-
urethane) films on the tensile properties has been studied in details
for the I-20 sample. Fig. 9 shows stress-strain curves of the I-20
films after various thermal treatment cycles for crosslinking. The
tensile properties data of I-20 including tensile modulus, tensile
strength, and elongation at break are tabulated in Table 2. It is
obvious that, as the thermal treatment temperature increases, both
tensile modulus and tensile strength increase whereas the elon-
gation at break decreases. This behavior is attributed to the increase
of crosslinking density coupled with the increase of the thermal
treatment temperature, which leads to further ring opening poly-
merization of benzoxazine structures in the main-chain to take
place and the formation of a more rigid three dimensional network
structure. As a result, both tensile modulus and tensile strength
increased whereas elongation at break decreases.
The effect of benzoxazine content as a crosslinking agent on the
tensile properties of poly(benzoxazine-co-urethane) films has also
been studied. Fig. 10 shows the stress-strain curves of copolymers
after thermally treated up to 180 ꢀC. Table 3 summarizes the results
of the tensile properties. Both the tensile modulus and tensile
strength increase with an increase in the benzoxazine content in
copolymers as depicted in Fig. 11. For example, the tensile modulus
values are 2360, 737, 517, 54 and 18 MPa whereas the tensile
strengths are 78, 40, 32, 13 and 3 MPa for samples poly(I-0), poly
(I-10), poly(I-20), poly(I-30), and poly(I-40) respectively. On the
other hand, the elongation at break increased with decreasing
the benzoxazine content to be 4,12,16, 73 and 109% for samples poly
(I-0), poly(I-10), poly(I-20), poly(I-30), and poly(I-40) respectively.
Fig. 10. Stress-strain curves of crosslinked copolymer films at different benzoxazine
Fig. 9. Stress-strain curves of I-20 treated at different temperatures.
molar ratio.

M. Baqar et al. / Polymer 52 (2011) 307e317
315
Table 3
Tensile properties of copolymer films of different benzoxazine content after thermal
treatment at 180 ꢀC.
Sample code
Modulus
(MPa)
Tensile
strength (MPa)
Elongation
at break (%)
Poly(I-0)
2358
737
517
54
78
40
32
13
3
4
12
16
73
109
Poly(I-10)
Poly(I-20)
Poly(I-30)
Poly(I-40)
18
polybenzoxazine/PU materials. This high value of the elongation at
break is attributed to the use of benzoxazine monomer that leads to
lower crosslinking density than the current concept through the
main-chain type benzoxazine polymers. In addition, the lower
elongation at break in this method for polybenzoxazine/PU mate-
rials is presumably due to the role of the inter- and intra-molecular
hydrogen bonding of free phenolic groups of polybenzoxazine that
acts as additional physical crosslinks between the chains of the
copolymer [22,44]. Moreover, most of the previously reported
approaches for polybenzoxazine/PU materials used toluene diiso-
cyanate (TDI) and 1,6-hexamethylene diisocyanate (HDI) to prepare
PU prepolymer. However, in this study methylene diphenyl diiso-
cyanate (MDI) of more rigid aromatic content has been used that
adds more rigidity in the molecular structure [1,44]. Also, in
comparison to well-known high performance poly(urethane-imide),
the crosslinked poly(benzoxazine-co-urethane) copolymers showed
comparable values for samples containing similar PU content. For
example poly(I-20) exhibited higher tensile strength but lower
elongation at break than poly(urethane-imide) of the same urethane
content [5,8].
3.7. Thermal properties of crosslinked poly(benzoxazine-co-
urethane)s
Fig. 12. DMA analysis for poly(benzoxazine-co-urethane) films polymerized at 180 ꢀC.
Dynamic mechanical analysis (DMA) of crosslinked copolymer
films has been performed and the results are shown in Fig. 12. The
DMA results indicate that the initial storage moduli (E`) of the
copolymer films increased with increasing the content of benzox-
azine in the main-chain. It was also observed that the storage
modulus maintained at higher value by increasing the benzoxazine
content, indicating the increase in the stiffness of the copolymers.
In addition, the Tg of crosslinked poly(benzoxazine-co-urethane)s
shifted to lower temperature with increasing the content of PBA as
a soft segment and decreasing the crosslinking density. For
example, Tg decreased from 230 to 219 ꢀC by increasing the polyols
content in the main-chain from 0 to 10 mol% for samples poly(I-0)
2500
2000
1500
1000
500
80
60
40
20
0
Modulus
Tensile Strength
100
80
60
Poly(I-0)
Poly(I-10)
40
20
0
Poly(I-20)
Poly(I-30)
Poly(I-40)
0
0
20
40
60
80
0
200
400
600
800
Benzoxazine content (%)
Temperature (°C)
Fig. 11. The effect of benzoxazine content on the modulus and tensile strength.
Fig. 13. TGA analysis of cured poly(benzoxazine-co-urethane)s films.

316
M. Baqar et al. / Polymer 52 (2011) 307e317
Table 4
4. Conclusions
TGA data of poly(benzoxazine-co-urethane) films cured up to 180 ꢀC.
A new concept to produce polybenzoxazine/PU copolymers has
been developed by incorporating oxazine and urethane groups in
one molecule through main-chain type poly(benzoxazine-co-
urethane)s. These copolymers have been synthesized via copoly-
merization of diisocyanate monomer with a dimethylol-functional
benzoxazine monomer and polyol to afford a linear main-chain
type polymer. The produced copolymers were further crosslinked
through thermally activated ring opening polymerization of ben-
zoxazine to produce network structure. Films were subjected to
mechanical and thermal tests and the results were associated with
the chemical structure of corresponding films according to the
content of benzoxazine in the moiety. This novel class of poly-
benzoxazine/PU materials is characterized by the following:
Sample
Td5 (ꢀC)
Td10 (ꢀC)
Char yield (%)
Poly(I-0)
280
300
293
287
285
311
333
323
310
309
53.0
44.3
35.0
27.5
17.5
Poly(I-10)
Poly(I-20)
Poly(I-30)
Poly(I-40)
and poly(I-10), respectively. This behavior is attributed to the
increased chain mobility of the copolymer as a result of decreasing
the crosslinking density that leads to decreased Tg.
Thermogravimetric analysis (TGA) has been used to investigate
the thermal stability of the crosslinked poly(benzoxazine-co-
urethane) films. Fig. 13 shows the TGA profiles of copolymer films
after 180 ꢀC treatment. The results of TGA are summarized in Table
4. The thermal stability of poly(I-0), which can be used as a control,
shows an early weight loss from 280 ꢀC. 4HBA-ddm monomer
which has been used in this study is a methylol functional end-
capped benzoxazine which, by thermal treatment, will condensate
to produce water at about 200 ꢀC, similar to traditional resole. Since
the copolymer poly(I-0) contains the highest content of 4HBA-ddm
and the thermal treatment did not exceed 180 ꢀC, the early weight
lost is attributed to the condensation reaction of methylol end
groups in the copolymer which will be discussed in details else-
where [41]. For the rest of the crosslinked copolymers, a higher
benzoxazine content led to an improvement in the onset of the
degradation as can be seen from the 5 and 10% weight loss
temperatures, Td5 and Td10. For example, Td5s are 300, 293, 287,
and 285 ꢀC whereas Td10s are 333, 323, 310, and 309 ꢀC for samples
poly(I-10), poly(I-20), poly(I-30), and poly(I-40), respectively. The
onset of degradation is higher than the control due to the absence
of methylol groups that are consumed for urethane linkage
formation. Although depolymerization of urethane bonds occurred
at about 240 ꢀC [8], the incorporation of benzoxazine in the poly
(benzoxazine-co-urethane) copolymer shifted the decomposition
temperature to w285e300 ꢀC, depending on the benzoxazine
content. The onset of the degradation did not improve significantly,
since the increase of the benzoxazine content will shift the initial
weight to low temperature due to methylol condensation. On the
other hand, the increase of polyol content as aliphatic component
will contribute to the shift of initial weight loss to lower temper-
ature. Despite the aforementioned reasons of the initial weight loss
of the copolymers, the thermal stability of this new system is still
comparable with the reported similar polybenzoxazine/PUs. For
example, Td5 changed from 285 ꢀC for poly(I-40) copolymer of
8.6 wt.% benzoxazine weight percent to 300 ꢀC for poly(I-10)
copolymer of 46.8 wt.%, respectively. The char yield also showed
significant increase upon increasing the benzoxazine content in the
main-chain. For example samples poly(I-0), poly(I-10), poly(I-20),
poly(I-30) and poly(I-40) have char yield of ca. 53.0, 44.3, 35.0, 28.0
and 18.0%, respectively. Thus, this new concept of crosslinked poly
(benzoxazine-co-urethane) copolymer is shown to be effective for
producing polybenzoxazine/PU materials of significant higher
thermal stability than the previously reported approaches. For
example, sample poly(I-20) of ca. 70 wt.% of PU has char yield of
35.0%, whereas the previously reported polybenzoxazine/PU
materials of similar PU content showed a char yield in the range of
22-25% [27,28,36] and not more than 30% for systems of ca. 30 wt.%
of PU [32,33]. This enhancement in thermal stability is attributed to
the presence of polybenzoxazine as a part of the copolymer main-
chain and the thermal stability of urethane linkage that result in
better thermal properties than the reported polybenzoxazine/PU
materials.
ꢂ One-pot synthesis of phenolic-urethane materials that avoids
the drawbacks associated with the use of preformed NCO-
terminated PU prepolymers.
ꢂ Superior thermal stability compared to previous approaches.
ꢂ Excellent mechanical integrity as can be seen from high tensile
modulus and tensile strength due to the presence of poly-
benzoxazine as part of the main-chain.
ꢂ These copolymers showed thermal and mechanical properties
that are comparable to the well-known high performance poly
(urethane-imide).
Acknowledgement
We thank the Ministry of Education of Libya for the financial
support of M. Baqar. We also thank Mr. Saeed Alhassan, Case
Western Reserve University, for his kind help in taking AFM images.
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