Biobased, Porous Poly(high internal phase emulsions): Prepared from

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Materials and Interfaces

Bio-based Porous PolyHIPEs: Prepared from Biomasses Vanillin and Laurinol and Applied as Oil-adsorbent Huanyu Zhang, Ran Zhao, Ming Pan, Jianping Deng, and Youping Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00515 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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Industrial & Engineering Chemistry Research

Bio-based Porous PolyHIPEs: Prepared from Biomasses Vanillin and Laurinol and Applied as Oil-adsorbent Huanyu Zhang,1, 2, 3, ‡ Ran Zhao,2, 3, ‡Ming Pan,2, 3 Jianping Deng*, 2, 3 and Youping Wu*, 1, 3 1State

Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China.

2State

Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China.

3College

of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China.

Keywords: Biomass; high internal phase emulsions template; oil-adsorption; porous materials; vanillin

ABSTRACT: This contribution reports a novel type of bio-based porous functional materials (polyHIPEs) prepared from vanillin- and laurinol-derivatives. The polyHIPEs demonstrate high porosity (90%), low density (0.0935 g/cm3), and high specific surface area (38.6 m2/g). Owing to the multilevel pore structures (cell, window and skeleton pores) and the chemical compositions, the polyHIPEs demonstrate suprahydrophobicity with water contact angle exceeding 160o and

ACS Paragon Plus Environment

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excellent lipophilicity. The polyHIPEs are further explored as oil adsorbent. The oil adsorption capacity increases with increasing the water-oil ratio in the recipe for forming HIPEs, and increases up to 40.9 g/g towards chloroform. The recycling use experiments prove that the materials’ oil adsorption ability can be maintained well for at least 10 times. The materials also demonstrate rapid oil adsorption capability. The polyHIPEs’ advantages, i.e. being derived from biomass and showing high oil-adsorption capacity and satisfactory recycling usability, endow them with promising potentials as sustainable oil adsorbents.

1. Introduction Various types of functional polymers have been designed to satisfy the diverse needs of today’s economic and technological development. The polymers of the kind are generally smart and multifunctional, and have high added-values. Indeed, microstructure design is also one of the effective ways for developing functional materials. As far as adsorbents are concerned, porous structures are essentially significant and often required to achieve high adsorption capacity. Oiladsorbing resins are typical functional polymer materials with a large demand at present, due to the increasingly serious oil pollution.

1-3

High internal phase emulsion (HIPE) template

technology is a powerful method for preparing functional porous materials. polyHIPEs, have proved to be excellent oil adsorbents.

5–7

4

The products,

Just like other oil adsorption resins

such as powders, aerogels, sponges, and foams, polyHIPEs contain similar advantages including high porosity, low cost, and easy preparation. Furthermore, the advantages of polyHIPEs, in particular the connected and conveniently adjustable pore structures, make them highly attractive. The applications of polyHIPEs are not limited to oil adsorption. Due to the high specific surface area and through-hole structures, polyHIPEs can be further effectively functionalized through grafting modification to significantly extend the range of applications. 8–11


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By utilizing new emulsifiers (e.g., fluorochemical derivatives), more monomers can be used for constructing polyHIPEs.

12, 13

Up to date, a large number of polyHIPEs and even functional

polyHIPEs have been reported, but they are still mainly based on petroleum-derivatives, e.g. polystyrene (PS) and polymethyl methacrylate (PMMA). The shortcomings of such polyHIPEs are obvious due to the limited petroleum sources. Therefore, opening up new bio-based polyHIPEs has become an urgent and important research topic. Preparing high value-added functional materials from biomasses is expected to achieve a win-win situation in both sustainable materials and economy. To meet the demands of sustainable development, various kinds of bio-based polymers were prepared and even commercialized,

14-17

well exemplified by polyesters, e.g. polylactic acid

(PLA) 18, 19 and poly(hydroxybutyrate-co-hydroxyvalerate). 20, 21 As far as aromatic polymers are concerned, e.g. polystyrenes (PS), they have found extensive uses as general-purpose plastics, 22 thermoplastic elastomers

23, 24

and functional materials.

25

However, it still remains as a big

academic challenge to achieving their bio-based substituents, since most of the bio-based resources so far explored well are carbohydrates such as vegetable oils and polysaccharides. 26-28 Fortunately, lignin is an exception and provides potential raw materials for developing sustainable aromatic polymer materials. 29-31 As a typical lignin derivative, vanillin has attracted much interest in diverse research areas.

32-34

It inherits the multiple advantages of lignin,

including abundant aromatic structures. Especially, it does not compete with human feeding. Meanwhile, using vanillin as raw material can overcome the difficulties in directly using lignin that contains crosslinked and complex structures.

35

Recently, rapidly developing refining

technologies (e.g., oxidative depolymerization) further promotes the advancements in using vanillin and other derivatives from lignin to open up new materials.

32

Employing vanillin as a


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raw material, a number of bio-based polymers have been developed with examples of polybenzoxazines,

36

polyesters,

37

and polymethacrylates.

38–40

Nonetheless, creating new bio-

based functional polymers is still significant in both scientific and practical application viewpoints. To combine the concepts of “bio-based” and “functional” in one polymeric material, in our earlier work vanillin-based aldehyde-containing microspheres were prepared via suspension polymerization and used as chelating resin, 41 scavenger resin 42 and metal corrosion inhibitor. 43 We have demonstrated the polymerization ability of the monomer (vanillin methacrylate, VMA), the high Tg temperature of the resulting polymer (PVMA),

44

and the functionalization of the

aldehyde groups on polymers. In this work, VMA is employed as a bio-based styrene substituent to prepare porous functional materials via HIPE method and the resulting polyHIPEs are used as oil adsorption resins. Another bio-derivative, lauryl methacrylate (LMA), is employed as a comonomer for preparing the oil adsorbents, since it can provide affinity for low polarity organic reagents. To further examine the polyHIPEs’ potentials as oil adsorbents, three usual organic chemicals together with kerosene are used as representatives to be adsorbed. To our delight, the oil adsorption experiments clearly justify our hypothesis, namely, the porous materials demonstrate high oil adsorption and recycling usability. Accordingly, the polyHIPEs are expected to find practical uses as oil adsorbents. It is also worth mentioning that the aldehyde groups contained in the as-prepared polyHIPEs also provide a versatile platform for further functionalizing the materials through Schiff-base reaction. 2. Experimental section 2.1. Materials.


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All chemicals in this work were of reagent grade. Lauryl methacrylate (LMA) and divinylbenzene (DVB) were supplied by Aldrich and purified by distillation under reduced pressure. Span-80 (with HLB value, 4.3), toluene, n-hexane, chloroform, kerosene, and calcium chloride were purchased from Beijing Chemical Reagents Company. 2,2-Azobis(isobutyronitrile) (AIBN) was supplied by Aldrich and recrystallized from methanol, dried under vacuum at room temperature. Monomer vanillin methacrylate (VMA) was synthesized according to a method reported before.

41

All the solvents were distilled by standard methods. Water was freshly

deionized before use. 2.2. Measurements. The FT-IR spectra were measured using a Nicolet NEXUS 670 spectrophotometer (KBr pellet). The scanning electron microscope (SEM, S4800, Japan) was used to investigate the microstructures of the products. The microscope (XSP-1600, Fenghuang, China) was used to investigate the HIPEs. Pore structures of the polyHIPEs were measured by mercury porosimetry (PoreMasterGT 60, USA). Water contact angles (WCAs) were obtained by a contact angle instrument (JY-PHa, Chengde) at room temperature; 4 μL deionized water was dropped down each time, which was recorded by computer. Thermal gravimetric analysis (TGA) was performed with a STA 449C thermal analyzer (Netzsch) under N2 at a heating rate of 10 °C/min. 2.3. Preparation of polyHIPEs. In the preparation process, four factors largely affecting the microstructure of polyHIPEs were investigated (monomer ratio, emulsifier content, polymerization temperature and water-oil ratio; more information can be found in Table 1). Taking polyHIPE-W/O-20 as an example, the major preparation process is presented below. In a 20 mL bottle, 0.015 g initiator of AIBN and 0.2 g


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surfactant of Span-80 were dissolved in the mixture of oil phase containing of 0.2 g VMA, 0.2 g LMA, 0.1 g DVB, and 0.5 mL toluene. The aqueous solution of calcium chloride (10 mL, with W/O ratio in volume as 20/1) was then added stepwise under vigorously stirring at 1000 rpm (magnetic stir bar). After that, the system was stirred vigorously at 1500 rpm for 2 h to form a viscous and homogeneous HIPE. Herein, since VMA monomer is a solid at room temperature, we define the W/O ratio as the ratio of the water-phase volume to the solvent toluene volume (the amount of toluene kept constant as 0.5 mL). The reaction system was then heated to 65 °C and retained at the temperature for 8 h. After polymerization, the unreacted monomer and Span80 were removed by Soxhlet extraction using ethanol as solvent. Finally, polyHIPE-W/O-20 in white was collected and dried (98 wt%, yield). The other polyHIPEs were prepared in the same way. 2.4 Adsorption studies of polyHIPEs towards oil. All the oil adsorption values were determined based on the average values of three parallel measurements. Typical experimental processes are briefly stated below. Adsorption capacity. The oil adsorption capacities of the polyHIPEs towards hexane, toluene, chloroform, and kerosene were measured. Four polyHIPEs prepared with different water-oil ratio, i.e. polyHIPE-E2, polyHIPE-W/O-10, polyHIPE-W/O-20 and polyHIPE-W/O-40 (see below for more details), were used as adsorbent. A typical oil adsorption procedure is described below. About 0.1 g of a polyHIPE sample (m1) in a filter paper was placed into the oil (20 mL) at room temperature. After the oil adsorption achieved saturation, the polyHIPE was taken away from the oil and weighed as m2. The total mass of the oil adsorbed by the polyHIPE was


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determined as m2‒m1, and the oil-intake capacity, k (g/g), can be calculated by the following formula: k = (m2 ‒ m1) / m1

(1)

where m1 is the mass of dry polyHIPE (g) and m2 is the total mass of polyHIPE (g) after adsorbing oil. Time-adsorption studies. Hexane and toluene were used as adsorbates. Two polyHIPEs prepared with varied water-oil ratio, polyHIPE-E2 and polyHIPE-W/O-20, were used as adsorbent. The oil-adsorption tests keep the same as described above. At pre-set time intervals, the polyHIPE was taken away from oil and weighed as mt, and then put back to the oil. The oil intake capacity, kt, can be calculated by the following formula: kt = (mt – m3) / m3

(2)

where m3 is the mass of polyHIPE (g) and mt is the mass of polyHIPE (g) at preset adsorption time. Oil-water separation. About 0.3 g sample (using polyHIPE-E2 as an example) was placed into the mixture of water and oil (toluene, 1 mL, dyed with oil red; water, 20 mL). The polyHIPE floated on the water and absorbed toluene in less than one minute. The experiment procedure of oil-water separation was tracked by taking digital photos and video. Recycling adsorption. Toluene was used as the oil in this case. PolyHIPE-W/O-20 was used as representative adsorbent. The adsorption process was the same as in testing adsorption capacity. The polyHIPEs after adsorption were regenerated by centrifugation (10000 rpm, 5 min),


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referring to the literature. 5 Briefly, in a 50 mL centrifuge tube, a 2 mL centrifuge tube was used as a hard holder to prevent the cotton and polyHIPEs from moving down to the bottom of the tube. Cotton was used as a soft holder to make the oil passing through. After centrifugation, the polyHIPEs were took out and weighed to calculate the oil adsorption capacity. The centrifuge tube system was weighed to calculate the oil removal rate. The adsorption-desorption process was repeated for 10 cycles. 3. Results and Discussion 3.1 Preparation and morphology of bio-based polyHIPEs.

Figure 1. Schematic strategy for preparing the polyHIPEs starting from vanillin and lauryl methacrylate (LMA) (A); SEM images of polyHIPE-E1 (B); typical photographs of forming polyHIPE-E2, continuous phase solution (C), HIPE (D) and polyHIPE-E2 (E); the contact angle measurement of polyHIPE-E2 (F). The overall strategy to form the polyHIPEs is illustratively presented in Figure 1(A). The polyHIPEs were prepared via HIPE polymerization approach using VMA and LMA as bio-based comonomers, DVB as crosslinking agent, AIBN as initiator, and Span-80 as surfactant. Since monomer VMA and initiator AIBN are solid at room temperature, we employed toluene as co-


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solvent. The preparation process of the polyHIPEs can be divided into two major steps. The first step is emulsification: A large amount of CaCl2 aqueous solution was added dropwise into the toluene solution containing monomers and DVB under a vigorous stirring, as shown in Figure 1(C). With the assistance of Span-80, a stable water-in-oil emulsion system was formed and denoted as HIPE (as shown in Figure 1(D)). With an increase in water content, the emulsion system gradually lost its flowability and became wholly frozen. Although the appearance of becoming wholly frozen means the formation of HIPE, the emulsion system still can be easily deformed by external force before polymerization. In addition, considering that the total volume of the HIPE system was about 20 mL and the length of magnetic stirrer was 1.5 cm, the external force from magnetic stirrer was enough for forming HIPE. The frozen HIPE was stirred vigorously at 1500 rpm for 2 h to form a homogeneous HIPE. The formation of water droplet in HIPE was demonstrated by optical microscopy image, as shown in Figure S1 (Supporting Information, SI). Discrete droplets with clear boundaries can be observed in the HIPE. The second step was polymerization: The co-monomers underwent polymerization in the organic continuous phase, and the microstructures of the HIPEs were cured, leading to the formation of porous monolith, denoted as polyHIPEs, as shown in Figure 1(B and E). In the HIPE polymerization system, forming a stable water-in-oil system which can undergo polymerization is indispensable. 4 The classic oil-soluble emulsifier Span-80 plays a key role in stabilizing the emulsion system. Firstly, Span-80 dissolves in the continuous phase (oil phase); because its concentration is far greater than the critical micelle concentration, it exists in the form of blank micelles. Subsequently, with adding the dispersed phase (aqueous phase), the emulsifier stabilizes the system by reducing the surface tension through regular arrangement at the oil-water interface. Finally, due to the disperse phase volume ratio greater than the maximum bulk density


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(74%), squeezing deformation occurs among the droplets, and the morphology of the dispersed phase of the templates is achieved by gelation of the co-monomers in continuous phase. Four parameters effecting the stability and polymerization of HIPE were investigated in detail. They are co-monomer ratio (Table 1, polyHIPE-M1, 2 and 3) affecting the hydrophobicity of the organic continuous phase; the content of emulsifiers (Table 1, polyHIPE-E1, 2 and 3) affecting the interface stability between water and oil phases; polymerization temperature (Table 1, polyHIPE-T1, 2 and 3) impacting on the speed of polymerization; and the ratio of water-oil (Table 1, polyHIPE-W/O-10, -20, -30 and -40) influencing the microstructure of the eventually formed polyHIPEs. All the above polyHIPEs were smoothly obtained and further observed by SEM (see below). Table 1. Parameters of the polyHIPEs. Sample a polyHIPE-M1 polyHIPE-M2 polyHIPE-M3c polyHIPE-T1c polyHIPE-T2d polyHIPE-T3 polyHIPE-E1 polyHIPE-E2d polyHIPE-E3 polyHIPE-W/O-10 polyHIPE-W/O-20 polyHIPE-W/O-30 polyHIPE-W/O-40

VMA (g)

LMA (g)

DVB (g)

Span80 (g)

W/Ob (V/V)

T(oC )

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.2 0.2 0.2 0.2 0.2 0.2 0.15 0.20 0.30 0.20 0.20 0.20 0.20

1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:10 1:20 1:30 1:40

55 55 55 55 65 75 65 65 65 65 65 65 65

a) For all the samples, AIBN 3 wt% of the total mass of the monomers and the cross-linker. Toluene was used as solvent (0.5 mL). b) Volume ratio of the CaCl2 aqueous solution and toluene. c, d) Samples were prepared in a same way but named differently for comparing (c, polyHIPE-M3 = polyHIPE-T1; d, polyHIPE-T2 = polyHIPE-E2).


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Figure 2. SEM images of the polyHIPEs prepared from varied experimental parameters (for detailed parameters, see Table 1). As shown in Figure 2, all the polyHIPEs exhibit hierarchical porous structures, which can be defined by three types of pores. They are the cells formed by removing water template with sizes around 1 μm; the windows formed by squeezing between droplets of dispersion phase with sizes around 200 nm; and the pores on the skeleton of polyHIPEs formed by removing Span-80 in the skeleton with sizes below 100 nm. More details will be presented below. For the bio-based monomer VMA, it has a similar molecular structure to that of styrene. The aromatic ring in styrene is known to provide oleophylic structure for preparing high oiladsorbing resins. 5, 6 Moreover, the abundant functional groups in VMA including aromatic ring,


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methyl ether, aldehyde, and ester bond, improve the oleophylicity of the corresponding polymer to multiple oils. However, in our experiment we found that it is difficult to prepare polyHIPEs with VMA as the only monomer. This phenomenon can be explained as the hydrophilicity of aldehyde groups making the HIPE system instable when undergoing polymerization. To solve this problem, we further employed the bio-based LMA as a co-monomer. The addition of the non-polar hydrophobic co-monomer LMA improves the stability of the HIPE; LMA also can provide affinity for the resulting polyHIPE when adsorbing organic reagents with low polarity. The cross-linking agent, DVB, was used, because a non-crosslinked polymer will dissolve and hence lose the ability to adsorb oils. Three polyHIPEs with different co-monomer ratio were successfully prepared and observed by SEM, as shown in Figure 2 (A,B,C). When only using VMA as monomer and DVB as cross-linker, the cells (holes formed after removing the internal-phase water during post-treatment) in polyHIPE-M1 are irregular since the hydrophilicity of aldehyde groups resulted in the instability of HIPE. With the increase of LMA content, the cells became clear, and dense skeletons are observed with significant reduced pores on them. It means that the nonpolar LMA helps to increase the stability of HIPE. According to our detailed experiments, a co-monomer mixture consisting of VMA/LMA/DVB= 2:2:1 (mass ratio) seems to be the most suitable formula and thus was used for preparing polyHIPEs in the subsequent experiments. Temperature is another important parameter largely influencing the formation and microstructure of polyHIPEs. Due to the large area of interface between water and oil, HIPE is a thermodynamically unstable system. The dispersed phase tends to undergo Ostwald ripening and agglomeration.

4

It means that there is a competition between the monomers undergoing

polymerization and the emulsion system tending to become more stable. Enhancing


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polymerization temperature can accelerate the polymerization and gelation of monomers in the continuous phase and promote the retention of template pores. On the other hand, it also can reduce the stability of the HIPE leading to the agglomeration of dispersion phase. Figures 2(D, E and F) show the SEM images of the three polyHIPEs prepared at different temperature. PolyHIPE-T1 prepared at 55 °C has a wider size distribution in the cell structures. It demonstrates that at a low temperature, the practical polymerization time (gelation time) is prolonged and the aggregation among the internal dispersed phase becomes severe. When the temperature is increased, the pore size distribution becomes narrow, with a distribution of around 1-2 μm (polyHIPE-T2, prepared at 65 °C). However, when the temperature increases to 75 °C, cells’ pore size distribution widens, with 4-5 μm pores appearing. It shows that although the gelation time becomes shorter at a higher temperature, the stabilization of the emulsifier decreases significantly, and the rate of aggregation among internal water droplets increases. To balance the stability of HIPE and polymerization, the polymerization temperature of 65 oC was selected for preparing the polyHIPEs. Figures 2(G to I) show the polyHIPEs prepared with varied emulsifier contents but with a same W/O ratio. According to the SEM images, although these polyHIPEs were prepared with the same W/O ratio, the size of their cell structures was different. It means that the emulsifier concentration contributed to the formation of holes between cells. With the emulsifier content increasing, the size of cells decreases and more open cells network formed. Interestingly, significant differences were observed on the skeleton structure of polyHIPE-E1, E2 and E3. In more detail, polyHIPE-E1, which was prepared with less emulsifier, showed a smooth skeleton surface in Figure 2G. When the amount of emulsifier increased, rough surfaces were formed in polyHIPE-E2 (Figure 2H) and polyHIPE-E3 (Figure 2I). Herein we define oleic phase as the


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polymerization solution containing monomers, crosslinker, AIBN and toluene. As a low HLB surface active agent, Span-80 is dissolved in the oleic phase. Nonetheless, it can be distributed both at oil-water interface layer and in the oleic phase as blank micelles. When excess emulsifier exists, the number of blank micelles increases. After polymerization and Soxhlet extraction, these blank micelles imprinted on the skeleton, leading to the rough surface of polyHIPE-E2 (Figure 2H). Accordingly, polyHIPE-E3 showed the roughest skeleton structure (Figure 2I). The advantages of preparing porous materials by HIPE method lie in that the pore structures can be easily adjusted through controlling the ratio of disperse phase to the continuous phase. The proportion of dispersion in HIPE can be increased to more than 90%,

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so polyHIPEs are

usually ultra-lightweight with high porosity. This feature is advantageous for preparing superabsorbent materials. Hence, in this work, polyHIPEs with varied water/oil ratio (polyHIPEW/O-10, -20, -30 and -40; the numbers mean the volume ratio of water to toluene) were further prepared. We also determined the limit water / oil ratio to be 36, with the phenomenon that excessive water (CaCl2 aqueous solution) floats on the HIPE and cannot enter the system. The SEM images of the four polyHIPEs are shown in Figures 2 (J to M). With the ratio of water / oil increases, the diameter of the cell structures decreases, and the structure of the window pores increases accordingly. According to the investigations and discussion above, three types of pores were observed in the obtained polyHIPEs. This multi-level pore structure in the polyHIPEs provides high porosity, high specific surface area, and low density, and hence provides advantages for further oil adsorption applications. Furthermore, the advantages together with the aldehyde groups also provide possibility for next preparing a wide variety of functional polymer materials.


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Table 2. Data of the polyHIPEs.

Water contenta (vol %)

Total intrusion volumeb (cm3/g）

Specific surface areab (m2/g)

Porosityb (%)

Apparent densityb,c (g/cm3)

Bulk densityb,d (g/cm3)

Cells diametere (μm)

diameter e (μm)

polyHIPEE1

75

1.85

15.93

80.1

0.43

2.18

6.5

0.16

0.03

polyHIPEE2(T2)

75

3.20

38.60

74.3

0.23

0.90

1.1

0.07

0.03

polyHIPEW/O-20

95

9.65

33.74

90.1

0.09

0.94

10.8

0.07

0.03

Sample

Windows

Pore on skeleton diameter e (μm)

a) calculated from the water/oil ratio in Table 1; b) obtained by mercury injection analysis; c) measured at 0.21 psia; d) measured at 29997.08 psia; e) obtained by pore size distribution from mercury injection analysis (Figure S1)

Considering the purpose of this work, i.e. preparing polyHIPEs from VMA and LMA and using them as high oil adsorption resin, it is important to fully understand the pore structures of polyHIPEs. Hence, three polyHIPEs prepared from different emulsifiers and water-oil ratio (polyHIPE-E1, E2 and polyHIPE-W/O-20) were selected as representatives to characterize their pore structures by mercury intrusion analysis. The data of porosity, average pore size, specific surface area, and density are summarized in Table 2. The mercury intrusion/extrusion isotherms and pore size distribution of the materials are presented in Figure S2. As shown in Table 2, a high porosity (over 74%) is observed in all the three polyHIPEs. With W/O ratio increased, a low apparent density (0.0935 g/cm3) with a high porosity (90.1%) are observed in polyHIPE-W/O20. For mercury intrusion/extrusion isotherms, increasing pressure caused the mercury intake to increase rapidly, which demonstrates that the polyHIPEs contain multiple types of pore structures. Two different equations were employed to analyze the pore size distributions, as shown in Figure S1 (D, E and F), and the size of three types of pores are summarized in Table 2. All of the three types of pores were observed in the figures of pore size distribution. Comparing the pore size distributions between polyHIPE-E1 and polyHIPE-E2, the size of cell structures


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decreased from 6.5 μm to 1.1 μm. Meanwhile, the size of window structures reduced from 160 nm to 70 nm. For the skeleton holes, the size of them around 30 nm did not change remarkably, but the number of them reduced significantly, with the decrease in specific surface area (Table 2, reduced from 38.6 m2/g to 15.93 m2/g). The observations keep consistent with those acquired in SEM images discussed above. PolyHIPE-E2 (which can be named as polyHIPE-W/O-4) and polyHIPE-W/O-20 were taken for comparing the polyHIPEs with different W/O ratio. Due to the Oswald effect, with a higher W/O ratio, the apparent density changed from 0.2321 g/cm3 to 0.0935 g/cm3 with a higher porosity being observed. For the distribution of the three types of pores, the size of skeleton holes around 15 nm changed little, but the size of cell structures increased from 1 μm to 10.8 μm with a wide distribution. Bulk density measured at high pressure reflects the density of the skeleton structure. The bulk density of polyHIPE-E2 (0.90 g/cm3) was similar to that of polyHIPE-W/O-20 (0.94 g/cm3), and both of them were much smaller than that of polyHIPE-E1 (2.18 g/cm3). This phenomenon also proves the skeleton holes contained in polyHIPE-E2 and polyHIPE-W/O-20. The co-monomers (VMA, LMA), cross-linking agent (DVB), and the obtained polyHIPEs (taking polyHIPE-E2 as an example) were characterized by FT-IR spectroscopy (Figure 3). For polyHIPE-E2, characteristic peaks appear at 2940 (methyl and methylene), 2750 (−(C=O)H), 1760, 1732 and 1135 (−(C=O)O−), 1700 (−(C=O)H), and 1600 cm-1(phenyl). The peak at 1700 cm-1 reflects the ketone of aldehyde groups from VMA. The peak at 1732 cm-1 indicates the ketone of ester linkage from LMA. The peak at 1760 cm-1 means the ketone of ester linkage from VMA. The disappearance of the peak around 1640 cm−1 in polyHIPEs indicates the transformation of −C(CH3)=CH2 and –CH2=CH2 groups in VMA, LMA, and DVB to the


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saturated polymer main chains in polyHIPEs. Also due to the transformation, the peak of ester group in VMA moved from 1735 to 1760 cm−1. Accordingly, FT-IR measurement strongly demonstrates the successful preparation of polyHIPEs.

Figure 3. Typical FT-IR spectra of LMA, DVB, VMA and polyHIPE-E2 (KBr tablet). The thermal stability of the polyHIPEs was subsequently measured by thermogravimetric analysis (TGA) technique. TGA measurement was performed from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen. The relevant results are presented in Figure S3. Similar to previous results,

44

the polymeric materials were found to decompose in the

temperature range of 270−450 °C demonstrating that the polyHIPEs can be used at moderately high temperatures. 3.2 The superhydrophobicity and oleophilicity of polyHIPEs PolyHIPEs as oil adsorbents have aroused considerable research interest for the advantages of high porosity, low density and the superhydrophobicity as well. The first two advantages of the


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as-prepared polyHIPEs in this work have been demonstrated above, and next we investigate their superhydrophobic and oil-adsorbing properties. In the WCA test, all the polyHIPEs demonstrate superhydrophobicity. Taking polyHIPE-E2 as an example, as shown in Figure 1 (F), the water drop cannot adhere to the surface of polyHIPE. An external force was applied to increase the contact of the water drop with the material surface; when the injector moved up, however the water drop closely adhered to the injector and instantly left the surface of the material. We also prolonged the contact time to about 1 min, but the phenomenon remained the same. This phenomenon demonstrates the hydrophobicity of the material. The WCA of polyHIPE-E2 at the moment of Figure 1(F) was measured as 160o. Furthermore, when a drop of oil (such as chloroform) meets the material surface, it was quickly adsorbed into the monoliths once it arrived at the surface of the material. This phenomenon indicates that the as-prepared polyHIPE materials have high affinity to oil. The chemical compositions and the microstructures of the polyHIPEs both had contribution to the superhydrophobicity and excellent lipophilicity. Hence, the multi-level pore structures, regular pore sizes distribution at the scale of micrometers together with hydrophobicity of the chemical compositions were considered to contribute to the superhydrophobicity. The high lipophilicity together with superhydrophobicity provide the materials with selectivity and adsorption efficiency when used to separate oils from water. As demonstrated above, polyHIPEs with different porosity and pore structures can be constructed by adjusting W/O ratio. The as-prepared polyHIPEs with varied W/O ratio (polyHIPE-E2, polyHIPE-W/O-10, polyHIPE-W/O-20, polyHIPE-W/O-30, polyHIPE-W/O-40) were employed to study their oil adsorbability. Hexane, toluene, and chloroform were chosen as representatives of organic solvents, and kerosene served as a representative of daily oil pollutant. To characterize the maximum adsorption capacity of polyHIPEs, we immersed them in an excess


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of oil and determined the adsorption capacity by calculating the increase in mass. During the oil adsorption process, all the polyHIPEs expanded in volume proportionally while maintaining the profile well. As shown in Figure 4, the oil adsorption capacity of polyHIPE-E2 towards the four organic solvents was 4.5, 5.5, 10.4 and 4.3 g/g, respectively. PolyHIPE-W/O-40 shows the highest adsorption (19.2, 20.7, 40.9, and 25.7 g/g for the four organic solvents, respectively). Considering that the density of hexane, toluene, chloroform, and kerosene is 0.66, 0.87, 1.5 and 0.8 g/cm3, respectively, the corresponding volumetric adsorption rate is 29.1, 23.9, 27.3, and 32.1 mL/g for the four chemicals. The different oil adsorption means that both the encapsulation of pore structures and the swelling effect of the skeleton contributed to it. This result also demonstrates that the polyHIPEs have adsorption capacity for a wide variety of oils.

Figure 4. Saturated oil adsorption of polyHIPEs with different water-oil ratio, 1:4 (polyHIPEE2), 1:10 (polyHIPE-W/O-10), 1:20 (polyHIPE-W/O-20), 1:30 (polyHIPE-W/O-30), and 1:40 (polyHIPE-W/O-40). Next, the adsorption time curves were studied for further understanding the oil adsorption process. Interestingly, the adsorption process of the present polyHIPEs is quite different from the


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counterparts in literature. 4 As shown in Figure 5, the oil adsorption rate of all the polyHIPEs as a function of time can be divided into three stages. In stage-Ι, from 0 to about 20 min, the oil adsorption rate rises rapidly, due to the porous and lipophilic structures of the material sucking oil through capillary action. In stage-П, from about 20 min to 2 h, the amount of adsorbed oil decreases considerably. For polyHIPE-E2 (polyHIPE-W/O-4) and polyHIPE-W/O-20, all the reduced amounts are about 4 g/g and change little as the water to oil ratio increased. We consider that the reduction in adsorbed oil should be due to the swelling of the polymer material’ surface layer. In the end, when swelling occurs inside the materials, the overall volume of polyHIPEs increases, resulting in increased enveloping capacity of both cell voids and skeleton. Hence, stage-Ш (from 2 to 12 h, with a slowly rising of oil adsorption) appears reasonably.

Figure 5. Oil adsorption process for hexane and toluene. (A) polyHIPE-E2 with parameter of water-oil ratio 4-1 towards hexane. (B) polyHIPE-E2 with parameter of water-oil ratio 4-1 towards toluene. (C) polyHIPE-W/O-20 towards hexane. (D) polyHIPE-W/O-20 towards toluene.


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Figure 6. The effects of oil-water separation using polyHIPE-E2 (polyHIPE-E2, 0.30 g; toluene, 1 mL, dyed with oil red; the video of oil-water separation is provided in SI). Due to the through-hole pore structure and lipophilic structure, instantaneous oil adsorption was observed in the polyHIPEs, as illustrated in Figure 5. A video is also provided in SI. Accordingly, we propose that the polyHIPEs may find practical applications in wastewater treatment, so we further performed oil adsorption tests using the polyHIPEs towards oil-water mixture. As shown in Figure 6, in the designed oil intake process, the monolithic material adsorbed oil extremely fast and the adsorption saturation completed in about 1 min. The fast adsorption speed of the polyHIPEs foams is attributed to the excellent oleophilicity and open pore structures of the materials. The polyHIPEs are anticipated to be recycled and reused not only for the material itself, but also for the valuable oil pollutants. To further elucidate this hypothesis, we subsequently restored the polyHIPEs after adsorbing oil through centrifugation.

5

The polyHIPE-W/O-20 was

employed as adsorbent and toluene was employed as a representative of oil pollutants. The results are presented in Figure 7. The oil adsorption during the 10 time-cycles largely kept unchanged. The oil removal rate of the polyHIPE remained more than 94%. This small portion of residual oil (about 6%) is due to the swelling effect of toluene on the skeleton of polyHIPE.


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During the 10 time-cycles, the high oil absorption maintained and no mass loss was observed for the polyHIPE after centrifugation. The results demonstrate the good reusability of the polyHIPEs. A digital image for the regeneration experiment is shown in Figure S4. Since the polyHIPEs underwent a large degree of swelling during the adsorption of toluene and then a vigorous centrifugal force in the course of centrifugation at 10000 rpm, the materials may break up in the regeneration test. This problem was successfully solved by using a filter paper bag (Figure S4). Of course, other solutions for enhancing the mechanical property of the polyHIPEs will be explored next through optimizing the composition, structure and morphology of the materials.

Figure 7. Recyclability performance of polyHIPE-W/O-20 (towards toluene). 4. Conclusions A novel type of bio-based porous polymer materials (polyHIPEs) were successfully prepared


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by using two bio-based derivatives, vanillin methacrylate (VMA) and lauryl methacrylate (LMA), as co-monomers and divinylbenzene (DVB) as cross-linking agent. Three types of porous structures (cell, window, and skeleton holes) in the obtained polyHIPEs were analyzed by SEM and mercury intrusion analyses. The pores can be adjusted by changing emulsifier content and/or water-oil ratio. The polyHIPEs demonstrate significant advantages: high porosity (90%, polyHIPE-W/O-20), low density (0.0935 g/cm3, polyHIPE-W/O-20), high specific surface area (38.6 m2/g, polyHIPE-E2), superhydrophobicity (with WCA exceeding 160o), together with abundant aldehyde groups and high thermostability. Using the polyHIPEs as super oil adsorption resin towards hexane, toluene, chloroform, and kerosene as representatives, high adsorption capacity was demonstrated and can be up to 40.9 g/g in the case of chloroform. Oil adsorption capacity of the materials also maintained well during recycling use. Also interestingly, the materials demonstrate excellent instantaneous oil adsorption capability. The advantages facilitate the polyHIPEs to be promising materials with potentials as high oil adsorption resin. Further worthy to be mentioned is that the polyHIPEs provide a versatile platform for preparing diverse functional polymer materials by forming reversible Schiff base, taking advantage of the active aldehyde groups. We are currently continuing our studies along the interesting research directions. ASSOCIATED CONTENT Supporting Information. The optical microscopy images of the HIPE system; the mercury intrusion/extrusion isotherms of polyHIPEs; the pore size distribution figures of polyHIPEs; the TGA curve of polyHIPE-E2; the regeneration of polyHIPE-W/O-20 and the video of oil-water separation (MP4). AUTHOR INFORMATION


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Corresponding Author *Tel: +86-10-6443-5128. Fax: +86-10-6443-5128. E-mail: [email protected] (Deng); Email: [email protected] (Wu). Author Contributions ‡These authors contributed equally. (H. Y. Zhang and R. Zhao) Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21774009). REFERENCES (1) Wang S.; Peng, X. W.; Zhong, L. X.; Tan, J. W.; Jing, S. S.; Cao, X.; Chen, W.; Liu, C. F.; Sun, R. C. An Ultralight, Elastic, Cost-Effective, and Highly Recyclable Superabsorbent from Microfibrillated Cellulose Fibers for Oil Spillage Clean up. J. Mater. Chem. A 2015, 3, 8772– 8781. (2) Liao, C. Y.; Chiou, J. Y.; Lin, J. J. Temperature-Dependent Oil Absorption of Poly(Oxypropylene)Amine-Intercalated Clays for Environmental Remediation. RSC Adv. 2015, 5, 100702–100708. (3) Wang, G.; Zeng, Z. X.; Wu, X. D.; Ren, T. H.; Han, J.; Xue, Q. J.; Three-Dimensional Structured Sponge With High Oil Wettability for the Clean-up of Oil Contaminations and Separation of Oil–Water Mixtures. Polym. Chem. 2014, 5, 5942–5948.


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Biobased, Porous Poly(high internal phase emulsions): Prepared from

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