Open Cell Aerogel Foams via Emulsion Templating - ACS Publications

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Open Cell Aerogel Foams via Emulsion Templating Nicholas Teo, and Sadhan C Jana Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03139 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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Open Cell Aerogel Foams via Emulsion Templating Nicholas Teo, and Sadhan C. Jana* Department of Polymer Engineering The University of Akron, 250 South Forge Street, Akron, OH 44325-0301 *Corresponding author email: [email protected]

Abstract Water-in-oil emulsion-templating method is used in this work for fabrication of open cell aerogel foams from syndiotactic polystyrene (sPS). A surfactant-stabilized emulsion is prepared at 60 C100 C by dispersing water in a solution of sPS in toluene. sPS gel, formed upon cooling of the emulsion to room temperature, locks the water droplets inside the gel. The gel is solvent exchanged in ethanol and then dried under supercritical condition of carbon dioxide to yield the aerogel foams. The aerogel foams show significant fraction of macropores of diameter of a few tens of micrometer, defined as macrovoids that originated from the emulsified water droplets. In conjunction, customary macropores of diameter 50-200 nm are derived from sPS gels. The macrovoids add additional openness to the aerogel structures. This paper evaluates the structural characteristics of the macrovoids, such as diameter distribution, macrovoid interconnect density, and skin layer density in conjunction with the final aerogel foam properties.

Keywords: aerogels; macroporous; mesoporous; aerogel foams; macrovoids

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Introduction Porous materials have been used in a variety of industrial applications such as membranes for gas and liquid filtration,1,2 catalyst supports,3 substrates for electrical components,4 gas sequestration reservoirs5 and absorbents for chemical and oil spills.6 The versatility of porous materials in undertaking multiple roles stems from their high porosity, large surface areas, and low density, all the while offering sufficient mechanical integrity to be free-standing. Polymeric foams were first reported in patent literature in 19357 and covered extensively in a number of authoritative monographs8–10 as a class of widely used porous materials with micrometer-sized, air-filled voids organized in either open-celled or closed-celled forms inside a host polymer. Two key methods dominate fabrication of polymer foams: (1) in-situ chemical foaming of thermoset polymers, e.g., polyurethanes11 and (2) foaming via the use of physical blowing agents in conjunction with extrusion, e.g., foaming of polystyrene using hydrocarbons.12 Attempts have been made in literature to incorporate hierarchical porosity into material structures to improve their properties, mimicking natural systems such as wood, bamboo, and bone.13 For example, Wong et al.

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fabricated hierarchical polyHIPEs consisting of surfactant-

stabilized droplets (~15 µm) and particle-stabilized Pickering emulsion droplets (>100 µm), while Elsing et al.

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produced hierarchical porous structures in polystyrene foams via foaming of

styrene-in-water emulsions. Gu and Jana 16 recently fabricated open cell polyurea aerogel foams by using the templates obtained from a co-continuous immiscible polymer blend system. These authors dissolved one polymer phase from a co-continuous blend to create a template material with micrometer-sized cavities in which polyurea gel was synthesized via sol-gel system. The second polymer phase was then removed by using a solvent and the gel was supercritically dried to yield the aerogel foam structures with significant mesopores and micrometer size voids. It was found that the small amounts of template polymers remaining in the aerogel foam structure 2 ACS Paragon Plus Environment

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affected the bulk density and the total porosity. To circumvent the above issue and to develop a more versatile method of fabrication of hierarchical polymer aerogel foam structures, an emulsion-templating method was used in this work in conjunction with sol-gel transition of the precursor polymer. Aerogels, first reported in 193117, are obtained by replacing the liquids in the gel structures with air. The precursor gels are synthesized via sol-gel reactions18 or a thermo-reversible gelation process.19 Kistler pioneered the process of fabrication of silica aerogels that find widespread use as thermal insulation17,20 and in aerospace applications.21,22 Over time, aerogels have been synthesized from polymeric materials such as polyurea,23 polyimide,24 and polystyrene25 to overcome the mechanical limitations of silica aerogels.26 One advantage of polymeric aerogels is a large variety of different structural morphologies that can be accessed from an array of different monomers undergoing step-growth polymerization reactions27 or thermodynamic phase transitions following different kinetic pathways.28,29 The aerogel foams were fabricated in this work by water-in-oil emulsion templating30–33 of a polymer sol that eventually turned into a gel via sol-gel transition.34 Syndiotactic polystyrene (sPS) solution in toluene was considered as the continuous phase in this work due to its fast crystallization kinetics, enabling rapid thermo-reversible gelation, which relaxes the constraints on time limits of emulsion stability.35 The regular tacticity of sPS enables the incorporation of solvent molecules and allows crystallization into the helical δ-from intercalate. A solution of sPS and toluene prepared at high temperature and subsequently cooled under ambient conditions passes through crystallization, binodal, and spinodal curves and undergoes liquid-liquid demixing via spinodal decomposition.36 The polymer-rich phase then crystallizes via nucleation and growth process. The system is vitrified as the temperature drops below the glass transition temperature of the polymer, thus locking in the fibrillar polymer structure37 and resulting in a physically cross3 ACS Paragon Plus Environment

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linked network of an average 50 nm diameter polymer strands. The sol-gel transition typically takes place at above 50 C, making the gel structure experimentally accessible at room temperature. In contrast, atactic polystyrene is only able to form gels at sub-zero temperatures and its crystallization is significantly harder due to its irregular conformation.38 Isotactic polystyrene is able to form stable gels at room temperature, but the long gelation times of up to two weeks makes the system impractical due to the issues of emulsion stability in emulsion templating step.39 The aqueous phase in the emulsion system used in this study is responsible for creating large fractions of macropores with typical diameter 20 µm, defined in this work as macrovoids. In conjunction, the 50-200 nm diameter macropores that customarily form in sPS gels, lead to a structure reminiscent of open cell polymer foams with porous skin layers. The additional openness due to macrovoids may lead to possible adaptation of aerogels in air or liquid filtration applications. Experimental Section Materials. Syndiotactic polystyrene (Mw ~ 300,000 g/mol) was acquired from Scientific Polymer Producers Inc. (Ontario, NY). Surfactants SPAN 80® (trademark of Croda Inc) and F127® (trademark of BASF), chlorophenylsilane, and toluene were purchased from Sigma Aldrich (Milwaukee, WI). Ethanol was acquired from Decon Laboratories Inc. (King of Prussia, PA). Chemical structures of the surfactants are provided in Figure S1. Fabrication of Aerogel Foams. Water-in-oil (W/O) emulsion-templating method was used in this work to obtain micrometer size water droplets dispersed in an organic sol. The sol transitions into an organic gel, thus locking the emulsified water droplets within the gel structure. This process


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resembles the fabrication of polyHIPEs 40–42, with the exception that the oil-phase in this study is an organic sol which later transitions into a gel with its inherent pores filled by the organic solvent. The continuous phase organic sol was prepared by dissolving syndiotactic polystyrene (sPS) in toluene at 100 C at a solid concentration of 0.06 g/mL. A typical sample with dispersed phase content of 33 vol% and surfactant concentration of 1.5 vol% of the continuous phase was prepared as follows. Toluene (2.67 mL), 1.33 mL of water, 0.04 mL of SPAN 80®, and 0.16 g sPS pellets were added into a sealed vial of internal diameter 1.92 cm. The amounts were adjusted accordingly to obtain samples of varying dispersed phase content (25, 33 and 50 vol%) and surfactant concentrations (1.5, 7.5 and 15 vol%). Next, the vial was heated in an oil bath at 100 C for one hour to enable dissolution of sPS in toluene. The resultant material was removed from the oil bath and left to cool under ambient conditions while the components were continuously mixed using a magnetic stir bar of length 1.27 cm and diameter 3.175 mm. In the process, a water-in-oil emulsion was formed. It was found that sPS gelation occurred at around 50 C. In view of this, the emulsion was allowed to cool to 60 C in approximately 2 min under ambient cooling and poured into glass molds. This facilitated material handling before the viscosity increased appreciably near the gelation point. The glass molds were immediately quenched in an ice-water bath and left to stand for 5 hours. The resultant gels were soaked successively in ethanol to remove water and surfactants and then in liquid carbon dioxide to replace ethanol. The interconnected macropores of sPS and the macrovoids facilitated removal of water and surfactants and exchange of ethanol with liquid carbon dioxide. The gel was subsequently dried under supercritical condition of carbon dioxide at 50 C and 11 MPa. In the rest of the paper, the numbers in specimen designation “sPS_x_y_z” indicate a dispersed phase content of x vol%, surfactant concentration of y vol%, and a magnetic stirrer mixing speed


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of z rpm. Recall that numbers x and y represent the vol% with respect to the volume of the continuous phase. Characterization of Water-in-Oil Emulsions Emulsion Droplet Size. The water droplet size in W/O emulsions prepared without sPS and with varying surfactant concentrations was measured using an Olympus BX51 optical microscope fitted with a heating stage controlled by an Instec STC200 temperature controller. A drop of the emulsion was first placed onto a microscope slide with a cover slip and inserted into the heating stage at the required temperatures of 50, 60, and 70 C. Optical images were recorded at prescribed time periods. The droplet size distribution was obtained from the analysis of the optical images using the ImageJ software. In each case, more than 200 droplets were considered. Interfacial Tension Measurements. The interfacial tension between water and toluene was measured using a Du Noy tensiometer (Interfacial Tensiometer 70545, Central Scientific Co., VA) at several temperatures. First, the surfactant was dissolved in toluene or water for 30 minutes, depending on its hydrophilic lipophilic balance (HLB) value. Next, 20 mL of the aqueous solution was added into a clean glass container and the toluene solution was gently added onto the top of the water phase by pouring the toluene solution down a clean glass slide to prevent any emulsification or droplet formation. The glass container was then heated to 80 C, below the boiling points of both water and toluene, and transferred to the tensiometer for measurement of interfacial tension values at temperatures of 50, 60, and 70 C. Characterization of Aerogel Foams Aerogel Foam Morphology and Macrovoid Size Distribution. The morphology of the aerogel foams was studied using a scanning electron microscope (JSM5310, JEOL, MA). An accelerating voltage of 2 kV and emission current of 20 mA was used to capture the SEM images. The aerogel foams


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were fractured and a representative piece was mounted on an aluminum stub using carbon tape, followed by sputter coating with silver (ISI-5400 Sputter Coater, Polaron, UK). The macrovoid size distribution was obtained from the analysis of SEM images using ImageJ software. Typically, more than 150 macrovoids were considered for determining the macrovoid size distribution for each specimen. Porosity: Porosity was calculated from the values of skeletal (ρs) and bulk density (ρb) as shown in equation 1. The values of skeletal densities were obtained using a helium pycnometer (AccuPyc II 1340, Micromeritics Instrument Corp., GA). Bulk density of the aerogel foams was calculated from the mass and volume of the foam specimens. 𝜌

𝑝𝑜𝑟𝑜𝑠𝑖𝑡𝑦 = (1 − 𝜌𝑏) × 100% 𝑠

(1)

BET surface area: Brunauer-Emmett-Teller (BET) surface area of the aerogel foams was obtained from N2 adsorption-desorption isotherms at 77 K using a Micromeritics Tristar II 3020 analyzer (Micromeritics Instrument Corp. GA). Results and Discussion Emulsion Stability Emulsion stability in a 10-minute window was important in this work as sPS sol turned into gel in approximately 3 minutes from the time the emulsion was poured into the glass mold. In view of this, emulsion stability over a period of time and corresponding water droplet size distribution at various temperatures and surfactant concentrations were investigated. Effect of Temperature. Emulsions with surfactant concentration of 1.5 vol% SPAN 80® were prepared at 50, 60 and 70 C. This was achieved by first heating the emulsion to 100 C in an oil bath, mimicking the gel formation process. The emulsion was then removed from the oil bath and allowed to air cool, while maintaining mixing. Once the emulsion reached appropriate 7 ACS Paragon Plus Environment

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temperature (50, 60 or 70 C), a droplet was placed on the glass slide, covered with a cover slip, and the assembly was allowed to reach desired equilibrium temperature (in less than a minute). Optical images of the droplets were taken as soon as the emulsion was first placed on the slide and after three minutes. It was seen that emulsions prepared at higher temperatures led to a shift of droplet size distribution to larger size. This was exhibited by an increase of the average droplet size registered right after placement of the emulsion droplet from 11.8 to 13.9 to 19.4 µm respectively for emulsion temperatures of 50, 60, and 70 C, as shown in Table 1. In addition, emulsions prepared at lower temperature were found to be more stable. For example, droplet size shifted to higher values after 3 minutes for emulsions prepared at higher temperature as shown in Figure S2. This larger droplet size observed at higher temperature can be attributed to coalescence events. This trend is discussed in a detail in the supporting information (SI), using the data presented in Figure S3 and Table S1. The experiment in SI focused on heating of the preformed emulsions and observing the change in droplet size distributions in absence of mixing or agitation. Interfacial Average Maximum Minimum Droplet Tension with Droplet Droplet Diameter surfactant of 1.5 Diameter (µm) Diameter (µm) (µm) vol% (mN/m) 50 2.0 11.8 ± 9.0 58.0 2.3 60 3.6 13.9 ± 8.8 56.1 3.0 70 5.7 19.4 ± 17.7 163.7 3.7 Table 1: Interfacial tension and water droplet size with 1.5 vol% SPAN 80®

Temperature (oC)

In an effort to understand why coalescence events increased with an increase of temperature, the interfacial tension values were measured in the case of water/toluene and water/toluene/SPAN 80® system at 50-75 C and presented in Figure 1.


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Interfacial Tension (mN/m)

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water/toluene water/toluene/SPAN 80

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7

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33 4 32 3

Interfacial Tension (mN/m)

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31 2 30 45

50

55

60

65

70

75

80

Temperature (oC)

Figure 1: Effect of temperature on interfacial tension of water/toluene (filled square) and water/toluene/1.5 vol% SPAN 80®(filled triangle) The data in Figure 1 presents two interesting trends. First, for water/toluene system without the surfactant, interfacial tension reduced with an increase of temperature, e.g., from 33.5 mN/m at 50 C to 30.8 mN/m at 75 C. This can be attributed to an increase of the mutual solubility between toluene and water at higher temperature. Second, as expected, the addition of SPAN 80® surfactant lowered the value of interfacial tension from 33.5 mN/m for water/toluene system to 2 mN/m for water/toluene/1.5 vol% SPAN 80® system, both measured at 50 C. However, the interfacial tension increased several folds with temperature in the presence of SPAN 80®, e.g., from 2 mN/m at 50 C to 7.6 mN/m at 73 C. One can attribute this to loss of solubility in water of the nonionic surfactant SPAN 80® with an increase of temperature and simultaneous increase


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of solubility in the organic phase; this also resulted in a reduction of HLB value of the surfactant at higher temperature.43 The hydrophilic heads of the surfactant that bind to the aqueous phase dehydrates at higher temperature due to weakening of hydrogen bonding.44,45 As a consequence, the surfactant molecules migrate away from the water/toluene interface into the bulk toluene phase and an increase of the interfacial tension ensues as is evident in Figure 1. In this scenario, water droplets experience higher frequency of coalescence events and larger water droplets are obtained. Effect of Surfactant Concentration. The data in Table 2 taken for emulsions prepared at 60 C indicate that an increase of surfactant concentration led to formation of smaller droplets and narrowing of the droplet size distribution, as shown in Figure S4. In addition, the emulsions were found to be more stable at higher surfactant concentration and experienced a smaller shift in droplet size distribution over a 3-minute period. As is expected, the interfacial tension value reduced with an increase of surfactant concentration (Table 2), which favored creation of additional interfacial area between water and toluene phases as ramified via creation of smaller water droplets as shown in Figure S4. The diameter distribution is quite broad with an average diameter of 20.6 m and standard deviation 14.7 m for surfactant concentration of 1.5 vol% (Table 2). The diameter distribution became much narrower with average diameter of 13.0 m and 12.1 m for surfactant concentration of 7.5 and 15 vol% respectively, also shown in Table 2. The maximum and minimum droplet size also showed strong dependence on surfactant concentration, e.g., both became smaller with an increase of surfactant concentration (Table 2). The smaller size droplets also settled at much slower rate as presented in Figure S5, thus contributing to the stability of the emulsions. Water droplets in emulsion with


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1.5 vol% SPAN 80® settled appreciably within a minute; in view of this, emulsions with 1.5 vol% SPAN 80® were not considered for preparation of aerogel foams.

Surfactant Interfacial Average Maximum Minimum Droplet o Concentration Tension at 60 C Droplet Droplet Diameter (vol%) (mN/m) Diameter (µm) Diameter (µm) (µm) 1.5 3.5 20.6 ± 14.7 84.6 3.7 7.5 1.0 13.0 ± 7.6 46.1 3.3 15 0.8 12.1 ± 9.0 53.3 1.6 Table 2: Interfacial tension and droplet size distribution of water/toluene/SPAN 80® for different surfactant concentrations at 60 C. Effect of Surfactant Type. Up to this point, the data produced with SPAN 80® were discussed. In a number of cases, emulsions were prepared with F127® as the surfactant. The presence of F127®, a PEO-PPO-PEO block copolymer surfactant, produced much higher reduction of interfacial tension at 60 C compared to SPAN 80®, e.g., 0.6 mN/m for F127® vs. 3.5 mN/m for SPAN 80® for the same 1.5 vol% surfactant. The average water droplet diameter with 1.5 vol% F127® was 2.5 ± 1.6 µm. The effect F127® on water droplet size distribution will be discussed later. Morphology of Aerogel Foam The fabrication method adopted in this work yielded aerogel foam structures with typical macrovoid diameters of ~20 m embedded in the inherently produced aerogel structure of the host polymer sPS. These macrovoids were occupied by the dispersed water droplets in W/O emulsion and locked in the sPS gel structure. As is apparent from a representative SEM image presented in Figure 2, the macrovoids were interconnected. One representative macrovoid and one interconnect are labelled in Figure 2a for clarity. The materials between adjoining macrovoids were due to macroporous sPS aerogel, as shown in Figure 2b. The macroporous sPS, the


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macrovoids, and the macrovoid interconnects in conjunction produced the open cell aerogel foam structures. The W/O interfaces in the emulsion turned into porous skin layers after sPS gelation. The SEM image presented in Figure 2b shows that the skin layer had much higher density than the bulk material. The macroporous nature of sPS domains between two adjoining macrovoids distinguishes the aerogel foams from the open cell conventional foam structures reported in literature.46 The skin materials in traditional polymer foams do not show inherent, volumetric porous networks, as exhibited in Figure 2b. The aerogel foams reported in this work consistently exhibit porosity values greater than 95%, e.g., a dispersed phase content of 50 vol% in the templating emulsion possessed the highest overall porosity of around 97.5% (sPS_50_7.5_1600), as listed in Table 3.

Figure 2: SEM images of aerogel foam with (a) macrovoids and interconnects and (b) macroporous skin layer.

Specimen

Porosity (%)

sPS_0_0_1600 sPS_33_1.5_1600 sPS_33_7.5_1600

92.5 ± 0.2 95.4 ± 0.4 96.0 ± 0.1

BET surface area (m2/g) 267 ± 6 247 ± 8 222 ± 2

Macrovoid Mean size (µm)

Macrovoid Size Distribution (%) < 20 m

Between 20 and 50 m

> 50 m

36 77

24 21

40 1

61.5 ± 61.6 14.3 ± 10.8 12

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BET Macrovoid Size Distribution (%) Macrovoid Porosity surface Specimen Mean size Between 20 (%) area < 20 m > 50 m (µm) and 50 m (m2/g) sPS_33_15_1600 95.5 ± 0.3 208 ± 9 11.9 ± 8.8 85 15 0 sPS_33_7.5_800 95.5 ± 0.1 225 ± 4 22.6 ± 26.4 68 17 14 sPS_33_7.5_1200 95.6 ± 0.4 243 ± 6 20.2 ± 17.5 68 24 8 sPS_25_7.5_1600 95.5 ± 0.2 262 ± 4 12.3 ± 8.7 86 13 1 sPS_50_7.5_1600 97.5 ± 0.1 204 ± 6 17.8 ± 11.5 66 33 1 Table 3: Porosity, BET surface area, and macrovoid size distribution of emulsion-templated aerogel foam specimens. Macrovoid Size Distribution The emulsion-templated aerogel foams showed significant degree of tunability of macrovoid size via changes in processing parameters, such as dispersed phase content, mixing speed, and surfactant concentration. Effect of Surfactant Concentration. Macrovoid size showed the strongest dependence on surfactant concentration. For example, the macrovoid mean size of the specimens sPS_33_1.5_1600, sPS_33_7.5_1600, and sPS_33_15_1600 were ~61.5, 14.3, and 11.9 m respectively for surfactant concentrations of 1.5, 7.5, and 15 vol% (Table 3). The macrovoid size distribution became narrower with an increase of the surfactant concentration, as shown in Figure 3. For some specimens, e.g., sPS_33_1.5_1600, the standard deviation of the macrovoid diameter distribution was greater than the mean values, indicating a large spread. In view of this, the macrovoid size distribution data was split into three fractions before analysis, e.g., those with diameter less than 20 m, between 20 and 50 m, and greater than 50 m, as listed in Table 3.


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Figure 3: SEM images of aerogel foams fabricated from emulsions with SPAN 80® concentrations of (a) 1.5 vol%, (b) 7.5 vol%, and (c) 15 vol%. Images in (d), (e), and (f) represent higher magnification. The initial emulsion droplet size and corresponding macrovoid size of the aerogel foams are shown for various surfactant concentrations, (g) 1.5 vol%, (h) 7.5 vol%, and (i) 15 vol%. We now examine the trend of macrovoid size distribution. First, the SEM images in Figure 3a-c indicate that the macrovoid size reduced with an increase of surfactant concentration, similar to what was found for water droplet size distribution discussed using the data presented in Table 2 and Figure S4. Second, the higher magnification SEM images in Figure 3d-f reveal that the population density of macrovoid interconnects increased with an increase of surfactant concentration. 14 ACS Paragon Plus Environment

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The water droplet size distributions of the starting emulsions are compared with the resultant macrovoids in Figure 3g-i. Two distinct observations can be made. First, the macrovoid size is larger than that of dispersed water droplets. The average water droplet size for emulsion systems with 1.5, 7.5, and 15 vol% SPAN 80® concentration was 20.6, 13.0, and 12.1 µm respectively (Table 2). The corresponding average macrovoid sizes are 61.5, 14.3, and 11.9 µm (Table 3). Second, the gap in starting emulsion water droplet size and macrovoid size reduced with an increase of surfactant concentration. At lower surfactant concentration and for higher interfacial tension values, the emulsion system is more prone to coalescence events, as discussed earlier. The time for sPS gelation, typically 3 minutes, was much longer than about 1 minute to observe appreciable coalescence and settling in these cases (Figure S5) and as a result, coalescence events were more abundant. In contrast, at higher surfactant concentrations, the emulsions were much more stable and resistant to coalescence events. Thus, in these cases, the resultant macrovoid size distributions closely mirrored the starting emulsion droplet size distribution shown in Figure 3i. It is to be noted that for polyHIPE formation, fabrication of micrometer sized macrovoids were typically achieved with high SPAN 80® surfactant concentrations of 20 vol%.

47–49

There was one

reported case of polyHIPEs formed with surfactant concentration of 5 vol%, but these resulted in macrovoids of 500 µm to 8mm, orders of magnitude larger than macrovoids achieved in this study. 50

Effect of Mixing Speed. The data listed in Table 3 showed that the fraction of macrovoids with diameter greater than 50 m reduced with an increase of the mixing speed. For example, for specimens sPS_33_7.5_800, sPS_33_7.5_1200, and sPS_33_7.5_1600, the fractions with diameter greater than 50 m were respectively 14%, 8%, and 1% for stirring speeds 800, 1200,


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and 1600 rpm. The fractions in diameter range 20-50 m increased from 17% to 24% with an increase of stirring speed from 800 to 1200 rpm, while the fractions with diameter smaller than 20 m increased from 68% to 77% with an increase of mixing speed from 1200 to 1600 rpm. This indicated that at higher mixing speeds, a greater proportion of smaller macrovoids were formed, as shown in Figure 4a. This trend can be explained as follows. At higher rotational speed of the magnetic stir bar, effective shear rate was higher. As a consequence, the shear stress of the continuous phase outweighed the restoring interfacial stresses on water droplets. Accordingly, larger water droplets underwent break up into smaller ones; these in turn produced smaller size macrovoids in the resultant aerogel foam. However, one cannot increase the stirring speed indefinitely as associated shear heating may result in boiling of the liquids and promote liquid to solid phase transition of the emulsion system. Effect of Dispersed Phase Content. The macrovoid size distribution was shown to be weakly influenced by the dispersed phase content, as shown in Table 3 and Figure 4b. For example, the specimen sPS_25_7.5_1600 with 25% water phase and sPS_50_7.5_1600 with 50% water phase showed close mean macrovoid diameter values of 12.3 ± 8.7 m and 17.8 ± 11.5 m respectively. This is an expected trend as there was no change in either the shear stress or the interfacial stress with the increase of water content in the system. A small increase in mean macrovoid diameter can be attributed to higher frequency of coalescence events in the presence of greater fractions of water.


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Figure 4: Effect of (a) mixing speed and (b) dispersed phase content on macrovoid size distribution. Surfactant concentration was kept constant at 7.5 vol%. The polydisperse nature of size distributions of droplet and macrovoid presented in Figure S2, S3, S4, 3 and 4 are due to a dynamic equilibrium of breakup and coalescence events in emulsion preparation process achieved via mechanical agitation. One can, however, obtain monodisperse macrovoids from other emulsion-templating methods, e.g., via using microfluidics, as demonstrated by Quell et al.51,52 Open Cell Structure It was alluded to earlier that the macrovoids, macroporous sPS domains, and the macrovoid interconnects are responsible for producing an open cell structure. The degree of openness can be inferred from the pore interconnect density and from the total projected surface area of the pore interconnects, as apparent in SEM images. The SEM images presented in Figure 5a-c show that the degree of openness of the cellular structures increased due to an increase of the surfactant concentration. An increase in the volume fraction of the dispersed phase also resulted in a more open cell structure, although, as inferred earlier in reference to Figure 4b, the macrovoid size was only weakly dependent on the dispersed phase content. 17 ACS Paragon Plus Environment

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Figure 5: SEM images showing pore interconnects density for SPAN 80® concentrations of (a) 1.5%, (b) 7.5%, and (c) 15%. (d) Macrovoid interconnect exhibiting frustum geometry and (e) proposed interconnect formation mechanism. The pore interconnect structures seen in Figure 2a and 5 a-c can be interpreted as follows. We invoke similarity with pore interconnect formation observed in polyHIPEs. In polyHIPEs, predominantly open cell structures were reported, although the polymer skins, unlike in this work, remained non-porous. The interconnects in polyHIPEs originate either from the shrinkage of the polymer skin due to density difference between the polymer and the precursor monomer or from the rupture of the thin polymer films during the dispersed phase extraction process. 53 However, these two mechanisms cannot adequately explain pore interconnect formation in aerogel foam systems described in this paper. First, a pre-made polymer was used in this work. Second, the customary volumetric shrinkage of the monolithic sPS gels during the supercritical drying step remained small, ~4%. The polymer aerogel monoliths did not show fracture at such low levels of shrinkage during the drying process.


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A closer examination of interconnects in Figure 5 also revealed no angular tears caused by stresscracking. Instead, interconnects were found sandwiched between two larger macrovoids with a circular cross-sectional shape exhibiting a spherical frustum geometry, as shown in Figure 5d. The unique geometry of interconnects highlighted in Figure 5, and not found in polyHIPEs, leads us to suggest that a different mechanism was at play for their genesis in the current study. It is apparent that interconnects are of much smaller in size than the macrovoids. It is also apparent that sPS strands wrapped around the interconnects, creating close to spherical hole patterns. Such regular structures quite possibly developed during gelation of sPS and we believe were due to phase separated droplets of water formed upon cooling of the emulsion to room temperature. This is supported by the data from Jou and Mather54 who indicated that mass fraction of dissolved water in toluene varies from 0.02 at 100 oC to 0.001 at 15 oC. Based on the above observation, we present that interconnects reported in Figure 5 were kinetically controlled. Recall that the emulsion templated sol system has four components - sPS, toluene, surfactant, and water. In view of data of Jou and Mather,54 water had higher solubility in toluene at emulsion preparation temperature of 100 C than at room temperature. As the temperature was reduced prior to gel formation, a polymer-rich phase was formed via liquidliquid demixing. In conjunction, water dissolved in toluene also lost its solubility and formed new water droplets inside the organic liquid. The increasing viscosity of the gel with time and the soluble surfactants in the organic phase stabilized the newly phase-separated water droplets, as schematically presented in Figure 5e. The polymer-phase vitrified via thermos-reversible gelation and surrounded the newly formed water droplets in the midst of coalescence, resulting in the spherical frustum structure as seen in Figure 5d and schematically presented in Figure 5e.


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Macroporous polymer skin structure The concentration of sPS in the continuous phase is another parameter that can be used to tune the open structures in aerogel foams. The interstitial space between the polymer strands are considered macroporous, with an average pore size of 200 nm. An increase in polymer concentration increases the volume fraction of the polymer-rich region during spinodal decomposition, and correspondingly, increases the diameter of the polymer strands. This, in turn, reduces the pore size. Such an effect is evident from the SEM images in Figure 6, where sPS concentration in the continuous phase was increased from 0.02 to 0.08 g/mL. Kim et al.34 explored the effect of sPS concentration on permeability of sPS aerogel monoliths and showed that at low polymer concentrations of 0.01 g/mL, air permeability was 9.74 x 10-10 m2, which reduced to 5.29 x 10-10 m2 at polymer concentration of 0.08 g/mL. This reduction of permeability is attributed to thicker polymer strands, smaller pore size, and lower pore volume fraction.

Figure 6: SEM images of macroporous matrix of varying polymer concentration: a) 0.02 g/mL, b) 0.04 g/mL, c) 0.06 g/mL and d) 0.08 g/mL. 20 ACS Paragon Plus Environment

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We now revisit the issue of skin layers. As briefly mentioned earlier, skin layers formed at two different locations in the aerogel foam structure- first, at the interfaces between the sPS sol and water droplets and second, at the surfaces of the container of the mold. These skin layers are regions where sPS strands were aggregated together, sealing off the macropores. In both cases, heterogeneous surfaces were involved. Gu and Jana 16 also observed dense skin layers in polyurea aerogel foams attributed to heterogeneous nucleation of polyurea domains on PEO surfaces. In this case, the skin layer thickness was measured to be 62 nm, which is the same order of magnitude of the diameter of one, typical sPS strand (50 nm). The BET surface area values presented in Table 3 are directly affected by skin layer formation. For example, monolithic aerogel specimen sPS_0_0_1600 produced with no emulsion-templating shows a BET surface area of 267 m2/g compared to 204 m2/g for an emulsion-templated aerogel specimen sPS_50_7.5_1600 with 50% dispersed water phase. Such a reduction in BET surface area with the addition of dispersed water is indicative of the formation of denser skin layers that are not readily accessible for adsorption of nitrogen gas. This also suggests that in the skin layers, sPS strands underwent higher degree of aggregation within a given volume compared to the bulk. The above scenario was further examined by controlling the interfacial tension between the two phases. In the first study, different types of surfactants were used in emulsion formation. Surfactants which causes largest reduction of interfacial energy of the water-toluene interface also should result in thinnest possible skin layers at the macrovoid boundaries yielding an open porous structure very much akin to the bulk phase. This was verified using aerogel foam specimens prepared with SPAN® 80 and F127®. Note that F127® is a PEO-PPO-PEO triblock copolymer with average Mw of 12,600 g/mol. For a 1.5 vol% surfactant concentration, SPAN® 80 provided higher interfacial tension value of 3.5 mN/m compared to 0.6 mN/m for F127®, both at 60oC. The F127® triblock copolymer lowers the interfacial tension more than SPAN 80® due to its 21 ACS Paragon Plus Environment

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increased solubility of the PEO and PPO blocks in the aqueous and organic phases of the emulsion respectively. For example, SPAN 80® is insoluble in water at room temperature, while the measured cloud point of F127® in water was measured at 102 oC. Specimens prepared with SPAN® 80 exhibited a denser skin layer compared to the skin layers of macrovoids prepared with F127® surfactants, as shown in Figure 7a and 7b respectively. Such an observation presents an interesting scenario for future work to delineate the relationship between interfacial tension and skin layer density.

Figure 7: Skin layer formation with (a) SPAN 80 as surfactant, (b) F127 as surfactant and c) The initial emulsion droplet size and corresponding macrovoid size of the aerogel foams with F127 surfactant concentration of 1.5 vol%. The greater reduction of interfacial tension due to F127® surfactant also shifted the macrovoid diameter distribution to smaller sizes, due to the smaller dispersed phase droplets formed in the emulsion, as shown in Figure 7c. The macrovoids obtained with F127® as the surfactant had diameters in the narrow range of 0-15 m. A second study was conducted to investigate the effect of surface energy of the glass mold surface on the properties of the skin layer formed. Two glass slides with different surface energies were prepared as substrates for gel formation. The first glass slide was plasma treated to produce a hydrophilic surface (water contact angle of 18.2O ± 2O), while a second glass slide was coated with 22 ACS Paragon Plus Environment

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chlorophenylsilane to produce a more hydrophobic surface (water contact angle of 67.7 O ± 1.1O) that has high compatibility with both sPS and toluene solvent. SEM images in Figure 8 show the skin layers formed during gel formation in direct contact with the glass substrate. It can be seen that aerogels formed on the hydrophilic surface (Figure 8a) show a denser skin layer compared to gels formed on the hydrophobic surface (Figure 8b). This reinforces the hypothesis that sPS preferentially nucleates at high energy interfaces, forming a denser skin layer. This also shows that both internal and external skin layer morphology can be manipulated through the interfacial energy between the two phases of the emulsion and substrate respectively.

Figure 8: Skin layer formation of monolithic sPS aerogel on a) hydrophilic substrate and b) hydrophobic substrate. The two phenomena discussed previously, i.e., pore interconnect density and skin layer formation, are good avenues to control the accessibility of the structures to fluid flow. For example, to create a hierarchical structure impervious to fluid flow, one can use a system with low surfactant concentration, which in turn can reduce the number of pore interconnects and the surface area, as well as create a denser skin layer with smaller pore size. In contrast, a reduction of interfacial energy in the system results in a more open and interconnected network of macrovoids with less dense skin layers at the aerogel surfaces and macrovoid walls. In addition, polymer concentration 23 ACS Paragon Plus Environment

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can also be another design factor to increase or decrease the permeability of the macroporous networks. Conclusions The emulsion-templating method was successfully implemented to prepare open cell aerogel foams. These structures show significant flexibility to tune the macrovoid size, macroporous nature of the skin, and the skin layer structure and morphology found at the interfaces between the voids and the polymer. This flexibility opens up this structure for a variety of applications. For example, these aerogels, or their corresponding gels, can be used as a membrane for gas and liquid filtration, where selectivity can be controlled through both the skin layer structure and the density of the macroporous matrix. Permeability of these membranes can also be improved through the inclusion of the macrovoids and the openness of the foam cell structure. In another possible application, these aerogels can potentially be used to attenuate elastic waves. Their low density, high porosity, and critical ability to tailor macrovoid dimensions allow these hierarchical aerogels to be a potential candidate for acoustic and blast wave attenuation applications.

Supporting Information. Additional data on water droplet size dependence on surfactant concentration and temperature and water droplet settling rate.


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