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Highly Concentrated W/O Emulsions Prepared by the PIT Method as Templates for Solid Foams Jordi Esquena,* GSR Ravi Sankar, and Conxita Solans Departament de Tecnologia de Tensioactius, Institut d’Investigacions Quı´miques i Ambientals de Barcelona (IIQAB/CSIC), Jordi Girona, 18-26, 08034 Barcelona, Spain Received June 26, 2002. In Final Form: December 11, 2002 The main aim of this work was to use highly concentrated W/O (water-in-oil) emulsions, prepared by the PIT (phase inversion temperature) method as templates for solid polystyrene foams with a narrow pore size distribution. The highly concentrated emulsions were prepared by a method based on the PIT principle, which consists of increasing rapidly the temperature across the conditions where the hydrophilic and lipophilic properties of the surfactant in the system are balanced (THLB). This method allows one to obtain emulsions with small droplets which are relatively homogeneous. The solid foams were obtained by polymerization, initiated by potassium persulfate, of the continuous phase of the highly concentrated water-in-styrene emulsions, stabilized by nonionic surfactants. Water and surfactant were removed by washing and drying. The pore volume of the obtained monoliths was very high (>15 mL/g). The mechanical properties of such solid foams were characterized by means of compression tests. The properties were dependent on both composition and emulsification parameters. For comparative purposes, the polymerization was also carried out in highly concentrated emulsions prepared by a conventional method. The strength and the toughness of the solid foams obtained from highly concentrated emulsions prepared by the PIT method were 400 and 50% higher, respectively, than that of solid foams obtained from emulsions prepared by conventional methods.
Introduction Highly concentrated emulsions are characterized by their large internal phase volume fraction, which can be as high as 99%. They were of much interest and welldiscussed in the literature.1-4 But systematic studies about their formation, stability, structure, and reactivity have been undertaken more recently.5-13 The internal or dispersed phase in highly concentrated emulsions can be either polar or nonpolar, and, as ordinary emulsions, they can be classified in two types: oil-in-water (O/W) or waterin-oil (W/O). The nature of the phases in W/O highly concentrated emulsions, developed with water/poly(ethylene glycol) alkyl ether nonionic surfactant/aliphatic hydrocarbon systems, are an aqueous phase and a swollen reverse micellar solution phase (or W/O microemul* Corresponding author. Telephone: +34 93 4006159. Fax: +34 93 2045904. E-mail:
[email protected]. (1) Ostwald, W. Kolloid Z. 1910, 6, 103; 1910, 7, 64. (2) Lissant, K. J. J. Colloid Interface Sci. 1966, 22, 462. (3) Lissant, K. J.; Mayhan, K. G. J. Colloid Interface Sci. 1973, 42, 201. (4) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160. (5) Solans, C.; Comelles, F.; Azemar, N.; Sanchez Leal, J.; Parra, J. L. Jorn. Com. Esp. Deterg. 1986, 17, 109. (6) Kunieda, H.; Solans, C.; Shida, N.; Parra, J. L. Colloids Surf. A 1987, 24, 225. (7) Solans, C.; Azemar, N.; Parra, J. L. Prog. Colloid Polym. Sci. 1988, 76, 224. (8) Kunieda, H.; Evans, D. F.; Solans C.; Yoshida, M. Colloids Surf. A 1990, 47, 35. (9) Solans, C.; Pons, R.; Zhu, S.; Davies, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (10) Pons, R.; Carrera, I.; Erra, P.; Kunieda, H.; Solans, C. Colloids Surf. A 1994, 91, 259. (11) Ozawa, K.; Solans, C.; Kunieda, H. J. Colloid Interface Sci. 1997, 188, 275. (12) Solans, C.; Pons, R.; Kunieda, H. In Modern Aspects of Emulsion Science; Binks B. P., Ed.; Royal Society of Chemistry: Cambridge, U.K., 1998; pp 367-394. (13) Solans, C.; Pinazo, A.; Caldero´, G.; Infante, M. R. Colloids Surf. A 2001, 176, 101.
sion).6,9,14 The droplets in highly concentrated emulsions are not spherical, but they are deformed and have become polyhedral due to its high degree of packing. The most compact packing ratio of monodispersed spherical drops is 0.74. Above this ratio, the drops are either deformed or polydispersed.4,8,9 The classical method of preparation consists of dissolving a suitable emulsifier in the component that will constitute the continuous phase, followed by stepwise addition of the components which will constitute the dispersed phase, with continuous stirring over a relatively long period of time. In addition, highly concentrated emulsions can be prepared according to other methods.5,6,10,11,15 Formation of W/O highly concentrated emulsions can be achieved, without the need for mechanical stirring, by quickly heating an oil-in-water microemulsion (one single phase system), from a temperature lower than the phase inversion temperature (PIT) to a temperature above it.15 The PIT or hydrophile-lipophile temperature (THLB), is the temperature at which the hydrophilic and lipophilic properties of the surfactant are balanced in the system.16 Preparation methods based on this PIT emulsification principle may allow one to obtain highly concentrated emulsions with small and relatively narrow droplet size distributions, generally in the range 0.5-0.8 µm.10,15 This method also allows one to reduce the time required for preparation and the energy input. Highly concentrated emulsions are interesting for both scientific and practical studies. One of the most interesting applications is their use as reaction media. Polymerization in the continuous and/or dispersed phase leads to highly porous materials such as solid foams. Such materials, with densities smaller than 0.1 g/mL, can be prepared by (14) Solans, C.; Domı´nguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S. E. Colloid Polym. Sci. 1988, 266, 570. (15) Kunieda, H.; Fukui, Y.; Uchiyama, H.; Solans, C. Langmuir 1996, 12, 2136. (16) Shinoda, K. J. Colloid Interface Sci. 1967, 24, 4.
10.1021/la026129z CCC: $25.00 © 2003 American Chemical Society Published on Web 03/01/2003
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polymerization in the continuous phase of concentrated emulsions followed by the removal of the dispersed phase components.17-22 The first patent on this subject17 describes the production of low-density solid foams using a concentrated W/O emulsion, stabilized by sorbitan fatty esters, with monomers such as styrene and the crosslinker divinylbenzene in the continuous phase. Williams studied the preparation of styrene-divinylbenzene crosslinked solid foams, by polymerization in water-in-monomer highly concentrated emulsions.18 It demonstrated the great influence of the volume fraction of the dispersed phase and the oil-to-surfactant ratio on the cellular structure of the foams. It seems that the increase in both the volume fraction of the dispersed phase and the surfactant concentration produces a thinning of the films with separate adjacent droplets, increasing the necks which connect the cells that constitute the solid foams.18 Williams and Sherington studied the formation of the connecting necks between the cells.19,20 It seems that the monomers are adsorbed preferentially on the parts of the surfactant monolayer located in the interstices between adjacent drops.19 Cameron et al. followed the polymerization process by cryo-SEM.20 Their results suggested that the formation of necks between the adjacent cells is due to the contraction of the thin monomeric films during the conversion of monomer to polymer. Ruckenstein et al. reported the preparation of composite polymers by simultaneous polymerization of both hydrophilic and lipophilic monomers in highly concentrated emulsions, stabilized either by ionic or nonionic surfactants.21,22 Two reviews have also appeared on this subject.23,24 A very wide variety of monomers may be polymerized in any phase of highly concentrated emulsions, but styrene and other vinyl monomers have been extensively used as a model monomer.23 The density of the foams can be as low as 0.02 g/mL, and the specific surface area can be as high as 350 m2/g, by addition of an inert oil (porogen).25 The cell size in the foams is typically in the range of 1-20 µm. Foam cell sizes are highly dependent on coalescence during polymerization, and addition of electrolyte has been found to greatly decrease the cell size.26 In all these reports,17-26 the cells were large, with diameters generally bigger than 10 µm, and highly nonhomogeneous due to the high polydispersity of the precursor emulsion. In the present work, polymerization has been carried out in the continuous phase of highly concentrated W/O emulsions, prepared by a method based on the phase inversion temperature principle, in which emulsification was made by increasing rapidly the temperature across the PIT temperature, and simultaneously applying agitation. Emulsions with smaller droplet size (≈1 µm) and lower polydispersity could be obtained, and consequently more homogeneous solid foams can be produced. For comparative purposes, the polymerization has also been carried out in highly concentrated emulsions, prepared by conventional methods. (17) Barby D.; Haq. Z. European patent 0060138 (Unilever), 1982. (18) Williams, J. M.; Wrobleski, D. A. Langmuir 1988, 4, 656. (19) Williams, J. M. Langmuir 1988, 4, 44. (20) Cameron, N. R.; Sherington, D. C.; Albiston, L.; Gregory, D. P. Colloid Polym. Sci. 1996, 274, 592. (21) Ruckenstein, E.; Park, J. S. Polymer 1992, 33, 405. (22) Ruckenstein, E.; Park, J. S. J. Polym Sci., Part C: Polym Lett. 1988, 26, 529. (23) Ruckenstein, E. Adv. Polym. Sci. 1997, 127, 3. (24) Cameron, N. R.; Sherington, D. C. Adv. Polym. Sci. 1996, 126, 163. (25) Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherington, D. C.; Tetley, L. Macromolecules 1991, 24, 117. (26) Williams, J. M.; Gray, A. J.; Wilkerson, M. H. Langmuir 1990, 6, 437.
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Materials and Methods (a) Materials. Hexaethylene glycol n-hexadecyl ether and octaethylene glycol n-dodecyl ether surfactants, abbreviated as C16(OE)6 and C12(EO)8, respectively, were purchased from Nikko Chemicals Co. (Japan). Synperonic L64 was purchased from ICI (Belgium). All surfactants were used without further purification. Styrene and divinylbenzene, either from Merck or Aldrich, were purified by distillation under vacuum or by passing through neutral chromatographic aluminum oxide, to eliminate the inhibitor, which is a nonvolatile polar compound. Initiator potassium persulfate 99%, as well as tetradecane 99%, used as received, were purchased either from Merck or Aldrich. (b) Methods. Preparation of Highly Concentrated Emulsions. Samples were weighted into glass ampules or bottles. If not otherwise stated, the emulsions had a weight fraction of the disperse phase equal to 0.9 and were prepared by first cooling to 0 °C. They were manually stirred, and temperature was quickly increased to either 60 or 70 °C by placing the samples into a water bath of such temperature, with constant manual agitation. Polymerization. Solid polystyrene foams were prepared by polymerization in the continuous phase of concentrated W/O emulsions. Sealed glass ampules or bottles containing the emulsions were kept at 60 °C for 48 h, allowing the polymerization to take place. The wet polystyrene monoliths were then removed by carefully breaking the glass containers. These monoliths were handled cautiously as they were very fragile at this stage. They were washed at 60 °C, first with water and later with ethanol, at least with a solvent volume 10 times higher than the sample volume, for 2 h for each solvent, to remove all the products but polystyrene. Samples with a volume larger than 2 mL were washed twice with each solvent. Very large samples, more than 10 mL, were washed for 6 h each time. Finally, samples were dried by placing in an oven at 70 °C, until weight was constant. Small pieces of macroporous polystyrene monoliths could be cut with a sharp blade. PIT Determinations. Crison 525 instrument, equipped with Pt electrodes, was used to determine the phase inversion temperature, by measuring sample conductivity as a function of the temperature. For this purpose NaCl (0.02 M) was added to samples without polymerization initiator. Optical Microscopy. Digital Images were acquired from a Reichert Polyvar 2 microscope, supplied by Leica. It was equipped with video, polarizers, and an interference contrast prism. Scanning Electron Microscopy. The instrument was a Stereoscan 360, supplied by Leica. Samples were coated by sputtering with a gold layer of 25 nm thickness. Determination of Mechanical Properties. Compression tests, at 0.5 mm/min rate, were carried out in an Instron 5500 R apparatus to determine the stress versus strain curve. The monoliths were cylindrical in shape with approximately 14 mm diameter and 10 mm height. Toughness (total energy of compression from 0 to 90% compression, per initial volume) was calculated from the area underneath the curve, and strength modulus was calculated from the slope at 90% compression. The results were normalized by dividing by the initial density.
Results and Discussion Preparation of W/O emulsions by the PIT method requires a system whose phase inversion temperature is above the freezing point of water and below the temperature at which polymerization takes place. This can be achieved by choosing the nonionic surfactant with the appropriate HLB value for the system. Another condition is that the emulsions must be stable during polymerization at a relatively higher temperature. Aromatic molecules, such as styrene and divinylbenzene, tend to produce less stable emulsions than aliphatic hydrocarbons, because more polar and flat molecules are able to penetrate into
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Figure 1. Conductivity as a function of temperature, for different weight percentages of monomer in the oil, for 0.02 M NaCl/C16(EO)6/styrene/divinylbenzene/tetradecane system (C16(EO)6 ) 2 wt %, W:O ) 90:10, styrene:divinylbenzene ) 4:1).
surfactant monolayers modifying its properties.27 C16(EO)6, which possesses approximately the right HLB value to obtain both styrene-in-water and water-in-styrene emulsions near room temperature, was chosen as the main stabilizing surfactant. C12(EO)8 was added in some formulations to increase PIT. The two monomers used were styrene and divinylbenzene. Tetradecane, a more lipophilic oil than these monomers, was also used to increase PIT. Divinylbenzene, a cross-linking agent, was also added in order to obtain better mechanical properties of the polymers. The radical initiator used was potassium persulfate. (a) Influence of Composition on PIT. The PIT of the different compositions was determined by conductivimetry. Figure 1 shows the conductivity as a function of the temperature, for different weight percentages of the monomer mixture (styrene:divinylbenzene ) 4:1) in the oil, for the system 0.02 M NaClaq/ C16(OE)6/styrene/divinylbenzene/tetradecane, with 2 wt % C16(OE)6 and water:oil weight ratio (W:O) equal to 90:10. The PIT is the temperature at which the conductivity decreases abruptly. It can be seen that the monomers have a strong influence in lowering the PIT. Such molecules may be able to penetrate into the surfactant monolayers, making the surfactant more lipophilic and thus lowering the PIT. The influence of the PIT as a function of the weight fraction of monomer in the oil phase, at two dispersed phase volume fractions, is shown in Figure 2. In Figure 2, the starting point (monomer ) 0), for W ) 50 wt %, has been taken from the literature.28 This point is in good agreement with the rest of the data. It also shows that the PIT has a linear relationship as a function of oil composition. This result is consistent with the equation developed by Kunieda,28 which is valid for systems with poly(ethylene glycol) alkyl ether type surfactants:
PIT ) Koil(HLB - Noil)
(1)
where Koil is a constant equal to 17 °C,28 HLB is the Griffin hydrophilic-lipophilic balance,29 and Noil, which represents the lipophilicity of the oil, depends linearly on oil composition, assuming that the oil is a mixture of ideal miscible liquids, which do not interact with each other. (27) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (28) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (29) Griffin, W. C. J. Soc. Cosmet. Chem. 1949, 1, 311.
Figure 2. Phase inversion temperature as a function of the weight fraction of monomer for the 0.02 M NaCl/C16(EO)6/ styrene/divinylbenzene/tetradecane system, at two dispersed phase (W) percentages, 50 and 90 wt %. (C16(EO)6 ) 2 wt %, styrene:divinylbenzene ) 4:1).
Figure 3. Conductivity as a function of temperature, for the system 0.02 M NaCl/C16(EO)6/tetradecane (NaClaq ) 90 wt %, C16(EO)6 ) 2 wt %). Heating and cooling are indicated by solid and open circles, respectively.
Figure 2 further shows that the PIT is also dependent on the fraction of the dispersed phase, being it slightly higher at 90 wt % than at 50 wt %. This result cannot be explained in terms of the natural curvature of the surfactant, since for a ternary system of pure components, as for zero weight fraction of monomer, PIT should be the same for both diluted and concentrated emulsions. The possible explanation can be kinetic reasons. The temperatures of inversion can be different because the mechanisms of inversion, for concentrated and diluted emulsions, are not the same and may have completely different kinetics. A highly concentrated W/O emulsion can become a diluted O/W emulsion by coalescence of the droplets. Therefore, the activation energy of this process is related to the rupture of the films that surround the droplets. However, the transition from a diluted O/W to a highly concentrated W/O emulsion cannot be achieved simply by coalescence, because this leads to complete phase separation, without emulsion formation. In this case, agitation has to be applied to deform the W/O emulsion droplets. Experimentally, hysteresis loops have been found for the formation of highly concentrated emulsions. Figure 3 shows that the PIT determined when heating the sample to 47.5 °C is higher than that determined if cooling to 44 °C. Therefore, a hysteresis loop results between these two temperatures. The system shown in Figure 3, 0.02 M NaClaq/C16(EO)6/tetradecane, NaClaq ) 90 wt %, C16(EO)6 ) 2 wt %, can be considered a ternary system with a pure surfactant and a pure oil. The hysteresis loop cannot be explained by changes in the natural curvature of the surfactant monolayers. However, it can be due to the different energy barriers which should be overcome when changing from a diluted O/W emulsion to a concentrated
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Figure 4. Phase inversion temperature as a function of the styrene/(styrene + divinylbenzene) ratio and the weight fraction of both monomers in the oil, for the system 0.02 M NaCl/C16(EO)6/styrene/divinylbenzene/tetradecane (W ) 90 wt %, C16(EO)6 ) 2 wt %).
W/O emulsion with the increase in temperature. An additional increase of temperature is needed for such inversion, as the energy barrier may be higher. PIT, in the system 0.02 M NaClaq/C16(OE)6/tetradecane/ styrene/divinylbenzene, depends on two oil composition parameters, which are the styrene/(styrene + divinylbenzene) ratio and the total monomer weight fraction in the oil. The PIT values were determined in two styrene/ total monomer weight ratios, 0.2 and 0.8, for different concentrations of monomer in the system. Figure 4 shows the best fit to the experimental PIT points, according to eq 1 and assuming that Noil varies linearly with oil composition, in a three-dimensional plot. The top line represents a ternary system with tetradecane as oil. The bottom right corner indicates styrene as the only monomer and the bottom left corner indicates divinylbenzene as the only monomer. It can be seen that pure tetradecane provides the highest PIT value, at 47.5 °C, because it is the most lipophilic oil. Styrene and divinylbenzene, with similar properties, are less lipophilic. The extrapolations to pure styrene and pure divinylbenzene are -57.5 and -53 °C, respectively. These values cannot be confirmed experimentally since they are well below the freezing point of the sodium chloride solution. However, Noil can be calculated by applying eq 1. The respective values, at 90 wt % of the dispersed phase, for tetradecane, styrene, and divinylbenzene are 7.63, 13.8, and 13.5. The value reported in the literature for tetradecane, which was determined by phase behavior, is 7.85,28 similar to the value obtained in our experiments. The Noil values for styrene and divinylbenzene, 13.8 and 13.5, respectively, which are higher than those of alkanes and long chain alcohols reported in the literature,28 indicate that the former are relatively more hydrophilic than such oils. This result is consistent with available solubility data. The Hildebrand solubility parameters for styrene and benzene are 9.2 and 9.3 (cal cm-3)1/2, respectively.30 These values are very similar, indicating that styrene has solubility properties very close to those of benzene, which its known solubility in water is relatively high, 1.9 g/L.31 Figure 4 also shows that the PIT extrapolates to negative values for monomer (styrene or divinylbenzene) concentrations higher than approximately 50 wt % in the oil phase. Therefore, highly concentrated emulsions cannot be obtained by the PIT emulsification method, in this (30) CRC Handbook of Chemistry and Physics, 70th ed.; CRC Press Inc.: Boca Raton, FL, 1990. (31) The Merck Index, 11th ed.; Merck & Co. Inc.: Rahway, NJ, 1989.
Figure 5. Micrographs of emulsions prepared in the system H2O/K2S2O8/C16(EO)6/C12(EO)8/Synperonic L-64/styrene/divinylbenzene/tetradecane (89.9/0.1/1.2/0.7/0.1/4.0/1.0/3.0): (a) W/O emulsion obtained after heating the sample from 0 to 70 °C, while stirring very gently by hand; (b) W/O emulsion obtained after heating emulsion a up to 70 °C and simultaneously applying strong agitation by hand; (c) scanning electron micrographs of sample a after polymerization and drying; (d) micrograph of sample b after polymerization and drying.
system. However, PIT was increased by addition of a more hydrophilic surfactant, such as C12(EO)8. The highly concentrated emulsions of the system H2O/ K2S2O8/C16(EO)6/C12(EO)8/Synperonic L-64/styrene/divinylbenzene/tetradecane (89.9/0.1/1.2/0.7/0.1/4.0/1.0/3.0), were used to carry out the polymerizations. The PIT of this system has been calculated to be approximately 2.4 °C by eq 1. If the temperature is changed from 0 to 70 °C without agitation, highly concentrated emulsions are not formed. However, the system transforms from a diluted O/W emulsion at approximately 0 °C to a highly concentrated W/O emulsion, if slight agitation is applied, while the temperature is being raised. Figure 5a shows that the droplets of this emulsion are relatively large, between 3 and 20 µm approximately, similar to highly concentrated emulsions prepared by conventional methods.1-4 The droplet size is much smaller, ≈1 µm, and homogeneous if stronger agitation is applied to the sample, as shown in Figure 5b. Therefore, the final properties of the emulsion obtained after increasing the temperature, from 0 to 70 °C, are dependent on the agitation applied during the thermal treatment of the sample. It should be pointed out that the foam shown in Figure 5d consists of smaller and less polydispersed cells than foams from emulsions prepared by conventional methods, which are generally bigger than 10 µm, as described in the literature.17-26 It has been described in the literature that emulsion instability and coalescence are high around the PIT temperature,11,15 which has also been observed for the present system. The role of agitation may be to deform the droplets, thus allowing the thinning of the oil phase, which becomes the continuous phase of the highly concentrated emulsion. The energy barriers during the transition from diluted O/W to concentrated W/O emulsions, which may cause the hysteresis loop as shown in Figure 3, may also produce inefficient emulsification if temperature is increased without proper agitation. Therefore, increasing temperature across the PIT, and applying
W/O Emulsions as Templates for Solid Foams
agitation should be simultaneous, to achieve a high degree of emulsification, because the starting composition, a diluted O/W emulsion, is not a single phase. (b) Polymerization and Foam Characterization. The polymerization was carried out by keeping the highly concentrated emulsions at 60 °C for 48 h, as described in the Experimental Section. The water soluble initiator, K2S2O8, was used because the highly concentrated emulsions were found to be more stable than using an oil soluble compound such as azoisobutyronitrile (AIBN) as initiator. This result agrees with those described in the literature, showing that the coalescence of highly concentrated emulsions is reduced by the presence of electrolytes.32 Parts c and d of Figure 5 show the micrographs, at approximately the same magnification, of the dried solid polystyrene foams obtained from the emulsions shown in Figure 5a,b, respectively. The cell size of the solid foam shown in Figure 5c is much larger than that shown in Figure 5d. These solid structures are replicas of the highly concentrated emulsions, shown in Figure 5a,b, in which polymerizations were carried out. Therefore, the size of the water-in-styrene emulsion droplets is preserved, producing polystyrene cells with approximately the same size distribution. The micrograph in Figure 5c also shows that the structure of the solid foams consists of polyhedral cells interconnected through narrow necks. This structure is similar to others described for polymerization in highly concentrated emulsions.17-22 The necks would form where the flat films between adjacent drops are, because the distance between surfactant monolayers is relatively small and the volume of styrene to polymerize is also small. The approximate pore volumes of the polystyrene foams, determined by the weight of ethanol that the sample can hold, are 16 and 19 mL/g for 6 and 5 wt % monomer, respectively. Obviously, the smaller the amount of monomer in the oil phase, the higher the pore volume. It should be noted that these values are relatively high, and therefore the samples can be considered as aerogels. Shrinkage of the monoliths can take place during drying, if the framework is not strong enough. Figure 6 shows the monoliths obtained in the system H2O/K2S2O8/C16(EO)6/ styrene/divinylbenzene/tetradecane, at 2 wt % surfactant, styrene:divinylbenzene ratio equal to 7:3, and for two weight fractions of the emulsion dispersed phase. For comparative purposes, the emulsions were also prepared by the conventional method of stepwise addition of the aqueous dispersed phase to the oil continuous phase, under vibromixer agitation. Independent of the emulsification method, no shrinkage was observed if the volume fraction of the dispersed phase was 0.9. For a higher volume fraction of 0.95, the shrinkage was dependent on the emulsification method, as shown in Figure 6a,b. The sample prepared by the PIT method did not have shrinkage at this volume fraction (Figure 6a). However, the emulsions prepared by the conventional method lead to shrinkage of the polystyrene monolith during drying (Figure 6b). These results indicate that emulsifying by the PIT method may lead to stronger frameworks. Much shrinkage was observed in both methods when the volume fraction of the dispersed phase was 0.97, as shown in Figure 6c,d. This dependence on the volume fraction of the dispersed phase can be due to the formation of thicker walls at lower volume fractions. (32) Kunieda, H.; Yano, N.; Solans, C. Colloids Surf. 1989, 36, 313.
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Figure 6. Photographs of the polystyrene monoliths, obtained from highly concentrated emulsions in the system H2O/K2S2O8/ C16(EO)6/styrene/divinylbenzene/tetradecane (2 wt % surfactant, styrene:divinylbenzene ) 70:30, for two emulsification methods: (a) PIT method, weight fraction of the dispersed phase equal to 0.95; (b) vibromixer method, weight fraction of the dispersed phase equal to 0.95; (c) PIT method, weight fraction of the dispersed phase equal to 0.97; (d) vibromixer method, weight fraction of the dispersed phase equal to 0.97.
Figure 7. Density and normalized mechanical properties as a function of the weight fraction of the dispersed phase, in the system H2O/K2S2O8/C16(EO)6/styrene/divinylbenzene/tetradecane (2 wt % surfactant, styrene:divinylbenzene ) 50:50), for the two preparation methods: solid line, PIT method; dashed line, conventional method.
The mechanical properties of the monoliths can be improved by increasing the divinylbenzene content, which results in heavily cross-linked polymers. No shrinkage was observed for styrene:divinylbenzene ) 1:1, at a volume fraction of the dispersed phase equal to 0.95. The mechanical properties of unshrunk monoliths were studied by means of compression tests, as described in the Experimental Section. Strength and toughness were determined from the stress/strain curve. Both parameters were normalized by dividing by the monolith bulk densities, to allow better comparisons. The reaction conditions, as described in the Experimental Section, were kept constant in order to avoid reaction parameters, such as the monomer conversions, influencing the results of mechanical properties. These results, shown in Figure 7, indicate that samples prepared by the PIT method lead to monoliths with higher
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mechanical properties than those prepared by the vibromixer method. In the PIT method, strength was approximately 4 times higher and toughness was approximately 50% higher. Therefore, these experiments may also indicate that smaller and less polydispersed sizes can lead to higher mechanical performance, despite the fact that the bulk monolith density has little dependence on the emulsification method, as also shown in Figure 7. In addition, both strength and toughness decrease with the increase in the volume fraction of the dispersed phase, due to a thinner framework. Similar results have already been described for other systems.18 A more detailed analysis, concerning the mechanical properties of the monoliths as a function of preparation method, composition, and polymerization parameters, will be the subject of a future paper. Conclusions Solid polystyrene foams, with very large pore volumes (15 mL/g) were prepared with a narrow pore size distribution. Such foams have been obtained by polymerization in the continuous phase of highly concentrated waterin-styrene emulsions stabilized by nonionic surfactants. These emulsions were prepared by a method based on the phase inversion temperature principle, in which emul-
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sification was produced by increasing rapidly the temperature across the HLB temperature and simultaneously applying agitation, which is required since the system is unstable near the inversion temperature. The role of the agitation may also be to deform the droplets and thus allow the thinning of the oil phase, which becomes the continuous phase of the highly concentrated emulsion. This process is greatly facilitated by the low interfacial tension values, which are reached at the PIT. The structure of the foams consisted of polyhedral cells, interconnected to each other through narrow necks. The size of the cells could be controlled by composition parameters. The mechanical properties of the polystyrene foams were dependent on the emulsion droplet size distribution and thus on the emulsification method. Acknowledgment. The authors greatly acknowledge the financial support by CICYT (Grant QUI99-0997-CO201) and Generalitat de Catalunya (Grant 2001SGR00357). J.E. acknowledges Generalitat de Catalunya for a RED grant. The authors greatly acknowledge Angela Pascual for her valuable assistance and Dr. R. Fontarnau, from “Serveis Cientı´fico-Te`cnics” of the University of Barcelona for helping in the scanning electron microscopy. LA026129Z
