Surface Morphology, Thermomechanical and Barrier Properties of Poly(Ether Sulfone)-Toughened Epoxy...

8/2/2019 Surface Morphology, Thermomechanical and Barrier Properties of Poly(Ether Sulfone)-Toughened Epoxy Clay Ternar

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Research Article

Received: 18 June2009 Revised: 8 October 2009 Accepted: 2 November 2009 Published online in Wiley Interscience: 7 Apri l 2010

(www.interscience.wiley.com) DOI 10.1002/pi.2817

Surface morphology, thermomechanical

and barrier properties of poly(ethersulfone)-toughened epoxy clay ternarynanocompositesAbdul Azeez Asif, Bibin John, Vattikuty Lakshmana Rao

and Kovoor Ninan Ninan

Abstract

Poly(ether sulfone) (PES)-toughenedepoxy clay ternary nanocompositeswere prepared by melt blending of PES with diglycidyl

ether of bisphenol A epoxy resin along with Cloisite 30B followed by curing with 4,4 -diaminodiphenylsulfone. The effect oforganoclay and thermoplastic on the fracture toughness, permeability, viscoelasticity and thermomechanical properties ofthe epoxy system was investigated. A significant improvement in fracture toughness and modulus with reduced coefficient ofthermal expansion (CTE) and gas permeability were observed with the addition of thermoplastic and clay to the epoxy system.Scanning electron microscopy of fracture-failed specimens revealed crack path deflection and ductile fracture without phaseseparation. Oxygen gaspermeability wasreduced by 57%and fracture toughnesswas increasedby 66%with theincorporationof 5 phr clay and 5 phr thermoplastic into the epoxy system. Optical transparency was retained even with high clay content.The addition of thermoplastic and organoclay to the epoxy system had a synergic effect on fracture toughness, modulus, CTEand barrier properties. Planetary ball-milled samples gave exfoliated morphology with better thermomechanical propertiescompared to ultrasonicated samples with intercalated morphology.c 2010 Society of Chemical Industry

Keywords: fracture toughness; gas permeability; organoclay; rheology

INTRODUCTIONIn the recent past, extensive research has been carried out onlayered silicate epoxy nanocomposites including the processing,preparation and exfoliation mechanisms112 and the incorpora-tion of these epoxy nanocomposites as a matrix into traditionalcarbon fibre-reinforced composites.13,14 Numerous reports in theliterature describe the improvement in barrier properties, tensilestrength, modulus, glass transition temperature and toughness,and a decrease in flammability and residual stress by the incor-poration of 210 wt% of organoclay into epoxy resins.112 Fullyexfoliated morphologies, where individual silicate layers are ho-mogeneously and uniformly dispersed throughout the polymer

matrix, aremostpreferredfor theenhancementof such properties.The main reasons for the improved properties of nanocompositesis thevery large interfacial interaction between matrixand layeredsilicates, the high aspect ratio of the dispersed clay particles andthe hindered tortuous diffusing paths created by the dispersedsilicate nanolayers.Layered silicatenanocompositesare becomingattractive in cryogenicstorage tanks based on theresults reportedby the NASA Glenn Research Center15 after the failure of X-33composite fuel tanks due to microcracking.16

There are many factors that influence the morphology ofnanocomposites; whether phase-separated, intercalated or ex-foliated depends upon the type of resin,17 curing agent (curative)used,1,18 rate of curing, methods of preparation (melt mixing, solu-tionmixingor in situ polymerization)9 andprocessingmethod.11,12

The addition of clay particles can also have an effect on thekinetics of the cure process. The addition of clay often leadsto an increased rate of gelation.19,20 Zunjarrao etal.11 reportedthat high-shear mixing resulted in better dispersion of clay parti-cles and better mechanical properties over ultrasonication, even ifbothmethodsgaveanexfoliatedmorphology.Itwasreportedthat4,4-diaminodiphenylsulfone (DDS)-cured epoxy nanocompositesshowed an exfoliated morphology with improved mechanicalpropertieswhen dispersedby exerting shear force on epoxy mont-morillonite solution using ball milling.12 Liu etal.21 reported anincreaseof65%infracturetoughnessforepoxy/claynanocompos-ites with the incorporation of 4 wt% Cloisite 93A (d001 = 25.8 ).

Also, an increase in fracture toughness of more than 100% wasobserved by us22,23 and others24,25 for thermoplastic-toughenedepoxy systems.

The addition of thermoplastic into epoxy clay nanocompositesprovides substantial improvement in fracture toughness. Not

Correspondenceto:VattikutyLakshmana Rao,Adhesivesand AdvancedMatrix

ResinSection, Propellants andSpecial ChemicalsGroup, Propellants, Polymers,

Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Trivandrum-

695022, India. E-mail: v [email protected]

Adhesives and Advanced Matrix Resin Section, Propellants and Special

ChemicalsGroup,Propellants,Polymers,ChemicalsandMaterialsEntity,Vikram

Sarabhai Space Centre, Trivandrum-695022, India

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PES-toughened e poxy clay ternary nanocomposites www.soci.org

many reports are available on thermoplastic-toughened epoxyclay nanocomposites in the open literature. Frohlich etal.26

reported an improved toughness in rubber-toughened hybridepoxy nanocomposites compared with nanocomposites withoutrubber, but the glass transition temperature (Tg) was lowereddrastically. Balakrishnan etal.27 reported that the addition of bothelastomers andnanoclay toan epoxyresinresultedin anenhancedductilityandimprovedtoughnesswithoutcompromisingstrengthand modulus. Isik etal.28 studied the mechanical propertiesof epoxypolyether polyol organically treated montmorillonitenanocompositeswithrespecttopolyetherpolyolandclaycontent.They observed an increased Tg and Youngs modulus with respectto clay content. Peng etal.29 reported the influence of organicallymodifiedclayonthereaction-inducedphaseseparationbehaviourof epoxy/poly(ether imide) ternary nanocomposites. Recently,we have reported the toughening by a hydroxyl-terminatedpoly(ether ether ketone) with pendant methyl groups (PEEKMOH)of epoxy clay ternary nanocomposites based on quaternary alkylammonium-modified montmorillonite without polar functionalgroups (Cloisite 25A)30 and montmorillonite with functional alkylammonium modifier (Cloisite 30B).31

In the present study, we report poly(ether sulfone) (PES)-toughened epoxy clay ternary nanocomposites based on mont-morillonite with functional alkyl ammonium modifier (Cloisite30B). We studied the effect of PES and processing techniqueson phase morphology, surface morphology, fracture toughness,barrier properties and thermomechanical properties of the epoxyclay ternary nanocomposites.

EXPERIMENTALMaterials

Diglycidyl ether of bisphenol A epoxy resin (LY 556, Ciba Geigy)with an epoxide equivalent weight of 188.68, DDS (Purity: 99%)(Merck) and PES having a Tg value of 201

C were used asreceived. Cloisite 30B (Southern Clay Product; methyl tallow(bis-2-hydroxyethyl) quaternary ammonium-modified montmorillonite,d001 = 1.85 nm, Cation Exchange Capacity (CEC)= 95 meq (100 gclay)1, specific gravity = 1.98 g cm3) was dried under vacuumat 70 C before use.

Preparation of PES-toughened epoxy clay ternarynanocomposites

PES-toughened epoxy clay ternary nanocomposites were pre-pared as shown in Scheme 1. For all compositions, PES con-centration was maintained at 5 phr. The preparation methodsincluded two processing techniques: planetary ball milling andultrasonication. A Fritsch Planetary Mono Mill Pulverisette 6 with

10 mm diameter balls (2.98 g each) of zirconium dioxide materialat 500 rpmwas used forplanetary ball milling. Ultrasonication wasperformed using an Elma ultrasonic bath (Elma Transonic, TI-H-5)at a frequency of 35 kHz and a power of 100 W.

Characterization

Fourier transform infrared (FTIR) spectra of cured and uncuredsamplesinKBrpelletswererecordedusingaPerkinElmerSpectrumGXA FTIR spectrometer. The curing behaviour of PES-toughenedepoxy clay DDS ternary mixtures was studied using DSC operatedin standard mode with a TA Instruments model DSC-2920from room temperature to 350 C at 10 C min1 in nitrogenatmosphere. Isothermal rheological experiments were carried

Epoxy resin

1. PES (5phr) at 180C

Epoxy-PES blend

1. Cloisite 30B (1-8 phr)

2. 80C, mechanical stirring 2 hours

3. 80C ultrasonication /Planetary ball milling

Epoxy-PES-clay ternary mixture

1. DDS at 180C, mechanical stirring

2. Evacuation

Epoxy-PES-clay-DDS ternary mixture (Uncured)

1. 180C for 3 hours

2. 200C for 2 hours

Epoxy-PES-clay ternary nanocomposite

Scheme 1. Flow chart for the preparation of PES-toughened epoxy clayternary nanocomposite.

out at 180 C using a Rheologica Stresstech rheometer, modelRheologic Viscotech QC, with a parallel plate assembly operatedin oscillation mode at a frequency of 1 Hz and controlled strain of0.01. Wide-angle XRD data were obtained with powder samples ofthe Cloisite 30B and nanocomposites using a PANalytical modelX-raydiffractometerwith Xpert pro software and nickel-filtered CuK radiation at 30 kV and20 mA. The morphology of cryogenicallyfractured tensile and fracture-failed samples was analysed using aPhilips XL 20 SEMinstrument.The failedsurfaces were etchedwithdimethylsulfoxide for 24 h to removethe thermoplastic phase. Thespecimens were dried in a vacuum oven at 100 C overnight toremove the solvent. All the specimens were sputter-coated withgold before obtaining the micrographs.

A thermomechanical analyser (PerkinElmer TMA-7) was usedto determine the coefficient of thermal expansion (CTE) and Tg .Scans were run at a heating rate of 10 C min1 from roomtemperature to 250 C under nitrogen atmosphere. The thermalstability of the nanocomposites was analysed using TGA with aTA Instruments model SDT 2960 thermal analyser. The sampleswere heated from room temperature to 900C ata heating rateof

10

C min1

in nitrogen atmosphere.The viscoelastic propertiesofthe nanocomposites were measured using a TA Instruments DMA2980 dynamic mechanical thermal analysis (DMTA) instrument.DMTA specimens of size 50 10 3 mm3 were cut from a3 mm thick cured laminate of nanocomposite using a diamondcutter. The analysis was done in three-point bending mode at afrequency of 1 Hz from room temperature to 300 C at a heatingrate of 2 C min1. Experiments on the permeation of oxygen gaswere carriedout using a LYSSY (model 1005000)manometergaspermeabilitytesterwithmeasuring range1 10 000 mL m2 day1

and flow rate of 10 L h1 at room temperature.Fracture toughness and tensile and flexural properties were

determined as per ASTM 5045, ASTM D638 (Type V) and ASTMD790,respectively.Themeasurementsweredoneusingauniversal

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www.soci.org A Asif etal.

4000.0 3000 2000 1500 1000 500

cm-1

%

T

Initial

180C / 1 h

180C / 3 h

180C / 3 h + 200C / 2 h

914

400.0

Figure 1. FTIR spectra of uncured sample and various cured samples.

testing machine (model TNE 5000) at a crosshead speed of10 mm min1. Rectangular specimens of 100 10 3 mm3 wereused for determining flexural strength. Flexural modulus wasdetermined from the slope of the initial portion of the flexuralstressstrain curve.

Flexural strength was calculated using32

Flexural strength =3Pl

2bd2(1)

where P is the load at break, l is the span length and b andd are the breadth and thickness of the specimen, respectively.Dogbone-shaped specimens were used for the evaluation of

tensile properties.Single edge notch specimensof 4663 mm3 (spanlength=24 mm) were used to measure the fracture toughness of theepoxy clay ternary nanocomposites. A notch of 2.7 mm wasmade at one edge of the specimen. A natural crack was madeby pressing a fresh razor blade into the notch. The fracturetoughness was expressed as stress intensity factor (KIC) calculatedusing33

KIC =L

BW1/2f(x) (2)

where 0 < x< 1 and

f(x) = 6x1/21.99 (1x)(2.15 3.93x 2.7x2)

(1+ 2x)(1 x)3/2

and L is the load at crack initiation, B is the specimenthickness, W is the specimen width, a is the crack length and

x= a/W.

RESULTS AND DISCUSSIONFTIR studies

The FTIR spectra of the nanocomposites (before curing and aftercuring at various time intervals) are shown in Fig. 1. The epoxypeak at 914 cm1 reduces in intensity for the samples cured at180 C for 1 h and it totally disappears for samples cured for3 h and for samples post-cured at 200 C for 2 h confirming the

Table 1. Tg values of the epoxy/PES/DDS/clay mixture cured atvarious temperatures and for various times

Cure temperature and time Tg (C)

180 C/1 h 151

180 C/3 h 192

180 C/3 h+ 200 C/2 h 208

0 50 100 150 200 250 300 350

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Heatflow(Wg

1)

Temperature (C)

0 h180 / 1 h180 / 3 h180 / 3 h + 200 / 2 h

Figure 2. DSC traces of epoxy/DDS/PES/clay mixture.

completion of the cure reaction. Post-curing is performed in orderto ensure full curing, as evidenced from Tg values obtained fromDSC (Table 1).

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Table 2. CTE and Tg values of the ternary nanocomposites

Coefficient of thermal expansion (106 C1) Tg (C)

Composition 30200 C TMA DMTA (from tan ) DSC

Epoxy 102 3 199 221 212

Epoxy/5 phr PES 100 2 197 216 209

Epoxy/5 phr PES/1 phr Cloisite 30B 98 2 198 214 208Epoxy/5 phr PES/3 phr Cloisite 30B 96 1 199 214 208

Epoxy/5 phr PES/5 phr Cloisite 30B 91 3 199 209 205

Epoxy/5 phr PES/8 phr Cloisite 30B 96 2 199 205 204

0 5000 10000

0

2

4

0

2

4

1400 1600 1800 2000 2200 2400 2600

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

Epoxy

Epoxy/5 phr PES

Epoxy/3 phr Cloisite 30B

Epoxy/5 phr PES/3 phr Cloisite 30B

LossModulus(MPa)

StorageModulus(MPa)

Time (s)

Figure 3. Storage and loss moduli as a function of time at 180 C for the nanocomposites.

DSC studies

The cure behaviour of PES-toughened epoxy clay DDS mixture(before curing and after curing at various time intervals) isshown in Fig. 2. A sharp exotherm is observed for uncured

samples with initial cure temperature at 160 C and maximumcure temperature at 224 C indicates the epoxy amine reaction.As curing proceeds at 180C for 3 h, the exothermic peaktotally disappears. However, post-curing at 200 C for another2 h results in high Tg (Table 1). Thus, increasing the temperaturein the post-curing stage makes the reaction more favourable andallows the polymerization reaction to be completed since thecuring process becomes diffusion controlled at a high degree ofconversion of epoxy. Tg of the epoxy system (Table 2) is foundto decrease with an increase in clay concentration, which maybe due to the plasticizing effect of organic moieties of clayparticles thereby decreasing the crosslink density of the epoxynetwork.

Rheological characterization

The application of thermoset-based nanocomposites is highlydependent upon their processproperty relationships. The typeof resin, curing agent, nanoparticles, cure kinetics and dispersion

techniques will play a major role during gelation and vitrificationstages in the final morphology of the crosslinked structure.34

Thermoset polymers are usually cured using isothermal curecycles.Theeffectsofisothermalcuringontheintergalleryspacingsof epoxy/layered silicate nanocomposites investigated using XRDhavebeenreportedbyseveralauthors.Thesereportsrevealedthat,once the clay is swollen with the epoxy-curing agent prepolymer,no further expansion occurs until the temperature of the cure isincreased to the onset of cure.1,34

The effect of thermoplastic and clay on the rheology of epoxyclay nanocomposites isothermally cured at 180 C is shown inFig. 3. There isa rapidincreasein storage modulus atgelationand alevellingoffasvitrificationoccursirrespectiveofthecomposition.Itis further observed that storage modulus is lower for theclay-filled



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0

5

10

15

20

25

30

35

40

Time(min)

Composition

Epoxy

Epoxy/5phrPES

Epoxy/3phrCloisite 30B

Epoxy/5phr PES/

3phr Cloisite 30B

Figure 4. Gel time versus composition of epoxy systems.

system compared to neat resin and thermoplastic-toughenedepoxy resin at all time intervals. Similar observations are noticedfor loss modulus versus time for the epoxy clay nanocomposites(Fig. 3).

The effect of the thermoplastic/clay on the gel time can bestudied by plotting the storage modulus (G) and loss modulus(G) as function of time at 180 C for the epoxy system. The G/G

crossover in Fig. 3 is taken as the gel point for these systems andis shown as a function of thermoplastic and clay in Fig. 4. The geltime of the epoxy system decreases from 36 to 24 min on additionof 3 phr clay. The reduction in gel time is due to the catalytic effectof clay on the curing reaction. Similar observations were made byseveral authors for epoxy and cyanate ester systems.34,35 Duringthe curing process the diffusion of the unreacted prepolymer intothe intergalleries of the clay layers followed by polymerization

play a vital role in the ultimate morphology of the system. Itis generally believed that the relative rate of extragallery andintergallery polymerization must be balanced in order to obtainan exfoliated structure.3638 This balance depends on severalfactors, including prepolymer viscosity and the nature of silicates(i.e. CEC, intergallery polarity).1,34,39 The gel time of the epoxysystem increases from 36to 41 min on addition of 5 phr PES,whichmay be due to retardation of the cure reaction of the epoxy bythe high molecular weight thermoplastic. However, the additionof both thermoplastic and clay to the epoxy system has a synergiceffect on gel time.

Wide-angle XRD

Tofurtherinvestigatethemorphologyoftheclayinsidetheternarynanocomposites, the diffraction pattern of pristine Cloisite 30Bwas compared with those of nanocomposites having various clayfractions and nanocomposites processed with various processingtechniques. Figure 5 reveals that the [001] diffraction peak ofClosite 30B clay appears at 2 = 4.81 with an interlamellarspacing (d001) of 1.81 nm, as calculated from Braggs law:

n = 2d sin (3)

where n is an integer, the wavelength, the glancing angle ofincidence and dthe interplanar spacing of clay layers.

Dean etal.35 have reported the effect of clay concentration andcuring temperature on the intergallery spacing for epoxy/silicate

0 10 20 30

0

200

400

Int

ensity

2 ()

Cloisite 30B

Epoxy/3 phr Cloisite 30B

Epoxy/5 phr PES /1 phr Cloisite 30B

Epoxy/5 phr PES /3 phr Cloisite 30BEpoxy/5 phr PES /3 phr Cloisite 30B(Planetary ball milling)

Figure 5. XRD patterns of Cloisite 30B and various nanocomposites.

nanocomposites. They found that intergallery spacing increaseswith an increase in cure temperature due to lower prepolymerviscosity and faster intergallery curing. Here we examined theeffect of clay concentration and thermoplastic on the d spacingfor the epoxy/curing agent/silicate systems. The diffractionpeak of epoxy/3phr Cloisite 30B without thermoplastic modifiershifts to 2 = 2.49 and the d spacing increases to 3.54 nm,

since the epoxy/hardener matrix enters the clay galleries. Thediffraction peak of epoxy/3phr Cloisite 30B nanocomposite with5 phr PES modifier is further shifted to 2 = 2.35 with anincrease of d spacing to 3.75 nm. The increase in d spacingon incorporation of PES is not substantial. This may be due tothe fact that high molecular weight PES (inherent viscosity =0.38 dL g1) is expected to diffuse less in between the claylayers compared with low molecular weightconstituents of epoxy(188 g mol1) and DDS (248 g mol1). Similar observations weremade by us30,31 for PEEKMOH-toughened epoxy clay ternarynanocomposites and by Isik etal.28 for epoxypolyether polyolorganically treated montmorillonite ternary nanocomposites. Asthe clay concentration increases from1 to 3 phrin PES-toughenedepoxy clay ternary nanocomposites, the diffraction angle peak

shifts from 2 = 2.31 to 2.35 and the d spacing marginallydecreases from 3.81 to 3.75 nm. However, the planetary ball-mill dispersion technique facilitates an exfoliated morphologywith total disappearance of the peak in the low-angle regionfor PES-toughened epoxy clay ternary nanocomposites. Hai-jun etal.12 also observed an exfoliated morphology for epoxyclay nanocomposites with better mechanical properties whendispersed by high-shear mixing using ball milling.

Tensile, flexural and fracture toughness properties

Tensile, flexural and fracture toughness properties of thePES-toughened epoxy and PES-toughened epoxy clay ternarynanocomposites are given in Table 3. The data reveal that the



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Table 3. Tensile, flexural and fracture toughness properties of ternary nanocomposites

Composition

Tensilestrength

(MPa)

Tensilemodulus

(GPa)Elongation

(%)

Flexuralstrength

(MPa)

Flexuralmodulus

(GPa)

Fracturetoughness,

KIC (MN m3/2)

Epoxy 60 4 1.67 0.02 3.4 0.3 122 6 2.95 0.11 1.16 0.16

Epoxy/5 phr PES 80 6 1.55 0.04 8 0.4 135 6 2.96 0.12 1.62 0.19

Epoxy/5 phr PES/1 phr Cloisite 30B 64 3 1.77 0.02 5.3 0.1 134 8 3.03 0.03 1.53 0.07


Epoxy/5 phr PES/3 phr Cloisite 30B (1 h ultrasonication) 60 5 1.79 0.03 4.2 0.4 111 10 3.20 0.01 1.65 0.10

Epoxy/5 phr PES/3 phr Cloisite 30B (1 h planetary ball milling) 65 2 1.92 0.04 4.1 0.3 132 9 3.59 0.05 1.93 0.14



tensile strength of the PES-toughened epoxy system is higherthan that of the neat epoxy systems by 33%, which may be due tothe plasticization of PES. But the tensile strength decreases withan increase in clay content at constant PES content (5 phr). Thisis because of the stress concentration effect of clay agglomer-ates at higher clay contents and thus the claypolymer surfaceinteraction decreases. Another reason is that as the clay contentincreases, the viscosity of the system increases resulting in het-erogeneity and nanovoid formation due to the entrapment of airbubbles during sample preparation.40 Similar observations havebeen reported for polyether polyol-modified epoxy montmoril-lonite ternary nanocomposites.28 The tensile and flexural moduliof the ternary nanocomposites are found to increase with theincorporation of Cloisite 30B. An increase of 10 and 18.7% in thetensile modulus is observed for PES-toughened epoxy clay ternarynanocomposites with respect to neat epoxy and PES-toughenedepoxy system, respectively, with 3 phr Cloisite 30B incorporation.Surprisingly the increase in tensile modulus is decreased to 5 and12% with 8 phr Cloisite 30B incorporation. This is due to the fact

that, as the clay concentration increases, the constraining effectof clay agglomeratesinhibits the plastic deformation of the matrixas reported by Wang etal.41 Qi etal.42 observed a 15.1% improve-ment in tensile modulus on addition of 10% montmorillonitemodified with methyl tallow (bis-2-hydroxyethyl) quaternary am-monium salt to diglycidyl ether of bisphenol A epoxy system.Similarly Basara etal.43 found a 17.2% enhancement in tensilemodulus on addition of 7 wt% Cloisite 30B to the epoxy sys-tem. Isik etal.28 observed an increase in tensile modulus withrespect to clay concentration for polyether polyol-modified epoxymontmorillonite ternary nanocomposites. The flexural modulusis increased by 17 and 21% for 8 phr Cloisite 30B incorpora-tion with respect to epoxy and PES-toughened epoxy matrix,

respectively.The fracture toughness values of the nanocomposites are givenin Table 3, which are expressed in terms of the stress intensityfactor (KIC). Fracture toughness values are found to be higher forPES-toughened epoxy clay ternary nanocomposites comparedto neat epoxy resin. The enhancement in toughness of thenanocomposites can be attributed to the good dispersion ofthe clay particles, efficient stress transfer through the clay layersas well as increase in interfacial interaction between clay particlesand epoxy. It has been reported that the increase of fracturesurface area due to crack path deflection is the major tougheningmechanism in the epoxy clay nanocomposites.41 However,as the clay concentration increases the fracture toughnessdecreases steadily for ternary nanocomposites revealing that the

0 2 31 4 5 6 7 80

10

20

30

40

50

60

70

80

90

Tensilestress(MPa)

Tensile Strain (%)

Epoxy

Epoxy/5 phr PES

Epoxy/5 phr PES/1 phr Cloisite 30BEpoxy/5 phr PES/3 phr Cloisite 30B


Figure 6. Tensile stressstrain behaviour of the ternary nanocomposites.

thermoplastic is the main contributing factor to the increase intoughness of the nanocomposites rather than the clay alone.At higher clay concentration there is a possibility of isolatedagglomeration of clay particles. These isolated clay agglomeratescan act as failure sites from where the crack initiates and thisreduces the stress intensity factor. Nanocomposites for whichthe clay particles are dispersed by planetary ball milling show a

greater improvement in tensile, flexural and fracture toughnessproperties compared to ultrasonicated systems. This may be dueto theshearforceexerted by ball milling being moreeffective thanultrasonication in dispersing the clay layers in the polymer matrixresulting in higher surface area of interaction between clay layersand matrix. Zunjarrao etal.11 reported that the fracture toughnessof an epoxy/clay system was increased by 35 and 20% with2% volume fraction of montmorillonite modified with octadecylammonium salt incorporatedby shear mixing and ultrasonication,respectively.

Tensile and flexural stressstrain behaviour of the ternarynanocomposites is shown in Figs 6 and 7, respectively. It canbe seen that the area under thestressstrain curve decreases withan increase in clay concentration for the PES-toughened epoxy



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0 2 4 70

20

40

60

80

100

120

140

Flexural

Strength(MPa)

Flexural Strain (%)

Epoxy

Epoxy/5 phr PES





81 3 5 6

Figure 7. Flexural stressstrain behaviour of the ternary nanocomposites.

0 200 400 600 800 10000

20

40

60

80

100

%

weight

Temperature (C)

Epoxy/5 phr PES




Figure 8. TGA curves of the nanocomposites.

system resulting in a gradual decrease in fracture toughness(Table 3). It is also observed that, as the clay content increases,the slope of the stress strain curve also increases, indicating anincrease in modulus (Table 3).

Thermogravimetric analysis

Thermograms of PES-toughened epoxy clay ternary nanocom-posites are shown in Fig. 8. No appreciable change is observedin the initial degradation temperature and overall thermal stabil-ity except a slight increase in percentage char residue at highertemperature.

0 90 180 270

0.0

0.2

0.4

0.6

T

an

Temperature (C)

Epoxy

Epoxy/5phr PES

Epoxy/5phr PES/1 phr Cloisite 30B



Epoxy/5phr PES/3 phr Cloisite 30B(Planetary ball milling)

Figure 9. Tan versus temperature for PES-toughened epoxy clay ternarynanocomposites.

Thermomechanical analysis (TMA)

TMAwasusedtodeterminetheCTEand Tg ofthenanocomposites.CTEand Tg of theternary nanocomposites aregivenin Table 2. Thedata reveal that CTE values decrease marginally with an increasein clay content up to 5 phr. This decrease may be due to theuniform and nanolevel distribution of clay particles in the epoxy

matrix resulting in a restriction of mobility of the epoxy chainsand also due to efficient stress transfer to clay layers with largesurface area. The reduction in CTE with the incorporation of clayplatelets was also reported by us30,31 and several other authors.15

No appreciable change in Tg is observed with the incorporationof clay into the epoxy matrix. However, DMTA and DSC analysesreveal a considerable decrease in Tg on incorporation of clay(Table 2).

Dynamic mechanical thermal analysis

The viscoelasticproperties of the epoxy resin, PES-modified epoxyresin and PES-modified epoxy resin clay ternary nanocompositeswere investigated using DMTA. The tan curves for the PES-

modified epoxy clay ternary nanocomposites are shown inFig. 9. The curves for the nanocomposite show only a singleTg value. Although two Tg values corresponding to epoxy-richand thermoplastic-rich regions are expected, only one Tg value isobserved because of the close proximity ofTg of the PES and Tg ofthe DDS-cured epoxy resin, and also due to homogeneity withoutphase separation. Tg of the nanocomposites decreases by about7 C (from 216 C for epoxy to 209 C) on incorporation of 5 phrclay.During intercalation/exfoliation,the interfacial surface areaofthe clay particles and the interaction with polymer can drasticallyalter thechain kineticsin theregionssurroundingthem andleadtoa lower crosslink density as suggested by Yasmin etal.44 Anotherpossible reason for the decrease of Tg of the nanocompositeswith increasing clay loading is due to the plasticizing effect of the



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0 50 100 150 200 250

0

1000

2000

3000

Storagem

odulus(MPa)

Temperature (C)

Epoxy

Epoxy/5phr PES



Epoxy/5phr PES/5 phr Cloisite 30BEpoxy/5phr PES/3 phr Cloisite 30BPlanetary ball milling)

Figure 10. Storage modulus versus temperaturefor PES-toughened epoxyclay ternary nanocomposites.

organic modifier of the clay layers as reported by Chen etal.36

They also reported a lower Tg with higher dspacing. In the presentsystem, exfoliated nanocomposites show a lower Tg compared tointercalated nanocomposites (Fig. 9). The decrease in Tg may alsobe due to the formation of matrix interface between the silicatelayers36 where properties are different from those of the bulk of

the matrix. A lower Tg with higher clay content was also reportedby us and by many other investigators.30,31,36,45

The storage modulus (E) values, recorded as a function of tem-perature, for PES-toughened epoxy clay ternary nanocompositesare shown in Fig. 10. The storage moduli of the PES-modifiedepoxy resin and PES-modified epoxy resin clay nanocompositesare found to be higher than that of neat resin below Tg. Withan increase in temperature, the storage modulus decreases, witha sharp decrease being observed near Tg. The storage moduliof the PES-modified epoxy and PES-modified epoxy clay ternarynanocomposites are slightly lower than that of the neat epoxyresin in the rubbery plateau region, indicating lower crosslinkdensity of the modified systems. The storage modulus of thenanocomposites increases with an increase in clay concentra-

tion irrespective of the processing techniques because of an

enhancement effect from the addition of rigid inorganic clay par-ticles. Ternary nanocomposites processed using the planetary ballmill dispersion technique with 3 phr clay show an increase of 21%in storage modulus, whereas the ultrasonication dispersion tech-nique leads to a 12% increase for the neat or PES-modified epoxyresin. This clearly indicates an enhanced interfacial interactionbetween clay layers and the matrix due to exfoliated morphology(Fig. 5). However, the increase in storage modulus is much lessin the present system compared to PEEKMOH-toughened epoxyclay ternary nanocomposites reported earlier31 due to the ab-sence of any chemical reaction/phase separation between PESand epoxy.

Scanning electron microscopy

The phase morphology of the PES-toughened epoxy and PES-toughened epoxy clay ternary nanocomposites were examinedusing SEM. SEM micrographs of tensile-failed surfaces of epoxyand epoxy/5 phr PES/3 phr Closite 30B and fracture toughness-failed surfaces of epoxy, epoxy/5 phr PES, epoxy/3phr Cloisite 30Band epoxy/5 phr PES/3 phr Cloisite30B are shownin Figs 11(a) and(b) and 12(a)(d), respectively. All the blends and nanocompositesare found to be homogeneous. This is due to the intermolecularhydrogen bonding between sulfonyl groups of PES and hydroxylgroups of epoxy along with the intramolecular hydrogen bondingbetween sulfonyl groups of DDS and hydroxyl groups of epoxy,as shown in Scheme 2, restricting phase separation. Similarly, Niand Zheng also reported the effect of inter- and intramolecularhydrogen bonding between sulfonyl groups of diaryl sulfone andhydroxyl groups of epoxy on phase separation in epoxy/poly(-caprolactone) blend systems.46

The SEMmicrographsof tensile-failed specimens(Fig. 11) revealthat the fracture surface of the neat epoxy system shows typicalcharacteristic of brittle fracture. The surface is smooth withuninterrupted crack propagation. The failure surface of epoxy/5

phr PES/3 phr Closite 30B is rough with ridge patterns, andriver markings can also be seen on the fracture surface. Theroughness of the fracture surface is for two reasons. First, it isan indication of crack path deflection. Second, the roughnessindicates the ductile nature of the crack. The matrix becomes lessbrittle in comparison with the unmodified epoxy resin becauseof a decrease in crosslink density. The polar sulfone groupsof PES enhance the interfacial adhesion through the hydrogenbonding interaction between epoxy matrix and PES. Similarly astrong interfacial/polar interaction between functional hydroxylgroups of the organic moiety of clay/epoxy and PES of ternarynanocomposites influences the crack path deflection. Anotherfactor responsible for the increase in the fracture toughness isthe local plastic deformation of the matrix. According to Hedricketal.,47 river markings on the fracture surface are an indication

S

O

O

O

H

Secondary hydroxyl group of epoxy/hydroxyl group of clay/hydroxyl group of clay modifier

Sulfone group of PES/sulfone group of DDS

Scheme 2. Chemical interactions of hydroxyl groups with sulfone groups.



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(a) (b)

Figure 11. SEM images of tensile-failed surfaces of (a) epoxy and (b) epoxy/5 phr PES/3 phr Cloisite 30B.

(a) (b)

(c) (d)

Figure 12. SEM images of fracture-failed surfaces of (a) epoxy, (b) epoxy/5 phr PES, (c) epoxy/3 phr Cloisite 30B and (d) epoxy/5 phr PES/3 phr Cloisite30B.

of the plastic deformation of the matrix. On careful observationof Fig. 11(b), there are more crack path deflection sites withthermoplastic inclusion.

Thefracturesurfaceoftheneatepoxysystem(Fig. 12(a))exhibits

an uninterrupted and featureless crack propagation resultingfrom the brittle failure of epoxy. In contrast, the fracture surfaceof 5 phr PES-toughened epoxy (Fig. 12(b)) shows an improvedtoughness characterized by crack path deflection, rough andridge patterns with river marking lines. The fracture surface ofthe epoxy incorporating 3 phr clay (Fig. 12(c)) shows only aslight improvement in fracture toughness as evidenced fromthe agglomerated and debonded clay particles around the onsetof crack deflection. Liu etal.48 also observed similar fractographicappearance for nanoclay-filled epoxy systems. The micrograph of3 phrclayand 5 phrPES-reinforced epoxy ternary nanocomposites(Fig. 12(d)) shows a better dispersion of clay particles as well asthe presence of PES in the ternary nanocomposites. The fracturetoughness of the PES-toughened epoxy clay system is improved

compared with epoxy clay ternary nanocomposites, but inferiorto that of the PES-toughened epoxy blend. This may be becauseof the presence of debonded clay particles. However, the additionof clay particles to the PES-toughened epoxy blend reduces the

toughness, but increases the modulus.SEM images of fracture-failed surface of epoxy/5 phr PES/3phr clay ternary nanocomposite processed using ultrasonicationand planetary ball mill clay dispersion techniques are shown inFigs 13(a) and (b), respectively. The fracture surface of the ternarynanocomposite processed usingthe planetaryball mill to dispersethe clay shows multiple fracture with rougher surface resulting inhigh fracture toughness (Table 3) compared to samples with claydispersed using ultrasonication (Fig. 13(a)).

Optical transparency

Photographs of neat epoxy, PES-toughened epoxy and PES-toughened epoxy clay ternary nanocomposites are shown



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(a) (b)

Figure 13. SEMimages of fracture-failedsurface of epoxy/5 phrPES/3phrclay ternary nanocomposite processedusing (a) ultrasonication and(b) planetaryball mill clay dispersion techniques.

(a) (b)

(c) (d)

Figure 14. Photographs of (a) epoxy, (b) epoxy/5 phr PES, (c) epoxy/5 phr PES/3 phr Cloisite 30B and (d) epoxy/5 phr PES/5 phr Cloisite 30B.

in Fig. 14. As the clay concentration increases, the opticaltransparency of the PES-toughened epoxy decreases due to the

occasional presence of agglomerated clay particles in intercalatedmorphology of the ternary nanocomposites. However, opticaltransparency is retained even with 5 phr clay concentration unlikeconventional microfilled composites. The retention of opticaltransparency is due to the nanolevel distribution of clay platelets.Anotherplausiblereasonisthatthedimensionsoftheclayplateletsare less than the wavelength of the light, allowing the passage oflight without hindrance.

Oxygen gas permeability

Oxygen gas permeability data for neat epoxy, epoxy clay andPES-toughened epoxy clay ternary nanocomposites are shown inFig. 15. The oxygen gas permeability increases to 26% on addition

of 5 phr PES to the neat epoxy system, indicating the amorphousnature of thethermoplastic.Espuche etal.49 alsoobservedthatthe

permeability of thermoplastic/thermoset blends increases with anincrease in thermoplastic content. Butthe permeability is reducedto 70% on incorporation of 3 phr clay to the neat epoxy system,indicating the layered nature of the clay particles, which act asa barrier to penetrants. When PES is added to the epoxy claynanocomposites, the reduction in oxygen gas permeability isonly 30% of that of the neat epoxy system. It is interesting tonote that as the clay concentration increases from 3 to 5 phr inPES-toughened epoxy clay ternary nanocomposite, the reductionin permeability increases from 30 to 57% of that of the neatepoxy system. It is important to state that gas permeability iscontrolled by the clay morphology of the nanocomposite50 andthe interaction between polymer matrix and organic moieties ofclay layers. The reduction in gas permeability is due to tortuous



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a c d e0

20

40

60

80

100

120

140

PermeabilitymL

m2day1

Composition

b

Figure 15. Oxygen gas permeability of the nanocomposites: (a) epoxy;(b) epoxy/5 phrPES;(c) epoxy/3 phrCloisite30B; (d) epoxy/5phr PES/3phrCloisite 30B; (e) epoxy/5 phr PES/5 phr Cloisite 30B.

paths; in other words, a longer diffusion path through which gaspenetrants travel in the presence of layered silicates, as shown inFig. 15.

CONCLUSIONS PES-toughened epoxy clay ternary nanocomposites were

processed by melt mixing of PES with epoxy along with clay bymechanical stirring followed by ultrasonication/planetary ballmilling.

Storage modulus was increased rapidly at gelation point andlevelled off as vitrification occurs. Similarly loss modulus was

also increasedto a maximum at a particular time andthereafterdecreaseddrastically in therheological plots of theblends. Thegel time of epoxy was increased on addition of PES anddecreased on addition of clay.

XRD revealed that the ultrasonication technique gave interca-lated morphology while the planetary ball milling techniquegave exfoliated morphology.

Tensile, flexural and storage moduli and fracture toughnesswere increased on addition of clay platelets. However, there isa marginal decrease in tensile and flexural strength as the claycontent increases.

The increase in fracture toughness and storage modulus, andimprovement in tensile and flexural properties were found to

be much greater for planetary ball-milled samples comparedto ultrasonicated samples. The CTE values decreased with an increase in clay content up

to 5 phr; thereafter they increased. Oxygen gas permeability was decreased considerably on

incorporation of clay into the epoxy matrix. But with the incor-porationofPESthepermeabilityofepoxyclaynanocompositeswas increased. However, with an increase in clay concentra-tion, the permeability of PES-toughened epoxy clay ternarynanocomposites was further decreased showing a synergiceffect.

SEM revealed no phase separation due to strong polar in-teractions and hydrogen bonding between PES and epoxymatrix. However, the area of crack path deflection and river

markings increased with nanoclay incorporation confirmingthat the nanoclay not only takes part in improving thestiffness, but also in improving the toughness of the nanocom-posites along with the thermoplastic. In addition, planetaryball-milled samples showed multi-plane fracture compared toultrasonicated ones resulting in high fracture toughness.

A marginal improvement in char residue was observed withan increase in clay content. The optical transparency of PES-toughenedepoxy blends was retained even with 5 phr loadingof clay particles.

ACKNOWLEDGEMENTSThe authors thank the Director, Vikram Sarabhai Space Centre(VSSC) for giving permission to publish the article. One of theauthors (A.Asif) is thankful to Chairman, Indian Space ResearchOrganisation and Director, VSSC for providing ISRO researchfellowship. Thanks are due to all colleagues of the Analyticaland Spectroscopy Division and Material Characterisation Divisionof VSSC, Sree Chitra Tirunal Institute for Medical Sciences andTechnology, Biomedical Technology Wing, Hindustan Latex Ltd,Thiruvananthapuram for analytical support.

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