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Catastrophic Emulsification of Epoxy Resin Using Pluronic Block Copolymers: Preinversion Behavior Jingrong Xu,† Alexander M. Jamieson,‡ Syed Qutubuddin,*,†,‡ Prasad V. Gopalkrishnan,‡ and Steven D. Hudson‡ Departments of Chemical Engineering and Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Received August 14, 2000. In Final Form: November 2, 2000
Introduction Phase inversion techniques are used to produce concentrated aqueous emulsions of fine droplet size distribution.1-3 The method is especially useful for emulsifying high-viscosity materials such as epoxy resin and other polymeric components.4 Catastrophic inversion of an emulsion is induced by increasing the dispersed phase volume.5,6 Kinetic modeling of catastrophic inversion based on droplet breakup and coalescence is qualitatively consistent with experimental observation in situations where coarse emulsions are produced and droplet size distribution is broad.8,9 However, fine emulsions (submicrometer droplet size) can also be produced from a catastrophic inversion path.10 The current study seeks to gain insight into the inversion mechanism leading to fine emulsions. We prepare high volume fraction epoxy emulsions with small (submicrometer) droplet size. This involves creation of a large interfacial area and requires use of high surfactant concentration. Thus the process resembles spontaneous emulsification, where the driving force is thermodynamic, requiring no mechanical energy.11-14 Of particular interest is a distinct structural transition in the epoxy-continuous emulsion which we observe prior to the inversion point. With increase of water content in a water-in-oil microemulsion, Greiner and Evans11 observed a transition in electrical conductivity prior to phase inversion under * To whom correspondence should be addressed. E-mail: sxq@ po.cwru.edu. † Department of Chemical Engineering. ‡ Department of Macromolecular Science. (1) Fo¨rster, Th.; Schambil, F.; Rybinski, W. v. J. Dispersion Sci. Technol. 1992, 13 (2), 183. (2) Min˜ana-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; WileyInterscience: 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. In Encyclopedia 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.
quiescent conditions (spontaneous emulsification). The authors observed a nonconducting to conducting to nonconducting behavior in a mixture of a high-viscosity resin with an anionic surfactant in water and interpreted this in terms of a percolation-antipercolation process. Percolation phenomena are well-known in emulsions, stabilized with ionic or nonionic surfactants.15-21 In waterin-oil microemulsions, water droplets sometimes percolate to form large networklike structures, accompanied by a sudden conductivity increase above a percolation threshold, φp. In particular, a water-in-p-xylene microemulsion stabilized by a Pluronic surfactant was found to exhibit such behavior.20 Using dynamic light scattering and timeresolved luminescence quenching, Mays et al.20 deduced that attractive interactions between surfactant layers lead to a dynamic percolation, forming an extended network of water droplets and facilitating ionic conductance at a low water fraction. The exact nature of the percolation state is still a matter for debate. It has been argued that surfactant micelles in the oil phase act to facilitate the aggregation of water droplets, either by a “bridging”21 or “depletion” mechanism.22 In what follows, we investigate changes in conductance and viscoelasticity during catastrophic emulsification of an epoxy resin. We observe in both properties a distinct transition prior to the inversion point. Morphological analysis in the preinversion region by flow video microscopy supports that the effect is due to percolation of water droplets. Experimental Section Pluronic 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-bPEO) triblock copolymer, the PPO center block being hydrophobic and the PEO end blocks hydrophilic. Micelle formation and interfacial properties of this surfactant type have been investigated.23-25 The total molecular weight of P65 is 3400, and the PPO/PEO ratio is one. The oil phase, bisphenol A diglycidyl ether epoxy resin (EPON 828), was supplied by Shell Chemical (Houston, TX). EPON 828 is a Newtonian fluid with a viscosity at room temperature that is 4 orders of magnitude higher than that of water. It is difficult to emulsify by direct emulsification or by catastrophic emulsification using conventional surfactants. A catastrophic inversion experiment involves addition of water to 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 mixture was visually homogeneous and transparent. Deionized water was added dropwise to the mixture at a fixed rate of approximately 1 mL/min. In-situ ac conductivity measurements were performed to determine the water volume fraction at the phase inversion (15) Boned, C.; Peyrelasse, J. J. Surf. Sci. Technol. 1991, 7, 1. (16) Schlicht, L.; Spilgies, J.-H.; Lipgens, S.; Boye, S.; Schu¨bel, 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.
10.1021/la001174x CCC: $20.00 © 2001 American Chemical Society Published on Web 01/17/2001
Notes
Figure 1. Conductance evolution when inversion is carried out at 30 °C, P65/resin ratio 0.17. (Filled symbols represent conductance readings when shear is applied, and unfilled symbols represent quiescent conductance values. The solid line indicates the initial slope.)
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Figure 2. Comparison of conductance evolution with and without shear during a catastrophic inversion at 50 °C (P65/ resin ratio 0.17).
point (φc), using two stainless steel electrodes 2 cm apart. The inversion point is characterized by a sudden, order-of-magnitude increase in the electrical conductance of the sample and a transformation from a milky suspension with moderate viscosity to a highly viscoelastic white gel. Droplet size was measured by dynamic light scattering (Brookhaven Instruments) after dilution of the emulsion with water. Rheological characterization was carried out with a Carri-Med CLS 50 controlled stress rheometer, using cone and plate geometry. Video microscopic examination under shear was performed using a CSS-450 Cambridge shearing cell mounted upon an Olympus BX-60 optical microscope with a CCD video camera.
Structural Transition Prior to Phase Inversion Electrical Conductance. The in-situ electrical conductance prior to the inversion point exhibits a common pattern of behavior, with variations, depending on surfactant-to-epoxy ratio and temperature. Prior to the inversion point, one typically observes a slow monotonic increase in conductance, approximately proportional to the amount of water added. Close to the inversion point, a deviation from this proportionality is seen as a more rapid increase in conductance. In this regime, the magnitude of the conductance becomes dependent on the mixing conditions. When shear is removed (quiescent condition), the mixture becomes more conducting. This behavior is illustrated in Figure 1 for a catastrophic inversion carried out at 30 °C using P65 at a surfactant/ oil ratio of 0.17. Above a net water volume fraction (φ) of 0.10, within seconds, the quiescent conductance increases sharply over that observed under shear. Under some conditions, a discrete peak in conductance is observed prior to the inversion point. This behavior is shown in Figure 2 for a catastrophic inversion on the same mixture as in Figure 1 but at 50 °C. A rapid increase of conductance is observed at φ ∼ 0.09 and proceeds until φ ∼ 0.17, when the conductance drops suddenly to a low value. This low conductance persists until φc ) 0.19, where phase inversion occurs. A particularly dramatic transitional peak in conductance is observed under quiescent conditions, as also shown in Figure 2. The increase in quiescent conductance disappears just before the inversion point. These observations prompt characterization of the mixtures at intervals during the inversion process by rheology and video microscopy. The results provide evidence for formation of a percolation network of water microspheres in the preinversion mixture.
Figure 3. Evolution of tan δ and storage modulus during catastrophic inversion at 50 °C, with P65/resin ratio 0.17.
Dynamic Rheological Measurements. Measurement of dynamic moduli at low deformation indicates that the initial resin-block copolymer mixture in Figures 1 and 2 is a Newtonian fluid. With addition of water, as shown in 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 reproducible maximum before the critical inversion point, which remarkably correlates to the location of the conductance peak in Figure 2. Evidently, as water fraction is increased, the elasticity increases, then declines, and subsequently increases again. The increase of elasticity and conductance indicate the formation of a three-dimensional network with addition of water under vigorous shear. Likewise, the subsequent decrease of elasticity and conductance prior to the inversion point indicates destruction of this 3-D network. Thus, rheology and conductance provide a self-consistent picture that a network forms and subsequently disappears before the emulsion inversion point. Video Microscopy. Macroscopic structure formation in the mixture is evident visually, as a change from a smooth to a “textured” appearance. Upon addition of water, the mixture, at first uniformly white, develops a spongelike pattern that coarsens as the inversion proceeds. Such pattern formation occurs at water contents where the quiescent and dynamic conductance diverge in Figures 1 and 2. To assess the microscopic nature of this “texture”, video microscopy was performed with a shear cell to monitor
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Figure 4. Images obtained from video microscopy after the sample is sheared at 20 s-1, P65/resin ratio 0.17, water content 12 % 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.
the morphology evolution following application of shear. A small aliquot from the emulsification reactor prior to the inversion point was loaded onto the shear stage. The emulsification process investigated corresponds to that in Figure 2, where catastrophic inversion is carried out at 50 °C with P65/resin ratio 0.17. The sample, with water content φ ) 0.12, maintained at the mixing temperature between parallel plates (gap ) 100 µm) in the shear cell, was subjected to a shear rate of 20 s-1 for 5 min. A sequence of pictures (Figure 4a-f) was taken immediately following cessation of shear. These show formation of a percolation network of water droplets as time elapses. Under phase contrast viewing, the brighter water droplets (∼4 µm in diameter) are evenly distributed in the dark epoxy matrix under shear (Figure 4a). Once shear is removed, aggregation proceeds rapidly. Within seconds, a fine percolation network forms (Figure 4b,c) and coarsens rapidly into a meshlike pattern (Figure 4d-e). The mesh size slowly increases with time and isolated water droplets inside the mesh disappear, similar to a diffusion-limited cluster aggregation (DLCA). The structure evolves into a coarse network by breaking some network links and coarsening of network strands (Figure 4e). The kinetics of network formation occurs on the same time scale as that of the conductance increase on cessation of shear. Percolation of water droplets is therefore proposed as the mechanism for formation of the visible macroscopic pattern in the preinversion region and, further, to be the source of the corresponding increases in conductance and elasticity (Figures 1-3). Discussion Evidence for a percolation network of water droplets in bulk oil was previously reported by Greiner and Evans11 for a quiescent microemulsion. Here we have demonstrated that such phenomenon appears to play a
Notes
role in emulsification by catastrophic phase inversion during turbulent mixing. It seems likely that the percolation network is a direct precursor of phase inversion. However, the observation, in some cases, of a distinct decrease in conductance immediately prior to inversion suggests that an intermediate mechanistic step may have to be considered. We propose the following mechanistic model for phase inversion emulsification. Due to the high continuous phase viscosity (and low viscosity ratio of water/oil), a high shear stress is generated, and water added to the oil-surfactant mixture is efficiently dispersed into small droplets. As the number of water droplets increases, the interdroplet distance decreases, and attractive interaction of the droplets (originating from adsorbed surfactant molecules or from surfactant micelles in the bulk) leads to the formation of a linear percolation network, as described by Greiner and Evans.11 Such a network of water droplets explains the observed peak in conductivity and the minimum in tan δ. As more water is added, new interface is generated, and redistribution of the block copolymer promotes coalescence of water droplets. Hence, the percolation network is disrupted by coalescence into clusters, within which, due to the high water content, local phase inversion occurs. The formation of local phaseinverted domains is the origin of the loss of conductivity and elasticity illustrated in Figures 2 and 3. In spontaneous emulsification, the conductometric peak prior to phase inversion11 was associated with a percolation-antipercolation-inversion mechanism. The physical picture given suggests a transformation from spherical microemulsion droplets into interconnected water conduits (percolation). Subsequent growth of the diameter of the conduits upon increase of water content leads to disconnection of conduits and formation of larger spheres due to requirement for optimum curvature (antipercolation).11 The common features between this phenomenon and present observations include a highly viscous oil phase and a distinct conductivity peak. However, the percolation process in our experiments involves formation of a much coarser structure than that in the microemulsion domain, as demonstrated by the macroscopic texture formation and the video microscopic evidence of a percolation network of individual water droplets. The decrease in conductance just before inversion (antipercolation) is frequently not seen, for example, in the P65 system at 0.17 surfactant/oil ratio, when inverted at 30 °C (Figure 1). At low temperature, water has an increased affinity for PEO, and therefore, there is an increased driving force for a change in sign of the mean monolayer curvature. This is consistent with the observed decrease in φc at 30 °C and disappearance of the intermediate (antipercolation) stage of local phase inversion. Concluding Remarks Prior to the catastrophic inversion point in epoxy/ Pluronic P65/water emulsion, there exists a distinct conductance increase, sometimes evidenced as a conductance peak. Such a peak is accompanied by changes in viscoelastic moduli that indicate a buildup and loss of elasticity before the inversion point. These results suggest a structural transition prior to the inversion from waterin-oil to oil-in-water emulsion, not previously reported in the context of catastrophic phase inversion emulsification. Flow video microscopy suggests that, analogous to previous work on spontaneous emulsification,11 a network is formed via percolation of water microdroplets in the resin. Subsequent collapse (antipercolation) of the network
Notes
promotes local nucleation of phase-inverted water domains, which ultimately leads to global inversion. Thus, in phase inversion emulsification of a high-viscosity oil using a high concentration of block copolymer, the intense shear stress coupled with a strong thermodynamic driving force (i.e., spontaneous emulsification) leads to efficient formation of a fine emulsion.
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Acknowledgment. The authors are grateful to Dr. Yuanze Xu and Professor J. Adin Mann, Jr., for useful discussions and suggestions. J.X. acknowledges the financial support from the Department of Chemical Engineering, CWRU. LA001174X
