Functionalized MWCNTs, an efficient reinforcement for the preparation of eugenol based high performance PBz/BMI/CNT nanocomposites exhibiting outstanding thermo-mechanical properties

Article abstract of DOI:10.1039/c7nj00357a

The chemistry of benzoxazine synthesis offers wide molecular design flexibility and thus facilitates preparation of various polybenzoxazine based composites. In this paper, we reported the preparation of polybenzoxazine/BMI blend and subsequent preparation of nanocomposites with amine functionalized multiwalled carbon nanotubes (F-MWCNTs). The curing mechanism and the material properties of the blend and nanocomposites were studied extensively. DSC analysis showed two exothermic maxima, corresponding to benzoxazine and allyl group polymerization. Furthermore, by the incorporation of F-MWCNTs into the PBz/BMI system, the properties of the PBz/BMI/F-MWCNT nanocomposites were improved significantly. The enhancement in the material properties is due to the strong interaction between the F-MWCNTs and PBz, resulting in the formation of interpenetrating polymer networks. The morphology and fracture surface of the nanocomposites were studied by scanning electron microscopy. The thermal, mechanical and dielectric properties of the nanocomposites are found to be outstanding when compared with the neat PBz and PBz/BMI blend.

Full text of DOI:10.1039/c7nj00357a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Functionalized MWCNT, an efficient reinforcement for the
preparation of eugenol based high performance PBz/BMI/CNT
nanocomposite exhibiting outstanding thermo-mechanical
properties
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Thirukumaran Periyasamy a*, Shakila Parveen Asrafali b, and Seong-Cheol Kim a*
Abstract The chemistry of benzoxazine synthesis offers wide molecular design flexibility and thus facilitates preparation of
various polybenzoxazine based composites. In this paper, we reported the preparation of polybenzoxazine/BMI blend and
subsequent preparation of nanocomposites with amine functionalized multiwalled carbon nanotubes (F-MWCNTs). The
curing mechanism and the material properties of the blend and nanocomposites were studied extensively. DSC analysis
showed two exothermic maxima, corresponding to benzoxazine and allyl group polymerization. Furthermore, by the
incorporation of F-MWCNT into PBz/BMI system, the properties of PBz/BMI/F-MWCNT nanocomposites were improved
significantly. The enhancement in the material properties is due to the strong interaction between F-MWCNTs and PBz,
resulting in the formation of interpenetrating polymer network. The morphology and fracture surface of the
nanocomposites were studied by scanning electron microscopy. The thermal, mechanical and dielectric properties of the
nanocomposites are found to be outstanding when compared with the neat PBz and PBz/BMI blend.
Keywords: Eugenol, polybenzoxazine, bio-composite
does not require strong acid catalyst for curing v) high thermal
stability and vi) good mechanical and excellent electrical
properties2,3. The straight and easy synthesis of benzoxazine
1. Introduction
The development of environmentally friendly and sustainable
polymers is one of the current challenges in polymer industry,
nowadays. And therefore, both academics and industrialists
show particular interest in the synthesis of organic materials
from renewables. Bio-polymers are sustainable and renewable
as they are made from plant materials, which can be grown
indefinitely. Various renewable materials (such as diphenolic
acids, cardanol, furfurylamine, stearylamine and guaiacol)
have been used for the synthesis of bio-polymers1. Recently,
eugenol (4-allyl-2-methoxy phenol) has been extensively
studied as a starting material for the synthesis of novel bio-
based polymeric materials. Eugenol is relatively cheap which
makes it economically suitable for the synthesis of bio-based
resins (in particular bio-based benzoxazines).
monomer through Mannich condensation is another
interesting fact to consider. This provides more flexibility to
design their molecular structure and allows tailoring the
properties of polybenzoxazine by adding specific
functionalities. Although PBz based materials offer several
advantages, they are also associated with some disadvantages
that includes i) requirement of high temperature for curing ii)
the obtained products are brittle in nature iii) most of the
monomers are solid, which limits their process-ability and iv)
difficulty to prepare films (3). Several strategies such as i)
synthesis of benzoxazine monomers with additional
functionality4-8 ii) incorporation of benzoxazine in polymer
chains and9-11 iii) synthesis of benzoxazine-based composites
or alloys have been adopted.
Polybenzoxazines (PBz), a novel class of phenolic resins, have
gained a lot of attention these days because of their attractive
characteristics, which includes i) near-zero volumetric change
upon curing ii) low water absorption iii) glass transition
temperature (Tg) much higher than the cure temperature iv)
Bismaleimide (BMI) has been used to stabilize the benzoxazine
moiety as it undergoes co-polymerization through an ene-
addition. BMIs are well known for their excellent mechanical
properties, thermal stability, solvent resistance and electrical
insulation properties over a wide range of temperature1,12
.
BMI can undergo self-addition reaction resulting in the
formation of dense crosslinked networks with high thermo-
mechanical stability. However, the cured resins are brittle
because of high crosslink density of these networks, which
School of Chemical Engineering, Yeungnam University, South Korea.
E-mail: sckim07@ynu.ac.kr
School of Chemical Engineering, Yeungnam University, South Korea.
E-mail: thiru.kumaran999@gmail.com
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limits their application. And therefore, the toughness or in the frequency range of 1 kHz – 6 MHz at room temperature.
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DOI: 10.1039/C7NJ00357A
impact strength of the materials has to be improved. This The dielectric constant and dielectric loss of the samples were
could be achieved by the formation of composites.
determined using ɛʹ and ɛ" as the standard relations. The
Carbon nanotubes (CNTs) hold the promise of delivering morphology of the material was examined by scanning
exceptional mechanical properties and multifunctional electron microscopy (FESEM, Hitachi S-4800) at an accelerating
characteristics [Young’s modulus as high as 1.2 TPa and tensile voltage of 30 kV, Dynamic mechanical analysis (DMA) was
strength of 50-200 GPa]. The addition of CNTs to the polymer carried out using a Netzsch 242 DMA at a heating rate of 10
o
matrix can generally improve the strength and modulus of the oC/min from 30 to 250 C. Thermogravimetric analysis (TGA)
polymer only when there is uniform dispersion of the was performed using a TA Q 600 thermal analyzer. Cured
nanoparticles (NPs) in the matrix, with the existence of strong samples were analyzed in open silicon pan at a heating rate of
interfacial bond between the NPs and the polymer. It is well 20 oC/min in N2 atmosphere, up to a maximum temperature of
known that dispersing NPs in a polymer matrix is a difficult 800 oC.
task due to their tendency to form agglomerates13,14. It is
therefore necessary to functionalize the nanoparticles in order
to obtain the expected improvements in the property of
3. Synthesis of diamine
polymer nanoparticles. This study investigates the effect of
functionalization of CNTs in the ternary system (Bzo/BMI/F-
MWCNTs). The curing behavior and the material properties of
the ternary system have been studied extensively.
3.1 Synthesis of 1, 4 [bis (4’-aminophenoxy) benzene] [BAPB]
The diamine 1, 4 [bis (4’-aminophenoxy) benzene] was
synthesized according to Zhang et al and Thirukumaran et
al15,16 as shown in Scheme 1.
FT-IR (KBr, cm-1): 3042 (C-H, aromatic), 1495 (C=C, aromatic),
1271 (C-O-C), 3313 (N-H, asymmetric stretching), 3225 (N-H,
symmetric stretching); H-NMR (DMSO-d6, ppm): 4.5 (s, -NH2),
2. Experimental
2.1 Materials
1
Eugenol was purchased from Sigma-Aldrich, Indonesia; 7.3 (d, -CH=C-N), 7.7 (d, -CH=C-O), 8.0 (d, -CH=C-O); 13C-NMR
paraformaldehyde was purchased from Sigma-Aldrich, (DMSO-d6, ppm): 125 (C=*C-NH2), 100 (*C=C-NH2), 115 (*C=C-
Germany; N,N- dimethylformamide (DMF), Ferric chloride, O), 143 (C=*C-O).
dioxane, sodium hydroxide, THF and ethanol were purchased
from Merck Limited, India; potassium carbonate (K2CO3) was
purchased from s d fine-chem limited, India, hydrazine
monohydrate was purchased from Spectrochem Pvt. Ltd., India;
p-chloronitrobenzene and 10% Pd/C from Sisco Research
Laboratories, Pvt. Ltd. India; hydroquinone was purchased
from Fisher Scientific (a part of Thermo Fisher Scientific, Pvt.
Ltd., India; Multiwalled cabon nanotube was purchased from
Iljin Nanotech Co. Ltd., Korea. All chemicals and solvents were
used without further purification.
2.2 Characterization methods
Fourier transform infrared (FTIR) spectra of the samples were
obtained using an ABB Bomen (Model MB 3000) Spectrometer.
The samples were ground with spectroscopy grade KBr and
made into pellets. 1H (600 mHz) & 13C (125 mHz) nuclear
magnetic resonance (NMR) spectra were recorded on a Joel
Spectrometer with tetramethylsilane (TMS) as the internal
standard. Solutions were prepared in DMSO-d6. Differential
Scanning Calorimetry was performed in a TA instrument Q10
model using 5-10 mg of the sample at a heating rate of 10
oC/min in N2 atmosphere. Dielectric constant and dielectric
loss measurements were carried out with an impedance
analyzer (Solatron 1260 Impedance/Gain-phase Analyzer) at
room temperature. Samples of 1 mm thickness × 12 mm dia.
were prepared using platinum (Pt) electrode sandwich model
Scheme 1: Synthesis of diamine
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4. Synthesis of benzoxazine monomers
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4.1 Synthesis of bis (6-allyl-8-methoxy-3-phenyloxy-3, 4-dihydro-
2H-1, 3-benzoxazinyl) oxane [Bzo-PHE]
The benzoxazine monomer bis (6-allyl-8-methoxy-3-phenyloxy-
3, 4-dihydro-2H-1, 3-benzoxazinyl) oxane [Bz-PHE] was
synthesized as described16 with some modifications as shown
in Scheme 2.
In a 500 mL round bottomed three necked flask 1, 4 [bis (4’-
aminophenoxy) benzene] [BAPB] (0.05 m, 16.0 g), eugenol (0.1
m, 16.4 g) and 1, 4-dioxane (150 mL) were taken and stirred
well to get a homogenous solution. Paraformaldehyde (0.3 m,
12.0 g) was added to the solution and the mixture was heated
at 120 oC for 8 h in an oil bath with reflex condenser, and then
reaction mixture was cooled to room temperature and finally
poured into 1 L of aqueous sodium hydroxide solution (0.5 N).
The precipitate formed was collected by filtration, washed
o
several times with water. After drying at 60 C in a vacuum
oven, pale yellow powder was obtained (86% yield, m.p. 90 oC).
FT-IR (KBr, cm-1) 3044 (C-H), 1593 (C=C), 1327 (CH2), 942
(oxazine ring), 1029, and 1233 (C-O-C), 1193 (C-N-C); H-NMR
Figure 1: FT-IR spectra of a) MWCNT b) Acid functionalized
MWCNT and c) Amine functionalized MWCNT
1
(DMSO-d6, ppm) 4.6 (s, Ar-CH2-O), 5.3 (s, O-CH2-N), 3.6 (s, -
*
OCH3), 3.3 (d, -CH2 -CH=CH2), 5.9 (m, -CH2-CH*=CH2), 5.0 (d, -
4.3 Preparation of Bzo/BMI blend and Bzo/BMI/CNT
nanocomposites with different ratio
*
CH2-CH=CH2 ), 6.5-7.5(m, aromatic protons); 13C-NMR (DMSO-
d6, ppm) 49 (Ar-CH2-O), 80 (O-CH2-N), 56.0 (-OCH3), 116.0 (-
*CH2-CH=CH2), 137 (-CH2-*CH=CH2), 40 (-CH2-CH=*CH2).
Synthesized benzoxazine monomer (5 g) and BMI (5 g) was
dissolved in THF (10 mL) and stirred for 1 h at room
temperature. To the solution 0.1 g of amine functionalized
o
MWCNT was added and stirred at 80 C to evaporate most of
the solvent (THF).
Scheme 2: Synthesis of benzoxazine monomer
4.2 Functionalization of MWCNT
Multiwalled carbon nanotube was functionalized with
ethylene amine by following the synthesis procedure of S. M.
Yeun et al18. The FT-IR spectra of CNT and functionalized CNT
are shown in Figure 1.
Scheme 3: Preparation of PBz/BMI/F-MWCNT
nanocomposites
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Then the mixture was subjected to sonication to ensure that F- The singlet at 3.6 ppm is due to the –OCH3 pVrioewtoAnrtsic.le OTnlhinee
MWCNT was well dispersed in the mixture. After complete doublets at 3.3, 5.0 and 5.9 ppm are DaOssI:ig10n.e10d39t/oC7NthJe003a5ll7yAl
dispersion (of F-MWCNT) and evaporation of the solvent, a protons [-CH2=CH-CH2-]. The multiplets between 6.5 and 7.5
dark brown colored resin was obtained. Bzo/BMI/F-MWCNT ppm are assigned to the aromatic protons 23
nanocomposites were prepared by varying the F-MWCNT
Fig. 4 shows the 13C-NMR spectrum of the benzoxazine
.
content (say 0, 1, 3 and 5 wt %). Each resin was introduced monomer. The characteristic carbon resonances of the oxazine
into a stainless steel mould and then step cured in an air ring are found at 49 and 80 ppm, corresponding to the
o
circulating oven by following the cure cycle (100, 150, 200 C methylene carbons of Ar-CH2-N and N-CH2-O respectively. The
o
each for an hour) and then finally post cured at 220 C/1 h. All peak at 56 ppm is attributed to the -OCH3 carbon of eugenol
the cured samples are black colored with 3mm thickness moiety and the peaks at 116, 137 and 40 ppm are assigned to
(Scheme 3).
the allyl carbons [CH2=CH-CH2-], respectively.
5.Results and Discussion
5.1 Structural analysis of benzoxazine
Chemical structure of the monomers was confirmed by FT-IR,
1H NMR & 13C NMR spectral methods.
Eugenol based benzoxazine monomer (Bzo-PHE) was
synthesised by the condensation reaction of eugenol with
formaldehyde and amine as shown in Scheme 2. The FT-IR
spectrum (Fig. 2) of the benzoxazine monomer shows
characteristic absorption bands at 1233 cm-1 (asymmetric
stretching of C-O-C), 1029 cm-1 (symmetric stretching of C-O-C)
and 942 cm-1 which is typical of benzoxazine ring structure 19
.
The spectrum also shows a band at 1376 cm-1 due to tetra
substituted benzene ring. Other aromatic vibrations are
observed at 1593 and 1504 cm-1. C-H stretching vibration of
oxazine ring appears at 3044 cm-1 and the peaks at 2934 and
2899 cm-1 are owing to the asymmetric and symmetric
stretching vibrations of the methylene group of the oxazine
Figure 3: 1H-NMR spectrum of benzoxazine monomer
ring as well as alkyl side chain of eugenol 20,21
.
Figure 4: 13C-NMR spectrum of benzoxazine monomer
Figure 2: FT-IR spectrum of benzoxazine monomer
5.2 Curing profile of Bzo, Bzo/BMI blend and Bzo/BMI/F-MWCNT
nanocomposites
Fig. 3 shows the 1H-NMR spectrum of the benzoxazine
monomer. The characteristic oxazine ring protons appear as
two singlets at 4.6 and 5.3 ppm which are assigned to the
DSC Analysis
The curing behavior of Bzo, Bzo/BMI blend and Bzo/BMI/F-
MWCNT nanocomposites investigated by differential scanning
22
methylene protons of C-CH2-N and O-CH2-N respectively
.
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calorimetry is shown in Fig. 5 and their values are tabulated in Table 1: DSC values of PBz, PBz/BMI blend and PBz/BMI/CNT
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Table 1. The DSC thermogram of the benzoxazine monomer nanocomposites
showed an endotherm and an exotherm. The endotherm at
90 °C is due to the melting point of the benzoxazine
monomer.
The exotherm at 250 °C is attributed to the ring opening of
the benzoxazine monomer.
S.
No.
Samples
Tm
Tonset
Tmax
(°C)
∆H1
(kJ/
mol)
-
∆H2
(kJ/mol)
(°C) (°C)
1.
2.
3.
4.
5.
Bzo
90
90
90
90
90
225
226
227
230
234
248
250
251
252
254
29.8
The influence of BMI in the polymerizability of the Bzo/BMI
blend and Bzo/BMI/F-MWCNT nanocomposites was studied
by DSC. All the DSC curves showed an endotherm at 90 °C,
corresponding to the melting point of the benzoxazine
monomer. This shows that there is complete miscibility of all
the compounds (i.e., Bzo, BMI and F-MWCNT) in the blend
and nanocomposites. Two exotherms were observed in the
thermograms, one at 175 °C and the other at 250 °C,
representing the curing reaction. As there are two
exothermic curves, we can say that two different types of
curing reactions should have taken place 1.
The first exotherm is associated with the reaction between the
maleimide group (of BMI) and allyl group (of Bzo monomer).
The second exotherm is associated with the ring opening
polymerization of benzoxazine. Moreover, due to the presence
of allyl group (in Bzo), all the maleimide groups (of BMI) reacts
with this allyl group and further there are no more maleimide
group available for homopolymerization to occur24.
The exothermic curing profile of Bzo/BMI/F-MWCNT
nanocomposites is similar to that of Bzo/BMI blend, except for
the one with high ratio of F-MWCNT. The Bzo/BMI/F-MWCNT
(5%) nanocomposite showed a slight increase in the peak
curing temperature. The processing window for Bzo/BMI blend
and Bzo/BMI/F-MWCNT nanocomposites were around 60 °C,
showing wide range of processability. Moreover, the blend
(Bzo/BMI) and the nanocomposites (Bzo/BMI/F-MWCNT)
showed another exotherm centered at 345 °C, indicating the
degradation of aliphatic chains present in it.
Bzo/BMI (1:1)
88.7 30.6
89.4 31.4
90.5 31.2
89.3 30.6
Bzo/BMI/CNT
(1%)
Bzo/BMI/CNT
(3%)
Bzo/BMI/CNT
(5%)
∆H1: Amount of heat liberated of BMI react with allyl group
∆H2: Amount of heat liberated of Bzo ring opening
5.3 FT-IR analysis
The curing reaction of Bzo and Bzo/BMI/F-MWCNT at
different temperatures (say 100, 150, 200 and 250 °C) was
monitored by FT-IR and are depicted in Figs. 6 & 7, respectively.
The curing profile of the Bzo/BMI blend system was monitored
by observing the changes in the peaks at 834 cm-1 (=C-H of
maleimide), at 939 cm-1 (oxazine ring), at 1634 and 996 cm-1
(allyl group), at 1711 cm-1 (-C=O), at 1387 cm-1 (-CN) and at 692
and 758 cm-1 (imide ring), respectively. Upon curing, the
intensity of the allyl peaks at 1634 and 996 cm-1 and imide ring
at 1387 and 834 cm-1 decreased, which indicates that reaction
occurred between allyl and maleimide groups. And further, the
bands due to the imide ring at 692 and 758 cm-1 disappears.
On the other hand, the typical benzoxazine band at 939 cm-1
decreased its intensity and finally disappeared during the
curing process. Meanwhile, the –OH band at 3300 cm-1
increased its intensity as the cure reaction advanced indicating
that benzoxazine ring opening could be considered as one
mode of polymerization24. Thus, the spectral evidences
mentioned above are in supportive of the expected cure
sequence. Benzoxazine polymerization was completed
between 200 - 250 °C, whereas the allyl group polymerization
was comleted at 200 °C.
In the FT-IR spectra of Bzo/BMI/F-MWCNT nanocomposites,
there are two regions of interest to study, in which one is the
carbonyl region (between 1750 - 1650 cm-1) and the other is
the hydroxyl region (between 3500
-
3000 cm-1). The
characteristic carbonyl band was observed at 1711 cm-1. In
addition to it, two weak bands were observed at 1689 and
1704 cm-1, respectively. These bands are a result of two types
of intermolecular H-bonding interactions. Of which, one is
between the carbonyl group of BMI and the hydroxyl group of
PBz and the other is between the hydroxyl groups of PBz 25
.
From the above interpretation, it is clearly seen that the
allyl group plays an active role in the polymerization of
PBz/BMI/F-MWCNT nanocomposites. Benzoxazine ring also
Figure 5: DSC curves of a) Bzo b) Bzo/BMI c) Bzo/BMI/F-
MWCNT (1%) d) Bzo/BMI/F-MWCNT (3%) and e) Bzo/BMI/F-
MWCNT (5%)
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involves in the curing of blend and the nanocomposites and 3.1, PBz/BMI: 3.2] due to its high organic content.
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thus forms a well crosslinked polymer network.
Nevertheless, on the addition of F-MWCNT, their
conductivities have been significantly enhanced [PBz/BMI/F-
MWCNT (1, 3 & 5%): 4.8, 6.7 & 8.4]. The functionalized
MWCNT, even in less quantity (say
5 %) disperse more
homogeneously in the polymer matrix and thus results in
producing polybenzoxazine nanocomposites with high
conductivity 26,27
.
Figure 6: FT-IR spectra of benzoxazine monomer at different
curing temperatures
Figure 8: Dielectric constant of a) PBz b) PBz/BMI c)
PBz/BMI/F-MWCNT (1%) d) PBz/BMI/F-MWCNT (3%) and e)
PBz/BMI/F-MWCNT (5%)
Figure 7: FT-IR spectra of Bzo/BMI/F-MWCNT at different
curing temperatures
5.4 Di-electric profile of PBz, PBz/BMI blend and PBz/BMI/F-
MWCNT nanocomposites
The effect of BMI and F-MWCNT on the dielectric constant
and dielectric loss of polybenzoxazine were measured by using
an impedance analyzer in the frequency range of 1 - 6 Hz at
30 °C. Fig. 8 and Table 2 show the dielectric curves and
dielectric values of PBz, PBz/BMI blend and PBz/BMI/F-
MWCNT nanocomposites. The dielectric constant values of the
PBz/BMI/F-MWCNT nanocomposites increased incredibly with
increasing content of F-MWCNT. Neat PBz and PBz/BMI blend
have relatively poor ability for electrical conductivity [Neat PBz:
Figure 9: Dielectric loss of a) PBz b) PBz/BMI c) PBz/BMI/F-
MWCNT (1%) d) PBz/BMI/F-MWCNT (3%) and e) PBz/BMI/F-
MWCNT (5%)
The dielectric loss of the nanocomposites also increased on
increasing the weight percentage of F-MWCNT, as shown in
Figure 9. Generally, the dielectric loss value of the material is
contributed from three distinct factors, which includes direct
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current, conduction space charge migration and the dipole The SEM micrograph of neat PBz displays a smooth and
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moment. The dielectric loss of neat PBz was found to be 1.5, homogeneous surface. Whereas, the surface of PBz/BMI blend
for PBz/BMI blend it was found to be 1.7 and for the is quite different, that shows dual phase morphology indicating
PBz/BMI/F-MWCNT (1, 3 & 5%) nanocomposites it was found apparent microphase separation. The degree of intermixing of
to be 2.3, 3.1 and 3.7, respectively. This indicates that the benzoxazine and BMI is mainly affected by the compatibility
network structure of PBz/BMI/F-MWCNT nanocomposites is between the two compounds24. The degree of dispersion of F-
highly crosslinked. Moreover, F-MWCNT restricted the MWCNT within the PBz/BMI matrix was investigated and their
mobility of the chains in the nanocomposites and thus images are shown in Fig. 10 (d-f). The images show uniform
hindered the curing of BMI, resulting in an increased dielectric and random dispersion of nanotubes in all the PBz/BMI/F-
loss factor. On the whole, both the dielectric constant and MWCNT nanocomposites. Moreover, the nanocomposites with
dielectric loss of PBz/BMI/F-MWCNT nanocomposites low content of F-MWCNT i.e., 1 and 3 %, exhibits a
increased when compared with the neat PBz and PBz/BMI homogeneous behavior. This fact is attributed to the
blend. It could be concluded that suitable content of F- miscibility of F-MWCNT with PBz/BMI matrix due to strong
MWCNT (say
5
wt %) in the PBz/BMI/F-MWCNT interfacial interaction (H-bonding) as well as chemical
nanocomposites results in good dielectric property and retains compatibility of PBz/BMI matrix with F-MWCNT. At higher F-
good dielectric stability28.
MWCNT content (say 5 %), the surface of the nanocomposites
seems to possess less amount of agglomeration.27 These
observations imply that even
a minimum amount of
5.5 Morphology profile of PBz/BMI/F-MWCNT nanocomposites.
functionalized MWCNT, when reinforced into the polymer
matrix results in excellent thermal, electrical and mechanical
properties of the polymer.
Fig. 10 illustrates the morphology of the fractured surface of
PBz,
PBz/BMI,
F-MWCNT
and
PBz/BMI/F-MWCNT
nanocomposites.
5.6 Dynamic mechanical properties of PBz, PBz/BMI blend and
PBz/BMI/F-MWCNT nanocomposites
There are several important material parameters that
could be derived from the DMA data. The storage modulus, ε’
at 35 °C provides a measure of material’s stiffness under shear
deformation. The storage modulus of polybenzoxazine, their
blend and nanocomposites are shown in Fig. 11 and the data
are summarized in Table 2. The initial storage modulus of PBz
was found to be 5.25 MPa, for PBz/BMI blend it was found to
be 5.47 MPa and for PBz/BMI/F-MWCNT (1, 3 & 5%)
nanocomposites it was found to be 5.78, 6.08 & 6.32 MPa,
respectively. It was found that the storage modulus of the
blend and the nanocomposites were found to be superior
when compared with the neat PBz. This is attributed to the
fact that in the case of PBz/BMI blend, the imide group (of BMI)
and allyl group (of Bzo) involves in co-polymerization and
forms a well cross-linked network1,24,29-31. However, in the case
of PBz/BMI/F-MWCNT nanocomposites, reaction between -
NH2 of F-MWCNT and C=C of BMI by Michael addition takes
place. And there also exists π-π interaction between Bzo and
CNT and as the CNT is functionalized with –NH2 group, and H-
bonding interaction between the –NH2 group of F-MWCNT and
–OH group of PBz. And due to these types of interactions, the
movement of the main chain segments is restricted resulting in
the formation of polymer nanocomposites with high crosslink
densities.
The tan δ plots of PBz, PBz/BMI blend and PBz/BMI/F-
MWCNT nanocomposites are shown in Fig. 12. The maximum
peak temperature in the tan δ plots represents the glass
transition temperature (Tg) of a material32. The Tg value of the
PBz/BMI [186 °C] and PBz/BMI/F-MWCNT (1, 3 & 5%)
nanocomposites [221, 234 and 246 °C] increased when
compared with neat PBz (162 °C).
Figure 10: SEM images of a) PBz b) PBz/BMI c) F-MWCNT d)
PBz/BMI/F-MWCNT (1%) e) PBz/BMI/F-MWCNT (3%) and f)
PBz/BMI/F-MWCNT (5%)
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Table 2: Dielectric, DMA and cross-link density values of PBz, PBz/BMI blend and PBz/BMI/CNT
nanocomposites
S.No. Samples
Dielectric
constant, ε’ loss, ε”
(1 MHz)
Dielectric
DMA
CLD x105
(mol/m3)
Storage
Modulus, ε’
(GPa)
Tg (°C)
(1 MHz)
1.
2.
3.
4.
5.
PBz
3.24
3.32
4.97
6.63
8.54
0.16
0.19
0.26
0.31
0.37
5.25
162
186
227
234
246
6.83
7.12
7.52
7.91
8.23
PBz/BMI (1:1)
PBz/BMI/CNT (1%)
PBz/BMI/CNT (3%)
PBz/BMI/CNT (5%)
5.47
5.78
6.08
6.32
Figure 12: Curves showing tan
PBz/BMI/F-MWCNT (1%) d) PBz/BMI/F-MWCNT (3%) and e)
PBz/BMI/F-MWCNT (5%)
δ
values of a) PBz b) PBz/BMI c)
Figure 11: Storage modulus of a) PBz b) PBz/BMI c)
PBz/BMI/F-MWCNT (1%) d) PBz/BMI/F-MWCNT (3%) and e)
PBz/BMI/F-MWCNT (5%)
5.7 Thermal stability of PBz, PBz/BMI blend and PBz/BMI/F-
MWCNT nanocomposites
The crosslink density (CLD) of the polymer, their blend and
nanocomposites was calculated from the storage modulus
value at 35 °C. CLD is defined as the number of crosslink points
per unit volume. The crosslink density may be in the order of
10-3 to 10-5 mol/cm3. For a typical rubbery material, there are
about 15 to 15,000 monomeric unit between the crosslinks33.
The crosslink density of highly crosslinked thermoset can be
determined by modulus measurement by using the following
equation,
Thermogravimetric analysis was performed to examine the
effect of BMI and F-MWCNT on the thermal stability and
decomposition behavior of PBz, as shown in Fig. 13 and their
values are tabulated in Table 3. The thermal stability of
PBz/BMI/F-MWCNT nanocomposites with increasing F-
MWCNT content triggers a significant positive shift towards its
degradation temperature. A noticeable increase in initial
degradation temperature (Ti) was observed for the blend and
nanocomposites when compared with pristine PBz. The initial
degradation temperature was shifted from 251 °C (for PBz) to
280 °C [for PBz/BMI blend] and 294, 308 & 315 °C [PBz/BMI/F-
MWCNT (1, 3 & 5) nanocomposites], respectively. In a similar
way, degradation temperature at 5 and 10% weight loss (T5
and T10) were also shifted towards higher temperatures.
υc = ε’/3RT
where ε’ is the tensile storage modulus (MPa) of the polymer,
T is the temperature (K) corresponding to the storage modulus
value and R is the gas constant. The CLD values are tabulated
in Table 2. The CLD of the PBz/BMI blend and PBz/BMI/F-
MWCNT nanocomposites increased when compared with PBz
and for the nanocomposites, it increased with higher loadings
of F-MWCNT.
8 | J. Name., 2012, 00, 1-3
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The incorporation of F-MWCNT into the PBz/BMI matrix Table 3: TGA values of PBz, PBz/BMI blend and PBz/BMI/CNT
View Article Online
DOI: 10.1039/C7NJ00357A
was found to have a significant impact on the thermal
properties of PBz. Indeed, it is well known that both F-
MWCNT and Bzo will form π-π stacking with each other. As a
result of this π-π stacking, there is a strong interfacial
interaction between F-MWCNT and PBz in the
supramolecular level. On the other hand, PBz/BMI/F-MWCNT
nanocomposites
S.No. Samples
Ti
T5
T10
CY LO
(°C) (°C) (°C) (%)
I
1.
2.
3.
4.
5.
PBz
251 260 310 36.5 32.1
280 347 391 39.2 32.7
294 362 403 42.8 34.6
308 378 412 44.1 35.1
315 391 428 46.3 36.0
PBz/BMI (1:1)
PBz/BMI/CNT (1%)
PBz/BMI/CNT (3%)
PBz/BMI/CNT (5%)
nanocomposites have a network structure with high crosslink
34-36
density that restricts the mobility of the chain segments
.
In addition to this, there is a network of H-bonding
interactions, which makes the nanocomposites more stable
than PBz. Moreover, the co-reaction (i.e., an alder-ene type of
reaction) between PBz and BMI, results in the formation of
single sphere system. This leads to an increase in char yield (at
800 °C) from 36.5 % (for PBz) to 39.2 % [for PBz/BMI] and 42.8,
44.0 & 46.3 % [for PBz/BMI/F-MWCNT (0.1, 0.3 & 0.5)
nanocomposites], respectively.
6.Conclusion
The curing behavior, thermal, mechanical and dielectric
properties of PBz/BMI blend and PBz/BMI/F-MWCNT
nanocomposites have been investigated. BMI was used to
stabilize the benzoxazine moiety, as it limits the thermal
degradation of PBz by the formation of dense crosslinked
network through ene-addition. During the curing process, the
allyl functionality of eugenol undergoes heteropolymerization
with BMI that prevents the homopolymerization of BMI itself.
When combined as composites, the attractive characteristics
of all the three components i.e., PBz, BMI and F-MWCNT are
apparent. The introduction of F-MWCNT into PBz/BMI
copolymer changed the chemical cross-linked network, which
exerted a tremendous influence on the mechanical and
thermal properties. Compared to PBz, the higher crosslinking
density of PBz/BMI/F-MWCNT nanocomposites led to higher
degradation temperature [T5 of PBz/BMI/F-MWCNT (5%):
391 °C], mechanical stability [storage modulus of PBz/BMI/F-
MWCNT (5%): 6.32 GPa] and glass transition temperature [Tg
of PBz/BMI/F-MWCNT (5%): 246 °C] indicating better thermo-
mechanical stability of the nanocomposites at service
temperature. These enhancements are attributed to the
effective participation of F-MWCNT in the formation of
nanocomposite network, as these F-MWCNTs act as an
internal reinforcing framework hindering volume changes in
the nanocomposites. In addition, the excellent electrical
property of CNT is preserved during curing, that results with
materials possessing high conductivity [ε’ of PBz/BMI/F-
MWCNT (1, 3 & 5%): 4.97, 6.63 & 8.54]. In summary, F-
MWCNT along with BMI paves a way for the preparation of
sustainable high performance bio-based PBz/BMI/F-MWCNT
nanocomposites which will find use for advanced high
technology purposes.
Figure 13: TGA curves of a) PBz b) PBz/BMI c) PBz/BMI/F-
MWCNT (1%) d) PBz/BMI/F-MWCNT (3%) and e) PBz/BMI/F-
MWCNT (5%)
5.8 Flame retardant property of PBz/BMI/F-MWCNT
nanocomposites
The flame retardancy or the non-flammability of neat PBz,
PBz/BMI (1:1) blend and PBz/BMI/F-MWCNT nanocomposites
is explained in terms of limiting oxygen index (LOI) value. LOI is
defined as the minimum fraction of oxygen in a mixture of O2
and N2 that will support flaming combustion. The LOI values
are calculated from the char yield (CY) values obtained from
TGA analysis by using van Krevelan and Hofytzer equation 37 as
shown below,
LOI = 17.5 + 0.4 (CY)
The LOI value of PBz, PBz/BMI blend and PBz/BMI/F-
MWCNT (1, 3 & 5%) nanocomposites were found to be 32.1,
32.7, 34.6, 35.1 and 36.0, respectively. As these values are
greater than 26 (threshold value), we can say that these
materials have good self-extinguishing and flame retardant
properties 38
.
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26. Y. H. Wang, C. M. Chang and Y. L. Liu, Polymer, 2012,
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Functionalized MWCNT, an efficient reinforcement for the preparation of
eugenol based high performance PBz/BMI/CNT nanocomposite exhibiting
outstanding thermo-mechanical properties
Thirukumaran Periyasamy* a, Shakila Parveen Asrafali b, and Seong-Cheol Kim* a
This work contemplates the stabilization of eugenol-based benzoxazine networks with the addition
of BMI and amine functionalized multiwalled carbon nanotube.