An epoxy-diamine system was simultaneously modified with two immiscible thermoplastic polymers, polysulfone (PSF) and poly (ether imide) (PEI), to develop tough materials without adding high quantities of modifiers, in order to avoid the processibility problems caused by the high initial viscosity of the mixtures. The mechanical behavior of blends containing 10 and 15 wt% total thermoplastic was analyzed and compared with the generated morphologies. The scanning electron micrographs (SEM) of the broken surfaces showed that when a small part of PEI is replaced by PSF, drastic changes in morphology, leading to co-continuity between the phases, occurred together with fracture (critical stress intensity factor, KIC) improvements. As an additional advantage, no noticeable decrease in the elastic modulus (E) of final materials was observed.Fil: Giannotti, Marina Inés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Bernal, Celina Raquel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Oyanguren, Patricia Angelica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Galante, Maria Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Bernal, Celina Raquel

Galante, Maria Jose

Giannotti, Marina Inés

Oyanguren, Patricia Angelica

Polymer Engineering & Science

CONICET Digital

Morphology and Fracture Properties Relationship ofEpoxy-Diamine Systems Simultaneously Modified WithPolysulfone and Poly(ether imide)M.I. Giannotti, C.R. Bernal, P.A. Oyanguren, M.J. GalanteInstitute of Materials Science and Technology (INTEMA), University of Mar del Plata and National ResearchCouncil (CONICET), J.B. Justo 4302, 7600 Mar del Plata, ArgentinaAn epoxy-diamine system was simultaneously modi-fied with two immiscible thermoplastic polymers,polysulfone (PSF) and poly (ether imide) (PEI), to de-velop tough materials without adding high quantitiesof modifiers, in order to avoid the processibility prob-lems caused by the high initial viscosity of the mix-tures. The mechanical behavior of blends containing10 and 15 wt% total thermoplastic was analyzed andcompared with the generated morphologies. Thescanning electron micrographs (SEM) of the brokensurfaces showed that when a small part of PEI isreplaced by PSF, drastic changes in morphology, lead-ing to co-continuity between the phases, occurredtogether with fracture (critical stress intensity factor,KIC) improvements. As an additional advantage, nonoticeable decrease in the elastic modulus (E) of finalmaterials was observed. POLYM. ENG. SCI., 45:1312–1318, 2005. © 2005 Society of Plastics EngineersINTRODUCTIONThe two methods used to enhance the macroscopictoughness of thermoset polymers are: 1) plasticization and2) the increase in the number of toughening mechanisms bythe generation of a heterogeneous structure.The first method is based on the incorporation of a lowglass transition temperature (Tg) component, soluble in thethermoset polymer. In this case, the increase in the fracturetoughness is achieved, causing a decrease in Tg and in yieldstress. For these reasons, this is not a technique that istypically used.In the second case, the most frequently used methodsto enhance toughness are the addition of particles, or thein situ formation of rubber or thermoplastic (TP) particlesdispersed in the thermoset matrix. The principal limita-tion of using a rubbery modifier is the fact that, as in thecase of a soluble component, the improvement takesplace at expenses of a decrease in other desirable prop-erties. Then, the toughening induced by modificationwith TP was an alternative explored during the lastdecade [1–3] that allowed obtaining a tough materialwithout a decrease in Tg or stiffness. The tougheninginduced by adding a TP is closely related to diversestructural factors, such as volume fraction and morphol-ogy of the dispersed phase [4].The simultaneous modification of the thermosetting pre-cursors with two TP polymers is an alternative to obtaindifferent properties, according to the diverse morphologiesgenerated due to different phase behavior of the blends[5–7].Generally, the epoxy-amine systems modified with onlyone TP show toughness improvements when more than 15wt% modifier is added. In these cases the high Tg of the TPleads to high viscosity reactive mixtures that are, in conse-quence, difficult to process.In this work the simultaneous modification of an epoxy-diamine (DA) system with two immiscible TPs was studiedas a possibility of developing toughened materials withoutincorporating high quantities of modifiers that otherwisemay lead to blends with high initial viscosity and thereforepoor processibility.EXPERIMENTALMaterialsThe selected monomers were a diglycidyl ether ofbisphenol-A (DGEBA) epoxy (DER 332, Dow Chemi-cals, number-average molecular mass, Mn, equal to 349g/mol and a hydroxyl/epoxy ratio of 0.015) and twoaromatic diamines 4,4-diaminodiphenyl sulfone (DDS,HT 976 Ciba-Geigy) and 4,4-methylenbis(3-chloro-2,6-diethyl aniline) (MCDEA, Lonza). They were cured in astoichiometric epoxy/amine equivalents relationship. TheTPs used were polysulfone (PSF, Udel P1700, AmocoCorrespondence to: M.J. Galante; e-mail:galant@fi.mdp.edu.arContract grant sponsors: ANPCyT; CONICET; University of Mar delPlata; Fundación Antorchas (Argentina).DOI 10.1002/pen.20407Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics EngineersPOLYMER ENGINEERING AND SCIENCE—2005Chemicals, Mn  38,000 g/mol, Mw  63,000 g/mol)and poly(ether imide) (PEI, ULTEM 1000, General Elec-tric, Mn  26,000 g/mol, Mw  50,000 g/mol). Theirchemical structures are shown in Fig. 1.Samples PreparationModified epoxy resins containing 0 –15 wt% TP wereprepared using the following procedure. The TP weredissolved in methylene chloride (CH2Cl2) and mixedwith the resin. Most of the solvent was evaporated atroom temperature and then the samples were heated to80°C and kept at this temperature during 24 h undervacuum to eliminate the residual amount of solvent. Thesolution was heated (at 135°C for DDS and 90°C forMCDEA) and the diamine was dissolved by stirring forabout 5 min. Air bubbles introduced during this processwere removed, keeping the sample under vacuum at150°C for 10 min. The mixtures were immediatelypoured into molds to prepare the specimens to be used inthe mechanical characterization.Thick plates were obtained by casting the mixture intomolds consisting of two rectangular glass plaques cov-ered by a thin Teflon layer, spaced by a Teflon spacerand held together with clamps. The mixture was pouredinto the mold that was previously heated to 200°C andFIG. 2. Elastic modulus (E) as a function of the relative fraction of PSF,for 10 wt% PSF/PEI modified DGEBA-DA systems: (●) DDS; (E) MC-DEA; () PEI, PSF (obtained from 1General Electric catalog; 2PolymerHandbook, 1999 Oxford University Press [17]); (dotted lined) neat system.FIG. 1. Chemical structures of the epoxy monomer (DGEBA), the amines (DDS, MCDEA), and the TPs (PEIand PSF).POLYMER ENGINEERING AND SCIENCE—2005 1313degassed under vacuum during 10 min to remove trappedair. The chosen curing schedules were 2 h at 200°Cfollowed by 1 h at 220°C for DDS, and 4 h at 200°Cfollowed by 2 h at 220°C for MCDEA, in order to reachfull epoxy groups conversion. The cured plates wereslowly cooled down to room temperature and removedfrom the mold to be machined, in order to producerectangular bars for mechanical testing.Mechanical CharacterizationIn order to measure flexural modulus, neat and modifiedepoxy unnotched specimens were tested in three-pointbending configuration. The specimen cross-section was 5.5mm  11 mm with a support span of 80 mm. Flexuralmodulus was determined following the ASTM D790M-93standard recommendations [8] from the initial load-deflec-tion slope as:E  S3m/4BW3 (1)where B is the thickness, W is the width, S is the supportspan, and m is the slope of the initial straight-line portion ofthe load-deflection curve.Single-edge notched (SENB) fracture specimens werealso cut from plates 5.5 mm thick using a diamond saw.Sharp notches were obtained by tapping a fresh razor bladeinto a machined slot so as to grow a natural crack of 0.45 a/W  0.55, where a is the total crack length and W is thewidth of the specimen. The span-to-width (S/W) and thethickness-to-width (B/W) ratios were kept equal to 4 and0.5, respectively.Fracture tests were also performed in three-point bend-ing. The critical stress intensity factor at the onset of crackgrowth, KIC, was calculated in accordance with the ASTMD5045-91 standard [9], as:KIC  PQ/BW1/ 2 f x (2)f x  6x1/ 21.99  x1  x2.15  3.93x  2.7x2/1  2x1  x3/ 2 (3)where PQ is the critical fracture load as determined in theASTM standard, and x  a/W. The total crack length a wasmeasured from the fracture surface using a profile projector(10).All mechanical tests were carried out in an INSTRONdynamometer 4467 at a crosshead speed of 2 mm/min, atroom temperature. A minimum of five specimens weretested and average values were reported along with theirdeviations.Scanning Electron Microscopy (SEM)Fracture surfaces of TP modified SENB samples wereexamined by SEM, after coating the broken surfaces with athin gold layer, using a Jeol JXA-8600 microscope.RESULTS AND DISCUSSIONFigure 2 shows the flexural modulus values obtained forDGEBA-DA/PSF/PEI blends, containing 10 wt% total TPin different PSF/PEI proportions, as a function of PSFcontribution to the total TP concentration, along with thecorresponding value to the neat system (dotted line), curedat 200°C and postcured at 220°C.It can be seen from the graph that modulus values arehigher when DDS is used as hardener than in the case ofMCDEA. This behavior can be explained taking intoaccount the effect of the lower number of stable confor-mations afforded by the –SO2 group than the –CH2 group[10], which leads to higher Tg and modulus values as aFIG. 3. Critical stress-intensity factor (KIC) (a) and percent increase inKIC (	KIC/KIC (neat)) (b), as a function of the relative fraction of PSF for 10wt% PSF/PEI modified DGEBA-DA systems: (● DDS; (E) MCDEA;(dotted line) neat system. Morphology is indicated as: P, particulate; CC,co-continuous; D, “island” domains.1314 POLYMER ENGINEERING AND SCIENCE—2005FIG. 4. SEM of fracture surfaces for 10 wt% PSF/PEI modified DGEBA-MCDEA systems in differentPSF/PEI ratios: 0/100 (a); 25/75 (b); 50/50 (c); 25/75 higher magnification (d); and 50/50 higher magnification(e).POLYMER ENGINEERING AND SCIENCE—2005 1315consequence of the higher rigidity of the DGEBA-DDSnetwork.For both the DDS and MCDEA blends, the variationof the PSF proportion in the TP mixture did not show aclear influence over modulus values, even though PEImodulus is higher than PSF one. Similar values weredetermined for modified and unmodified systems. Then,the loss of stiffness that is a necessary consequence ofadding rubber particles is avoided when high Tg TPs areused.The critical stress intensity factor, KIC, and the relativetoughness improvement (	KIC/KIC(neat)), where KIC(neat)is the neat resin value) vs. wt% PSF for 10% of totalTP data are plotted in Fig. 3, for DDS and MCDEAblends, and compared with the neat systems values (dot-ted line). For DDS cured samples the resulting morphol-ogies are particulate, with TP rich particles dispersed inan epoxy-amine rich matrix, for all the PSF/PEI propor-tions. The replacement of part of the PEI by PSF regard-less of the PSF/PEI proportion leads to a slight, practi-cally constant, but clear increase in toughness. As anexception, the 75/25 blend, with an initial compositionclose to the miscibility limit, i.e., close to the composi-tion where the samples are initially phase separated, doesnot show a significant change in KIC value with respect tothe neat system, probably due to the fact that a non-uniform particle distribution is present. As demonstratedbefore [11] for PSF/PEI ratios near the miscibility limit,slight temperatures changes provoked the generation ofmorphology gradients, even in initially homogeneousblends.A quite different behavior is observed when MCDEA isused as hardener. Replacing 25% of PEI by PSF (25/75mixture) drives to a substantial change in morphology, froma particulate type (0/100 blend) (Fig. 4a), to a structurewhere the DGEBA-MCDEA rich-phase is continuous andthe TP rich phase shows co-continuity (Fig. 4b). Thischange in morphology leads to a 30% increase in toughness,as can be observed in Fig. 3b.In these modified-epoxy blends, the crack is forced toadvance through the TP-rich phase that is obviouslytougher than the epoxy-rich phase leading to an increasein toughness [12]. On the other hand, it has been wellestablished in literature [12, 13] that co-continuous mi-crostructures are more effective in toughening than par-ticulate morphologies.Modification with 10 wt% PSF/PEI—50/50 and 75/25ratios—leads to double-phase morphologies, characteris-tic of initially heterogeneous blends, consisting in mac-roscopic irregular co-continuous TP-rich domains (“is-land” domains) dispersed in an epoxy-rich matrix,containing dispersed TP-rich particles (Fig. 4c, 50/50).The TP-rich domains show morphologies where epoxydomains are connected by threads of TP. As it waspointed out in a previous work [14] based on X-raymicroanalysis, this TP-rich phase may be ascribed to becomposed of PEI. Ductile drawing of the TP-rich phase isalso observed on the fracture surfaces of both blends withco-continuous and island domains morphologies (Fig. 4dand e). Therefore, crack bridging by these ductile phasesis expected to contribute to the overall toughness. Inaddition, tails behind the island domains are clearly ob-served in Fig. 4c, evidencing the presence of the crack-pinning mechanism that also would consume additionalenergy [13–15]. However, based on SEM analysis offracture surfaces only, we cannot identify the majortoughening mechanism operating in our blends as dem-onstrated by Pearson and Yee [16]. Microcracking mightalso be occurring and crack bridging would contribute totoughening mostly by stabilizing the microcracks and,hence, by controlling the size of the microcracking zone[16].Nevertheless, the increase in the PSF/PEI ratio does notdrive to a toughness improvement, compared to the unmod-ified DGEBA-MCDEA system.In addition, the energy release rate at crack initiation(GIC) was calculated using KIC and E values [9]:GIC  KIC2 1  2/E. (4)Poisson’s ratio () was taken equal to 0.4 [15]. The obtainedGIC values vs. PSF wt% are plotted in Fig. 5. The tendencyis similar to the one observed in KIC curves.A quite different behavior is observed when blendswith 15 wt% of modifier and using MCDEA as hardenerare polymerized at 200°C. For these systems a smallincrease in PSF/PEI ratio provokes a drastic change inmorphology. While a mixture containing 15 wt% PEIshows a particulate morphology, blends with PSF/PEIratio 5/95 and 10/90 develop phase-inverted and co-continuous morphologies, respectively. In the first ofthese two cases (5/95), epoxy-rich domains appear dis-FIG. 5. Energy release rate at crack initiation (GIC) as a function of therelative fraction of PSF for 10 wt% PSF/PEI modified DGEBA-DA sys-tems: (F) DDS; (E) MCDEA; (dotted line) neat system.1316 POLYMER ENGINEERING AND SCIENCE—2005persed within a TP-rich matrix (not shown). In the secondcase (10/90), a cellular structure with certain degree ofco-continuity is achieved (Fig. 6). When a plate of thiscomposition was immersed in methylene chloride, it wasswollen by the solvent, and the continuous phase andpart of the epoxy-amine particles precipitated on it wereextracted. However, the specimen kept its shape, con-firming the existence of the above mentioned morphol-ogy.The described changes in morphology also affect thematerials fracture toughness (Fig. 7a and b). As Fig. 7bshows, replacing 5 and 10% of PEI by PSF, leads to a 25and 40% enhancement in KIC values, respectively. Whena 15% mixture in a 50/50 PSF/PEI ratio is analyzed,fracture surfaces SEM micrographs show that the finalstructure is formed by a high concentration of phase-inverted PEI-rich domains with an average diameter be-tween 200 and 700 m (“island domains”) (Fig. 8),which lead to an improvement in fracture toughness of35%. For a 90/10 blend (not shown) a particulate mor-phology is obtained again and, as would be expected, nonoticeable enhancement in KIC was observed.In agreement with the results obtained for 10 wt% ofmodifier, SEM analysis of the fracture surfaces confirmsthat the TP-rich phase undergoes ductile tearing; hence,crack bridging exists. Furthermore, tails near the islanddomains are observed in Fig. 8, suggesting that crack-pinning is also occurring in this microstructure and,hence, this mechanism would also contribute to the over-all toughness for this composition.CONCLUSIONSWe have analyzed the mechanical behavior of samplescontaining 10 and 15 wt% TP modifiers. We observedFIG. 8. SEM of fracture surface for 15 wt% PSF/PEI modified DGEBA-MCDEA system in 50/50 PSF/PEI ratio.FIG. 6. SEM of fracture surface for 15 wt% PSF/PEI modified DGEBA-MCDEA system in 10/90 PSF/PEI ratio.FIG. 7. KIC and GIC (a) and percent increase in KIC (	KIC/KIC (neat)) (b)as a function of the relative fraction of PSF for 15 wt% PSF/PEI modifiedDGEBA-DA systems: (● DDS; (E) MCDEA; (dotted line) neat system.Morphology is indicated as: P, particulate; I, inverted; CC, co-continuous;D, “island” domains.POLYMER ENGINEERING AND SCIENCE—2005 1317that when a small part of PEI is replaced by a secondpolymer like PSF, drastic changes in the final morphol-ogies [14] and then in fracture properties were obtainedwhen compared to the systems modified only with PEI.When the TP-rich phase shows continuity, great im-provements in toughness (up to 40%) are achieved.Hence, we can conclude that by varying PSF/PEI ratio, ata constant modifier total concentration, appropriate mor-phology changes can be induced. Therefore, materialswith better fracture properties can be obtained, keepingformulations viscosity constant. As an additional advan-tage, no important changes in elastic modulus of finalmaterials are found.REFERENCES1. J.L. Hedrick, I. Yilgör, G.L. Wilkes, and J.E. Mc Grath,Polym. Bull., 13, 201 (1985).2. J.H. Hodgkin, G.P. Simon, and R.J. Varley, Polym. Adv.Tech.,9, 3 (1998).3. J.P. Pascault and R.J.J. Williams, Polymer Blends: Volume I:Formulation and Characterization of Thermoset-Thermoplas-tic Blends, D.R. Paul and C.B. Bucknall eds., John Wiley &Sons, New York, 379 (2000).4. C.B. Bucknall and A.H. Gilbert, Polymer, 30, 213 (1989).5. E. Girard-Reydet, H. Sautereau, and J.P. Pascault, Polymer,40, 1677 (1999).6. M.J. Galante, J. Borrajo, R.J.J. Williams, E. Girard-Reydet,and J.P. Pascault, Macromolecules, 34, 2686 (2001).7. G. Peña, A. Eceiza, P. Valea, P. Remiro, P. Oyanguren, and I.Mondragón, Polym. Int., 52, 1444 (2003).8. ASTM Specification D790M-93, “Standard Test Methods forFlexural Properties of Unreinforced and Reinforced Plasticsand Electrical Insulating Materials [Metric],” in Annual Bookof ASTM Standards, American Society for Testing and Mate-rials, Philadelphia (1993).9. ASTM Specification D5045–93, “Standard Test Methods forPlane-Strain Fracture Toughness and Strain Energy ReleaseRate of Plastic Materials,” in Annual Book of ASTM Stan-dards, American Society for Testing and Materials, Philadel-phia (1991).10. E. Girard-Reydet, C.C. Riccardi, H. Sautereau, and J.P. Pas-cault, Macromolecules, 28, 7599 (1995).11. M.I. Giannotti, I. Mondragón, M.J. Galante, and P.A. Oyan-guren, Polym. Int., 54, 897 (2005).12. A.J. Kinloch, M.L. Yuen, and S.D. Jenkins, J. Mater. Sci., 29,3781 (1994).13. A.F. Yee, J. Du, and M.D. Thouless, “Toughening of Ep-oxies,” in Polymer Blends. Volume 2: Performance, D.R. Pauland C.B. Bucknall, eds., John Wiley & Sons, New York, 250(1999).14. M.I. Giannotti, I. Mondragon, M.J. Galante, and P.A. Oyan-guren, J. Polym. Sci., Part B: Polym. Phys., 42, 3964 (2004).15. E. Girard-Reydet, V. Vicard, J.P. Pascault, and H. Sautereau,J. Appl. Polym. Sci., 65, 2433 (1997).16. R.A. Pearson and A.F. Yee, Polymer, 34, 3658 (1993).17. J. Brandrup, ed., Polymer Data Handbook, Oxford UniversityPress, Oxford (1999).1318 POLYMER ENGINEERING AND SCIENCE—2005

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Morphology and fracture porperties relationship of epoxy-diamine systems simultaneously modified with polysulfone and poly(ether imide)

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Morphology and fracture porperties relationship of epoxy-diamine systems simultaneously modified with polysulfone and poly(ether imide)

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