Silica

Subscriber access provided by University of Chicago Library

Article

Pore structure of macroporous polymers using PS/ silica composite particles as Pickering stabilizers Shuhua Tu, Chenxu Zhu, Lingyun Zhang, Haitao Wang, and Qiangguo Du Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03285 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Pore structure of macroporous polymers using PS/silica composite particles as Pickering stabilizers †

†

Shuhua Tu, Chenxu Zhu, Lingyun Zhang, Haitao Wang,* and Qiangguo Du

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China

ABSTRACT A novel approach for preparation of interconnected macroporous polymers with controllable pore structure was reported. The method was based on polymerization of a water-in-oil (w/o) Pickering high internal phase emulsion (HIPE) stabilized by polystyrene/silica composite particles. The composite Pickering stabilizers were facilely obtained by mixing positively charged polystyrene (PS) microspheres and negatively charged silica nanoparticles, and their amphiphilicity could be delicately tailored by varying the ratio of PS and silica. The droplet size of Pickering HIPEs was characterized by an optical microscope. The pore structure of polymer foams was observed by a scanning electron microscope (SEM). The interconnectivity of macroporous polymers was evaluated upon their gas permeability, which was greatly improved after etching PS microspheres included in Pickering stabilizers with tetrahydrofuran. As a result, fine tailoring of pore structure of polymer foams could be realized by simply tuning the ratio of PS to silica particles in the composite stabilizer.

1

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

INTRODUCTION Porous polymers have been widely used in many fields such as catalysts,1,2 separation,3,4 storage,5,6 tissue engineering scaffolds,7-9 and sound and heat insulation10-12 owning to their low density and high porosity. Various applications of porous polymers are heavily relied on their pore structures. Although many methods have been reported on fabrication of porous polymers,13-16 emulsion templates attract great attentions due to the facile tailoring of pore size and size distribution of resulting polymer materials. With a minimum internal phase volume fraction of 74%, high internal phase emulsions (HIPEs) are often used as templates to produce high porosity polymers.17-22 Conventional

HIPEs

are

commonly

stabilized

by

surfactants

including

cetyltrimethylammonium bromide (CTAB), sorbitan monooleate (Span 80), and amphiphilic polymers, which are toxic and not environmental friendly. A large amount of surfactant is required to produce stable HIPEs (5~50 wt %23 relative to the continuous phase), which gives rise to the additional producing cost. Moreover, macroporous polymers prepared from HIPEs generally show undesirable mechanical strength.24 Thus these poly-HIPEs are obviously restricted for further applications, especially in biomedical materials. Instead of molecular surfactants, solid particles with appropriate surface property such as silica,25-28 titania29,30 and iron oxide nanoparticles31-33 can also stabilize HIPEs, which are often referred to as Pickering HIPEs. However, pristine solid particles are often too hydrophilic or too hydrophobic to effectively stabilize HIPEs, and thus they still need to be modified using certain surfactants34,35, thus making the modification process more tedious.36,37 Many efforts have been devoted to obtaining stable Pickering HIPEs by using surfactant-free stabilizers. Colloidal celluloses and various cellulose derivatives were found to be capable of stabilizing simple oil-in-water systems.38 Some other colloidal particles, including chitosan nanocrystals,39,40 protein particles,41,42 fat crystals and wax microparticles43,44, could also successfully stabilize HIPEs. However, these reports mostly focus on the stability of Pickering HIPEs rather than the pore structures of poly-HIPEs, which have rarely been explored. 2


Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Polymer foams with interconnected pores have been reported in the systems where HIPE templates are stabilized by surfactants.24,45-47 In the case of Pickering HIPEs, though a small quantity of pores are interconnected by pore throats formed during polymerization process, control over the interconnectivity in poly-Pickering HIPEs still remains challenging.24 Thus, applications of such macroporous polymers in the fields of adsorption, filtration and tissue engineering scaffolds are restrained. There have been only several reports about Pickering HIPEs concentrating on the formation of open-cell structure so far. Interconnected porous polymers are formed by introducing the molecular surfactants,45,46 or drawing the solid particles from the interface through monomer-particle reaction.48 Recently, we prepared polymer foams with open pore structures by extracting the thin monomer layer between two adjacent emulsion droplets using the flocculation with lipophilic core.26 It’s also an effective method to form pore throats by using the monomer with high polymerization shrinkage.25 In this work, a surfactant-free composite Pickering stabilizer is facilely prepared only by mixing positively charged polystyrene (PS) microspheres and negatively charged silica nanoparticles. The wettability of the composite stabilizer is easily tailored by varying the ratio of PS microspheres and silica nanoparticles. Stable water-in-oil HIPEs with controllable droplet size can be produced when PS/silica composite particles are used as Pickering stabilizers. Open-cell macroporous polymers are achieved after polymerization of Pickering HIPEs with relatively high PS microsphere concentration and their interconnectivity is further improved by etching with tetrahydrofuran.

EXPERIMENTAL SECTION Materials. Silica hydrosol (28 nm, 40.0%) was supplied by Fujian Sanbang Chemical Co. (China). Tetrahydrofuran (THF, 99%) was purchased from Sinopharm Chemical Reagent

Co.

(China).

Styrene

(St,

99%),

divinylbenzene

(DVB,

80%),

[2-(methacryloyloxy) ethyl] trimethylammonium chloride (MATMAC, 75%), 3


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2,2'-azobis

(2-methylpropionamidine)

dihydrochloride

Page 4 of 26

(AIBA,

99%)

and

2,2'-azobisisobutyronitrile (AIBN, 99%) were provided by Aladdin Chemistry Co., Ltd. Styrene and divinylbenzene were purified by distillation under vacuum and AIBN was recrystallized with ethanol prior to use. Deionized water was used throughout the experiments. Synthesis of PS/silica composite Pickering stabilizers. PS microspheres were prepared by a soap-free emulsion polymerization method. In a typical procedure, AIBA (0.05 g) was dissolved in water (107.5 g) in a 250 mL three-necked flask equipped with magnetic stirring and nitrogen protection, and then St (9.375 g) and MATMAC (0.277 g) were added. After heating at 70 oC for 12 h, PS microspheres dispersed in water were obtained. Crosslinked PS microspheres were prepared in a similar route, only instead of 9.375 g St a mixture of 8.438 g St and 0.937g DVB was used. PS/silica composite particles were fabricated by dropping silica hydrosol into PS microspheres aqueous dispersion according to the recipe listed in Table 1. Then the mixture was sonicated for 10 min before preparing Pickering HIPEs.

Table 1. Recipes for the preparation of PS/silica composite Pickering stabilizers sample

silica particles (wt %)a

PS microspheres (wt %)a

1 2 3 4 5 6 7 8

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55

0.80 0.75 0.70 0.65 0.60 0.55 0.50 0.45

a

With respect to aqueous phase.

Preparation of Pickering HIPEs. Pickering emulsions were prepared using PS/silica composite particles as the 4


Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

stabilizers. The oil phase (1.25 mL) consisting of 80 vol% St and 20 vol% DVB was added to 5mL aqueous phase containing PS/silica composite Pickering stabilizer. The mixture was then vigorously emulsified with Vortex-BE1 at 500 rpm for 10 min to form Pickering HIPEs. Preparation of Poly-Pickering HIPEs. Initiator AIBN (1.0 wt %) was dissolved in the oil phase prior to the preparation of Pickering HIPEs when the samples would undergo the polymerization. The Pickering HIPEs were transferred into centrifuge tubes, sealed and placed in a 65 oC oil bath for 24 h. The poly-HIPEs were then obtained after drying in a convection oven at 60 oC for 48 h. Characterization. Both hydrodynamic diameter and zeta potential of silica nanoparticles and PS colloidal particles were measured on a Malvern ZS90. The hydrodynamic diameter was measured in a PS cell at 25 °C. The zeta potential was measured in a universal dip cell in a PS cuvette for potential measurements at 25 °C under pH 7. The concentration of the suspension was 0.1 wt %. Photographs of Pickering HIPEs, PS colloidal particles and silica hydrosol were taken by a Sony Xperia Z5P digital camera. The morphology of the Pickering emulsion droplets was observed by an EV5680 optical microscope. The type of Pickering emulsions (w/o or o/w) was assessed by the drop test.49 The pore structure of the macroporous polymers was observed by a Tescan 5136MM scanning electron microscope (SEM). The morphology of PS microspheres was observed by a Zeiss Ultra 55 field-emission scanning electron microscope (FE-SEM). All samples were placed onto carbon-coated lacy substrates and the polymer samples were sputtered with gold before observation. The mean pore size of the poly-Pickering HIPEs was obtained by averaging size of at least 100 pores estimated from SEM images. The permeability of the poly-Pickering HIPE was characterized by measuring the flow rates of nitrogen passing through the sample at 5000 Pa. The macroporous polymers were cut into cylinders with a length of 10 mm and a diameter of 14 mm. 5


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

Each sample was measured three times.

RESULTS AND DISCUSSION Preparation of PS/silica composite Pickering stabilizers Silica hydrosol is normally too hydrophilic to stabilize Pickering emulsions, and thus surfactants are used to tailor its amphiphilicity.25,50,51 Polystyrene (PS) microspheres obtained via a soap-free emulsion polymerization are also very hydrophilic due to the charges on the surface. The charge neutralization of negatively charged silica nanoparticles and positively charged PS microspheres hereby is developed as a new way to endow the composite particles with appropriate amphiphilicity for the subsequent stabilization of Pickering HIPEs. The average diameters of PS microspheres and silica nanoparticles are 103 nm and 57 nm, respectively (see Figure 1). The white flocculation is clearly observed when silica hydrosols are dropped into PS microspheres aqueous dispersion, indicating the strong electrostatic attraction between polymer and inorganic particles (see Figure 2). The zeta potentials of PS particles and silica nanoparticles are measured to be +49.7 ± 0.6 and -20.0 ± 0.8 mV, respectively. Obviously, zeta potential of PS particles decreases gradually as increasing content of silica particles and approaches neutral when the concentration of silica reaches 60 wt % (see Figure 3). Therefore, the extremely hydrophilic surface of PS particles can be suitably hydrophobized via this facile and efficient route for the subsequent preparation of stable Pickering HIPEs. It is worth noting that no other molecular surfactants are involved, indicating an environmental friendly modification process.

6


Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1. Size distribution of (a) silica nanoparticles and (b) PS microspheres in water.

Figure 2. Photographs of (a) PS microspheres aqueous dispersion, (b) silica hydrosol, and (c) PS/silica composite particles (sample 2, Table 1).

7


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

Figure 3. Zeta potentials of PS/silica composite particles with different composition of silica nanoparticles in a constant particle concentration of 0.1 wt %.

Preparation of HIPEs Figure 4 shows the emulsions stabilized by the composite stabilizers with different ratios of silica particles to PS microspheres. With a low silica/PS ratio of 1:4, the PS/silica composite particles are still too hydrophilic to stabilize water-in-oil HIPE due to the strong positive charges on PS surface (see Figure 4a). Pickering HIPEs are prepared successfully when silica/PS ratio in PS/silica composite particles increases (Figure 4b and c), although they flow down slowly when the centrifuge tubes are inverted. With further increasing the silica/PS ratio, HIPEs with high viscosity and stability are obtained (Figure 4d-g). However, when the silica/PS ratio increases as high as 11:9, no stable HIPEs are formed probably due to the extreme flocculation of silica particles and PS microspheres (Figure 4h). During this neutralization process, PS surface is believed to be covered by silica particles in steps as increasing silica amount. The naked surface of PS microspheres renders the composite particles 8


Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

lipophilic while the packed silica particles promise a good affinity toward water. With a proper flocculation extent, the PS/silica composites have optimal stabilization effort for HIPEs. But when extremely large aggregations of silica and PS are produced, the zeta potential approaches minus, meaning that the surfaces of PS microspheres are fully covered by hydrophilic silica particles. It has been reported that solid particles with extreme hydrophilicity or hydrophobicity are unable to stabilize Pickering emulsions.52-54 Meanwhile, the gravity of such large aggregates cannot be neglected anymore at oil/water interface, which is bad for stabilization of Pickering emulsions. Therefore, PS/silica composite particles with too high silica content (sample 8, Table 1) are not able to stabilize HIPEs (Figure 4h).

Figure 4. Photographs of w/o Pickering HIPEs stabilized by composite particles with different weight ratios of silica particles to PS microspheres: (a) 1:4, (b) 1:3, (c) 3:7, (d) 7:13, (e) 2:3, (f) 9:11, (g) 1:1, and (h) 11:9 (samples 1-8, Table 1).

Pickering HIPEs stabilized by PS/silica composite particles were observed by an EV5680 optical microscope. As shown in Figure 5a-d, the droplet size of Pickering HIPEs decreases with increasing silica/PS ratio in PS/silica composite particles from 1:3 to 2:3. Notably, further increment of silica/PS ratio to 9:11 or more (1:1) leads to a larger droplet size of formed HIPEs, instead (see Figure 5e and f). We attribute the size difference of emulsion droplets to the different stabilization effect of composite Pickering stabilizers. As demonstrated above, weak flocculation of oppositely charged PS microspheres and silica nanoparticles can vary the PS surface from extremely 9


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

hydrophilic to amphiphilic due to charge neutralization, which facilitates the effective stabilization of HIPEs. However, over flocculated composite particles sacrifice their surface amphiphilicity due to full coverage of PS surface by hydrophilic silica nanoparticles, indicating a poor stabilization for water-in-oil HIPEs. In short, the size of Pickering HIPEs could be adjusted effectively by varying the ratio of silica to PS in the composite particles.

10


Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 5. Optical microscopy images of Pickering HIPEs stabilized by composite particles with different weight ratios of silica particles to PS microspheres: (a) 1:3, (b) 3:7, (c) 7:13, (d) 2:3, (e) 9:11, and (f) 1:1 (samples 2-7, Table 1). Preparation of poly-Pickering HIPEs After polymerization of Pickering HIPE templates, porous polymers are obtained, as shown in Figure 6. The pore size of macroporous polymers are measured by averaging size of at least 100 pores estimated from SEM images and plotted in Figure 7. It decreases first, and then increases with the content of silica in the composite stabilizer. The pore size of poly-Pickering HIPEs is in a good agreement with the droplet size of the original emulsions, indicating that the coalescence of adjacent droplets is suppressed during polymerization procedure. Interestingly, some pore throats are clearly observed, as shown in Figure 6a-d. To our knowledge, polymerization of Pickering HIPEs mainly produces closed-cell pores. Although some pores are interconnected by pore throats, the interconnectivity is still out of control because the solid particles surround the emulsion droplets acting as barriers.55-57 Considering that PS/silica composites play as Pickering stabilizers of HIPEs in our system, PS microspheres are swollen by St before polymerization. Thus, the monomer layer between two emulsion droplets becomes thinner initially and the pore throats are then produced due to the split on the weak walls when polymerization shrinkage occurs.

11


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

Figure 6. SEM images of poly-Pickering HIPEs stabilized by composite particles with different weight ratios of silica particles to PS microspheres: (a) 1:3, (b) 3:7, (c) 7:13, 12


Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(d) 2:3, (e) 9:11, and (f) 1:1 (samples 2-7, Table 1).

Figure 7. Pore size of poly-Pickering HIPEs stabilized by composite particles with different concentration of silica nanoparticles in PS/silica composite stabilizer (samples 2-7, Table1). In order to verify the formation mechanism of pore throats, crosslinked PS microspheres which are hardly swollen by monomer, were prepared in comparison with uncrosslinked ones. As seen in Figure 8, there is no obvious difference in appearance between uncrosslinked and crosslinked PS microsphere powders. Besides, the size and the morphology of the microspheres are almost the same (see Figure 9). However, the solution of uncrosslinked PS microspheres in monomer becomes transparent and the viscosity increases a lot, while a turbid dispersion is obtained when crosslinked PS particles are mixed with monomer. This result again confirms that uncrosslinked PS microspheres are easily swollen by St, but crosslinked ones are not. Crosslinked PS colloidal particles are also mixed with silica hydrosol as the 13


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

recipe of sample 2 in Table 1 to prepare the Pickering stabilizer and the subsequent Pickering HIPEs. In our case, the size of uncrosslinked PS microspheres after swelling is larger than that of crosslinked ones which are hardly swollen by monomer. It has already been demonstrated that larger Pickering stabilizer often stabilizes larger emulsion droplets, and vice versa.58 Therefore, smaller droplet size of HIPEs as well as smaller pore size of poly-HIPEs is anticipated when substituting crosslinked PS/silica composite particles for uncrosslinked PS/silica composite particles as Pickering stabilizer (see Figure 10a and 10b). Moreover, as shown in Figure 10b few open throats are observed, which is significantly different from the pore structure in the previous case (see Figure 6a). We think one possible reason is that the oil layer between two water droplets is too thick to be torn up during polymerization because crosslinked PS microspheres cannot extract monomer from the oil phase. Gas permeability test (Figure 11) confirms that the interconnectivity of poly-Pickering HIPEs is greatly improved by using uncrosslinked PS/silica composite stabilizers.

Figure 8 Images of (a) uncrosslinked and (b) crosslinked PS microspheres in dried state (left) and after dispersed in St (right).

14


Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 9. FE-SEM images of (a) uncrosslinked and (b) crosslinked PS microspheres.

Figure 10. (a) Optical microscopy image of Pickering HIPEs and SEM images of poly-Pickering HIPEs stabilized by crosslinked PS/silica composite particles (b) before and (c) after etching with THF (sample 2, Table 1). 15


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

Figure 11. Gas permeability of poly-Pickering HIPEs stabilized by PS/silica composite particles containing (a) uncrosslinked or (b) crosslinked PS microspheres (sample 2, Table 1).

The results above have demonstrated that polymer foams with interconnected pore structures can be achieved when uncrosslinked PS/silica composite particles are used as Pickering stabilizers. Since THF is a good solvent for dissolving uncrosslinked PS, which are immobilized initially at the monomer/water interface and then at the pore wall after polymerization, more and larger pore throats would be expected when exposing the poly-Pickering HIPEs to THF. Figure 12 shows morphology of poly-Pickering HIPEs after treatment with THF. It’s clearly seen that the samples have many larger pore throats compared with those prior to THF treatment as shown in Figure 6. The amount of pore throats decreases with lowering content of PS microspheres in the composite particles. While etching the poly-Pickering HIPE stabilized by crosslinked PS/silica composite particles with THF, there was no obvious difference before (Figure 10b) and after (Figure 10c) THF treatment, 16


Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

indicating that the crosslinked PS microspheres are hardly washed up. The interconnectivity of polymer foams before and after THF etching is also characterized by the gas permeability test and the result is plotted in Figure 13. It is clear that the permeability of macroporous polymers is enhanced obviously after THF treatment, especially for those using the composite stabilizer with high PS content. This is in good agreement with SEM images. In a word, both the gas permeability and SEM results suggest that the interconnectivity of macroporous polymers can be facilely tailored by simply varying the weight ratio of uncrosslinked PS in the composite particles upon treatment with THF.

17


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Figure 12. SEM images of poly-Pickering HIPEs stabilized by composite particles with different weight ratios of silica particles to PS microspheres after etching with THF: (a) 1:3, (b) 3:7, (c) 7:13, (d) 2:3, (e) 9:11, and (f) 1:1 (samples 2-7, Table 1).

Figure 13. Gas permeability of poly-Pickering HIPEs stabilized by uncrosslinked PS/silica composite particles with different PS/silica weight ratios before and after THF treatment (samples 2-7, Table 1). 18


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

CONCLUSIONS Environmental friendly PS/silica composites are facilely prepared by mixing positively charged PS colloidal particles and negatively charged silica nanoparticles as Pickering stabilizer to produce stable w/o HIPEs without any other surfactants. The amphiphilicity of the composite stabilizer is tailored by varying the ratio of PS to silica, and thus the droplet size of Pickering HIPES as well as the pore size of poly-Pickering HIPEs is tuned conveniently. Dominantly over other Pickering stabilizer, PS/silica composite particles immobilized on the oil/water interface can extract monomer from the continuous phase, thus thinning the monomer layer between adjacent water droplets and finally producing open-cell structure due to the split of week interface during polymerization. The interconnectivity of polymer foams can be further improved by etching with THF, especially for the samples with high PS content in composite stabilizer. This kind of polymer/silica composite Pickering stabilizer probably opens a new gateway for tailoring the pore structure, especially interconnectivity of polymer foams, meeting the requirements of various applications.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] Author Contributions †

These authors contributed equally to this work.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51373038).

REFERENCES (1) Chen, Z.; Zhao, C.; Ju, E.; Ji, H.; Ren, J.; Binks, B. P.; Qu, X. Design of 19


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

Surface-Active Artificial Enzyme Particles to Stabilize Pickering Emulsions for High-Performance Biphasic Biocatalysis. Adv. Mater. 2016, 28, 1682-1688. (2) Zhang, Y.; Chen, Y.; Shen, Y.; Yan, Y.; Pan, J.; Shi, W.; Yu, L. Hierarchically Macro-/Mesoporous Polymer Foam as an Enhanced and Recyclable Catalyst System for the Sustainable Synthesis of 5-Hydroxymethylfurfural from Renewable Carbohydrates. ChemPlusChem 2016, 81, 108-118. (3) Jonathan, M. H.; Peter, M. B.; Karen, T.; John, L. Polymerized High Internal Phase Emulsion Monoliths for the Chromatographic Separation of Engineered Nanoparticles. J. Appl. Polym. Sci. 2014, 132, 41229-41236. (4) Li, Y.; Li, D.; Rao, Y.; Zhao, X.; Wu, M. Superior CO2, CH4 and H2 Uptakes over Ultrahigh-Surface-Area Carbon Spheres Prepared from Sustainable Biomass-Derived Char by CO2 Activation. Adv. Mater. 2016, 105, 454-462. (5) Li, B.; Huang X.; Liang, L.; Tan, B. Synthesis of Uniform Microporous Polymer Nanoparticles and Their Applications for Hydrogen Storage. J. Mater. Chem. 2010, 20, 7444-7450. (6) Silverstein, M. S.; PolyHIPEs: Recent Advances in Emulsion-Templated Porous Polymers. Prog. Polym. Sci. 2014, 39, 199-234. (7) Robinson, J. L.; Robert, S.; Moglia; Melissa, C. S.; McEnery, M. A. P.; Cosgriff-Hernandez, E. Achieving Interconnected Pore Architecture in Injectable PolyHIPEs for Bone Tissue Engineering. Tissue Eng. Part A 2013, 20, 1103-1112. (8) Tang, M.; Purcell, M.; Steele, J. A. M.; Lee, K.; McCullen, S.; Shakesheff, K. M.; Bismarck, A.; Stevens, M. M.; Howdle, S. M.; Williams, C. K. Porous Copolymers of ε-Caprolactone as Scaffolds for Tissue Engineering. Macromolecules 2013, 46, 8136-8143. (9) Hu, Y.; Gu, X.; Yang, Y.; Huang, J.; Hu, M.; Chen, W.; Tong, Z.; Wang, C. Facile Fabrication of Poly(L-lactic Acid)-Grafted Hydroxyapatite/Poly(lactic-co-glycolic Acid) Scaffolds by Pickering HighInternal Phase Emulsion Templates. ACS Appl. Mat.

Interfaces 2014, 6, 17166-17175. (10) Sueki, T.; Takaishi, T.; Ikeda, M.; Arai, N. Application of Porous Material to Reduce Aerodynamic Sound from Bluff Bodies. Fluid Dyn. Res. 2010, 42, 1-14. 20


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(11) Nakajima, H. Fabrication, Properties and Application of Porous Metals with Directional Pores. Prog. Mater Sci. 2007, 52, 1091-1173. (12) Spinnler, M.; Winter, E. R. F.; Viskanta, R. Studies on High-Temperature Multilayer Thermal Insulations. Int. J. Heat Mass Transfer 2004, 47, 1305-1312. (13) Aram, E.; Mehdipour-Ataei, S. A Review on the Micro- and Nanoporous Polymeric Foams: Preparation and Properties. Int. J. Polym. Mater. 2016, 65, 358-375. (14) Sveca, F. Porous Polymer Monoliths: Amazingly Wide Variety of Techniques Enabling Their Preparation. J. Chromatogr. A 2009, 1217, 902-924. (15) Dorin, R. M.; Sai, H.; Wiesner, U. Hierarchically Porous Materials from Block Copolymers. Chem. Mater. 2014, 26, 339-347. (16) Ziminsk, M.; Dunne, N.; Hamilton, A. R. Porous Materials with Tunable Structure and Mechanical Properties Via Templated Layer-by-Layer Assembly. ACS

Appl. Mat. Interfaces 2016, 8, 21968-21973. (17) Cameron, N. R. High Internal Phase Emulsion Templating as a Route to Well-Defined Porous Polymers. Polymer 2005, 46, 1439-1449. (18) Ikem, V. O.; Menner, A.; Bismarck, A. High Internal Phase Emulsions Stabilized Solely by Functionalized Silica Particles. Angew. Chem. Int. Ed. 2008, 47, 8277-8279. (19) Kovacic, S.; Matsko, N. B.; Jerabek, K.; Krajnc, P.; Slugovc, C. On the Mechanical Properties of HIPE Templated Macroporous Poly(Dicyclopentadiene) Prepared with Low Surfactant Amounts. J. Mater. Chem. A 2013, 1, 487-490. (20) Zhang, S.; Chen, J.; Perchyonok, V. T. Stability of High Internal Phase Emulsions with Sole Cationic Surfactant and Its Tailoring Morphology of Porous Polymers Based on the Emulsions. Polymer 2009, 50, 1723-1731. (21) Zhu, Y.; Zhang, R.; Zhang, S.; Chu, Y.; Chen, J. Macroporous Polymers with Aligned Microporous Walls from Pickering High Internal Phase Emulsions. Langmuir 2016, 32, 6083-6088. (22) Hua, Y.; Zhang, S.; Zhu, Y.; Chu, Y.; Chen, J. Hydrophilic Polymer Foams with Well-Defined Open-Cell Structure Prepared from Pickering High Internal Phase Emulsions. J. Polym. Sci.,Part A: Polym. Chem. 2013, 51, 2181-2187. 21


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

(23) Williams, J. M.; Wrobleski, D. A. Spatial Distribution of the Phases in Water-In-Oil Emulsions - Open and Closed Microcellular Foams From Cross-Linked Polystyrene. Langmuir 1988, 4, 656-662. (24) Ikem, V. O.; Menner, A.; Horozov, T. S.; Bismarck, A. Highly Permeable Macroporous Polymers Synthesized from Pickering Medium and High Internal Phase Emulsion Templates. Adv. Mater. 2010, 22, 3588-3592. (25) Xu, H.; Zheng, X.; Huang, Y.; Wang, H.; Du, Q. Interconnected Porous Polymers with Tunable Pore Throat Size Prepared Via Pickering High Internal Phase Emulsions.

Langmuir 2015, 32, 38-45. (26) Zheng, X.; Zhang, Y.; Wang, H.; Du, Q. Interconnected Macroporous Polymers Synthesized from Silica Particle Stabilized High Internal Phase Emulsions.

Macromolecules 2014, 47, 6847-6855. (27) Yang, Y.; Ning, Y.; Wang, C.; Tong, Z. Capsule Clusters Fabricated by Polymerization Based on Capsule-in-Water-in-Oil Pickering Emulsions. Polym. Chem. 2013, 4, 5407-5415. (28) Ning, Y.; Wang, C.; Ngai, T.; Tong, Z. Fabrication of Tunable Janus Microspheres with Dual Anisotropy of Porosity and Magnetism. Langmuir 2013, 29, 5138-5144. (29) Song, X.; Yin, G.; Zhao, Y.; Wang, H.; Du, Q. Effect of an Anionic Monomer on the Pickering Emulsion Polymerization Stabilized by Titania Hydrosol. J. Polym. Sci.,

Part A: Polym. Chem. 2009, 47, 5728-5736. (30) Zhao, Y.; Wang, H.; Song, X.; Du, Q. Fabrication of Two Kinds of Polymer Microspheres

Stabilized

by

Modified

Titania

During

Pickering

Emulsion

Polymerization. Macromol. Chem. Phys. 2010, 211, 2517-2529. (31) Tang, M.; Wang, X.; Wu, F.; Liu, Y.; Zhang, S.; Pang, X.; Li, X.; Qiu, H.

Au

Nanoparticle/Graphene Oxide Hybrids as Stabilizers for Pickering Emulsions and Au Nanoparticle/Graphene Oxide@Polystyrene Microspheres. Carbon 2014, 71, 238-24 8. (32) Kovacic, S.; Matsko, N. B.; Ferkc, G.; Slugovc, C. Macroporous Poly(Dicyclopentadiene) Fe2O3/Fe3O4 Nanocomposite Foams by High Internal Phase 22


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Emulsion Templating. J. Mater. Chem. A 2013, 1, 7971-7978. (33) Lin, K. A.; Yang, H.; Petit, C.; Lee, W. Magnetically Controllable Pickering Emulsion Prepared by a Reduced Graphene Oxide-Iron Oxide Composite. J. Colloid

Interface Sci. 2015, 438, 296-305. (34) Qiao, X.; Zhou, J.; Binks, B. P.; Gong, X.; Sun, K. Magnetorheological Behavior of Pickering Emulsions Stabilized by Surface-Modified Fe3O4 Nanoparticles. Colloids

Surf., A 2012, 412, 20-28. (35) Akartuna, I.; Studart, A. R.; Tervoort, E.; Gonzenbach, U. T.; Gauckler, L. J.; Stabilization of Oil-in-Water Emulsions by Colloidal Particles Modified with Short Amphiphiles. Langmuir 2008, 24, 7161-7168. (36) Hu, Z.; Ballinger, S.; Pelton, R.; Cranston, E. D. Surfactant-Enhanced Cellulose Nanocrystal Pickering Emulsions. J. Colloid Interface Sci. 2015, 439, 139-148. (37) Yi, W.; Wu, H.; Wang, H.; Du, Q. Interconnectivity of Macroporous Hydrogels Prepared Via Graphene Oxide-Stabilized Pickering High Internal Phase Emulsions.

Langmuir 2016, 32, 982-990. (38) Paunov, V. N.; Cayre, O. J.; Noble, P. F.; Stoyanov, S. D.; Velikov, K. P.; Golding, M. Emulsions Stabilised by Food Colloid Particles: Role of Particle Adsorption and Wettability at the Liquid Interface. J. Colloid Interface Sci. 2007, 312, 381-389. (39) Shi, W.; Tang, C.; Yin, S.; Yin, Y.; Yang, X.; Wu, L.; Zhao, Z. Development and Characterization of Novel Chitosan Emulsion Films Via Pickering Emulsions Incorporation Approach. Food Hydrocolloids 2006, 52, 253-264. (40) Mwangi, W. W.; Ho, K. W.; Tey, B. T.; Chan, E. S.; Effects of Environmental Factors on the Physical Stability of Pickering Emulsions Stabilized by Chitosan Particles. Food Hydrocolloids 2016, 60, 543-550. (41) Folter, J. W. J.; Ruijven, M, W. M.; Velikov, K. P. Oil-in-Water Pickering Emulsions Stabilized by Colloidal Particles from the Water-Insoluble Protein Zein.

Soft Matter 2012, 8, 6807-6815. (42) Romoscanu, A. I.; Fenollosa, A.; Acquistapace, S.; Gunes, D.; Deuchande, T. M.; Clausen, P.; Mezzenga, R.; Nyden, M.; Zick, k.; Hughes, E. Structure, Diffusion, and Permeability of Protein-Stabilized Monodispersed Oil in Water Emulsions and Their 23


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

Gels: A Self-Diffusion NMR Study. Langmuir 2010, 26, 6184-6192. (43) Binks, B. P.; Rocher, A. Effects of Temperature on Water-in-Oil Emulsions Stabilised Solely by Wax Microparticles. J. Colloid Interface Sci. 2009, 335, 94-104. (44) Li, C.; Liu, Q.; Mei, Z.; Wang, J.; Xu, J.; Sun, D. Pickering Emulsions Stabilized by Paraffin Wax and Laponite Clay Particles. J. Colloid Interface Sci. 2009, 336, 314-321. (45) Ling, L.; Wong, C.; Ikem, V. O.; Menner, A.; Bismarck, A. Macroporous Polymers with Hierarchical Pore Structure from Emulsion Templates Stabilised by Both Particles and Surfactants. Macromol. Rapid Commun. 2011, 32, 1563-1568. (46) Ikem, V. O.; Menner, A.; Bismarck, A. Tailoring the Mechanical Performance of Highly Permeable Macroporous Polymers Synthesized Via Pickering Emulsion Templating. Soft Matter 2011, 7, 6571-6577. (47) Menner, A.; Salgueiro, M.; Shaffer, M. S. P.; Bismarck, A. Nanocomposite Foams Obtained by Polymerization of High Internal Phase Emulsions. J. Polym. Sci.,

Part A: polym. Chem. 2008, 46, 5708-5714. (48) Yang, Y.; Wei, Z.; Wang, C.; Tong, Z. Lignin-Based Pickering HIPEs for Macroporous Foams and Their Enhanced Adsorption of Copper(Ii) Ions. Chem.

Commun. 2013, 49, 7144-7146. (49) Binks, B. P.; Lumsdon, S. O. Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica. Langmuir 2000, 16, 2539-2547. (50) Zhu, Y.; Jiang, J.; Liu, K.; Cui, Z.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant. Langmuir 2015, 31, 3301-3307. (51) Zou, S.; Yang, Y.; Liu, H.; Wang, C. Synergistic Stabilization and Tunable Structures of Pickering High Internal Phase Emulsions by Nanoparticles and Surfactants. Colloids Surf., A 2013, 436, 1-9. (52) Binks, B. P. Particles as surfactants--similarities and differences. Curr. Opin.

Colloid Interface Sci. 2002, 7, 21-41. (53) Zhou, J.; Wang, L.; Qiao, X.; Binks, B. P.; Sun, K. Pickering Emulsions Stabilized by Surface-Modified Fe3O4 Nanoparticles. J. Colloid Interface Sci. 2012, 24


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

367, 213-224. (54) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions Stabilised Solely by Colloidal Particles. Adv. Colloid Interface Sci. 2003, 100, 503-546. (55) Ikem, V. O.; Menner, A.; Bismarck, A. High-Porosity Macroporous Polymers Sythesized from Titania-Particle-Stabilized Medium and High Internal Phase Emulsions. Langmuir 2010, 26, 8836-8841. (56) Ikem, V. O.; Menner, A.; Bismarck, A. High Internal Phase Emulsions Stabilized Solely by Functionalized Silica Particles. Angew. Chem. Int. Ed. 2008, 47, 8277-8279. (57) Gurevitch, I.; Silverstein, M. S. Nanoparticle-Based and Organic-Phase-Based Aget Atrp PolyHIPEs Synthesis within Pickering Hipes and Surfactant-Stabilized HIPEs. Macromolecules 2011, 44, 3398-3409. (58) Binks, B. P.; Lumsdon, S. O. Pickering Emulsions Stabilized by Monodisperse Latex Particles: Effects of Particle Size. Langmuir 2001, 17, 4540-4547.

25


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

Table of Contents Graphic and Synopsis

Pore structure of macroporous polymers using PS/silica composite particles as Pickering stabilizers

Shuhua Tu,† Chenxu Zhu,† Lingyun Zhang, Haitao Wang,* and Qiangguo Du

26


Silica

Recommend Documents