Gel Emulsion Based on Amphiphilic Block Copolymer: A Template to

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Gel-Emulsion based on Amphiphilic Block copolymer: A Template to Develop Porous Polymeric Monolith for the Efficient Adsorption of VOCs Arindam Chakrabarty, Monali Maiti, Kazuma Miyagi, and Yoshikuni Teramoto ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00068 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Nano Materials

Gel-Emulsion based on Amphiphilic Block copolymer: A Template to Develop Porous Polymeric Monolith for the Efficient Adsorption of VOCs Arindam Chakrabarty a, *, Monali Maiti a, Kazuma Miyagi a, Yoshikuni Teramoto a, b a

Department of Applied Biological Sciences, Gifu University, Gifu 501-1193, Japan

b

Center for Highly Advanced Integration of Nano and Life Sciences (G-CHAIN), Gifu

University, Gifu 501-1193, Japan

ABSTRACT The fabrication of adsorbent for volatile organic compounds (VOCs) always attains significant research interest for the industrialized civilization. Herein, we report the fabrication of a new class of porous polymeric monolith for the efficient adsorption of VOCs. In this case, a gelemulsion stabilized by poly(oligo(ethylene glycol) methyl ether methacrylate)-b-polystyrene (POEGMA-b-PS), an amphiphilic block copolymer (BCP) was successfully used as the template for the development of porous monolith having porosity in the range of 5–50 µm. Reversible addition-fragmentation chain transfer (RAFT) polymerization technique was adopted for the synthesis of BCP with varying PS block length. The BCPs were used as the stabilizer for waterin-oil gel-emulsion consists of >90% dispersed phase without using any co-stabilizer. Morphology, thermal and rheological behavior of the prepared gel-emulsions were found to be dependent on the PS block length in the BCP as analyzed in FE-SEM, DSC and rheometer respectively. Porous polymeric monoliths were prepared by polymerizing gel-emulsions with a mixed oil phase containing styrene monomer, crosslinker and radical initiator. The resulting monoliths had the minimum density of 0.08 g/cm3. Those monoliths were found to have very high resistance to water, showing water contact angle ~120° while, they can effectively adsorb wide range of VOCs like benzene, toluene, xylene, ethyl benzene, chloroform, tetrahydrofuran, acetone, formaldehyde, hexane etc. and also attain reusability with similar efficiency.

Keywords: Gel-emulsion, Monolith, VOC Adsorption, Block copolymer

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INTRODUCTION Volatile organic compounds (VOCs) can be defined as a class of toxic compounds contributing to environmental pollution in the forms of photochemical smog, greenhouse effect, and depletion of stratospheric ozone etc. VOCs are produced from low-boiling synthesized chemicals that are being used for the manufacture of paints, pharmaceuticals, adhesives, petroleum products, and refrigerants [1,2]. Thus, it is a rising problem for the industrially developed countries. For example, In Japan, most of the VOCs are produced by chemical (32,662 tons in FY 2010) and printing (42,700 tons in FY 2010) industries, thereby the measures taken by the Government of Japan include the voluntary actions by those industries to capture the produced VOCs [3]. In order to get rid of the VOCs, researchers are involved to develop the technology which includes biofiltration [4], catalytic oxidation [5], membrane separation [6], adsorption [7], and so on. Among them, adsorption was recognized as the most effective and economic method to remove VOCs. In this regard, scientists have developed many types of adsorbents like activated carbon fiber [8], granular activated carbon [9], modified clay [10], alumina [11], etc. However, those adsorbents suffer from certain limitations with regard to the sensitivity to high temperature and humidity, chemical instability, and non-reusability. As a result, the development of polymeric monolith has drawn significant research interest in recent years [12–14]. The important features of such monoliths are high porosity, physicochemical stability, and reusability among other things. The fabrication of porous monolith generally requires the use of certain templates. In this regard, gel-emulsions achieved much popularity as template because their ease of preparation and controllable properties [15,16]. Gel-emulsions are defined as emulsions in which the occupancy of dispersed phase is greater than 74 % of total volume, resulting the maximum volume occupied by uniform spheres [17,18]. They are also called as high internal phase emulsions (HIPEs) that are being used to produce ‘polyHIPE’, an important class of porous polymers [19]. Gel-emulsions are composed of two immiscible liquids, like oil (organic solvent) and water, producing water-in-oil (W/O) or oil-in-water (O/W) gel-emulsions in the presence of suitable emulsifier. The commonly used method for preparing gel-emulsion consists of dissolving the emulsifier in the component which constitutes the continuous phase, followed by stepwise addition of the component which forms the dispersed phase under continuous shaking 2


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[20,21]. If we are able to prepare a gel-emulsion using a polymerizable monomer as the continuous phase, it provides the opportunity to polymerize that continuous phase producing a porous monolith, where the dispersed phase contributes to the porosity [19]. Therefore, the density and porosity of the monolith exclusively depend on the nature of the template i.e. gelemulsion, which can be easily tailored by the type of emulsifier and oil/water ratio. In general, a gel-emulsion may encounter the problem of phase separation upon storage and increased temperature. In this regard, the choice of emulsifier is crucial, where it will produce very stable gel-emulsion even in monomer/water system. Moreover, the stability of the gel-emulsion should not deteriorate by the temperature during the polymerization. The well-known stabilizers for gelemulsions are based on the amphiphilic derivatives of cholesterol [22], boronic acid [23], carboxylic acid [24], metal-organic framework [25], poly(styrene-co-methacrylic acid) particles [26] etc. The gel-emulsions prepared using cholesterol derivatives were successfully used as the template for the preparation of porous monoliths that were used not only in adsorption of VOCs but also in the selective absorption of oil in an oil-water mixture [27–30]. However, there was certain concern over the stability of those gel-emulsions, as it sometimes required small amount co-stabilizer together with the emulsifier to achieve desired stability [31]. Moreover, the stable gel-emulsions contained about 80% dispersed phase volume which was bit lower to achieve very high porosity and lower density in the monolith. In this regard, we found interest to use polymeric stabilizer for the gel-emulsion to achieve much higher porosity and lower density compared to the reported system. Herein, we report the preparation of gel-emulsion using amphiphilic block copolymer (BCP) which attains the potential to act as a surfactant for emulsion polymerization [32,33]. Since the inception of reversible deactivation radical polymerization (RDRP) in 1990s, the synthesis of BCP became easier due to the mild reaction condition in comparison with the existing ionic polymerization techniques [34]. In the present study, we adopted RAFT process under RDRP to prepare poly(oligo(ethylene glycol) methyl ether methacrylate)-b-polystyrene (POEGMA-b-PS) which was introduced as a sole stabilizer in w/o gel-emulsion with very high volume fraction of droplet (aqueous) phase. We studied the effect of different parameters like molecular weight of BCP, concentration of BCP, type of oil (solvent), concentration of oil etc. on the formation of stable gel-emulsion. Thermal, rheological and microscopic characterizations were carried out to study the effect of the length of PS-segment in the BCP. The prepared gel-emulsions containing 90% 3



volume of dispersed phase were used as template for the fabrication of monolith with higher porosity and much lower density.

EXPERIMENTAL Materials The monomers, oligo(ethylene glycol) methyl ether methacrylate (OEGMA; the average degree of polymerization (DP) of the oligo(ethylene glycol) side chain was 4–5, namely, the number average molecular weight (Mn) was 300), styrene (S) and the cross-linker, ethylene glycol dimethacrylate (EGDMA) were purchased from Sigma-Aldrich Co. LLC. (USA) and were purified by passing through activated basic alumina column prior to use. The free radical initiator 2,2’-azobisisobutyronitrile (AIBN, Sigma Aldrich) was used after recrystallization from methanol. The RAFT agent, 2-cyano-2-propyl dodecyl trithiocarbonate (CPDTC, Sigma Aldrich) was used as received. The solvents like 1,4-dioxane, tetrahydrofuran (THF), chloroform, formaldehyde, hexane, acetone, benzene, ethyl benzene, toluene, and p-xylene were purchased from Wako Pure Chemical Industries, Ltd. (Japan) and used without further purification. RAFT polymerization of OEGMA Following a typical RAFT polymerization procedure, the RAFT agent CPDTC (0.023 g, 0.066 mmol) and the initiator AIBN (0.002g, 0.013 mmol) were weighed out in a glass tube (having the dimension of 85mm x 15mm). The monomer, OEGMA (1 g, 3.3 mmol) was then added to the glass tube and kept under stirring by magnetic bar to make a homogeneous solution. The tube was then sealed with a rubber septum and nitrogen gas was purged for 15min to drive out the oxygen present inside the glass tube. Finally, it was placed in an oil bath preheated at 65 °C. The reaction was carried out for 2h until the magnetic bar stops stirring due to increased viscosity. The reaction was stopped by removing the rubber septum followed by cooling the tube in a cold-water bath. The polymer was then dissolved in small amount of THF and purified by precipitating it into a large volume of hexane. This precipitation process was repeated 2 times. The purified polymer was dried in a vacuum oven at 40 °C for 24 h. The obtained polymer was further used for the chain extension experiment. Yield 0.943g (Conv. 94.3%). Mn,NMR = 16,800 g/mol.

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Preparation of block copolymer, POEGMA-b-PS using the prepared POEGMA as macroRAFT agent The prepared POEGMA was further used as the macro-RAFT agent for the polymerization of styrene to obtain the block copolymer. In a typical procedure, about 0.25 g (0.015 mmol) macro-RAFT agent was taken in a 20 mL round-bottom flask and dissolved in 1,4-dioxane (0.5 g). About 0.25 g (2.4 mmol) styrene monomer was added to the solution. The flask was then sealed with a rubber septum and purged with nitrogen for 30 min to ensure replacement of oxygen present inside. A solution of AIBN (0.5 mg, 0.003 mmol) in 1,4-dioxane (0.25 g) was prepared in glass vial and injected to the round-bottom flask containing the reaction mixture. The purging of nitrogen gas was continued for another 10 min. Finally, the flask was placed in an oil bath preheated at 80 °C. The reaction was carried out for 48h. The product polymer was obtained by precipitating into a large volume of hexane followed by drying in a vacuum oven at 40 °C for 24 h. Yield 0.385 g (Conv. 54.0%). Mn,NMR = 28,600 g/mol. Preparation of w/o Gel-emulsion About 5 mg of BCP was weighed out in a test tube and dissolved in 150-µL organic solvent. Then de-ionized water was added slowly to that test tube sealed with a rubber septum followed by shaking at room temperature. After adding 2–4-mL water, a viscous and milky emulsion was produced. The formed gel emulsion was allowed to stand for 5–7 min, then the stability was evaluated by the “stable to inversion of a test tube” method. Gel-emulsion templated preparation of porous polymeric monolith A polymeric monolith was obtained by the polymerization in the gel-emulsion. In a septumcapped test tube, the gel-emulsion having about 93% volume of water, was prepared using a mixture of styrene (150 µL), BCP (5 mg, 3.3% w/v), EGDMA (5 µL, 3.3% v/v), and AIBN (1.0 mg, 0.6% w/v) instead of organic solvent. Then, the gel-emulsion was purged with nitrogen gas for 30 min. Finally, the tube was placed in an oil bath preheated at 50 °C. The polymerization reaction was carried out for 24 h. Finally, a polymeric monolith was obtained as a white solid substance which was thoroughly washed with water and ethanol followed by drying in a vacuum oven at 40 °C for 48 h. General characterization of polymers 5



Molecular characterization (NMR and GPC), thermal analysis (DSC), rheological testing (DMA), morphological observation (optical and scanning electron microscopy) and contact angle measurements were performed in the ordinary procedures. Details of them are listed in Supporting Information. VOC adsorption of monoliths For VOC adsorption test, about 2 mL of organic solvent was taken in a glass vial (l × d = 45 mm × 20 mm) and the vial was kept into a glass bottle (l × d = 125 mm × 65 mm). A known weight of monolith was placed inside the glass bottle before it was closed. At certain time intervals, the monolith was taken out of the glass bottle and its weight was recorded. This test was performed at ambient temperature at 25° C and the absorption capacity was calculated using following equation:

where q is the absorption capacity (g/g), m0 is the initial weight (g) of the monolith, and m is the weight (g) of the monolith after the VOC adsorption. Reusability test of monoliths In order to check the reusability of the monolith, the swelled monolith obtained after the VOC adsorption was washed with ethanol followed by drying in vacuum oven at 40 °C for 24h. Finally, the regenerated porous monolith was again introduced for the VOC adsorption. This cycle was repeated ten times and the corresponding adsorption capacity was recorded.

RESULTS AND DISCUSSION Preparation of amphiphilic BCP In the present study, a series of amphiphilic BCP was prepared by the combination of PEGcontaining segment and polystyrene (PS). At first, the RAFT polymerization of OEGMA containing OEG-pendant groups was carried out and the produced polymer was further used as the macro-RAFT agent for the polymerization of styrene. Thus, POEGMA-b-PS was produced where the POEGMA-block belonged to the hydrophilic segment and PS-block constructed the hydrophobic part (Scheme 1). The chemical structures of the prepared polymers were confirmed 6


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by the 1H NMR measurements. Figure S1 (in Supporting Information) shows the 1H NMR spectrum of the POEGMA prepared using trithiocarbonate-based RAFT agent. The chemical shift at 4.1 ppm (d) refers to the first methylene protons (-O-CH2) in the pendant PEG unit, while the chemical shifts at 3.3–3.7 ppm (e and f) are due to the remaining methylene protons (-OCH2-CH2-) and methoxy unit (-O-CH3). The chemical shift at 3.2 ppm (g) corresponds to the methylene protons (-S-CH2-) in the RAFT-end group. Therefore, the molecular weight of the prepared POEGMA was calculated by the following equation utilizing the integral areas of d and g, MPOEGMA = MRAFT + [(Id/2)/(Ig/2)] × MOEGMA where MRAFT and MOEGMA denote the molecular weights of RAFT agent and OEGMA monomer, respectively, and Id and Ig are the integral areas of peak d and g respectively in Figure S1. Similarly, Figure S2 (in Supporting Information) shows the

1

H NMR spectrum of

POEGMA-b-PS with different PS-chain lengths. In this case, the chemical shifts at 6.5 ppm and 7.0 ppm represent different aromatic protons in the PS-segment. The degree of polymerization (DP) of different blocks in the BCP was analyzed with the integral areas of the chemical shifts at 6.5 ppm (l) and 4.1 ppm (d). The estimated compositions of the prepared BCPs are summarized in Table 1. The table demonstrates that the BCPs have controlled molecular weights as observed from the little difference in the theoretical ones and those obtained from the NMR spectra and GPC analysis.

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Scheme 1 Schematic pathways for the preparation of amphiphilic BCPs via RAFT polymerization.

Table 1 Results of molecular characterization for the BCPs prepared by using POEGMA as macro-RAFT agent. Sample [M]:[macro-

% Conv. b

Mn,theoc

Mn,NMR

Mn,GPC

Ð

Block Compositiond

RAFT]:[I]a BCP-1

160:1:0.2

54.0

25,800

28,600

23,500

1.35

POEGMA55-b-PS114

BCP-2

480:1:0.2

53.7

43,600

54,300

47,300

1.55

POEGMA55-b-PS361

a

[M], [macro-RAFT], and [I] are the molar equivalents of styrene, POEGMA having Mn of 16,800, and the initiator AIBN, respectively.

b

Determined gravimetrically

c

Theoretical Mn was calculated as the sum of Mn (16,800) of the macro-RAFT and 104x[M]/[Macro-RAFT], where x was conversion of styrene and 104 was molecular weight of styrene.

d

Calculated from 1H NMR. 8


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In the BCPs, the POEGMA-segment provides hydrophilicity whereas the PS-section is hydrophobic. The two BCPs obtained here differed only in the lengths of the PS segment that can distort their amphiphilic nature. That is, one contained a shorter PS-chain (denoted as BCP1) and the other had a longer PS-chain (denoted as BCP-2). In the present study, these amphiphilic BCPs were further used for the fabrication of w/o gel-emulsions.

Preparation of Gel-emulsion Using Amphiphilic BCPs The prepared amphiphilic BCPs differing in the PS-chain length were introduced to form w/o gel-emulsions. The ability of gel-emulsion formation by both BCP-1 and BCP-2 was investigated by using various organic solvents. We found that both the BCPs were able to form gel-emulsions in toluene, xylene, mesitylene, and benzene and the results of their respective critical gelation concentration (CGC) values are tabulated in Table S1 (in supporting information). CGC refers to the minimum concentration of gelator required for the gelation and the lower the CGC value, the higher the gelation ability [24]. We found considerable difference in CGC values of the BCPs; BCP-2 had lower CGC values compared to BCP-1 in all solvents. It indicates that BCP-2 having longer PS-segment is a better gelator than the BCP-1. We observed the stability of the prepared gel-emulsions by using “stable to inversion of a test tube” method. In this case, BCP-1 was able to form a stable gel-emulsion only in toluene, whereas those in benzene, xylene and mesitylene were unstable as shown in Figure 1a. Interestingly, the BCP-2 was found to form stable gel-emulsion in toluene, benzene and xylene (Figure 1b). However, the gel-emulsions of both BCP-1 and BCP-2 in mesitylene were unstable due to the increased insolubility of the BCPs in mesitylene. Therefore, we can say that BCP-2 has high gelation ability than BCP-1, and this was also reflected in their respective CGC values listed in Table S1. This was due to the difference in the length of PS-segment in the BCPs. During the preparation of gel-emulsion using BCPs, the BCPs stayed at the oil-water interface where the PS-segment resided at the continuous oil phase and the POEGMA-segment remained in the water droplets. The PS-segment provided stability to the water droplets preventing the coalescence by steric repulsion [35]. Coalescence between the water droplets led to the instability in the gel-emulsion. Thus, the BCP-2 having longer PS-segment could provide stronger steric stabilization compared to the BCP-1 with short PS-segment. 9



Figure 1 Photographs of the gel-emulsions prepared in different solvents (150 µL) by using (a) BCP-1 (5 mg) and (b) BCP-2 (5 mg) as gelators, respectively.

Thermal Characterization of Gel-emulsion Thermal analysis of gel-emulsions was performed in order to obtain the information on their thermo-reversible nature and gel-to-sol transitions. In the present study, differential scanning calorimetric (DSC) analysis was carried out with the mechanically stable gel-emulsions formed by the BCPs (5 mg) in water/toluene (2.0 mL/150 µ L). Figure S3 (in Supporting Information) shows the DSC thermograms of the prepared gel-emulsions with heating and subsequent cooling experiment curves. During the heating of gel-emulsion, the DSC thermogram showed two distinct endothermic peaks. Both the gel-emulsions showed the first endothermic peak at around 68 °C, which corresponds to a solubilization transition as observed for the surfactants that exhibit a Krafft temperature [36,37]. The second endotherm at 85 °C can be attributed to the gel-to-sol transition of the gel-emulsions. These DSC thermograms confirmed the absence of thermoreversible nature in the gel-emulsions, because on heating two endotherm peaks were observed but no exotherm or endotherm peak was obtained on cooling.

Effect of Concentration on Gelation Generally, the formation of a stable gel-emulsion is very much dependant on the concentration of its components. In the present study, we are interested to observe the effect of the concentration of BCPs and volume of added toluene as a function of maximum amount of water that can be used for the preparation of stable gel-emulsion. Figure S4a (in Supporting 10


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Information) shows the relationship between the volumes of water and toluene for the formation of gel-emulsion using fixed amount (5 mg) of BCPs. In this case, the relationship exhibited a non-linear trend where the volume of water that can be added sharply increased with the small increase in the volume of toluene and reached maxima, then decreased slowly. Irrespective to the molecular weight of BCP, the observed maxima in volume of water were obtained when the corresponding volume of toluene was 150 µL. At this point, a stable gel-emulsion of BCP-1 was prepared using 3.3-mL water and this amount of water corresponds to the 95.6% dispersed phase volume in the gel-emulsion. Similarly, about 4.2-mL water was used for BCP-2 which provides 96.5% dispersed phase volume. As the number of BCP molecules are fixed in this case, the maxima in the volume of water indicates the most efficient inter-molecular hydrogen bonding among BCP molecules [38]. Thus, the volume of toluene (150 µL) corresponding to the maxima in volume of water was considered as the best gelation condition which was followed in further experiments. Figure S4b (in Supporting Information) shows the plots of the volume of water as a function of the amount of BCP-1 and BCP-2. In this case, different amounts of BCPs were dissolved in fixed volume (150 µL) of toluene and gel-emulsions were prepared by slow addition of water. The addition of water was continued until the gel-emulsion start flowing and the corresponding volume of water was recorded as the maximum volume of water that can be used to obtain a stable gel-emulsion for the respective amounts of BCP. The figure indicates that the maximum volume of water is directly proportional to the amount of BCP taken, although the values are dependant to the molecular weight of BCPs.

Rheological Properties The study of rheological behavior of w/o gel-emulsions always attains substantial importance from the view-point of their real-life applications [39,40]. In this study, the visco-elastic behavior of the gel-emulsions formed by the BCP-1 and BCP-2 was characterized by the storage modulus (G') and loss modulus (G"). Detailed discussion is described in Supporting Information (Figure S5–S7), but both the gel emulsions exhibited sufficient mechanical stabilities under oscillatory stress.

Microscopic studies of gel-emulsions 11



Optical Microscopy A preliminary investigation on the microstructures of the gel-emulsions was carried out using optical microscopy (OM). Figure 2 shows the microstructures of the gel-emulsions prepared using BCP-1 and BCP-2. We could see that both the gel-emulsions appeared in the form of bubbles with different sizes. For the BCP-1 gel emulsion, connected polygonal networks having the size of the compartment from 2–25 µm in diameter were formed (Figure 2a). However in the case of BCP-2, the oil-water interface of the droplets became round (Figure 2b). Hence, we could observe a striking difference in the microstructures arising from the difference in the molecular weights of the BCPs. The difference in the OM images is closely associated to the different interfacial interaction arising from the different volume fractions of the components of BCP. The BCP-1 (POEGMA55-b-PS114) having shorter PS length than BCP-2 (POEGMA55-b-PS361) provided less steric stabilization to the water droplets [35]. As a result, the water droplets of the gel-emulsion of BCP-1 were more prone to coalescence, exhibiting a close-packed structure, whereas the more sterically stabilized water droplets in the gel-emulsion of BCP-2 remained spherical. Moreover, an increase in the interfacial tension in gel-emulsion is also expected with the increase in the mol. wt. of BCPs [35, 41]. Thus, we can see the differences in the shape of the water droplets due to their differences in interfacial tension. Nonetheless, the nature of microstructure suggested that the water-droplets are closely organized and provided the essential condition of very high dispersed phase volume to produce the gel-like characteristics.

Figure 2 Optical micrographs of the gel-emulsions as prepared using (a) BCP-1 (5 mg) and (b) BCP-2 (5 mg) in water/toluene (2.0 mL/150 µ L). (Scale bar: 20 µm)

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Field emission scanning electron microscopy (FE-SEM) In order to characterize the different self-assembled microstructures in the gel-emulsions, FESEM observation was carried out. Figure 3 shows the FE-SEM images of the freeze-dried gels i.e. cryogel. Figures 3a and 3b represent BCP-1 cryogel at different magnifications exhibiting the polygonal microstructures; the texture resembled the OM of the corresponding gel-emulsion. Similarly, the cryogel of BCP-2 showed spherical microstructures in Figures 3c and 3d, which can also be related to the OM observation. Thus, by means of FE-SEM as well, we could clearly observe the porous network structure in the cryogels and also could differentiate the nature and size of the microstructures between the same of BCP-1 and BCP-2. The cryogel of BCP-1 exhibited the porosity with the diameter in a narrow range of 10–30 µm, whereas BCP-2 formed the porosity having the diameter in a broad range of 5–50 µm. As a whole, the nature and size of the microstructures in cryogels were found to be dependent on the molecular weights of BCP; specifically the amount of porosity increased substantially with the increase in polymer chain lengths. In this case, the pores reflect that water droplets were present in the gel-emulsion state. The water droplets were stabilized by the steric stabilization effect of PS-block in the BCPs. The shorter PS block of BCP-1 provided less steric stabilization compared to the longer PS block in BCP-2 [35]. As a result, the water droplets in the gel-emulsion of BCP-1 tended to coalesce and thus after freeze-drying the cryogel exhibited lower amount of porosity. On the other hand, the water droplets in the gel-emulsion of BCP-2 were less prone to coalescence and thus exhibited higher amount of porosity.

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Figure 3 FE-SEM images of the freeze-dried gel-emulsions formed by BCP-1 (a and b) and BCP-2 (c and d) in water/toluene (2.0 mL/150 µL).

Preparation of Porous Monolith As the gel-emulsions exhibits porous structures, researchers found interest to use them as the template for the preparation of porous monoliths [42,43]. A w/o gel-emulsion can be converted to a porous monolith by the polymerization of its continuous phase i.e. oil. In our present study, to carry out the polymerization in the gel-emulsion, it was prepared by using the mixture of styrene (S), EGDMA and AIBN as the oil phase. Interestingly, the mixture did not affect the stability of the prepared gel-emulsion. The AIBN-initiated polymerization of styrene was conducted at 50 °C, and the growing polystyrene (PS) chains were cross-linked due to the presence of EGDMA. Figure 4 displays the observation on the preparation of polymeric monolith by the polymerization in gel-emulsion. Figure 4a shows the photograph of the w/o gelemulsion prepared using a mixture of S (150 µL), EGDMA and AIBN as the oil phase, water (2 mL) and BCP-2 as stabilizer. Figure 4b shows the picture of the monolith obtained after the polymerization. The obtained monolith was very uniform and closely resembled the dimension 14


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of gel-emulsion. In this case, we could also see the comparable height of the gel-emulsion (2.8 cm) and its respective monolith (2.6 cm). However, in the case of BCP-1, we found little more shrinkage in the dimension of monolith because of the lower stability in the gel-emulsion. As a result, we obtained much lower density of 0.08 g/cm3 for the monolith of BCP-2, whereas the monolith of BCP-1 had the density of 0.11 g/cm3 even though in both cases the gel-emulsions had 93% volume of droplet phase (water).

Figure 4 Photographs to demonstrate the preparation of porous monolith from the gel-emulsion: (a) gel-emulsion of BCP-2 in water/styrene (2.0 mL/150 µL) and (b) the solid monolith produced after polymerization.

Microstructures of the Monolith In order to investigate the internal structure of the PS-monoliths as prepared, FE-SEM study was performed with the cross-sections of the prepared monoliths, and the results are shown in Figure 5. In the case of the monolith formed by incorporating BCP-1 gel-emulsion, it was observed as cell-like structures with diameter in the range of 20–50 µm (Figure 5a–5c). Those cell-like structures could be considered as pores in the monolith. However, the monolith prepared from BCP-2 gel-emulsion exhibited porous cage-like structures similar to the conventional polyHIPEs. It seemed like the spherical cavities are interconnected by numerous intracellular pores having the diameter in the range of 5–20 µm (Figure 5d–5f). Thus, the FESEM images clearly indicated the amount of porosity in the monolith of BCP-2 was considerably higher than that of BCP-1.

15



Figure 5 FE-SEM images of the porous microstructures in the monoliths prepared from the gelemulsions of BCP-1 (a–c) and BCP-2 (d–f), respectively.

Water and Oil Absorption In order to investigate the affinity of the PS-monoliths towards water or oil, the contact angle (CA) analysis was performed, and the results were shown in Figure S8 (in Supporting Information). The respective CA values were recorded in every second up to 5 s, immediately after placing the drop on the cross-sectional surface of the monolith. The series of figure exhibits the rates of water and oil absorption for the monoliths prepared using BCP-1 and BCP-2, respectively. Both the monoliths had very good hydrophobicity having water contact angle value of around 120° which remained almost constant with time. On the other hand, they showed very high oleophilicity with CA value of maximum 42.4° which rapidly decreased with time. This observation confirmed the very high oleophilic nature which gave the scope to use the prepared monoliths for specific application.

Adsorption of VOCs The extreme oleophilic property provided a possibility for the application of the prepared porous monolith in the adsorption of VOCs. Thus, they were introduced for the adsorption of VOCs produced by the solvents such as chloroform, tetrahydrofuran (THF), benzene, toluene, mesitylene, p-xylene, formaldehyde, acetone and n-hexane. For this purpose, a small specimen 16


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of the monolith was weighed out and kept inside a closed vessel where an opened glass vial containing about 2 mL of organic solvent was also placed (Figure S9 in Supporting Information). The closed vessel was kept at room temperature for 60 h and the monolith was taken out to record its weight at certain time intervals. The VOC adsorption capacity (g/g) was calculated from the initial weight of the monolith and the weight after VOC adsorption for the respective solvents. Figure 6a and 6b display the results of the VOC adsorption experiment of the monolith in presence of various volatile organic solvents. The maximum amount of VOC adsorbed per gm of the monolith, derived from BCP-1 and BCP-2 are shown in Figure 6a. The monolith obtained from the gel-emulsion of BCP-2 exhibited better adsorption behavior in comparison with the series of BCP-1. The higher adsorption ability for the monolith of BCP-2 can be attributed to the presence of increased amount of intracellular porosity, providing the path for the diffusion of VOCs. Moreover, the PS-block in the BCPs has higher affinity towards the organic solvent vapors compared to the POEGMA-block, causing the difference in the VOC adsorption by the monoliths. Thus, the BCP-2 having higher PS fraction can adsorb more solvent vapor than BCP-1 with lower PS fraction.

Figure 6 VOC adsorption profiles of the porous monoliths prepared using the gel-emulsions of BCP-1 and BCP-2: (a) adsorption of different solvents after 60 h by both the monoliths and (b) time-course of adsorption for the monolith of BCP-2.

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Figure 6b shows the plot of the adsorption capacity at certain time interval during the adsorption experiment carried out with the monolith of BCP-2. The plot exhibited linear increase in the VOC adsorption with time. The experiment was carried out for 60 h, where the adsorption capacity of the monolith reached the equilibrium at 48 h. The VOC adsorption capacity of the porous monolith was not only dependent to the PS fraction of the BCPs, but also to the other parameters of monolith preparation such as amount of BCP, volume of added styrene monomer and the volume% of water. Figure S10 (in Supporting Information) shows the trend of chloroform adsorption capacity of BCP-2 with the mentioned parameters. When amount of BCP2 was gradually increased in the monolith keeping the amount of styrene and vol% of water constant at 150 µL and 87% respectively, there was very little improvement in the adsorption capacity. However, the increase in the volume of styrene keeping the amount of BCP-2 and vol% of water constant at 8 mg and 87% respectively, provided a dramatic improvement in the adsorption capacity. A similar result was also obtained when vol% of water was changed keeping the amount of BCP-2 and volume of styrene constant at 8 mg and 150 µL respectively. Figure 7 shows the photographs of the BCP-2 monolith before and after adsorption of VOC. The photographs clearly indicate the considerable increase in the dimension of the monolith after absorbing chloroform vapor.

Figure 7 (a) Monolith prepared from the gel-emulsion of BCP-2 before the adsorption of VOC, and (b) swelled monolith after the adsorption of VOC (chloroform vapor) for 48h.

18


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Table 2 Examples of VOC adsorption by different monoliths. Monoliths made

Density 3

Adsorption

References

from

(g/cm )

Capacity (g/g)

Silica aerogel

5.0

1.34 (Toluene)

Standeker et al. [44]

Carbon aerogel

0.26

0.365 (Toluene)

Fairén-Jiménez et al. [45]

Activated carbon

0.29

0.988 (Benzene)

Balathanigaimani et al.

0.977 (Toluene)

[46]

-

0.225 (Toluene)

Bajwa et al. [47]

0.15–0.35

0.4 (Benzene)

Silvestre-Albero et al.

(Corn grain) Activated carbon (Bagasse ash) Activated carbon (Petroleum pitch) Activated carbon fibre

[48] 0.25–0.16

0.364 (Toluene)

Fuertes et al. [49]

0.368 (Benzene) Zeolite

-

0.147 (Toluene)

Saini et al. [50]

Gel-emulsion of

0.17–0.99

0.500 (Toluene)

Miao et al. [31]

0.08–0.11

0.777 (Toluene)

This work

cholesterol derivative Gel-emulsion of Block copolymer

0.907 (Benzene)

In order to compare the results obtained in the present study, we have summarized few examples of VOC adsorption by different monoliths as shown in Table 2. The evaluation of the re-usability of an absorbent is crucial for its practical uses. Therefore, we analyzed the reusability of the PS monolith formed by the gel-emulsion of BCP-2 and the results of adsorption are shown in Figure S11 (in Supporting Information). In this case, the swelled monolith obtained after the adsorption of VOC was washed with ethanol followed by drying in vacuum oven. This led to obtain the monolith in the form as it was before the adsorption. Now, the monolith was again introduced to adsorption experiment and maximum adsorption capacity was recorded. We repeated this cycle of adsorption and desorption for ten times and we found that there was almost

19



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no change in the maximum adsorption capacity in each cycle. This observation refers to the retained efficiency of adsorption making the monolith a reusable one.

CONCLUSION An amphiphilic block copolymer, POEGMA-b-PS was successfully synthesized by RAFT polymerization and was used for the preparation of stable w/o gel-emulsion. The formation of this type of polymer-stabilized gel-emulsion was studied by using various organic solvents. We found that the increase in the length of PS-segment had beneficial effect on the stability, morphology and rheological behavior of the prepared gel-emulsions. Due to the formation of highly porous structures as observed in SEM, the gel-emulsions were used as the template for the preparation of porous monoliths. These monoliths had much lower density and also exhibited better adsorption of VOCs in comparison with the previous reports. Moreover, they can also be reused for many cycles without significant loss in the adsorption capacity. This approach involving the preparation of porous monolith from a block copolymer-stabilized gel-emulsion will open up the scope to develop highly efficient VOC-adsorbent in near future.

ASSOCIATED CONTENT Supporting Information 1

H NMR spectra of the macro-RAFT and BCPs, CGC values of the BCPs in different organic

solvents, DSC results of the gel-emulsions, plot to show the effect of the concentration of different

components

on

the

formation

of

gel-emulsion,

results

of

rheology

by

stress/frequency/strain sweep experiments, results of contact angle analysis, image of VOC adsorption experiment, plot of chloroform adsorption capacity, and the plot of reusability study are included.

AUTHOR INFORMATION Corresponding Author *Tel: +81-58-293-2922. Email: [email protected], [email protected] Notes The authors declare no competing financial interest. 20


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ACKNOWLEDGEMENT The authors wish to acknowledge Prof. Shinya Takahashi of Faculty of Engineering, Gifu University for his help with contact angle measurement. This work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 17H01480 to Y.T.) from the Japan Society for the Promotion of Science and by the Environment Research and Technology Development Fund (3K153010 to Y.T.) of the Ministry of the Environment, Japan.

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Amphiphilic block copolymer-stabilized w/o gel-emulsion as the template for the preparation of low density porous polymeric monolith for the efficient adsorption of VOCs. 181x126mm (96 x 96 DPI)


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Gel Emulsion Based on Amphiphilic Block Copolymer: A Template to

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