Studies on Stress Relaxation and Thermomechanical Properties of

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Studies on Stress Relaxation and Thermomechanical Properties of Poly(acrylonitrile-butadiene-styrene) Modified Epoxy-Amine Systems Jyotishkumar P,† J€urgen Pionteck,‡ R€udiger H€assler,‡ Sajeev Martin George,† Uros Cvelbar,§ and Sabu 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 with poly(acrylonitrile-butadiene-styrene) (ABS) were prepared according to two different cure schedules, one with a single step curing and the other with two step curing. The samples were carefully analyzed by thermomechanical analysis (TMA) to understand the physical aging phenomenon. The TMA runs on samples with single curing step are strained and show “bumps” in the expansion traces indicating that internal stress relaxation takes place during heating. On the other hand, the samples prepared by two-step curing were not strained and hence no bumps occurred. The ABS modified epoxy blends were further characterized by Fourier transform 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 electron microscopy (FESEM). The FTIR spectroscopy study confirms that the epoxy/amine reaction was complete, irrespective of the cure schedule used. TEM micrographs reveals heterogeneous morphology for all the blends studied. DSC and TGA were employed to evaluate the thermal stability of epoxy/ABS blends. The mechanical properties of both strained and unstrained samples were investigated in detail and are correlated to the blend morphologies. The result shows that the mechanical and morphological properties 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 tensile toughness compared to neat cross-linked epoxy. FESEM micrographs of fractured surfaces proved fracture mechanisms such as nanocavitation, 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 than those formed for the neat epoxy system.

1. INTRODUCTION Thermosetting resins are widely used in engineering applications because of their high modulus and easy processability. Among the various thermosetting resins, epoxy resins are extensively used as matrices for high performance composites in the aerospace and automotive industry.1-5 However, cured resins are highly brittle and hence limit their application in some areas. Improvement of the fracture toughness of thermosetting epoxy polymers is achieved by using a most common method of forming a blend with reactive liquid rubber such as carboxylterminated butadiene (CTBN) and amine terminated butadiene (ATBN), etc., where rubber undergoes phase separation from the matrix during curing, leading to different morphologies.6-14 The advantage of rubber toughening in thermosets is that the fracture toughness can be improved. In these systems, the toughening is considered to be mainly from shear deformation in the matrix and the cavitation process of the phase separated rubber particles.9-11 However, rubber modification will lead to significant reduction in modulus and thermal stability of the material and increase the tendency for water absorption. In fiber composite applications, the loss of modulus, thermal stability, r 2011 American Chemical Society

and solvent resistance is a concern. An alternative approach to toughen epoxy polymers for fiber composite applications is the use of epoxy blends as a matrix with high performance thermoplastics.15-28 The advantage of using thermoplastic is that the glass transition temperature and mechanical properties of epoxy resin are retained after blending. Thermoplastic toughening is used commercially with PES/epoxy systems in aircraft composites with continuous carbon fiber.26 It is demonstrated that the incorporation of thermoplastics can provide an opportunity to significantly toughen an inherently brittle thermoset matrix for composite industrial applications.27 The final properties of epoxy blends greatly depend on final morphology of the polymer blends, which depends on the selection of the thermoplastic 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, 2010 Accepted: February 21, 2011 Revised: January 19, 2011 Published: March 09, 2011 4432

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

Industrial & Engineering Chemistry Research thermoplastic cocontinuous28 or phase inverted phase structure20 yields greater fracture toughness. However, some of the recent studies have reported greater toughness for droplet/matrix morphology.29 It is generally accepted that the epoxy shrinkage may lead to the generation of excess internal stress while cooling during the processing cycle.4,30-35 This internal stress reduces adhesive strength and may even cause cracks in the casting material. Physical aging is the time-dependent approach toward equilibrium and can be viewed as a recovery phenomenon. During annealing at higher temperature, generated excess internal stress approaches the equilibrium values.31,34 Even though epoxies are widely used for industrial applications, there were very few studies on physical aging. In this study, a thermally stable and tough thermoplastic, namely poly(acrylonitrile-butadienestyrene) (ABS) has been used for modifying diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The present investigation concentrates on the importance of the cure schedule on the final thermomechanical behavior of the blends of epoxy and ABS. The importance of two step curing is illustrated in the manuscript. The thermal, mechanical, and morphological properties of epoxy/ABS blends were investigated as a function of composition. The relationship between the morphology and the thermomechanical properties of epoxy/ABS blends are discussed. Further, the toughening mechanism was also investigated in detail. The resulting blends were found to have superior toughness while retaining the thermomechanical properties of the neat epoxy system.

2. EXPERIMENTAL SECTION 2.1. Materials. The matrix material used in the experiments consists of diglycidyl ether of bisphenol A (DGEBA) (Lapox L-12, Atul Ltd., India) and 4,40 -diamino diphenyl sulfone (DDS) (Lapox K-10, Atul Ltd., India). The epoxy content in Lapox L-12 varies between 5.25 and 5.40 eq/kg. The toughener ABS (Poly lac PA-757K) was manufactured by Chi Mei Corporation, Taiwan. The used poly(acrylonitrile-butadiene-styrene) (ABS) is a commercially available thermoplastic polymer consisting of 70 wt % polystyrene (PS), 25 wt % acrylonitrile (AN), and 5 wt % polybutadiene (PB). The molecular weight of the soluble part of ABS was determined to be Mn = 51300 g/mol and Mw = 125200 g/mol (PDI = 2,4 GPC, PS standard) and the density was determined to be 1.05 g/cm3 by means 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 using the melt mixing technique. ABS was mixed with epoxy resin at 180 C under constant stirring. After proper mixing, DDS was added to epoxy/ABS mixture with a stoichiometric epoxide: amine ratio of 2:1 (100 epoxy þ35 DDS þ X ABS, by weight, X = 5, 10, 15, and 20). The solution was evacuated, if necessary, and transferred to the open mold. Two different cure schedules were used: (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 allowed to cool slowly to room temperature. (2) The blends were cured in the air oven at 180 C for 3 h, followed by slow cooling to room temperature. 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 separation.

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2.3. Characterization Techniques. 2.3.1. Thermomechanical Analysis. The thermomechanical properties of neat epoxy and

epoxy blends were measured using a TA Instruments Q 400 thermomechanical analyzer. The samples were scanned from 50 to 250 C at a heating rate of 1 K/min. Rectangular specimens of 20 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 powdered and these samples in the form of KBr pellets were scanned from 4000 to 400 cm-1 using a FTIR-8400S spectrometer (Shimadzu). Each interferogram was generated by signal averaging 32 scans at a resolution of 4 cm-1 and the spectra was obtained as percentage transmittance against wavenumber. 2.3.3. Transmission Electron Microscopy. Due to the lower penetration power of electrons, it was necessary to mount objects for 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 nm thick ultrathin films. After treating in the vapor of OsO4, the stained samples were examined in the EM 902 transmission electron microscope (Zeiss, Germany) with an accelerating voltage of 80 kV. 2.3.4. Differential Scanning Calorimetry. The glass transition temperatures (Tg) of neat epoxy and epoxy blends were determined using a Perkin-Elmer, Diamond DSC. The measurements were performed using 2-10 mg of the samples in nitrogen atmosphere using heating and cooling rates of 10 K/min in the temperature range of 30-300 C. 2.3.5. Thermogravimetric Analysis. Thermal stability of neat epoxy and epoxy blends were analyzed by means of a thermogravimetric analyzer (TGA), that is, a Mettler Toledo TGA/SDTA/851. The measurements were performed using 2-10 mg of the samples in the temperature interval from 25 to 700 C at a heating rate of 20 K/min in nitrogen atmosphere. 2.3.6. Tensile Properties. Specimens for mechanical testing were machined to the required dimensions from the cast laminates using cutting with a cutting machine. Tensile measurements were performed according to ASTM D 638. The measurements were taken with a universal testing machine (Tinius Olsen) model H 50 KT at a cross head speed of 10 mm/minute. Rectangular specimens of 100 10 3 mm3 were used for determining the tensile strength. The tests were performed on six different specimens of the same sample and the average was taken as the final value. 2.3.7. Fracture Toughness. Fracture toughness of the specimens was determined according to ASTM D 5045-99. The measurements were taken with a universal testing machine (Tinius Olsen) model H 50 KT. Rectangular specimens of dimension 60 10 4 mm3 were used for fracture toughness measurements. A notch of 5 mm was made at one edge of the specimen. A natural crack was made by pressing a fresh razor blade into the notch. The analysis was done in bending mode at room temperature. The value of stress intensity factor (KIc) was calculated using eq 1. stress intensity factor, KIc ¼

QPa1=2 bd

ð1Þ

where P is the load at the initiation of crack, a is the crack length, b is the breadth of the specimen, d is the thickness of the specimen, and Q is a geometry constant. Q is calculated using the following 4433

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

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).

equation: Q ¼ 1:99 - 0:41ða=bÞ þ 18:7ða=bÞ2 - 38:48ða=bÞ3 þ 53:85ða=bÞ4

ð2Þ

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

3. RESULTS 3.1. Thermal Expansion Behavior of Strained and Unstrained Samples. 3.1.1. Thermal Expansion Behavior of Strained Samples. As shown in Figure 1a the TMA run on the

strained sample (cured and annealed in a single step) shows a “bump” in the expansion trace. With an increase in sample temperature, the change in dimension also increases and above the Tg of the ABS, the change in dimension increases quickly. When the sample temperature reaches 180 C, the change in dimension begins to drop with temperature. Again as the sample temperature reaches 210 C the sample begins to expand linearly

with temperature, showing the typical thermal expansion behavior for the rubbery state. The above phenomenon can be explained as follows. There will be some excess internal stress in the cured epoxy network because some of the polymer chains cannot attain the equilibrium conformation during the cooling process in the processing cycle. The abrupt increase of linear dimension is considered to be related to the relaxation of residual stress during heating (TMA running).34 When the sample temperature reaches a particular value (above the ABS Tg), the motion capacity becomes large enough to begin to unfreeze the molecular chains, and hence the molecular chain segments start rearranging the local network and thus the residual stress releases. As a result, the metric dimension increases disproportionately high, this means that as the sample temperature reaches a certain value (above the ABS Tg), the release of residual stress can occur and the dimension begins to exhibit a sharp increase and hence a “bump” appears in the TMA analysis. According to above point, the “bump” could be attributed to the internal stress generated during the cooling process in the processing cycle. Similar bumps have been reported in the literature.34 When approaching epoxy Tg, the network structure moves toward the equilibrium state and above the Tg, that is, in the rubbery state, the material is in equilibrium and the change in dimension 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 residual stress is absent as revealed by Figure 1b. After the first heating and cooling step, some polymer chains may exist in nonequilibrium conformation. However, during the second heating for a period of 2 h at 200 C, most of the polymer chains will attain the equilibrium conformation by the release of residual stress. This in fact reduces the bumps in the TMA analysis and consequently reduces the formation of internal cracks in the cast material, so that one can expect improved mechanical properties. The plot clearly shows the glassy to rubbery transformation followed by the linear expansion in the rubbery state. The changes in dimension for neat epoxy and blends at temperatures below the Tg of ABS are similar; however, at temperatures above the Tg of ABS, the blends possess a greater linear expansion. This suggests that the dimensional stability of epoxy blends is slightly lower than that of neat epoxy when the temperature of the samples reaches the Tg of ABS. When the sample temperature reaches the Tg of epoxy phase the change in dimension increases quickly with temperature, showing the typical expansion behavior of the rubbery state. On the basis of the above observation, a schematic representation 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. 4434



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.

Upon heating (TMA running) the molecular vibration increases, and the motion capacity becomes large enough to begin to unfreeze the molecular chains, and hence the molecular chain segments start rearranging the local network to approach the equilibrium 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 of the blends at high temperatures (above the ABS Tg) and hence may 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 to make sure that curing reaction reached completion. In the present study FTIR spectra of cured DGEBA/ABS/DDS systems were taken to examine the completion of the cure reaction. The FTIR spectra for epoxy monomer and cross-linked epoxy blends of the unstrained samples (cured and annealed in two steps) are given in Figure 3a. The spectra of cross-linked epoxy blends did not show any characteristic absorption of an epoxy group at 913 cm-1. The absence of a characteristic epoxy peak revealed that all the epoxy monomers took part in the reaction.37 Similar to the spectra of unstrained samples, the spectra of strained cross-linked epoxy blends (cured and annealed in one step) did not show any characteristic absorption of an epoxy group at 913 cm-1 (Figure 3b). However, one has to consider the fact that their may be minute traces of

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oxirane-rings which are below the sensitivity limit of the FTIR instrument. 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 TEM micrographs of unstrained samples are given in the manuscript (Figure 4). All the blends show phase separated microstructured morphologies. For blends modified with lower ABS contents (e6.9 wt %) matrix-droplet morphology was observed in which thermoplastic phase dispersed in the continuous matrix of crosslinked epoxy network. Figure 4a shows the TEM micrograph of cross-linked epoxy. Figure 4b represents the TEM micrograph with 3.6 wt % ABS modified epoxy blend, which shows a dispersed phase morphology in which dark domains, that is, ABS domains around 500 nm in size, are uniformly dispersed in the continuous epoxy matrix. Figure 4c reveals the TEM micrograph of the 6.9 wt % ABS-containing epoxy blend, which again shows normal dispersed phase morphology with ABS domains around 800 nm in size dispersed in the continuous epoxy phase. On the other hand, the TEM micrographs of 10 wt % ABSmodified-completely cross-linked epoxy blend (Figure 4d) shows very interesting morphology with three different phases: two continuous phases forming a cocontinuous structure with substructures (epoxy continuous phase containing dispersed SAN particles and the SAN continuous phase containing epoxy particles dispersed) and, the most important feature, the PB phase appears as dispersed small agglomerates at the blend interface between the cocontinuous structures. The driving force for the PB segregation at the interface between the SAN and epoxy continuous phases is the minimization of the specific interfacial energy of the system. Similar structures were observed for 12.9 wt % ABS-modified epoxy blends (Figure 4e). A similar type of morphology has been observed previously by other authors.38,39 3.4. Thermal Properties. 3.4.1. Differential Scanning Calorimetry. Thermal properties of the unmodified and thermoplastic modified systems were measured by means of DSC. The Tg values obtained from DSC measurements during the first and second 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 the first heating run are due to postcuring of the epoxy phase during the DSC measurement. When heating to higher temperatures (300 C) in the first DSC scan, the frozen state can relax and the Tg shifts to its true value. Single Tg was observed for neat epoxy as well as for blends. The Tg corresponding to PB and SAN phase are not detectable because of the low sensitivity of the DSC instrument. The Tg of the epoxy phase depends on the concentration of ABS and decreases slightly by increased incorporation of the thermoplastic (ABS). The decrease in matrix Tg is mainly originated by the ABS remaining dissolved in the epoxy-amine polymer.40 3.4.2. Thermogravimetric Analysis. Thermal stability of the blends was analyzed using TGA in nitrogen atmosphere. TGA curves for both strained and unstrained samples seem to be identical irrespective of the curing conditions. For avoiding overlapping of the results we are giving only the TGA cures for the unstrained samples. TGA curves for all the cross-linked blends are given in Figure 6. There is no deterioration in thermal stability of the blend as compared with that of the neat material. Thermal stability can be expressed in terms of parameters like 4435



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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 6. TGA curves of unstrained cross-linked epoxy/ABS blends. Figure 5. Variation of Tg with respect to ABS content.

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

3.5. Mechanical Properties of DDS-Cured Epoxy/ABS Blends. 3.5.1. Tensile Properties. Tensile properties of DDS

cured epoxy/ABS blends, for both strained and unstrained samples are given in Table 1. However, the tensile results are comparable irrespective of the cure schedule. In any case, the data revealed a remarkable increment in tensile strength and tensile elongation for 3.6 wt % and 6.9 wt % ABS containing blends with 4436



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a maximum for 3.6 wt % ABS containing blend (unstrained samples). The Young’s modulus remains the same for neat epoxy and epoxy/ABS blends. Representative, tensile stress/strain curves for both strained and unstrained samples are given in Figure 7. To our surprise the tensile toughness obtained from the area under the stress/strain curve shows an increment of more than 100% for the 3.6 wt % ABS modified epoxy system with respect to the neat cross-linked epoxy (for both strained and unstrained samples) (Table 1). The toughening mechanism is elaborated in the Discussion section. 3.5.2. Fracture Toughness. Fracture toughness is the resistance of material to crack initiation and propagation. Previous scientific reports 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, for both strained and unstrained samples, are shown in Figure 8. In any case, the fracture toughness of the epoxy resin is improved by blending 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 also important to mention that the KIC values of strained samples are slightly lower than the unstrained samples, but with in the limit of errors. 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 getting improved fracture toughness.28 Since the fracture micrographs were identical, irrespective of the curing conditions and also irrespective of the test performed, the fracture micrographs of the unstrained samples after the KIC fracture test were shown in Figure 9, to discuss the mechanical properties of the ABS modified epoxy blends. All the blends in this study are heterogeneous and thus satisfy one of the important conditions for improved fracture toughness. The presence of heterogeneous morphology is evident from the SEM micrographs of the fractured samples. For the neat epoxy system, cracks spread freely and regularly and orient in the direction of loading. This indicates typical characteristics of brittle fracture as revealed by Figure 9a. Although, the samples were well mixed, we noticed steplike structures as would be observed in poorly mixed aminecured epoxy systems.41 Figure 9 panels b and c show the FESEM micrographs of the fracture surfaces of the cured blends

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

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

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

a

tensile strength (MPa)

tensile modulus (GPa)

0

48 ( 4

a

51 ( 4

2.3 ( 0.1

3.6

61 ( 4

a

6.9

tensile elongation (%)

tensile toughness

2.3 ( 0.1

4.49 ( 0.2

a

3.99 ( 0.2

138a

121

65 ( 3

a

2.3 ( 0.1

2.3 ( 0.1

6.71 ( 0.3

a

6.93 ( 0.3

272a

279

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

85

a

Values of the strained samples (cured and annealed in one step). 4437



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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

0.49

a

Dn and Dw were calculated from SEM micrographs; at least 200 particles were measured to calculate Dn and Dw. b Volume fraction of ABS is calculated from total mass fraction; density of ABS is 1.05 g/cm3.

containing 3.6 and 6.9 wt % ABS, clearly exhibiting features of ductile drawing phenomenon on the fracture surfaces to some extent, which appear mainly in the ABS phase. The ductile tearing of thermoplastic is one of the factors responsible for the increase in fracture toughness of the thermoplastic-toughened epoxy resin. In the case of 3.6 and 6.9 wt % ABS content, spherical ABS particles may act as stress concentrators upon the application of external load and will lead to plastic deformation of the matrix surrounding the ABS particles. This will contribute to river marks and hence offer more roughness to the fracture surface and hence more ductility to the epoxy matrix. The high degree of roughness on the fractured surface also indicates the

crack deviation from its original plane, resulting in an increased surface area of the crack, which may also increase the toughness. Moreover, the interface between the epoxy phase and ABS phase remains intact. This is evidence for good adhesion between the matrix and dispersed domains. Hence, the stress is transferred more effectively to the thermoplastic domains from the cross-linked epoxy phase. Another important factor to be mentioned is the formation of nanocavities around 100 nm during the fracture process. The nanocavities in the ABS domains are very clear from the micrographs; the formation of nanocavities may take up a significant amount of applied stress and hence elevate the fracture toughness.42 The cavitation process in ABS is due to the presence of rubber (5 wt %) 4438



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in the ABS phase and is a phenomenon frequently observed in rubber-modified epoxy blends. Another important parameter influencing the fracture toughness is the domain size. For a more comprehensive consideration, the number average and weight average domain diameters and polydispersity index (PDI), interparticle distance and interfacial area per unit volume for the 3.6 and 6.9 wt % ABS modified blends were calculated using the following equations:24

∑ni di =∑ni

ð3Þ

∑ni di2 =∑ni di

ð4Þ

number average diameter, Dn ¼ weight average diameter, Dw ¼

polydispersity index, PDI ¼ Dw =Dn

ð5Þ

where ni is the number of domains having diameter di. The interparticle distance and interfacial area per unit volume were calculated using the following equations:24 interparticle distance ¼ Dn ½ðπ=6aTP Þ1=3 - 1

ð6Þ

interfacial area per unit volume ¼ 3aTP =r

ð7Þ

where Dn is the number average diameter of the domains, r is the number average radius of the domains, and aTP is the volume fraction 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 with increasing ABS content while the polydispersity index remains constant indicating uniform particle size distribution. The interparticle distance and interfacial area per unit volume increased with ABS content in the blends. The increase in interparticle distance at higher thermoplastic content is due to a strong coalescence effect of the ABS phase. But the domain size influences the efficiency of the initiation of energy absorbing mechanisms. Hence the smaller domains in 3.6 wt % ABS containing blends are effective in initiating energy absorbing mechanisms in comparison with other blends with larger domains. In the previous reports, better toughness values were also obtained for blends with cocontinuous phase structures as reported in other high performance thermoplastic-modified systems.28 In the present case, for 10 and 12.9 wt % ABSmodified blends, cocontinuous morphologies were observed as revealed by Figure 9d,e, hence the advancing crack had to propagate through the continuous ABS and epoxy phase which should offer more resistance to crack propagation. However, the cocontinuous phase structures for the blends containing 10 and 12.9 wt % ABS did not exhibit any improved toughness in the present system. Thus, the fracture properties of the cocontinuous blends seem to depend predominantly on the inherent property of 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 when approaching the glass transition during the TMA (heating) run. The presence of internal stress affects the dimensional stability of the blends at high temperatures (above the ABS Tg). The internal stress can be removed by two-step curing. The impact of the cure schedules and the increasing ABS concentration on properties were

carefully analyzed. Irrespective of the cure schedule, the thermal and mechanical properties remain comparable. On the other hand, the mechanical and morphological properties are affected by blending with the thermoplastic.The influence of increase in ABS content on the final mechanical properties was carefully analyzed and was correlated with blend morphology. In the blends with lower ABS content (3.6 and 6.9 wt % ABS), ABS domains were spherically dispersed in the continuous epoxy matrix and possess uniform size. The cured blends containing 10 and 12.9 wt % ABS exhibited a typical cocontinuous phase structure. These differences in morphology are responsible for the big differences in the mechanical properties. While the particle-matrix structures result in improved toughness and strength, with best results at 3.6 wt % ABS content, the cocontinuous blends containing 10 and 12.9 wt % ABS exhibit poor mechanical properties as compared to the neat cross-linked epoxy. In many applications dimensional stability is very important. In this context a two step curing is always recommended to alleviate the dimensional variation. It is important to add that experiments are in progress to investigate the effect of physical aging of other thermosetting systems such as epoxy/SAN blends and epoxy nanocomposites, etc.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ91-481-2730003. Fax: þ91-481-2731002. E-mail: [email protected]; [email protected]. Address: Centre for Nanoscience & Nanotechnology, Mahatma Gandhi University, Priyadarshini Hills P.O., Kottayam, Kerala686560, India.

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(37) Sabra, A.; Lam, T. M.; Pascalt, J. P.; Grenier-Loustalot, M. F.; Grenier., P. Characterization and behaviour of epoxy-based diaminodiphenylsulphone networks. Polymer 1987, 28, 1030–1036. (38) Woo, E. M.; Bravence, L. D.; Seferis, J. C. Morphology and properties of epoxy alloy system containing thermoplastics and a reactive rubber. Polym. Eng. Sci. 1994, 34, 1664–1673. (39) Oyanguren, P A.; Riccardi, C. C.; Williams, R. J.; Mondragon, I. Phase separation induced by a chain polymerization: Polysulfonemodified epoxy/anhydride systems. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1349–1359. (40) Jyotishkumar, P.; Koetz, J.; Tierisch, B.; Strehmel, V.; Hassler, R.; Ozdilek, C.; Moldenaers, P.; Thomas, S. Complex phase separation in poly(acrylonitrile-butadiene-styrene)-modified epoxy/4,40 -diaminodiphenyl sulfone blends: Generation of new micro- and nanosubstructures. J. Phys. Chem. B 2009, 113, 5418–5430. (41) Daly, J.; Pethrick, R. A.; Fuller., J.; Cunliffe, A. V.; Datta, P. K. Rubber-modified epoxy resins: 1. Equlibrium physical properties. Polymer 1981, 22, 32–36. (42) Bubeck, R. A.; Buckley, D. J., Jr.; Karmer, E. J.; Brown, H. R. Modes of deformation in rubber-modified thermoplastic during tensile impact. J. Mater. Sci. 1991, 26, 6249–6259.

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Studies on Stress Relaxation and Thermomechanical Properties of

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