Thermo-mechanical and dielectric properties of graphene reinforced caprolactam cardanol based benzoxazine-epoxy nanocomposites

Article abstract of DOI:10.1039/c4ra14383c

In the present work a new type of benzoxazine was synthesized using caproamine with cardanol and the molecular structure was characterized by FTIR and NMR spectroscopy. The graphene reinforced epoxy-polybenzoxazine nanocomposites were prepared by incorporating varying weight percentages of graphene and benzoxazine to an epoxy resin. Data obtained from the mechanical study infer the significant improvement in the values of tensile strength, impact strength and flexural strength of the composites reinforced with different weight percentages of benzoxazine and graphene. The values of the dielectric constant of the composite samples are appreciably enhanced and are higher than that of the neat epoxy matrix. Data from SEM, TEM and XRD ascertain the existence of homogeneous distribution of graphene in the composites. Data obtained from thermal, mechanical, dielectric and surface free energy studies infer that 20 wt% Bz-g reinforced composites possess better properties than those of the neat epoxy matrix and other weight percentages of composites.

Full text of DOI:10.1039/c4ra14383c

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Page 1 of 23
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Thermoꢀmechanical and dielectric properties of graphene reinforced caprolactam
cardanol based benzoxazineꢀepoxy nanocomposites
K. Sethuramana and M. Alagara*
a Polymer Composite Lab, Department of Chemical Engineering, Anna University, Chennai-600 025,
India
a) Tel.: + 91 44 2235 9164; Fax: +91 44 2235 9164; Electronic mail: mkalagar@yahoo.com and
muthukaruppanalagar@gmail.com
Abstract
In the present work a new type of benzoxazine was synthesized using caproamine with cardanol and
their molecular structure was characterized by FTIR and NMR spectroscopy. The graphene reinforced
epoxyꢀpolybenzoxazine nanocomposites was prepared by incorporating varying weight percentages of
graphene and benzoxazine to epoxy resin. Data obtained from mechanical study infer the significant
improvement in the values of tensile strength, impact strength and flexural strength of the composites
reinforced with different weight percentages of benzoxazine and graphene. The values of dielectric
constant of composite samples are appreciably enhanced and are higher than that of neat epoxy matrix.
Data from SEM, TEM and XRD ascertain the existence of homogeneous distribution of graphene in the
composites. Data obtained from thermal, mechanical, dielectric and surface free energy studies infer
that 20 wt% Bzꢀg reinforced composites possesses better properties than those of neat epoxy matrix and
other weight percentages of composites.
Key Words: Cardanol, caprolactam, benzoxazine, impact strength, dielectric constant.
Introduction
Polybenzoxazines are a class of thermosetting phenolic resins obtained from benzoxazine through ring
opening polymerization of oxirane ring in the absence of a catalyst, releasing no byproducts. The
Polybenzoxazine have many advantages like nearꢀzero shrinkage upon polymerization, low water
absorption, high char yield, excellent dimensional stability, flame retardance, stable dielectric constant,
and low surface free energy. Although, polybenzoxazines offer a variety of advantages, neat

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polybenzoxazineꢀbased polymers suffer a certain disadvantages too. The high temperature required for
curing and the brittleness of the cured materials limit their potential applications. To properly address
these issues and overcome the associated disadvantages, the most successful strategies are the
preparation of monomers with suitable molecular structure and blending them with high performance
polymer and suitable reinforcement1.Caprolactam (CPL) is a cyclic amide of caproic acid and is widely
used as precursor for synthetic polymer. Caprolactam possesses better combination of thermo
mechanical properties suitable for the fabrication of polymer composites because of its flexible
structure (ꢀNHꢀCOꢀ) formed during in situ reaction of cyclic aliphatic caprolactam with benzoxazine.
Cardanol is a well known phenolic compound obtained as a renewable bio source. Polybenzoxazine
developed using cardanol have received much research attention because they overcome many of the
deficiencies commonly associated with conventional novolac and resolꢀtype phenolic resins2,3. They
have demonstrated various attractive properties including high thermal stability, high char yield, high
glass transition temperature (Tg), nearꢀzero volumetric change upon curing, good mechanical and
dielectric properties, low water absorption, low flammability, and excellent solubility in common
organic solvents. These characteristics make benzoxazine polymers become an excellent materials for
high performance composites4ꢀ7.
Thermosetting resins can be used as an efficient dielectric material in embedded capacitor technology if
their electrical property is improved with suitable reinforcements8. Thermosetting polymers with metals
and ceramics are widely used in capacitor technology, however their high density, poor processability
and low adhesion strength restrict their use in lightweight aerospace applications9. If these materials are
blended with light weight materials like capron and graphene it will be suitable for high performance
light weight dielectric applications. Graphene exhibits fascinating electronic, thermal, and mechanical
properties due to its unique structure and also possesses an ability to be dispersed in various polymer
matrices make graphene become most suitable for the development of nextꢀgeneration polymer hybrid
nanocomposites10ꢀ12. Epoxy resins are well known industrial material used in the form of electronic
encapsulants, electrical laminates, structural adhesives and composite materials. Like Polybenzoxazine,
epoxy also exhibit an inherent brittle nature and poor crack resistance and limits its utility for high
performance applications. Many attempts have been made to improve toughness, stiffness and strength
of epoxy resins by blending with numerous chemical modifiers, reinforcements etc.13ꢀ16 An
improvement property of cardanolꢀepoxy composites and graphene–benzoxazine/epoxy
nanocomposites developed for capacitor applications are also reported in the literature17ꢀ19
.

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In the present work an attempt has been made to develop hybrid composite materials based on
industrially potential and commercially competitive resin ie epoxy and benzoxazine with graphene
reinforcement. The prime objective of the present work involves 1. Toughening of epoxy and
benzoxazine resin, 2. The development of benzoxazine graphene reinforced epoxy composites, 3. To
enhance thermo mechanical and dielectric behaviour to make them useful for efficient capacitive
applications by reinforcing with graphene, 4. To utilize renewable resource cardanol biomaterial for
synthesis of structurally modified benzoxazine suitable for toughening of epoxy resin, 5. To utilize
resilient caprolactam to enhance thermo mechanical behaviour of hybrid composites. With those
objectives in view the hybrid epoxy based composites reinforced with varying weight percentages of
Bzꢀg were developed and characterized by different sophisticated analytical techniques and the data
resulted are discussed and reported.
Experimental methods
Materials
Cardanol was obtained from Sathya Cashew products Chennai. Formaldehyde, Nꢀmethyl 2ꢀ
pyrrolidone, sodium hydroxide, potassium permanganate, chloroform, and hydrazine hydrate were
purchased from Merck and were used as received. Diglycidyl ether of bisphenol – A (DGEBA epoxy
resin) (LY556) was utilized as matrix resin in the present study and it was received from CibaꢀGeigy
Ltd. Natural graphite was purchased from Active Carbon India Pvt. Ltd., India. Sulfuric acid (H2SO4,
95ꢀ98%) was purchased from Ranbaxy Chemicals, India. Hydrogen peroxide (H2O2) was purchased
from SD Fine Chemicals, India. Sodium nitrate (NaNO3), caprolactam, sodium hypo phosphate,
dimethylformamide (DMF) and diphenyldiaminomethane (DDM) were purchased from SRL
chemicals, Chennai.
Synthesis of N, N’ꢀ(4,4'ꢀmethylenebis (4,1ꢀphenylene))bis(6ꢀaminohexanamide)ꢀ(CPA)
CPA was synthesized as per modified procedure reported earlier by our research group20. In a 250 mL
two necked round bottomed flask equipped with condenser, 100 mL of Nꢀmethyl pyrrolidone (NMP),
10g (0.0883 mol) of caprolactam, 7.8g (0.0442 mol) of 4, 4’ methylenedianiline and 5mg of alkali hypo

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0
phosphate catalyst were added. The mixture was allowed react at 100 C for 24 h with efficient
agitation to facilitate completion of reaction. The product was filtered by separation. (Yield 85%).
(Scheme 1)
Scheme 1. Synthesis of CPA.
Scheme 2. Synthesis of CPAꢀ cardanol benzoxazine (CBZ).
Synthesis of CPAꢀ cardanol benzoxazine (CBZ)
To the CPA (5g, 0.011mol) in chloroform, formaldehyde (1.4g, 0.047mol) was added and stirred for 30
o
minutes at 0 C. Subsequently, 7.42g of cardanol (0.023mol) was added to the reaction mixture and
stirred for overnight at 70 oC. After the completion of reaction, the reaction mixture was extracted with

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ethyl acetate and washed with 2N NaOH, water, brine and concentrated the organic layer to obtain the
product.(Yield 95 %).( Scheme 2) and preserved for further use.
Synthesis of graphene
Graphene Oxide (GO) was prepared from natural graphite through modified Hummer’s method21.
About 1 g of graphite and 1.2 g of NaNO3 were added to 69mL of concentrated H2SO4, and then 3 g of
0
KMO4 was gradually added with stirring and then the reaction mixture was heated at 35 C for 6 h. In
the next step, about 80mL distilled water was slowly added to the mixture, which was maintained at 60
0C for 30 min. Finally, 200mL distilled water and 6mL of 30 wt% H2O2 were added. The product was
washed repeatedly with Ultra pristine MilliꢀQ water using centrifugation method until the
homogeneous colloidal suspension was obtained. An aqueous suspension of exfoliated GO, with ~20
mg/mL was obtained by centrifugation. To the exfoliated GO 250 mL of water is added to get a
homogeneous dispersion. This dispersion was sonicated using an ultrasonicator. Hydrazine hydrate
0
(3.5ml. 0.1 mol) was then added and the solution was heated in an oil bath at 100 C under a waterꢀ
cooled condenser for 24 h over which the reduced GO gradually precipitated out as a black solid. The
product was isolated by filtration and washed with water and then with methanol and dried in hot air
oven and preserved for further use.
Development of benzoxazineꢀgraphene/epoxy nanocomposites
The benzoxazineꢀgraphene/ epoxy nanocomposites was developed by following method.The purified
benzoxazine monomer (0.2 mol) and 1 wt% of graphene was dispersed in 100 mL of DMF and taken in
a round bottomed flask and refluxed at 120 0C at 24 h. After cooling to room temperature, the above
product was filtered and used as the precursor for the composite preparation. The blend of benzoxazineꢀ
graphene (Bzꢀg) was added with varying percentages (5wt%, 10wt%, 15wt% and 20wt %) to epoxy
resin. The stiochometric amount of DDM curative was added (Table 1) to epoxy resin and then cured at
1400C for 1 h, 1800C for 1 h, and post cured at 2200C for 2 h.

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Table 1. The composition material used sfor the development of neat epoxy and Bzꢀg/Ep
nanocomposites.
Epoxy resin Benzoxazine ꢀ graphene
Sample code
DDM (g)
(g)
40
38
36
34
32
blend (g)
Neat Epoxy
5wt% Bzꢀg/Ep
10wt% Bzꢀg/Ep
15wt% Bzꢀg/Ep
20wt% Bzꢀg/Ep
0
2
4
6
8
11.65
11.06
10.50
9.90
9.32
Scheme 3. Development of benxoxazineꢀgrahene/ epoxy nanocomposites.

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Characterization
Fourierꢀtransform infrared (FTIR) spectra of KBr disks were obtained using a Bruker Tensor 27 FTꢀIR
spectrophotometer. Proton nuclear magnetic resonance (1HꢀNMR) and 13CꢀNMR of the benzoxazine
monomer were obtained from Bruker AV III FTꢀNMR spectrometer operating at a frequency of 500
MHz. The sample was dissolved in deuterated chloroform and tetramethylsilane was used as an internal
standard. Thermogravimetric and differential scanning calorimetric analyses of the composites were
carried out with a Exstar 6300 at a heating rate of 10 °C/min under nitrogen atmosphere.
Raman spectra were measured on a Lab RAM HR UV–VIS–NIR Raman Microscope from HORIBA
Jobin–Yvon (633 nm laser source). The dielectric studies of the neat epoxy and Bzꢀg/Ep
nanocomposites were determined using Broad band Dielectric Spectrometer (BDS), NOVOCONTROL
Technologies GmbH & Co. (Model Concept 80) at 30ºC in the range of 1Hz to 1MHz.For dielectric
measurement the samples were made as pellet and directly used.
The surface morphology of the fractured surface of the samples was examined using a scanning
electron microscope (SEM; JEOL JSM Model 6360). A JEOL JEMꢀ3010 analytical transmission
electron microscope operating at 80 kV with a measured pointꢀtoꢀpoint resolution of 0.23 nm was used
to characterize the phase morphology of the developed nanocomposites. TEM samples were prepared
by dispersing powder sample of composites under sonication in ethanol and were mounted on carbonꢀ
0
coated Cu TEM grids and dried for 1 h at 70 C to form a film of <100 nm. Contact angle
measurements were carried out using 210 a Rameꢀhart Inc. goniometer (Succasunna, NJ, USA) with 5
ml of deionised water and diiodimethane (DIM).
Xꢀray diffraction patterns were recorded at room temperature, by monitoring the diffraction angle 2θ
from 10 to 80˚ as the standard, on a Rich Seifert (Model 3000) Xꢀray powder diffractometer. The
tensile strength was determined, using INSTRON (Model 6025 UK) as per ASTM D 3039 at 10 mm
minꢀ1 crossꢀhead speed, using specimens with dimensions of 100 mm×25 mm ×3 mm. The flexural
strength was measured by the INSTRON (Model 6025 UK) as per ASTM D 790, using specimens with
dimensions of 100 mm ×10 mm × 3 mm at 10 mm minꢀ1 crossꢀhead speed. The unꢀnotched Izod impact

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strength of sample was studied as per ASTM D 256, using specimen dimensions of 65 mm × 10 mm ×
3 mm.
Results and Discussion
FTIR Spectra
Figure 1a shows the FTIR spectra of caproamine (CPA). In the figure the peak appeared at 3347 cmꢀ1
represents for NꢀH stretching vibration. Peaks appeared at 2931, 1670 and 1609 cmꢀ1 represent for C=O
amide, NH2 deformation and aromatic CꢀN stretching respectively. Figure 1b presents the FTꢀIR
spectrum of GO and graphite. The O–H stretching is appeared at 3300–3500 cmꢀ1 as a broad peak, C=O
and CꢀO stretching vibration occurs at 1717 cm−1 and 1204 cm−1 respectively22 and confirm the
formation of graphene oxide.
CPA
b
a
Graphite
Graphene Oxide (GO)
1670
1609
2931
3347
1717
1204
1045
1000 500
4000
3500
3000
2500
Wave number(cmꢀ1)
2000
1500
4000
3500
3000
2500
2000
1500
1000
500
Wave number(cmꢀ1)
Figure 1. (a) FTIR spectra of CPA amine and (b) Graphite and Graphene oxide.
Figure 2a shows the FTꢀIR spectra of cardanol benzoxazine (CBZ). The peaks appeared at 1227 cmꢀ1
and 1035 cmꢀ1 represent asymmetric and symmetric stretching vibrations of CꢀOꢀC bond in the
benzoxazine respectively. The appearance of new peak at 945 cmꢀ1 corresponds to stretching vibration
of CꢀO moiety from benzoxazine.

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20% Bzꢀg/Ep
10% Bzꢀg/Ep
Neat Epoxy
15% Bzꢀg/Ep
5% Bzꢀg/Ep
Cardanol benzoxazine (CBZ)
a
b
3320
1035
1227
2865
2925
945
3410
1627
2000
2853
2500
1495
1500
1115
3500
3000
2500
1000
3500
3000
2000
1500
1000
Wavenumer (cmꢀ1
)
Wavenumer (cmꢀ1
)
Figure 2. FTIR spectra of (a) Cardanol benzoxazine (b) Neat epoxy and Bzꢀg/Ep composites.
The peak appeared at 1495 cm−1 is due to the trisubstituted benzene ring of benzoxazine. The peak
appeared at 1627 cmꢀ1 represents C=C stretching vibration in benzene ring. Further the characteristic
absorptions at 3320 cmꢀ1 represents the functional hydroxyl group and the peaks appeared in ranges
between 2925 and 2865 cmꢀ1 corresponds to alkyl side chain of cardanol 23, 24
.
Figure 2b illustrates the FTꢀIR spectra of the neat epoxy matrix and Bzꢀg/Ep nanocomposites. After
thermal curing, the Oꢀ. . ..H+N intramolecular hydrogen bonding was observed around 2850 cmꢀ1 and
OH. . ..O intermolecular hydrogen bonding was observed around 3410 cmꢀ1.25 The disappearance of
peak at 946 cmꢀ1 further confirms the formation of network structure. From 5wt% Bzꢀg/Ep to 20wt%
Bzꢀg/Ep the characteristic absorption peaks again observed at 1115 cmꢀ1 (symmetric stretching CꢀOꢀC)
and 1258 cmꢀ1 (asymmetric stretching CꢀOꢀC) are due to the formation of ether linkages in the Bzꢀg/Ep
system. The intensity of absorption peaks at 1252 cmꢀ1and 1007 cmꢀ1 is increased with increases of Bzꢀ
g/Ep content and this may be explained due to the formation of strong ether linkage between the
hydroxyl groups of graphene and Polybenzoxazine.

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NMR Spectra
Figure 3. (a) 1H NMR and (b) 13C NMR spectra of CPA.
1
Figure 3a shows the H NMR spectra of CPA, in the spectra aromatic protons appeared between 6.6
and 6.9 ppm, amine protons appeared between 3.1 and 3.5 ppm and the aliphatic protons appeared
between 1.6 and 2.4 pm. In 13C NMR (Figure 3b) the carbonyl carbons appeared at 179 ppm, aromatic
carbons appeared between 115 and 144 ppm and aliphatic carbons appeared between 23 and 42 ppm.
Figure 4. (a) 1H NMR and (b) 13C NMR spectra of CBZ.

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1
Figure 4a shows the H NMR spectra of cardanol benzoxazine (CBZ). From the spectra the peaks
appeared from 6.75 ppm to 7.23 ppm were assigned to the aromatic protons. The characteristic protons
of oxazine ring appeared at 4.5 ppm and 5.2 ppm are assigned to ꢀOꢀCH2ꢀNꢀ and Arꢀ CH2ꢀNꢀ
respectively. The amine protons appeared at 3.2 ppm and the peak at 5.36 ppm are due to the vinyl
double bond C =CH protons of long alkyl side chain present in cardanol skeleton. In addition, all other
13
aliphatic protons are appeared from 0.8ppm to 2.8 ppm. In C NMR (Figure 4b) the carbonyl carbons
appeared at 179 ppm, aromatic carbon appeared from 115 ppm to 144 ppm and aliphatic carbon atoms
are appeared from14 ppm to 36 ppm 26.
XRD analysis
In the diffractogram (Figure 5a) of pristine graphite sample the peak appeared at 26.5⁰, indicates the
(002) plane with interlayer spacing of 0.34 nm, whereas (002) plane of the graphene appeared at 11.7⁰,
with increase in interlayer spacing of 0.75 nm. The shifting of (002) plane of graphite ascertains the
formation of typical GO, and confirms the exfoliation of graphite during the oxidation process. The
hybrid network structure of neat epoxy, 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep and 20wt% Bzꢀg/Ep
nanocomposites were characterized with XRD and the data obtained are presented in Figure 5b. The
XRD patterns exhibit a broad amorphous with peak maximum at 2θ = 18.5⁰ which implies that the
graphene molecules are homogeneously dispersed in epoxy benzoxazine networks through covalent
bonding in the hybrid nanocomposites.
20wt% Bzꢀg/Ep
b
10wt% Bzꢀg/Ep
a
5wt% Bzꢀg/Ep
Neat Epoxy
Graphite
Graphene oxide
5
10
15
20
25
30
10
20
30
40
50
60
2θ (degree)
2θ (degree)
Figure 5. XRD spectra of (a) Graphene oxide and Graphene (b) Neat epoxy and Bzꢀg/Ep composites.

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Raman Spectra
1347
D band
1598
G band
Grapene
1577
1343
Graphene Oxide
500
1000
1500
ꢀ1
2000
Wave number (cm )
Figure 6. Raman spectra of graphene oxide and graphene.
Figure 6 represents the Raman spectra of graphene oxide and graphene. The G peak and 2D peak are
the characteristics of sp2 hybridized carbon–carbon bonds in graphene. In graphene oxide, the D band
appeared at 1343 cmꢀ1 with lower intensity than that of the G band due to the decrease in the size of the
sp2 plane. The G band appeared at 1573 cmꢀ1 indicates the presence of isolated double bond
resonances. After reduction of graphene oxide, the D and G bands of graphene are appeared at 1347
cmꢀ1 and 1598 cmꢀ1 respectively. (ID/IG) is an indicator for the disordered structure of the graphitic
structure.The ID/IG value of graphene oxide is 0.89 after reduction of GO the value becomes 1.16. The
increased ID/IG ratio of graphene after the chemical reduction of GO results modified structure of GO
and this shows the occurrence of graphitisation in the reduced product27,28
.
Mechanical Properties
The mechanical properties of neat epoxy matrix, 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep, 15wt% Bzꢀg/Ep and
20wt% Bzꢀg/Ep are presented in Table 2. The impact strength of DGEBA matrix with 5wt% Bzꢀg/Ep,
10wt% Bzꢀg/Ep, 15wt% Bzꢀg/Ep and 20wt% Bzꢀg/Ep shows an improvement of 15%, 24%, 40% and
56% respectively when compared with that of neat epoxy matrix.

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Table 2. Mechanical properties of neat epoxy and Bzꢀg/Ep nanocomposites
Tensile
strength
(MPa)
Flexural
strength
(MPa)
Impact
strength
(J/m)
Sample
Code
Dielectric
Constant (k)
Neat epoxy matrix
5wt% Bzꢀg/Ep
61.34
65.26
69.36
71.26
82.37
106.82
109.34
134.64
152.26
168.61
101.85
117.37
126.74
142.27
159.30
3.62
5.81
7.68
9.21
10.48
10wt% Bzꢀg/Ep
15wt% Bzꢀg/Ep
20wt% Bzꢀg/Ep
The improvement of impact strength is mainly attributed to the presence of flexible amide linkages (ꢀ
NHꢀCOꢀ) present in capron and the aliphatic skeleton of cardanol based benzoxazine 20, 29. Thus the
caprolactam based benzoxazine contributes to more flexibility to Bzꢀg/Ep composites. Thus the excess
of free volume caused by the chain entanglement with high energy absorption is the key factor which
contributes to the toughness behavior and the same was found more predominant in the case of capron
based benzoxazine.
The values of flexural strength of 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep , 15% Bzꢀg/Ep and 20% Bzꢀg/Ep
nanocomposites are increased to 2.3%, 26.04%,42.51% and 57.84%, respectively, when compared
with that of neat epoxy matrix system. This is due to the formation of an interpenetrating network
between the molecular chains of epoxy and caprolactam cardanol based benzoxazine and contributes an
enhanced stress dissipating behaviour. The neat epoxy matrix shows the value of tensile strength of
61.34 MPa. Whereas the tensile strength values of 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep, 15wt% Bzꢀg/Ep
and 20wt% Bzꢀg/Ep are 65.26, 69.36, 71.26 and 81.37 MPa respectively. The 20wt% Bzꢀg/Ep
composite material shows the higher value of tensile strength than that of other composites, this infers
that the highest interfacial interaction resulted between epoxy and Bzꢀg layers30, 31. In addition to
interfacial interaction, the enhancement in the value of tensile strength is also due to the

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intermolecular hydrogen bonding and long chain polar polyamide (PA6) and aliphatic skeleton in the
benzoxazine, are these contributes to significant improvement in load bearing properties for the
resulting composites.
Thermal stability of Bzꢀg/Ep nanocomposites
Thermo Gravimetric Analysis (TGA)
The thermal stability data obtained for neat epoxy matrix, polybenzoxazine (PCBZ) and Bzꢀg/Ep
composites are shown in Figure 7 and Table 3. The neat epoxy matrix found to show an inferior
0
thermal stability, whose char yield became zero at 550 C, where as that of polybenzoxazine and Bzꢀ
g/Ep composites are increased to an appreciable extent. There is a small amount of weight loss below
o
240 C in all the systems was noticed due to the removal of removal of water molecules, unreacted
hydroxyl groups and carbonyl groups of acid from the graphene moiety of composites.
20wtzꢀg/Ep
15wtzꢀg/Ep
10wtzꢀg/Ep
100
5wtzꢀg/Ep
PCBZ
80
Neat Epoxy
60
40
20
0
100
200
300
400
500
600
700
Temperature (oC)
Figure 7. TGA profile of neat epoxy and Bzꢀg/Ep composites

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However the degradation occurred between 320⁰C to 430⁰C was explained due to the decomposition of
flexible amide segments of caprolactam of benzoxazine. The polybenzoxazine which contains flexible
long chain with aromatic group degrades completely at 725⁰C. Whereas, on increasing the wt% of
benzoxazineꢀgraphene in epoxy system the significant improvement in thermal stability was noticed.
The char yield values of 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep, 15wt% Bzꢀg/Ep and 20wt% Bzꢀg/Ep are
4.5%, 8.0%, 11.6% and 19.8% respectively. This is due to the long aliphatic chain with aromatic
moiety present in the cardanol which imparts more steric hindrance and further the reinforcement of
graphene improves thermal stability with restricted the chain moility32,33
.
Table 3. Thermal properties of neat epoxy and Bzꢀg/Ep nanocomposites.
Weight loss
Sample
Char
Tg (⁰C)
20 (%) 40(%) 60 (%)
Yield at
Code
Temperature (⁰C)
750 (⁰C)
Neat epoxy
Polybenzoxazine(PCBZ)
5wt% Bzꢀg/Ep
339
370
358
367
391
370
410
386
0
167
ꢀ
330
340
372
338
364
394
400
410
413
452
0
4.5
8.0
11.6
19.8
183
189
199
210
10wt% Bzꢀg/Ep
15wt% Bzꢀg/Ep
20wt% Bzꢀg/Ep
Differential Scanning Calorimetry (DSC)
The DSC analyses of epoxy reinforced with different weight percentages of Bzꢀg/Ep composites are
illustrated in Figure 8 and Table 3. From the Table it is observed that the neat epoxy matrix exhibits
the value of glass transition temperature of 167 ⁰C and the Tg value is increased with increase in weight
percentages of Bzꢀg/Ep. This is due to the enhanced chain entanglement resulted between the graphene

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reinforced benzoxazine and epoxy matrix as well as with polyamide and cardanol segment of
benzoxazine. The Bzꢀg restricts the chain mobility of the matrix due to the inter flake interaction and
piꢀpi stacking between the graphene with epoxy and benzoxazine. The 20wt% Bz –g shows the highest
0
value of Tg of 210 C and is due to the enhanced crosslinking and entanglement of long molecular
chains present in the network.
Figure 8. DSC profile of neat epoxy matrix and Bzꢀg/Ep composites.
Dielectric constant (k)
The value of dielectric constant of the neat epoxy matrix and Bzꢀg/Ep nanocomposites are presented in
Figure 9a and Table 2. With the increasing wt% of Bzꢀg to epoxy, the value of dielectric constant was
increased. The value of dielectric constant of neat epoxy matrix is 3.6 and is increased to 5.8 for 5wt %
Bzꢀg/Ep at 1 MHZ and subsequently increased to 7.6, 9.2 and 10.5 with increasing concentration of Bzꢀ
g 10, 15 and 20wt% respectively. An incorporation of filler and/or reinforcement in polymer matrix
influences the dielectric behaviour of the reinforcing composites. The reinforcing materials have a
significant effect on the migration and accumulation of charge carriers at the interfaces. An
improvement of average electric field of the polymer matrix, attained with the reinforcement of
graphene contributes to the enhanced value of dielectric constant. The increase in the value of dielectric

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constant was attributed to the motion of free charge carriers due to the formation of continuous
conductive pathway throughout the nanocomposites 34.
0.03
Neat Epoxy
a
b
20wt% Bzꢀg/Ep
10wt% Bzꢀg/Ep
Neat Epoxy
15wt% Bzꢀg/Ep
5wt% Bzꢀg/Ep
5% BzꢀG/Ep
10% BzꢀG/Ep
15% BzꢀG/Ep
20% BzꢀG/Ep
12
9
0.02
0.01
0.00
6
3
10000
100000
1000000
10000
100000
1000000
Frequency
(
Hz)
Frequency (Hz)
Figure 9. A plot of electrical properties dependent on frequency for neat epoxy matrix and Bzꢀg/Ep
nanocomposites (a) Dielectric constant (b) Dielectric loss.
Homogeneous dispersion of graphene into the epoxy matrix creates a local amplified electric field and
thus contributes to an enhanced value of capacitance to the nanocomposites. Hence, an increase in the
value of dielectric constant was observed with an increase in wt % benzoxazineꢀ graphene content,
where as it was also found to be decreased with increase in frequency, which is attributed to the
occurrence of leakage current from the longꢀrange transport of charge carrier in the low frequency
range35.
The values of dielectric loss are presented in Figure 9b for neat epoxy and Bzꢀg/Ep nanocomposites. At
1 MHz the tan δ value for neat epoxy matrix is about 0.012, whereas in the case of 20wt% Bzꢀg/Ep
shows the tan δ value is 0.001. It was observed that there was no appreciable change in the value of
dielectric loss for 20wt% BzꢀgꢀEp at higher frequency range. Therefore, the lowering of dielectric loss
tangent of a graphene reinforced benzoxazine/epoxy can be explained due to formation of insulating
layer outer to the dielectric centre and thus results the restricted migration and accumulation of charges

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within the nanocomposites. Data from dielectric studies infers that these materials can be used as an
effective and efficient high k material for microelectronic applications.
Effect of Bzꢀg/Ep loading on Morphological Properties.
10 µm
10 µm
10 µm
10 µm
Figure 10. SEM images of (a) 5wt% Bzꢀg/Ep, (b) 10wt%s Bzꢀg/Ep, (c) 15wt% Bzꢀg/Ep and (d) 20wt%
Bzꢀg/Ep composites.
Figure 10 illustrates the SEM images of fractured surfaces of 5wt% Bzꢀg/Ep, 10wt% Bzꢀg/Ep, 15wt%
Bzꢀg/Ep and 20wt% Bzꢀg/Ep composites. From the images it is inferred that the graphene layers are
well dispersed in the benzoxazine/epoxy matrix without any significant aggregation. The fractured
surface of the sample shows higher amount of rough surface which might be due to the presence of
graphene in the polymer composite. It is clearly ascertained that with an increase in Bzꢀg wt%, there
was the formation of small clusters due to polyamide linkage of benzoxazine and were found in the
epoxy matrix at lower level of reinforcement. Thus the nanocomposites hold two phases such as
nanofibril phase and normal matrix phase and these results are concur with the literature reported36.

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Transmission Electron Microscope (TEM)
TEM images of 20% Bzꢀg/Ep nanocomposites are presented in Figure 11. The presence of graphene
layer in the epoxy/benzoxazine network is clearly seen in Figure 11(a and b). These results indicate that
the Bzꢀg/Ep nanocomposites possess good interfacial interaction through the chemical bonding. The
nanometerꢀlevel dispersion of the graphene molecules in the epoxy matrix influences the thermal and
electrical properties of the composites.
a
b
Figure11. TEM image of 20% Bzꢀg/Ep composites.
Surface free energy
The contact angle measurements were performed using a goniometer and the values are presented in
Figure 12 and Table 4. The values of contact angle of neat epoxy matrix and different weight
percentages (5,10,15 and 20 wt%) of Bzꢀg/Ep reinforced composites are 75.6°, 78.4°, 80.2°, 84.7° and
90.0 respectively using 5 µl of deionised water as probe liquid, which infers that the hydrophobic
nature increased with increase in wt% of benzoxazine. Similarly when diiodomethane was used as a
dispersive liquid the value of contact angle was found to be 40.5°, 48.3°, 56.1°, 57.6° and 60.3o
respectively.

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Table 4. The values of contact angle and surface free energy of neat epoxy matrix and Bzꢀg /Ep
composites.
Contact angle (⁰)
Surface free energy (J/mꢀ1)
Sample
Code
Water
DI
ϒd
ϒp
6.4
5.2
2.6
1.9
1.2
Γ
Neat epoxy
5% Bzꢀg/Ep
10% Bzꢀg/Ep
15% Bzꢀg/Ep
20% Bzꢀg/Ep
75.6
78.4
80.2
84.7
90.0
40.5
48.3
57.6
50.5
60.3
39.4
35.2
33.9
32.2
30.5
45.8
40.4
36.5
34.1
31.7
The value of contact angle can be used to estimate the surface free energy of composites surfaces. The
surface free energy (┌) of neat epoxy matrix and Bzꢀg/Ep nanocomposites were calculated according to
the geometric mean model37.
d
p
Cosθ = 2/γL [(γL γsd)1/2 + (γL γsp)1/2
γs = γsd + γsp
(1)
(2)
Where θ is the contact angle and γL is the liquid surface tension; and γsd and γsp are the dispersive and
polar components of γL, respectively
From Table 4, it can be seen that the 20% Bzꢀg/Ep nanocomposites exhibited the lowest value of
surface free energy (31.7 mJ/m2) when compared to that of other percentages of Bzꢀg reinforced
nanocomposites. This is attributed due to the effective interaction occurred between benzoxazine and
the epoxy matrix, which favors the enhanced hydrophobic behaviour of the resulted matrix.

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(ii)Diiodomethane
(i)Water
a
c
b
a
b
c
d
e
e
d
Figure 12. Contact angle images of (a) Neat epoxy matrix (b) 5%Bzꢀg/Ep,(c) 10%Bzꢀg/Ep,(d) 15%Bzꢀ
g/Ep and (e) 20%Bzꢀg/Ep composites.(i) Water (aꢀe) and (ii) Diiodomethane(aꢀe).
Conclusion
Epoxy based toughened nanocomposites have been prepared by casting technique, with varying weight
percentages of Bzꢀg. Data obtained from thermoꢀmechanical and dielectric properties show that 20%
Bzꢀg/Ep possess improved properties than those of neat epoxy matrix and other weight percentages of
Bzꢀg/Ep reinforced composites. The contact angle studies show that 20% Bzꢀg/Ep possesses the lowest
surface free energy than that of other samples. TEM analysis further supports the homogeneous
dispersion of graphene to benzoxazine/epoxy composites. Data obtained from different studies indicate
that, these materials can be used in the form of coatings, adhesives, sealants, encapsulants etc., as
dielectric materials in microelectronic applications for better performance with improved longevity.
Acknowledgment
The authors thank CSIR G. No: 02(0112)/12/EMRꢀII CSIR, New Delhi , Govt. of India., for the
financial support and also thank to Dr. M.R.Vengatesan, South Korea, Dr. S. Devaraju, Pusan National

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University, Busan, R.Sasi Kumar and Mrs. Kanimozhi, Research Scholar Dept. of Chemical
Engineering, AC Tech, Anna University for their support.
References
1. Y. Yagci, B. Kiskan and N.N. Ghosh, J Polym Sci Part A: Polym Chem, 2009, 47, 5565.
2. N.N. Ghosh, B. Kiskhan, and Y. Yagci, Prog. Polym. Sci., 2007, 32, 1344.
3. C.P. Reghunadhan Nair, Prog. Polym. Sci., 2004, 29, 401.
4. A. Chernykh, T. Agag and H. Ishida, Polymer, 2009, 50, 382.
5. Y. Liu, S. Zhao, H. Zhang, M. Wang, and M. Run, Thermochim.Acta, 2012, 42, 549.
6. C. Andronescu, S.A. Garea, C. Deleanu, and H. Iovu, Thermochim.Acta, 2012, 4, 530.
7. H. Ishida and D.J. Allen, Polymer, 1996, 35, 4487.
8. D.D.L. Chung, Polym. Polym. Compos. 2000, 8, 219.
9. Z.M. Dang, J. K. Yuan, J.W. Zha, T. Zhou, S.T. Li and G.H. Hud, Prog. Mater Sci., 2012, 57, 660.
10. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva
and A. A. Firsov, Science, 2004, 306, 666.
11. L.M. Veca, M. J. Meziani,W. Wang, X. Wang, F. S. Lu, P. Y. Zhang, Y. Lin, R. Fee, J. W. Connell
and Y. P. Sun, Adv. Mater., 2009, 21, 2088.
12. V. E. Muradyan, A. A. Arbuzov, E. A. Sokolov, S. D. Babenko, and G. V. Bondarenko, Tech Phys
Lett., 2013, 39, 1.
13. L.C. Tang,Y. J.Wan, K. Peng, Y.B. Pei, L.B. Wu and L.M. Chen, Compos Part A, 2013, 45, 95.
14. S. Nagendiran, M. Alagar and I. Hamerton, Acta Materialia, 2010, 58, 3345.
15. M.R. Vengatesan, S. Devaraju and M. Alagar, High performance Polymers, 2011,23,3.
16. S. Nagendiran, S. Premkumar and M. Alagar, Journal of Applied Polymer Science 2007,106, 1263.
17. R. Verdejo, M.M. Bernal, L.J. Romasanta and M.A. LopezꢀManchado, J Mater Chem 2011, 21,
3301.
18. K. Huanga, Y. Zhanga, L. i. Mei, J. Lian, Y. Xiaohua and X. Jianling, Progress in Organic
Coatings, 2012, 74, 240.
19. S. Sahila, L.S. Jayakumari, Polymer Composites, 2014, DOI 10.1002/pc
20. K.Sethuraman, P. Prabunathan and M. Alagar, RSC Advances, 2014, 4, 45433.
21. X.F. Zhou and Z.P. Liu, Chem. Commun. 2010, 46, 2611.

Page 23 of 23
RSC Advances
View Article Online
DOI: 10.1039/C4RA14383C
22. K. Sethuraman, P. Prabunathan and M. Alagar, RSC Advances, 2014, 4, 30485.
23. T. Agag and T. Takeichi, Macromolecules, 2003, 36, 6017.
24. Y.J. Lee, J.M. Huan, S.W. Kuo, J. K. Chen and F.C Chang, Polymer, 2005,46, 2320.
25. J. Wang, X. Fang, M.Q. Wu, X. Y. He, W.B. Liu, and X. D. Shen, Eur. Polym. J. 2011, 47, 2158
26. B.S. Rao and Aruna Palanisamy, Progress in Organic Coatings, 2012, 74, 427.
27. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T.
Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558.
28. V. C. Tung, M. J. Allen, Y. Yang and R. B. Kaner, Nat. Nanotechnol., 2008, 4, 25.
29. S. Premkumar, C. K. Chozhan and M. Alagar, Eur. Polym. J., 2008, 44, 2599.
30. L.C. Tang a, Y.J. Wan, D. Yan, Y.B. Pei, L. Z, Y.B. Li, L.B. Wu, J.X. Jiang and G.Q. Lai, Carbon,
2013, 60, 16.
31. Y.J. Wan, L.C. Tang, L.X. Gong, D. Yan, Y.B. Li, L.B. Wu, J.X. Jiang and G.Q. Lai, Carbon,
2014, 69,467.
32. M. Selvi, S. Devaraju, M. R. Vengatesan, J. S. Go, Manmohan Kumar and M. Alagar, RSC
Advances, 2014, 4, and 8238.
33. W. Gao, L. B. Alemany, L. Ci and P. M. Ajayan, Nat. Chem., 2009, 1, 403.
34. Y. Zhang, Y. Wang, Y. Deng, M. Li, and J. Bai, Mater. Lett., 2012, 9 72.
35. N. J. Delollis and O. Montoya, J. Appl. Polym. Sci., 1967, 11, 983.
36. S. Ahmadi, J. Morshedian, S.A. Hashemi, P.J. Carreau, and W. Leelapornpisit, Iran Polym
J., 2010, 19, 229.
37. S. Devaraju, M. R. Vengatesan, M. Selvi, A. Ashok Kumar, I. Hamerton, J. S. Go and M.
Alagar, RSC Adv., 2013, 3, 12915.