Interconnected Porous Polymers with Tunable Pore Throat Size

Dec 16, 2015 - Abstract Image. Interconnected macroporous polymers were prepared by copolymerizing methyl acrylate (MA) via Pickering high internal ph...
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Interconnected Porous Polymers with Tunable Pore Throat Size Prepared via Pickering High Internal Phase Emulsions Hongyun Xu, Xianhua Zheng, Yifei Huang, 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 S Supporting Information *

ABSTRACT: Interconnected macroporous polymers were prepared by copolymerizing methyl acrylate (MA) via Pickering high internal phase emulsion (HIPE) templates with modified silica particles. The pore structure of the obtained polymer foams was observed by field-emission scanning electron microscopy (FE-SEM). Gas permeability was characterized to evaluate the interconnectivity of macroporous polymers. The polymerization shrinkage of continuous phase tends to form open pores while the solid particles surrounding the droplets act as barriers to produce closed pores. These two conflicting factors are crucial in determining the interconnectivity of macroporous polymers. Thus, poly-Pickering HIPEs with high permeability and well-defined pore structure can be achieved by tuning the MA content, the internal phase fraction, and the content of modified silica particles.

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INTRODUCTION Porous polymers are normally low in density and high in porosity.1 High permeability and a large effective surface area can be achieved when the polymer framework is interconnected. This kind of porous polymers with appropriate pore structure may have potential applications in catalysts,2,3 tissue engineering scaffolds,4,5 gas storage,6,7 and separation.8,9 To meet the various application requirements, they commonly should have an adjustable pore structure and a high porosity. In that case, preparing porous materials by high internal phase emulsion (HIPE) templates has its advantages in controllable pore size as well as inherent high porosity due to the high dispersed phase fraction.10,11 HIPEs were defined as the concentrated emulsions with an internal volume ratio of 74.05%12 or higher. The conventional method to prepare HIPEs is based on surfactants, such as sorbitan monooleate (Span 80),13−15 cetyltrimethylammonium bromide (CTAB),16 and amphiphilic polymers.17−19 Upon adding surfactants with the amount up to 5−50%13 into the continuous phase, a stable HIPE can be achieved. After the polymerization of HIPEs, macroporous polymers with interconnected voids are obtained. However, this method requires a large consumption of expensive surfactants and a series of posttreatments to remove these surfactants.20 Furthermore, polyHIPEs prepared via conventional HIPEs commonly have undesirable mechanical property and low permeability due to their small size of pore and pore throat.21 It has been proven that many solid particles, such as silica particles,22,23 titania particles,24,25 iron oxide nanoparticles,26,27 polymer particles,28−31 and graphene oxide flakes,32 can also be used to stabilize HIPEs, which is named as Pickering HIPEs. © XXXX American Chemical Society

Because of the almost irreversible adsorption of solid particles onto the oil−water interface, only a small amount of stabilizer22,32 is needed to produce stable Pickering HIPEs. The incorporation of solid particles into the organic framework endows the porous materials with improved mechanical properties.33,34 The use of magnetic,35,36 electrical, or thermal conductive particles37,38 can also introduce corresponding properties to materials. Unfortunately, the polymerization of Pickering HIPEs commonly produces closed-cell22,24,32,39 or slightly open pore structure.37,40 Adding a large amount of surfactants into the Pickering HIPEs21,36,41−43 is a common strategy effective to form pore throats in polymer foams. However, it still brings the inherent problems of porous polymers synthesized from conventional surfactant stabilized HIPEs. The mechanism of the formation of pore throats is crucial for the comprehension and the application of poly-HIPEs, but it is still under debate now. For surfactant-contained HIPEs, researches are focused on the property of surfactants,21 the polymerization procedure44 and the postsynthesis process.45 With addition of surfactants, the interfacial tension and the viscosity reduce. As a result, smaller droplets are formed with increased overall surface area of droplets, leading to thinner films more vulnerable to break.21 During polymerization, the formation of open pores can be contributed to several factors, such as partial coalescence between neighboring droplets, phase separation at surfactant rich phase and polymer rich phase,45 Received: August 14, 2015 Revised: November 17, 2015

A

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Langmuir and volume contraction in the process of conversion from monomer to polymer.44 Besides, some thin polymeric films may rupture to form throats at the postsynthesis process such as sonication and Soxhlet extraction.45 However, unlike surfactant-contained HIPEs, the larger droplets of Pickering emulsions leads to thicker monomer layer between droplets. Meanwhile, it has been proven that droplets of Pickering emulsions are surrounded by closely packed solid particles. The particles at the oil−water interface form a solid shell to protect the emulsion droplets against any structural imperfection during polymerization, including coalescence and film rupture.46 Therefore, poly-Pickering HIPEs are normally with closed pores. To open the pores in poly-Pickering HIPEs, research so far is focused on introducing novel stabilizers. The use of lignin particles47 and specific poly(urethane urea) stabilizer30 can produce interconnected porous polymers but is not versatile in most HIPEs. The use of polymer Janus nanoparticles48 can form open-cell structures, but it relies on strong post treatments that dramatically extended the preparation period. Recently, we reported a strategy to extract monomer at the oil−water interface during polymerization by functional silica aggregates with hydrophobic cores, and thus interconnected polyPickering HIPEs were obtained.49 One can expect that the pore throats may also be achieved if sufficient volume shrinkage occurs or greater force is employed to tear the films between the pores. Styrene, with low polymerization shrinkage of 14.4%, has been reported to prepare poly-Pickering HIPEs with closed pore structure. Herein, methyl acrylate with higher volume shrinkage of 22.2% during the polymerization is utilized as the comonomer to open the pores and tailor the pore structure. As a result, the interconnectivity of poly-Pickering HIPEs is greatly improved. Emphasized on utilizing monomer with high volume contraction during the conversion of monomers to polymers, this paper proposes a facile and universal strategy to synthesize interconnected macroporous polymers with well-defined pore structures.

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Table 1. Composition of Pickering Emulsions organic phasec

aqueous phase sample

internal phase volume (%)

1 2 3 4 5 6 7 8 9 10 11 12 13

80 80 80 80 80 55 60 70 75 80 80 80 80

MA St DVB modified silica particles (wt %)a (vol %)b (vol %)b (vol %)b 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.7 1.5 3.0

0 20 40 60 80 60 60 60 60 60 60 60 60

80 60 40 20 0 20 20 20 20 20 20 20 20

20 20 20 20 20 20 20 20 20 20 20 20 20

a With respect to the aqueous phase weight. bWith respect to the organic phase volume. cAll organic phases contain 1.0 wt % initiator BPO.

Characterization. Pictures of Pickering HIPEs were taken with a Casio EX-Z800 Digital camera. The size and the shape of the emulsion droplets were observed with an EV5680 optical microscope. Fieldemission scanning electron microscopy (FE-SEM, Zeiss Ultra 55) was applied to observe the pore structure of the poly-Pickering HIPEs. Prior to the test, the poly-HIPEs were fixed to substrate via a carbon sticker and sputtered with gold. The corresponding SEM images were used to measure the void size and the throat size by counting at least 100 pores per sample with Nano Measurer 1.2.0. N2 adsorption/ desorption measurement was carried out on a Micromeritics Tristar3000 (Quantachrome) apparatus at 77 K and the surface area of the macroporous polymer was calculated by the Brunauer−Emmet−Teller (BET) method. The permeability of the poly-Pickering HIPE was characterized by measuring the flow rates of nitrogen passing through the sample at 5,000 Pa. The macroporous polymers were cut into cylinders with a length of 10 mm and a diameter of 14 mm. Each sample was characterized three times. The permeability was calculated by Darcy’s law50 and recorded in unit D (10−12 m2). The volume shrinkage (K) of monomers during polymerization was calculated as K = 1 − ρmonomer/ρpolymer. In the equation, ρmonomer is the density of the organic phase, measured directly with a liquid densitometer. And ρpolymer is the density of the polymerized organic phase (without stabilizers and water). It was obtained by measuring the density of three polymer samples prepared by bulk polymerization with the same composition as the organic phase and under the same condition as the corresponding emulsion samples. The density of each sample was measured by suspension method at 25 °C. Different concentrations of NaNO3 solution (density ranging from 1.000 to 1.350 g/cm3) was employed to measure the polymer densities in the range from 1.062 to 1.291 g/cm3. All polymer samples were immersed in the dilute solution overnight. The solution was mixed with appropriate liquids until the polymer floated in the center of the solution steadily for at least 10 min. The density of the resulting solution was measured as the density of the polymer sample with a liquid densitometer. The foam densities (ρfoam) of the poly-HIPE samples were calculated by mass and volume upon five samples for each poly-HIPE according to ISO-845. The porosity (p) was calculated as p = 1 − ρfoam/ρpolymer. The compression test was conducted at 25 °C using a SANS CMT4102 universal testing machine (Shenzhen, China) equipped with a 10 kN load cell. Five samples of each poly-HIPE were tested as cylinders with a length of 10 mm and a diameter of 14 mm. The samples were loaded at a rate of 1 mm/min according to ISO-844 until 50% relative deformation was achieved. After compression, the samples were left in room temperature for 24 h. Their recovery δ was calculated as δ =

EXPERIMENTAL SECTION

Materials. Silica hydrosol (28 nm, 40.0 wt %) was generously provided by Fujian Sanbang Chemical Co. (China). Styrene (St, 99%), methyl acrylate (MA, 98%), cetyltrimethylammonium bromide (CTAB, 99%), and hydrofluoric acid (HF, 40%) were purchased from Sinopharm Chemical Reagent Co. (China). Divinylbenzene (DVB, 80%) and benzoyl peroxide (BPO, 99%) were supplied by Aladdin Chemistry Co., Ltd. All chemicals were used as received. Deionized water was used throughout the experiments. Preparation of Modified Silica Particles. In a typical procedure, silica hydrosol (0.125 g, 40 wt %) was dispersed into distilled water, and then the CTAB aqueous solution (0.4 g, 1.0 wt %) was added drop by drop with sonication. The modified silica particles (0.05 g) were obtained by centrifugation. Preparation of Pickering HIPEs. A typical emulsion with an 80 vol % aqueous internal phase was prepared as follows. The modified silica particles were dispersed in distilled water, which made up the 5 mL aqueous phase. The oil phase (1.25 mL) consisted of BPO (1.0 wt % to the total weight of oil), MA, St, and DVB. The organic phase was added to the water phase and then vigorously emulsified with VortexBE1 at 1000 rpm for 20 min to form a highly viscous Pickering HIPE. Specific compositions of all samples are summarized in Table 1. Preparation of Poly-Pickering HIPEs. The obtained emulsions were transferred into centrifuge tubes, sealed and placed in a 73 °C oil bath for 24 h. The poly-HIPEs were obtained after drying in a convection oven at 65 °C to a constant weight. B

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Langmuir (2h1/h0 − 1) × 100% (h0 is the height of samples before compression, and h1 is the height of the compressed samples after recovery). The filtration test was performed by separating polystyrene microspheres through poly-HIPE samples with a diameter of 25 mm and thickness of 3 mm. The size distribution of polymer particles dispersed in water was obtained by counting ∼400 microspheres in optical microscope images.

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RESULTS AND DISCUSSION MA Content. It is well-known that Pickering emulsions should be stabilized by solid particles with the appropriate amphiphilicity. The as-received silica particles are too hydrophilicity to produce Pickering emulsions as shown in Figures S1a and S2a. The surface of silica particles is functionalized by a slight amount of CTAB to tailor the lipophilicity of solid particles (Figure S1), which has also been reported by a number of published works51−54 for the preparation of Pickering emulsions. Figure S2 indicates that only the modified silica particles with appropriate amphiphilicity can stabilize a Pickering HIPE. It should be noted that CTAB molecules are firmly adsorbed on the surface of silica particles via electrical attraction as proved in Figure S3 and Table S1. Silica particles are clearly seen on the pore wall of the polymer foam (Figure S4), indicating that they stabilize the oil−water interface of the emulsion throughout polymerization. However, only CTAB cannot be used to produce a stable HIPE in our experiments (Figure S2e). Sample 1−5 have various MA contents of 0, 20, 40, 60, and 80 vol % based on organic phase, with the same silica content and internal phase fraction. The five Pickering HIPEs are all highly viscous as shown in Figure S5. The optical microscopy images (Figure S6) show no obvious difference in droplet size, deformation of droplets, and thickness of the oil layer between adjacent droplets among these samples. After polymerization, the porosity of the polymer foams calculated according to their densities are identical within errors to the initial internal volume fraction (listed in Table 2),

Figure 1. Void diameter distributions of samples 1−5 (poly-HIPEs obtained with varied MA contents).

Table 2. Foam Density, Porosity, and Average Void Diameter (Dv) and Throat Diameter (Dt) of Samples 1−5 sample

MA (vol %)

1 2 3 4 5

0 20 40 60 80

density (g·cm−3)

porosity (%)

± ± ± ± ±

82 82 83 83 83

0.191 0.192 0.191 0.193 0.195

0.018 0.014 0.028 0.004 0.019

Dv (μm) 31 35 32 33 34

± ± ± ± ±

6 8 5 5 6

Dt (μm) 6.8 6.5 8.0 9.3

± ± ± ±

3.0 2.1 2.7 2.9

Figure 2. FE-SEM images of samples 1−5 (poly-HIPEs obtained with varied MA contents).

indicating high stability of Pickering HIPEs during polymerization. However, the five poly-HIPEs which have the same porosity and similar sizes and size distributions of pores (summarized in Figure 1) are with obviously different pore interconnectivity. Sample 1 in Figure 2a shows a typical pore morphology of a closed-cell polystyrene foams, which is coincided with other reports.22,32 No real pore throats are formed in sample 1 but some crumpled thin films can be observed between neighboring pores. These vulnerable points fail to break during the polymerization, while they become breakable after the introduction of comonomer MA, as shown in Figure 2b−e. The higher the content of MA is, the more throats form between the voids. For sample 2, only some of the thin films rupture into irregularly shaped throats while most remain closed. More and rounder open throats are formed in

sample 3 and 4 while some of the unbroken and half-broken films can still be seen. With a further increased amount of MA in sample 5, the polymer foam with a thorough open-cell structure is produced and it shows a low BET specific surface area of 6 m2/g due to its macroporous structure. It is shown in Figure 3 that the polymerization shrinkage K correlates positively with the amount of MA. When the content of MA is comparatively low (sample 2), only some films between the droplets can be torn apart and form irregularly shaped throats. The limited force from volume shrinkage during the polymerization can only cause the films become thinner and crumpled. When the MA content is further C

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Figure 3. Polymerization shrinkage and gas permeability of polyPickering HIPEs of samples 1−5 with MA content from 0 to 80%.

increased, the polymerization shrinkage of the monomer layers becomes notable. Thus, the pore throats are more likely to form with increased sizes, as listed in Table 2, during the polymerization. The interconnectivity of polymer foams is characterized by their gas permeability which is plotted in Figure 3. It is clear that the permeability of macroporous polymers is enhanced with the increasing polymerization shrinkage. This is in good agreement with FE-SEM results. The gas permeability increases significantly when the MA content is above 20 vol %, which can be regarded as the threshold for the preparation of open-cell polymer foams in our experiments. The enhanced permeability of polymer foams is ascribed to the formation of more pore throats with larger size. Moreover, it is known that a common method to improve the mechanical property of polystyrene foams is to introduce acrylate derivates, like 2-ethylhexyl acrylate,14 methyl methacrylate,55 and poly(ethylene glycol) dimethacrylate.43,56 In this work, the copolymerization of the soft monomer (MA) endows the poly-HIPE with an remarkably improved elasticity and even an intact pore morphology after 50% deformation from compression as shown in Table S2 and Figure S7. Internal Phase Fraction. It has been discussed above that the polymerization shrinkage of the monomer layer between two neighboring droplets should be responsible for the formation of interconnected pores. As the internal phase fraction significantly influences the thickness of oil layers in w/o emulsions, the interconnectivity and throat size of the porous materials should be tuned accordingly. The following five Pickering HIPE samples (sample 6−9 and sample 5) are of various internal phase fractions of 55, 60, 70, 75, and 80 vol %, respectively. All of the samples have the same silica content and organic composition. With the increasing internal phase fraction, the viscosity increases (Figure S8); the water droplets gradually get closer to each other, and the distinct deformation of the droplets is clearly found in samples 9 and 5 (Figure 4). The oil phase of these five emulsions all consists of 60 vol % MA, which provides a polymerization shrinkage force strong enough to create open pores even in the middle internal phase emulsions (MIPEs) (sample 6 and 7) as displayed in Figure 5. For sample 6, most water droplets are surrounded by a thick oil layer, and thus the polymerization shrinkage of the continuous phase can neither break nor crumple the polymer films. Nonetheless, some open throats still form between closely adjacent droplets. When the internal phase fraction is further increased, more interconnected pores can be found in corresponding porous polymers. Especially for samples 9 and

Figure 4. Optical microscopy images of Pickering emulsions of samples (a) 6, (b) 7, (c) 8, (d) 9, and (e) 5 (MIPEs and HIPEs with varied internal phase fractions).

Figure 5. FE-SEM images of samples (a) 6, (b) 7, (c) 8, (d) 9, and (e) 5 (poly-MIPEs and poly-HIPEs with varied internal phase fractions).

5, the oil layers are so thin that the deformation of emulsion droplets becomes obvious, resulting in the formation of bigger pore throats. Based on above, an increase of the internal phase fraction leads to the formation of thinner monomer layers in w/ o emulsions, and thus more and larger throats are produced by the polymerization of Pickering emulsions. D

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volume contraction of the oil phase caused by the polymerization. The following five Pickering HIPE samples (samples 10, 11, 5, 12, and 13) (Figure S9) are of various silica contents of 0.5, 0.7, 1.0, 1.5, and 3.0 wt %, respectively, based on aqueous phase. All of the samples have the same internal phase fraction and organic composition. From the optical microscopy images of HIPEs (Figure 7), a distinct decrease of droplet size can be

As plotted in Figure 6, the gas permeability of polymer foams can be improved by increasing the internal phase fraction. For

Figure 6. Gas permeability and pore throat size of samples (a) 6, (b) 7, (c) 8, (d) 9, and (e) 5 (poly-MIPEs and poly-HIPEs with varied internal phase fractions).

the internal phase fraction below 70%, the increase of gas permeability is smooth. However, the increase becomes significant if further elevating the internal phase level. This is consistent with the transition from MIPEs to HIPEs. By tuning the internal phase fraction, the pore throat size can be tailored in a wide range from 2 to 8 μm as shown in Figure 6. Besides improving the mechanical properties of porous polymers by introducing soft monomer, another effective solution which has been thoroughly discussed by many articles is to increase the continuous phase level to obtain materials with increased foam density.21,56 In emulsion templates, the decreased internal phase fraction of emulsions are matched with porosities of polymer foams and lead to an effective increase in foam densities as listed in Table 3. Now the

Figure 7. Optical microscopy emulsion images of samples (a) 10, (b) 11, (c) 5, (d) 12, and (e) 13 (Pickering HIPEs with varied contents of silica particles).

observed when the silica content increases from 0.5 to 1.0 wt %. This is attributed to the formation of larger stable interface when adding more silica particles. For samples 12 and 13, the surplus silica particles sharply increase the viscosity of the emulsions. Energy provided by emulsification is insufficient to further break the droplets into smaller ones. Thus, a firm barrier created by silica particles can be expected between the emulsion droplets. At lower silica contents, the polymerization shrinkage effectively creates bigger and more spherical pore throats in sample 10 and 11 as shown in Figure 8a, b. However, the halfopen throats in Figure 8d and the completely closed pores in Figure 8e indicate that the enhanced hindrance from the increased stabilizer at the oil−water interface can outweigh the strong polymerization shrinkage of MA. It has been proven that the shrinkage of the oil phase with 60 vol % MA during polymerization is able to produce interconnected pores even in poly-MIPEs. However, the barrier effect of the solid particles with high content is efficient to constrain the formation of pore throats. The results listed in Table 4 display that the pore structure of polymer foams can be effectively tuned by the content of silica particles in the premise of identical foam porosities and densities. The interconnectivity of macroporous polymers is also characterized by gas permeability which is positively correlated with larger pore and pore throat size.21 As mentioned above, volume shrinkage of the monomer during the polymerization form interconnected pore structure in poly-Pickering HIPEs, though the silica particles surrounding the droplets can serve as barriers to prevent the formation of pore throats. The combined effect of these two conflicting factors acts on the pore structure of polymer foams, with the pore and pore throat size tuned from 22 to 71 μm and 4 to 18 μm, respectively. Along with other conditions such as the oil phase composition and internal phase fraction, the pore

Table 3. Foam Density and Porosity of Samples 5−9 sample

internal phase fraction (%)

6 7 8 9 5

55 60 70 75 80

density (g·cm−3)

porosity (%)

± ± ± ± ±

55 64 73 79 83

0.508 0.411 0.313 0.243 0.193

0.017 0.020 0.021 0.014 0.004

interconnected structure has proved to be feasible even in polyPickering MIPEs due to the high polymerization shrinkage when MA content is adequate. This can provide more possibility to seek for the balance between permeability and mechanical strength. Content of Silica Particles. In contrast to porous polymers prepared by surfactant-stabilized HIPEs, poly-Pickering HIPEs normally have closed-cell pore structure due to the extreme stability of Pickering emulsions. However, it has been proven that sufficient volume shrinkage of polymer framework during the polymerization results in the formation of open throats even when the internal phase fraction is as low as 55%. Herein, the interconnectivity of poly-Pickering HIPEs is tuned by the two conflicting factors, the hindrance of silica particles and the E

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stronger polymerization shrinkage of polymer frameworks tends to form interconnected pore structures. We hope the Pickering HIPE-templated macroporous polymers can satisfy the requirements of various applications with an enhanced permeability, a well-defined pore structure, and an improved 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.5b03037. Digital pictures of modified silica nanoparticles; irreversible adsorption of CTAB; pore surface morphology of the polymer foam; digital pictures and optical microscopy images of emulsions of various MA content; improved mechanical properties of polymer foams with increased MA content; digital pictures of emulsions with various internal phase fraction and various silica content; filtration of polymer particles by poly-Pickering HIPEs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51373038).

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Figure 8. FE-SEM images of samples (a) 10, (b) 11, (c) 5, (d) 12, and (e) 13 (poly-Pickering HIPEs with varied contents of silica particles).

REFERENCES

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structure of poly-HIPEs is well-defined from pore to throat. This is crucial for applications such as living cell growth,57 filtration, and separation8 as demonstrated in Figure S10.

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CONCLUSION In this work, we introduce a facile and versatile method to prepare interconnected poly-Pickering HIPEs where no surfactant is needed. The open-cell structure of polymer foams is induced by the distinct volume shrinkage of polymer frameworks during the polymerization by using comonomer MA. The broken pore throats can be produced even when the internal phase fraction is as low as 55%. The interconnectivity of macroporous polymers depicted by gas permeability shows a positive correlation with the polymerization shrinkage of the monomer phase. Silica particles surrounding the droplets can serve as barriers to maintain the structural integrity, while the

Table 4. Average Void Diameter (Dv), Throat Diameter (Dt), Permeability (P), Foam Density, and Porosity of Sample 5 and Samples 10−13 sample

silica content (wt %)

10 11 5 12 13

0.5 0.7 1.0 1.5 3.0

Dv (μm) 71 38 33 22 23

± ± ± ± ±

8 7 5 3 4

Dt (μm) 18.3 10.1 8.0 4.5

± ± ± ±

7.4 3.9 2.7 2.2

F

P (D) 2.48 1.22 0.96 0.09 0

± ± ± ±

0.21 0.25 0.15 0.00

density (g·cm−3)

porosity (%)

± ± ± ± ±

83 83 83 83 84

0.196 0.189 0.193 0.192 0.188

0.016 0.018 0.004 0.006 0.004

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DOI: 10.1021/acs.langmuir.5b03037 Langmuir XXXX, XXX, XXX−XXX