Catastrophic Emulsification of Epoxy ResinUsing Pluronic Block Copolymers:

Preinversion Behavior

Jingrong Xu,† Alexander M. Jamieson,‡Syed Qutubuddin,*,†,‡ Prasad V. Gopalkrishnan,‡ and

Steven D. Hudson‡

Departments of Chemical Engineering andMacromolecular Science, Case Western Reserve University,

Cleveland, Ohio 44106

Received August 14, 2000.In Final Form: November 2, 2000

IntroductionPhase inversion techniques are used to produce con-

centrated aqueous emulsions of fine droplet size distri-bution.1-3 The method is especially useful for emulsifyinghigh-viscosity materials such as epoxy resin and otherpolymeric components.4 Catastrophic inversion of anemulsion is induced by increasing the dispersed phasevolume.5,6 Kinetic modeling of catastrophic inversion basedon droplet breakup and coalescence is qualitatively consis-tent with experimental observation in situations wherecoarse emulsions are produced and droplet size distribu-tion is broad.8,9 However, fine emulsions (submicrometerdroplet size) can also be produced from a catastrophicinversion path.10 The current study seeks to gain insightinto the inversion mechanism leading to fine emulsions.

We prepare high volume fraction epoxy emulsions withsmall (submicrometer) droplet size. This involves creationof a large interfacial area and requires use of high sur-factant concentration. Thus the process resembles spon-taneous emulsification, where the driving force is ther-modynamic, requiring no mechanical energy.11-14 Ofparticular interest is a distinct structural transition inthe epoxy-continuous emulsion which we observe prior tothe inversion point.

With increase of water content in a water-in-oil mi-croemulsion, Greiner and Evans11 observed a transitionin electrical conductivity prior to phase inversion under

quiescent conditions (spontaneous emulsification). Theauthors observed a nonconducting to conducting tononconducting behavior in a mixture of a high-viscosityresin with an anionic surfactant in water and interpretedthis in terms of a percolation-antipercolation process.

Percolation phenomena are well-known in emulsions,stabilized with ionic or nonionic surfactants.15-21 In water-in-oil microemulsions, water droplets sometimes percolateto form large networklike structures, accompanied by asudden conductivity increase above a percolation thresh-old, φp. In particular, a water-in-p-xylene microemulsionstabilized by a Pluronic surfactant was found to exhibitsuch behavior.20 Using dynamic light scattering and time-resolved luminescence quenching, Mays et al.20 deducedthat attractive interactions between surfactant layers leadto a dynamic percolation, forming an extended networkof water droplets and facilitating ionic conductance at alow water fraction. The exact nature of the percolationstate is still a matter for debate. It has been argued thatsurfactant micelles in the oil phase act to facilitate theaggregation of water droplets, either by a “bridging”21 or“depletion” mechanism.22

In what follows, we investigate changes in conductanceand viscoelasticity during catastrophic emulsification ofan epoxy resin. We observe in both properties a distincttransition prior to the inversion point. Morphologicalanalysis in the preinversion region by flow video micros-copy supports that the effect is due to percolation of waterdroplets.

Experimental SectionPluronic surfactant (P65) was obtained from BASF

Corp. (Mt. Olive, NJ). P65 is a symmetric poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymer, the PPO center block being hydrophobicand the PEO end blocks hydrophilic. Micelle formation andinterfacial properties of this surfactant type have been inves-tigated.23-25 The total molecular weight of P65 is 3400, and thePPO/PEO ratio is one. The oil phase, bisphenol A diglycidyl etherepoxy resin (EPON 828), was supplied by Shell Chemical(Houston, TX). EPON 828 is a Newtonian fluid with a viscosityat room temperature that is 4 orders of magnitude higher thanthat of water. It is difficult to emulsify by direct emulsificationor by catastrophic emulsification using conventional surfactants.

A catastrophic inversion experiment involves addition of waterto a mixture of oil and surfactant while vigorous shear is applied.The epoxy phase was first mixed with surfactant at a surfactant/oil weight ratio of 0.17. After thermal equilibration, the mixturewas visually homogeneous and transparent. Deionized waterwas added dropwise to the mixture at a fixed rate of approximately1 mL/min. In-situ ac conductivity measurements were performedto determine the water volume fraction at the phase inversion

* To whom correspondence should be addressed. E-mail: sxq@po.cwru.edu.

† Department of Chemical Engineering.‡ Department of Macromolecular Science.(1) Forster, Th.; Schambil, F.; Rybinski, W. v. J. Dispersion Sci.

Technol. 1992, 13 (2), 183.(2) Minana-Perez, M.; Gutron, C.; Zundel, C.; Anderez, J. M.; Salager,

J. L. J. Dispersion Sci. Technol. 1999, 20 (3), 893.(3) Shinoda, K.; Friberg, S. E. Emulsions and Solubilization; Wiley-

Interscience: New York, 1986.(4) Akey, G. Chem. Eng. Sci. 1998, 53 (2), 203.(5) Dickinson, E. J. Colloid Interface Sci. 1981, 84, 284.(6) Salager, J. L. Phase transformation and emulsion inversion on

the basis of catastrophe theory. InEncyclopedia of Emulsion Technology;Becher, P., Ed.; Dekker: New York, 1988; Vol. 3.

(7) Vaessen, G. E. J.; Visschers, M.; Stein, H. N. Langmuir 1996, 12,875.

(8) Groeneweg, F.; Agterof, W. G. M.; Jaeger, P.; Janssen, J. J. M.;Wieringa, J. A.; Klahn, J. K. Trans. Inst.Chem. Eng. 1998, 26, 55.

(9) Zerfa, M.; Sajjadi, S.; Brooks, B. W. Colloids Surf. 1999, 155, 323.(10) Greiner, R.; Evans, D. F. Langmuir 1990, 6, 1793.(11) Shahidzadeh, N.; Bonn, D.; Meunier, J. Europhys. Lett. 1997,

40 (4), 459.(12) Shahidzadeh, N.; Bonn, D.; Aguerre-Chariol, O.; Meunier, J.

Colloids Surf. A 1999, 147, 375.(13) Rang, M.-J.; Miller, C. A. J. Colloid Interface Sci. 1999, 209,

179.(14) Borkovec, M.; Eicke, H.-F.; Hammerich, H.; Gupta, B. D J. Phys.

Chem. 1988, 92, 206.

(15) Boned, C.; Peyrelasse, J. J. Surf. Sci. Technol. 1991, 7, 1.(16) Schlicht, L.; Spilgies, J.-H.; Lipgens, S.; Boye, S.; Schubel, D.;

Ilgenfritz, G. Biophys. Chem. 1996, 58, 39.(17) Lehnert, S.; Tarabishi, H.; Leuenberger, H. Colloids Surf. 1994,

91, 227.(18) Antalek, B.; Willians, A. J.; Texter, J.; Feldman, Y.; Garti, N.

Colloids Surf. A 1997, 128, 1.(19) Cazabat, A.-M.; Chatenay, D.; Langevin, D.; Meunier, J. Faraday

Discuss. Chem. Soc. 1982, 76, 291.(20) Mays, H.; Almgren, M.; Brown, W. Ber. Bunsen-Ges. Phys. Chem.

1998, 102, 1648.(21) Hazlett, R. D.; Schechter, R. S. Colloid Surf. 1988, 29, 53.(22) Leal-Calderon, F.; Gerhardi, B.; Espert, A.; Brossard, F.; Alard,

V.; Tranchant, J. F.; Stora, T.; Bibette, J. Langmuir 1996, 12, 872.(23) Alexandradis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1.(24) Buckton, G.; Machiste, E. O. J. Pharm.. Sci. 1997, 86, 163.(25) Nolan, S. L.; Phillips, R. J.; Cotts, P. M.; Dungan, S. R. J. Colloid

Interface Sci. 1997, 191, 291.

1310 Langmuir 2001, 17, 1310-1313

10.1021/la001174x CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 01/17/2001

point (φc), using two stainless steel electrodes 2 cm apart. Theinversion point is characterized by a sudden, order-of-magnitudeincrease in the electrical conductance of the sample and atransformation from a milky suspension with moderate viscosityto a highly viscoelastic white gel. Droplet size was measured bydynamic light scattering (Brookhaven Instruments) after dilutionof the emulsion with water. Rheological characterization wascarried out with a Carri-Med CLS 50 controlled stress rheometer,using cone and plate geometry. Video microscopic examinationunder shear was performed using a CSS-450 Cambridge shearingcell mounted upon an Olympus BX-60 optical microscope witha CCD video camera.

Structural Transition Prior to Phase Inversion

Electrical Conductance. The in-situ electrical con-ductance prior to the inversion point exhibits a commonpattern of behavior, with variations, depending on sur-factant-to-epoxy ratio and temperature. Prior to theinversion point, one typically observes a slow monotonicincrease in conductance, approximately proportional tothe amount of water added. Close to the inversion point,a deviation from this proportionality is seen as a morerapid increase in conductance. In this regime, the mag-nitude of the conductance becomes dependent on themixing conditions. When shear is removed (quiescentcondition), the mixture becomes more conducting. Thisbehavior is illustrated in Figure 1 for a catastrophicinversion carried out at 30 °C using P65 at a surfactant/oil ratio of 0.17. Above a net water volume fraction (φ) of0.10, within seconds, the quiescent conductance increasessharply over that observed under shear.

Under some conditions, a discrete peak in conductanceis observed prior to the inversion point. This behavior isshown in Figure 2 for a catastrophic inversion on the samemixture as in Figure 1 but at 50 °C. A rapid increase ofconductance is observed at φ ∼ 0.09 and proceeds until φ∼ 0.17, when the conductance drops suddenly to a lowvalue. This low conductance persists untilφc ) 0.19, wherephase inversion occurs. A particularly dramatic transi-tional peak in conductance is observed under quiescentconditions, as also shown in Figure 2. The increase inquiescent conductance disappears just before the inversionpoint.

These observations prompt characterization of themixtures at intervals during the inversion process byrheology and video microscopy. The results provideevidence for formation of a percolation network of watermicrospheres in the preinversion mixture.

Dynamic Rheological Measurements. Measure-ment of dynamic moduli at low deformation indicates thatthe initial resin-block copolymer mixture in Figures 1 and2 is a Newtonian fluid. With addition of water, as shownin Figure 3, the storage modulus G′ increases, and tan δ(≡G′′/G′) decreases, indicating an increase of elasticity.Interestingly, in Figure 3, tan δ exhibits a reproduciblemaximum before the critical inversion point, whichremarkably correlates to the location of the conductancepeak in Figure 2. Evidently, as water fraction is increased,the elasticity increases, then declines, and subsequentlyincreases again.

The increase of elasticity and conductance indicate theformation of a three-dimensional network with additionof water under vigorous shear. Likewise, the subsequentdecrease of elasticity and conductance prior to the inver-sion point indicates destruction of this 3-D network. Thus,rheology and conductance provide a self-consistent picturethat a network forms and subsequently disappears beforethe emulsion inversion point.

Video Microscopy. Macroscopic structure formationin the mixture is evident visually, as a change from asmooth to a “textured” appearance. Upon addition of water,the mixture, at first uniformly white, develops a spongelikepattern that coarsens as the inversion proceeds. Suchpattern formation occurs at water contents where thequiescent and dynamic conductance diverge in Figures 1and 2.

To assess the microscopic nature of this “texture”, videomicroscopy was performed with a shear cell to monitor

Figure 1. Conductance evolution when inversion is carriedout at 30 °C, P65/resin ratio 0.17. (Filled symbols representconductance readings when shear is applied, and unfilledsymbols represent quiescent conductance values. The solid lineindicates the initial slope.)

Figure 2. Comparison of conductance evolution with andwithout shear during a catastrophic inversion at 50 °C (P65/resin ratio 0.17).

Figure 3. Evolution of tan δ and storage modulus duringcatastrophic inversion at 50 °C, with P65/resin ratio 0.17.

Notes Langmuir, Vol. 17, No. 4, 2001 1311

the morphology evolution following application of shear.A small aliquot from the emulsification reactor prior tothe inversion point was loaded onto the shear stage. Theemulsification process investigated corresponds to thatin Figure 2, where catastrophic inversion is carried outat 50 °C with P65/resin ratio 0.17. The sample, with watercontent φ ) 0.12, maintained at the mixing temperaturebetween parallel plates (gap ) 100 µm) in the shear cell,was subjected to a shear rate of 20 s-1 for 5 min. A sequenceof pictures (Figure 4a-f) was taken immediately followingcessation of shear. These show formation of a percolationnetwork of water droplets as time elapses. Under phasecontrast viewing, the brighter water droplets (∼4 µm indiameter) are evenly distributed in the dark epoxy matrixunder shear (Figure 4a). Once shear is removed, aggrega-tion proceeds rapidly. Within seconds, a fine percolationnetwork forms (Figure 4b,c) and coarsens rapidly into ameshlike pattern (Figure 4d-e). The mesh size slowlyincreases with time and isolated water droplets insidethe mesh disappear, similar to a diffusion-limited clusteraggregation (DLCA). The structure evolves into a coarsenetwork by breaking some network links and coarseningof network strands (Figure 4e). The kinetics of networkformation occurs on the same time scale as that of theconductance increase on cessation of shear. Percolation ofwater droplets is therefore proposed as the mechanismfor formation of the visible macroscopic pattern in thepreinversion region and, further, to be the source of thecorresponding increases in conductance and elasticity(Figures 1-3).

Discussion

Evidence for a percolation network of water dropletsin bulk oil was previously reported by Greiner andEvans11 for a quiescent microemulsion. Here we havedemonstrated that such phenomenon appears to play a

role in emulsification by catastrophic phase inversionduring turbulent mixing. It seems likely that the percola-tion network is a direct precursor of phase inversion.However, the observation, in some cases, of a distinctdecrease in conductance immediately prior to inversionsuggests that an intermediate mechanistic step may haveto be considered.

We propose the following mechanistic model for phaseinversion emulsification. Due to the high continuous phaseviscosity (and low viscosity ratio of water/oil), a high shearstress is generated, and water added to the oil-surfactantmixture is efficiently dispersed into small droplets. Asthe number of water droplets increases, the interdropletdistance decreases, and attractive interaction of thedroplets (originating from adsorbed surfactant moleculesor from surfactant micelles in the bulk) leads to theformation of a linear percolation network, as described byGreiner and Evans.11 Such a network of water dropletsexplains the observed peak in conductivity and theminimum in tan δ. As more water is added, new interfaceis generated, and redistribution of the block copolymerpromotes coalescence of water droplets. Hence, thepercolation network is disrupted by coalescence intoclusters, within which, due to the high water content,local phase inversion occurs. The formation of local phase-inverted domains is the origin of the loss of conductivityand elasticity illustrated in Figures 2 and 3.

In spontaneous emulsification, the conductometric peakprior to phase inversion11 was associated with a percola-tion-antipercolation-inversion mechanism. The physicalpicture given suggests a transformation from sphericalmicroemulsiondroplets into interconnectedwater conduits(percolation). Subsequent growth of the diameter of theconduits upon increase of water content leads to discon-nection of conduits and formation of larger spheres dueto requirement for optimum curvature (antipercolation).11

The common features between this phenomenon andpresent observations include a highly viscous oil phaseand a distinct conductivity peak. However, the percolationprocess in our experiments involves formation of a muchcoarser structure than that in the microemulsion domain,as demonstrated by the macroscopic texture formationand the video microscopic evidence of a percolationnetwork of individual water droplets.

The decrease in conductance just before inversion(antipercolation) is frequently not seen, for example, inthe P65 system at 0.17 surfactant/oil ratio, when invertedat 30 °C (Figure 1). At low temperature, water has anincreased affinity for PEO, and therefore, there is anincreased driving force for a change in sign of the meanmonolayer curvature. This is consistent with the observeddecrease in φc at 30 °C and disappearance of the inter-mediate (antipercolation) stage of local phase inversion.

Concluding Remarks

Prior to the catastrophic inversion point in epoxy/Pluronic P65/water emulsion, there exists a distinctconductance increase, sometimes evidenced as a conduc-tance peak. Such a peak is accompanied by changes inviscoelastic moduli that indicate a buildup and loss ofelasticity before the inversion point. These results suggesta structural transition prior to the inversion from water-in-oil to oil-in-water emulsion, not previously reported inthe context of catastrophic phase inversion emulsification.Flowvideomicroscopysuggests that,analogous topreviouswork on spontaneous emulsification,11 a network is formedvia percolation of water microdroplets in the resin.Subsequent collapse (antipercolation) of the network

Figure 4. Images obtained from video microscopy after thesample is sheared at 20 s-1, P65/resin ratio 0.17, water content12 % vol: (a) t ) 0 s; (b) t ∼ 5 s; (c) t ∼ 10 s; (d) t ∼ 15 s; (e)t ∼ 30 s; (f) t ∼ 120 s.

1312 Langmuir, Vol. 17, No. 4, 2001 Notes

promotes local nucleation of phase-inverted water do-mains, which ultimately leads to global inversion. Thus,in phase inversion emulsification of a high-viscosity oilusing a high concentration of block copolymer, the intenseshear stress coupled with a strong thermodynamic drivingforce (i.e., spontaneous emulsification) leads to efficientformation of a fine emulsion.

Acknowledgment. The authors are grateful to Dr.Yuanze Xu and Professor J. Adin Mann, Jr., for usefuldiscussions and suggestions. J.X. acknowledges the fi-nancial support from the Department of Chemical En-gineering, CWRU.

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Notes Langmuir, Vol. 17, No. 4, 2001 1313