Article pubs.acs.org/Langmuir
Diving into the Finestructure of Macroporous Polymer Foams Synthesized via Emulsion Templating: A Phase Diagram Study Aggeliki Quell, Thomas Sottmann, and Cosima Stubenrauch* Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany S Supporting Information *
ABSTRACT: During our studies on emulsion-templated monodisperse polymer foams we found significant differences in the finestructure if the locus of initiation is changed. This motivated us to study the phase behavior of the liquid template. Our studies indicate that the template consists of droplets of three different length scales: The water droplets generated via microfluidics (∼70 μm) are surrounded by a continuous phase in which a w/o emulsion (≤100 nm) coexists with a w/o microemulsion (∼5 nm). We speculate that the w/oemulsion droplets act as seeds for the porous finestructure observed in AIBNinitiated polymer foams. We have experimental evidence that the w/o emulsion inverts to an o/w emulsion with progressing polymerization. This explains the granular texture observed in KPS-initiated polymer foams. The control of the finestructure is important in the preparation of tailor-made polymer foams because it directly impacts the material’s density and thus, in turn, its mechanical stability. (isobutyronitrile)) leads to spherical and open-cell foams12,16 (see Figure 1). However, the macroporous pore morphology was not the only property that changed. Zooming into the structure, that is, looking at the samples with a higher magnification (Figure 1, middle and right), a finestructure becomes visible for both polyhedral and spherical pore morphologies. Note that a finestructure of polydisperse interface-initiated emulsion-templated polymer foams was observed as early as 1962,1 but an explanation has not been provided. What we call finestructure in the paper at hand had previously been described as “secondary pore structure”17 or “additional porosity”.18 It was found that the finestructure can be fine-tuned if porogens such as toluene (solvating porogen) or petroleum ether (precipitating porogen) are added to the templating system.17−21 However, the origin of an additional porosity in the absence of a porogen was never investigated. To fill this gap, we studied the origin of the two different finestructures. For this purpose, we carried out an extensive phase diagram study that reveals how the structure of the templating emulsion changes with progressing polymerization. On the basis of the respective phase diagrams and the different loci of initiation, we propose a mechanism that explains both finestructures.
1. INTRODUCTION Emulsion templating is a widespread technique to generate lowdensity polymer foams.1−4 The protocol includes two main steps, namely, emulsion formulation and solidification. First, a nonpolymerizable dispersed phase is emulsified in a liquid matrix of monomers or polymers. Solidification of the template via polymerization1,2,5,6 or gelation7−9 and removal of the internal phase results in polymer foams whose macroporous morphology is well-defined by the morphology of the liquid emulsion template. The properties of the polymer foams can be altered by changing the emulsion composition. Emulsion templating is therefore a promising technique to produce tailor-made polymer foams for specific applications. By controlling the emulsification process using droplet-based microfluidics, monodisperse, highly ordered polymer foams were synthesized as well.10−12 The big advantage of the monodispersity is the fact that the droplet size of the emulsion template can be kept constant. This is beneficial when investigating the effect of different emulsion components on the final polymer foam properties, as demonstrated in Quell et al.12 One of the most studied templates is a water-in oil (w/o) emulsion with water being the dispersed phase and styrene/DVB (divinylbenzene) being the hydrophobic monomers.1,2,5,6,13 The locus of initiation in emulsion-templated polymer foams was found to have a remarkable effect on the polymer foam morphology.9,14,15 In the case of monodisperse emulsiontemplated polymer foams, it was found that initiating from the interface using water-soluble KPS (potassium persulfate) results in honeycomb-shaped closed-cell foams, whereas initiating from the continuous phase using oil-soluble AIBN (azobis© XXXX American Chemical Society
2. MATERIALS AND METHODS Materials. Styrene (99%) and divinylbenzene (DVB, technical grade, 80%) were purchased from Sigma-Aldrich, distilled under vacuum, and kept under nitrogen. Water was bidistilled. The initiators Received: October 15, 2016 Revised: December 17, 2016
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DOI: 10.1021/acs.langmuir.6b03762 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. SEM pictures of emulsion-templated polymer foams with the locus of initiation being at the o/w interface using KPS (top) or in the bulk phase using AIBN (bottom) at different magnifications. The total amount of initiator was 1.28 mol % with respect to the monomers. The respective emulsion templates had the following composition: 1:1 mixture of styrene/DVB (containing 10 wt % Hypermer 2296 as surfactant) as continuous phase and water as dispersed phase. potassium persulfate (KPS, 99%) and azobis(isobutyronitrile) (AIBN, 99%) as well as the pure surfactant octaethylene glycol monodecyl ether (C10E8, >98%) were purchased from Sigma-Aldrich and used as received. The surfactant Hypermer 2296 was kindly supplied by Croda and used as received. The microfluidics setup consists of a pressure controller from Elveflow (OB1MKII), a droplet junction chip from Dolomite Microfluidics with constriction dimensions of 100 μm × 105 μm, and a microscope (Nikon SMZ 745T) with attached high-speed camera (Optronis CL600X2). Methods. Polymer Foam Synthesis. The generation process was described in detail recently.11 In brief, a monodisperse water-inmonomer emulsion is generated by means of a microfluidic device. The continuous phase of the emulsion consists of a 1:1 mixture of styrene and DVB containing 10 wt % of the surfactant Hypermer 2296. For interface-initiated polymerization, 1.28 mol % KPS (potassium persulfate) with respect to the monomers was dissolved in water. For bulk initiated polymerization, 1.28 mol % AIBN (azobis(isobutyronitrile)) was added to the monomer phase. The generated emulsion leaves the microfluidic chip and is collected in a glass tube. The water droplets are allowed to sediment so that neatly packed water droplets in a continuous monomer matrix were obtained. Assuming a close hexagonal packing, the final emulsion templates consisted of 74 vol % internal phase and 26 vol % continuous phase. The emulsion templates were then placed in an oil bath and polymerized at 70 °C for 48 h. The resulting polymer foam was purified by Soxhlet extraction with ethanol for 12 h and dried at 80 °C for 72 h in an oven until constant weight. Investigation of the Phase Behavior. The study of the phase behavior was carried out using a procedure that is described in more detail elsewhere.22 In brief, all components were weighed into test tubes that were then sealed with a PE stopper. The samples were homogenized by shaking or stirring and placed in a temperature controlled water bath with an accuracy of ±0.02 K. While setting the temperature, the sample is stirred continuously. Stirring is turned off after the thermal equilibrium is reached. Then, the number and kind of coexisting phases were determined by visual inspection in both transmitted and scattered light, using crossed polarizers to recognize anisotropic phases. This process is repeated for each temperature setting. After recording all
phase transition temperatures the sample is diluted with water and oil. In the case of the styrene-containing system a new sample was prepared at each composition to avoid uncontrolled polymerization of styrene, which will affect the phase behavior of the sample. Scanning Electron Microscopy. For scanning electron microscopy (SEM) measurements the polymer foams were cut with a razor blade to obtain a flat surface. The specimens were subsequently coated with a thin gold layer and investigated with the CamScan CS44 SEM. (Relative) Foam Density. The relative foam density Rf was calculated by dividing the foam density by the bulk density. For the determination of the foam density, small cubes with an edge length of ∼0.5 cm were cut out of the polymer foam. The exact cube dimensions were then measured with a calliper to calculate the sample’s volume. The foam density was obtained by dividing the sample’s weight by the sample’s volume. The bulk density (ρbulk = 1.14 g cm−3) was obtained by polymerizing a pure styrene/DVB (1:1) monolith and measuring its density by helium displacement pycnometry. Young’s Modulus. The Young’s modulus of the polymer foams was determined by measuring a stress−strain curve in compression using the rheometer Physica MCR 501 from Anton Paar. The Young’s modulus was extracted from the linear portion of the stress−strain curve.
3. RESULTS AND DISCUSSION During our previous studies on monodisperse water-in-styrene/ DVB emulsion-templated polymer foams, we observed the following: If KPS is used to initiate the polymerization from the interface, then one obtains smooth and nonporous pore walls but rough pore faces (compare Figure 1, top). If AIBN was used to initiate the polymerization from the bulk phase, the finestructure was “inverted”; that is, the pore faces were smooth and the pore walls were porous (see Figure 1, bottom). Although the emulsion template only differed in the nature of the initiator (water-soluble vs oil-soluble), the resulting polymer foams varied not only in their morphology but also in their mechanical properties. Note that as a consequence of close-packing of the emulsion droplets the final emulsion templates consisted of 74 vol % dispersed B
DOI: 10.1021/acs.langmuir.6b03762 Langmuir XXXX, XXX, XXX−XXX
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Langmuir phase in 26 vol % continuous phase. (See the Supporting Information for experimental details.) The theoretically expected relative foam density was therefore 0.26. The KPS-initiated polymer foam had a relative density of Rf = 0.21 ± 0.1 and a Young’s modulus of E = 26 ± 5 MPa. Both values are within the expected range. The AIBN-initiated polymer foams, however, had values of Rf = 0.13 ± 0.01 and E = 8 ± 2 MPa. The strong difference in the mechanical stability is attributed to (1) the pore morphology (spherical vs honeycombs), because even if the foam densities were the same, honeycomb-like structures are known to better withstand compression compared with spherical structures,23 and (2) the finestructure, which obviously leads to a much lower foam density due to the porous holes in AIBNinitiated polymer foams. The lower density, in turn, directly affects the Young’s modulus. In a personal communication, Prof. N. R. Cameron, an expert in the field of emulsion-templated polymer foams, hypothesized that the finestructure could arise from microemulsions (μe) that coexist within the actual emulsion.24 Because the finestructure seems to impact directly the foam density and thus, in turn, the mechanical stability of the material, we were highly motivated to study the phase behavior of the liquid templates in detail. The knowledge of how to control the finestructure might play a key role in the preparation of tailormade polymer foams. To countercheck that the finestructure does not arise from a specific component of the emulsion template, three simple experiments were performed before we started a detailed study of whether microemulsions play a role. (1) The two monomers (styrene and DVB) were polymerized in bulk with the oil-soluble initiator AIBN to find out if the open-porous finestructure is caused by the initiator. We found that this was not the case because the polymer’s surface was smooth. (2) The same setup as above was used, but 10 wt % of surfactant (Hypermer 2296) was added to the mixture with the outcome that the obtained polymer surface was still smooth. Thus neither the initiator AIBN nor the surfactant is responsible for the porous finestructure. (3) For the third experiment, the system from experiment (2) was prepared (styrene/DVB (1:1) containing 10 wt % of surfactant and 1.28 mol % AIBN as initiator) and drop-wisely added to a water-containing glass vessel by constant stirring. In this way an oil-in-water emulsion was formed in contrast with the usually formed water-in-oil emulsion that is the preferred emulsion type if Hypermer 2296 is used as surfactant.25 The mixture was polymerized at 70 °C for 48 h by constantly stirring. SEM micrographs of the obtained polymer (see Figure 2) show that the monomer droplets were polymerized and then agglomerated. Additionally, the resulting polymer spheres had a porous finestructure (see inlet of Figure 2). In conclusion, the porosity of the polymer did not arise until water was added to the system. Thus the finestructure occurs in ternary (water−oil−surfactant) systems only, which encouraged us to study the phase behavior of the ternary emulsion template water−styrene−Hypermer 2296 in detail. Note that the second monomer DVB was neglected for the phase behavior studies to keep the system as simple as possible. The phase behavior was studied by measuring a so-called “fishcut” through the phase prism.26,27 This means that at a constant water-to-styrene mass ratio (which was 1:1) the surfactant concentration (or surfactant mass fraction γ) and the temperature T were varied. The goal was to determine the phase boundaries as well as the X-point to specify the conditions under which microemulsions form. The microemulsion community uses the following notation to label the different regions
Figure 2. SEM micrographs of agglomerated polymer spheres. The respective emulsion template was an oil-in-water emulsion with the following composition: 1:1 mixture of styrene/DVB (containing 10 wt % Hypermer 2296 as surfactant) as dispersed phase and water as continuous phase.
occurring in a fish cut: At very low surfactant concentrations, the surfactant is monomerically solubilized in the oil and water phases, respectively. Above a certain concentration the surfactant starts to mix water and oil, forming a microemulsion phase (socalled 3-phase region, 3). This region is surrounded by two twophase regions at low and high temperatures, namely, an oil-inwater microemulsion that coexists with an oil excess phase (2) and a water-in-oil microemulsion with an excess water phase (2̅). By further increasing the surfactant concentration, one reaches the point where the system contains enough surfactant to form a single microemulsion phase (1). The three-phase region and the one-phase region meet at the so-called X-point. The X-point is a measure for the efficiency of the surfactant because it determines the least amount of surfactant needed to solubilize all water and oil in one phase. Determining the phase behavior of the ternary system water− styrene−Hypermer 2296 left us with some unexpected problems. First, the technical surfactant Hypermer 2296, which is a blend of poly(isobutylene)succinic anhydride (PIBSA) and sorbitanester,28 forms an extensive lamellar phase that makes it extremely difficult to detect the phase transitions of the ternary system. Second, styrene polymerized over time, although an inhibitor was used so that it was not possible to determine a stationary phase diagram. To resolve these issues, we substituted styrene with toluene and the technical surfactant Hypermer 2296 with the pure surfactant C10E8. Toluene was chosen because it is chemically similar to styrene but does not polymerize. Hypermer 2296 was substituted with C10E8 to eliminate the lamellar phases and to shift the phase behavior of the ternary system to measurable temperatures. A step-by-step resubstitution of C10E8 by Hypermer 2296 as well as of toluene with styrene allowed us to extrapolate the phase behavior of the base system water− styrene−Hypermer 2296. Figure 3 shows the phase behavior of the system water−toluene−C10E8/Hypermer 2296 as a function of the temperature T and the overall surfactant mass fraction γ at a constant water to toluene ratio of 1:1 and at different surfactant C
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= 0.115 and a temperature of T̃ = 23.8 °C. One can see that this point is shifted to significantly lower temperatures and slightly lower surfactant mass fractions with increasing amount of Hypermer 2296 in the surfactant mixture. When 35% of the surfactant C10E8 is substituted with Hypermer 2296 (δ = 0.35), the X-point is shifted to γ̃ = 0.088 and T̃ = 11.8 °C. Extrapolating to δ = 1.00 (100% Hypermer 2296) reveals that the X-point (T ≈ −11 °C and γ ≈ 0.04) would be beneath the freezing point of water. The ternary system water−toluene−C10E8 was further compared with the ternary system water−styrene−C10E8. It was found that styrene shifts the X-point to lower temperatures (2 K) and to two times higher surfactant mass fractions due to the higher monomeric solubility of C10E8 in styrene compared with toluene.29 The results so far allow us to conclude that in every emulsion template at room temperature as well as at the polymerization temperature (Tinit = 70 °C) the two-phase system 2̅ is formed. In recent studies on the formation kinetics of related oil-rich nonionic microemulsions, it was observed that w/o emulsion droplets that coexist with w/o microemulsion droplets can already form 20 ms after the surfactant-containing oil phase and water get into contact.30 These measurements were carried out with a stopped-flow device and are thus comparable to our flowfocusing device. Therefore, in droplet emulsion generation using microfluidics (MF emulsion droplets, 70 μm), similar structures are expected to be formed in the oil-rich continuous phase, which surrounds the MF emulsion droplets. We assume that this is indeed the case; that is, the continuous phase is a w/o emulsion (∼100 nm) coexisting with a w/o microemulsion (∼5 nm), as shown schematically in Figure 4. The w/o emulsion droplets act as seeds, as they continue taking water from the MF emulsion droplets (see Supporting Information Figure S1). The kinetic studies reported by Klemmer et al.30 combined with the phase behavior studies in the work at hand explain the porous finestructure obtained in AIBN-initiated polymer foams (see Figure 1, bottom). As the continuous monomer phase solidifies, the water droplets of the MF emulsion leave the bulk polymer with the desired macroporosity, and the w/o emulsion droplets cause the porous finestructure. However, the results obtained so
Figure 3. Phase behavior of the system water−toluene−C10E8/ Hypermer 2296 as a function of the temperature T and the surfactant mass fraction γ at a constant water to toluene ratio of 1:1. A gradual substitution of C10E8 with Hypermer 2296 (δ = 0.00 to 0.35) shifts the phase boundaries to significant lower temperatures and to slightly lower surfactant mass fractions.
ratios δ. The black-dotted data points represent the phase boundaries of the ternary system water−toluene−C10E8 (no Hypermer 2296 added, δ = 0.00). The gray and white data points show the phase behavior with increasing substitution of the pure surfactant C10E8 by Hypermer 2296. The data points are extrapolated to determine the X-points. The results are summarized in Table 1, in which the surfactant mass fraction γ̃ Table 1. Surfactant Mass Fraction γ̃ and Temperature T̃ of the X-Point Given As a Function of the Surfactant Ratios δ δ
0.00
0.10
0.20
0.35
γ̃ T̃
0.115 23.8 °C
0.105 21.4 °C
0.105 18.6 °C
0.088 11.8 °C
and the temperature T̃ of the X-point are given as a function of the surfactant ratios δ. For the system with solely C10E8 as surfactant, the X-point is located at a surfactant mass fraction of γ̃
Figure 4. (left) Scheme of the microfluidic-assisted emulsion generation. The water phase and the surfactant-containing monomer phase meet at a junction where the water phase is broken up into monodisperse droplets that are surrounded by the surfactant containing monomer phase. In the monomer phase a w/o emulsion and a w/o microemulsion form instantly. (middle) Scheme of the collected emulsion template. (right) Zoom into the emulsion template with visualization of the coexisting emulsion. The initially formed w/o emulsion solidifies when AIBN is used as initiator. The w/o emulsion inverts into a o/w emulsion with progressing polymerization when KPS is used as initiator. D
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emulsion droplets the polymerization leads to a finestructure arising from additional polymer (granular texture) and not from voids (porous texture). A scheme of the coexisting o/w emulsion and o/w microemulsion droplets within the emulsion template is also shown in Figure 4 (right).
far do not explain the granular texture in KPS-initiated polymer foams. By studying the phase behavior of n-alkane-containing microemulsions, it has been found that the phase boundaries of the “fish-cut” shift toward higher temperatures with increasing chain length of the n-alkane.31 Therefore, another factor that needs to be accounted for and which could explain the inverted finestructure in KPS initiated polymer foams is the rising hydrophobicity of the oil phase with progressing polymerization. To simulate the polymerization in a controlled manner, defined amounts of polystyrene were solubilized in styrene. Figure 5
4. CONCLUSIONS We propose that the different finestructure in AIBN- and KPSinitiated polymer foams can be attributed to the following reasons. AIBN initiation (bulk-phase initiation) results in a fast increase in the overall viscosity in the system because the polymerization starts “everywhere” at the same time. The initial w/o emulsion droplets grow and at some point freeze due to the increasing viscosity of the oil phase, which results in a porous finestructure. This happens before a phase inversion can occur. KPS initiation, on the contrary, starts locally at the o/w interface. We speculate that this leads to a moderate increase in the viscosity of the bulk phase so that the system has time to invert from 2̅ to 2 before it solidifies. Therefore, a granular texture is observed in KPS initiated polymer foams because now oil emulsion droplets are polymerized in addition to the continuous phase. We are aware that the inverted oil droplets somehow need to pass the polymerization front to solidify as granules at the o/w interface. Note that both the granules and the voids have diameters up to 1 μm, which is one order of magnitude larger than the size of the emulsion droplets (≤100 nm).26,32 However, as explained above, the emulsion droplets can be seen as seeds for the bigger structures because they can grow as a function of time while they take up either water or oil. We conclude that the finestructure in both AIBN- and KPSinitiated polymer foams arises from spontaneously formed emulsion droplets in the continuous phase of the MF emulsion template. Our investigations of the phase behavior of the emulsion template indicate that the continuous phase of the MF emulsion is a w/o emulsion (≤100 nm) coexisting with a w/o microemulsion (∼5 nm). The w/o emulsion droplets grow as they take water from the MF emulsion droplets, thus forming bigger water droplets over time. On the one hand, the fast initiation from the bulk-phase using AIBN leads to a fast increase in the emulsion viscosity and the template solidifies as it is. Hence the pore walls of the resulting polymer foam are left with small pores (voids), which arose from the w/o emulsion droplets. On the other hand, using KPS to initiate the polymerization from the interface, the viscosity of the MF emulsion increases more slowly. Therefore, the w/o emulsion has time to change into an o/w emulsion with progressing polymerization. Our investigations of the phase behavior have proved that this is the preferred state with increasing degree of polymerization. Our experiments reveal that the finestructure can be controlled by the type of initiator. We found that initiating with AIBN results in polymer foams with an additional porosity inside the pore walls while initiating with KPS results in dense pore walls with additional granules on the polymer surface. By using a 1:1 mixture of AIBN/KPS, the additional porosity as well as the granules were almost eliminated (see Supporting Information). We therefore think that the ratio of the initiators is a potent tuning parameter for the finestructure. Furthermore, it has been shown that the viscosity has a considerable influence on the kinetics of structural transformations in microemulsions.33 Therefore, the finestructure of the resulting polymer foam might be controllable by the viscosity of the initial emulsion using more viscous monomers or by adding cosolvents with a higher viscosity. Additionally, a faster polymerization reaction using
Figure 5. Phase behavior of the system water−styrene/polystyrene− C10E8 as a function of the temperature T and the polystyrene concentration csurf at constant surfactant mass fraction of γ = 0.20. With increasing polystyrene content the phase boundaries of the fishdiagram shift to higher temperatures.
shows the phase behavior of the system water−styrene/ polystyrene−C10E8 as a function of the temperature T and the polystyrene concentration cPS. The surfactant mass fraction was kept constant at γ = 0.20, because typical emulsion templating recipes contain between 10 and 20 wt % surfactant.1,11,13 It can be seen that with increasing polystyrene content the phase boundaries are shifted to higher temperatures and the threephase region becomes larger. Both observations are direct consequences of the increasing hydrophobicity of the oil phase. Please note that because of the solubility limit of PS in styrene the phase behavior of samples with a PS concentration larger than 5.2 wt % in styrene could not be measured. Furthermore, the values for the phase boundaries of the sample with the highest PS concentration are very inaccurate because the phase separation took many days and PS started to precipitate during the measuring time. However, it can be assumed that increasing the hydrophobicity of the oil phase even further, one would obtain an extensive three-phase region at very high temperatures. These findings provide a plausible explanation for the granular finestructure observed in KPS-initiated polymer foams. With increasing degree of polymerization the three-phase region shifts to higher temperatures such that o/w emulsion droplets in coexistence with an o/w microemulsion are formed within the water droplets (70 μm) generated by the microfluidic. This change is possible in the KPS system only because here the viscosity of the bulk phase presumably does not increase as quickly as in the AIBN system, due to the slower polymerization reaction. If we now have oil emulsion droplets instead of water E
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(13) Manley, S. S.; Graeber, N.; Grof, Z.; Menner, A.; Hewitt, G. F.; Stepanek, F.; Bismarck, A. New insights into the relationship between internal phase level of emulsion templates and gas-liquid permeability of interconnected macroporous polymers. Soft Matter 2009, 5, 4780− 4787. (14) Robinson, J. L.; Moglia, R. S.; Stuebben, M. C.; McEnery, M. A. P.; Cosgriff-Hernandez, E. Achieving Interconnected Pore Architecture in Injectable PolyHIPEs for Bone Tissue Engineering. Tissue Eng., Part A 2014, 20, 1103−1112. (15) Gitli, T.; Silverstein, M. S. Bicontinuous hydrogel-hydrophobic polymer systems through emulsion templated simultaneous polymerisations. Soft Matter 2008, 4, 2475−2485. (16) Quell, A.; Heitkam, S.; Drenckhan, W.; Stubenrauch, C. Controlling material distribution in porous polymers by osmotic transport: a possible explanation why Nature forms honeycombs. ChemPhysChem 2016, DOI: 10.1002/cphc.201600834. (17) Hainey, P.; Huxham, I. M.; Rowatt, B.; Sherrington, D. C.; Tetley, L. Synthesis and ultrastructural studies of styrene-divinylbenzene polyhipe polymers. Macromolecules 1991, 24, 117−121. (18) Ceglia, G.; Mahéo, L.; Viot, P.; Bernard, D.; Chirazi, A.; Ly, I.; Mondain-Monval, O.; Schmitt, V. Formulation and mechanical properties of emulsion-based model polymer foams. Eur. Phys. J. E: Soft Matter Biol. Phys. 2012, 35, 31−42. (19) Santora, B. P.; Gagné, M. R.; Moloy, K. G.; Radu, N. S. Porogen and Cross-Linking Effects on the Surface Area, Pore Volume Distribution, and Morphology of Macroporous Polymers Obtained by Bulk Polymerisation. Macromolecules 2001, 34, 658−661. (20) Barbetta, A.; Cameron, N. R. Morphology and Surface Area of Emulsion-Derived (PolyHIPE) Solid Foams Prepared with Oil-Phase Soluble Porogenic Solvents: Span 80 as Surfactant. Macromolecules 2004, 37, 3188−3201. (21) Barbetta, A.; Cameron, N. R. Morphology and Surface Area of Emulsion-Derived (PolyHIPE) Solid Foams Prepared with Oil-Phase Soluble Porogenic Solvents: Three-Component Surfactant System. Macromolecules 2004, 37, 3202−3213. (22) Jakobs, B.; Sottmann, T.; Strey, R. Efficiency boosting with amphiphilic block copolymers: A new approach to microemulsion formulation. Tenside, Surfactants, Deterg. 2000, 37, 357−364. (23) Ashby, M. F. The properties of foams and lattices. Philos. Trans. R. Soc., A 2006, 364, 15−30. (24) Cameron, N. R., Personal communication, 2015. (25) Bancroft, W. D. The theory of emulsification. J. Phys. Chem. 1912, 17, 501−519. (26) Stubenrauch, C. Microemulsions: Background, New Concepts, Applications, Perspectives; Wiley-Blackwell: Oxford, 2009. (27) Kahlweit, M.; Strey, R. Phase Behavior of Ternary Systems of the Type H2O−Oi −Nonionic Amphiphile (Microemulsions). Angew. Chem., Int. Ed. Engl. 1985, 24, 654−668. (28) Graeber, N. A Study of Fundamentals in Emulsion Templating for the Preparation of Macroporous Polymer Foams. Ph.D. Thesis, Imperial College London, 2013. (29) Burauer, S. Diploma Thesis, Universität Köln, 1997. (30) Klemmer, H. F. M.; Harbauer, C.; Strey, R.; Grillo, I.; Sottmann, T. Formation Kinetics of Oil-Rich, Nonionic Microemulsions. Langmuir 2016, 32, 6360−6366. (31) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. On microemulsion phase behavior and the monomeric solubility of surfactant. Phys. Chem. Chem. Phys. 1999, 1, 4299−4306. (32) Strey, R. Microemulsion microstructure and interfacial curvature. Colloid Polym. Sci. 1994, 272, 1005−1019. (33) Gotter, M.; Sottmann, T.; Baciu, M.; Olsson, U.; Wennerström, H.; Strey, R. A comprehensive, time-resolved SANS investigation of temperature-change induced sponge-to-lamellar and lamellar-to-sponge phase transformations in comparison to 2H-NMR results. Eur. Phys. J. E: Soft Matter Biol. Phys. 2007, 24, 277−295.
other initiators, a higher initiator concentration, or a higher polymerization temperature would lead to a faster increase in the viscosity and thus to a faster fixation of the structure. In other words, finetuning the time scales of the phase inversion and the polymerization, respectively, one has another way via which it should be possible to control the finestructure.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03762. Experimental procedures and additional observations. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Cosima Stubenrauch: 0000-0002-1247-4006 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Jeffrey Appiah for measuring the phase behavior, Prof. Dr. R. Strey for fruitful discussions, and Dr. A. Fels for his SEM support. We acknowledge funding from the German Research Foundation (DFG) (STU 287/4-1).
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b03762 Langmuir XXXX, XXX, XXX−XXX
