Effect of Cure Conditions on the Generated ... - ACS Publications

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Effect of Cure Conditions on the Generated Morphology and Viscoelastic Properties of a Poly(acrylonitrile−butadiene−styrene) Modified Epoxy−Amine System Jyotishkumar Parameswaran Pillai,† Jürgen Pionteck,‡ Rüdiger Haß̈ ler,‡ Christophe Sinturel,§ Viju Susan Mathew,∥ and Sabu Thomas*,†,⊥,¶,▽ †

School of Chemical Sciences and ⊥Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Priyadarshini Hills, Kottayam, Kerala-686560, India ‡ Leibniz Institute of Polymer Research Dresden, Hohe Straße 6, 01069 Dresden, Germany § Centre de Recherche sur la Matière Divisée, UMR 6619 CNRS Université d’Orléans, 1 B Rue de la Férollerie, 45071 Orléans Cedex 2, France ∥ Department of Chemistry, St. Thomas College, Kozhencherry, Kerala-689641, India ¶ Universiti Teknologi MARA, Faculty of Applied Sciences, 40450 Shah Alam Selongor, Malaysia ▽ Center of Excellence for Polymer Materials and Technologies, Tehnoloski park 24, 1000 Ljubljana, Slovenia S Supporting Information *

ABSTRACT: The curing behavior, phase morphology, and dynamic mechanical characteristics of an epoxy system based on the diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diaminodiphenylsulfone (DDS), modified with different amounts of poly(acrylonitrile−butadiene−styrene) (ABS), were investigated by employing differential scanning calorimetry (DSC), fieldemission scanning electron microscopy (FESEM), and dynamic mechanical thermal analysis (DMTA). The effects of different curing conditions on the generated morphologies and viscoelastic properties were evaluated. The amounts of ABS in the epoxy blends were 3.6, 6.9, 10, and 12.9 wt %. The rate of the curing reaction decreased with increasing thermoplastic content and with decreasing curing temperature. Morphological analysis revealed a phase-separated morphology for the blend systems. The storage modulus (E′), loss modulus (E″), and tan δ values of the systems were measured as functions of temperature and are discussed based on the morphological behavior of the epoxy blends with different amount of ABS.

1. INTRODUCTION Epoxy resins are most important among the thermosetting polymers and find a wide range of applications such as in adhesives, coatings, and matrixes for high-performance composites. The wide range of applications arises from the desirable properties of epoxy resins, including easy processability, high tensile strength and modulus, good chemical and corrosion resistance, dimensional and thermal stability, good creep resistance, excellent fatigue properties, low shrinkage on curing, good adhesion to various substrates, long pot life period, and easy curing.1,2 However, applications of epoxy monomers usually require a high level of cross-linking, which results in brittle behavior. Considerable efforts have been made to improve the toughness of cross-linked epoxy by blending with rubber,3−7 but the incorporation of rubber adversely affects the thermal and mechanical properties of the system. Recently, high-performance thermoplastics have been widely used as toughening agents for epoxy systems that can improve the toughness without affecting the mechanical and thermal properties. Engineering thermoplastics such as poly(acrylonitrile−butadiene−styrene) (ABS),8−10 poly(styrene− acrylonitrile) (SAN),11 poly(ethersulfone)s (PESs),12 and poly(etherimide)s (PEIs)13,14 have been widely used as toughening agents. © 2011 American Chemical Society

The curing process is a chemical reaction in which the epoxide groups in the epoxy resin react with a curing agent (amine) to form a highly cross-linked three-dimensional network. The curing of epoxy/amine takes place through three steps: (1) addition reactions that occur during the curing of the diglycidyl ether of bisphenol A (DGEBA) with primary amines, resulting in a secondary amine and a secondary hydroxyl group; (2) addition reactions that occur during the curing of DGEBA with secondary amines results in the formation of a tertiary amine and a secondary hydroxyl unit; and (3) further etherification reactions between epoxy units and the secondary hydroxyl groups formed in steps 1 and 2. The first two steps take place in the initial stages of the reaction, whereas etherification takes place in the later stages, which might be retarded by the gelling network.15,16 The reaction of epoxy curing is very complex because curing in thermoset/thermoplastic blends is coupled with gelation, vitrification, and phase separation with the advancement of curing.17 In the beginning of the curing process, the curing reactions are controlled by chemical kinetics; however, in the Received: Revised: Accepted: Published: 2586

February 1, 2011 December 28, 2011 December 28, 2011 December 28, 2011 dx.doi.org/10.1021/ie2011017 | Ind. Eng.Chem. Res. 2012, 51, 2586−2595

Industrial & Engineering Chemistry Research

Article

Figure 1. AFM micrographs of ABS (smoothed cryo cuts): (a) topology, (b) phase contrast. Size = 4 × 4 μm2.

100 g of DGEBA was mixed with 5, 10, 15, or 20 g of ABS at 180 °C under constant stirring. After proper mixing, 35 g of DDS corresponding to a stoichiometric epoxy/amine ratio of 2:1 was dissolved in the blend at 180 °C in less than 40 s to minimize the curing reaction during mixing, and the mixture was immediately plunged into liquid nitrogen to freeze the curing reaction. The freshly prepared mixtures were directly used for differential scanning calorimetry (DSC) analysis. For morphological and mechanical evaluation, samples cured for 5 h in the temperature range of 150 −180 °C were used. 2.2. Characterization Techniques. 2.2.1. Differential Scanning Calorimetry. The calorimetric measurements were performed on a Perkin-Elmer Pyris DSC 6 differential scanning calorimeter. The instrument was calibrated with indium and dry cyclohexane standards. Dry nitrogen was used as the purge gas. Curing was performed in aluminum pans with sample weights between 15 and 20 mg. Nonisothermal DSC measurements were performed at heating rates of 2.5, 5, 7.5, and 10 °C/min in the temperature range from 20 to 350 °C for the epoxy/DDS mixture to determine the total heat of the reaction (ΔHtotal) of the epoxy system. ΔHtotal was determined by the integration of the DSC nonisothermal signal for neat epoxy (data not shown); the temperature range for calculating ΔHtotal was from 100 to 300 °C. The determined values of 403 J/g or 106 kJ/ee (in terms of equivalent epoxy) were taken as ΔHtotal for calculating the fractional conversion α at time t from the isothermal measurements and hence the rate of the reaction. Isothermal measurements were performed at 150, 165, and 180 °C. The heating rate used was 100 °C/min to attain the isothermal curing temperature. The curing was assumed to be complete when the isothermal curve leveled off to a straight line. The area under the peak during isothermal curing for various times was used to determine the conversion α as a function of time for neat epoxy and epoxy blends. The conversion, α, was calculated as α = ΔHt/ΔHtotal, where ΔHt is the heat of curing at time t, calculated directly by integration of the DSC isothermal signal for the blend systems, and ΔHtotal is the total heat of curing of the epoxy monomer, calculated by the integration of the DSC nonisothermal signal for neat epoxy. The time ranges for calculating the ΔHt values were ∼90, 175, and 250 min at 180, 165, and 150 °C respectively. 2.2.2. Oscillatory Shear Rheology. The complex viscosity of the ABS/epoxy blends at different temperatures was studied by oscillatory shear measurements using a stress-controlled ARG2 rheometer (TA Instruments). The samples were placed

later stages, the curing is controlled by diffusion, provided that the curing temperature is below the glass transition (Tg) of the thermosets.18 For a thermoplastic-modified epoxy system, the important factors controlling the final morphology are the thermodynamics and kinetics of phase separation during curing. Both can be modified by changing the thermoplastic content and curing conditions.19−23 Depending on the thermoplastic content and curing conditions, dispersed, co-continuous, or phase-inverted morphologies can be generated.24,25 The mechanical properties of the blends depend on the generated blend morphology.11 The ABS/epoxy mixture used for these studies is heterogeneous before curing; SAN-g-polybutadiene (PB) exists in a homogeneous epoxy/SAN phase [polystyrene (PS) and SAN are miscible with the epoxy prepolymer, unlike PB]. The initially homogeneous epoxy/SAN phase undergoes reactioninduced phase separation.25 In this article, an attempt has been made to investigate the effects of curing conditions on the generated phase morphology and viscoelastic properties as a function of ABS content.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The matrix material used in the experiments consisted of diglycidyl ether of bisphenol A (DGEBA) (Lapox L-12, Atul Ltd., India) and 4,4′diaminodiphenylsulfone (DDS) (Lapox K-10, Atul Ltd., India). The epoxy content in Lapox L-12 varies between 5.25 and 5.40 equiv/kg. The toughener ABS (Poly lac PA-757K) was manufactured by Chi Mei Corporation, Taiwan. The poly(acrylonitrile−butadiene−styrene) (ABS) used was a commercially available thermoplastic polymer consisting of 70 wt % polystyrene (PS), 25 wt % acrylonitrile (AN), and 5 wt % polybutadiene (PB). Figure 1 shows atomic force microscopy (AFM) images of a cut surface of ABS taken as a topology and in phase-contrast mode. The AFM micrographs reveal that the SAN-g-PB particles are finely dispersed in the rigid SAN matrix phase. The SAN-g-PB particle size ranges from 50 to 700 nm. The molecular weight of the soluble part of ABS was determined to be Mn = 51300 g/mol and Mw = 125200 g/ mol [polydispersity index (PDI) = 2.4, gel permeation chromatography (GPC), PS standard], and the density was determined to be 1.051 g/cm3 by means of a helium pycnometer. Blends of ABS/epoxy containing 3.6, 6.9, 10, and 12.9 wt % ABS were prepared using the melt mixing technique. Typically, 2587

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between a disposable parallel-plate system, consisting of two 25-mm aluminum plates, one standard upper plate and one lower plate with a drip channel, that were replaced at the end of each measurement. Temperature control was achieved by a sealed environmental testing chamber (ETC system, TA Instruments). All experiments were carried out with 5% strain, and an angular frequency of 1 Hz was used to measure the complex viscosity. 2.2.3. Field-Emission Scanning Electron Microscopy. The morphologies of the cross-linked epoxy and the epoxy blends were examined by field-emission scanning electron microscopy (FESEM; model ultra plus, Nano Technology Systems Division, Carl Zeiss SMT AG, Oberkochen, Germany). The fractured samples were smoothed with an ultramicrotome, and the SAN-rubber phase was etched out by immersing the cut in chloroform for 2 h. The samples were coated with platinum by vapor deposition using a vacuum sputterer. 2.2.4. Atomic Force Microscopy. AFM measurements were performed in tapping mode on a Dimension 3100 Nanoscope V (Veeco, Plainview, NY). We used silicon SPM sensors (Budget Sensors, Sofia, Bulgaria) with a spring constant of ca. 40 N/m and a resonance frequency of ca. 280 kHz; the tip radius was lower than 10 nm. According to Magonow et al.,26 we chose the scan conditions (free amplitude, >100 nm; setpoint amplitude ratio, 0.8) to obtain stiffness contrast in the phase image, which means that bright features in the phase image are stiffer than dark areas. 2.2.5. Dynamic Mechanical Thermal Analyzer. The viscoelastic properties of the pure epoxy network and epoxy blends were measured using a TA Instruments DMA 2980 dynamic mechanical thermal analyzer. Rectangular specimens of 40 × 10 × 3 mm3 were used for the analysis. The analysis was done in single-cantilever mode at a frequency of 1 Hz, and the samples were scanned from −100 to 300 °C at a heating rate of 1 °C/min.

3. RESULTS AND DISCUSSION 3.1. Kinetics of Curing. The neat epoxy resin and its blends with different weight percentages of ABS were cured with amine at three different isothermal temperatures, namely, 150, 165, and 180 °C. Figure 2a shows the DSC curves of isothermal curing of epoxy blends at 180 °C, which is representative for all blends studied at various temperatures. For the blends, a rapid increase in the reaction rate was observed that passed through a maximum in the exothermic heat flow. This increase in reaction rate is attributed to the autoacceleration process arising from the hydroxyl groups formed during the epoxy/amine reactions. A careful examination of the exothermic peak reveals broadening, depression, and the appearance of a shoulder with increasing ABS content. The broadening and depression reflect a decrease in the reaction rate. The shoulder in 10 and 12.9 wt % ABS-containing blends is due to the generation of an epoxy/amine-rich phase during the process of phase separation because of the increase in epoxy molecular weight upon the application of high temperature, which raises the curing kinetics.24 In addition to the broadening and depression and the appearance of a shoulder in the exothermic peak, the peak maximum is slightly shifted toward longer curing times as the concentration of ABS increases. This again reflects a reduction in the reaction rate at higher ABS concentrations, because of the decrease in probability of the reaction between epoxy and diamine due to the dissolution effect produced by the thermoplastic phase.

Figure 2. (a) Isothermal DSC curves of the curing of epoxy blends at 180 °C. (b) Plot of conversion versus time for 3.6 wt % ABS/epoxy blends at 150, 165, and 180 °C.

The effect on the epoxy/amine curing reaction rate of the change in viscosity upon ABS addition was analyzed by rheological measurements and permitted the characterization of the complex viscosity. The complex viscosities measured for the ABS-modified epoxy/DDS blend systems at various temperatures are given in Table 1. From this table, one can see that the Table 1. Viscosities of Epoxy Blends at Different Curing Temperatures complex viscosity (η*, Pa·s) sample

150 °C

165 °C

180 °C

neat epoxy 3.6 wt % 6.9 wt % 10 wt % 12.9 wt %

0.22 0.17 0.33 0.49 0.86

0.35 0.19 0.32 0.43 0.73

0.05 0.25 0.15 0.47 0.67

complex viscosity (Pa·s) increased with increasing thermoplastic addition and with decreasing temperature. These results suggest that the delay in curing reaction at higher thermoplastic 2588



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Figure 3. SEM micrographs (extraction with chloroform for 2 h) of the samples cured at 150, 165, and 180 °C for 5 h. (For the 3.6 and 6.9 wt % ABS-modified blends, the scale is 21 × 15 μm2, and for the 10 and 12.9 wt % ABS-modified blends, the scale is 105 × 75 μm2.)

SAN phase, which has been dissolved away from the crosslinked epoxy matrix by chloroform. On the other hand, irrespective of the curing temperature, the morphologies of the 10 and 12.9 wt % ABS-containing cross-linked epoxy specimen are co-continuous-like. Both the epoxy phase and the ABS phase appear continuous. In the continuous epoxy phase, SAN particles (average diameter ≈ 400−500 nm) are dispersed (etched holes), whereas in the continuous SAN phase, clusters of epoxy particles (average diameter ≈ 2−2.5 μm) are dispersed. The “tomato-slice effect” on the epoxy particles is due to the ultramicrotome cutting. For better understanding of the final generated complex morphology, we recorded SEM micrographs without the extraction process. Figure 4 shows the micrographs of 3.6 and 12.9 wt % ABS-containing epoxy blends without the extraction process. The SEM micrographs of the 3.6 wt % ABScontaining sample (Figure 4a) show the thermoplastic-rich phase as dispersed particles in the epoxy-rich matrix. However, a careful examination reveals the occurrence of few agglomerates of SAN-g-PB dispersed in SAN droplets. SEM micrographs of the cross-linked epoxy specimen containing 12.9 wt % ABS exhibits a co-continuous structure in which both the epoxy phase and the ABS phase are continuous (Figure 4b). In the continuous epoxy phase, the SAN particles are dispersed,

content can be attributed not only to the dilution effect exerted by the thermoplastic but also to the viscosity increase produced by ABS addition. These results are in agreement with studies of Martinez et al. on epoxy/amine systems.22 To study the effect of curing temperature on the final conversion, a comparison of curing conversion versus curing time for the 3.6 wt % ABS-modified epoxy blends cured at 150, 165, and 180 °C is shown in Figure 2b. All of the conversion curves show a rapid increase of the extent of reaction in the early stages of the curing reaction followed by the leveling off to a plateau value, indicating a cessation of the curing reaction as the system vitrifies. At lower curing temperatures, the epoxy/ amine reaction is slower, and the final conversion is lower. 3.2. Phase Morphology of Epoxy Blends. The generated complex morphologies of ABS-modified epoxy blends after curing at different temperatures were carefully examined by SEM. SEM micrographs of ABS/epoxy blends cured at 150, 165, and 180 °C for 5 h after ultramicrotome cutting followed by etching in chloroform for 2 h are shown in Figure 3. The micrographs reveal phase-separated morphologies for the ABS/ epoxy blends. Irrespective of the curing temperature, the thermoplastic-rich phase (average diameter ≈ 610−920 nm) is dispersed in the epoxy-rich phase in 3.6 and 6.9 wt % ABScontaining blends. The spherical hollows correspond to the 2589



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the SEM. AFM micrographs were recorded for 3.6 and 12.9 wt % ABS blends that were prepared by curing for 5 h at 180 °C. Figure 5 shows the AFM micrographs of a cut surface of 3.6 or 12.9 wt % ABS-modified epoxy/DDS system taken as a topology (Figure 5a,c) and in phase-contrast mode (Figure 5b,d). The AFM micrographs of the 3.6 wt % ABS-containing blend reveal matrix droplet morphology in which the SAN-rich phase are dispersed in the epoxy-rich matrix, and the SAN-g-PB phase appears as dispersed small agglomerates at the blend interface (Figure 5a,b). The AFM micrographs of the cut surface of the 12.9 wt % ABS-modified epoxy/DDS system reveal three different phases: two continuous phases forming a co-continuous structure with substructures (the epoxy continuous phase containing dispersed SAN particles and the SAN continuous phase in which epoxy particles are dispersed) and the SAN-g-PB phase appearing as dispersed small agglomerates at the blend interface between the co-continuous structures (Figure 5c,d). These results are in full agreement with the SEM micrographs. 3.3. Dynamic Mechanical Behavior of Epoxy Blends. The dynamic mechanical analysis (DMA) profile of ABS is shown in Figure 6a, which indicates two Tg values, one for the polybutadiene part at −80 °C and the other for the SAN matrix phase at 100 °C. This confirms that the ABS is not homogeneous but exhibits a heterogeneous structure in which PB particles are dispersed in the SAN matrix. The plots of log E″ versus temperature for epoxy and ABS/ epoxy blends cured at 180 °C are shown in Figure 6b. A single Tg value is observed for unmodified cross-linked epoxy, indicating the homogeneous nature of cross-linked epoxy system. Careful examination of log E″ curves reveals a peak at around 60 °C, called the ω-relaxation peak, which is due to some sites with lower cross-link density in the epoxy network

Figure 4. SEM micrographs (21 × 15 μm2) of ABS-modified epoxy blends (without extraction) cured at 180 °C for 5 h.

whereas in the continuous SAN phase, the epoxy particles are dispersed. From the micrographs of the microtome sample, it is clear that the immiscible agglomerates of SAN-g-PB particles tend to stay at the interface between the co-continuous structures. AFM investigations were made for selected samples to provide supporting evidence for the above results obtained by

Figure 5. AFM micrographs of ABS-modified epoxy blends cured at 180 °C for 5 h. 2590



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for 10 and 12.9 wt % ABS-containing blends, phase separation takes place through nucleation and growth followed by a phase inversion process. This finally results in co-continuous-like structures, in which both the epoxy and the ABS phase appear continuous. For a more comprehensive consideration of the morphology generated at different curing temperatures, the number-average and weight-average domain diameters of the SAN dispersedphase particles in 3.6 and 6.9 wt % ABS-modified blends were calculated from the SEM micrographs using the following equations27 (approximately 200 particles from different SEM micrographs of the same sample were taken to calculate the number-average and weight-average domain diameters)

number‐average diameter, Dn̅ =

∑ nidi ∑ ni

(1)

weight‐average diameter, D̅w =

∑ nidi 2 ∑ nidi

(2)

polydispersity index, PDI =

D̅w Dn̅

(3)

where ni is the number of domains having diameter di. The thermoplastic domain diameter and the other parameters calculated from the above-mentioned equations are summarized in Table 2. The domain size distributions of Table 2. Number-Average Diameter (Dn), Weight-Average Diameter (Dw), and Polydispersity Index (PDI) of Dispersed SAN Particles in Cross-Linked ABS/Epoxy Blends temperature (°C)

Figure 6. (a) DMA profile of ABS. (b) Plot of log E″ versus temperature for epoxy and ABS/epoxy blends cured at 180 °C.

150 165 180

or β-relaxation overtones in the regions of higher cross-link density matrix, which are occluded in the matrix with lower cross-link density.27 On the other hand, two distinct Tg values are observed for ABS/epoxy blends, indicating the heterogeneous nature of the blends. The peak at around 100 °C is due to the SAN-rich phase, whereas the peak at around 180−190 °C is due the cross-linked epoxy-rich phase. The increase in peak height of the SAN-rich phase becomes more prominent at higher ABS content, because the dimensions of the separated SAN-rich phase are higher at higher ABS content.

150 165 180

Dn (nm)

Dw (nm)

ABS/epoxy (3.6 wt %) 659 627 619 ABS/epoxy (6.9 wt %) 895 868 805

PDI

710 679 678

1.08 1.08 1.09

914 892 827

1.02 1.03 1.03

the blends are shown in Figure 7a,b. From the table and distribution curves, it follows that the dispersed particle size increased with the lowering of the curing temperature and with the increase of the thermoplastic content. At high curing temperatures, the viscosity of the thermoplastic should be lower; however, the rate of epoxy/amine curing reaction should be high, and hence, the epoxy blend viscosity increases rapidly (less time for gelation), and the mobility of the thermoplastic domains decreases. Therefore, coalescence is suppressed, which results in smaller particle sizes at high curing temperatures. Similarly, at higher thermoplastic contents, the rate of the reaction is comparatively lower because of the dissolution effect, and hence, the greater chance for coalescence results in larger domains dispersed in the epoxy matrix. It is important to add that the wider the peak, the higher the polydispersity. For 3.6 wt % ABS-modified epoxy blends, the number of particles with the maximum diameter decreases with increasing curing temperature, but for the 6.9% sample, the behavior is not as clear. It is not always true that the distribution of the diameters is more homogeneous with an increase of the curing temperature. Our results show this

4. DISCUSSION The curing study by DSC proved to be a valuable tool providing useful information on epoxy/amine curing reaction. The reaction rate decreases with the addition of ABS and with decrease of the curing temperature. Once the Tg of the epoxy phase equals the curing temperature, the system immediately starts to vitrify, resulting in the cessation of epoxy/amine reaction and, hence, a lower final conversion for the epoxy phase at lower curing temperatures. SEM and AFM micrographs for the blends cured at different temperatures reveal heterogeneous morphologies for the blend systems. For 3.6 and 6.9 wt % ABS-containing blends, the phase separation takes place through a nucleation and growth mechanism, which results in a matrix droplet morphology, with the SAN-rich phase dispersed in the epoxy-rich matrix. On the other hand, 2591



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curves. From the figure, the Cole−Cole plot of neat epoxy gives a semicircular curve indicating the homogeneity of the system. On the other hand, the blend systems give two semicircular curves, indicating the heterogeneity of the system; the one at higher modulus values is due to the ABS-rich phase, whereas that at lower modulus values is due to the epoxy-rich phase. The semicircular curve of the epoxy-rich phase is depressed and shifts to lower modulus values with the addition of ABS. On the other hand, the semicircular curve due to the ABS-rich phase increases in size and is shifted toward lower E′ values upon the addition of ABS because the dimensions of the thermoplasticrich phase are higher at higher ABS contents. The tan δ versus temperature plots for the 6.9 wt % ABScontaining epoxy system cured at 180, 165, and 150 °C are shown in Figure 8b. Two peaks corresponding to the epoxyrich and thermoplastic-rich phases appeared. The tan δ peak for the epoxy-rich phase is shifted to higher temperature, but the peak height decreases with increasing curing temperature. This phenomenon is due to the increase in conversion at higher curing temperatures that indicates the influence of curing temperature on the Tg value, as well as segmental mobility of the epoxy chains at Tg. These results are in agreement with our previous study.18 For the blends cured at 180 °C, the Tg value of the epoxy phase is around 200 °C; this compares reasonably with literature reports for the completely cured epoxy blends.30 At lower curing temperatures, the Tg values of the epoxy phase are lower. The glass transition temperatures (Tg) of the SAN-rich phase and epoxy-rich phase in ABS/epoxy blends measured under different curing conditions by DMA (tan δ) are shown in Figure 8c. The relaxation peak of the SAN-rich phase is observable at around 100 °C, and the relaxation peak of the cross-linked epoxy-rich phase is observable at around 180−205 °C, depending on the curing conditions. The Tg value of the pure cross-linked epoxy-rich phase remains more or less the same irrespective of the blend concentration. However, the Tg value of the epoxy phase decreases with decreasing curing temperature. This is due to the earlier vitrification of the continuous epoxy phase at lower curing temperatures. The decrease in Tg at lower curing temperature is an indication of a corresponding decrease in cross-link density. On the other hand, the Tg value of the thermoplastic-rich phase in epoxy blends remains same, irrespective of the ABS loading and curing temperature. The molecular weight between the cross-links (Mc), which is an indirect measure of cross-link density of epoxy resin, can be calculated from the Tg of the epoxy-rich phase (tan δ) using the equation31

Figure 7. Domain size distributions of (a) 3.6 and (b) 6.9 wt % ABSmodified epoxy blends.

dependency only for the 6.9 wt % ABS-containing sample, where the peaks are narrower at higher temperatures. The dimensions of the particles depend on both the curing temperature and the percentage of ABS because these factors affect the viscosity of the blend. Dynamic mechanical thermal analysis of the blends also supported the two-phase morphology of the blends. As mentioned in section 3.3, the two peaks in the log E″ versus temperature profile (Figure 6b) corresponding to the epoxyrich and thermoplastic-rich phases indicate the heterogeneous nature of the blends. As shown in Figure 8a, a Cole−Cole plot was constructed by plotting the loss modulus against the storage modulus for ABS-modified epoxy blends cured at 180 °C. Cole−Cole plots are extensively used to investigate the structure of polymer systems,28,29 The representation of E′ and E″ in a Cole−Cole plot provides information about relaxation processes taking place in multiphase blends or about phase separation in polymer blends. It is assumed that, when the blends are homogeneous, the corresponding Cole−Cole plot gives an almost semicircular curve; if the blends are phaseseparated, it will result in a larger number of semicircular

Mc =

3.9 × 104 Tg − Tg0

(4)

where Tg is the glass transition temperature of the cross-linked epoxy resin and Tg0 is the glass transition temperature of uncross-linked polymer having same composition as the crosslinked polymer. The value of Tg0 was taken as 91 °C for DGEBA/DDS system.32 The effective cross-link density (Ve) was calculated from Mc using the equation31

υe =

ρNA Mc

(5)

where ρ is the density and NA is Avogadro’s number. 2592



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Figure 8. (a) Cole−Cole plot for epoxy and ABS/epoxy blends cured at 180 °C. (b) Plot of tan δ versus temperature for the 6.9 wt % ABScontaining epoxy cured at 150, 165, and 180 °C. (c) Dependence of Tg of both the epoxy phase and ABS phase on the curing temperature and ABS loading. (d) log E′ curves of the cross-linked neat epoxy system and the blends cured at 180 °C. (e) Plot of E′ versus temperature for the 12.9 wt % ABS-containing epoxy cured at 150, 165, and 180 °C.

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studied polymer blends. Dynamic mechanical spectrum analysis reveals always two relaxation peaks, corresponding to the epoxy-rich and thermoplastic-rich phases, which is consistent with SEM and AFM observations. The relaxation peak height of the SAN-rich phase from E″ becomes more prominent at higher ABS content. The change of the morphology with the changing of the curing conditions is not well distinguishable by DMA because of the high dimension of the separated phases. However, cured epoxy blends reveals a decrease in Tg and E′ with the decrease in curing temperature and with thermoplastic addition due to decreased epoxy conversion. Based on the results, we conclude that effective control over curing kinetics and phase morphology is essential to control the thermal and mechanical properties of this system.

The molecular weight between the cross-links (Mc) and the effective cross-link density (Ve) are summarized in Tables 3 and Table 3. Molecular Weights between the Cross-Links from the tan δ Profiles of Epoxy Blends Mc (g/mol) curing temperature (°C)

0 wt %

150 165 180

438 379 345

3.6 wt % 6.9 wt % 438 379 342

433 375 348

10 wt %

12.9 wt %

438 379 358

438 394 358

Table 4. Effective Cross-Link Densities from the tan δ Profiles of Epoxy Blends

■

υe (×1027 chains/m3) curing temperature (°C)

0 wt %

3.6 wt %

150 165 180

1.65 1.91 2.09

1.65 1.91 2.11

6.9 wt % 10 wt % 1.67 1.93 2.07

1.65 1.91 2.02

ASSOCIATED CONTENT

S Supporting Information *

12.9 wt %

SEM micrographs (lower magnification) of 12.9 wt % ABSmodified epoxy blend cured at 180 °C for 5 h. Schematic representation of phase separation in the epoxy/ABS blend system. This material is available free of charge via the Internet at http://pubs.acs.org.

1.65 1.83 2.02

■

4, respectively. The decrease in Mc and consequent increase in cross-link density with respect to the increase in curing temperature is evident from these tables. These results are consistent with the DSC observations. The log E′ values of the neat epoxy and the blends cured at 180 °C are shown in Figure 8d. The log E′ values for the blends are lower than that of the neat epoxy system. This is possibly due to the presence of less stiff ABS thermoplastic. The storage modulus decreases slightly at the Tg of the SAN phase when the ABS amount is small. However, the drop in the storage modulus at the Tg of the SAN phase for the 10 and 12.9 wt % ABS blend systems is quite large because the dimensions of the thermoplastic-rich phase are high and the associated softening of this phase leaves the continuous epoxy/amine phase as the remaining glassy component. As a result, the log E′ values for the 10 and 12.9 wt % ABS blend drops to lower values than for the other blends. Figure 8e shows the storage moduli of 12.9 wt % ABS-containing blends cured at different temperatures. It is important to mention that the storage modulus increases with increase in curing temperature. The increase in storage modulus is due to the more complete epoxy/amine reaction at higher temperatures.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-481-2730003. Fax: +91-481-2731002. E-mail: [email protected], [email protected].

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REFERENCES

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5. CONCLUSIONS Calorimetric and dynamic mechanical analysis reveals that the conversion reached in ABS/epoxy blends upon curing at various temperatures decreases with the addition of ABS and with a decrease in curing temperature. The formation of a cured epoxy network causes a reaction-induced phase separation. A deep analysis of the morphologies generated after phase separation for the blends at various curing temperatures revealed that all of the blends studied here are heterogeneous in nature. In the blends with lower ABS contents (3.6 and 6.9 wt % ABS), a normal matrix droplet morphology was observed. The dispersed ABS particle size increased slightly with decreasing curing temperature and with increasing ABS content. The cured blends containing 10 and 12.9 wt % ABS exhibited a co-continuous like morphology with substructures. The higher curing temperature results in a higher epoxy conversion and a more complete phase separation for the 2594



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