Synergistic effect of three-dimensional multi-walled carbon nanotube-graphene nanofiller in...

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eSynergistic effect of three dimensional multi-walled carbon

nanotube/graphene nanofiller in enhancing the mechanical and thermal

properties of high performance silicone rubber

Bratati Pradhan and Suneel Kumar Srivastava�

Inorganic Materials and Nanocomposite Laboratory Department of Chemistry Indian Institute of Technology Kharagpur-721302, India

Correspondence to: Suneel Kumar Srivastava (E-mail: [email protected])

Tel.: (+91-03222) 283334; Fax: (+91-3222) 255303

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/pi.4627

This article is protected by copyright. All rights reserved.

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eAbstract:

The homogeneous dispersion of nanofillers and filler–matrix interfacial interaction are

important factors in the development of high-performance polymer materials for various

applications. In the present work, a simple solution mixing method was used to prepare multi-

walled carbon nanotube (MWCNT)-graphene (G) (3:1, 1:1, 1:3) hybrids followed by their

characterization through wide angle X-ray diffraction, transmission electron microscopy and

thermogravimetric analyses. Subsequently, MWCNT-G (1:1) hybrid was used as reinforcing

filler in the formation of silicone rubber (VMQ) nanocomposites by solution intercalation and

investigated their morphology and properties. Our findings showed that MWCNT-G (0.75 wt

%)/VMQ composite exhibited significant improvements in tensile strength (110 %) and

Young’s modulus (137 %) compared to neat VMQ. Thermal stability of MWCNT-G (1 wt

%)/VMQ was maximum improved by 154 °C compared to neat VMQ. Differential scanning

calorimetry demonstrated the maximum improvement of glass transition temperature (4 °C),

crystallization temperature (8 °C) and melting temperature (5 °C) in 1 wt % MWCNT-G

filled VMQ nanocomposites with respect to neat VMQ. Swelling measurements confirmed

that the crosslink density and solvent resistance property was found to be maximum for

hybrid nanocomposites. Such improvements in the properties of MWCNT-G/VMQ

nanocomposites could be attributed to a synergistic effect of the hybrid filler.

Keywords: silicones, MWCNT, graphene, nanocomposites, mechanical properties, thermal

properties.

INTRODUCTION

In the last few years, 1D carbon nanotube (CNT)1-4 and 2D graphene5-8 have

generated considerable amount of interest due to their excellent properties. Though, the

chemical composition of both these graphitic forms are identical, graphene has the major

advantage due to abundant availability of graphite as a naturally occurring precursor and its

production cost compared to CNT. These graphitic materials find applications in solar cells,

actuators, batteries, super capacitors, optical/electrochemical devices, sensors, catalysis,

organic light emitting diodes, and field emission transistors etc.9-13 In addition, they have also


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ebeen used as effective nanofillers in order to improve the electrical, mechanical, thermal

properties of the polymers.14,15 However, the agglomeration remains one of the most common

problems in CNT as well as graphene due to the presence of inter-tubular interaction and

restacking graphene sheets respectively.16,17 In addition, the chemical stability and lack of

available functional sites on the surface also accounts for their poor dispersion in many

common organic solvents as well as in the polymer matrix.18,19 Though, the covalent

functionalization can overcome these difficulties,20 it further introduces the serious damage

on the graphitic structure deteriorating its properties. Therefore, it remains a major challenge

in finding out the ways to enhance the dispersion of graphene and CNT in the polymer

matrix/solvent preserving their intrinsic properties

Recently, the 3D materials have been receiving the considerable amount of attention

due to their interesting properties and applications.21-34 In this regard; 3D CNT/graphene

hybrids have been obtained by the hybridization of 1D MWCNT with 2D graphene, which

can overcome the dispersion problem faced by the individual fillers. It is suggested that the

nanoscale separation of the individual CNT and graphene sheets takes place due to the π−π

interaction between the 1D CNT and 2D graphene.35,36 As a result, the long and tortuous

CNT entangles with the graphene nanosheets inhibiting their aggregation and could result in

the synergistic characteristics of the hybrid filler on morphological dispersion and its

enhanced physical properties.30-33 Accordingly, the preparation of polymer nanocomposites,

e.g. polystyrene,37 epoxy,38 poly (styrene-co-butadiene-co-styrene),39 polyvinyl alcohol,40

filled with the hybrid fillers has been receiving the momentum for their advanced

applications. Recently, the effect has been made to use 3D hybrid materials to enhance the

mechanical and electrical properties of polymer nanocomposits.23,32,33 Hu et al.34 investigated

the effect of dispersion of graphene and MWCNT in silicone rubber. They concluded that the


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estrong interaction between the silicone rubber matrix and graphene/MWCNT facilitates the

excellent dispersion of MWCNT assisted by graphene.

Silicone rubber (VMQ) is one of the unique high performance elastomer due to its

excellent physical and chemical properties. However, the poor mechanical strength restricts

its multifaceted applications in many fields.41 Therefore, VMQ has been reinforced with

clay,42 nanosilica,43 carbon nanotubes,44 graphene,44 and layered double hydroxides,45 etc.

However, the further improvement in the mechanical and thermal properties of VMQ still

remains a challenge, especially in medial and aerospace applications. Recently, the electrical

conductivity and thermal diffusivity measurements have been reported on SR/MWCNT/G

nanocomposite.34 Motivated by this, we focused our work on synthesis of 3D MWCNT-

graphene hybrid reinforced VMQ nanocomposites prepared by solution intercalation and

investigated their morphology, mechanical properties, thermal stability and swelling

behavior. It is anticipated that MWCNT-graphene hybrid, due to its homogeneous dispersion

and its strong interaction with the VMQ matrix, could impart the extraordinary mechanical

and thermal properties simultaneously compared to that attained by individually filled

MWCNT or graphene nanocomposites. It is also anticipated that composites could also be

effective for electromagnetic shielding effect due to the high conductivity of the fillers

used.46,47

EXPERIMENTAL

Materials

Surface enhanced natural graphite AKA SEFG 3777 (purity 99%, particle size 19 µm,

surface area 23 m2/g) was received as a gift from the Asbury Graphite Mills, INC, Asbury,

Warren County, NJ. Carbon nanotube, multiwalled 724769 (carbon > 95%, O.D × L 6-9 nm

× 6 μm) were purchased from Sigma-Aldrich. Sodium nitrate (NaNO3) was supplied from S.

D. Fine Chemicals, India. Potassium permanganate (KMnO4), concentrated sulphuric acid


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e(H2SO4), hydrogen peroxide (H2O2) and tetrahydrofuran (THF) were procured from Merck,

India. Hydroquinone was obtained from SRL, Mumbai, India. Commercially available vinyl

terminated, linear polydimethylsiloxane base polymer with a vinyl content 0.05 mmol/g

(Baysilone U10), Baysilone U crosslinking agent 430, and Pt catalyst complex were supplied

by G.E, Bangalore, India. Ethynyl cyclohexanol (Inhibitor) was obtained from Sigma-

Aldrich. Millipore water has been used through the course of all the experiments.

Synthesis of graphite oxide (GO) and graphene (G)

Graphite oxide (GO) was prepared from graphite by the Hummers method as reported

earlier.48 According to this, 1.0 g graphite, 500 mg NaNO3 and 40 mL H2SO4 were mixed in a

three-neck flask in an ice bath (0 °C) and 3 g KMnO4 was slowly added to prevent the

temperature of the suspension from exceeding 20 °C. Then, the suspension was transferred to

a 35 ± 3 °C water bath and stirred for 30 minutes. After that, 80 mL millipore water was

added, stirred for 15 minute at 98 °C and further diluted with warm water. Subsequently, the

minimum quantity of H2O2 was added, which turned the color of the above solution from

dark brown to yellow. The suspension was filtered, rinsed with millipore water and vacuum

dried to get brown GO powder.

GO prepared earlier was suspended in millipore water and exfoliated through

ultrasonication for 1 h. The exfoliated GO was reduced to graphene nanoplatelets by

refluxing the GO solution with hydroquinone for 36 h at 120 °C and the product was

centrifuged, washed and vacuum-dried to obtain graphene denoted as G.

Preparation of MWCNT-graphene (MWCNT-G) hybrid

The appropriate amount of MWCNT and graphene were separately dispersed in THF

for 15 min sonication. The respective suspensions were mixed together and again sonicated

for 30 min followed by vacuum drying to produce MWCNT-G (3:1, 1:1 and 1:3 weight

ratios) hybrids.


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ePreparation of MWCNT/VMQ, G/SVMQ and MWCNT-G/VMQ hybrid composite

Initially, the desired amount (0.375, 0.5, 0.75, 1 and 1.5 wt %) of MWCNT-G (1:1)

hybrid was dispersed in anhydrous THF. After that, the vinyl terminated linear

polydimethylsiloxane base polymer was added to the above suspension and subjected to

sonication for 2 h. This is followed by the addition of appropriate amounts of catalyst,

inhibitor and cross-linker (V430) in a 3:1 mole ratio of hydride (crosslinker) to the vinyl

group of base polymer under continuous stirring for 1 h. The resultant mixture was cast on a

Teflon Petri dish, degassed in vacuum for 24 h at room temperature followed by curing at

165 °C (15 min) and 200 °C (4 h) in an air oven. Similar methodology was adopted for the

preparation of neat VMQ, G/VMQ and MWCNT/VMQ nanocomposites. The preparation of

MWCNT-G hybrid and MWCNT-G/VMQ hybrid nanocomposites has also been displayed in

Scheme 1.

Characterization

Wide angle X-ray diffraction (WAXD) patterns were recorded in the range of

diffraction angle 2θ = 5-40 ° and 5-60 ° with PANalytical (PW3040/60), model ‘X’ pert pro

with Cu Kα radiation (λ=0.1542 nm) at a scanning rate of 3 °/min at room temperature.

Transmission electron microscopy (TEM) analysis was taken on JEOL 2100 TEM 80

instruments operated under an acceleration voltage of 200 keV. TEM samples of MWCNT,

G, and MWCNT-G were prepared by mounting the THF suspension on the copper grid and

drying in air. TEM samples of as prepared VMQ composite films were obtained with a

thickness of a section 100 nm by ultramicrotomy using a diamond knife under cryogenic

condition.

The mechanical properties of neat VMQ and its nanocomposites were measured using

Tinius Olsen h10KS universal testing machine at 25 °C with a crosshead speed of 300


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emm/min; dumbbell shape specimens with overall length of 100 mm and width of 3 mm. In

each system, more than three specimens were tested.

Field emission scanning electron microscopy (FESEM) images were obtained on a

Carl Zeiss Supra 40 instrument at an accelerating voltage of 20 kV.

Thermo gravimetric analysis (TGA) of the MWCNT, G, MWCNT-G, neat VMQ and

VMQ nanocomposites was performed using Redcroft 870 thermal analyser, Perkin-Elmer at a

heating rate of 10 °C/min over a temperature range of 50-800 °C under nitrogen atmosphere.

Differential scanning calorimetry (DSC) of neat VMQ and VMQ nanocomposites was

conducted using Perkin Elmer Pyris differential scanning calorimetric instrument at scan rate

of 10 °C/min under nitrogen atmosphere over a range of temperature -150 to +150 °C.

The volume fraction and crosslink density of polymer and polymer composite in the

swollen network were determined using the Flory-Rehner approach.49 Previously weighed

samples were allowed to swell in toluene for 72 h at room temperature to reach equilibrium

swelling. Sample masses in the equilibrium swollen state were measured by removing it from

toluene, blot-drying with a tissue paper, weighing it and dried to a constant weight in a

vacuum oven for minimum of 24 h. According to this, the volume fraction (Vr) of neat

polymer and polymer nanocomposites has been estimated from the relation:

11

1

−−

−

+=

ssrr

rrr WW

WV

ρρρ

,where ρr (wr) and ρs (ws) are the density (weight) of polymer/composite and solvent

respectively. The crosslink density (nc) and toluene uptake (mol %) of the polymer and

polymer composite are also calculated from the relationship:

[ ])

2(

)1ln(

31

0

2

rr

rrrc VVV

VVVn

−

++−−=

χ


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ewhere, nc, V0, χ and ρr are the crosslink density, molar volume of solvent (Toluene, V0=106.8

cm3/mol), Flory-Huggins polymer–solvent interaction parameter and density of the

polymer/polymer nanocomposites respectively.

RESULTS AND DISCUSSION

Characterization of MWCNT-G hybrid

FTIR spectra of the pristine graphite, graphite oxide and graphene are represented in

Fig. S1. It is noted that observed no significant peak appears in case of graphite, which also

agree well with the reported literature.50 However, the different oxygen functionalities are

developed in its oxidation product corresponding to O-H stretching vibrations (at 3406 cm-1),

stretching vibrations from C=O (1720 cm-1), skeletal vibrations from unoxidized graphitic

domains (1626 cm-1), C-OH stretching vibrations (1218 cm-1), and C-O stretching vibrations

(1050 cm-1).51 It is also noted that intensity of the 3434 cm-1 band corresponding O-H

stretching vibration of graphene is reduced due to de-oxygenation and removal of the other

oxygen containing functional groups.52

Wide angle X-ray diffraction (WAXD) patterns of graphite, graphite oxide, and

graphene are displayed in Fig. S2. A sharp diffraction peak appears at 2θ≈26° corresponding

to the 002 plane of graphite (d002 ≈0.34 nm).53 When it is oxidized to graphite oxide, a

diffraction peak at 2θ≈13° (d001=0.68 nm) is arised.54 The presence of a diffraction peak at

2θ≈26.09° (d002 =0.341 nm) in its reduction product confirmed the formation of graphene.54

Fig. 1 shows WAXD patterns of multi walled carbon nanotube (MWCNT), graphene

(G) and MWCNT-G (3:1; 1:1; 1:3) hybrids in the range of 2θ = 5- 40 ° at the scan rates of 3

°/min. It is noted that the diffraction peak of (002) plane of graphitic carbon appear at 25.80 °

(d002 = 0.344 nm) and 26.09 ° (d002 = 0.341 nm) in MWCNT and G (JCPDS No.41-1487)

respectively. The slow scan (0.5 °/min) of the graphene and MWCNT-G hybrids in Fig. 2

clearly show that (002) peak of graphene in MWCNT-G (1:1) is maximum shifted to lower


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e2θ angle due to the π-π interaction between MWCNT and graphene. It is also noted that the

intensity of this peak is considerably reduced compared to either graphene or other MWCN-

G (3:1 and 1:3) hybrids. However, this peak is not completely disappeared suggesting that the

few graphene layers could be fused together. However, this could lead the exfoliation of

MWCNT and graphene,18 which needs further verification by TEM.

TEM image and corresponding selected area diffraction (SAED) pattern (Inset) of

respective MWCNT, G, and MWCNT-G (1:1) hybrid are displayed in Fig. 3. It clearly shows

that MWCNT are entangled into bundled and the corresponding SEAD pattern exhibits the

presence of small but strong arcs for (002), together with a ring for (100) and weak arc

corresponding to (004) diffractions.55 TEM image of graphene reveals the staking of graphene

nanosheets as expected, whereas its crystalline nature is confirmed by the presence of well-

defined diffraction spots in the SAED pattern. 54 This is also inferred from TEM images in

Fig. 3 (and Fig. S3 under supporting information) that the nanostructure of hybrids can be

tuned depending on the wt. ratios of MWCNT and graphene. The bundled MWCNT are

attached on the surface of garphene layers in MWCNT-G (3:1), whereas individual MWCNT

seems to be confined on the stacked graphene in MWCNT-G (1:3). Interestingly, TEM image

of MWCNT-G (1:1) hybrid shows that MWCNT and graphene sheets at nanoscale are

individually separated/attached on the surface of the graphene sheets. In addition, these

MWCNT are also seen to be interconnected through graphene sheets in 3D hybrid network

structure preventing the aggregation of graphene sheets in MWCNT-G.54,56 Therefore,

MWCNT-G (1:1) hybrid are likely to show an enhanced combination of thermal and

mechanical properties due to strong bonding within 1D/2D extended to extended

π−conjugated network. SAED pattern of 3D MWCNT-G (1:1) hybrid also shows the

presence of unresolved diffraction rings and spots confirming the hybridization of 1D

MWCNT and 2D graphene.


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eRoom temperature stability of MWCNT, G and MWCNT-G (3:1), MWCNT-G (1:1)

and MWCNT-G (1:3) hybrid suspensions in THF has been investigated and the

corresponding findings are displayed in Fig. 4. It is anticipated that such finding could

provide a guideline in the processing of VMQ nanocomposites. It is distinctly clear that

MWCNT as well as graphene tends to sediment within 1h at the bottom of the vial. Though,

the suspension of MWCNT-G (3:1) and MWCNT-G (1:3) exhibit better colloidal stability in

THF than individual fillers, the hybrid suspensions have been precipitated within 24 h. On

the contrary, the suspension of MWCNT-G (1:1) hybrid is found to be stable even up to 7

days. This could be attributed to π-π interaction between 2D graphene and 1D MWCNT

inhibiting their aggregation and exhibiting stable colloidal suspension of 3D MWCNT-G

hybrid in THF.57 Therefore, TEM and suspension stability of MWCNT-G (1:1) hybrid further

reaffirms our inference based on XRD analysis earlier.

Thermal behaviour of MWCNT, G and MWCNT-G has been examined under

nitrogen atmosphere and the findings are displayed in Fig. 5. MWCNT are found to be more

or less stable in the temperature range of 30-600 °C, whereas graphene exhibits three steps

degradation under the identical condition. The first stage (30-150 °C) corresponds to 4 %

weight loss due to the presence of adsorbed water. 58 The second stage degradation (250-500

°C) is assigned to the decomposition of residual -COOH groups present on the graphene

sheets.58,59 Finally, it is followed by the another weight loss in TG in the range of 550-600 °C

due to the restoration of conjugated structures of graphene sheets at high temperature.40,60

Interestingly, MWCNT-G hybrid exhibits more or less similar thermal degradation behaviour

upto 150 °C. However, the further weight loss is significantly low above this temperature

when compared with graphene. Fig. 5 also suggests that overall residue left at 600 °C in

MWCNT-G hybrid is 86 % in contrary to either MWCNT (97 %) or graphene (78 %). This


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ecould be simply due to a balance between residual percentage of MWCNT and graphene in

the hybrid.

Nanostructure of VMQ nanocomposites

The choice of MWCNT-G (1:1) as reinforcing filler in the development of

VMQ nanocomposites is mainly guided by its exfoliation as evident from XRD, which has

been further established by its TEM and suspension stability in THF. TEM images of

MWCNT (0.375 wt %)/VMQ, G(0.375 wt %)/VMQ, MWCNT-G (0.375 wt %)/VMQ,

MWCNT-G(0.75 wt %)/VMQ and MWCNT-G(1.5 wt %)/VMQ nanocomposites are

displayed in Fig. 6. It clearly shows that the graphene and MWCNT filled, when

indivisuallay filled in VMQ , are agglomerated. Interestingly, TEM image of 0.375 wt % and

0.75 wt % MWCNT-G filled VMQ composite shows better dispersion and homogeneity of

MWCNT-G in VMQ matrix. Hu et al34 also recently observed that graphene is well dispersed

in the silicone rubber matrix without any aggregation of carbon nanotubes in the matrix

(silicone rubber/graphene/MWCNT=100/1/3). It is suggested that the graphene has a good

interaction with silicone rubber and carbon nanotubes facilitating the dispersion of MWCNT

in silicone rubber. When MWCNT-G loading is 1.5 wt % in VMQ, the agglomeration of the

filler is clearly evident through TEM.

WAXD patterns of VMQ and its nanocomposites of 0.375, 0.50, 0.75, 1.0 and 1.5 wt

% filled MWCNT-G are shown in Fig. 7 and the corresponding nanocomposites of MWCNT

and graphene are presented in Fig. S4 and S5 respectively (under Supporting Information). It

is observed that neat VMQ exhibits a broad diffraction peak at 21.31° due to the amorphous

regimes of silicone rubber and a strong peak at 12.28° suggesting the inherent crystallinity of

PDMS arising from the un-stretched chain conformations.61 The crystalline packing of

MWCNT, G and MWCNT-G filled VMQ nanocomposites remain more or less unaltered. In

addition, the full width at half maximum of VMQ diffraction peak (2θ≈12.28°) in MWCNT-


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eG/VMQ nanocomposites are relatively low compared to VMQ and its individually filled

graphene and MWCNT nanocomposites prepared at the same filler loading. This is a clear

indication that the ordered-ness of VMQ segment increases in presence of MWCNT-G

hybrids due to the interfacial interaction between the hybrid filler and VMQ matrix.62

Mechanical properties of VMQ nanocomposites

Fig. 8 shows the stress-strain plots of neat VMQ, MWCNT-G(0.75 wt %)/VMQ,

MWCNT-G(1.0 wt %)/VMQ and MWCNT-G(1.5 wt %)/VMQ nanocomposites. The stress-

strain plots of individually filled 0.375 wt % of MWCNT and G in VMQ are also included in

Fig. 8. The corresponding data related to the tensile strength (TS), elongation at break (EB)

and modulus at different % of strain of neat VMQ and its nanocomposites are presented in

Table 1. It is noted that MWCNT-G(0.75 wt %)/VMQ exhibits maximum enhancement in TS

of 0.67 MPa compared to neat VMQ (0.32 MPa). When VMQ is individually filled with

0.375 wt % of MWCNT and G, TS follows the order: MWCNT/VMQ (TS: 0.42 MPa) >

G/VMQ (TS: 0.33 MPa). This clearly suggests that TS of MWCNT-G(0.75 wt %)/VMQ is

higher than the corresponding MWCNT or G nanocomposites. Table 1 also shows that the

modulus at 50 and 100% strain is increased in VMQ nanocomposites compared to neat

VMQ. It attains a maximum value in MWCNT-G/VMQ nanocomposites in all probability

due to the development of shear zones in nanocomposites under stress and strain conditions,63

or better dispersion of MWCNT-G leading to its enhanced interaction with VMQ.64 In

addition, EB of VMQ is considerably reduced when filled with 0.375 wt % loaded MWCNT

or G. Interestingly, such loss in flexibility of individually filled MWCNT and G

nanocomposites of VMQ has been recovered in case of MWCNT-G(0.75 wt %)/VMQ

nanocomposites. All these findings clearly demonstrate the synergistic effect of MWCNT-G

hybrid on the TS as well as EB of VMQ composite. This could be mainly attributed to the

homogeneous dispersion of 3D MWCNT-G hybrid filler in VMQ matrix. Alternatively, 3D


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ehybrid filler can become entangled with the polymer chains resulting better interaction

between MWCNT-G and VMQ matrix facilitating the efficient load transfer from polymer

matrix to nanofillers. It is also inferred from the data in Table 1 that modulus gradually

increases, while EB and tensile strength correspondingly decreases with increasing MWCNT-

G loadings (1.0 and 1.5 wt %) in VMQ. Such enhanced rigidity in composite is attributed to

the restriction effect of hybrid filler on the VMQ segments, similar results have also been

observed with previously reported CNT-graphene reinforced nanocomposites.39,40

The improvements in mechanical properties of MWCNT-G filled VMQ

nanocomposites can also be correlated in terms of fracture behavior using FESEM analysis.

Fig. 9 displays the corresponding image of tensile fractured surface of MWCNT-G(0.75 wt

%)/VMQ. The image in Fig. 9 distinctly shows the homogeneous dispersion of hybrid filler

without involving any aggregation in the VMQ matrix. It is also observed that the hybrid

filled VMQ composite shows a rough fractured surface with bright spots, which are

developed due to the breaking of MWCNT rather than being just pulled out from the VMQ

matrix.65 These results indicate the presence of strong interfacial interaction between

MWCNT-G and VMQ matrix. All these facts are likely to contribute towards the

improvement in the mechanical properties of VMQ when reinforced with the hybrid filler.

Thermal properties of VMQ nanocomposites

TG analysis has been performed to evaluate the thermal stability of neat VMQ

and its nanocomposites. Fig. 10 shows the thermogram of neat VMQ and its 0.5, 1.0 and 1.5

wt % MWCNT-G filled nanocomposites. Neat VMQ and its all nanocomposites exhibit two

steps degradation behaviour with different threshold. The first step degradation at about 410-

440 °C is regard as the depolymerisation of the siloxane chains and second step weight loss

due to the breakdown of the polymer backbone.66 In order to verify the synergistic effect of

hybrid filler on VMQ, thermograms of individually 0.5 wt % of MWCNT and G filled VMQ


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enanocomposites are also recorded and shown in Fig. 10. All the thermal data related to the

initial decomposition temperatures (T0), temperature corresponding to 25 % weight loss (T25)

and char residue at 800 °C of neat VMQ and its nanocomposites with MWCNT, G and

MWCNT-G are summarized in Table 2. It is evident from this that T0 and T25 of

MWCNT/VMQ, G/VMQ and MWCNT-G/VMQ are always higher than the neat VMQ.

However, MWCNT-G(1.0 wt %)/VMQ composite exhibits the maximum improvements in

T0 (441 °C) and T25 (629 °C) with respect to neat VMQ (T0: 421 °C, T25: 475 °C). Such

significant enhancement in T0 and T25 could be attributed to the hindering effect of the

confined geometry of 3D MWCNT-G hybrid structure on the diffusion of nitrogen and

volatile products throughout the composite materials by reducing the initiation of VMQ chain

scission. Alternatively, the possibility of the nano level dispersion of the hybrid filler and

enhanced interfacial interaction between VMQ and MWCNT-G suppressing the heat flux and

polymer backbone degradation also cannot be ruled out. It is also observed that the thermal

stability of hybrid filled VMQ nanocomposites is decreased with high hybrid (1.5 wt %)

content due to the aggregation tendency of the MWCNT-G in VMQ.67,68 The % of residue

left at 800 °C in VMQ and its composite in TGA suggest the following order in terms of

thermal stability: VMQ < G/VMQ < MWCNT/VMQ < MWCNT-G/VMQ. This is also in

agreement with our earlier findings deduced on the basis of the temperature referring to 25 %

weight loss (T25) of the samples in TG analysis. The maximum char residue in case of

MWCNT-G/VMQ nanocomposites could be attributed to the confined geometry and

interfacial interaction between MWCNT-G hybrid and VMQ. There is an obvious synergistic

effect of MWCNT-G hybrid on the retardance of thermal decomposition and enhancement of

carbonaceous char formation. On the other hand, the char residue of hybrid filled VMQ

nanocomposites is decreased with increasing filler loading. This is probably due to the

integrated effect of the interaction between the MWCNT-G and the polymer matrix.69


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eTherefore, the prepared MWCNT-G hybrid may find potential application as flame retardant

nano fillers.

The effect of MWCNT, G and MWCNT-G loading on the crystallization and melting

behaviour of neat VMQ has been studied by DSC analysis and findings are presented in Fig.

11 and all the corresponding data are summarized in Table 3. It is noted that the glass

transition temperature (Tg) follows the order: VMQ < MWCNT(0.5 wt G(0.5 wt %)/VMQ <

MWCNT-G(1.0 wt %)/VMQ nanocomposites. The maximum improvement in Tg of

MWCNT-G/VMQ composite could be attributed to the restricted mobility of the VMQ

chains in presence of 3D MWCNT-G hybrid filler. Fig. 11 also shows that the cold

crystallization peak (Tc) is absent in neat VMQ in all probability due to its complete

crystallization.70 Interestingly, the cold crystallization peak appeared at -100.57, -100.74, and

-98 °C in MWCNT(0.5 wt %)/VMQ, G(0.5 wt %)/VMQ and MWCNT-G(1.0 wt %)/VMQ

nanocomposites respectively. It is also noted that the sharpness of the peak is highest in

MWCNT-G filled VMQ possibly due to the homogeneous dispersion of hybrid filler in VMQ

and strong interfacial interaction between MWCNT-G and polymer segments. Though, DSC

plots show that the melting temperature (Tm) of VMQ is increased compared to neat VMQ by

2 and 5 °C in MWCNT/VMQ and MWCNT-G/VMQ respectively, Tm remains more or less

same in G/VMQ. The enthalpy of melting (ΔHm) of VMQ, MWCNT(0.5 wt %)/VMQ, G(0.5

wt %)/VMQ, MWCNT-G(1.0 wt %)/VMQ and MWCNT-G(1.5 wt %)/VMQ have also been

calculated from DSC analysis and found to be 22.05, 21.05, 21.61, 25.35 and 23.65 J/g

respectively. This is inevitable that both the melting temperature and ΔHm are higher in

MWCNT-G/VMQ compared to either neat VMQ or its individually filled MWCNT and

graphene nanocomposites. All these findings clearly demonstrate that 3D MWCNT-G hybrid

contributes the formation of more ordered VMQ by nucleation and growth processes. In

addition, the presence of the interfacial interaction between the hybrid and VMQ matrix


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edecreases the entanglement of the VMQ. As a result, MWCNT-G/VMQ composite exhibits

the increase in the crystallinity as well as melting temperature.71 However, the Tg, Tc, Tm and

degree of crystallinity of hybrid filled VMQ nanocomposites decreases with higher hybrid

(1.5 wt %) content. This may be ascribed to the fact that, dispersions of MWCNT-G become

poor at relatively higher hybrid content and this hinders to increase the crystallinity of

nanocomposites.72,73

Swelling properties of VMQ nanocomposites

In order to evaluate the correlation between mechanical property and equilibrium

solvent uptake, swelling experiments have been carried out in toluene and the variation of

crosslink density (nc) and toluene uptake (mol %) of neat VMQ, MWCNT/VMQ, G/VMQ,

and MWCNT-G/VMQ nanocomposites are presented in Fig. 12 and 13 respectively. This

suggests that the crosslink densities of MWCNT-G/VMQ nanocomposites are always higher

compared to neat VMQ or individually MWCNT and graphene filled VMQ nanocomposites.

When, polymer blended with carbon nanotube or graphene, one can expect three networks:

the temporary polymer network formed by entanglements, the carbon nanotube (graphene)

network and a combined carbon nanotube (graphene)-polymer network. In all

probability,‘entanglements’ between the polymer chains and the nanotubes (grahene) could

form carbon nanotube–polymer or graphene-polymer network.74-76 As a consequence, the

crosslink density of carbon nanotube–polymer or graphene-polymer network is higher than

that of the entangled polymer network alone. The higher cross-link density of SWCNT/ VMQ

(and G/ VMQ) composite compared to pure SR could be attributed to the physical cross-

links/constraints of MWCNTs (and G) in the composites.77-79 In our case, MWCNT and G are

non functionalized suggesting the absence of any chemical bonding with SR. However, the

possibility of physical interactions between the filler and polymer due to the polymer

wrapping and/or coating of the filler can not be ruled out.80, 81 Alternatively, the sonication


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ecould lead the generation of free radicals in the polymer chains providing an additional

source of interaction with the filler.82,83

It is also inferred that crosslink density as well as Young’s modulus show increasing

trend with MWCNT-G loadings in VMQ. Fig. 13 shows that the solvent uptake ability of

MWCNT-G/VMQ nanocomposites is relatively less than neat VMQ, MWCNT/VMQ or

G/VMQ nanocomposites. This is probably due to the homogeneous dispersion and strong

interfacial interaction between MWCNT-G and VMQ polymer segments forming a bound

polymer. As a result, the solvent uptake is restricted from surface to bulk and the rigidity of

the polymer is also enhanced.84

CONCLUSIONS

The high performance MWCNT-G(0.75 wt %)/VMQ hybrid exhibits

significant improvements in tensile strength (110 %) and Young’s modulus (137 %), whereas

as elongation at break is retained compared to neat VMQ. It also shows the improvement in

glass transition temperature (4 °C), melting temperature (5 °C), crystallization temperature (8

°C) and thermal stability corresponding to 25 % weight loss (154 °C) in comparison neat

VMQ. Swelling measurements confirmed that the crosslink density and solvent resistance

property is also found to be maximum for hybrid nanocomposites. We also established that

the synergistic effect of MWCNT-G hybrid could account for such improvement in properties

of neat VMQ due to co dispersion of MWCNT and graphene nanosheets in VMQ matrix as

well as the strong interfacial interaction between 3D hybrid and VMQ segments.

ACKNOWLEDGMENTS

Author gratefully acknowledge Dr. Anubhav Saxena, GE India, Bangalore for

Baysilone U10, Baysilone U 430, Pt catalyst complex and Asbury Graphite Mills, Asbury, NJ


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efor providing natural graphite AKA SEFG 3777. Lastly, the financial help from CSIR and

DRDO, New Delhi, India is also greatly acknowledged.

Supporting Information Available

FTIR patterns of graphite, graphite oxide, graphene; WAXD patterns of graphite,

graphite oxide, graphene; TEM images of MWCNT-G (3:1) hybrid, MWCNT-G (1:3)

hybrid; WAXD patterns of MWCNT/ VMQ and graphene/ VMQ nanocomposites.

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eA. Caption to Schemes

Scheme 1. Presentation for the preparation of VMQ nanocomposites with co-dispersion of

MWCNT and graphene nanoplatelets.

B. Caption to Figures

Figure 1. WAXD patterns of MWCNT (a), G (b), MWCNT-G (3:1) (c), MWCNT-G (1:3)

(d) and MWCNT-G (1:1) (e).

Figure 2. WAXD profiles for 002 reflection of G (a), MWCNT-G (3:1) (b), MWCNT-G

(1:3) (c) and MWCNT-G (1:1) (d).

Figure 3. TEM images of MWCNT (a), G (b) and MWCNT-G (1:1) hybrid (c). (The inset

shows the corresponding SAED pattern of the fillers).

Figure 4. Digital images showing room temperature dispersion of MWCNT (a), G (b),

MWCNT-G (3:1) in THF (c), MWCNT-G (1:1) in THF (d), MWCNT-G (1:3) in THF (e),

(The photographs were taken after 1 day of storage at room temperature).

Figure 5. TGA curves of MWCNT (a), G (b) and MWCNT-G (1:1) (c).

Figure 6. TEM image of MWCNT(0.375 wt %)/ VMQ (a), G(0.375 wt %)/ VMQ (b),

MWCNT-G(0.375 wt %)/ VMQ (c), MWCNT-G(0.75 wt %)/ VMQ (d) and MWCNT-G(1.5

wt %)/ VMQ (e).

Figure 7. WAXD patterns of VMQ (a), MWCNT-G(0.375 wt %)/ VMQ (b), MWCNT-G(0.5

wt %)/ VMQ (c), MWCNT-G(0.75 wt %)/VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and

MWCNT-G(1.5 wt %)/ VMQ (f).


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eFigure 8. Typical Stress-strain curves of neat VMQ (a), MWCNT(0.375 wt %)/VMQ (b),

G(0.375 wt %)/VMQ (c), MWCNT-G(0.75 wt %)/VMQ (d), MWCNT-G(1.0 wt %)/VMQ

(e) and MWCNT-G(1.5 wt %)/ VMQ (f).

Figure 9. FESEM image of tensile fracture surface of MWCNT-G(0.75 wt %)/ VMQ

nanocomposites.

Figure 10. TGA analysis of neat VMQ (a), MWCNT(0.5 wt %)/ VMQ (b), G(0.5 wt %)/

VMQ (c) MWCNT-G(0.5 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and MWCNT-

G(1.5 wt %)/ VMQ (f).

Figure 11. DSC curves of neat VMQ (a), MWCNT(0.5 wt %)/ VMQ (b), G(0.5 wt %)/ VMQ

(c), and MWCNT-G(0.5 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and MWCNT-

G(1.5 wt %)/ VMQ (f).

Figure 12. Variation of crosslink density of neat VMQ (a), VMQ filled with 0.375 and 0.75

wt % of MWCNT (b and c), VMQ filled with 0.375 and 0.75 wt % of G (d and e) and VMQ

filled with 0.375, 0.50, 0.75, 1.0 and 1.5 wt % filled MWCNT-G (f, g, h, i and j).

Figure 13. Variation of toluene uptake (mole %) of neat VMQ (a), VMQ filled with 0.375

and 0.75 wt % of MWCNT (b and c), VMQ filled with 0.375 and 0.75 wt % of G (d and e)

and VMQ filled with 0.375, 0.50, 0.75, 1.0 and 1.5 wt % filled MWCNT-G (f, g, h, i and j).


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eC. Caption to the Tables

Table 1 Summery of the mechanical properties of VMQ and its nanocomposites.

Table 2 Thermal properties of neat VMQ, MWCNT (0.5 wt %)/ VMQ, G(0.5 wt %)/ VMQ

and MWCNT-G(1.0 wt %)/ VMQ nanocomposites.

Table 3 Glass transition temperature (Tg), melting temperature (Tm), crystallization

temperature (Tc), enthalpy of melting (ΔHm) and degree of crystalinity (Xc) of VMQ and its

nanocomposites.


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e

Scheme 1. Presentation for the preparation of VMQ nanocomposites with co-dispersion of

MWCNT and graphene nanoplatelets.


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e

Table 1 Summery of the mechanical properties of VMQ and its nanocomposites.

Sample TS (MPa) EB (%) 50% Modulus (MPa)

100% Modulus (MPa)

Neat VMQ

MWCNT(0.375 wt %)/ VMQ

MWCNT (0.75 wt %)/ VMQ

G (0.375 wt %)/ VMQ

G (0.75 wt %)/ VMQ

MWCNT-G (0.375 wt %)/ VMQ





0.32 ± 0.02

0.42 ± 0.03

0.44 ± 0.03

0.33 ± 0.01

0.37 ± 0.01

0.48 ± 0.02

0.53 ± 0.01

0.67 ± 0.03

0.61 ± 0.02

0.50 ± 0.01

192 ± 8

113 ± 7

104 ± 6

88 ± 7

90 ± 5

144 ± 2

158 ± 3

194 ± 4

165 ± 5

123 ± 3

0.15

0.23

0.26

0.25

0.27

0.26

0.28

0.28

0.29

0.30

0.22

0.39

0.40

--------

--------

0.40

0.43

0.43

0.44

0.45


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e

Table 2 Thermal properties of neat VMQ, MWCNT (0.5 wt %)/ VMQ, G(0.5 wt %)/ VMQ

and MWCNT-G(1.0 wt %)/ VMQ nanocomposites.

Sample T0 T25 % char residue

Neat VMQ


G(0.5 wt %)/ VMQ

MWCNT-G(0.5 wt %)/ VMQ



421

430

434

438

441

436

475

590

575

614

629

621

0.08

48

41

53

56

53


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eTable 3 Glass transition temperature (Tg), melting temperature (Tm), crystallization

temperature (Tc), enthalpy of melting (ΔHm) and degree of crystalinity (Xc) of VMQ and its

nanocomposites.

Sample

Tg (˚C)

Tm (˚C)

Tc (˚C)

ΔHm (J/g)

Xc (%)

Neat VMQ


G(0.5 wt %)/ VMQ




-129.00

-126.75

-126.71

-125.75

-124.46

-126.00

-46.67

-44.42

-46.17

-43.55

-41.81

-43.97

-106.00

-100.57

-100.74

-100.21

-98.00

-100.58

22.05

21.05

21.61

24.05

25.35

23.65

58.92

56.23

57.73

64.25

67.77

63.18


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e

Figure 1. WAXD patterns of MWCNT (a), G (b), MWCNT-G (3:1) (c), MWCNT-G (1:3) (d)

and MWCNT-G (1:1) (e).


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e

Figure 2. WAXD profiles for 002 reflection of G (a), MWCNT-G (3:1) (b), MWCNT-G (1:3)

(c) and MWCNT-G (1:1) (d).


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e

Figure 3. TEM images of MWCNT (a), G (b) and MWCNT-G (1:1) hybrid (c). (The inset

shows the corresponding SAED pattern of the fillers).


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e

Figure 4. Digital images showing room temperature dispersion of MWCNT (a), G (b),

MWCNT-G (3:1) in THF (c), MWCNT-G (1:1) in THF (d), MWCNT-G (1:3) in THF (e),

(The photographs were taken after 1 day of storage at room temperature).


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e

Figure 5. TGA curves of MWCNT (a), G (b) and MWCNT-G (1:1) (c).


Figure 6. TEM image of MWCNT(0.375 wt %)/ VMQ (a), G(0.375 wt %)/ VMQ (b),

MWCNT-G(0.375 wt %)/ VMQ (c), MWCNT-G(0.75 wt %)/ VMQ (d) and MWCNT-G(1.5

wt %)/ VMQ (e).

d e

b a c


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e

Figure 7. WAXD patterns of VMQ (a), MWCNT-G(0.375 wt %)/ VMQ (b), MWCNT-G(0.5

wt %)/ VMQ (c), MWCNT-G(0.75 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and

MWCNT-G(1.5 wt %)/ VMQ (f).


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e

Figure 8. Typical Stress-strain curves of neat VMQ (a), MWCNT(0.375 wt %)/ VMQ (b),

G(0.375 wt %)/ VMQ (c), MWCNT-G(0.75 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ

(e) and MWCNT-G(1.5 wt %)/ VMQ (f).


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e

Figure 9. FESEM image of tensile fracture surface of MWCNT-G(0.75 wt %)/ VMQ

nanocomposites.


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e

Figure 10. TGA analysis of neat VMQ (a), MWCNT(0.5 wt %)/ VMQ (b), G(0.5 wt %)/

VMQ (c) MWCNT-G(0.5 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and MWCNT-

G(1.5 wt %)/ VMQ (f).


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e

Figure 11. DSC curves of neat VMQ (a), MWCNT(0.5 wt %)/ VMQ (b), G(0.5 wt %)/ VMQ

(c), and MWCNT-G(0.5 wt %)/ VMQ (d), MWCNT-G(1.0 wt %)/ VMQ (e) and MWCNT-

G(1.5 wt %)/ VMQ (f).


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e

Figure 12. Variation of crosslink density of neat VMQ (a), VMQ filled with 0.375 and 0.75

wt % of MWCNT (b and c), VMQ filled with 0.375 and 0.75 wt % of G (d and e) and VMQ

filled with 0.375, 0.50, 0.75, 1.0 and 1.5 wt % filled MWCNT-G (f, g, h, i and j).


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e

Figure 13. Variation of toluene uptake (mole %) of neat VMQ (a), VMQ filled with 0.375

and 0.75 wt % of MWCNT (b and c), VMQ filled with 0.375 and 0.75 wt % of G (d and e)

and VMQ filled with 0.375, 0.50, 0.75, 1.0 and 1.5 wt % filled MWCNT-G (f, g, h, i and j).


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