How the Locus of Initiation Influences the Morphology and the Pore

Article pubs.acs.org/Macromolecules

How the Locus of Initiation Influences the Morphology and the Pore Connectivity of a Monodisperse Polymer Foam Aggeliki Quell,† Benedetta de Bergolis,† Wiebke Drenckhan,‡ and Cosima Stubenrauch*,† †

Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany Laboratoire de Physique des Solides, CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France

‡

S Supporting Information *

ABSTRACT: The properties of polymer foams are tightly linked to the porous structure of the material: two of the most decisive parameters are the interconnectivity of the pores and the thickness of the pore walls. Despite the vital importance of these parameters, a deep understanding of the processes that control the wall thickness and the pore opening is still lacking. We tackle these questions by studying monodisperse, highly ordered polymer foams which are generated via emulsion templating using microfluidic lab-on-a-chip techniques. We explore the influence of different processing parameters, and we show that the most crucial parameter is the locus where the polymerization is initiated. If initiation starts within the continuous monomer matrix, the morphology of the liquid template is “frozen in” with pore openings arising where neighboring drops are separated by thin films. However, if the locus of initiation is at the interface, not only do the pores remain closed, but we evidence a hitherto unexplained mechanism which leads to an osmotically driven redistribution of monomer in the walls during polymerization. This changes dramatically the pore morphology (polyhedral pores with thick walls) and therefore the final material propertiesopening the pathway to new applications of low weight or acoustic bandgap materials.

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described.3 The reported inverse emulsion was a water-instyrene emulsion whose continuous phase was polymerized. After removal of the aqueous phase one obtains a polymer foam. Emulsion templated polymer foams were later on referred to as polyHIPEs, with HIPE being the abbreviation for high internal phase emulsion.4 In such emulsions the volume fraction of the dispersed phase is 74 vol % or higher. To date, the most frequently studied emulsion templates are surfactantstabilized styrene/divinylbenzene HIPEs with water being the dispersed droplet phase. Over the years many researchers have taken up this templating route and started to tune the solid foam properties via the composition of the templating emulsion. It has been found that by modifying the template, the pore connectivity, the average pore size, the pore density, and thus the density of the polymer foam as well as the strut thickness can be varied.5−17 What is important for the study at hand is the fact that the resulting polymer foams nearly always are open-cell foams with spherical pores. The few exceptions we are aware of are described in the following. Most surfactant-stabilized HIPEs are polymerized with the water-soluble initiator KPS (potassium persulfate). In 1988, Williams and Wrobleski studied the influence of the HIPE’s

INTRODUCTION Polymer foams consist of a gaseous dispersed phase in a solid polymer matrix. The presence of the gas phase, which can be larger than 90 vol %, does not only reduce the weight but also gives the material interesting thermal, acoustic, and mechanical properties. Thus, polymer foams are of particular interest for packaging and insulation materials because low weight packaging materials save transport energy and low weight insulation materials save both transport and thermal energy. Besides large-scale industrial manufacturing techniques for polymer foams like foam extrusion1 there are also small-scale routes based on the use of templates. Depending on the physical state of the internal phase, one can distinguish between hard and soft templating processes. In the first case, solid particles build up a pattern which is immersed in a continuous phase. After the continuous phase is solidified, the solid particles are removed, and thus an open-cell foam is formed.2 As regards soft templating, the template consists of “soft matter”, e.g., of micelles or droplets. Soft templating has the advantage that the templating internal phase can permeate through the solid polymer network without necessarily perforating it. Soft templating can therefore lead to both open-cell and closed-cell foams. A well-known type of soft templating is emulsion templating. The first emulsiontemplated polymer foam ever reported dates back to 1962 when the polymerization of so-called inverse emulsions was © XXXX American Chemical Society

Received: March 8, 2016 Revised: May 8, 2016

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DOI: 10.1021/acs.macromol.6b00494 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Robinson et al. did not address is the fact that the pores have a different shape (spherical or polyhedral) if one changes the locus of initiation. The study at hand is a follow-up of our previous work in which we showed for the first time that monodisperse, highly ordered water-in-styrene HIPEs can be generated with a microfluidic device and that the polymerization of these HIPEs leads to monodisperse, highly ordered polymer foams.19 While carrying out this work we made two observations which were in contrast to our expectations. (1) All obtained polymer foams were closed-cell foams, and (2) the pores had a polyhedral shape. We explained the formation of closed-cell foams with the fact that “the ordered monodisperse sample is at its jamming fraction and, therefore consists of neatly packed spheres which are not compressed enough to form thin films in the liquid state. The water-soluble initiator can therefore ensure sufficient polymerization of the droplet surfaces before the shrinkage of the continuous phase leads to the formation of the flat films”,19 which are expected to rupture during polymerization. In other words, a polymer skin surrounding the water droplets is formed, which, in turn, suppresses the formation of windows. As regards the polyhedral shape, we argued that this is due to shrinkage during polymerization, which, in turn, leads to a decrease of the volume fraction of the continuous phase. While the explanation for the closed-cell foam still holds true, we realize now that the explanation for the polyhedral shape is only half the truth. What we did not discuss is the fact that the pore walls have a homogeneous thickness along most of the film, while the template consists of spherical droplets which are separated by thick nodes and thin films. What we need to obtain polyhedral pores with homogeneous pore wall thicknesses is a transport of material from the nodes to the films. The paper at hand therefore aims at answering two questions: (1) How can monodisperse, highly ordered opencell foams be generated? (2) Can open-cell foams be synthesized whose pores have polyhedral shape? To answer these questions, we investigated systematically the influence of the surfactant concentration, the amount of the dispersed phase, the temperature of polymerization, and the locus of initiation on the final foam structure.

surfactant concentration on the resulting polyHIPE and found that closed-cell foams are obtained if the surfactant concentration is below 7 vol % (with respect to the oil phase).5 Increasing the surfactant concentration, one obtains interconnected pores, i.e., an open-cell foam. They argued that by increasing the surfactant concentration, the monomer films separating adjacent emulsion droplets become thinner and eventually break at a critical film thickness. In other words, the more surfactant is present, the less material can be polymerized. In order to gain a better understanding of the interconnection (or “window”) formation in polymer foams, Cameron et al. used cryo-SEM, and they concluded that the pore connectivity arises from the shrinkage of the continuous phase during polymerization.7 The same observation was made by Costantini et al., who investigated the pore connectivity in monodisperse dextran−methacrylate-based polyHIPEs.8 Reducing the surfactant concentration, one obtains a closed-cell foam at surfactant concentrations around 1 wt %. For higher surfactant concentrations they observed an increasing pore connectivity with increasing surfactant concentration, which is in line with the study of Williams et al. Gurevitch et al. obtained closed-cell polyhedral pores by initiating from the interface (using KPS). They argued that the HIPE maintained its initially polyhedral droplet shape during polymerization, thus forming polyhedral closed-cell pores.17 Wu et al. studied MIPEs (medium internal phase emulsions) as template with initially spherical droplets. They obtained closedcell polyhedral pores via interface initiation and explain their findings by a rapid polymerization at the interface which prevents pore opening.10 However, the origin of the polyhedral pore shape was not addressed. Livshin et al. reported polyhedral closed-cell foams as well and argued that they form due to a destabilization of the HIPE.12 However, it is not explained why coarsening and Ostwald ripening should result in closed-cell polymer foams with pores of polyhedral shape. By replacing the water-soluble initiator KPS by the oil-soluble initiator AIBN (azobis(isobutyronitrile)), the influence of the locus of initiation in surfactant-stabilized HIPEs was investigated. The main effect associated with the change of the initiator is reported to be a lower mechanical stability of the AIBN-initiated polyHIPEs,6,9,10 while the structure of the resulting polymer foams was similar in most cases, namely, open-cell foams with spherical pores. However, thedifferent(?)fundamental processes behind KPS- and AIBNinitiated polymerization are hardly addressed. In a recent study from Robinson et al. the influence of the locus of initiation in surfactant-stabilized HIPEs was investigated in more detail.18 If the polymerization is initiated in the bulk phase (corresponding to the AIBN-initiated systems mentioned above), the resulting polyHIPEs are open-cell foams with spherical pores. If the polymerization is initiated at the interface (corresponding to the KPS-initiated systems mentioned above), the resulting polyHIPEs are closed-cell foams with polyhedral pores. Robinson et al. explain their findings as follows: interfacial initiation directly stabilizes the thin film between two adjacent water droplets, while bulk initiation induces “forces that pull and tear the thin film between the pores”, which, in turn, “results in interconnects between the pores”. The force they refer to is “the force of densification upon conversation of macromer to polymer”. Although they do not explain this force in more detail, one can interpret it as the force that occurs due to the shrinkage of the continuous phase during polymerization as already mentioned above. What

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MATERIALS AND METHODS

Microfluidics Setup. The microfluidic chip with a junction cross section of 100 μm × 105 μm and a channel length of 79 μm was purchased from Dolomite Microfluidics. The flow is pressure controlled, and control is provided by an OB1MkII pressure controller from Elveflow. For optical monitoring a Nikon SMZ 745T microscope is used with an Optronis CL600X2 high speed camera. Synthesis of Monodisperse Polymer Foams. The continuous phase of the emulsion consists of a 1:1 mixture of styrene and divinylbenzene (DVB) containing 10 wt % of the surfactant Hypermer 2296. For the interface initiated polymerization 1.28 mol % KPS (potassium persulfate) with respect to the monomers was dissolved in water. For the bulk initiated polymerization 1.28 mol % AIBN (azobis(isobutyronitrile)) was added to the monomer phase. The emulsion was generated with the microfluidic chip, collected in a glass tube, and allowed to sediment so that we obtained neatly packed water droplets in a continuous monomer matrix. The final emulsion templates consisted of 74 vol % internal phase and 26 vol % continuous phase. The emulsion template was 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 room temperature. Scanning Electron Microscopy (SEM). For SEM measurements the polymer foams were cut with a razor blade in order to obtain a flat B


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Macromolecules surface. The specimens were subsequently coated with a thin gold layer and investigated with the CamScan CS44 SEM. Material Characterization. The polymer foams were characterized by measuring their density and their Young’s modulus. The relative density 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 approximately 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. 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 via a linear regression.

Taking KPS 2 as starting and reference point, we wanted to find a way to open the pore walls, i.e., to synthesize a monodisperse, highly ordered, open-cell foam. For this purpose we changed various parameters, namely the surfactant concentration of the HIPE, the amount of the dispersed phase (via the emulsion height), and the polymerization temperature (see Table 1 for details). We will discuss the influence of each of these parameters with respect to window formation. As mentioned before, the polymerization of surfactantstabilized HIPEs was found to lead to closed-cell pores in polydisperse polyHIPEs at surfactant concentrations below 7 vol %5 and in monodisperse polyHIPEs at surfactant concentrations around 1 wt %.8 In both cases an increase of the surfactant concentration led to the formation of open-cell foams. In our systems, on the contrary, we never observed the formation of open-cell monodisperse polyHIPEs even upon increasing significantly the surfactant concentration. We increased the surfactant concentration from 5 up to 30 wt % (KPS 1−5) with respect to the oil phase and found that the pore wall thickness h decreases from ∼10 down to ∼3 μm, yet window formation was not observed (see Figure 2). Plotting the obtained pore wall thicknesses against the respective surfactant concentrations, one obtains a linear relation (compare Figure 2, right). The linear regression predicts a critical surfactant concentration of 36 wt %, at which the pore wall thickness would be zero. However, as our experiments at 40 wt % surfactant show (compare Figure S1 in the Supporting Information), at such high surfactant concentration the HIPE template disintegrates before or during polymerization without transferring its morphology to the polymer foam. The reason for the decreasing pore wall thickness with increasing surfactant concentration is the dilution of the monomers by the surfactant. We recall that the template consists of hexagonally close-packed water droplets in a continuous styrene/DVB phase with 74 vol % water and 26 vol % oil. For water droplets with a diameter of around 70 μm one can estimate that a total of 0.6 wt % surfactant with respect to the oil phase would be sufficient to form a densely packed layer around the droplets. Thus, concentrations of over 5 wt % used in the literature and in the paper at hand are not needed to stabilize the emulsion, i.e., that any excess surfactant acts as cosolvent in the monomeric oil phase. Consequently, the pore walls get thinner because there is less material present which can be polymerized. Note that diluents in polyHIPEs are used to gain porosity within the polymer matrix.20 Our second attempt to open the pores of the polymer foam was to decrease the amount of continuous phase of the emulsion which leads to the formation of increasingly polyhedral droplets separated by thin films in the liquid template. To obtain such emulsions, we used the effect of gravity: the heavier water droplets sediment over time in the lighter continuous phase (styrene/DVB). The water volume fraction at the bottom of the emulsion is therefore higher than on the top of the emulsion. The volume fraction at the top of the emulsion is always identical (74 vol %) corresponding to close-packed spherical droplets. However, the volume fraction at the bottom of the emulsion depends on the height of the emulsion or, more specifically, on the distance D from the top of the emulsion. Knowing the density difference between the dispersed and the continuous phase (Δρ = 0.075 g cm−3), the interfacial tension (γ = 3.3 mN m−1), and the droplet size (ddrop ≈ 75 μm), the variation of the water volume fraction with the

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RESULTS AND DISCUSSION PolyHIPE Synthesis with Water-Soluble Initiator. In the following we use as reference system (“KPS 2”) a HIPE consisting of 74 vol % dispersed water phase and 26 vol % oil phase (1:1 mixture of styrene/DVB containing 10 wt % surfactant). This system is the one we studied in our previous work.19 In our previous as well as in the present study, the HIPE was produced via microfluidics, which is why all water droplets have the same size and arrange themselves in a hexagonally closed-packed order. The polymerization of this monodisperse HIPE leads to a monodisperse and highly ordered polyHIPE. The polymerization was initiated from the aqueous phase using KPS. The obtained pore morphology of the reference system KPS 2 can be seen in Figure 1. All

Figure 1. Pore morphology of a KPS-initiated polyHIPE (referred to as reference system “KPS 2”).

obtained pore walls stayed intact (no interconnection or window formation), leading to typical closed-cell foams. As reportedbut not explainedin the literature,10−12 interfaceinitiated polymerization of a HIPE can lead to polyhedral pores. Because of the monodispersity of our template, the obtained pore morphology not only is polyhedral but also exhibits a regular honeycomb structure with a homogeneous pore wall thickness of 10 μm. The thick pore walls are associated with a redistribution of the continuous phase from the vertices to the struts upon polymerizationthe explanation of which is given further below. C


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Table 1. Summary of the Composition and Properties of the Different Foam Samples Generated with the Water-Soluble Initiator KPSa csurf (wt %)

emulsion height (cm)

Tinit (°C)

dpore (μm)

1 2 3 4 5 6

5 10 15 21 30 10

1 1 1 1 1 3

70 70 70 70 70 70

71 69 67 59 74 70

KPS 7

10

6

70

64

KPS 8

10

12

70

68

KPS 9 KPS 10

10 10

1 1

60 50

69 68

name KPS KPS KPS KPS KPS KPS

pore wall thickness h (μm) 10.4 7.6 6.2 3.1 2.5 8.7 5.6 8.3 4.9 8.0 4.0 5.4 5.0

(top) (bottom) (top) (bottom) (top) (bottom)

relative density 0.68 0.47 0.65 0.57 n.a. n.a.

± ± ± ±

0.05 0.06 0.05 0.06

Young’s modulus (MPa) 13 ± 26 ± 15 ± 25 ± n.a. n.a.

2 2 2 2

n.a.

n.a.

n.a.

n.a.

0.37 ± 0.01 0.23 ± 0.01

14 ± 2 14 ± 2

a

Surfactant concentration csurf, emulsion height, and temperature of initiation Tinit of the HIPES as well as pore diameter dpore, pore wall thickness h, relative density, and Young’s modulus of the resulting polyHIPEs. All HIPEs have a DVB content of 50 wt % (with respect to the overall continuous phase) and a KPS concentration of 1.28 mol % (with respect to the monomers (styrene and DVB)). KPS 2 is the reference system.

Figure 2. (left) SEM micrographs of monodisperse polymer foams (KPS 1, KPS 2, and KPS 4) prepared with surfactant concentrations ranging from 5 to 21 wt % with respect to the oil phase. The pores of all polymer foams have roughly the same diameter of dpore = 65 ± 5 μm. (right) Pore wall thickness h as a function of the surfactant concentration csurf used in the respective emulsion template prior to polymerization.

Figure 3. (a) Predicted variation of the water volume fraction with distance D from the top of the water-in-styrene/DVB emulsion (with Δρ ≈ 0.09 g cm−3, γ ≈ 35 mN m−1, ddrop ≈ 75 μm) using the semiempirical theory developed by Maestro et al.21 This can be used to predict the variation of the final film thickness with distance from the top of the emulsion assuming that all polymer is distributed homogeneously throughout the structure. The prediction (black line) is shown together with the experimental results for three different sample KPS 6−8 (circles). (b) SEM micrographs of the KPS 8 sample at the top and at the bottom of an emulsion which is 12 cm high.

D


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Macromolecules distance D from the top of the emulsion can be calculated numerically using the semiempirical theory developed by Maestro et al.21 The result of these calculations is shown in Figure 3a. A higher volume fraction of the dispersed phase results in thinner films separating the emulsion droplets which we expected to give rise to a pore opening in the polymer foams. We therefore prepared emulsion templates with heights from 3 to 12 cm (KPS 6−8) and polymerized them under standard conditions. SEM micrographs from the top and the bottom of each sample were taken to analyze and compare the obtained pore morphology. As expected, the pore walls were thicker at the top than at the bottom of each polyHIPE (see Table 1 and Figure 3b). However, even with the highest emulsion sample we still did not observe any window formation between the pores but only intact pore walls. Figure 3b shows SEM pictures taken at the top and at the bottom of a sample of KPS which had a height of 12 cm. As in the previous SEM micrographs, one observes polyhedral pores with fairly homogeneous film thicknesses even at different heights of the emulsion. We can use the prediction of the volume fraction of the emulsion in order to predict the film thickness, assuming that all polymer is distributed evenly throughout the structure. For this calculation we assume that the final emulsion structure can be approximated by a Kelvin structure which consists of truncated octahedra and which is commonly observed in monodisperse foams.22,23 Using this structure together with the assumption that the pores are bounded by films of homogeneous thickness h, the relationship between h, the droplet diameter ddrop, and the continuous volume fraction φ of our emulsions can be approximated as h ≈ 0.303ddrop((1 − φ)/φ)

To conclude this part, we can say that a variation of neither the surfactant concentration, the amount of the dispersed phase, nor the polymerization temperature led to the formation of an open-cell foam in our system. We must admit that these results astonished usespecially the fact that the variation of the parameters had no effect on the connectivity but only on the thickness of the pore walls. We like to remind the reader that we formulated a HIPE that contained 30 wt % surfactant and still did not observe window formation. We did not only obtain closed-cell foams in all cases, but the pores of these foams also all had a polyhedral shape. Whenever the initiation is interface initiated and the radical concentration is sufficiently high, the rapid polymerization prevents film rupture10 independent of other templating parameters. As a last resort we thus decided to change the locus of initiation. PolyHIPE Synthesis with Oil-Soluble Initiator. We used the same formulation and preparation protocol as in the previous section except for the initiator which is now the oilsoluble AIBN (instead of the water-soluble KPS) and therefore located in the continuous monomer phase. Looking at Figure 4,

(1)

We use φ from the numerical calculations shown on the left of Figure 3a. The obtained relationship for the film thickness with emulsion height is shown on the right of Figure 3a as black line together with the experimental data for three samples KPS 6− 8. The good agreement between theory and experiment raises a number of important questions to which we shall get back at the end of this article. The third attempt to obtain open-cell foams was a reduction of the initiation temperature Tinit from 70 to 50 °C. The results are shown and discussed in the Supporting Information (Figure S2). What is of importance for the study at hand is the observation that this change did still not lead to open-cell foams and/or to a significant change of the pore size and the pore morphology. Just like reported by Wu et al.,10 the rapid interface initiation obviously creates a solid polymer film around the droplets which prevents the generation of open-cell foams. Except for samples KPS 9 and KPS 10 (see Table 1), all other samples were polymerized at 70 °C. According to Wu et al., polymerization at 70 °C leads to a high radical concentration, and the water/monomer interface solidifies quickly without film rupture. In case of samples KPS 9 and KPS 10 which were polymerized at 60 and 50 °C, respectively, we also obtained closed-cell foams. Wu et al. argue that at T < 70 °C the radical concentration is too low to form a closed skin around the water droplet which is why polyHIPEs synthesized at T < 70 °C are nearly always open-cell foams. In our case, however, we obtain a closed-cell foam even at T = 50 °C because our KPS concentration is nearly 2 times higher than the one used by Wu et al.

Figure 4. Pore morphology of a bulk phase initiated polyHIPE (SEM micrograph of the reference system AIBN 1).

one sees that the polymerization of a monodisperse HIPE containing AIBN leads to open-cell foams with spherical pores! Thus, open-cell, monodisperse styrene/DVB polymer foams with spherical pores can be obtained if the locus of initiation is no longer at the interface (water-soluble initiator, KPS) but in the bulk phase (oil-soluble initiator, AIBN). A detailed discussion of the different pore morphologies is given further below. The formulations of all AIBN-containing HIPEs are listed in Table 2. After having obtained an open-cell structure, we tried to change the degree of interconnectivity by varying different template parameters, namely (a) the AIBN concentration, (b) the composition of the monomer phase by diluting it with nhexane, (c) the styrene/DVB ratio, and (d) the initiation temperature. As was the case for the KPS-containing samples, we will discuss the influence of each of these parameters with respect to the degree of interconnectivity. A selection of monodisperse AIBN initiated polyHIPE structures is given in Figure 5. (a) We found that the stability of the emulsion decreased significantly when the AIBN concentration is 2 mol % or higher (with respect to the monomers styrene and DVB). Reducing E


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Macromolecules

Table 2. Summary of the Composition and Properties of the Different Foam Samples Generated with the Oil-Soluble Initiator AIBNa name AIBN AIBN AIBN AIBN AIBN

1 2 3b 4c 5d

cinit (mol %)

dpore (μm)

dwindow (μm)

dwindow/dpore (%)

1.28 1.00 1.00 1.00 1.00

61 75 73 59 57

11.6 13.1 12.8 9.7 7.7

19 17 18 16 13

relative density 0.13 0.09 0.05 0.02 0.04

± ± ± ± ±

0.01 0.03 0.02 0.003 0.01

Young’s modulus (MPa) 8 6 2 2 3

± ± ± ± ±

2 2 1 1 1

a

Surfactant concentration csurf of 10 wt %, initiator concentration cinit, pore diameter dpore, and window diameter dwindow of the HIPEs with a DVB content of 50 wt %, an emulsion height of 1 cm, an initiation temperature of Tinit = 70 °C (if not stated otherwise), and properties of the resulting polyHIPEs; AIBN 1 Is the reference system; vol % and wt % refer to the overall continuous phase, and mol % refers to the monomer phase (styrene and DVB). bContinuous phase contains 20 wt % hexane as diluent. cDVB content of 25 wt %. dTinit = 60 °C.

(d) Reducing the initiation temperature from 70 to 60 °C increases the AIBN half-life time from 6 to 23 h.24 Although the solidification time was increased drastically, the template stayed stable and a monodisperse open-cell foam was obtained (AIBN 5, Figure 5d). Although the size of the windows decreased slightly compared to the reference system AIBN 1 (see Table 2), the change is not very significant. The main outcome of varying the parameters mentioned above is the fact that neither the morphology nor the size of the windows changes. It appears that the windows are only located at the points where neighboring droplets meet and therefore arise solely from shrinkage of the continuous phase during polymerization as already proposed by Cameron.7 However, all template variations led to a material with a much lower density and a much lower Young’s modulus (compare Table 1 and Table 2). Speculative as it may be, the following reasons may explain our observations: (1) By adding 20 wt % hexane to the monomers, we diluted the continuous phase so that there was less material present to polymerize which reduced the foam density as well as the Young’s modulus. (2) A lower DVB content is associated with a lower cross-linking density, which is also reflected in the foam’s density and the Young’s modulus. (3) Decreasing the initiator concentration or decreasing the initiation temperature is associated with a slower polymerization time. Thus, the template had time to partly phase separate, thus leaving less material in the continuous phase that can be polymerized, which is again reflected in the reduced densities and Young’s moduli. Note that the relative densities of AIBN-initiated polymer foams are about 5 times lower than those of KPS-initiated polymer foams. We believe that this difference is due to the finestructure of the pore walls: while the KPS-initiated foams have dense walls, the walls of the AIBNinitiated ones are porous. We will discuss this in a follow-up paper. The low densities obtained for the AIBN series are also reflected in the Young’s moduli which are much lower compared to the ones from the KPS series. Discussion of Window Formation. For the discussion of the window formation in the monodisperse polyHIPEs studied in the work at hand, we will compare the two reference polyHIPEs KPS 2 and AIBN 1. The respective emulsion templates only differ in the initiator that was used for polymerization. Thus, the only parameter that was changed was the locus of initiation. Just as reported in the literature for polydisperse polyHIPEs,11,18 changing the locus of initiation from the interface to the bulk phase has an important impact on the obtained pore morphology. Initiating the polymerization at the interface (using KPS), one obtains a closed-cell foam with polyhedral shaped pores and a homogeneous pore wall thickness. On the contrary, initiating the polymerization in

Figure 5. SEM micrographs of bulk phase initated polyHIPEs synthesized under different conditions: (a) AIBN concentration of 1 mol % with respect to the monomers (AIBN 2), (b) 20 wt % hexane in the oil phase (AIBN 3), (c) 25 wt % DVB (AIBN 4), and (d) initiation temperature of 60 °C instead of 70 °C (AIBN 5).

the AIBN concentration of the reference system AIBN 1 by half also resulted in a polymer foam, but its initial template morphology could not be retained. The lack in emulsion stability could be compensated by adding a salt to the aqueous phase.9 In the study at hand, monodisperse open-cell foams were obtained at AIBN concentrations between 1 and 1.28 mol %. Unfortunately we have no explanation why the emulsions stay stable in such a narrow concentration range only. Figure 5a shows a polyHIPE with 1 mol % AIBN as initiator (AIBN 2). (b) In order to increase the degree of interconnectivity, we diluted the monomer phase by adding 20 wt % of hexane to the monomer phase, leading to a total of 30 wt % of nonpolymerizable continuous phase (AIBN 3). In KPSinitiated polyHIPEs this dilution resulted in a decreased pore wall thickness because there was less material present which could be polymerized. Thus, we expected the windows in AIBN-initiated polyHIPEs to increase if less polymerizable material is available. Surprisingly, this attempt to increase the interconnectivity showed no remarkable effect (compare Figure 5b): the average window diameters remained the same. (c) Reducing the amount of cross-linker DVB such that the continuous phase of the template consists of a 3:1 mixture of styrene/DVB (AIBN 4) instead of a 1:1 mixture did also not change the resulting pore morphology (compare Figure 5c). F


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monodisperse emulsions one has φc = 74%. It is these thin films which will break upon solidification of the matrix.9 Often the created holes increase due to the shrinkage of the polymer matrix. While this analysis works rather well for the AIBN-initated polyHIPES (compare Figure 6, middle and bottom part), it completely fails for the KPS-initiated systems. Not only are the drops entirely surrounded by flat films at volume fractions where one expects spheres (φ = φc = 74%), but the films are also 3 orders of magnitude thicker than those present in a liquid emulsion (10 μm rather than 10 nm). This means that a force must arise during the solidification of the KPS-initiated system which redistributes the liquid from the nodes of the emulsion (the dense pockets between four neighboring drops) to the contact points between the drops (compare Figure 6, middle and top part); i.e., there must be a force which is sufficiently strong to counterbalance the capillary pressure. We assigned this force to an osmotic pressure gradient which arises between the nodes and the contact points during solidification. A more detailed explanation of this new mechanism has been given in a separate publication.26 Here we provide a short summary of the main ideas. We hypothesize that the osmotic pressure gradient arises from a concentration gradient during the polymerization process: it is well-known that different monomers have different tendencies to react in a copolymerization reaction (as it is in our case, since we polymerize a mixture of styrene and DVB). There is a clear tendency for DVB to incorporate more likely into a growing polymer chain than it is for styrene.27 Therefore, as the polymerization progresses, DVB is preferentially polymerized. This leaves the monomer mixture with a concentration gradient (hence an osmotic pressure gradient!) and the need to redistribute matter from the nodes to contact zones, i.e., the films. As a consequence, this mechanism leads to a homogeneous pore wall thickness and a polyhedral pore morphology. Note that the osmotic pressure driven material redistribution only arises if the polymerization is interface initiated. In bulk phase initiated polymerizations the initiator is homogeneously distributed in the monomer mixture. Concentration gradients arising during polymerization are only local so that no large-scale concentration gradient is induced and thus no osmotic transport. As a consequence, the structure of the bulk phase initiated emulsion is simply frozen in without changing the spherical emulsion droplet shape. The window formation in bulk phase initiated polyHIPEs occurs solely from shrinkage (∼17%28) of the continuous phase during polymerization at the points where neighboring droplets meet. Christenson et al.29 showed that interface-initiated polymerization can lead to both open-cell and closed-cell foams. However, open-cell foams were only obtained for droplet sizes below 50 μm. We do agree with their findings because the capillary pressure is inversely proportional to the droplet size, which means that the smaller the droplet size the higher the capillary pressure. Thus, one could argue that below a certain droplet size the capillary pressure counteracts the osmotic pressure gradient and open-cell polyHIPEs can be formed even if the template was interface initiated. In order to check this hypothesis, we used the same KPS-initiated formulation as above and generated monodisperse droplets of a smaller diameter, namely d = 27 μm (Figure 7, left). As one can see, the pores are still closed but spherical rather than polyhedral which means that the capillary pressure indeed increased due to the

the bulk phase (using AIBN), one obtains an open-cell foam whose pores are spherical (compare Figure 6).

Figure 6. (left) Pictures and (right) schemes of a monodisperse polymerizable emulsion template and the two obtained polymer foam morphologies if the locus of initiation is at the interface (using KPS) and in the bulk phase (using AIBN).

Emulsions with polyhedral drop shapes do exist at sufficiently high water volume fraction. For the kind of pore shapes we observe, one would need a water volume fraction of the order of 99 vol %. This, however, is not the case for the system at hand where we have a water volume fraction of approximately 74 vol %, which, in the liquid state, leads to a close packing of spheres. Moreover, in the liquid state the finite interfacial tension creates a capillary pressure (pressure difference between the drop and the continuous phase) which drains the liquid out of the contact zone between the drops. This drainage proceeds until repulsive interactions between the interfaces (e.g., electrostatic or steric repulsion) are as strong as the capillary pressure, which happens around film thicknesses of a few tens of nanometers. Hence, in the liquid state, the drops are separated by nanometric films, whose area is strictly correlated with the volume fraction of the emulsion.14 For our monodisperse emulsions one finds to a good approximation that the fraction f of the drop surface which is covered by such thin films is given by ⎛ f ≈ ⎜⎜1 − ⎝

1−φ 1 − φc

⎞2 ⎟ ⎟ ⎠

(2)

where φc is the water volume fraction at which the drops are closely packed spheres.25 Its value depends on the polydispersity and the organization of the drops. For ordered, G


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foams led us to a new mechanism for the material redistribution and window formation. We proposed that interface initiation leads to closed-cell foams with a homogeneous pore wall thickness because of a strong osmotic pressure gradient which redistributes the liquid from the nodes of the emulsion to the contact points between the drops right before the template solidifies. In bulk phase initiated polymerizations there is no osmotic pressure gradient arising, and the spherical emulsion droplet shape is simply frozen in with progressing polymerization. The window formation occurs due to shrinkage of the continuous phase at the points where neighboring droplets meet. In a follow-up work we will elaborate on the mechanical properties of the polymer foams. The mechanism of redistribution of material in the continuous phase in the interface-initiated systems leads to low-density materials in which most of the polymer is contained in the walls. These materials have elastic moduli which can be up to 2 orders of magnitude higher than those in which the material is contained mostly in the nodes. Moreover, it is commonly very difficult to make low density, closed-cell foams. The mechanism of redistribution of material to the walls could therefore be heavily exploited to develop new routes to low-density, closedcell materials with high elastic moduli. Last, but not least, the periodic nature of our materials might be very interesting for acoustic applications. Simulations31 have recently shown that with the kind of materials we generate with the KPS-initiated systems one can generate acoustic band gap materials which do not allow certain frequency ranges to pass. The drawback of microfluidics certainly is the fact that only small amounts of emulsion can be generated at a time. Moreover, our current setup only allows us producing droplets with diameters ranging from 30 to 120 μm. On the one hand, emulsions with droplets larger than 120 μm in diameter were found to be very unstable, and they could not be successfully polymerized. On the other hand, generating emulsions the droplet diameters of which are below 30 μm left us with very little amounts. In order to produce more material, suitable upscaling techniques such as microchannel emulsification might be promising.32,33

Figure 7. SEM micrographs of KPS-initiated polyHIPEs with the same composition and polymerization conditions as the reference system KPS 2. (left) Monodisperse with a pore diameter of d = 27 μm. (right) Polydisperse with d = 4−55 μm.

smaller droplet diameters. As a consequence, less material is transported from the nodes to the films which results in a more spherical shape. The same observation was made with a polydisperse polymer foam whose droplet diameters were in the range of d = 4−55 μm (Figure 7, right). We conclude that closed-cell foams are always obtained in the case of interfaceinitiated styrene//DVB polymer foams if the radical concentration is sufficiently high. However, the pore morphology is spherical rather than polyhedral if the capillary pressure is high (small droplet diameters). On the contrary, polyhedral pores are only obtained if the osmotic pressure gradient counteracts the capillary pressure. This can only happen if (1) an osmotic pressure gradient is present in the first place (requiring at least two monomers with different reaction kinetics) and (2) the capillary pressure is sufficiently low (big droplet diameters). We would also like the reader to know that it has come to our attention that some publications which reported polyhedral pores with thick pore walls had only one monomer in their emulsion templates.18,30 However, since the surfactant they used contained double bonds, we think that it might have acted as second monomer, which, in turn, gave rise to the osmotic pressure gradient discussed above.

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CONCLUSION AND OUTLOOK Our templating route via microfluidics is the perfect tool to understand and control the pore morphology of the final polymer foam. We were able to show how one can control the shape of the pores (spherical or polyhedral pores) and the connectivity (open-cell or closed-cell) in a reproducible way. For that purpose, we kept the droplet size of the template constant and varied the surfactant concentration, the water volume fraction (by varying the emulsion height), the initiation temperature, and the locus of initiation. Initiating from the interface led to polyhedral closed-cell foams with a homogeneous wall thickness around the pores. The obtained pore wall thicknesses varied from 10 to 3 μm. Thinner pore walls were reproducibly obtained by increasing the surfactant concentration and the water volume fraction or by decreasing the initiation temperature. Open-cell foams were obtained by initiating from the bulk phase. The main outcome of varying the template parameters for bulk phase initiated foams is the fact that neither the morphology nor the size of the windows changed. However, increasing the amount of nonpolymerizable diluent in the continuous monomer phase, decreasing the cross-linking density, and decreasing the initiation temperature had a huge impact on the Young’s modulus which decreased significantly compared to the reference system. The results obtained both from the interface-initiated and bulk-phase initiated polymer

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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.macromol.6b00494. Figures S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. A. Fels for his SEM support. We acknowledge funding from the German Research foundation (STU 287/4-1) and from the European Research Council (ERC, FP7/2007-2013, 307280-POMCAPS). H


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(22) Weaire, D.; Hutzler, S. In The Physics of Foams; Oxford University Press: Oxford, 1999. (23) Drenckhan, W.; Langevin, D. Monodisperse foams in one to three dimensions. Curr. Opin. Colloid Interface Sci. 2010, 15, 341−358. (24) Brandrup, J.; Immergut, E. H.; Grulke, E. A; Abe, A.; Bloch, D. R. In Polymer Handbook; John Wiley & Sons: Toronto, 2005. (25) Forel, E.; Rio, E.; Beguin, S.; Hutzler, S.; Weaire, D.; Drenckhan, W. Under review. (26) Quell, A.; Heitkam, S.; Drenckhan, W.; Stubenrauch, C. Submitted to Angew. Chem., Int. Ed.. (27) Schwachula, G. Calculation of the copolymerization parameters in the ternary copolymerization system of styrene/m-divinylbenzene/ p-divinylbenzene. J. Polym. Sci., Polym. Symp. 1975, 53, 107−112. (28) Brydson, J. A. In Plastic Materials; Butterworth-Heinemann: Oxford, England, 1995. (29) Christenson, E. M.; Soofi, W.; Holm, J. L.; Cameron, N. R.; Mikos, A. G. Biodegradable Fumarate-Based PolyHIPEs as Tissue Engineering Scaffolds. Biomacromolecules 2007, 8, 3806−3814. (30) Moglia, R. S.; Holm, J. L.; Sears, N. A.; Wilson, C. J.; Harrison, D. M.; Cosgriff-Hernandez, E. Injectable PolyHIPEs as High-Porosity Bone Grafts. Biomacromolecules 2011, 12, 3621−3628. (31) Spadoni, A.; Höhler, R.; Cohen-Addad, S.; Dorodnitsyn, V. Closed-cell crystalline foams: Self-assembling, resonant metamaterials. J. Acoust. Soc. Am. 2014, 135 (4), 1692−1699. (32) Nisisako, T.; Torii, T. Microfluidic large-scale integration on a chip for mass production of monodisperse droplets and particles. Lab Chip 2008, 8, 287−293. (33) Nisisako, T.; Ando, T. High-volume production of single and compound emulsions in a microfluidic parallelization arrangement coupled with coaxial annular world-to-chip interfaces. Lab Chip 2012, 12, 3426−3435.

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How the Locus of Initiation Influences the Morphology and the Pore

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