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Materials Science and Engineering A 487 (2008) 574–585

Fabrication and characterization of TiO2–epoxy nanocomposite

Amit Chatterjee a,b,∗, Muhammad S. Islam a

a Center for Composite Materials, University of Delaware, Newark DE-19716, DE, USAb Navel Materials Research Laboratory, DRDO, Ambernath (E) 421506, India

Received 27 August 2007; received in revised form 15 November 2007; accepted 15 November 2007

bstract

A systematic study has been conducted to investigate the matrix properties by introducing nanosize TiO2 (5–40 nm, 0.5–2% by weight) fillersnto an epoxy resin. Ultrasonic mixing process, via sonic cavitations, was employed to disperse the particles into the resin system. The thermal,

echanical, morphology and the viscoelastic properties of the nanocomposite and the neat resin were measured with TGA, DMA, TEM and Instron.he nano-particles are dispersed evenly throughout the entire volume of the resin. The nanofiller infusion improves the thermal, mechanical andiscoelastic properties of the epoxy resin. The nanocomposite shows increase in storage modulus, glass transition temperature, tensile modulus,exural modulus and short beam shear strength from neat epoxy resin. The mechanical performance and thermal stability of the epoxy nanocom-

osites are depending on with the dispersion state of the TiO2 in the epoxy matrix and are correlated with loading (0.0015–0.006% by volume). Inddition, the nanocomposite shows enhanced flexural strength. Several reasons to explain these effects in terms of reinforcing mechanisms wereiscussed.

2007 Elsevier B.V. All rights reserved.

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eywords: Epoxy; TiO2; Nanocomposite

. Introduction

High performance polymer composite materials are nowa-ays has wide application under hard working conditions. Theaterials must provide unique mechanical and thermal proper-

ies combined with a low specific weight and high resistance toegradation in order to ensure safety and economic efficiency.ne of the composite materials for technical applications may be

epresented by thermoset polymer matrix, epoxy resins, whichlready covers along some of the demanded properties. Epoxyesins are a class of versatile thermosetting polymers, which areidely used in structural composites, adhesives, surface coatings

nd electronic circuit board laminates [1]. However, the poly-er matrix must withstand high mechanical loads; it is usually

einforced with fillers. The addition of different fillers favorably

tiffens the material and may also increase the strength underertain load conditions. But the micro-fillers have a detrimentalffect on certain important properties such as impact resistance

∗ Corresponding author at: Gentex Corporation, 324 Main Street, Carbondale,A-18407, USA.

E-mail address: chatterjeamit@yahoo.com (A. Chatterjee).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.11.052

2]. A new approach aiming to overcome this basic problem iselated to the nanotechnology and uses of fillers in the nanometercale. Nanoparticles embedded in polymer matrix have attractedncreasing interest because of the unique mechanical, optical,lectrical and magnetic properties displayed by nanocomposites.ue to nanometer size of these particles, their physicochemi-

al characteristics differ significantly from those of molecularnd bulk materials [3,4]. Nanoparticle reinforced polymers, syn-rgistically combine the properties of both the host polymeratrix and the discrete nanoparticles [4]. The approach demon-

trates the potential to change characteristics of thermosettingnd thermoplastic polymers fundamentally to improve theireneral performance [2,5]. Nanocomposites include more thanne solid-phase, where at least one-phase dimension is in theanometer range. Typically, all solid phases, may be amorphous,emicrystalline or crystalline, are in the 1–20 nm range [6]. Theechanical properties of the nanocomposite are expected to beuch higher compared to the micro-filler because of the par-

icle size. The absolute number of particles would rise rapidly

y a factor of 1000 after imaginary changing the particle sizey a factor of only 10, e.g. 1 �m to 100 nm [7]. The resultantomposite properties should be more improved than expectedrom the individual components because the nanoparticles pro-

ence

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mtt[sure above the cavitation threshold creates numerous cavitiesin the liquid. Some of these cavities oscillate at a frequency ofthe applied field (usually 20 kHz) while the gas content insidethese cavities remain constant. However, some other cavities

Table 1Mechanical properties of TiO2

A. Chatterjee, M.S. Islam / Materials Sci

ide a very high specific surface area. This area can generate aew material behavior, which is widely determined by interfacialnteractions, offering unique properties and a completely newlass of materials [8]. The fundamental change in the materialroperties arises becomes obvious, if one considers for exam-le a small crack propagating through the material, which isnteracting with many nanoparticles instead of crossing only aew microparticles. So, higher energy consumption would bexpected. It was already verified by several research groups thatanoparticles are reinforced effectively into the thermoplasticnd thermosetting polymer matrices [9] and that enhances theroperties. Specially, the reinforcement covers improvement ofhe flexural modulus without loosing flexural strength. Thisffect is at the same time accompanied by improvements inracture toughness and impact energy which, however, dependtrongly on the filler volume content. The nano-filler inducedicromechanical deformation processes can be responsible for

his behavior [10].However, the unique nanocomposite effect can only be effec-

ive, if the nanoparticles are well dispersed in the surroundingolymer matrix. It is reported that a considerable amount ofmprovement in mechanical properties can be achieved usingery low amount of nanopartcicles loadings [1–10]. A neg-igible contribution made by the interphase provides diverseossibilities of performance tailoring and is able to influencehe properties of the matrices to a much greater extent underather low nanofiller loading. Significant improvement in theensile performance of polypropylene composites has also beeneported in terms of stiffening, strengthening and tougheningith a low filler content of about 0.5%. The mechanical per-

ormance significantly depends on the dispersion state and theicrostructural homogeneity of the nanoparticles [11]. The dis-

ersion method for the nanoparticle incorporation in polymeratrix must be very vital footstep and key point to receive the

esired material properties.Nanoparticles were generally introduced into polymer matrix

sing various techniques. The various dispersion processes areecessary in order to transfer the nanoparticles from the agglom-rated state into a homogeneously dispersed state. The directncorporation using chemical methods and by application ofigh shear forces during mechanical powder dispersion pro-ess [12–14] are the popular one. Ultrasound vibration wassed to disperse nanoparticles, which also helps to improve theispersion state of nanoparticles. Chemical methods are ableo generate individual and non-agglomerated nanoparticles “initu” within a thermosetting/thermoplastic polymer. However,n additional chemical treatment of the nanoparticle surfaceay further enhance the composite’s properties by improving

he filler-matrix coupling quality [12–14].The TiO2 is extensively used in the industries such as addi-

ives in plastics, agglomerates for thermal sprays, air/fuel ratioontroller in automobile, attenuation of ultraviolet light, cat-lysts and catalyst supports, demilitarization of chemical and

iological warfare agents, electrode materials in lithium bat-eries, energy converter in solar cells, gas sensors, inorganic

embranes, photo catalytic degradation of bacteria and grime,hotochemical degradation of toxic chemicals, piezoelectric

CHTT

and Engineering A 487 (2008) 574–585 575

apacitors, solid oxide fuel cell, UV protection and wasteater purification. So, epoxy–TiO2 nanocomposite may haveide application, particularly for bio-terrorism, organic photo-oltaic, fire retardant composite, composite nanomenbrane, etc.anocomposites using thermoplastic polymers are well known

or improving the mechanical, electrical, thermal and insulatingroperties. However, nanocomposite using thermosetting poly-ers have not been studied so extensively [15,16] particularly

sing TiO2. The influence of nanoclay on the mechanical andurface properties of epoxies has been characterized [15–17].he influence of TiO2 on epoxy resin, SC79 is reported in thistudy for the first time. We focused on the performance opti-ization of composites containing TiO2 nano-particles and to

nderstand the role of nanoparticles. The nanosized TiO2 par-icles have been infused through sonic cavitations in an epoxyesin matrix. An ultrasonic mixing procedure was developedo uniformly disperse the particles into the resin system. The

echanical and thermal performance of the nano-filled polymeratrix as a function of the nanopartcicle content (0.5–2%) was

valuated. The expected overall properties of nanocompositesay open the way towards new applications of high perfor-ance polymers, leading to an innovative product development

n the automotive industry, electronics for coatings and manyther applications.

. Experimental

.1. Materials and methods

The epoxy resin used in this investigation has two part,art A (diglycidylether of bisphenol A 60–70%, aliphaticiglycidylether 10–20%) and part B (cycloaliphatic 85–90%,roprietary amine 10–15%, toughener 10–20%) of Applied Pol-ramic Inc., Benecia, CA, USA. The nanoparticles used here isiO2 of 5 nm size supplied by Nanostructured & Amorphousaterials Inc., Los Alamos, NM, USA. The properties of the

articles are given in Table 1.

.2. Fabrication

There are various techniques such as melt mixing, solutionixing; shear mixing and mechanical stirring to infuse nanopar-

icles into a polymer are available. Acoustic cavitations is one ofhe efficient ways to disperse nanoparticles into virgin materials18]. In this case, the application of alternating acoustic pres-

omp. strength (MPa) 800–1000ardness (kg f mm−2) 980ensile strength (MPa) 350en. Mod. (GPa) 200–300

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76 A. Chatterjee, M.S. Islam / Materials Sci

row intensely under tensile stresses while yet another portionf these cavities which are not completely filled with gas, starto collapse under the compression stresses of the sound wave.n the later case, the collapsing cavity generates tiny particles ofebris and the energy of the collapsed one is transformed intoressure pulses. It is note worthy that the formation of the debrisurther facilitates the development of cavitation. It is assumedhat acoustic cavitation in liquids develops according to a chaineaction. Therefore, individual cavities on real nuclei developo rapidly that within a few microseconds an active cavitationegion is created close to the source of the ultrasound probe.he development of a cavitation processes in the ultrasonicallyrocessed melt creates favorable conditions for the intensifica-ion of various physio-chemical processes. Acoustic cavitationccelerates heat and mass transfer processes such as diffusion,etting, dissolution, dispersion and emulsification. Recently isas been reported that there is a clear acceleration of polymereaction under ultrasound in both catalyzed and uncatalysedeactions [19]. In the present investigation ultrasonic mixingas employed to infuse TiO2 nanoparticles into the epoxy

esin.Titanium nanoparticles were added to the epoxy part A at

arying percentage in a plastic beaker and mixed by hand as annitial dispersion. The mixer was then placed for the sonicationrocess. Vibracell ultrasonic processor shown in Fig. 1 was usedor dispersion. The maximum power output is 750 watts and ispplied at a frequency of 20 kHz using a 12.5 mm diameter tita-ium alloy tip. An amplitude of 15 �m is applied with a pulse (9 sn,1 s off) for about 20 min. During sonication, heat was buildsp. In order to avoid temperature rise during sonication, exter-al cooling was employed by submerging the mixing beaker inmixture of ice and water. The equivalent amount of part Bas added in the mixture, and mixing was continued for another0 min using a high speed mechanical stirrer. After degassing,he resin mixture was poured into a mold. The sample was curedor about 28 h at room temperature and postcured at 93 ◦C forbout 8 h. The postcured resin was removed from the mould andamples were prepared for various tests discussed below.

Transmission electron microscopy was conducted in JEM-

00 FX. Thin sections (70 nm) of the specimen were obtained byicrotome with diamond knife for TEM analysis. The filament

oltage was set at 200 KV to make a bright field image of theanostructure.

Fig. 1. Ultrasonic Vibracell.

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and Engineering A 487 (2008) 574–585

Flexural properties of the materials were obtained accordingo ASTM D-790. Samples length to depth ratio was kept higherhan 16:1. Flexural load was employed in an Instron under threeoint bend fixture and the loading rate was applied according tohe specimen dimension as defined in ASTM D-790.

Dynamic mechanical analysis was conducted in a TAynamic mechanical analyzer (model DMA 2980) at a frequencyf 1 Hz using dual cantilever clamp with amplitude of 15 �m.he samples were ramped from room temperature to 250 ◦Csing a ramp rate of 2 ◦C/min. The storage modulus and lossodulus were measured. The loss modulus peak is used foreasuring the Tg of the resin. The nanocomposite samples were

irectly cut into rectangular shapes 60 mm × 13 mm × 3 mm forMA analysis.Thermogravimetric analysis (TGA Q-500, TA Inc.) was used

o measure the degradation properties of the nanocomposites.he experimental parameters used for TGA included a heating

ate of 10 ◦C/min, room temperature to 900 ◦C and a flow ratef helium purging gas is 100 ml/min.

Tensile properties of the materials were obtained according toSTM D-638. Samples were prepared according to the ASTM

tandard. The bidirectional stain gages were mounted in the sam-les to measure the axial and the transverse stain. The tensileoad was employed in an Instron. The loading rate was appliedccording to the specimen dimension as defined in ASTM D-38. The short beam shear testing was conducted using ASTM-2344. The sample preparation, conditioning, testing and the

nalysis was done as defined in ASTM D-2344.

. Results and discussion

.1. Morphology

Dispersion of nanoparticles in the epoxy resin is very diffi-ult because of the Van Der Waal’s force between the particles.esin viscosity plays a significant role for the uniform disper-

ion of nano-particle. We have chosen a low viscosity VARTMpoxy resin (300 cp) for this work that helps to disperse thearticles uniformly. To optimize the dispersion the sonicationime was varied from 0 to 120 min. The best result was obtainedsing a total mixing time of 30 min. Further increment of disper-ion time increases the temperature and viscosity of the resin.he resin starts to cure that decreases the dispersion of theano-particles. Transmission electron microscopy (TEM) analy-is was conducted to verify the size and dispersion quality of theanocomposite. Fig. 2 is the TEM image of an agglomeration ofiO2 nanoparticles prior to adding to the epoxy. Fig. 3 shows theEM picture of the 1% TiO2–epoxy nanocomposite. In Fig. 3

t is shown that the particles have been effectively dispersed inhe epoxy. The nano-particles are mostly circular in shape. Theize varies from 5 to 10 nm. Most of the particles are distributedniformly. However, some residual agglomeration of particlesemains (20–50 nm) (Fig. 3). Van Der Waal force between the

articles is the leading reason for weak dispersion of the nano-articles in the epoxy resin matrix. The effect of loading onispersion is also observed. Increasing on loading creates non-niformity that clearly observed in the TEM pictures. However,

A. Chatterjee, M.S. Islam / Materials Science and Engineering A 487 (2008) 574–585 577

Fig. 2. TEM image of powder TiO2 (5 nm).

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% loading gives better dispersion with homogeny. Size effect

f nano-particle also plays a big role for uniform dispersion.arger particle size improves dispersion. Smaller size has morean Der Waal force between the particles and that decrease theispersion capability of the TiO2 nano-particles in the epoxy

chflb

able 2echanical properties for TiO2–epoxy Nanocomposite

aterials Flexure strength (KSi) Flexure mod. (KSi) Tensile strength (PS

eat resin 18.0 ± 0.58 487 ± 9.72 7321 ± 755.0% TiO2 18.9 ± 0.28 514 ± 9.64 5424 ± 728.0% TiO2 17.3 ± 0.69 440 ± 1.92 5024 ± 498

ig. 4. Loading effect on flexure properties of epoxy–TiO2 nanocomposite.

atrix. The particle–particle interaction is high for smaller size.he applied ultrasonic energy and time of dispersion are unable

o separate the all particle. So, the particles agglomerate andventually form lumps that we have seen here. Efforts are con-inuing to focus on improving more homogenous dispersion ofhe particles.

.2. Mechanical performance of nanocomposite

Two types of mechanical properties, flexural and tensile wereerformed to evaluate the stiffness and strength of the materials.ive samples of each type were tested for measuring the flexuralroperties of neat epoxy resin and nanocomposite samples usingSTM-D790. In the case of 1% loading, the 5 nm TiO2–epoxyanocomposite shows 5% increase in modulus and an increase inexural strength (Table 2). For the different amount of loading,

he modulus is observed to decrease as increasing the loading to% for the 5 nm particle. A typical stress versus displacementehavior from the flexural test is shown in Fig. 4. The energybsorption behavior of the neat resin and the nanocomposite arelso plotted and presented in Fig. 5. One percent infusion thetrain, stiffness and energy absorption of the nanocomposite isncreased. However, further increment of % loading decreaseshe mechanical properties of the nanocomposites from the neatesin. The influence of rigid micro-particulate fillers on thetress–stain behavior of polymers is well known. Micro-fillers

ommonly increase the stiffness, but on the other side they mayave a detrimental effect on the flexural stain to break [20]. Theexural strength of microparticle filled composites is known toe reduced with rising filler content [20–21]. The results mea-

i) Tensile mod. (KSi) SBS strength (KSi) % Elongation to break (T)

418 ± 122 2.41 ± 0.1 4.28456 ± 60 2.81 ± 0.05 3.22476 ± 45 2.15 ± 0.05 2.92

578 A. Chatterjee, M.S. Islam / Materials Science and Engineering A 487 (2008) 574–585

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ig. 5. Effect of nanoparticle loading on energy absorption of nanocomposite.

ured for the nanocomposites in this study offer an apparentonflict with the known behavior. Flexural properties increaseith addition of nano-particulates (Table 2).Some important characteristic of composites have to be con-

idered in order to explain this phenomenon. The quality of thenterface in composites, i.e. the static adhesion strength as wells the interfacial stiffness, usually plays a very important rolen the materials’ capability to transfer stresses and elastic defor-

ation from the matrix to the fillers [22]. This is especiallyrue for nanocomposites, because they impart a high portion ofnterface. If the filler matrix interaction is poor, the particles arenable to carry any part of the external load. In that case, thetrength of the composite cannot be higher than that of the neatolymer matrix. If the bonding between the fillers and matrix isnstead strong enough, the yield strength of a particulate com-osite can be higher than that of the matrix polymer [23]. In theame way corresponds a high interfacial stiffness and to a highomposite modulus. Hence, the gradual increase in stiffness andexural strength, as observed for the nanocomposites, reveals

he stresses are efficiently transferred via interface. The stain atreak usually declines with rising filler content. Low filler con-ent can cause dramatic drop in the fracture strain. The composites partly filler and part matrix. Due to rigid nature of the fillers,he deformation comes from the polymer. The actual deforma-ion experienced only by the polymer matrix is much largerhan the measured deformation of the sample with the resulthat the polymer reaches the failure strain limit at a lower totaleformation. Hence, total composite strain-to-break decreases.owever, it is surprisingly observed that for the nanocomposites

n the present study, the stain-to-break behaves contrary to con-entionally filled composites (Fig. 6). It tends rather to higheralues as long as the filler content does not surpass 1% loadingy weight (Fig. 6). This increase suggests that the nanoparti-les are able to introduce additional mechanisms of failure andnergy consumption without blocking matrix deformation. Par-icles may induce matrix yielding under certain conditions and

ay furthermore act as stoppers to crack growth by pinning the

reaks [24]. Nevertheless, if the fillers exceed 1% loading byeight the failure stain undergoes a slight decay. Such a reduc-

ion proposes that the large number of fillers now dominate, andhey reduce the matrix deformation by restraining mechanically.

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ig. 6. Effect of weight percent loading on the stain at break of the TiO2–epoxyanocomposite.

Another important aspect, which is more related to the dis-ersion state of the nanoparticles, may additionally play aignificant role. For the case that relatively large agglomeratesould remain in the matrix, a propagation crack could encounterstress concentration locally and then easily induce the ini-

iation of the final failure. The embrittling effects occur onlyt higher filler contents, where more agglomerates are likelyo be formed. The improved properties at low filler contents,estify lower stress concentrations and therefore, the remain-ng nanoparticle clusters should be small. This demonstrateshe importance of a homogeneous distribution of nanoparticlesithin the matrix. The more nanoparticles are in contact with theolymer, the higher is a probability of interaction. Large contactreas benefit an efficient stress transfer between both parts andesult in improved flexural stiffness, strength and failure stain.

Visual examination on the fracture surfaces of nanocompositey SEM methods shows detailed information on the cause andocation of failure. Some specimens were chosen to study thecting deformation mechanisms responsible for the reinforce-ent. Several mechanisms of crack are applicable for filled

poxy composites [24]. The crack may pass through particlesf the fillers are weak, known as trans-particulate fracture, orhey may pass around if the particles are string enough. Theailure may occur by interfacial debonding or by cohesive fail-re of the matrix. Some of the mechanisms are proven true foranocomposites in the present study. The SEM was also done forhe neat resin and the nanocomposite samples. The neat epoxy

atrix resin system shows brittle behavior characterized byarge smooth areas, large hyperbolic markings, ribbons and frac-ure steps in the direction of crack propagation. In contract theanocomposite surfaces are rougher structured, clearly revealsany hyperbolic markings opening in the direction of crack

ropagation. The similar observation was reported by scientistsnd those tallies well with our findings [2]. It is already reportedhat the void generation in the nanocomposite are less comparedo the neat resin system [25]. The reduction in void content in the

atrix increases the strength of the nanophase composite. With

ncremental of particle loading above 1%, the number of par-icles increases, that cause particle–particle interaction ratherhan the particle–matrix interaction. So, the particles begin to

ence and Engineering A 487 (2008) 574–585 579

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A. Chatterjee, M.S. Islam / Materials Sci

gglomerate and form lumps which eventually affect the Vaner Waals interaction between the polymer chains and that may

ause decrease in properties.The volume fractions for the 5 nm of 0.5, 1, 1.5, and 2%

y weight are 0.0015, 0.003, 0.0045, and 0.006, respectively.ased on typical properties of Ti02 given in Table 1, and consid-ring composite micromechanics [26], we have calculated theodulus of the nanocomposite having different volume frac-

ions. The values for 0.5, 1, 1.5 and 2% by weight of 5 nm TiO2re 4.88E5, 4.9E5, 4.91E5 and 4.93E5 PSI. Our experimentalalue for the 0.003 volume fraction is about 5% higher than thealculated value. The increase in the modulus is reflected dueo the nano-effect which directly influences the bulk materialroperties.

Nanoparticle infusion has been typical with the doping ofanoclays or nanoparticles in polymeric matrices [15,16,27–29].his is quite understandable from the fact that a highly stifferaterial is being mixed with relatively softer epoxy resin. A rule

f mixture calculation based on volume fraction and individualtiffness would certainly reflect the trend [30,31]. However, ouresult did reflect the improvement in modulus and strength ofhe nanocomposite. In the case of strength, the influence of par-icles will also be strongly dependent on the interfacial adhesionetween the particle surface and the epoxy molecule discussedreviously. No special sizing was used in the present study toromote adhesion. Results in Table 2 shows that the particlesignificantly reduce flexural strength except in the case of the% loading of 5 nm particles. Overall improvements might beossible with surface treatments. In addition, the large reductionor the 2% of 5 nm particles may be a consequence of a higherevel of agglomerated particles that serve as defects sites. The

echanical load imposed on the nanocomposites is effectivelyransferred to the TiO2 from the matrix. It is well known thatood interfacial adhesion between fillers and polymer matrixestricted the molecular motions of the polymer networks thatay cause the improvement of the flexural properties. However

fter 1% of the filler content, this particle gets agglomeratedhile processing the sample. Therefore, the dispersion becomesoor and the flexure properties decreases. As described in theiterature [32,33], modulus is a bulk property of the materialshat depends on the geometry, particle size distribution and con-entration of the filler. Higher particle loading increases Van Deraal force between the particles are high and that may lower the

ispersion. It will reduce the surface area and lower any inter-ction between the matrix and filler. Effective load transfer wille low, resulting the lower modulus value. The flexural strengths known to be reduced with raising the filler content [34]. Thetatic adhesion strength as well as the interfacial stiffness usuallylays a significant role to transfer stress and elastic deformationrom the matrix to the fillers [35]. This is especially true foranocomposite, because they impart a high portion of interface.f filler matrix interaction is poor, the particles are unable toarry out any part of external load. In this case the strength can-

ot be higher than the neat composite. If the bonding between theatrix and filler is strong then the strength would be higher than

hat of neat composite. If we consider the failure of flexure load,rack is generally initiate at the tension side along the width

sTen

Fig. 7. Tensile modulus of neat resin and nanocomposite.

hen it propagate towards the thickness finally it flailed at theension side. For both of the cases failure is identical, but fromhe experiment it is observed that the crack initiation occurred atigher load in nanocomposite compare to the neat resin. Sincehe particle size is very small, the surface area is higher. It offers

ore interfacial surface interaction, which is strong in this case,nd the stress transfer through this strong interfacial bonding;elay the crack propagation resulting a higher strength.

As started earlier, the investigation began with the infusionf nanoparticles into epoxy matrix. The tensile properties wereeasured. The tensile responses of these systems are shown inig. 7 (Table 2). It is seen in Fig. 7 that epoxy nanocomposite

mprove the tensile properties. A batch of epoxy matrix was alsoixed mechanically for equal length of time without any par-

icle infusion to evaluate the effect of sonication on the epoxy.t is observed that sonicated epoxy has better (10%) in stiffnessnd strength (5%) than the mechanically mixed epoxy system.he reason for such change in stiffness of the matrix due toltrasound irradiation comes from the fact that the ultrasoundrradiation enhances the homogeneity of the reaction mixture ofesin of the epoxy matrix. The epoxy resin used here is the con-ent the mixture of diglycidyl ether of bisphenol A and aliphaticiglycidyl ether. The ultrasound irradiation helps in molecu-ar mixing of these components together and the formation ofeactive species which ultimately leads to increase the cross-inking in the polymer when mixed with amine. This effect is

ore prominent when each of the highly reactive surfaces ofhe nanoparticle is effectively coated with the resin and reactedith the amine. When the matrix is sonicated with TiO2 nanopar-

icles in it, it is observed that the enhancement in stiffness has

ignificantly increased to around 15% over the neat resin system.he corresponding improvement however, over the sonicated-poxy system in stiffness is 10%, which we believe due to theanoparticle infusion along.

580 A. Chatterjee, M.S. Islam / Materials Science and Engineering A 487 (2008) 574–585

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tcsmoaRing fillers are pushed to a higher level relative to the neat resinsystem. On the other hand the modulus decreases with the tem-perature rising, but they retain a higher stiffness than the matrixwhen the temperature exceeds 200 ◦C. The effect of TiO2 con-

ig. 8. Relation between the tensile strength and flexure strength of the neat andiO2–epoxy nanocomposites.

The tensile strength is decreased than the neat resin sys-em. However, the modulus is increased compared to neat epoxyystem (Table 2). The decreased in the strength of the compos-tes with addition of the particulate fillers is also observed byther scientists [7,8]. The strong filler/matrix adhesion wouldead to enhanced strength of the nanocomposite. Accordinglyo their consideration [7,8], it is known that the improvementf the tensile modulus of the composites exhibited in Fig. 5,hould also be interpreted as the improvement of the interfacialnteraction. Especially epoxy and nano-TiO2 is incorporated,he physiochemical entanglement between the matrix polymernd the nano-particles guarantees effective interfacial bond-ng over the whole filler content range of interests. To obtainhe relation between the flexural and tensile strength we havelotted both the values (Fig. 8). A parabolic relation is found,= −3.3683X2 + 122.18X − 1100.6, where Y = tensile strength

n KSi and X = flexure strength, KSi. The correlation factor,2 = 1. However, this is a preliminary correlation, developedased on three data point. Additional study will be helpful toerify the correlation and is ongoing.

From engineering point of view, elongation-to-break is anmportant parameter describing the rapture behavior of compos-te materials. The addition of mineral particulates into polymersed to lower it, even through the matrix has high impact tough-ess [9]. The same trend is follows here also Table 2. Theeat resin has 4.27% compared to 1% nanocomposite 3.12%.t implies that in principle matrix polymer to be more involvedore plastic deformation than the nanocomposites. The chainexibility of the crosslinked polymer on the nanoparticles isestricted that causes the decreases in the elongation to breakf the nanocomposite. In contrast to tensile strength of theomposites that needs strong filler-matrix bonding, the micro-eformation mechanism involved in elongation-to-break of theomposites depends upon extensionality of the nanoparticu-ates agglomerates in the composites. The increment in theoading causes more agglomeration confirmed by the TEM anal-sis. Balanced viscoelasticity of the grafting polymers on theanoparticles might thus be necessary for the improvement of

he elongation-to-break of the composites. As started previouslyhe volume fractions for the 5 nm of 1% by weight is 0.003%.

ith such a low volume fraction it is very difficult to visual-ze 10% increase in stiffness of the nanoinfused system. The

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ig. 9. Behavior of storage modulus with temperature of the neat epoxy resinnd the TiO2–epoxy nanocomposites.

ain in mechanical properties is due to the reduction of voidontent [25] and that have discussed before. This reduction inoid content of the matrix provided strength for the nanophasedomposites. However, the increases in loading above 1%, thearticles started to lumps, greater than the size of voids. Thoseumps act as impurities and act as the source of failure. So,he properties decreases for higher loading and have discussedarlier.

.3. Dynamic mechanical thermal properties

Dynamical mechanical tests over a wide range of tempera-ure were performed to see the physical and chemical structuralhanges of the polymers and the nanocomposites. The glass tran-itions or secondary transitions and yield information about theorphology of polymers were determined. Experimental results

f dynamic tests conducted on nanocomposites and neat resinre presented in Figs. 9 and 10 of a temperature range fromT to 200 ◦C. The complex moduli for all composites contain-

ig. 10. Behavior of loss modulus with temperature of the neat epoxy resin andhe TiO2–epoxy nanocomposites.

A. Chatterjee, M.S. Islam / Materials Science and Engineering A 487 (2008) 574–585 581

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httcsc

Fig. 11. Variation of storage mo

ent at percentage of loading on the storage modulus of epoxyesin is shown in Fig. 11. With increase in loading the modulusncreases up to 1% loading. At 1% TiO2 dispersion in epoxyhows the highest value of storage modulus among all the mate-ials. This result is similar to the flexural response, which isentioned earlier. Because of the high modulus of TiO2 and the

mproved interfacial adhesion between nanoparticles and poly-er by physiochemical interaction, mechanical load imposed

n the nanocomposites transfers through the stronger interfacialurface to the strongest nanoparticles effectively. The storageodulus starts decreasing from 1.5% of loading. The decreas-

ng in modulus may be the poor dispersion of nano-particlesith the matrix resin as have discussed previously.Fig. 12 is representing the Tg of neat and nanophased compos-

tes. During the DMA analysis with increasing the percentagef nanoparticles loading, Tg of the materials increases linearly.t 1% loading of nanoparticles (5 nm) the epoxy resin give theighest Tg. After that it starts decreasing. So, with increasing thellers contents the Tg of nanocomposites tends to move to higheralues relative to the Tg of the neat system (118 ◦C) Fig. 10. Theanoparticles obsequiously influence the Tg. The increases in Tgay be attributed to a loss in the mobility of chain segments of

he epoxy systems resulting from the nanoparticle/matrix inter-ction. Impeded chain mobility is possible if the nanoparticlesre well dispersed in the matrix. The particle surface-to-surfaceistance should then be relatively small and chain segment

ovement may be restricted. Good adhesion of nanoparticlesith the surrounding polymer matrix would additional benefit

he dynamic modulus by hindering molecular motion to somextend. The hard particles incorporated into the polymer would

sces

Fig. 12. Tg values of neat resin and nanocom

with % of loading of the TiO2.

ct as additional virtual network nodes. Hence, the enhancementf this Tg is due to the incorporation of nanoparticles. In mostf the cases crosslink density is the key factor of controllinghe Tg for normal thermoset polymer systems. With increasinghe crosslink density Tg increases. The enhanced glass transi-ion temperature is related to the restriction effect of the TiO2n segmental motion. The TiO2 may chemically react with thepoxy–amine system. Interfacial layers formed during the curingrocess will exhibit a significantly different segmental dynam-cs from the bulk resin. Therefore, TiO2–epoxy-nanocompositeas exhibited a higher Tg than the corresponding neat resin sys-em. Smaller the particle size, the more surface reactivity thatmplify the restriction of the resin networks. Therefore, the Tgas increased for the smaller particles. With increasing the load-ng the Tg increases and optimum conditions were achieved at% by weight loading. Further increase in the loading causingoor dispersion that decreases the Tg (Fig. 12). An increasedn Tg has reported for nanocomposite using the epoxy networkystems [21,25,36,37], and tallies well with our findings.

The peak temperature in the loss modulus tends to shift toigher temperatures with increasing loading. On the other hand,he restriction effect of segmental motion is also manifest byhe change in peak width of the loss modulus. According to theoupling model theory suggested by Roland and Ngai [38], theegmental motion of polymers below Tg is a cooperative pro-ess and has to overcome the resistance from the surrounding

egments in order to accomplish the transformation betweenonfigurations. As more segments are restricted by the pres-nce of TiO2, the activation threshold for the motion of someegments becomes higher. As a consequence, the loss modulus

posites at different % of TiO2 loading.

5 ence and Engineering A 487 (2008) 574–585

piofltstcrponnsdcou

3

sTUtsiwnr5[cInw

Fig. 14. Variation of decomposition temperature with respect to weight loss forneat epoxy resin and the epoxy–TiO2 nanocomposites.

Table 3TGA data of epoxy nanocomposites

% of wt. remaining Temperature (◦C)

TiO2 (%)

0 0.50 1 2

90 342 349 355 35280 351 360 364 36370 360 366 372 3706C

TNtit

d

82 A. Chatterjee, M.S. Islam / Materials Sci

eak during the glass transition trends to become wider withncreasing restriction effect. The restricted segmental motionbserved herein not only causes an increase in the Tg and theexural modulus but also improves the dimensional stability of

he polymeric materials. For the polymeric materials, dimen-ional stability is closely related to the chemical structure ofhe molecular chains and the topological structure formed. Tohange in the stiffness of primary chains, one can adjust theesponse of segment motions to the change in surroundings tem-erature. The reduction of Tg above 1% loading may be becausef two reasons, the adsorption of matrix resin to the surface of theanoparticles that shifted the non-stoichiometric mixture. Theon-stoichiometry epoxy–amine system shifted the Tg to loweride as has been reported by several authors [39,40] and thus aecrease in the Tg is observed. The second and most probableause is the lack of uniform dispersion that has discussed previ-usly. The agglomerated particles decrease the crosslinking andltimately the Tg.

.4. Thermal analysis of nanocomposite

The thermo gravimetric analysis (TGA) was done to under-tand the thermal stability of the nanophase composite system.he real time characteristic curves were generated using theniversal Analysis Data Acquisitation System. Data from a

ypical TGA of the neat resin and the manocomposites arehown in Fig. 13. TGA graphs show that increases in TiO2 load-ng increases the thermal stability of the SC79 resin. The 5%eight loss for the neat SC79 resin, 0.5, 1, and 2% TiO2–SC79-anocomposite are occurred at 295, 300, 317 and 319 ◦C,espectively (Fig. 14). Researchers are commonly considered0% weight loss as an indicator for structural destabilization36]. So, in the present study 50% of the total weight loss is

onsidered as the structural destabilization point of the system.t is clearly observed that the neat resin sample without TiO2anoparticles is stable up to 171 ◦C (1% weight loss (Table 3)),here as the 1% loading the composite was stable up to 198 ◦C.

srna

Fig. 13. TGA responses of the neat resin a

0 369 375 380 378har yield % at 650 ◦C 1.17 1.47 4.4 2.43

he thermal stability drop down to 327 ◦C (5% weight loss).ano-particle loading was (0.5–2%) increased and measured

he thermal stability. At 1% loading the highest thermal stabilitys observed. Further increment in particle loading decreases thehermal stability (Table 3) (Fig. 13).

The increase in thermal stability may be the higher crosslinkensity of the epoxy-nanocomposite than the neat epoxy-resinystem. The TGA measurement shows at 550 ◦C the neat

esin has 5% char content. However, for 1% TiO2–SC79-anocomposite system about 20% weight is present at 550 ◦C,bout ∼15% higher weight retention than the other systems

nd the epoxy–TiO2 nanocomposites.

ence and Engineering A 487 (2008) 574–585 583

apdiTpimbsdtWpoTeStidworTtMtoifsoawcw

tr

wtw

whAba

FT

C

l

w

g

l

Fst(stoT

nif

l

wdlplot of ln [ln (1/(1 − α))]versus θ (Fig. 16), the activation energyfor decomposition was calculated from the slope of the straightline in Eq. (5). The calculated activation energy as a functionof TiO2 loading is presented in Fig. 17. The activation energy

Table 4Kinetic data of epoxy nanocomposites by using Coats–Redfern equation

% TiO2 loading by wt. Ea (kJ/mol)

A. Chatterjee, M.S. Islam / Materials Sci

round that high temperature (Fig. 13). Generally, thermosetolymer having higher crosslink density shows higher maximumecomposition temperature [39]. This may be another cause toncrease the decomposition temperature of the nanocomposite.he 2% nanocomposite shows the lower decomposition tem-erature than the 1% loading (Fig. 14). The crosslink densitys maximized when the complete stoichiometry of the epoxy is

aintained. As soon as the stoichiometry of epoxy matrix wasroken with an addition of nano-loading [39], the crosslink den-ity was reduced and lower crosslink density resulted in lowerecomposition temperature as observed in Figs. 13 and 14. So,he decomposition temperature is lowered for the 2% loading.

hen the TiO2 loading is very low (0.5 wt.%), the nanocom-osite dispersed uniformly and develops a barrier to heat andxygen in the epoxy matrix due to ceramic nature of the particles.he retardant effects of the composite to heat and oxygen in thepoxy matrix are strengthened when the TiO2 loading increases.ince the number of the particles is increasing with loading. Fur-

her enhancement of loading (>1%) causes particle-to-particlenteraction domination than the particle-to-matrix interaction,iscussed previously. The defect due to agglomeration increasesith loading and shows the less retardant effects to heat andxygen, but till higher than that of the neat resin system. Theeason for lowering of the temperature above 1% by weight ofiO2–epoxy systems is due to (macroscopically) simple colliga-

ive thermodynamics effect of an impurity on a bulk solution.icroscopically, it may be seen as the result of the perturba-

ion that the TiO2 introduces to the three-dimensional structuref the polymer. This perturbation weakens the Van Der Waalsnteraction between the polymer chains. This affects the stabilityor the polymer, and that is reflected in lowering of the thermaltability. This perturbation begins at a point when the numberf particle reaches a certain level and particle-to-particle inter-ction initiates leading to agglomeration of particles into lumps,hich acts as an impurity in the system [25]. The TEM picture

onfirms the results that the agglomeration of the nanoparticleith increasing the particle loading in our mixing system.To evaluate the thermal stability more detailed, nonisothermal

hermal degradation kinetics were calculated. For an n-ordereaction, the reaction rate can expressed as

dα

dt= k(1 − α)n (1)

here α is the apparent conversion, k the rate constant and n ishe reaction order. In dynamic mode, the rate expression can beritten as

dα

dT= A

φexp

(−E

RT

)(1 − α)n (2)

here α is the conversion at temperature T, dependent on the

eating rate φ, E the apparent activation energy and A is therrhenius frequency factor. Different kinetics expressions coulde obtained by integrating the above equation with differentpproximation treatments. Among those kinetics equations, the

0012

ig. 15. Kinetics of thermal decomposition of epoxy nanocomposites (0.5%iO2 by wt) by using Coats–Redfern equation.

oats–Dfern equation [41,42] is one of the well-known models:

n

[g(α)

T 2

]=

[AR/φE

1 − 2RT/E

]− E

RT(3)

here (g(α) = 1 − (1 − α)1−n/1 − α).If the order is equal to 1

(α) = ln (1 − α)

n

[ln (1 − α)

T 2

]=

[AR/φE

1 − 2RT/E

]− E

RT(4)

or the decomposition of thermosets, the mechanism of randomcission of molecules dominates. Therefore, the decomposi-ion could be treated as one order reaction. The curves ofg(α) = ln (1 − α) versus 1000/T were computed with conver-ion. The activation energy was calculated from the slope ofhe line. Fig. 15 shows the Coats–Dfern curve treated with onerder reaction. The apparent activation energies are tabulated inable 4.

Energy of activation for decomposition of epoxy-anocomposite is calculated from TGA curves by thentegral method of Horowitz and Metzger [43] according to theollowing equation:

n

[ln

(1

1 − α

)]=

[Eaθ

RT 2max

](5)

here α is the decomposed fraction, Ea the activation energy forecomposition, Tmax the temperature at maximum rate of weightoss, θ = T − Tmax and R the universal gas constant. From the

(neat resin) 186.5 197

205191

584 A. Chatterjee, M.S. Islam / Materials Science

Fig. 16. Kinetics of thermal decomposition of epoxy nanocomposites (0.5%TiO2) by using Horowitz–Metzger method.

FHd

ir

g

Ffo

I

wtln

bmpHeaiAtTti

tedthtAdi

ig. 17. Effect of TiO2 loading on activation energy (calculated by usingorowitz–Metzger method, Coats–Redfern method) and integral proceduralecomposition temperature (IPDT).

s a function of nano-loading (Table 4). The activation energyeaches a maximum at 1% TiO2 loading.

In addition, thermal stability has also been characterized byiven integral procedural decomposition temperature (IPDT).

Tbir

Fig. 18. The schematic diagram of the Doyl

and Engineering A 487 (2008) 574–585

ig. 18 shows the schematic diagram of Doyle’s proposition [44]or determining the IPDT of a major factor on thermal stabilitiesf the samples and the IPDT is calculated as follows:

PDT = (A1 + A2)3

(A1 + A2 + A3)2 (Tf − Ti) + Ti (6)

here Ti and Tf are the initial and final experimental tempera-ures, respectively. The relationship between IPDT and the TiO2oading is shown in Fig. 17. The IPDT clearly increases withano-loading.

The changes in activation energy of the nanocomposites maye the structural evolution. In minimum loading, the particle-atrix distribution is homogeneous in nature. The ceramic

articles block the heat as it is insulator as described previously.eat and oxygen movement paths are lingering due to the pres-

nce of the ceramic particles. So, the retardant effects to heatnd oxygen are observed and that enlarged further with load-ng, since the number of nano-particles increases (up to 1%).dditional increment of loading causes detrimental effect due

o the agglomeration of particles that has occurred, confirmed byEM. Agglomerated particles are non-uniformly distributed in

he matrix and are less effective to blocking the heat and oxygen,s still higher than the neat epoxy system.

The IPDT trend is same as for the evolution of the activa-ion energy. This may be the same physic natures of activationnergy and IPDT. The activation energy describes the fastecomposition process of the resin network, ignoring the ini-ial decomposition and char yield formation (Table 3). IPDT,owever, reflects the whole stability of the sample, includinghe initial, fast decomposition and final (char forming) steps.s the TiO2 in the epoxy matrix mainly contributes the retar-ant effects to heat and oxygen and char formation, the IPDTncreases with TiO2 loading and maximum at 1% loading.

he activation energy, also reaches a maximum at 1% loadingecause the dispersion state of the TiO2 is the main concernn determining the fast decomposition process of the epoxyesin.

e’s method for determining the IPDT.

ence

4

mdbTtNttpaiilasm

A

aocD

R

[

[

[

[

[

[[

[

[[[

[[

[

[

[

[

[

[[

[[[[[[[

[

[[[

A. Chatterjee, M.S. Islam / Materials Sci

. Conclusions

An ultrasonic cavitations process has been employed toanufacture the epoxy–TiO2-nanocomposite. The particles are

ispersed the entire volume of the resin. Resin properties haveeen influenced with percent of loading of the nanoparticles.he tensile modulus, flexural modulus, storage modulus, Tg and

hermal stability were increased with 1% TiO2 of 5 nm size.anoparticle infusion changes the morphology of the resin sys-

ems that increases the Tg of the bulk matrix, the mechanical andhermal properties of the nanocomposites. Nano-particle incor-oration enhances the activation energy of the nanocompositesnd that level off at 1% loading than decreases with increas-ng loading. The IPDT for the epoxy TiO2-nanocomposite isncreased with nano-loading and the trend is dissimilar from theiterature [41]. We have observed the same trend for the IPDTnd activation energy for the epoxy TiO2-nanocomposite. Thetudy reveals that the nanofiller infusion improves the thermal,echanical and viscoelastic properties of the epoxy resin.

cknowledgements

The authors would like to express their gratitude and sincereppreciation to Jack W. Gillespie, Jr. Director CCM, Universityf Delaware, for his endless help and encouragement. Finan-ial support from Center for Composite Materials, University ofelaware is highly appreciated.

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