Toward Maximizing the Mechanical Property of Interconnected

University, 38 Zheda Road, Hangzhou, Zhejiang 310027, P. R. China. Langmuir , Article ASAP. DOI: 10.1021/acs.langmuir.7b03176. Publication Date (W...
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Toward Maximizing the Mechanical Property of Interconnected Macroporous Polystyrenes Made from High Internal Phase Emulsions Song Wang, Jiaxu Li, Mengfei Qi, Xiang Gao, and Wen-Jun Wang* State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, P. R. China S Supporting Information *

ABSTRACT: Macroporous materials polymerized from high internal phase emulsions (PolyHIPEs) possess well-defined interconnected porous structures and tunable device shapes. This provides interesting property characteristics well-suited for a variety of applications. However, such materials also demonstrate poor mechanical performances, which limit their potential use. As will be demonstrated, this results from the high surfactant content required by PolyHIPEs. Herein, a new approach is introduced for designing a highly efficient polymeric surfactant, which generates interconnected pores in PolyHIPEs through designing an incompatible surfactant and skeleton material. The surfactant also possesses a hyperbranched topology, which combines the strong amphipathy of small molecular surfactants and the nanosphere structure of Pickering emulsifiers to provide an excellent colloidal stability to HIPEs. A hyperbranched polyethylene having pendant sodium sulfonate groups (HBPE−SO3Na) was thus designed and synthesized via chain walking copolymerization of ethylene and 2-trimethylsilyloxyethyl acrylate followed by sulfonation. Stable HIPEs of styrene/divinylbenzene and water at a weight ratio of 1 to 5 were obtained with using HBPE−SO3Na. The polymerization of HIPEs produced interconnected macroporous polystyrenes (PSs) at a substantially lower surfactant content, for example, 0.5 wt % HBPE−SO3Na. The compressive Young’s moduli of PolyHIPEs reached 104−111 MPa with 0.5−2 wt % HBPE−SO3Na, which is the first reported case of a PS-based PolyHIPE achieving its theoretical modulus. The PolyHIPE was used to support Au nanoparticles and embed in a column for oxidation of dimethylphenylsilane. A complete conversion of dimethylphenylsilanol was achieved with low column back pressure in a 50 h continuous reaction with no degradation of PolyHIPE integrity and mechanical property.

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polyethylene19 and 3.6 GPa for polystyrene (PS).20 It is speculated that the reduction in the mechanical property results from the surfactant residue in the polymer skeleton. In addition, the presence of high surfactant concentrations increases the toxicity of PolyHIPEs, limiting their use in biological applications.8 To inhibit the mechanical property loss in PolyHIPEs, polymeric and Pickering surfactants have been developed to produce PolyHIPEs.21−34 Engler and Battaglia first succeeded in introducing PS-b-poly(ethylene oxide) and PS-b-poly(acrylic acid) block copolymers for the preparation of divinylbenzene (DVB)-based water-in-oil PolyHIPE with interconnected pores.21 A 5 wt % linear PS-b-polyvinylpyridine was able to generate PolyHIPE having interconnected pores.22 In addition to the linear block copolymers, a dendritic polyethylenimine-b-PS was developed and used to stabilize HIPEs.23 A low-molecular-mass gelator35,36 and modified graphene oxide37 were also used as stabilizers for HIPEs. Although the Pickering emulsifiers were able to stabilize

INTRODUCTION Interconnected macroporous polymers are widely used in applications such as catalysis,1 separation,2,3 energy conversion and storage,4 biomedicine,5 gas sensing,6 and space science.7 Their high porosity and permeability is typically achieved using templating techniques8 such as high internal phase emulsion (HIPE) polymerization.9,10 The macroporous polymers generated with HIPE (PolyHIPEs) possess well-defined and adjustable interconnected pores of sizes larger than 1 μm with porosities greater than 74%11−13 and easily tunable device shapes. However, PolyHIPEs tend to be brittle even chalky with compressive Young’s moduli that are substantially lower than the theoretical values estimated from Gibson’s equation,14 which limits their use.15,16 This is believed to result from the approximately 20 wt % small molecular surfactant typically applied to stabilize HIPEs and help generate the interconnected pore structure in PolyHIPEs.17,18 Young’s moduli of PolyHIPEs produced with small molecular surfactants are normally in the range of 10 MPa, compared with values greater than 1 GPa for the polymer monolith, for example, 2.4 GPa for ultrahigh-molecular-weight © XXXX American Chemical Society

Received: September 11, 2017 Revised: October 25, 2017

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DOI: 10.1021/acs.langmuir.7b03176 Langmuir XXXX, XXX, XXX−XXX

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Langmuir HIPEs,30−34 the resulting PolyHIPEs contained collapsed pores in the absence of a pore-forming agent. PS-based PolyHIPEs with a Young’s modulus of 49 MPa and a 76% porosity were obtained using a Pickering emulsifier.32 However, this is approximately 24% of the theoretical modulus of the pure PS monolith.20 Polymerization approaches such as controlled/ living radical polymerization for PS-based PolyHIPEs,15 ringopening metathesis polymerization for poly(dicyclopentadiene)-based PolyHIPEs,16 and simultaneous polymerizations for PS-based PolyHIPEs38 have also been used in an attempt to enhance the mechanical property of PolyHIPEs. However, the maximum modulus observed with these approaches was only 83% of the theoretical value.15 It appears that improving the mechanical property of PolyHIPEs will require a significant reduction in the emulsifier content without destabilizing HIPEs, which would prevent the formation of an interconnected pore structure. Although it is possible to stabilize the water-in-oil HIPEs with only a small amount of a Pickering emulsifier, for example, 1 wt %,30−34 only closed pores are formed in PolyHIPEs.31 Here another option, the use of an amphipathic hyperbranched polymer having a spherical topological structure, is introduced. This emulsifier could mimic the Pickering emulsifiers for the HIPE applications in that it provides stability at low concentrations. The challenge for such amphipathic hyperbranched polymers is to introduce the pore-opening feature in PolyHIPEs while maintaining the HIPE stability for low dosages. Incompatibility, resulting in phase separation between the skeleton polymer and the amphipathic hyperbranched polymer, is speculated to assist in forming the interconnected pore structure during the formation of PolyHIPEs. It has been reported that hyperbranched polyethylene (HBPE) is not compatible with PS.39 Therefore, an amphipathic HBPE is designed for this purpose. Herein, an amphipathic HBPE40−49 having pendant sodium sulfonate (HBPE−SO3Na) was designed and synthesized through copolymerization of ethylene (E) and 2-trimethylsilyloxyethyl acrylate (HEA-TMS) with [(ArNC(Me)−(Me)CNAr)Pd(CH2)3C(O)OMe]+SbF6− (1) as a catalyst, followed by the deprotection of the trimethylsilyl group and sulfonation of the hydroxyl group. Styrene (St)-based HIPEs were prepared and polymerized with HBPE−SO3Nas as surfactants. The pore structure and the mechanical property of PolyHIPEs were investigated and compared with their theoretical modulus. Good mechanical properties of PolyHIPEs were further demonstrated by continuous oxidation of dimethylphenylsilane in a column packed with their supported Au nanoparticles (AuNPs).

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Sinopharm Chemical Reagent Co., Ltd. All of the chemicals were used directly as received, except DVB and St were passed through an alkaline aluminum oxide column to remove the inhibitor prior to use. Synthesis of 2-Trimethylsilyloxyethyl Acrylate. Twenty milliliters (174 mmol) of HEA and 29 mL (192 mmol) of TEA were charged into a three-necked flask containing 150 mL of dichloromethane immerged in an ice bath under N2 protection. Chlorotriethylsilane (24.5 mL, 192 mmol) dissolved in 50 mL of dichloromethane was then added dropwise. The reaction system was kept at room temperature overnight and filtered to remove NEt3·HCl. The resulted product was washed by NaHCO3 aqueous solution three times and NaCl saturation twice and further distilled at 100 °C under 35 mm Hg. A transparent liquid (28 g) was obtained. Copolymerization of Ethylene and HEA-TMS. Copolymerization was carried out following the reported procedure.55 Take HBPE− SO3Na1 as an example for readers’ convenience. A 50 mL Schlenk flask equipped with a magnetic stirrer and a rubber septum was flamedried under vacuum and refilled with ethylene. After three cycles of vacuum and ethylene refilling, 3.42 g of TMS-HEA solution (6 mmol in 10 mL of dichloromethane) was injected into the flask, followed by another 10 mL of dichloromethane and Pd-diimine catalyst 1 (0.17 g in 10 mL of dichloromethane). The polymerization was carried out at 25 °C for 24 h at 1 atm ethylene pressure and then terminated by pouring into methanol under vigorous stirring. The precipitate was dissolved in THF and precipitated in methanol for three times and dried under reduced pressure at 30 °C. A transparent sticky product HBPE−OH−TMS1 (6.7 g) was obtained. The number-average molecular weight (Mn) was 178 kDa, and the dispersity (Đ) was 1.17, whereas the HEA-TMS mole fraction was 1.53 mol %. Deprotection of the Trimethylsilyl Group in HBPE−OH− TMS. Deprotection of the trimethylsilyl group was carried out by the reaction of HBPE−OH−TMS with tetrabutylammonium fluoride. Take HBPE−OH−TMS1 as an example. HBPE−OH−TMS1 (6.3 g) was dissolved in 160 mL of THF and mixed with 15 mL of tetrabutylammonium fluoride in THF solution (1 M, 10 equiv) at room temperature overnight. Majority of THF in the solution was then removed under vacuum, and the concentrated solution was precipitated in methanol. The precipitate was dissolved in THF and precipitated in methanol for two more times to remove residual tetrabutylammonium fluoride and dried under reduced pressure at 30 °C. HBPE with a pedant −OH group (HBPE−OH1) (6.0 g) was obtained. The deprotection efficiency was 99%. Sulfonation of HBPE−OHs. Sulfonation was carried out by the reaction of HBPE−OH with NaH and 1,3-propanesultone. Take HBPE−OH1 as an example. P−OH-1 (5.9 g) was dissolved in 200 mL of superdry THF in a 500 mL flask with a rubber septum. NaH (0.25 g, 60% dispersion in mineral oil, 2 equiv) dissolved in 10 mL of dry THF was added dropwise at room temperature under N2 protection. The reaction mixture was then raised to 70 °C for 24 h, and 1.5 g (4 equiv) of 1,3-propanesultone was added for another 24 h reaction. The mixture was concentrated by removing THF under reduced pressure and poured into a large amount of methanol for precipitation. The precipitate was washed with methanol, and residual solvents were removed under reduced pressure at 30 °C. The product HBPE− SO3Na1 (4.5 g) was obtained. The sulfonation efficiency was 51%. Preparation of HIPE of St. Take run 2 as an example. CaCl2 (0.083 g) and KPS (0.012 g) were dissolved in 6 g of deionized water to form an aqueous phase. An oil phase composed of 0.84 g of St and 0.36 g of DVB. HBPE−SO3Na1 (12 mg) was dispersed in the oil phase in a 10 mL vial by homogenizing at 15 000 rpm for 30 s. The aqueous phase was then added dropwise at 15 000 rpm to produce the HIPE. The impact of varying the St/DVB ratio will be investigated in the future study. Preparation of PolyHIPE of St. HIPEs were polymerized at 60 °C for 24 h. The internal phase was removed under reduced pressure at 80 °C for 24 h, followed by Soxhlet extraction with water and then methanol for 24 h each to produce PolyHIPEs. Preparation of PolyHIPE-Supported AuNPs. CaCl2 (0.083 g) and KPS (0.012 g) were dissolved in 6 g of deionized water to make the aqueous phase. An oil phase composed of 0.65 g of St, 0.35 g of

EXPERIMENTAL SECTION

Materials. Experiments involving air- and/or moisture-sensitive compounds were conducted in a glovebox or employing Schlenk techniques. A Pd-diimine catalyst [(ArNC(Me)−(Me)CNAr)Pd(CH2)3C(O)OMe]+SbF6− (1) was synthesized following the reported procedure (ref S1). Ultrahigh-purity N2 and polymerization-grade ethylene (Sinopec China) were purified by passing through columns filled with CuO catalyst and 3 Å molecular sieve for purification, respectively. HEA (97%), trimethylamine (99%, TEA), chlorotriethylsilane (99%), tetrabutylammonium fluoride (1.0 M in tetrahydrofuran (THF), containing ca. 5% H2O), sodium hydride (60% in paraffin oil), 1,3-propanesultone (99%), gold(III) chloride trihydrate (49.0% Au), 1-vinyl-2-pyrrolidinone (99.5%, NVP), potassium persulfate (KPS, 99.%), and dimethylphenylsilane (97%) were purchased from J&K Scientific. DVB was received from Alfa Aesar, whereas styrene (St), basic solvents, and salts were from B

DOI: 10.1021/acs.langmuir.7b03176 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Theoretical weight percentages of Span 80 on the pore interface SI for the PS-based PolyHIPE possessing various pore radii R estimated using eq 1. The PolyHIPE has a σ = 80%. (b) Fraction of Young’s modulus of pure PS preserved for the compounded PS samples (Ep/Ep0) with different Span 80 loadings. The fit line is Ep/Ep0 = 1 − 3.26Ss. DVB, and 0.2 g of NVP. HBPE−SO3Na1 (60 mg) was dispersed in the oil phase in a 10 mL vial by homogenizing at 15 000 rpm for 30 s. The aqueous phase was then added dropwise at 15 000 rpm to prepare the HIPE. The emulsion was charged into a column with a length of 50 mm and a diameter of 7.5 mm. The column was sealed and maintained at 70 °C for 24 h. After polymerization, the internal phase was removed under reduced pressure at 80 °C and flushed with a 1:1 v/v water/acetone mixed solvent at a flow rate of 0.1 mL/min. A 20 mg/mL HAuCl4 solution in 1:1 v/v water/acetone was flowed through the column at a flow rate of 0.1 mL/min for 2 h. A 1 mg/mL NaBH4 solution in 1:1 v/v water/acetone was then pumped through at 0.1 mL/min for 1 h to reduce HAuCl4 to AuNPs. The column was further flushed with the 1:1 v/v water/acetone mixed solvent at 0.1 mL/min for 2 h. Continuous Oxidation of Dimethylphenylsilane. Dimethylphenylsilane (0.010 g/mL) and water (0.017 g/mL) in THF were flowed through the column at 30 °C and a flow rate of 0.1 mL/min. The outflow was characterized by 1H NMR to determine the conversion of dimethylphenylsilane. Characterization. The mixture of Span 80 and PS at various weight ratios was circulated at 180 °C and 60 rpm for 10 min using a HAAKE MiniLab and was molded into 60 × 10 × 1 mm3 specimens by a HAAKE MiniJet II at a jet pressure of 1000 bar and a post pressure of 950 bar. The dynamic mechanical analysis (DMA, DMA 242 NETZSCH) was used to determine Young’s modulus of the samples. The DMA measurements were conducted at 24 °C, amplitude of 1%, and a frequency of 10 Hz. The 1H NMR characterization was conducted on a Bruker AVANCE 2B 400 MHz spectrometer with CCl3D as a solvent. The infrared (IR) spectra were acquired from a Nicolet 5700 FT-IR instrument. The molecular weight and the dispersity of polymers were determined using a triple-detector gel permeation chromatography (PL-GPC50) equipped with a differential refractive index (DRI), a four-bridge capillary viscometer, and light scattering detectors (45° and 90°). One guard column (PL# 1110−1120) and three 30 cm columns (two PLgel 10 μm Mixed-B 300 × 7.5 mm and one PLgel 10 μm 500 Å 300 × 7.5 mm) were used. THF was used as an eluent at a flow rate of 1.0 mL/min. The measurement temperature was set at 30 °C. The signals collected by the laser detector were used to calculate the absolute molecular weights via Cirrus software. A DRI increment (dn/dc) value of 0.078 mL/g was applied for the HBPE copolymers.49 The particle sizes of the polymers were determined using a Malvern Zetasizer Nano ZS model ZEN 3690 instrument at 25 °C with THF as a solvent. This equipment was equipped with an argon-ion laser with a wavelength of 633 nm. The detection angle was 90°. The sulfur element content in HBPE−SO3Nas was determined by a GLC-200 microcoulometry sulfur analyzer (Jiangyan Yinhe Instrument Co., detection limit = 0.05 ppm). The images of PolyHIPEs were acquired using a Carl Zeiss ULTRA 55 scanning electron microscopy (SEM) instrument. PolyHIPEs were gold sputtered in an Emitech 7620 instrument under an argon atmosphere before the SEM measurements. The density and porosity of PolyHIPEs were determined by a Micromeritics AutoPore IV 9510 automated mercury porosimeter. The mechanical properties of PolyHIPEs were characterized by a Zwick/Roell Z020 universal

testing machine with a 25 kN load cell. The PolyHIPEs were compressed at a rate of 2 mm min−1 until reaching a 40% compression ratio.

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RESULTS AND DISCUSSION Traditional HIPE normally requires approximately 20 wt % surfactant based on oil monomers,17,18 which is greatly in excess of what is typically required for stabilizing a similar emulsion. Part of the reasons of using high surfactant loading is to generate the open pore structure in PolyHIPEs.17,18 To understand the theoretical surfactant amount required when the surfactant is closely packed on the interface of PolyHIPEs, a model was developed and introduced in the Supporting Information. The theoretical surfactant weight percentage on the interface of the pores (SI) can be estimated using eq 1 SI =

3 3σ ws × 100 2π (1 − σ )ρs r 2R

(1)

where σ is the porosity, R is the average pore radius, ρs is the skeleton density of the PolyHIPE, r is the individual surfactant radius, and ws is the weight of an individual surfactant species. A plot of SI as a function of R for a PS-based PolyHIPE produced with (Ss) 20 wt % Span 80 with σ = 80% is given in Figure 1a. Values were estimated using eq 1 with ρs = 1.05 g/ cm3, ws = 7.117 × 10−22 g/molecule, and πr2 = 0.46 nm2.50 From Figure 1a, it can be seen that, theoretically, only 0.3 wt % Span 80 locates on the interface of the PS PolyHIPE having the pore radius of 10 μm, that is, 19.7 wt % Span 80 (98.5% of the overall surfactant used) resides in the PS skeleton. For further understanding the effect of the surfactant residue on Young’s modulus of the skeleton, four Span 80 compounded PS samples with Ss values ranging from 1 to 3.76 wt % were prepared. A neat PS sample was also prepared for comparison. Their compressive Young’s moduli were determined by the dynamic mechanical analysis (DMA). The ratio of Young’s moduli of the compounded PS (Ep) to pure PS (Ep0) versus the Span 80 loading is plotted in Figure 1b. A linear decrease of the modulus ratio Ep/Ep0 with the addition of Span 80 was observed. When the Span 80 loading was 3.76 wt %, only 85% of Young’s modulus for the neat PS was preserved. Moreover, the fraction of Young’s modulus of the pure PS preserved for the compounded PS samples can be expressed as

Ep/Ep0 = 1 − ASs

(2)

where A is an empirical parameter specific to the surfactant and the polymer. Here, Ep/Ep0 = 1 − 3.26Ss for the Span 80 compounded PS. It is worth pointing out that the degree of aggregation and the distribution of surfactants play an important role in C

DOI: 10.1021/acs.langmuir.7b03176 Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. (a) Synthesis of HBPE Containing Pendant Sodium Sulfonate Functionalities (HBPE−SO3Na) via the Pd-Catalyzed Chain Walking Copolymerization of Ethylene and HEA-TMS Followed by Deprotection and Sulfonation; (b) Procedure of Fabricating PS-Based PolyHIPEs and Immobilizing AuNPs in the PolyHIPE

Table 1. Experimental Conditions and Results of Pd-Catalyzed Copolymerization of Ethylene and HEA-TMS Followed by Deprotection and Sulfonationa run

[HEA-TMS] (M)b

yield (g)

F (mol %)c

Mn (kDa)d

Đd

Ne

es (%)f

BD (C/103C)g

dz (nm)h

HPBE−SO3Na1 HPBE−SO3Na2

0.6 0.8

6.7 5.5

1.53 2.46

178 69.8

1.17 3.28

93.5 57.4

51 27

95 98

10.1 7.5

a

Other experimental conditions: Pd-diimine catalyst 1 loading 0.2 mmol, ethylene pressure 1 atm, solvent CH2Cl2 30 mL, polymerization temperature 25 °C, and reaction for 24 h. bHEA-TMS concentration. cHEA-TMS mole fraction in the polymer determined by 1H NMR. dAbsolute number-average molecular weight and dispersity determined by a triple-detector GPC. eN is the average number of incorporated comonomer per polymer chain. fSulfonation efficiency determined by a microcoulometry sulfur analyzer. gBranching density (BD) is the number of methyl groups per 1000 carbons of the polymer determined by 1H NMR. hThe z-average particle size of HBPE−OH in THF determined by a dynamic light scattering (DLS) analyzer.

OH samples were obtained with number-average molecular weight (Mn) values of 178 and 69.8 kDa, dispersity (Đ) values of 1.17 and 3.28, and average number of −OH group per polymer chain (N) values of 93.5 and 57.4 (Table 1). High HEA-TMS usage resulted in HBPE having broad dispersity.45 The z-average particle size (dz) values of HBPE−OHs in THF were 10.1 and 7.5 nm. After sulfonation, 51% of −OH groups in HBPE−OH1 was converted into −SO3 groups, whereas 27% of those in HBPE−OH2 was sulfonated. The low sulfonating efficiency (es) resulted from HBPEs having −SO3 groups possessed a poor solubility in THF, although HBPE−OHs were soluble in the same solvent. The HBPE−OH/THF solutions became cloudy with the progress of the sulfonation reaction. An appropriate solvent could not be found to dissolve HBPE−

determining the extent to which the mechanical performance of compounded PS samples are impacted. Figure 1b shows that the residue of large fraction of the surfactant in the polymer skeleton is responsible for causing the deterioration of the mechanical property in PolyHIPEs. It appears clear that the key to maintain a high modulus for PolyHIPEs is to decrease the surfactant content in the polymer skeleton. To reduce the surfactant usage, a high efficient HBPE-based surfactant HBPE−SO3Na was designed and tailored. The preparation process of HBPE−SO3Na is shown in Scheme 1a. A chain walking polymerization51,52 of ethylene and HEA-TMS catalyzed by a Pd-diimine catalyst 1 at 1 atm was conducted to synthesize HBPE with pendant −OH groups (HBPE−OH) after the deprotection of the trimethylsilyl group. Two HBPE− D

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Langmuir Table 2. Experimental Conditions and Characterization Results for PolyHIPE Samplesa run

surfactant

S (wt %)c

σ (%)d

ρ (g/cm3)d

1 2 3 4 5 6 7 8 9 10 11 12b

HBPE−SO3Na1 HBPE−SO3Na1 HBPE−SO3Na1 HBPE−SO3Na1 HBPE−SO3Na1 HBPE−SO3Na2 HBPE−SO3Na2 HBPE−SO3Na2 HBPE−SO3Na2 HBPE−SO3Na2 Span 80 HBPE−SO3Na1

0.5 1 2 5 10 0.5 1 2 5 10 20 5

78.2 78.0 78.0 77.4 77.4 70.1 78.1 77.9 77.6 77.5 77.6 82.3

0.183 0.184 0.185 0.185 0.190 0.183 0.183 0.185 0.191 0.193 0.178 0.190

Dp (μm)e

SI/S (%)f

± ± ± ± ± ± ± ± ± ±

66 45 24.5 11.8 7 32 43 26 10 2

20.4 14.6 13.4 10.9 8.8 18.4 10.7 8.7 8.9 6.2

15.3 7.2 4.5 3.8 2.2 9.2 4.1 3.7 3.3 1.7

4.8 ± 1.2

a

Other experimental conditions: an aqueous phase comprises 6 g of water with 0.083 g of CaCl2 and 0.012 g of KPS, and an oil phase has 0.84 g of St and 0.36 g of DVB. bSame aqueous phase but different oil phases: 0.7 g of St, 0.3 g of DVB, and 0.2 g of N-vinyl-2-pyrrolidone (VP). cSurfactant content (S) with respect to the mass of the oil phase. dPorosity (σ) and density (ρ) of the PolyHIPEs determined by a mercury intrusion porosimetry. eEstimated from SEM images. fPercentage of total surfactant usage locating on the pore interface of the PolyHIPEs estimated using eq 1.

SO3Na samples because of their distinct amphipathy between the hydrophilic and hydrophobic components. HBPE−SO3Nas were used to prepare W/O St HIPEs with an oil-to-water weight ratio of 1:5. Stable HIPEs were formed even at 0.2 wt % HBPE−SO3Na1 with respect to the oil phase, as shown in Figure S7 of the Supporting Information. When HBPE−SO3Na1 reached 1 wt %, gel HIPEs appeared and the emulsions could be held in the vials even when inverted. The emulsion maintained stability at room temperature for more than 2 months. HBPE−SO3Nas were found to be excellent emulsifiers for preparing W/O HIPEs. For the preparation of PS-based PolyHIPEs, 30 wt % St was substituted by DVB to prepare HIPEs. HIPEs with 0.5−10 wt % HBPE−SO3Nas (runs 1−10) were polymerized at 70 °C. A PolyHIPE sample using 20 wt % Span 80 as an emulsifier (run 11) was also produced for comparison. The characterization results of the obtained PolyHIPE are summarized in Table 2. Both St and DVB were found to be approximately 100% converted via gravimetric measurement and 1H NMR for residual monomers in CClD3 extract of the PolyHIPEs. The PolyHIPEs were produced at a variety of HBPE−SO3Na (S) dosages including 0.5 wt %, which was more than one order of magnitude lower than those using other polymeric surfactants.21−29 The fraction of the total HBPE−SO3Na usage located on the pore interface in PolyHIPEs (SI/S) was estimated using eq 1, following the procedure shown in the Supporting Information and summarized in Table 2. For run 1 having 0.5 wt % HBPE−SO3Na1, 66% of overall surfactant was distributed on the interface, demonstrating an extremely low residue of the surfactant in the PS skeleton. The SEM images in Figure 2 show runs 1−10 made with HBPE−SO3Nas possessing integrated pore structures, whereas run 11 with 20 wt % Span 80 had poor pore structure control, suggesting that HBPE−SO3Nas were capable of holding the pore structure through the polymerization at high temperature. The average pore sizes (Dp) of runs 1−10 were 6.2−20.4 μm, which were comparable to those employing small molecular surfactants.19,20,26 The Dps decreased, and the pore size distribution became narrower with more HBPE−SO3Na usage. Because HBPE−SO3Na1 had higher Mn than HBPE−SO3Na2, the resulted PolyHIPEs produced with HBPE−SO3Na1 possessed slightly larger Dp than those with HBPE−SO3Na2 at the same

Figure 2. SEM images of PS-based PolyHIPEs. HBPE−SO3Na1: 0.5 wt % (run 1), 1 wt % (run 2), and 5 wt % (run 4); HBPE−SO3Na2: 0.5 wt % (run 6), 1 wt % (run 7), and 5 wt % (run 9); Span 80: 20 wt % (run 11); and HBPE−SO3Na1: 5 wt % (run 12) for P(S-co-VP) PolyHIPE.

loading. The pore structure of runs 1 and 6 slightly warped because of the low HBPE−SO3Na usage (0.5 wt %). However, even at such a low level of HBPE−SO 3 Na content, E

DOI: 10.1021/acs.langmuir.7b03176 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. (a) Compressive stress−strain curves for PS-based PolyHIPEs. aEf is the experimental compressive Young’s modulus of the PolyHIPE sample; bEf0 is the theoretical compressive Young’s modulus of the PolyHIPE sample estimated from Gibson’s equation (eq 3) using an Ep0 of 3600 MPa20 as the pure PS skeleton modulus (without the consideration of the surfactant effect); cEp is the compressive Young’s modulus of the PolyHIPE skeleton (with the consideration of the surfactant effect) estimated using Gibson’s equation. (b) Comparison of Young’s moduli of PSbased PolyHIPEs from this work and other works with respect to theoretical optimum moduli given by Gibson’s equation. (c) Ratio of Young’s modulus of the PS skeleton to that of pure PS (Ep/Ep0) with respect to different HBPE−SO3Na1 loadings. The fit line is Ep/Ep0 = 1 − 8.41Ss.

⎛ ρ ⎞2 Ef = ⎜⎜ ⎟⎟ Ep ⎝ ρs ⎠

interconnected pores in run 1 sample can still be observed. Both HBPE−SO3Nas provided PolyHIPEs with interconnected pore structure in integrity when S was greater than 1 wt %. The pore throats and polymer “tongues” (burst polymer thin films around the interconnected pores) could be observed in the PolyHIPEs from the SEM images, which become clearer with increasing HBPE−SO3Na dosages (see Figure S9 in the Supporting Information). Energy-dispersive spectrometer (EDS) mapping was conducted on the sample generated in run 4. The sulfur concentrations around the pore throats were approximately five times higher than those on the other pore interfaces, suggesting that the interconnected pores were produced by HBPE−SO3Nas. Atomic force microscopy (AFM) characterization of PS samples blended with 5 wt % HBPE−SO3Na1 and Span 80 was carried out. A clear phase separation between PS and HBPE−SO3Na was observed from the AFM images, as shown in Figure S10 in the Supporting Information, further supporting the incompatibility between PS and HBPE−SO3Na. The concentrated HBPE−SO3Nas between the adjacent PS cells weaken the integrity and strength of cell walls, resulting in the formation of interconnected throats between the pores in addition to the contribution of contraction forces from the density change between St and PS during the polymerization.53,54 The incompatibility between PS and HBPE−SO3Na appears to be greatly beneficial to the formation of interconnected pores for the low surfactant level, while normally at approximately 20 wt % for the small molecular surfactant. Poor mechanical property of PolyHIPEs is a big issue limiting their applications. The compressive Young’s moduli of PolyHIPEs made from the small molecular surfactant are usually less than 10 MPa, which are well below their theoretical values estimated from the Gibson equation, eq 3.14

(3)

where Ef and Ep are the compressive Young’s moduli of the PolyHIPE and its PS skeleton, respectively, and ρ is the PolyHIPE density. The compression measurements for PolyHIPE runs 1−5 and 11 were conducted. Their compressive stress−strain curves and Young’s moduli Efs are provided in Figure 3a. On the basis of their densities (Table 2), the theoretically compressive Young’s moduli Ef0s of runs 1−5 (without the consideration of the surfactant effect) were estimated using eq 3 and summarized in Figure 3 for comparison, where Young’s modulus (Ep0) of 3600 MPa was used for the pure PS skeleton (without the surfactant).20 Samples from runs 1−3 containing 0.5−2 wt % HBPE−SO3Na1 had the moduli of 104−111 MPa and crush strengths of 4.61−5.06 MPa, whereas the modulus and crush strength of run 11 using Span 80 was only 5.31 and 0.18 MPa, respectively. The crush strengths of runs 1−3 are almost twice of the reported highest value of 2.8 MPa for the PS PolyHIPE having a similar foam density of 0.2 g/cm3.33 By comparing their corresponding theoretical Ef0 values, runs 1−3 samples achieved their theoretical optimum moduli. HBPE−SO3Na is shown to effectively produce PolyHIPEs of high mechanical property. It can also be seen that the modulus and crush strength of PolyHIPEs dropped to 23.4 and 1.23 MPa, respectively (run 5) with the increase of HBPE−SO3Na1 usage to 10 wt %, which further confirms the high surfactant residue in the skeleton resulting in the low mechanical property of the PolyHIPE. Because the pore size and size distribution decreased with more HBPE−SO3Na used, it appears that the smaller pore size and narrower size distribution did not enhance the mechanical property of the PolyHIPE. Young’s moduli of PS-based PolyHIPEs from this work are also compared with those from other studies, which are summarized F

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Langmuir

been developed. Majority of the surfactant was found to reside within the skeleton of the PolyHIPE when a high surfactant loading was used, leading to a linear decrease in the compressive Young’s modulus with the surfactant loading for both Span 80 and polymeric surfactant HBPE−SO3Na. The large amount of surfactant residue retained in the polymer skeleton has been shown to be the primary reason for the substantial deterioration in the mechanical property of PolyHIPEs. With the successful design and synthesis of high efficient HBPE−SO3Nas, St HIPEs could be stabilized with as low as 0.2 wt % HBPE−SO3Na, and interconnected PS-based PolyHIPEs were produced at 0.5 wt % loading, which was one order of magnitude lower than that of small molecular or polymeric surfactants. The resulted interconnected PolyHIPEs possessed 104−111 MPa of compressive Young’s moduli at 0.5−2 wt % HBPE−SO3Na, achieving the theoretical optimum moduli estimated from Gibson’s equation. The formation of the interconnected structure in PolyHIPEs at such low surfactant loadings was attributed to the phase separation between HBPE−SO3Na and PS skeleton. After filling a column with the PS-based PolyHIPE and immobilizing with AuNPs, a continuous oxidation of dimethylphenylsilane through the column was carried out with fully converted dimethylphenylsilanol obtained within 50 h reaction. The PolyHIPE after use still preserved its integrity with high mechanical property.

in Figure 3b. It can be seen that the PS-based PolyHIPEs produced in this work possess high modulus values, which are similar to their theoretical values calculated from Gibson’s equation (eq 3). The moduli of runs 1−5 skeletons Ep (without the consideration of the surfactant effect) were further estimated using eq 3 and summarized in Figure 3. A linear decrease between Ep/Ep0 and Ss existed with Ep/Ep0 = 1 − 8.41Ss, as shown in Figure 3c. This is consistent with the results reviewed previously for PS compounded with Span 80, providing further evidence supporting our hypothesis that the presence of large amount of residual surfactant in skeleton is responsible for lowering the mechanical property of the PolyHIPE. To examine the durability offered by the PolyHIPE having high mechanical property, a column embedded with PS-based PolyHIPE-supported AuNPs was prepared for continuous catalytic reaction. For better immobilization of AuNPs, 17% of VP was added in the oil phase with 5% of HBPE−SO3Na1 (run 12). The run 12 sample had 82.3% of porosity with an average pore size of 4.8 μm and supported with 50 mg of AuNPs/g PolyHIPE. A continuous silane oxidation of dimethylphenylsilane at 30 °C was carried out in the immobilized AuNP column through a feeding pump at a flow rate of 0.1 mL/min. The back pressure of the column was maintained at 0.01 MPa, suggesting the PolyHIPE provided for high throughput. The mean residence time (τ) of the reactants was controlled at 18 min. Within 50 h reaction (equivalent to 167 τ’s), complete conversion of dimethylphenylsilane was maintained as presented in Figure 4. The PolyHIPE after use still preserved its integrity with high mechanical property.

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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.7b03176. Details about the estimation of surfactant amount on the interface of PolyHIPEs, 1H and 13C NMR spectra of 2trimethylsilyloxyethyl acrylate and HBPE−OH2 after deprotection, IR spectra of HBPE−OH1 and HBPE− SO3Na1, appearance of HIPEs, EDS mapping for sulfur determination on the interface of PolyHIPEs, SEM images and element analysis of PolyHIPEs, and AFM images of PS films mixed with various surfactants (PDF)

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CONCLUSIONS To better understand the surfactant distribution in PolyHIPEs, a model for surfactant amount required on the interface has

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 571 87952772. Fax: +86 571 87952772. ORCID

Wen-Jun Wang: 0000-0002-9740-2924 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grants 21376211, 21536011, and 21420102008) and the Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (grants SKL-ChE15D03 and SKL-ChE-14D01).

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Figure 4. Continuous silane oxidation of dimethylphenylsilane in PSbased PolyHIPE-supported AuNPs in the column. Reaction conditions: a column with 50 mm long and 7.5 mm diameter; a flow rate of reactants 0.1 mL/min; 30 °C; reactants: 0.01 g/mL dimethylphenylsilane and 0.017 g/mL water in THF. Conversions were determined by 1H NMR.

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