PVT Behavior of Thermoplastic Poly(styrene-co-acrylonitrile)-Modified Epoxy Systems:Relating Polymerization-Induced Viscoelastic Phase Separation with the Cure ShrinkagePerformance

Jesmy Jose,† Kuruvilla Joseph,†,‡ Jurgen Pionteck,§ and Sabu Thomas*,†

School of Chemical Sciences, Mahatma Gandhi UniVersity, Priyadarshini Hills P.O., Kottayam, Kerala, India,Leibniz-Institute fur Polymerforschung Dresden e.V., Hohe Stra�e 6, 01069 Dresden, Germany

ReceiVed: March 7, 2008; ReVised Manuscript ReceiVed: August 4, 2008

The volume shrinkage during polymerization of a thermoplastic modified epoxy resin undergoing a simultaneousviscoelastic phase separation was investigated for the first time by means of pressure-volume-temperature(PVT) analysis. Varying amounts (0-20%) of poly(styrene-co-acrylonitrile) (SAN) have been incorporatedinto a high-temperature epoxy-diamine system, diglycidyl ether of bisphenol A (DGEBA)-4,4′-diamino-diphenyl sulfone (DDS) mixture, and subsequently polymerized isothermally at a constant pressure of 10MPa. Volume shrinkage is highest for the double-phased network-like bicontinuous morphology in the SAN-15% system. Investigation of the epoxy reaction kinetics based on the conversions derived from PVT dataestablished a phase-separation effect on the volume shrinkage behavior in these blends. From subsequentthermal transition studies of various epoxy-DDS/SAN systems, it has been suggested that the behavior ofthe highly intermixed thermoplastic SAN-rich phase is the key for in situ shrinkage control. Various microscopiccharacterizations including scanning electron microscopy, atomic force microscopy, and optical microscopyare combined to confirm that the shrinkage behavior is manipulated by a volume shrinkage of the thermoplasticSAN-rich phase undergoing a viscoelastic phase separation during cure. Consequently, a new mechanism forvolume shrinkage has been visualized for the in situ polymerization of a thermoplastic-modified epoxy resin.

Introduction

Addition of low amounts (up to 20 wt %) of engineeringthermoplastics (TP) is a promising way to overcome the intrinsicbrittleness of the classical epoxy resins, with the least compro-mise on the processability. Various TP-modified epoxy resinspossess enhanced fracture toughness,1-4 adhesion,5 thermome-chanical stability,6 etc. on account of their interesting micro-structural characteristics. With the epoxy glass transition as wellas the modulus being least affected upon TP addition, thesematerials are now increasingly replacing the conventionalunmodified high-temperature purpose epoxy resins. A keyapplication area includes the aerospace industry, where, thesehigh-impact resins make up superior matrices for the carbon,Kevlar, and Aramid fiber-reinforced composites that constitutethe various structural components of aircraft and rockets.7,8

During epoxy polymerization, the continuous replacement ofsecondary van der Waals bonds by primary covalent bondsresults in a volume shrinkage (ca. 4-7%). This is a seriousproblem in the area of material sciences, especially forcomposite materials. Being cured under geometrically con-strained environments, cure shrinkage generates large residualstresses that further lead to severe manufacturing problems suchas surface-quality flaws, shape distortions, warpage, etc., in theepoxy-based materials. When translated to thermoplastic incor-porated epoxies, the polymerization however proceeds via acompetitive series of transitions: the liquid-liquid phase separa-tion giving separate epoxy and TP-rich phases, gelation of the

epoxy phase, or a subsequent vitrification (Tg ) Tcure) of eitheror both of the epoxy- and thermoplastic-rich phases. Conse-quently, in addition to chemical cross-linking, the phase-separation characteristics, such as the mechanism and coarseningkinetics and nature and type of morphologies, would alsopresumably influence the in situ shrinkage behavior of thesemodified resins. Studies on internal stress in polyester-modifiedepoxies by Cao et al.9 revealed a composition-dependentbehavior that was attributed to the influence of thermoplasticpolyester on the epoxy cross-link density. In the case of TP-modified polyester resins, the phase separation favorably offeredan effective shrinkage control via a “low profile” mechanism,which is determined by the morphological changes duringcure.10-12 However, the volume-shrinkage behavior of epoxy/thermoplastic systems remains unexplored, even though it isvery significant from both scientific and industrial viewpoints.

It is now generally accepted that a predominant Viscoelasticphase separation13 follows the spinodal demixing during thepolymerization-induced phase separation (PIPS) in dynamicallyasymmetric epoxy/TP systems to give mechanically favorablefinal network-like and sponge-like morphologies.14-16 The largedifference in Tg, molecular weight, or both between the epoxyoligomer and thermoplastic component induces a dynamicviscoelastic asymmetry, which on coupling with concentrationfluctuations, results in a viscoelastic phase separation (VPS)during PIPS. Here, the relaxation time of the interaction networkof slow fluid (thermoplastic) being longer because of the lowmobility (or large size), therefore, fails to catch up with thecharacteristic time of phase separation and leads to a viscoelasticphase separation. VPS is characterized by unique coarseningbehaviors like phase inversion, TP volume shrinkage, etc., whichare never observed for conventional phase separation. The

* Corresponding author. E-mail: sabut552001@yahoo.com.† Mahatma Gandhi University.‡ Current address: Indian Institute of Space Science and Technology,

Department of Space, IIST Post, Thiruvananthapuram, Kerala, India.§ Leibniz-Institute fur Polymerforschung Dresden e.V.

J. Phys. Chem. B 2008, 112, 14793–14803 14793

10.1021/jp802015n CCC: $40.75 2008 American Chemical SocietyPublished on Web 10/31/2008

interplay between phase-separation (or critical) dynamics andthe slow dynamics of the thermoplastic material initiallygenerates transient phase-inverted systems having domains ofthe fast dynamic epoxy-rich phase in slow dynamic thermoplastic-rich phase matrix. Then these domains grow further to givenetwork-like or sponge-like structures. Finally, the slow dynamicepoxy-rich phase develops into a dispersed phase by volumeshrinkage. Among the few researchers, Zheng et al.14 studiedthe coarsening dynamics for PEI-modified epoxy resin anddescribed successfully the temperature-dependent relaxation timefor the viscoelastic flow of epoxy oligomers using the WLFequation. Additionally, a simultaneous secondary phase separa-tion (SPS), which yields complex phase-in-phase microstruc-tures, has also been visualized in several epoxy/TP systems.17-20

Therefore, any investigation on the epoxy resin properties byconsidering the possible effects of these complex events wouldcertainly serve toward better end-use applications.

In this context, we describe the in situ volume shrinkagebehavior of a thermoplastic SAN-modified diglycidyl ether ofbisphenol A (DGEBA) epoxy by means of PVT analysis. Theinitially homogeneous epoxy-4,4′-diaminodiphenyl sulfone(DDS)/SAN mixtures undergo a reaction-induced phase separa-tion during cure. Because the kinetic interplay between phaseseparation and cross-linking is the key to morphology selection,this paper exploits the phase separation-microstructure-cureshrinkage performance correlation in SAN-incorporated epoxyresins. The reasons for selecting SAN for the present study areas follows: (i) Since the SAN glass transition (Tg ) 105 °C) islower than the isothermal cure temperatures (Tcure )140 and160 °C) selected, the vitrification of the SAN-rich phase neverhappens during cure. (ii) Because of the very high SANmolecular weight (Mw) 54 800 g/mol), compared with theepoxy oligomer, the dynamic asymmetry would favorablyinduce a viscoelastic phase separation during cure.

Moreover, from our recent studies, the SAN-modified epoxymatrices were found capable of imparting augmented mechanicalstrength as well as thermomechanical stabilities in glass fiberreinforced composites.21

Experimental Section

Materials and Samples Preparation. The epoxy precursorused, a diglycidyl ether of bisphenol A (DGEBA), is a liquidat room temperature with a low degree of polymerization (n )0.15) (LY556 manufactured by Ciba Geigy and kindly suppliedby Vantico Polymers, India). The curing agent was an aromaticdiamine with intermediate reactivity, 4,4′-diaminodiphenylsulfone (DDS), supplied by Atul Products India, Ltd. Theamorphous thermoplastic styrene-co-acrylonitrile copolymer(Mw) 54 800 g/mol; Tg ) 105 °C, acrylonitrile content ) 33wt %) was procured from Bayer Chemicals, Belgium.

The blends were prepared by initially dissolving SAN inepoxy resin by continuous stirring at 180 °C in a N2 atmosphere.The DDS hardener, taken in stoichiometric ratio of epoxy toamino hydrogen’s (equal to one) was then added at a lowertemperature of 140 °C. The clear solution was then quenchedin liquid nitrogen and stored in a freezer to prevent any curingbefore the actual measurements started. Blends with SANcontent varying between 0 and 20 g in the epoxy-hardenermixture (100 g DGEBA + 32 g DDS) were prepared, and thesamples were named as neat epoxy, SAN-5%, SAN-10%, SAN-15%, and SAN-20% blends.

Measurements. Specific volumes were measured using afully automated GNOMIX high-pressure dilatometer capableof detecting volume changes as small as 0.0002 cm3/g within

an accuracy limit of 0.002 cm3/g (below 200 °C). The apparatusis described elsewhere.22 Ultrahigh-vacuum samples embeddedin a nickel foil cup were loaded in the liquid Hg filled PVTsample cell and were isothermally scanned for specific volumesat a constant pressure of 10 MPa. In order to check whetherany cross-linking has taken place during the sample preparationstages, the experimental initial specific volumes were comparedwith those calculated from the densities of the individualcomponents by assuming an additive behavior (Table 1). Thelower values for experimental specific volumes indicate a certaindegree of cross-linking; however, the deviation was within theacceptable range (less than 1%).

The cloud-point measurements were performed on a built-insmall-angle light scattering (SALLS) set up equipped with a 5mW He-Ne laser (λ ) 638.2 nm). After necessary calibrations,a few milligrams of the sample was placed between twomicroscopic glass slides and cured on the hot stage at 160 °C.The onset time of liquid-liquid phase separation was determinedfrom the surge in the scattering intensities being recorded as afunction of time using built-in software (custom-made TestPointapplication). Thermal transitions of the cured samples weredetermined by measuring the loss modulus (E′′ ) as a functionof temperature using a dynamic mechanical thermal analyzerat a heating rate of 3 °C/min (TA instruments DMA 2980operating in a dual cantilever mode at 1 Hz). Samples obtainedafter curing at 160 °C for 6 h with approximate dimensions of20 mm × 3 mm × 5 mm were used.

Morphology development during polymerization was studiedusing an optical microscope (Olympus BH2). A few milligramsof the sample was sealed between two glass slides and placedin a Mettler FP82-HT hot stage at 160 °C. Digital micrographswere taken at several curing times with a JVC TK-C1381 CCDcamera and analyzed by the program Qwin from LeicaCompany. The final blend morphologies were investigated usingscanning electron microscopy (SEM) and atomic force micros-copy (AFM). The cured samples from PVT run were cleanedto remove the mercury. To get a better phase contrast, thethermoplastic SAN was preferentially etched out using THF atroom temperature, and the samples were vaccuum-dried, sput-tered with gold, and scanned using a Leo 435 VP scanningelectron microscope. In blends where an etching with THFruined the original blend morphology, atomic force microscopy(AFM) was performed on the unetched cryo-smoothenedsamples. AFM pictures were captured in tapping mode by meansof a NanoScope Dimension 3100 (Veeco).

Results and Discussion

Determination of the Volume Shrinkage in Epoxy-DDS/SAN Blends from PVT Results. Pressure-volume-tem-perature analysis is a well-known probe for studying the volumechanges accompanying phase transitions in thermoplastic poly-mers. However, for thermosets, the specific volume variations(PVT behavior) could be used to investigate the volumeshrinkage during polymerization. Under isothermal polymeri-

TABLE 1: Experimental and Calculated Initial SpecificVolumes for Epoxy/SAN Blends at 160 °C

% SANspecific volume(exptl), cm3/g

specific volume(calcd), cm3/g

0 0.8867 0.89435 0.8891 0.897210 0.8887 0.899915 0.8978 0.902520 0.8983 0.9049

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zation conditions, the specific volume changes (at fixed T andP) directly reflect the internal structure and volume-averagedinteractions in the sample without any hindrance from the glasstransition and thermal expansion coefficient.

Figure 1 gives the specific volume data obtained for thevarious epoxy-DDS/SAN mixtures studied at 140 and 160 °C.Irrespective of blend composition, the volume decays exponen-tially due to polymerization and remains constant in the finalstages of cure. The beginning of the final plateau regioncorresponds to gelation or vitrification processes; hereafter, thespecific volume change is less sensitive. The raw specificvolume data seems to give some information of a SANconcentration effect. For both isothermal cure temperatures(Figure 1), at certain intermediate stages of cure, the specificvolume curves for the SAN-10% and SAN-15% systems showa cross-over, whereby the SAN-15% system gave slightly lowerspecific volumes than SAN-10% blend in the final cure stages.From the specific volume values, the percentage volumeshrinkage at any time t could be calculated as

% volume shrinkage) [1-V (t)

blend

V (t)0)SAN +V (t)0)

epoxy] × 100 (1)

where V(t ) 0)SAN and V(t ) 0)

epoxy are the initial volumes of pure SANand neat epoxy-DDS mixtures at time t ) 0. V (t)

blend is thevolume of the blend at any time t calculated from theexperimental blend specific volumes using eq 2:

Vtblend )V spt

blend�blend (2)

V sptblend is the experimental specific volume for blends at any time

t and �blend represents the total weight of the blend in grams.Equation 3 gives the final shrinkage values normalized withrespect to the epoxy volume content in the blend.

% shrinkagenormalized ) (1-V (t)

epoxy

V (t)0)epoxy) × 100 (3)

Figure 2 gives the shrinkage evolution profile for epoxy-DDS/SAN samples cured isothermally at 160 °C. Among the blends,with increasing SAN content, the percentage shrinkage de-creased progressively up to a certain cure time (around 50 min).Interestingly, thereafter, the SAN-15% system exhibited rela-

tively higher volume shrinkage than the SAN-5% and SAN-10% blends. In any case, in the absence of any phase separation,at any given cure time the thermoplastic SAN would merelydilute the epoxy-DDS reaction volume causing a progressivedecrease in the relative volume shrinkage with the increasingSAN concentration. Therefore, in epoxy-DDS/SAN samples,the simultaneous thermoplastic phase separation during epoxypolymerization would possibly account for the experimentallyobserved (Figure 2) anomalous trend in the shrinkage evolutionas determined by various factors, such as stoichiometricvariations, phase compositions, and interfaces. The inset inFigure 2 gives the final cure shrinkage (from eq 2) togetherwith the normalized values (from eq 3) as a function of theSAN content. In either case, the volume shrinkage decreasedlinearly up to the SAN-10% system, then shows an upturn givinga maximum shrinkage for the SAN-15% system, thereby furtherdemonstrating the profound thermoplastic phase separation effecton volume shrinkage.

Phase-Separation Effect on the Volume Shrinkage Kinet-ics of Epoxy-DDS/SAN Blends. In view of the PVT data ageneral equation of state for the specific volume (V) dependenceon temperature (T), pressure (P), and conversion (X) for epoxypolymerization could be expressed as23

dVV

) 1V(∂V

∂T )P, XdT+ 1

V(∂V∂X)P,T

dX+ 1V(∂V

∂P)T, XdP (4)

For curing at constant T and P the volume changes betweenthe times 0 and t could be then given by eq 5:

∫0

tdV)∫0

XVtR(X,T,P) dX (5)

where R(X,T,P) ) 1/V(∂V/∂X)P,T is a volumetric coefficient asso-ciated with polymerization. For neat epoxy systems, a likelyhypothesis is to assume a linear relationship between epoxyconversion and cure shrinkage, and then the conversion X couldbe represented as24,25

X)V t

′-V 0′

V∞ -V0(6)

where V t′ and V 0

′ are the specific volumes at time t and at t )0, respectively, for each cure temperature. V 0

′ and V∞ are the

Figure 1. PVT data for SAN-modified epoxy-DDS blends duringisothermal cure at 140 (closed symbols) and 160 °C (open symbols) ata constant pressure of 10 MPa: (9,0) neat epoxy; ([,]) SAN-5%;(2,4) SAN-10%; (b,O) SAN-15%; (1,3) SAN-20%.

Figure 2. Volume shrinkage vs time plots for epoxy-DDS/SANblends having different SAN concentrations (%) during the isothermalcure at 160 °C at a constant pressure of 10 MPa: (0) neat epoxy; (])SAN-5%; (4) SAN-10%; (O) SAN-15%; (3) SAN-20%. The insetgraph gives the net volume shrinkage in relation to the blend volume(b) and normalized to the volume of the epoxy phase (9) (the dottedlines are to guide the eyes).

PVT Behavior of Thermoplastic SAN-Epoxy Systems J. Phys. Chem. B, Vol. 112, No. 47, 2008 14795

initial and final specific volumes at a reference temperatureconsidered as the highest temperature used, which in our casewas 160 °C.

The step polymerization of an epoxy-amine system is wellestablished in the literature (Scheme 1): an oxirane ring openingby the primary amine hydrogen (reaction 1) gives a secondaryamine that further reacts with another epoxy group to create atertiary amine (reaction 2). Recently Ramos et al.26 have derivedthe epoxy reaction kinetics based the conversion values obtainedfrom the PVT analysis of an epoxy-MCDEA hardener mixtureby measuring the specific volumes at varying isothermaltemperatures and pressures. They noticed a predominant tem-perature effect on the kinetic rate constants. For epoxy-aminesystems, the phenomenological autocatalytic model proposedby Kamal27,28 is found to be the most conceivable.29-31 Thegeneralized rate expression for a non-zero initial reaction rateis given by eq 7:

dXdt

) (k1 + k2Xm)(1-X )n (7)

where k1 and k2 are the apparent rate constants for the reactionsautocatalyzed by the hydroxyl groups initially present and thosegenerated during cure; m and n are the order of the reaction.The above model gave very good fitting for neat epoxy at alltemperatures studied (results are not discussed here).

In thermoplastic SAN-modified epoxy-DDS blends, aninvestigation of the cure kinetics based on conversions fromPVT data would enable a quantitative determination of any phaseseparation effect on shrinkage. Figure 3a-c illustrates the resultsof the kinetics analysis for epoxy-DDS/SAN samples usingthe autocatalytic model. In the case of blends, the diluent effectof SAN has been taken into account during modeling becauseit does not affect the values of the apparent rate constants. Notethat no chemical reactions between SAN and either epoxy orDDS were noticed from our earlier IR studies. The symbols(Figure 3a-c) represent experimental conversion (from PVTmeasurements at 160 °C) for the 10%, 15%, and 20% SAN-modified blends measured at a constant pressure of 10 MPa.The solid lines in each graph represent the theoretical conversionvalues obtained by a differential of eq 7 resolved using a fourth-order semi-implicit Runge-Kutta method32,33 with k1 and k2 asfitting parameters. The cloud-point time determined by lightscattering (indicated by arrows) represents the beginning of thephase-separation process. In the homogeneous region (beforephase separation in the range of cloud-point arrows), a goodagreement between the experimental and theoretical curves hasbeen observed. In terms of volume shrinkage, the above findingevidently establishes the dilution effect by SAN on epoxyvolume shrinkage in the miscible state. However, at the onsetof phase separation (marked by arrows), the experimentalconversion curves deviated from the theoretical values, therebyestablishing for the first time a phase-separation effect on volume

shrinkage that is being manifested by a possible couplingbetween phase-coarsening kinetics and the reaction kinetics. Thismeans that in the phase-separated regime, the experimentalconversion values account for the combined volume changesassociated with both phase separation and polymerization. Asseen from Figure 3, beyond the onset of phase separation, therelative extent of conversion at a fixed time is found to vary as10% < 15% > 20%, which agrees with our earlier findings onthe relative shrinkage evolution among the blends (Figure 2).Since the extent of conversion due to volume changes may varyin the separate emerging epoxy- (R1) and SAN-rich (�1) phases,the term “average conversion” is better used to represent theexperimental conversion values. Considered in terms of chemicalreaction rates, the above observed deviations in experimentalconversions during the phase separation of nonreactive SAN-modified epoxy systems could be due to numerous factors,stoichiometric variations, phase compositions, etc., which arebeing further investigated.

Remarkably, a failure of the presumed linear relationshipbetween shrinkage and conversion would also contribute towardthe altered experimental conversion obtained for epoxy-DDS/

SCHEME 1: Curing Reactions in DGEBA-DDS Systems

Figure 3. Experimentally determined (symbols) average epoxy conver-sion (Xavg) vs time for various epoxy-DDS/SAN systems cured at 160°C and 10 MPa having (a) SAN-10%, (b) SAN-15%, and (c) SAN-20%. The solid lines are the theoretical values (see text). The arrowsindicate the onset of phase separation obtained from light scatteringstudies.

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SAN blends. In reactive systems undergoing a polymerizationinduced phase separation, a possible difference in reaction rateand modulus of the emerging phases would generate internalstresses at the interface. In low profile additive thermoplastic(LPA)-modified polyester resins, during PIPS, the release ofinternal stresses via microvoids caused a substantial volumeexpansion, which further counteracts the simultaneous polym-erization shrinkage and thereby causes a rapid decline in thenet volume shrinkage values.10-12 However, in epoxy-DDS/SAN systems, the rather monotonous evolution of the shrinkagecurves (Figure 2) would indicate the absence of any suchcompetitive volume expansion, which could be explained asfollows. During cure, the development of internal stresseshappens only when the polymerization shrinkage is greater thanthe “resistance” force possessed by the collective interfacialattributes constituting the phases. Generally in thermoplastic-based epoxy systems, the gelation happens at a much later stageof cure, usually around 57%, which is well beyond phaseseparation. Therefore in the initial and intermediate stages ofcuring, the stress generated at the interface is not sufficient toinitiate any volume expansion. Besides, in epoxy-DDS/SANblends, because the SAN glass transition is always much lowerthan the experimental isothermal temperature (140 and 160 °C),a higher stress relaxation due to the viscoelastic deformationof SAN chains tends to prevent the growth of microcracks. Thetopographical analysis of the SEM micrographs of the unetchedepoxy-DDS/SAN blend sample surfaces (not given here)obtained after PVT run failed to display any microvoids in bothepoxy- and SAN-rich phases and therefore further corroboratesthe proposed negligible interfacial contribution toward thevolume shrinkage.

Investigation of the Morphologies and Phase Behavior ofSAN-Modified Epoxy-DDS Blends. The morphologies of thesamples obtained after the isothermal PVT run at 160 °C werestudied using SEM and are illustrated in Figure 4A-G. In theSEM micrographs B, C, D, and F, the dark areas correspond tothe regions from which the SAN phase (�1) has been etchedout using THF, and the relatively bright area indicates the epoxy-rich (R1) phase. The unmodified neat epoxy gives an invariantmorphology quite evident of a homogeneous system (Figure4A). The microstructure of SAN-5% and SAN-10% blendsgiven in Figure 4C,D consists of primary spherical SAN-richdomains dispersed in the continuous epoxy-rich matrix. At ahigher magnification in the case of the SAN-10% sample (Figure4D), the primary SAN domains disclosed secondary intercon-nected epoxy globules (R2) that have emerged via a secondaryphase separation. For the sake of simplicity, the morphologicalimplications of SPS will be discussed separately.

The SAN-15% system exhibits “macrophase”-separated mor-phology (Figure 4E). In the figure, the relatively dark arearepresents the smoothened epoxy-rich (R1) region and the brightcoarse regions constitute the SAN-rich (�1) phase. Such aprimary network-like morphology consisting of large thermo-plastic-rich domains exhibiting irregular shapes and dispersedin an epoxy-rich continuous phase that is very close to thecocontinuous phase structure is characteristic of a predominantviscoelastic phase separation during PIPS.34,35 At an even higherSAN content, that is, for the SAN-20% system, the finalmorphology consists of a phase-inverted sponge-like morphol-ogy, again distinctive for viscoelastic phase separation. Figure5A shows the AFM picture of the unetched SAN-20% blendshowing large dispersed epoxy globules of average size of15-25 µm embedded in a continuous SAN-rich (�1) phase. Hereit was noticed that the dispersed epoxy globules are not

interconnected as evidenced by a complete destruction of thespecimen while etching with THF.

A careful analysis of the SEM, as well as AFM, micrographsreveals double-phase structures due to a secondary phaseseparation in either or both epoxy- and thermoplastic-richprimary phases. Figure 4C,D,G gives a closer view of secondaryepoxy particles (R2) in SAN-rich (�1) matrix. Except for theSAN-20% system, the secondary epoxy particles existed asinterconnected spherical globules in the primary thermoplasticSAN-rich phase as verified from the intact nature of the epoxyglobular structure after THF treatment. Again, the secondarySAN particles were seen to be randomly dispersed domains inthe epoxy-rich primary phases of SAN-15% (Figure 5F) andSAN-20% (Figure 5D) systems.

The phase behavior of the above complex double-phasestructures was investigated using dynamic mechanical thermalanalysis (DMTA). Two separate transitions in the loss modulusvs temperature plot (Figure 6) indicate phase separation; themodulus peak at lower temperature corresponds to the SAN-rich phase (�1) and that at higher temperature is due to therelaxation of the cross-linked epoxy-rich phase (R1). Theincrease in the height of the R relaxation peak of SAN withincrease in SAN content demonstrates the shift in primarymorphology from a particulate to phase inversion and thereforecorroborates the morphology results.

The glass transition is a parameter sensitive to the degree ofcure and, furthermore is a very good probe for stoichiometricimbalances. In any case, a deviation of 5% in the epoxy-aminemixture composition, for example, would result in a change of5 °C in the full cure glass transition of neat epoxy-DDSsystem.36 The above notion was considered to examine thepossible deviations in the epoxy-amine ratio of the primaryepoxy-rich phase in stoichiometric epoxy-DDS/SAN blends.It has been found that the composition of the primaryepoxy-DDS-rich phase of these systems is close to pureepoxy-amine for cross-linking beyond the onset of phaseseparation.37 For isothermal curing at 160 °C, upon thecompletion of cure, the absence of differential segregation wouldresult in an epoxy-phase glass transition close to that of theneat epoxy-DDS. The glass transition of the epoxy-rich phase(at 160 ( 2 °C) in epoxy-DDS/SAN blends shows nosignificant deviation from that of the neat epoxy-DDS; hence,denying any possible epoxy-DDS stoichiometric imbalancesin the epoxy-rich phase during reaction-induced phase separa-tion. Similar unshifted final epoxy glass transitions have beenoften reported for low Tg (Tg of thermoplastic < Tcure)thermoplastic-modified epoxy blends.38,39

In blends, the SAN-rich phase exhibited broad relaxationpeaks (their widths vary from 30 to 40 °C for 5% and 10%SAN containing blends, and 60 °C for the 15% SAN-containingsystem) having an onset around 100 °C that is very close to theTg of neat SAN (Figure 6). Besides, the peak maxima wereshifted to higher temperatures and the extent of Tg increasefollowed a descending trend with increasing SAN content.Earlier, the morphology studies on the various blend systemsrevealed secondary epoxy particles present in the highlyintermixed SAN-rich phase. So, the broader relaxation peak aswell as the increased thermoplastic glass transition observedfor the highly intermixed SAN-rich primary phase in variousepoxy-DDS/SAN blends could be attributed to the closerrelaxation temperatures corresponding to SAN phase and thatdue to the partially/fully phase-separated cross-linked secondaryepoxy substructures, the latter having a lower degree of cross-linking than the primary epoxy-rich phase. This finding is

PVT Behavior of Thermoplastic SAN-Epoxy Systems J. Phys. Chem. B, Vol. 112, No. 47, 2008 14797

particularly relevant for SAN-15% blend system, where theprimary epoxy phase relaxation, reduced to a shoulder of themain peak at 111 °C, contrary to an expected relatively dominantseparate peak for the primary epoxy-rich major phase (Figure6). As verified from the morphological results, in SAN-15%blend, the relatively large volume fraction and the possible highcross-link densities of the secondary epoxy substructure in theintermixed SAN-rich primary phase accounts for the aboveinteresting relaxation behavior.

The morphological results together with glass transition andthe final volume shrinkage of the completely cured epoxy-DDS/

SAN blends are summarized in Table 2. The glass transition isdirectly related to specific volume. Thus, depending on the glasstransition behavior, the corresponding cross-linking densitiesof the epoxy chains present in the separate primary epoxy-richphase, as well as for those in the highly intermixed SAN-richphase, would influence the relative volume shrinkage behaviorof epoxy-DDS/SAN blends. Having established an intact glasstransition for the primary epoxy-rich phase with an undisturbedepoxy-amine stoichiometry during phase separation and alsoconsidering the discussions pertaining to the broad relaxationbehavior of the highly intermixed SAN phase, we suggest that

Figure 4. SEM micrographs of various epoxy-DDS/SAN blends after 7 h of isothermal curing at 160 °C and 10 MPa from PVT analysis: (A)unmodified epoxy; (B) SAN-5% blend; (C) SAN-10% blend; (D) SAN-10% (magnified phase-in-phase structure); (E) SAN-15% blend; (F) SAN-15% (magnified view of the epoxy-rich phase with secondary SAN domains); (G) SAN-15% (magnified view of the SAN-rich phase with secondaryepoxy globules).

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the behavior of the highly intermixed SAN-rich phase illustratedby both the composition and morphological disposition duringpolymerization-induced phase separation would preferentiallydetermine the relative volume shrinkage behavior of the variousepoxy-DDS/SAN blends.

Phase Structural Evolution during PIPS in Epoxy-DDS/SAN Systems. It is common knowledge that the modifierconcentration has a strong influence on both the phase-separationmechanisms and the microstructural attributes of reactivepolymer systems. In epoxy-TP blends (usually for TP < 30%),a spinodal decomposition (SD) has been generally accepted asthe most conceivable mechanism of phase separation. Figure 7shows the optical micrographs during phase separation in SAN-10% blend at 160 °C. At the onset of phase separation, asvisualized by Figure 7a, the well-interconnected initial bicon-tinuous morphology establishes a spinodal demixing mechanismof phase separation. A consequent phase coarsening by inter-diffusion looses the interconnectivity of the minor SAN-richphase (Figure 7b) in the SAN-10% system to finally yield aSAN-dispersed microstructure (Figure 7c).

Careful analysis of the primary SAN-rich phase disclosed finesecondary epoxy substructures at the early stages of primaryphase separation. A consensus is yet to be established regardingthe possible reasons behind this scientifically debated complex

phenomenon observed for phase-separating reactive systems.In a recent study, Tang et al.18 have suggested that a predominanthydrodynamic effect induced by the faster phase separation rateinduced secondary phase separation in the phase-invertedmorphologies of epoxy-PES blends. In a broader sense,generally for polymer mixtures, the phenomenon of secondaryphase separation emerges through at least four processes: (a)noninstantaneous temperature changes;40,41 (b) altered thermo-dynamic conditions, for example, by cross-linking42 or confor-mational ordering;43 (c) mass transfer limitations;44 (d) rapidhydrodynamic coarsening.45 Process (a) is not feasible underthe present experimental conditions of isothermal curing.Because the gelation is well-separated from phase separation,no semipermeable membrane is formed during phase separationin epoxy-SAN blends, and hence process (c) is not interesting.Since the curing of epoxy involves a continuous quench, theequilibrium compositions are not constant; they move apart. Asa result, the coexisting phases have to change their compositioncontinuously by material exchange. Because of a rapid hydro-dynamic coarsening and the increasing distances, materialexchange by diffusion becomes more and more difficult, andat a certain time, the diffusion becomes too slow to follow thestructure coarsening resulting in secondary demixing inside theprimary domains. Thus in epoxy-SAN blends, a combinedeffect due to cross-linking and the rapid hydrodynamic coarsen-ing of the initial bicontinuous structures would probably be themain reason for secondary phase separation. Concomitantly,recent studies on the PIPS effect on reaction kinetics ofepoxy-TP systems with a high TP content (TP > 20 wt %)have indicated that the relative interdiffusion rates between thecoexisting heterogeneous primary epoxy- and thermoplastic-rich phases competes with chemical reaction rate duringPIPS.36,38,39 Due to its complexity, comprehensive analyses ofSPS in epoxy-TP blends are limited, which would be interestingtoward the development of novel materials with unique proper-ties. As far as the present study is concerned, the preferentialinterdiffusion between the primary and secondary phases as aresult of secondary phase separation might be the reason forthe earlier findings of unshifted epoxy-rich phase glass transitionas well as for the existence of a highly intermixed SAN-richphase in the SAN-modified systems.

Among the various blends, the SAN-15%, having a network-like macrophase structure, exhibited the maximum volumeshrinkage during cure. The phase structural evolution for SAN-15% blend at 160 °C was studied using optical microscopy and

Figure 5. AFM image of (A) SAN-20% blend, (B) SAN-20% (magnified view), (C) secondary structures in SAN-rich matrix, and (D) secondarystructures in epoxy-rich dispersed phase.

Figure 6. Loss modulus vs temperature plots of the epoxy-DDS/SAN samples having varying SAN contents being measured after 6 hcure at 160 °C: (0) neat epoxy; (9) neat SAN; (]) SAN-5%; (4) SAN-10%; (O) SAN-15%; (3) SAN-20%.

PVT Behavior of Thermoplastic SAN-Epoxy Systems J. Phys. Chem. B, Vol. 112, No. 47, 2008 14799

can be described as follows. As shown in Figure 8a, the onsetof phase separation for the SAN-15% system is visualized bythe formation of initial bicontinuous structures due to a spinodaldemixing mechanism. Thereafter, in contrast to the expectedspinodal coarsening dynamics, the minority SAN phase formsthe matrix containing nucleated epoxy dispersed spheres (Figure8b). This is due to viscoelastic phase separation.12

According to various theories of viscoelastic phase separa-tion13 and as also recently reported by Li and co-workers15,34

for TP-modified epoxies, the pattern evolution during viscoelas-tic phase separation is governed by a competitive dominancebetween deformation (τd, characteristic time of deformation thatrepresents the composition fluctuations between the SAN- andepoxy-rich phases) and the rheological time scales (τts, char-acteristic rheological time of the slower thermoplastic SANphase representing the elastic field). Upon phase separation, τd

decreases rapidly at first with increasing composition difference

and then increases with the domain size, while τts increasessteeply with the increased composition difference and becomesalmost constant in the late stage. Accordingly, VPS proceedsthrough three main stages as explained below for the SAN-15% blend.

Figure 8b represents the initial diffusion stage (τd > τt) wherethe SAN viscoelastic effects prevent the growth of the normalcomposition fluctuations resulting in epoxy nucleated spheres.Here it is noteworthy that the primary SAN-rich matrix existsas a coarse-grained highly intermixed phase comprising epoxysubstructures due to an early secondary phase separation.Subsequently (Figure 8c-e), the primary epoxy spheres growby the transport of the epoxy oligomers from the highlyintermixed SAN-rich primary matrix toward the epoxy-richholes making the SAN phase increasingly elastic. This isaccompanied by a volume shrinkage of the highly intermixedSAN-rich phase in the elastic regime (τd < τts). Interestingly, a

TABLE 2: Summary of the Volume Shrinkage, Glass Transition Behavior, and Morphological Observations Obtained forEpoxy-DDS/ SAN Blends Cured at 160 °C

glass transition temperature (°C)

sample temperature (°C) morphologyepoxy-richphase (R1)

SAN-richphase (�1) volume shrinkage (%)

volume shrinkagenormalized (%)

neat epoxy 160 homogeneous 160 7.3 7.3SAN-5% 160 SAN dispersed 160 116 6.6 7SAN-10% 160 SAN dispersed 161 113 5.7 6.4SAN-15% 160 network-like bi continuous 158a 111 6.2 7.2SAN-20% 160 sponge-like phase inversion 108 5.4 6.7SAN-100% 160 homogeneous 104

a The relaxation at 158 °C is reduced to a shoulder of the broad main relaxation peak at 111 °C.

Figure 7. Optical microscopy pictures of SAN-10% blend cured during different times at 160 °C: (a) 18, (b) 20, and (c) 35 min.

Figure 8. Optical microscopy pictures of SAN-15% blend cured during different times at 160 °C: (a) 21, (b) 22, (c) 23, (d) 24, (e) 25, and (f) 36min.

14800 J. Phys. Chem. B, Vol. 112, No. 47, 2008 Jose et al.

simultaneous diffusion of the epoxy oligomers toward thesecondary epoxy phase also happens in conjunction with thegrowth of the primary epoxy spheres as evident from the rapidcoarsening of the secondary epoxy globules visualized in Figure8c-e. Furthermore, the viscoelastic phase separation theorynecessitates the existence of a “transient gel-like” structure forthe more viscoelastic phase in order to undergo the so-calledvolume-shrinkage phenomenon during VPS.13 Therefore, therather “permanent gel-like” network structure constituting ahighly intermixed SAN-rich phase containing interconnectedsecondary epoxy globules validates the observed volumeshrinkage during viscoelastic phase separation in SAN structuralevolution for SAN-15% blend. In the final stages, the relaxationof dynamic asymmetry occurs, and the interfacial tensiondominated hydrodynamic regime (τd > τts) yields a finalnetwork-like morphology having a predominant epoxy majorphase (Figure 8f). Thus during viscoelastic phase separation,in addition to the cure shrinkage owing to polymerization, avolume shrinkage of the highly viscoelastic SAN-rich phaselikely contributes to the net volume shrinkage, which furthertoward the final cure stage is being manifested by an apparentcross-linking of the secondary epoxy substructures present inthe intermixed SAN-rich primary phase.

Volume Shrinkage Mechanism in Epoxy-DDS/SANBlends. In view of the results and discussions so far, we candivide the dynamics of volume shrinkage control in epoxy/SANblends into five stages as given by the schematic diagram inFigure 9. Stage 1 corresponds to the early induction stage, wherethe system starts as a homogeneous fluid mixture consisting ofepoxy resin, thermoplastic, and the hardener (assuming nomixing effects). Hereafter, due to the epoxy cross-linking, thevolume shrinkage increases progressively with time. Thethermoplastic SAN dilutes the oxirane ring concentration, and,therefore, in the homogeneous regime, the relative rate of

shrinkage among the blends decreases proportionally withincreasing SAN content. Here, no volume contributions fromSAN chain deformation are expected by the pressure effectbecause SAN exists in the melt state (SAN Tg being lower thanthe cure temperature).

At different varying extents of conversion, determined byvarious thermodynamic parameters, a deep quench into theunstable region through a spinodal mechanism (SD) yields initialbicontinuous morphologies as given by stage 2 in Figure 9. Fromthis stage onward, the total volume shrinkage is the average ofthe volume changes due to the separate primary epoxy- andthermoplastic-rich phases. The initial bicontinuous structuresundergo rapid coarsening, which when coupled with thecontinuous chemical quench induces a secondary phase separa-tion to yield highly intermixed SAN and epoxy phases.

Further, in the late stages of SD phase separation (stage 3),the volume shrinkage is determined by the contributions fromboth reaction and phase-coarsening kinetics. For a low TPcontent system (φSAN e 10%), a much faster percolation tocluster transition occurs according to the conventional SD latestage dynamics, whereby the continuous thermoplastic minorphase breaks up to reduce the interfacial tension. A differentsituation exists when the TP content is beyond that of the SAN-10% system (φSAN g 15%). Here, a simultaneous growth ofthe dynamic asymmetry suppresses the normal SD coarseningdynamics causing a viscoelastic phase separation. As a resultthe slow dynamic SAN intermixed primary phase becomes moreviscoelastic and forms the matrix, while the less viscoelasticepoxy-rich primary phase nucleates as spheres in the elasticregime (τd < τts) as shown in the pictures.

In the case of low TP content system (φSAN e 10%),equilibrium is rapidly attained in stage 4 followed by gelation,which freezes the morphology and subsequent vitrification ofthe epoxy phase. Here the nonvitrified SAN dispersed domains

Figure 9. Schematic representation of the mechanism of volume-shrinkage evolution during reaction-induced viscoelastic phase separation inthermoplastic SAN-modified epoxy systems.

PVT Behavior of Thermoplastic SAN-Epoxy Systems J. Phys. Chem. B, Vol. 112, No. 47, 2008 14801

might effectively resist the shrinkage. Meanwhile for higherSAN content systems in the elastic regime, the primary epoxynucleates as well as the secondary epoxy globules further growby normal diffusion. According to theories of viscoelasticity,this diffusion process must accompany a volume shrinkage ofthe more viscoelastic phase in order to avoid the inhomogeneousdeformation of the elastic network.13 This additional shrinkagedue to the thermoplastic-rich phase further contributes towardthe total specific volume changes during polymerization andtherefore accounts for the observed high volume shrinkagebeyond the SAN-10% system, that is, for the SAN-15% system(stages 4 to 5 for φSAN ) 15%) during phase separation.Theoretical studies have further suggested that such volumeshrinkage processes essentially involve a relaxation of thethermoplastic dynamic asymmetry.13,46 Now, the higher the TPconcentration, the slower will be the relaxation of this dynamicasymmetry during phase separation. Thus, in the case of SAN-20% blend (φSAN g 20%), the thermoplastic rheological time,τts, becomes longer than that for phase separation whereby theSAN dynamic asymmetry ceases to relax. The morphology thenpreferentially gets frozen in the elastic regime (stage 4 for φSAN

g 20%), and no volume shrinkage of the SAN-rich phase occurs.In the case of intermediate TP content systems (stage 5 for

φSAN ) 15%), the volume shrinkage of the more viscoelasticSAN thermoplastic phase further continues with a coarseningof both the anisotropic primary epoxy domains and thesecondary epoxy substructures (stage 5), thereby inducing aphase inversion until the gelation/vitrification commences andfinally yielding a network-like partially continuous SAN-richprimary phase with local phase inversion in the continuousmacrosized epoxy-rich matrix.

Conclusions

The in situ volume shrinkage behavior, determined by thepressure-volume-temperature analysis of poly(styrene-co-acrylonitrile)-modified epoxy thermosets has been shown to begoverned by the viscoelastic phase separation kinetics duringpolymerization, a first investigation of this kind amongthermoplastic-toughened epoxy resins. In contrast to anexpected linear dependency, the shrinkage is highest for theSAN-15% system with a double-phase network-like bicon-tinuous microstructure. Considering a linear relationshipbetween conversion and specific volume changes, an inves-tigation of the shrinkage kinetics of blends have shown thatin the miscible region, the diluent effect due to SAN causesa progressive decrease in relative shrinkage with its increasingconcentration among the blends. However, beyond phaseseparation, a coupling between the phase coarsening andreaction kinetics decided the shrinkage behavior. Owing toa continuous quench depth, the phase separation in theseblends are accompanied by secondary phase separation, andat higher SAN contents, the various double-phase morphol-ogies also possessed clear imprints of viscoelastic phaseseparation. Concomitantly, similar cross-link densities of thestoichiometric epoxy-rich phase, as revealed from DMTAstudies, as well as the broad relaxation behavior of the highlyintermixed SAN-ric phase estabilshed a prominent role ofthe thermoplastic phase toward determining the shrinkagecharacteristics of epoxy-DDS/SAN systems.

In the case of SAN-15% system, the morphology develop-ment studies visualizes a viscoelastic phase separation beingcharacterized by an additional volume shrinkage of the coarse-grained thermoplastic SAN-rich phase during polymerization,which further accounts for its enhanced volume shrinkage.

Accordingly a new volume shrinkage mechanism has beenformulated to understand the volume shrinkage in SAN-modifiedepoxy resins. In view of practical applications, it is noteworthythat the intrinsic viscoelastic properties of the thermoplasticmodifier, even though present in a very low amount, dodetermine the processing characteristics of these multiphaseresins.

Acknowledgment. The authors wish to thank ISRO, India,for the financial support to J.J. and G. Groeninckx for the DMTAand optical microscopy measurements.

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