Studies on Stress Relaxation and Thermomechanical Properties of...

Published: March 09, 2011

r 2011 American Chemical Society 4432 dx.doi.org/10.1021/ie1016915 | Ind. Eng. Chem. Res. 2011, 50, 4432–4440

ARTICLE

pubs.acs.org/IECR

Studies on Stress Relaxation and Thermomechanical Properties ofPoly(acrylonitrile-butadiene-styrene) Modified Epoxy-AmineSystemsJyotishkumar P,† J€urgen Pionteck,‡ R€udiger H€assler,‡ Sajeev Martin George,† Uro�s Cvelbar,§ andSabu Thomas*,†,^

†School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills, Kottayam, Kerala 686560, India‡Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, Germany§Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia^Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam, Kerala-686560, India

ABSTRACT: Epoxy networks based on diglycidyl ether of bisphenol A cured with diamino diphenyl sulfone and modified withpoly(acrylonitrile-butadiene-styrene) (ABS) were prepared according to two different cure schedules, one with a single step curingand the other with two step curing. The samples were carefully analyzed by thermomechanical analysis (TMA) to understand thephysical aging phenomenon. The TMA runs on samples with single curing step are strained and show “bumps” in the expansiontraces indicating that internal stress relaxation takes place during heating. On the other hand, the samples prepared by two-stepcuring were not strained and hence no bumps occurred. The ABS modified epoxy blends were further characterized by Fouriertransform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), differential scanning calorimetry (DSC),thermogravimetric analysis (TGA), mechanical tests using a universal testing machine, and field emission scanning electronmicroscopy (FESEM). The FTIR spectroscopy study confirms that the epoxy/amine reaction was complete, irrespective of the cureschedule used. TEM micrographs reveals heterogeneous morphology for all the blends studied. DSC and TGA were employed toevaluate the thermal stability of epoxy/ABS blends. The mechanical properties of both strained and unstrained samples wereinvestigated in detail and are correlated to the blend morphologies. The result shows that the mechanical and morphologicalproperties are affected by blending with the thermoplastic but not with the cure schedule used. The addition of low ABS amounts(e6.9 wt %) in the epoxy resin resulted in epoxy matrix/ABS particle morphologies leading to more than 100% increase in tensiletoughness compared to neat cross-linked epoxy. FESEM micrographs of fractured surfaces proved fracture mechanisms such asnanocavitation, crack path deflection, crack pinning, ductile tearing of the thermoplastic, and local plastic deformation of the matrix.In contrast, when cocontinuous morphologies are formed at higher ABS loadings the mechanical properties are much lower thanthose formed for the neat epoxy system.

1. INTRODUCTION

Thermosetting resins are widely used in engineering applica-tions because of their high modulus and easy processability.Among the various thermosetting resins, epoxy resins areextensively used as matrices for high performance compositesin the aerospace and automotive industry.1-5 However, curedresins are highly brittle and hence limit their application in someareas. Improvement of the fracture toughness of thermosettingepoxy polymers is achieved by using a most common method offorming a blend with reactive liquid rubber such as carboxyl-terminated butadiene (CTBN) and amine terminated butadiene(ATBN), etc., where rubber undergoes phase separation fromthe matrix during curing, leading to different morphologies.6-14

The advantage of rubber toughening in thermosets is that thefracture toughness can be improved. In these systems, thetoughening is considered to be mainly from shear deformationin the matrix and the cavitation process of the phase separatedrubber particles.9-11 However, rubber modification will lead tosignificant reduction in modulus and thermal stability of thematerial and increase the tendency for water absorption. In fibercomposite applications, the loss of modulus, thermal stability,

and solvent resistance is a concern. An alternative approach totoughen epoxy polymers for fiber composite applications is theuse of epoxy blends as a matrix with high performancethermoplastics.15-28 The advantage of using thermoplastic isthat the glass transition temperature and mechanical propertiesof epoxy resin are retained after blending. Thermoplastic tough-ening is used commercially with PES/epoxy systems in aircraftcomposites with continuous carbon fiber.26 It is demonstratedthat the incorporation of thermoplastics can provide an oppor-tunity to significantly toughen an inherently brittle thermosetmatrix for composite industrial applications.27 The final proper-ties of epoxy blends greatly depend on final morphology of thepolymer blends, which depends on the selection of the thermo-plastic polymer, content of thermoplastic, the epoxy precursors,the hardener, and the curing temperature. From previous studies,it is known that in thermoplastic modified epoxy resins usually

Received: August 15, 2010Accepted: February 21, 2011Revised: January 19, 2011

4433 dx.doi.org/10.1021/ie1016915 |Ind. Eng. Chem. Res. 2011, 50, 4432–4440

Industrial & Engineering Chemistry Research ARTICLE

thermoplastic cocontinuous28 or phase inverted phase structure20

yields greater fracture toughness. However, some of the recentstudies have reported greater toughness for droplet/matrixmorphology.29

It is generally accepted that the epoxy shrinkage may lead tothe generation of excess internal stress while cooling during theprocessing cycle.4,30-35 This internal stress reduces adhesivestrength and may even cause cracks in the casting material.Physical aging is the time-dependent approach toward equilib-rium and can be viewed as a recovery phenomenon. Duringannealing at higher temperature, generated excess internal stressapproaches the equilibrium values.31,34 Even though epoxies arewidely used for industrial applications, there were very fewstudies on physical aging. In this study, a thermally stable andtough thermoplastic, namely poly(acrylonitrile-butadiene-styrene) (ABS) has been used for modifying diglycidyl ether ofbisphenol A (DGEBA) epoxy resin. The present investigationconcentrates on the importance of the cure schedule on the finalthermomechanical behavior of the blends of epoxy and ABS. Theimportance of two step curing is illustrated in the manuscript.The thermal, mechanical, and morphological properties ofepoxy/ABS blends were investigated as a function of composi-tion. The relationship between the morphology and the thermo-mechanical properties of epoxy/ABS blends are discussed.Further, the toughening mechanism was also investigated indetail. The resulting blends were found to have superior tough-ness while retaining the thermomechanical properties of the neatepoxy system.

2. EXPERIMENTAL SECTION

2.1. Materials. The matrix material used in the experimentsconsists of diglycidyl ether of bisphenol A (DGEBA) (LapoxL-12, Atul Ltd., India) and 4,40-diamino diphenyl sulfone (DDS)(Lapox K-10, Atul Ltd., India). The epoxy content in Lapox L-12varies between 5.25 and 5.40 eq/kg. The toughener ABS (Polylac PA-757K) was manufactured by Chi Mei Corporation,Taiwan. The used poly(acrylonitrile-butadiene-styrene) (ABS)is a commercially available thermoplastic polymer consisting of70 wt % polystyrene (PS), 25 wt % acrylonitrile (AN), and 5 wt %polybutadiene (PB). The molecular weight of the soluble part ofABS was determined to beMn = 51300 g/mol andMw = 125200g/mol (PDI = 2,4 GPC, PS standard) and the density wasdetermined to be 1.05 g/cm3 bymeans of an helium pycnometer.2.2. Preparation of Blends. Blends of epoxy resin/ABS

containing 3.6, 6.9, 10, and 12.9 wt % ABS were prepared usingthe melt mixing technique. ABS was mixed with epoxy resin at180 �C under constant stirring. After proper mixing, DDS wasadded to epoxy/ABS mixture with a stoichiometric epoxide:amine ratio of 2:1 (100 epoxyþ35DDSþXABS, by weight, X =5, 10, 15, and 20). The solution was evacuated, if necessary, andtransferred to the open mold. Two different cure schedules wereused: (1) The blends were cured in the air oven at 180 �C for 3 h,followed by curing at 200 �C for additional 2 h, and then allowedto cool slowly to room temperature. (2) The blends were curedin the air oven at 180 �C for 3 h, followed by slow cooling to roomtemperature. Postcuring was performed at 200 �C for 2 h,followed again by slow cooling to room temperature.The cured epoxy/amine system is transparent; on the other

hand, cured blends are light yellow in color due to phase separa-tion.

2.3. Characterization Techniques. 2.3.1. ThermomechanicalAnalysis. The thermomechanical properties of neat epoxy andepoxy blends were measured using a TA Instruments Q 400thermomechanical analyzer. The samples were scanned from 50to 250 �C at a heating rate of 1 K/min. Rectangular specimens of20 � 10 � 3 mm3 were used for the analysis.2.3.2. FTIR. Infrared studies were conducted to investigate the

completion of curing reaction. Fully cured samples were pow-dered and these samples in the form of KBr pellets were scannedfrom 4000 to 400 cm-1 using a FTIR-8400S spectrometer(Shimadzu). Each interferogram was generated by signal aver-aging 32 scans at a resolution of 4 cm-1 and the spectra wasobtained as percentage transmittance against wavenumber.2.3.3. Transmission Electron Microscopy. Due to the lower

penetration power of electrons, it was necessary tomount objectsfor examination in the electron microscope on very thin films.Therefore, samples were cut by using an ultramicrotom(ULTRACUT E. from REICHERT-JUNG) into 50-80 nmthick ultrathin films. After treating in the vapor of OsO4, thestained samples were examined in the EM 902 transmissionelectron microscope (Zeiss, Germany) with an acceleratingvoltage of 80 kV.2.3.4. Differential Scanning Calorimetry. The glass transition

temperatures (Tg) of neat epoxy and epoxy blends were deter-mined using a Perkin-Elmer, Diamond DSC. The measurementswere performed using 2-10 mg of the samples in nitrogenatmosphere using heating and cooling rates of 10 K/min in thetemperature range of 30-300 �C.2.3.5. Thermogravimetric Analysis. Thermal stability of neat

epoxy and epoxy blends were analyzed by means of athermogravimetric analyzer (TGA), that is, a Mettler ToledoTGA/SDTA/851. The measurements were performed using2-10 mg of the samples in the temperature interval from 25to 700 �C at a heating rate of 20 K/min in nitrogen atmos-phere.2.3.6. Tensile Properties. Specimens for mechanical testing

were machined to the required dimensions from the castlaminates using cutting with a cutting machine. Tensile measure-ments were performed according to ASTMD 638. The measure-ments were taken with a universal testing machine (TiniusOlsen) model H 50 KT at a cross head speed of 10 mm/minute.Rectangular specimens of 100 � 10 � 3 mm3 were used fordetermining the tensile strength. The tests were performed on sixdifferent specimens of the same sample and the average wastaken as the final value.2.3.7. Fracture Toughness. Fracture toughness of the speci-

mens was determined according to ASTM D 5045-99. Themeasurements were taken with a universal testing machine(Tinius Olsen) model H 50 KT. Rectangular specimens ofdimension 60 � 10 � 4 mm3 were used for fracture toughnessmeasurements. A notch of 5 mm was made at one edge of thespecimen. A natural crack was made by pressing a fresh razorblade into the notch. The analysis was done in bending mode atroom temperature. The value of stress intensity factor (KIc) wascalculated using eq 1.

stress intensity factor, KIc ¼ QPa1=2

bdð1Þ

where P is the load at the initiation of crack, a is the crack length, bis the breadth of the specimen, d is the thickness of the specimen,and Q is a geometry constant. Q is calculated using the following



equation:

Q ¼ 1:99- 0:41ða=bÞ þ 18:7ða=bÞ2 - 38:48ða=bÞ3þ 53:85ða=bÞ4 ð2Þ

2.3.8. Field Emission Scanning Electron Microscopy. Thefracture morphology of the cross-linked epoxy as well as epoxyblends were examined using a ULTRA FESEM (model UltraPlus, Nano Technology Systems Division Carl Zeiss SMT AG,Germany). The samples were coated with platinum by vapordeposition using a SCD 500 Sputter Coater (BAL-TEC AG,Liechtenstein).

3. RESULTS

3.1. Thermal Expansion Behavior of Strained and Un-strained Samples. 3.1.1. Thermal Expansion Behavior ofStrained Samples. As shown in Figure 1a the TMA run on thestrained sample (cured and annealed in a single step) shows a“bump” in the expansion trace. With an increase in sampletemperature, the change in dimension also increases and abovethe Tg of the ABS, the change in dimension increases quickly.When the sample temperature reaches 180 �C, the change indimension begins to drop with temperature. Again as the sampletemperature reaches 210 �C the sample begins to expand linearly

with temperature, showing the typical thermal expansion beha-vior for the rubbery state.The above phenomenon can be explained as follows. There

will be some excess internal stress in the cured epoxy networkbecause some of the polymer chains cannot attain the equilibri-um conformation during the cooling process in the processingcycle. The abrupt increase of linear dimension is considered to berelated to the relaxation of residual stress during heating (TMArunning).34 When the sample temperature reaches a particularvalue (above the ABS Tg), the motion capacity becomes largeenough to begin to unfreeze the molecular chains, and hence themolecular chain segments start rearranging the local network andthus the residual stress releases. As a result, the metric dimensionincreases disproportionately high, this means that as the sampletemperature reaches a certain value (above the ABS Tg), therelease of residual stress can occur and the dimension begins toexhibit a sharp increase and hence a “bump” appears in the TMAanalysis. According to above point, the “bump” could be attrib-uted to the internal stress generated during the cooling process inthe processing cycle. Similar bumps have been reported in theliterature.34 When approaching epoxy Tg, the network structuremoves toward the equilibrium state and above the Tg, that is, inthe rubbery state, the material is in equilibrium and the change indimension begins to increase linearly with temperature again.3.1.2. Thermal Expansion Behavior of Unstrained Samples.

For the samples cured through the two-step process, the residualstress is absent as revealed by Figure 1b. After the first heatingand cooling step, some polymer chains may exist in nonequili-brium conformation. However, during the second heating for aperiod of 2 h at 200 �C, most of the polymer chains will attain theequilibrium conformation by the release of residual stress. This infact reduces the bumps in the TMA analysis and consequentlyreduces the formation of internal cracks in the cast material, sothat one can expect improved mechanical properties.The plot clearly shows the glassy to rubbery transformation

followed by the linear expansion in the rubbery state. Thechanges in dimension for neat epoxy and blends at temperaturesbelow the Tg of ABS are similar; however, at temperatures abovethe Tg of ABS, the blends possess a greater linear expansion. Thissuggests that the dimensional stability of epoxy blends is slightlylower than that of neat epoxy when the temperature of thesamples reaches the Tg of ABS. When the sample temperaturereaches the Tg of epoxy phase the change in dimension increasesquickly with temperature, showing the typical expansion beha-vior of the rubbery state.On the basis of the above observation, a schematic representa-

tion of the release of residual stress from the strained sample(cured and annealed in one step) is depicted in Figure 2.Figure 2a represents epoxy polymer chains that are strained.

Figure 1. TMA profiles for different cross-linked DGEBA/ABS blends;(a) cured and annealed in one step; (b) cured and annealed in two steps(see Experimental section).

Figure 2. A schematic representation of the release of internal stressesfrom the strained sample.



Upon heating (TMA running) the molecular vibration increases,and the motion capacity becomes large enough to begin tounfreeze the molecular chains, and hence the molecular chainsegments start rearranging the local network to approach theequilibrium state under the release of internal stresses(Figure 2b). From the experimental observation, it is clear that,the presence of internal stress affects the dimensional stability ofthe blends at high temperatures (above the ABS Tg) and hencemay limit its applications at high temperatures.3.2. FTIR Analysis.Maximum properties of epoxy network were

attained only by complete cross-linking.36 Therefore it is necessary tomake sure that curing reaction reached completion. In the presentstudy FTIR spectra of cured DGEBA/ABS/DDS systems weretaken to examine the completion of the cure reaction. The FTIRspectra for epoxy monomer and cross-linked epoxy blends of theunstrained samples (cured and annealed in two steps) are given inFigure 3a. The spectra of cross-linked epoxy blends did not show anycharacteristic absorption of an epoxy group at 913 cm-1. Theabsence of a characteristic epoxy peak revealed that all the epoxymonomers took part in the reaction.37 Similar to the spectra ofunstrained samples, the spectra of strained cross-linked epoxy blends(cured and annealed in one step) did not show any characteristicabsorption of an epoxy group at 913 cm-1 (Figure 3b). However,one has to consider the fact that their may be minute traces of

oxirane-rings which are below the sensitivity limit of the FTIRinstrument.3.3. Morphological Analysis. Phase morphology of both

strained and unstrained samples was investigated by TEM.However, the TEM micrographs of the blends are identical,irrespective of the curing schedule, and hence only the TEMmicrographs of unstrained samples are given in the manuscript(Figure 4). All the blends show phase separated microstructuredmorphologies. For blends modified with lower ABS contents(e6.9 wt %) matrix-droplet morphology was observed in whichthermoplastic phase dispersed in the continuous matrix of cross-linked epoxy network. Figure 4a shows the TEM micrograph ofcross-linked epoxy. Figure 4b represents the TEM micrographwith 3.6 wt % ABS modified epoxy blend, which shows adispersed phase morphology in which dark domains, that is,ABS domains around 500 nm in size, are uniformly dispersed inthe continuous epoxy matrix. Figure 4c reveals the TEM micro-graph of the 6.9 wt % ABS-containing epoxy blend, which againshows normal dispersed phase morphology with ABS domainsaround 800 nm in size dispersed in the continuous epoxy phase.On the other hand, the TEM micrographs of 10 wt % ABS-modified-completely cross-linked epoxy blend (Figure 4d)shows very interesting morphology with three different phases:two continuous phases forming a cocontinuous structure withsubstructures (epoxy continuous phase containing dispersedSAN particles and the SAN continuous phase containing epoxyparticles dispersed) and, the most important feature, the PBphase appears as dispersed small agglomerates at the blendinterface between the cocontinuous structures. The driving forcefor the PB segregation at the interface between the SAN andepoxy continuous phases is the minimization of the specificinterfacial energy of the system. Similar structures were observedfor 12.9 wt % ABS-modified epoxy blends (Figure 4e). A similartype of morphology has been observed previously by otherauthors.38,39

3.4. Thermal Properties. 3.4.1. Differential Scanning Calo-rimetry.Thermal properties of the unmodified and thermoplasticmodified systems were measured by means of DSC. The Tg

values obtained from DSC measurements during the first andsecond heating run are shown in Figure 5. From the plot, Tg

values for both strained and unstrained samples are comparable.Changes in the DSC values when comparing the second with thefirst heating run are due to postcuring of the epoxy phase duringthe DSC measurement. When heating to higher temperatures(300 �C) in the first DSC scan, the frozen state can relax and theTg shifts to its true value. SingleTg was observed for neat epoxy aswell as for blends. The Tg corresponding to PB and SAN phaseare not detectable because of the low sensitivity of the DSCinstrument. The Tg of the epoxy phase depends on the concen-tration of ABS and decreases slightly by increased incorporationof the thermoplastic (ABS). The decrease in matrix Tg is mainlyoriginated by the ABS remaining dissolved in the epoxy-aminepolymer.40

3.4.2. Thermogravimetric Analysis. Thermal stability of theblends was analyzed using TGA in nitrogen atmosphere. TGAcurves for both strained and unstrained samples seem to beidentical irrespective of the curing conditions. For avoidingoverlapping of the results we are giving only the TGA cures forthe unstrained samples. TGA curves for all the cross-linkedblends are given in Figure 6. There is no deterioration in thermalstability of the blend as compared with that of the neat material.Thermal stability can be expressed in terms of parameters like

Figure 3. FTIR data for different cross-linked DGEBA/ABS blends: (a)cured and annealed in two steps; (b) cured and annealed in one step.



initial decomposition temperature, final degradation tempera-ture, and final residue. From the graph, it is clear that the initialdecomposition temperature (Ti), final degradation temperature(Tmax), and residual weight fraction for all the blends remain thesame at various temperatures indicating that the thermal stabilityof cured epoxy resin was not affected by blending.

3.5. Mechanical Properties of DDS-Cured Epoxy/ABSBlends. 3.5.1. Tensile Properties. Tensile properties of DDScured epoxy/ABS blends, for both strained and unstrainedsamples are given in Table 1. However, the tensile results arecomparable irrespective of the cure schedule. In any case, the datarevealed a remarkable increment in tensile strength and tensileelongation for 3.6 wt % and 6.9 wt % ABS containing blends with

Figure 4. TEM micrographs of ABS-modified epoxy blends (cured and annealed in two steps): (a-c) 4 � 4 μm2; (d, e) 8 � 8 μm2.

Figure 5. Variation of Tg with respect to ABS content.

Figure 6. TGA curves of unstrained cross-linked epoxy/ABS blends.



a maximum for 3.6 wt % ABS containing blend (unstrainedsamples). The Young’s modulus remains the same for neat epoxyand epoxy/ABS blends. Representative, tensile stress/straincurves for both strained and unstrained samples are given inFigure 7. To our surprise the tensile toughness obtained from thearea under the stress/strain curve shows an increment of morethan 100% for the 3.6 wt % ABS modified epoxy system withrespect to the neat cross-linked epoxy (for both strained andunstrained samples) (Table 1). The toughening mechanism iselaborated in the Discussion section.3.5.2. Fracture Toughness. Fracture toughness is the resistance

of material to crack initiation and propagation. Previous scientificreports prove that the fracture toughness of the epoxy resins can

be effectively increased through blending with thermoplastics.28

Plots of KIC vs ABS content in the cured epoxy/ABS blends, forboth strained and unstrained samples, are shown in Figure 8. Inany case, the fracture toughness of the epoxy resin is improved byblending with 3.6 and 6.9 wt % ABS (thermoplastic). However,when the ABS content was 12.9 wt % there is a decrease in KIC,even lower than that of the control neat epoxy system. It is alsoimportant to mention that the KIC values of strained samples areslightly lower than the unstrained samples, but with in the limit oferrors.3.5.3. Discussion. The important factors that influence the

mechanical properties include the morphology of the blends,amount of the modifier, interfacial adhesion between the phases,molecular weight of the thermoplastic, and curing conditions.Heterogeneous morphology is very much important for gettingimproved fracture toughness.28 Since the fracture micrographswere identical, irrespective of the curing conditions and alsoirrespective of the test performed, the fracture micrographs of theunstrained samples after the KIC fracture test were shown inFigure 9, to discuss the mechanical properties of the ABSmodified epoxy blends. All the blends in this study are hetero-geneous and thus satisfy one of the important conditions forimproved fracture toughness. The presence of heterogeneousmorphology is evident from the SEM micrographs of thefractured samples. For the neat epoxy system, cracks spreadfreely and regularly and orient in the direction of loading. Thisindicates typical characteristics of brittle fracture as revealed byFigure 9a. Although, the samples were well mixed, we noticedsteplike structures as would be observed in poorly mixed amine-cured epoxy systems.41 Figure 9 panels b and c show the FESEMmicrographs of the fracture surfaces of the cured blends

Figure 7. Tensile stress-strain curves of epoxy/ABS blends: (a) curedand annealed in one step; (b) cured and annealed in two steps.

Table 1. Tensile Properties of the ABS Modified Epoxy Blends (for Both Strained and Unstrained Specimens)

(wt % ABS) tensile strength (MPa) tensile modulus (GPa) tensile elongation (%) tensile toughness

0 48( 4a 51( 4 2.3( 0.1a 2.3( 0.1 4.49( 0.2a 3.99( 0.2 138a 121

3.6 61( 4a 65( 3 2.3( 0.1a 2.3( 0.1 6.71( 0.3a 6.93( 0.3 272a 279

6.9 59( 5a 60( 3 2.3( 0.1a 2.4( 0.1 7.83( 0.3a 5.73( 0.3 310a 210

10 48( 3a 39( 4 2.3( 0.1a 2.2( 0.1 4.43( 0.2a 3.23( 0.2 131a 78

12.9 34( 3a 43( 3 2.3( 0.1a 2.3( 0.1 2.02( 0.2a 3.23( 0.2 39a 85aValues of the strained samples (cured and annealed in one step).

Figure 8. (Critical stress intensity factor) Fracture toughness of(strained and unstrained samples) cured epoxy/ABS blends.



containing 3.6 and 6.9 wt % ABS, clearly exhibiting features ofductile drawing phenomenon on the fracture surfaces to someextent, which appear mainly in the ABS phase. The ductiletearing of thermoplastic is one of the factors responsible forthe increase in fracture toughness of the thermoplastic-tough-ened epoxy resin. In the case of 3.6 and 6.9 wt % ABS content,spherical ABS particles may act as stress concentrators upon theapplication of external load and will lead to plastic deformation ofthe matrix surrounding the ABS particles. This will contribute toriver marks and hence offer more roughness to the fracturesurface and hence more ductility to the epoxy matrix. The highdegree of roughness on the fractured surface also indicates the

crack deviation from its original plane, resulting in an increasedsurface area of the crack, which may also increase the toughness.Moreover, the interface between the epoxy phase and ABS phaseremains intact. This is evidence for good adhesion between thematrix and dispersed domains. Hence, the stress is transferred moreeffectively to the thermoplastic domains from the cross-linked epoxyphase. Another important factor to bementioned is the formation ofnanocavities around 100 nm during the fracture process. Thenanocavities in the ABS domains are very clear from the micro-graphs; the formation of nanocavities may take up a significantamount of applied stress and hence elevate the fracture toughness.42

The cavitationprocess inABS is due to thepresenceof rubber (5wt%)

Figure 9. Scanning electron micrographs of the fractured surfaces of epoxy/ABS blends (cured and annealed in two steps).

Table 2. The Volume Fraction, Number Average Diameter (Dn), Weight Average Diameter (Dw), Polydispersity Index (PDI),Interparticle Distance, and Interfacial Area per Unit Volume of Crosslinked Epoxy/ABS Blendsa

ABS content (wt %) volume fractionb Dn (μm) Dw (μm) PDI interparticle distance (μm) interfacial area per unit volume (μm-1)

3.6 0.034 0.5 0.6 1.1 0.74 0.41

6.9 0.065 0.8 0.9 1.1 0.80 0.49a Dn andDw were calculated from SEMmicrographs; at least 200 particles were measured to calculateDn andDw.

bVolume fraction of ABS is calculatedfrom total mass fraction; density of ABS is 1.05 g/cm3.



in the ABS phase and is a phenomenon frequently observed inrubber-modified epoxy blends.Another important parameter influencing the fracture tough-

ness is the domain size. For amore comprehensive consideration,the number average and weight average domain diameters andpolydispersity index (PDI), interparticle distance and interfacialarea per unit volume for the 3.6 and 6.9 wt % ABS modifiedblends were calculated using the following equations:24

number average diameter, Dn ¼ ∑nidi=∑ni ð3Þ

weight average diameter, Dw ¼ ∑nid2i =∑nidi ð4Þ

polydispersity index, PDI ¼ Dw=Dn ð5Þwhere ni is the number of domains having diameter di. Theinterparticle distance and interfacial area per unit volume werecalculated using the following equations:24

interparticle distance ¼ Dn½ðπ=6aTPÞ1=3 - 1� ð6Þ

interfacial area per unit volume ¼ 3aTP=r ð7ÞwhereDn is the number average diameter of the domains, r is thenumber average radius of the domains, and aTP is the volumefraction of the dispersed phase.The domain diameter and the other parameters calculated

from the above-mentioned equations are summarized in Table 2.One can conclude that the domain diameter was increased withincreasing ABS content while the polydispersity index remainsconstant indicating uniform particle size distribution. The inter-particle distance and interfacial area per unit volume increasedwith ABS content in the blends. The increase in interparticledistance at higher thermoplastic content is due to a strongcoalescence effect of the ABS phase. But the domain sizeinfluences the efficiency of the initiation of energy absorbingmechanisms. Hence the smaller domains in 3.6 wt % ABScontaining blends are effective in initiating energy absorbingmechanisms in comparison with other blends with largerdomains.In the previous reports, better toughness values were also

obtained for blends with cocontinuous phase structures asreported in other high performance thermoplastic-modifiedsystems.28 In the present case, for 10 and 12.9 wt % ABS-modified blends, cocontinuous morphologies were observed asrevealed by Figure 9d,e, hence the advancing crack had topropagate through the continuous ABS and epoxy phase whichshould offer more resistance to crack propagation. However, thecocontinuous phase structures for the blends containing 10 and12.9 wt % ABS did not exhibit any improved toughness in thepresent system. Thus, the fracture properties of the cocontinuousblends seem to depend predominantly on the inherent propertyof the thermoplastic.25

4. CONCLUSION

TMA runs on strained samples show “bumps” in the TMA scans,which can be explained by the release of excess internal stress whenapproaching the glass transition during theTMA(heating) run. Thepresence of internal stress affects the dimensional stability of theblends at high temperatures (above the ABSTg). The internal stresscan be removed by two-step curing. The impact of the cureschedules and the increasing ABS concentration on properties were

carefully analyzed. Irrespective of the cure schedule, the thermal andmechanical properties remain comparable. On the other hand, themechanical and morphological properties are affected by blendingwith the thermoplastic.The influence of increase in ABS content onthe final mechanical properties was carefully analyzed and wascorrelated with blend morphology. In the blends with lower ABScontent (3.6 and 6.9 wt % ABS), ABS domains were sphericallydispersed in the continuous epoxy matrix and possess uniform size.The cured blends containing 10 and 12.9 wt % ABS exhibited atypical cocontinuous phase structure. These differences inmorphol-ogy are responsible for the big differences in the mechanicalproperties. While the particle-matrix structures result in improvedtoughness and strength, with best results at 3.6 wt % ABS content,the cocontinuous blends containing 10 and 12.9 wt % ABS exhibitpoor mechanical properties as compared to the neat cross-linkedepoxy. In many applications dimensional stability is very important.In this context a two step curing is always recommended to alleviatethe dimensional variation. It is important to add that experiments arein progress to investigate the effect of physical aging of otherthermosetting systems such as epoxy/SAN blends and epoxynanocomposites, etc.

’AUTHOR INFORMATION

Corresponding Author*Tel.: þ91-481-2730003. Fax: þ91-481-2731002. E-mail:[email protected]; [email protected]: Centre for Nanoscience & Nanotechnology, MahatmaGandhi University, Priyadarshini Hills P.O., Kottayam, Kerala-686560, India.

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