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Root-soil structure inspired hydrogel microspheres with high dimensional stability and anion-exchange capacity
Accepted Manuscript Root-soil Structure Inspired Hydrogel Microspheres with High Dimensional Stability and Anion-Exchange Capacity Haifeng Ji, Xin Song, Chao He, Chengqiang Tang, Lian Xiong, Weifeng Zhao, Changsheng Zhao PII: DOI: Reference:
S0021-9797(18)30951-2 https://doi.org/10.1016/j.jcis.2018.08.036 YJCIS 23969
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
18 May 2018 9 August 2018 10 August 2018
Please cite this article as: H. Ji, X. Song, C. He, C. Tang, L. Xiong, W. Zhao, C. Zhao, Root-soil Structure Inspired Hydrogel Microspheres with High Dimensional Stability and Anion-Exchange Capacity, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.08.036
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Root-soil Structure Inspired Hydrogel Microspheres with High Dimensional Stability and Anion-Exchange Capacity Haifeng Ji,a,# Xin Song,a,# Chao He, a Chengqiang Tang, a Lian Xiong, a Weifeng Zhao,a,b,* and Changsheng Zhao.a,*
a
College of Polymer Science and Engineering, State Key Laboratory of Polymer
Materials Engineering, Sichuan University, Chengdu, 610065, People’s Republic of China b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
Donghua University, Shanghai, 201620, People’s Republic of China *Corresponding author. E-mail: [email protected] (W-F. Zhao*); [email protected] (C-S. Zhao**) Tel.: +86-28-85400453; Fax: +86-28-85405402. #
These authors contributed equally to this work.
1
Keywords:
Root-soil
structure;
Poly(methacryloxyethyltrimethyl
ammonium
chloride); Hydrogel microsphere; Ultrahigh ion exchange capacity; Restricted swelling behavior; Ultrahigh mechanical strength
Abstract: Ion exchange materials show great advantages in water purification, food industry, pharmaceutical industry, etc. However, the ion exchange capacities of ion exchange materials, especially for anion-exchange materials, at present are still relatively low. Hydrogels own abundant functional groups and show high hydrophilicity, and thus are recognized as high-potential ion exchange materials, but may deform and even crush in use due to their low mechanical strength and unavoidable swelling behavior. In this study, inspired by the root-soil structure, novel poly(methacryloxyethyltrimethyl
ammonium
chloride)
composite
hydrogel
microspheres with ultrahigh ion exchange capacity (more than 3.8 meuqiv/g), low swelling ratio (less than 1.5 g/g under pH=7), and ultrahigh mechanical strength (more than 28.1 MPa) were prepared. The microspheres showed efficient adsorption for anionic dyes (1491 mg/g for methyl orange, 1693 mg/g for Congo red, and 204.7 mg/g for amaranth, respectively) and great adsorption for bilirubin (131.6 mg/g). Taken together, the hydrogel microspheres were qualified as stable and high-efficiency ion exchange materials. More importantly, the root-soil structure opens up avenues for enhancing the dimensional stability of functional hydrogels.
Introduction Ion exchange materials exhibit wide applications in water purification, food industry, pharmaceutical industry, etc., since they are firstly invented by Adams and Holms [1-3]. The main property of ion exchange materials is the ion exchange capacity (IEC), but commercial ion-exchange materials still do not process enough IEC, especially for anion exchange materials. For example, the IEC of the DOWEX 2
550A is less than 1.1 mequiv/g. Thus, great efforts have been made to enhance the IEC
of
anion
exchange
materials.
Lin
et
al.
synthesized
a
set
of
imidazolium-functionalized poly(arylene ether sulfone)s, and the IECs were in the range of 1.35-1.84 mequiv/g [4]. Recently, Knauss et al. reported an anion-exchange materials
of
poly(2,6-dimethyl-1,4-phenylene
oxide)-b-poly(vinylbenzyltrimeth
-lammonium), and the IEC was about 2.9 mequiv/g [5]. Though these works enhanced the IECs of the anion-exchange materials, but still relatively low. Functional hydrogels could show ultrahigh IECs owing to their abundant functional groups and high hydrophilicity, and thus are recognized as a new generation of ion exchange materials [6,7]. However, the weak mechanical strength and high swelling ratio of the hydrogels seriously restrict their applications, and they may deform and even crush in the use as ion exchange materials. Recently, many researchers make their efforts in enhancing the mechanical strength of hydrogels by means of semi interpenetrating structure, hydrophobic interactions, multivalent host-guest interactions, aromatic stacking, hydrogen bonds, etc. [8-10], but these methods cannot prevent the deformation of the hydrogels. Additionally, these methods usually have specific requirements in the monomers. Thus, new approach towards dimensionally stable ion exchange materials with commonly used monomers such as acrylic
acid
(AA),
2-acrylanmido-2-methylpropanesulfonic
acid
and
methacryloxyethyltrimethyl ammonium chloride (DMC), are strongly desired. In nature, soil is easily striked by water and which further triggers soil erosion. Afforestation is an effective way to prevent the soil erosion due to the strong interaction between root and soil. Additionally, in ancient Chinese architecture, pure soil could not be used to build house, but people could effectively build strong houses by using thatch reinforced soil. These phenomenons inspired us to use the materials with high mechanical strength and dimensional stability to reinforce the materials with poor mechanical strength [11-13]. If the hydrogels with poor mechanical strength and low dimensional stability were used as soil, and polymer materials with good 3
mechanical property and good dimensional stability were used as root, then root-soil structure materials could be prepared. Recently, the hydrogels could be reinforced by cellulose or glass fiber, but these methods still cannot effectively restrict the swelling behavior of the hydrogels, which were mainly attributed to the heterogeneous mixture of the materials [14,15]. Additionally, the hydrogels could not be produced in a microspherical shape as ion exchange materials by these methods. Thus, how to prepare hydrogel microspheres with high performance still be a challenge. In our previous work, we filled the PAA hydrogel into the skeletons with high mechanical strength and hydrophobicity uniformly, and we termed this structure as root-soil inspired structure [16]. As the results, the hydrogels were effectively protected by the skeletons, and the swelling behaviors of the hydrogels were also limited by the hydrophobic skeletons. But the universality of this method remained to be studied, we hoped that this method could also be used to prepared high-performanced hydrogel microspheres, especially for the anion-exchange hydrogel microspheres. The aim of this study is to prepare the root-soil structure inspired anion-exchange hydrogel
microspheres.
First,
polyethersulfone
(PES)
was
dissolved
in
N,N'-dimethylacetamide (DMAc), and then dropped into a reaction solution (containing the monomer DMC, the solvent deionized water, cross-linker and initiator), and the skeleton microspheres were obtained via phase inversion [16]. In the phase inversion process, the reaction solution replaced the DMAc rapidly to ensure the uniformly dispersion of the reaction solution in the skeleton. Then the reaction solution filled into the skeleton microspheres were heated to initiate the in-situ crosslinking polymerization, thus the hydrogels were mixed with the skeletons uniformly to form the root-soil structure. In this system, the hydrophobic PES were performed as roots to improve the mechanical properties and decrease the swelling of the PDMC hydrogels. We anticipated that this structure could endow the hydrogels with ultrahigh dimensional stability, but still retain the high IEC of the hydrogels. 4
Experimental Polyethersulfone (PES, Ultrason E6020P) was obtained from BASF chemical company.
N-vinyl-2-pyrrolidone
(VP),
methacryloxyethyltrimethyl ammonium
chloride (DMC), N,N'-methylidenebis (acrylamide) (MBA), 2,2-azobisisobutyronitrile (AIBN) and ammonium persulfate (APS) were purchased from Aladdin Reagent Co. Ltd. N,N'-Dimethylacetamide (DMAc) and sodium dodecyl sulfate (SDS) were obtained from Chengdu Kelong Chemical Reagent Co. Ltd. Dyes (methyl orange (MO), amaranth (AR), Congo red (CR)) were purchased from Aladdin reagent Co. Ltd. Deionized (DI) water was used through the whole study. All the chemicals were used as received. To fabricate PES/PVP skeleton, an in-situ crosslinking polymerization of PVP was performed. Typically, 4 g PES, 4 g PVP, 0.1g AIBN and 0.1 g MBA were dissolved in 46 g DMAc, the PVP was used to enhance the hydrophilia of PES. The polymerization was performed at 120 °C for 36 h. To fabricate PES skeleton, 4 g PES was dissolved in 46 g DMAc. To prepare the skeleton microspheres, different skeleton solutions were dropped into the mixture (1 g SDS, 280 g DMC, 120 g DI water, 6 g MBA and 1.2 g APS) via electrospraying, and the DMAc would be replaced by the reaction solution during the phase inversion. Then the received microspheres were kept at 80 °C for 1 h for the fabrication of the root-soil structure inspired hydrogel microspheres (RSHMs) (as shown in Figure. 1A). The RSHMs with PES and PES/PVP skeletons were termed as PES/Gel and PES/PVP/Gel, respectively. Then the hydrogel microspheres (HMs) were prepared by dissolving the skeletons of the RSHMs in DMAc, and the HMs fabricated from PES/Gel and PES/PVP/Gel were named as Gel/PES and Gel/PES/PVP, respectively. The two kinds of HMs showed similar properties, and we choose the Gel/PES/PVP for further characterization (except for the adsorption kinetics tests).
5
For Fourier transform infrared spectroscopy (FTIR) analysis, the RSHMs, the hydrogel and skeleton microspheres were completely dried, and then the FTIR spectra between 1000 and 4000 cm−1 were obtained from a FTIR spectrometer (Nicolet 560, USA). AQ500 thermogravimetric analyzer (TG209F1, Germany) was used for thermogravimetric analysis under a dry N2 atmosphere from 50 to 750 °C at the heating speed of 10 °C/min. In order to explore the contents of the hydrogels in the RSHMs, the RSHMs were dried and weighed (WR), and then applied into DMAc to dissolve the PES skeletons to prepare pure hydrogels, then the hydrogels were dried and weighed (WH), the content (C) of the hydrogels in the RSHMs was calculated using the following formula: C (%) =
× 100 %
(1)
The cross-section morphologies of the RSHMs were obtained from a scanning electron microscope (SEM, JSM-7500F, JEOL, Japan). Then, the mechanical properties of the microspheres were investigated. After being stored in DI water for 24 h, the compressive strengths of the RSHMs and HMs were measured by a universal tensile testing machine (SANS CMT4000) at room temperature with a constant speed of 1 mm/min under a 1000 N load cell. The swelling behaviors of the RSHMs and HMs were measured by a gravimetric method. The wet RSHMs and HMs were dried at 60 °C more than 2 days; and then the equilibrium water uptake (EWU) was defined as the following modified formula [17]:
EWU (%) =
× 100 %
(2)
where We1 and We2 are the weights of the wet RSHMs (or the HMs) and the skeletons (as for the HMs, We2 = 0), respectively; Wd1 and Wd2 are the weights of the
6
dry RSHMs (or the HMs) and the skeletons (as for the HMs, Wd2 = 0), respectively. In order to determine the ion exchange capacities (IEC), the RSHMs were firstly incubated with 0.01 mol/L NaOH and 0.01 mol/L HCl solutions in turns for 4 times. DI water was used in between to remove the residual HCl or NaOH solution. Then, the microspheres were put into 0.01 mol/L HCl solution for 24 h. The theoretical IEC was calculated by assuming that the ammonium groups on the microspheres could be fully protonated. Then the HCl solution was titrated with a standard NaOH solution (0.001 mol/L). Then the IEC of the RSHMs was calculated by the following formula:
IEC (mequiv/g) =
× 1000
(3)
where MHCl is the molar concentration of the HCl solution; VHCl is the volume of the HCl titrated by residual NaOH solution; MNaOH is the molar concentration of the NaOH solution; VNaOH is the volume of the NaOH solution; and m is the dried weight of the RSHMs. The adsorption capacities of the RSHMs for anionic dyes were measured after immersing the samples in the dye solutions at different temperatures and pH values for 48 h. A UV-vis spectrometer (UV-1750, Shimadzu Co. Ltd, Japan) was used to detect dye concentration (MO: 505 nm; CR: 488 nm; AR: 521 nm). The adsorption capacities of the RSHMs for inorganic anions were measured after immersing the samples in inorganic anion solutions at room temperature for 48 h (Na2CrO4, CaCl2, and Na2SO4 solutions were used (100 μmol/L)). Titration was used to determine the adsorption capacity of the RSHMs (as for Cl-, AgNO3 solution was used (100 μmol/L), as for CrO42- and SO42-, Ba(NO3)2 solution was used (100 μmol/L)). The precipitate was centrifuged and dried, and the adsorption capacity of the RSHMs for the inorganic anions was calculated by the following equation: C=
(4)
7
where C is the adsorption capacity of the of the RSHMs for the inorganic anions; M1 is the relative molecular mass of the inorganic anions (as for CrO42-, SO42- and Cl-, M1 is 116, 96 and 36.5 g/mol, respectively); M2 is the relative molecular mass of the precipitate (as for BaCrO4, BaSO4 and AgCl, M2 is 253, 234 and 143 g/mol, respectively); P1 and P2 are the masses of the precipitates come from the solutions before and after adsorption; m is the mass of the RSHMs used for adsorption. and m0 represent the relative molecular mass of MO (g/mol) and the dry weight of the adsorption column (g), respectively; Vtotal is the influent dye volume (mL). To verify whether the column filled with the RSHMs owned effective adsorption in practical application, 1 g RSHMs were placed into a 10-mL chromatographic column to prepare an adsorption column. We studied dynamic adsorption of the microspheres to evaluate MO breakthrough behaviour. In brief, 10 L of MO solution (500 μmol/L) was passed through the column with a fast streaming rate of 5.0 mL/min, a UV-vis spectrophotometer was used to detect the MO concentration of the effluent. Breakthrough curves were obtained by plotting V (mL) against Ct/C0, where V (mL) is the influent dye volume, Ct (μmol/L) is the effluent dye concentration and C0 (μmol/L) is the influent dye concentration. After reaching the adsorption equilibrium, the adsorption capacity of the column was calculated by the following equation:
qe =
×
(5)
where qe is the adsorption capacity of the column (mg/g); M and m0 represent the relative molecular mass of MO (g/mol) and the dry weight of the adsorption column (g), respectively; Vtotal is the influent dye volume (mL). In order to verify that the RSHMs owned ultrahigh clearance rate for anionic dye, MO solution (100 μmol/L) was filtered by the column with a faster streaming rate at 15.0 mL/min, a UV-vis spectrophotometer was used to detect the MO concentration 8
of the effluent, and the filtered water was directly used for raising goldfish (the goldfish were bought from local market), then the water was changed by the new filtered water every day, and the survival of the goldfish was observed. Bilirubin was dissolved in 0.1 M NaOH solution and then diluted with phosphate buffer solution (PBS (pH=7.4)) to 0.15 mg/mL. Then, 1 mg RSHMs were applied in 10 mL bilirubin solution with oscillation at 37 °C. An UV–vis spectrometer was used to determine the concentration of bilirubin (438 nm). Results and discussion In order to explore the influences of the skeletons’ hydrophilia on the properties of the RSHMs, the PES and PES/PVP skeleton solutions were firstly prepared, and which were used to prepare the skeleton microspheres with different hydrophilia via electrospraying (as shown in Figure. 1A and B). In order to explore the water contact angles of the PES and PES/PVP, we firstly prepared PES and PES/PVP membranes via an spin-coating method, and the water contact angles (measured by a contact angle goniometer od OCA20, Data physics, Germany) of the PES and PES/PVP membranes were 81° and 55°, respectively, which indicated that the introduction of PVP enhanced the hydrophilia of PES obviously. The average diameter of the RSHMs was approximately 550 μm (as shown in Figure. 1D), and the surface and inner part of microspheres exhibited a porous structure (as shown in the Figure. 1E and F). In order to explore whether the PDMC hydrogels were filled into the skeletons uniformly, the energy dispersive spectrometer mapping (EDS mapping) was used. As shown in Figure. 1F, the Cl and N elements were distributed in the microspheres uniformly, which indicated that the uniform distribution of the PDMC hydrogel (the Cl and N elements came from the DMC hydrogel only). In addition, S element was uniformly distributed, indicating the PDMC hydrogel was uniformly distributed in the PES skeleton (the S element came from the PES skeleton only).
9
Figure 1. (A) The sketch of the electrospraying. (B) The preparation process of the skeleton microspheres. (C) The preparation process of the RSHMs. (D) The digital photograph of the RSHMs (PES/PVP/Gel). (E) The surface morphologies of the RHSMs (PES/PVP/Gel) (the top-right corner is the enlarged surface morphologie of the RHSMs). (F) The cross-section morphologies and EDS mapping analysis of the RSHMs (PES/PVP/Gel) corresponding to the white circle. The surface and cross-section morphologies were obtained from SEM.
The presences of PVP and PDMC in the RSHMs were detected by FTIR spectra. The peak at 1670 cm-1 was attributed to the -C=O in PVP [18], and the peak at 2950 cm-1 was corresponding to the –CH2Cl in PDMC (as shown in Figure. 2B) [19]. TGA was used to measured the components of the skeletons, and the contents of PVP in the skeletons of PES and PES/PVP were 0 and 42.35 wt. %, respectively (as shown in Figure. 2C); and the mass fractions of the PDMC hydrogels in the PES/Gel, PES/PVP/Gel were 81.4 and 78.2 wt. %, respectively; the slightly decrease of the PDMC hydrogel content in the PES/PVP/Gel was due to the presence of the PVP. To explore whether the root-like skeletons could effectively enhance the mechanical properties of the hydrogels, related mechanical property tests were performed. The skeleton microspheres showed high mechanical strength, and the 10
introduction of PVP did not reduce the mechanical strength of the PES obviously (as shown in Figure. S1). The stresses of the PES/Gel and PES/PVP/Gel (Figure. 2D and E) reached up to 37.2 MPa and 25.1 MPa with a strain of 70%, respectively. However, after compression, the HMs were crushed, and the compression yield stress of the HMs was less than 0.025 MPa, which indicated that the skeletons could effectively protect the hydrogel and enhance the compressive strength of the hydrogels over 1000 times, this phenomenon further ensured that the RSHMs would keep the dimensional stability under high working pressure. As we known, most of the ion exchange materials must be stored in dry and dark environment to retain their properties, which was mainly due to their weak dimension stability. In this study, the root-soil inspired structure effectively enhanced the dimension stability of the hydrogels. As shown in Figure. 2F and G, the hydrogels showed obviously dimensional change in dried and wet states, while the RSHMs showed great dimension stability. In previous report, PES showed high dimension stability [20], which could restrict the swelling and shrinking behaviors of the hydrogels under dried and wet states. Figure. 2H further indicated that the RSHMs exhibited excellent dimension stability in acidic, alkaline and adsorbing states; while the hydrogels showed obviously dimensional change. As shown in Figure. 2I, the swelling ratio of RSHMs was obviously restricted and less than 1.75 g/g under different pH conditions, but the change of the swelling behavior for HMs was obvious, and the swelling ratio of HMs was even more than 80 g/g under pH = 7. Table 1 summarized the proportional relationship between the dry weight, wet weight and volume of RSHMs and HMs (measured in DI water), which further indicated that the RSHMs would be easy to restore and occupy a smaller space during the using process.
11
Figure 2. (A) The graphic symbols of (B), (C), (D), (E) and (H). (B) The FTIR spectra for the skeletons, RSHMs, and hydrogels. (C) The TGA curves for the skeletons. (D) The compressive stress-strain curves of the RSHMs and (E) HMs. (F) The digital photographs and schematics of the HMs under dried and wet states. (G) The digital photographs and schematics of the RSHMs (PES/PVP/Gel) under dried and wet states. (H) The digital photographs of the HMs and RSHMs (PES/PVP/Gel) under different states. (I) The swelling and drying schematic of the RSHMs (PES/PVP/Gel) and HMs. Table 1. The proportional relationship between the dry weight, wet weight and volume of the RSHMs and HMs. Sample
Dry weight (g)
Wet weight (g)
Volume (cm3)
PES/Gel
1
1.38
1.70
PES/VP/Gel
1
1.56
1.95
12
DMC Gel
1
56.25
129.98
The foremost property of the ion exchange materials is IEC, in order to explore whether the root-soil inspired structure would influence the IECs of PDMC hydrogels, we compared the practical and theoretical IECs of the RSHMs. Figure. 3A exhibited the practical and theoretical IECs of the RSHMs, as shown, the IECs of the RSHMs with hydrophobic skeletons were far below the theoretical values; interestingly, the IECs of the RSHMs with hydrophilic skeleton were similar to the theoretical values, indicating the RSHMs with hydrophilic skeletons exhibited fewer influence on the IECs of the PDMC hydrogels. What is more, the IEC of the PES/PVP/Gel was more than 3.8 mequiv/g, which is the highest IEC among all the anion-exchange materials at present [21-23]. Benefiting from the ultrahigh IEC, the RSHMs exhibited high adsorption capacities of anionic dyes (Figure. 3B). The adsorption capacities of MO, CR and AR for the PES/Gel were 1022, 483 and 107.6 mg/g (according to dry weight), respectively; while which for the PES/PVP/Gel were 1491, 1693 and 204.7 mg/g (according to dry weight); indicating the interactions between the dye molecules and adsorption sites were affected by the hydrophobic PES skeletons, resulting in the decrease in
the equilibrium adsorption amount. Additionally, the adsorption
capacities of the anionic dyes for the PES/PVP/Gel were one of the highest levels [24,25]. For closing to the actual condition, we calculated the adsorption capacity according to wet weight or volume, and the adsorption capacities of the MO, CR and AR for the PES/Gel were 956, 1085, 130.8 mg/g (according to wet weight) and 764.6, 868.2, 104.9 mg/cm3, respectively; which were much higher than those for other materials used as anion-exchange materials [26,27].
13
Figure 3. (A) The practical and theoretical IECs of the RSHMs. (B) The adsorption capacities of MO, CR, and AR for the RSHMs. (C) Photos of the MO, CR and AR solutions before and after contacting with the RSHMs. (D) The adsorption amounts of MO for the RSHMs, HMs, and skeleton microspheres. (E) The adsorption amounts of MO for the PES/Gel and Gel/PES. (F) The adsorption amounts of MO for the PES/PVP/Gel and Gel/PES/PVP.
Furthermore, to investigate the adsorption mechanisms of the RSHMs, adsorption kinetics tests were performed. Figure. 3D exhibited the adsorption amounts of MO for the RSHMs, HMs, and skeleton microspheres, as shown, the PES/PVP/Gel showed the fastest adsorption rate and the highest adsorption amount among all the samples, while the PES/Gel showed slower adsorption rate and lower adsorption amount, and the skeletons had few adsorption amount of MO, indicating the PDMC hydrogel contributed to the adsorption capacities of the RSHMs mainly. Interestingly, the hydrogels showed lower adsorption rate and adsorption amount comparing with the RSHMs because the ammonium groups of the hydrogels were
14
combined with MO during the adsorbing process, then the hydrogels shrinked and which further lead to the decrease in the porosity. However, the root-soil structure ensured that the hydrogels in the RSHMs would not shrink obviously, and which further resulted in the excellent adsorbing effect of MO for the RSHMs (Figure. 2I).
Figure 4. (A) The adsorption capacities of Cl-, SO42-, and CrO42- for the RSHMs (the concentrations of the Cl-, SO42-, and CrO42- solution were 500 μM). (B) The effects of temperature on adsorption amounts. (C) The effects of pH value on adsorption amounts. (D) The photos of the adsorption and desorption for the RSHMs. (E) The recyclability of the RSHMs. (Cycle number: 3)
Afterwards, the separation of inorganic anions by the RSHMs was also explored. As shown in Figure. 4A, the adsorption amounts of Cl-, SO42-, and CrO42- for the RSHMs were 72, 54, and 288 mg/g, respectively. Besides, the effects of temperature on adsorption amounts were explored, as shown in Figure. 4B, the adsorption amounts decreased slightly with the increase in temperature, because the adsorption process was a exothermic process, this phenomenon further indicated that the RSHMs might have a wide applicable range of temperature. After, the effects of pH value on adsorption amounts were also explored, as shown in Figure. 4C, the adsorption amounts decreased obviously with the increase in pH value, and the adsorption
15
amount for the RSHMs even decreased to 92 mg/g, which was meanly due to the deprotonation under an alkaline environment [16]. Since the adsorption amount of the RSHMs was very sensitive to the pH value of the environment, it was worthy to explore the recycling ability of the RSHMs by changing the pH value of the environment. As shown in Figure. 4D, after adsorption, the color of solution changed from orange to colourless, then the desorption process of the RSHMs was carried out by changing the pH of the solution, and the color of solution changed from colourless to orange. The adsorption and desorption ratios of the RSHMs reached more than 90% for three times (Figure. 4E). These results demonstrate that the RSHMs can be reused again by washing with NaOH solution. Considering the low cost of DMC and the high removal ratio of the RSHMs, the microspheres may be suitable for use in industrial wastewater treatment. For further exploring whether the RSHMs could be qualified for the practical water purification, the PES/PVP/Gel were filled into a chromatographic column (10 mL) for fabricating the adsorption column (column diameter = 1 cm, column height = 1.8 cm). As shown in Figure. 5A (Video S1), the adsorption column exhibited a high removal ratio (more than 95 %) for the MO solution (more than 2 L), then the removal ratios decreased with the increase in the volumes for the MO solutions because the adsorption of the RSHMs gradually became saturated with the increment of the solution volumes; the maximum adsorption capacity for MO of the adsorption column was 1151 mg/g, which was similar to the static adsorption capacity of the RSHMs (1491 mg/g). The breakthrough behavior of the adsorption column further indicated that the RSHMs might exhibit stable and efficient adsorption capacity for anionic dyes in actual industrial applications. In order to explore the clearance rate of MO for the RSHMs, goldfish were placed in the filtered water and original MO solution. As shown in Figure. 5C (Video S2), the filtered water was clean and colourless, and the goldfish placed in the filtered
16
water was alive even after a week, the UV-vis spectra of the MO solution before and after filtration also suggested that the MO dye was almost removed completely (nearly 100%). These phenomenons indicated that the adsorption column owned ultrahigh clearance rate for MO, which further demonstrated that the RSHMs exhibited great advantages in water remediation. For further expanding the application of the RSHMs in biomedicine application, bilirubin adsorption tests of the RSHMs were performed. Bilirubin is a pathogenic toxin in the blood of patients. Until now, heamoperfusion is the most common method for removing bilirubin from the blood, and high-preformanced bilirubin adsorbent is the key component in the heamoperfusion [28-30]. Many researches have been carried out to prepare the bilirubin adsorbents, but their adsorption capacities are still relatively low [31,32]. According previous reports, bilirubin could be removed via electrostatic interaction (Figure. 5D) [33-35]. As shown in Figure. 5E, the adsorption capacities could reach about 72.4 mg/g for the PES/Gel and 131.6 mg/g for the PES/PVP/Gel, respectively; and the skeletons showed no obviously adsorption capacities for bilirubin. Benefiting from the ultrahigh IEC of the PES/PVP/Gel, the PES/PVP/Gel exhibited a relatively high adsorption capacity for bilirubin. Thus the RSHMs may also show great advantages in heamoperfusion.
17
Figure 5. (A) The breakthrough curves of adsorption column (MO concentration = 500 μM). (B) The filtering photo of the adsorption column. (C) Photos of goldfish lived in the filtered MO solutions and UV-vis spectra of the MO solution before and after filtration,. (D) The schematic of the bilirubin adsorption. (E) The bilirubin adsorption capacities of the skeletons and RSHMs.
Conclusion Hydrogel materials commonly exhibited poor dimensional stability. In this study, inspired by the root-soil structure, reinforced PDMC hydrogel microspheres were successfully fabricated via combining electrospray and phase conversion. Comparing with pure PDMC hydrogel, this novel structure effectively increased the compression strength and decreased the swelling ratios of the hydrogels. Additionally, the reinforced PDMC hydrogel microspheres owned the highest IEC comparing with other anion-exchange materials [21-23] and exhibited effective adsorption for anionic dyes. What is more, according to the breakthrough curves, the adsorption column filled with the microspheres also showed high clearance ratio and stable adsorption for anionic dyes. Also, the microspheres showed high adsorption capacities for bilirubin. The raw materials to prepare the reinforced PDMC hydrogel microspheres were cheap industrial chemicals. Thus, it is believed that the root-soil structure inspired PDMC hydrogel microspheres with ultra-high performances have great advantages in the application as anion-exchange materials. In our further study, we will fabricate the reinforced heparin-mimcking hydrogel microspheres with anticoagulation property, and we except they can be further used in heparin-free blood purification.
Acknowledgments This work was financially sponsored by the National Natural Science Foundation of China (No. 51673125 and 51773127), the State Key Research Development Programme of China (Grant Nos. 2016YFC1103000), State Key Laboratory for 18
Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1619) and State Key Laboratory of Polymer Materials Engineering (No. sklpme2017-3-07).
Supporting Information The
following
are
the
supplementary
data
related
to
this
article:
pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models, Figure.S1, and Video.S1-S2.
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[20] M. Lapelosa, T.W. Patapoff, I.E. Zarraga, Modeling of protein-anion exchange resin interaction for the human growth hormone charge variants, Biophys. Chem., 207 (2015) 1-6. [21] P. Rekha, R. Muhammad, V. Sharma, M. Ramteke, P. Mohanty, Unprecedented adsorptive removal of Cr2O72- and methyl orange by using a low surface area organosilica, J. Mater. Chem. A, 4 (2016) 17866-17874. [22] W.C. Wan, R.Y. Zhang, W. Li, H. Liu, Y.H. Lin, L.N. Li, Y. Zhou, Graphene-carbon nanotube aerogel as an ultra-light, compressible and recyclable highly efficient absorbent for oil and dyes, Environ.-Sci. Nano, 3 (2016) 107-113. [23] L. Gan, S.M. Shang, E.L. Hu, C. Wah, M. Yuen, S.X. Jiang, Konjac glucomannan/graphene oxide hydrogel with enhanced dyes adsorption capability for methyl blue and methyl orange, Appl. Surf. Sci., 357 (2015) 866-872. [24] G.A. Mahmoud, S.E. Abdel-Aal, N.A. Badway, A.A. Elbayaa, D.F. Ahmed, A novel hydrogel based on agricultural waste for removal of hazardous dyes from aqueous solution and reuse process in a secondary adsorption, Polym. Bull., 74 (2017) 337-358. [25] K.G. Bhattacharyya, A. Sharma, Kinetics and thermodynamics of Methylene Blue adsorption on Neem (Azadirachta indica) leaf powder, Dyes Pigment., 65 (2005) 51-59. [26] Y.S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem., 34 (1999) 451-465. [27] A. Pal, T. Nasim, A. Giri, A. Bandyopadhyay, Polyelectrolytic aqueous guar gum for adsorptive separation of soluble Pb(II) from contaminated water, Carbohydr. Polym., 110 (2014) 224-230. [28] E.S. Kim, S.W. Lee, E.Y. Mo, S.D. Moon, J.H. Han, Inverse association between serum total bilirubin levels and diabetic peripheral neuropathy in patients with type 2 diabetes, Endocrine, 50 (2015) 405-412. [29] M. Mayer, Association of serum bilirubin concentration with risk of coronary artery disease, Clin. Chem., 46 (2000) 1723-1727. [30] Z.T. Li, X. Song, S.Y. Cui, Y.P. Jiao, C.R. Zhou, Fabrication of macroporous reduced graphene oxide composite aerogels reinforced with chitosan for high bilirubin adsorption, Rsc Adv., 8 (2018) 8338-8348. [31] R. Zhao, Y.M. Li, X. Li, Y.Z. Li, B.L. Sun, S. Chao, C. Wang, Facile hydrothermal synthesis of branched polyethylenimine grafted electrospun polyacrylonitrile fiber membrane as a highly efficient and reusable bilirubin adsorbent in hemoperfusion, J. Colloid Interface Sci., 514 (2018) 675-685.
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[32] I. Mazurenko, K. Monsalve, J. Rouhana, P. Parent, C. Laffon, A. Le Goff, S. Szunerits, R. Boukherroub, M.T. Giudici-Orticoni, N. Mano, E. Lojou, How the Intricate Interactions between Carbon Nanotubes and Two Bilirubin Oxidases Control Direct and Mediated O-2 Reduction, ACS Appl. Mater. Interfaces, 8 (2016) 23074-23085. [33] X. Jiang, D.X. Zhou, X.L. Huang, W.F. Zhao, C.S. Zhao, Hexanediamine functionalized poly (glycidyl methacrylate-co-N-inylpyrrolidone) particles for bilirubin removal, J. Colloid Interface Sci., 504 (2017) 214-222. [34] X. Song, K. Wang, C.Q. Tang, W.W. Yang, W.F. Zhao, C.S. Zhao, Design of Carrageenan-Based Heparin-Mimetic Gel Beads as Self-Anticoagulant Hemoperfusion Adsorbents, Biomacromolecules, 19 (2018) 1966-1978. [35] Y. Wang, X. Huang, C. He, Y. Li, W. Zhao, C. Zhao, Design of carboxymethyl chitosan-based heparin-mimicking cross-linked beads for safe and efficient blood purification, Int. J. Biol. Macromol., 117 (2018) 392-400.
22
S0021-9797(18)30951-2 https://doi.org/10.1016/j.jcis.2018.08.036 YJCIS 23969
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
18 May 2018 9 August 2018 10 August 2018
Please cite this article as: H. Ji, X. Song, C. He, C. Tang, L. Xiong, W. Zhao, C. Zhao, Root-soil Structure Inspired Hydrogel Microspheres with High Dimensional Stability and Anion-Exchange Capacity, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.08.036
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Root-soil Structure Inspired Hydrogel Microspheres with High Dimensional Stability and Anion-Exchange Capacity Haifeng Ji,a,# Xin Song,a,# Chao He, a Chengqiang Tang, a Lian Xiong, a Weifeng Zhao,a,b,* and Changsheng Zhao.a,*
a
College of Polymer Science and Engineering, State Key Laboratory of Polymer
Materials Engineering, Sichuan University, Chengdu, 610065, People’s Republic of China b
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
Donghua University, Shanghai, 201620, People’s Republic of China *Corresponding author. E-mail: [email protected] (W-F. Zhao*); [email protected] (C-S. Zhao**) Tel.: +86-28-85400453; Fax: +86-28-85405402. #
These authors contributed equally to this work.
1
Keywords:
Root-soil
structure;
Poly(methacryloxyethyltrimethyl
ammonium
chloride); Hydrogel microsphere; Ultrahigh ion exchange capacity; Restricted swelling behavior; Ultrahigh mechanical strength
Abstract: Ion exchange materials show great advantages in water purification, food industry, pharmaceutical industry, etc. However, the ion exchange capacities of ion exchange materials, especially for anion-exchange materials, at present are still relatively low. Hydrogels own abundant functional groups and show high hydrophilicity, and thus are recognized as high-potential ion exchange materials, but may deform and even crush in use due to their low mechanical strength and unavoidable swelling behavior. In this study, inspired by the root-soil structure, novel poly(methacryloxyethyltrimethyl
ammonium
chloride)
composite
hydrogel
microspheres with ultrahigh ion exchange capacity (more than 3.8 meuqiv/g), low swelling ratio (less than 1.5 g/g under pH=7), and ultrahigh mechanical strength (more than 28.1 MPa) were prepared. The microspheres showed efficient adsorption for anionic dyes (1491 mg/g for methyl orange, 1693 mg/g for Congo red, and 204.7 mg/g for amaranth, respectively) and great adsorption for bilirubin (131.6 mg/g). Taken together, the hydrogel microspheres were qualified as stable and high-efficiency ion exchange materials. More importantly, the root-soil structure opens up avenues for enhancing the dimensional stability of functional hydrogels.
Introduction Ion exchange materials exhibit wide applications in water purification, food industry, pharmaceutical industry, etc., since they are firstly invented by Adams and Holms [1-3]. The main property of ion exchange materials is the ion exchange capacity (IEC), but commercial ion-exchange materials still do not process enough IEC, especially for anion exchange materials. For example, the IEC of the DOWEX 2
550A is less than 1.1 mequiv/g. Thus, great efforts have been made to enhance the IEC
of
anion
exchange
materials.
Lin
et
al.
synthesized
a
set
of
imidazolium-functionalized poly(arylene ether sulfone)s, and the IECs were in the range of 1.35-1.84 mequiv/g [4]. Recently, Knauss et al. reported an anion-exchange materials
of
poly(2,6-dimethyl-1,4-phenylene
oxide)-b-poly(vinylbenzyltrimeth
-lammonium), and the IEC was about 2.9 mequiv/g [5]. Though these works enhanced the IECs of the anion-exchange materials, but still relatively low. Functional hydrogels could show ultrahigh IECs owing to their abundant functional groups and high hydrophilicity, and thus are recognized as a new generation of ion exchange materials [6,7]. However, the weak mechanical strength and high swelling ratio of the hydrogels seriously restrict their applications, and they may deform and even crush in the use as ion exchange materials. Recently, many researchers make their efforts in enhancing the mechanical strength of hydrogels by means of semi interpenetrating structure, hydrophobic interactions, multivalent host-guest interactions, aromatic stacking, hydrogen bonds, etc. [8-10], but these methods cannot prevent the deformation of the hydrogels. Additionally, these methods usually have specific requirements in the monomers. Thus, new approach towards dimensionally stable ion exchange materials with commonly used monomers such as acrylic
acid
(AA),
2-acrylanmido-2-methylpropanesulfonic
acid
and
methacryloxyethyltrimethyl ammonium chloride (DMC), are strongly desired. In nature, soil is easily striked by water and which further triggers soil erosion. Afforestation is an effective way to prevent the soil erosion due to the strong interaction between root and soil. Additionally, in ancient Chinese architecture, pure soil could not be used to build house, but people could effectively build strong houses by using thatch reinforced soil. These phenomenons inspired us to use the materials with high mechanical strength and dimensional stability to reinforce the materials with poor mechanical strength [11-13]. If the hydrogels with poor mechanical strength and low dimensional stability were used as soil, and polymer materials with good 3
mechanical property and good dimensional stability were used as root, then root-soil structure materials could be prepared. Recently, the hydrogels could be reinforced by cellulose or glass fiber, but these methods still cannot effectively restrict the swelling behavior of the hydrogels, which were mainly attributed to the heterogeneous mixture of the materials [14,15]. Additionally, the hydrogels could not be produced in a microspherical shape as ion exchange materials by these methods. Thus, how to prepare hydrogel microspheres with high performance still be a challenge. In our previous work, we filled the PAA hydrogel into the skeletons with high mechanical strength and hydrophobicity uniformly, and we termed this structure as root-soil inspired structure [16]. As the results, the hydrogels were effectively protected by the skeletons, and the swelling behaviors of the hydrogels were also limited by the hydrophobic skeletons. But the universality of this method remained to be studied, we hoped that this method could also be used to prepared high-performanced hydrogel microspheres, especially for the anion-exchange hydrogel microspheres. The aim of this study is to prepare the root-soil structure inspired anion-exchange hydrogel
microspheres.
First,
polyethersulfone
(PES)
was
dissolved
in
N,N'-dimethylacetamide (DMAc), and then dropped into a reaction solution (containing the monomer DMC, the solvent deionized water, cross-linker and initiator), and the skeleton microspheres were obtained via phase inversion [16]. In the phase inversion process, the reaction solution replaced the DMAc rapidly to ensure the uniformly dispersion of the reaction solution in the skeleton. Then the reaction solution filled into the skeleton microspheres were heated to initiate the in-situ crosslinking polymerization, thus the hydrogels were mixed with the skeletons uniformly to form the root-soil structure. In this system, the hydrophobic PES were performed as roots to improve the mechanical properties and decrease the swelling of the PDMC hydrogels. We anticipated that this structure could endow the hydrogels with ultrahigh dimensional stability, but still retain the high IEC of the hydrogels. 4
Experimental Polyethersulfone (PES, Ultrason E6020P) was obtained from BASF chemical company.
N-vinyl-2-pyrrolidone
(VP),
methacryloxyethyltrimethyl ammonium
chloride (DMC), N,N'-methylidenebis (acrylamide) (MBA), 2,2-azobisisobutyronitrile (AIBN) and ammonium persulfate (APS) were purchased from Aladdin Reagent Co. Ltd. N,N'-Dimethylacetamide (DMAc) and sodium dodecyl sulfate (SDS) were obtained from Chengdu Kelong Chemical Reagent Co. Ltd. Dyes (methyl orange (MO), amaranth (AR), Congo red (CR)) were purchased from Aladdin reagent Co. Ltd. Deionized (DI) water was used through the whole study. All the chemicals were used as received. To fabricate PES/PVP skeleton, an in-situ crosslinking polymerization of PVP was performed. Typically, 4 g PES, 4 g PVP, 0.1g AIBN and 0.1 g MBA were dissolved in 46 g DMAc, the PVP was used to enhance the hydrophilia of PES. The polymerization was performed at 120 °C for 36 h. To fabricate PES skeleton, 4 g PES was dissolved in 46 g DMAc. To prepare the skeleton microspheres, different skeleton solutions were dropped into the mixture (1 g SDS, 280 g DMC, 120 g DI water, 6 g MBA and 1.2 g APS) via electrospraying, and the DMAc would be replaced by the reaction solution during the phase inversion. Then the received microspheres were kept at 80 °C for 1 h for the fabrication of the root-soil structure inspired hydrogel microspheres (RSHMs) (as shown in Figure. 1A). The RSHMs with PES and PES/PVP skeletons were termed as PES/Gel and PES/PVP/Gel, respectively. Then the hydrogel microspheres (HMs) were prepared by dissolving the skeletons of the RSHMs in DMAc, and the HMs fabricated from PES/Gel and PES/PVP/Gel were named as Gel/PES and Gel/PES/PVP, respectively. The two kinds of HMs showed similar properties, and we choose the Gel/PES/PVP for further characterization (except for the adsorption kinetics tests).
5
For Fourier transform infrared spectroscopy (FTIR) analysis, the RSHMs, the hydrogel and skeleton microspheres were completely dried, and then the FTIR spectra between 1000 and 4000 cm−1 were obtained from a FTIR spectrometer (Nicolet 560, USA). AQ500 thermogravimetric analyzer (TG209F1, Germany) was used for thermogravimetric analysis under a dry N2 atmosphere from 50 to 750 °C at the heating speed of 10 °C/min. In order to explore the contents of the hydrogels in the RSHMs, the RSHMs were dried and weighed (WR), and then applied into DMAc to dissolve the PES skeletons to prepare pure hydrogels, then the hydrogels were dried and weighed (WH), the content (C) of the hydrogels in the RSHMs was calculated using the following formula: C (%) =
× 100 %
(1)
The cross-section morphologies of the RSHMs were obtained from a scanning electron microscope (SEM, JSM-7500F, JEOL, Japan). Then, the mechanical properties of the microspheres were investigated. After being stored in DI water for 24 h, the compressive strengths of the RSHMs and HMs were measured by a universal tensile testing machine (SANS CMT4000) at room temperature with a constant speed of 1 mm/min under a 1000 N load cell. The swelling behaviors of the RSHMs and HMs were measured by a gravimetric method. The wet RSHMs and HMs were dried at 60 °C more than 2 days; and then the equilibrium water uptake (EWU) was defined as the following modified formula [17]:
EWU (%) =
× 100 %
(2)
where We1 and We2 are the weights of the wet RSHMs (or the HMs) and the skeletons (as for the HMs, We2 = 0), respectively; Wd1 and Wd2 are the weights of the
6
dry RSHMs (or the HMs) and the skeletons (as for the HMs, Wd2 = 0), respectively. In order to determine the ion exchange capacities (IEC), the RSHMs were firstly incubated with 0.01 mol/L NaOH and 0.01 mol/L HCl solutions in turns for 4 times. DI water was used in between to remove the residual HCl or NaOH solution. Then, the microspheres were put into 0.01 mol/L HCl solution for 24 h. The theoretical IEC was calculated by assuming that the ammonium groups on the microspheres could be fully protonated. Then the HCl solution was titrated with a standard NaOH solution (0.001 mol/L). Then the IEC of the RSHMs was calculated by the following formula:
IEC (mequiv/g) =
× 1000
(3)
where MHCl is the molar concentration of the HCl solution; VHCl is the volume of the HCl titrated by residual NaOH solution; MNaOH is the molar concentration of the NaOH solution; VNaOH is the volume of the NaOH solution; and m is the dried weight of the RSHMs. The adsorption capacities of the RSHMs for anionic dyes were measured after immersing the samples in the dye solutions at different temperatures and pH values for 48 h. A UV-vis spectrometer (UV-1750, Shimadzu Co. Ltd, Japan) was used to detect dye concentration (MO: 505 nm; CR: 488 nm; AR: 521 nm). The adsorption capacities of the RSHMs for inorganic anions were measured after immersing the samples in inorganic anion solutions at room temperature for 48 h (Na2CrO4, CaCl2, and Na2SO4 solutions were used (100 μmol/L)). Titration was used to determine the adsorption capacity of the RSHMs (as for Cl-, AgNO3 solution was used (100 μmol/L), as for CrO42- and SO42-, Ba(NO3)2 solution was used (100 μmol/L)). The precipitate was centrifuged and dried, and the adsorption capacity of the RSHMs for the inorganic anions was calculated by the following equation: C=
(4)
7
where C is the adsorption capacity of the of the RSHMs for the inorganic anions; M1 is the relative molecular mass of the inorganic anions (as for CrO42-, SO42- and Cl-, M1 is 116, 96 and 36.5 g/mol, respectively); M2 is the relative molecular mass of the precipitate (as for BaCrO4, BaSO4 and AgCl, M2 is 253, 234 and 143 g/mol, respectively); P1 and P2 are the masses of the precipitates come from the solutions before and after adsorption; m is the mass of the RSHMs used for adsorption. and m0 represent the relative molecular mass of MO (g/mol) and the dry weight of the adsorption column (g), respectively; Vtotal is the influent dye volume (mL). To verify whether the column filled with the RSHMs owned effective adsorption in practical application, 1 g RSHMs were placed into a 10-mL chromatographic column to prepare an adsorption column. We studied dynamic adsorption of the microspheres to evaluate MO breakthrough behaviour. In brief, 10 L of MO solution (500 μmol/L) was passed through the column with a fast streaming rate of 5.0 mL/min, a UV-vis spectrophotometer was used to detect the MO concentration of the effluent. Breakthrough curves were obtained by plotting V (mL) against Ct/C0, where V (mL) is the influent dye volume, Ct (μmol/L) is the effluent dye concentration and C0 (μmol/L) is the influent dye concentration. After reaching the adsorption equilibrium, the adsorption capacity of the column was calculated by the following equation:
qe =
×
(5)
where qe is the adsorption capacity of the column (mg/g); M and m0 represent the relative molecular mass of MO (g/mol) and the dry weight of the adsorption column (g), respectively; Vtotal is the influent dye volume (mL). In order to verify that the RSHMs owned ultrahigh clearance rate for anionic dye, MO solution (100 μmol/L) was filtered by the column with a faster streaming rate at 15.0 mL/min, a UV-vis spectrophotometer was used to detect the MO concentration 8
of the effluent, and the filtered water was directly used for raising goldfish (the goldfish were bought from local market), then the water was changed by the new filtered water every day, and the survival of the goldfish was observed. Bilirubin was dissolved in 0.1 M NaOH solution and then diluted with phosphate buffer solution (PBS (pH=7.4)) to 0.15 mg/mL. Then, 1 mg RSHMs were applied in 10 mL bilirubin solution with oscillation at 37 °C. An UV–vis spectrometer was used to determine the concentration of bilirubin (438 nm). Results and discussion In order to explore the influences of the skeletons’ hydrophilia on the properties of the RSHMs, the PES and PES/PVP skeleton solutions were firstly prepared, and which were used to prepare the skeleton microspheres with different hydrophilia via electrospraying (as shown in Figure. 1A and B). In order to explore the water contact angles of the PES and PES/PVP, we firstly prepared PES and PES/PVP membranes via an spin-coating method, and the water contact angles (measured by a contact angle goniometer od OCA20, Data physics, Germany) of the PES and PES/PVP membranes were 81° and 55°, respectively, which indicated that the introduction of PVP enhanced the hydrophilia of PES obviously. The average diameter of the RSHMs was approximately 550 μm (as shown in Figure. 1D), and the surface and inner part of microspheres exhibited a porous structure (as shown in the Figure. 1E and F). In order to explore whether the PDMC hydrogels were filled into the skeletons uniformly, the energy dispersive spectrometer mapping (EDS mapping) was used. As shown in Figure. 1F, the Cl and N elements were distributed in the microspheres uniformly, which indicated that the uniform distribution of the PDMC hydrogel (the Cl and N elements came from the DMC hydrogel only). In addition, S element was uniformly distributed, indicating the PDMC hydrogel was uniformly distributed in the PES skeleton (the S element came from the PES skeleton only).
9
Figure 1. (A) The sketch of the electrospraying. (B) The preparation process of the skeleton microspheres. (C) The preparation process of the RSHMs. (D) The digital photograph of the RSHMs (PES/PVP/Gel). (E) The surface morphologies of the RHSMs (PES/PVP/Gel) (the top-right corner is the enlarged surface morphologie of the RHSMs). (F) The cross-section morphologies and EDS mapping analysis of the RSHMs (PES/PVP/Gel) corresponding to the white circle. The surface and cross-section morphologies were obtained from SEM.
The presences of PVP and PDMC in the RSHMs were detected by FTIR spectra. The peak at 1670 cm-1 was attributed to the -C=O in PVP [18], and the peak at 2950 cm-1 was corresponding to the –CH2Cl in PDMC (as shown in Figure. 2B) [19]. TGA was used to measured the components of the skeletons, and the contents of PVP in the skeletons of PES and PES/PVP were 0 and 42.35 wt. %, respectively (as shown in Figure. 2C); and the mass fractions of the PDMC hydrogels in the PES/Gel, PES/PVP/Gel were 81.4 and 78.2 wt. %, respectively; the slightly decrease of the PDMC hydrogel content in the PES/PVP/Gel was due to the presence of the PVP. To explore whether the root-like skeletons could effectively enhance the mechanical properties of the hydrogels, related mechanical property tests were performed. The skeleton microspheres showed high mechanical strength, and the 10
introduction of PVP did not reduce the mechanical strength of the PES obviously (as shown in Figure. S1). The stresses of the PES/Gel and PES/PVP/Gel (Figure. 2D and E) reached up to 37.2 MPa and 25.1 MPa with a strain of 70%, respectively. However, after compression, the HMs were crushed, and the compression yield stress of the HMs was less than 0.025 MPa, which indicated that the skeletons could effectively protect the hydrogel and enhance the compressive strength of the hydrogels over 1000 times, this phenomenon further ensured that the RSHMs would keep the dimensional stability under high working pressure. As we known, most of the ion exchange materials must be stored in dry and dark environment to retain their properties, which was mainly due to their weak dimension stability. In this study, the root-soil inspired structure effectively enhanced the dimension stability of the hydrogels. As shown in Figure. 2F and G, the hydrogels showed obviously dimensional change in dried and wet states, while the RSHMs showed great dimension stability. In previous report, PES showed high dimension stability [20], which could restrict the swelling and shrinking behaviors of the hydrogels under dried and wet states. Figure. 2H further indicated that the RSHMs exhibited excellent dimension stability in acidic, alkaline and adsorbing states; while the hydrogels showed obviously dimensional change. As shown in Figure. 2I, the swelling ratio of RSHMs was obviously restricted and less than 1.75 g/g under different pH conditions, but the change of the swelling behavior for HMs was obvious, and the swelling ratio of HMs was even more than 80 g/g under pH = 7. Table 1 summarized the proportional relationship between the dry weight, wet weight and volume of RSHMs and HMs (measured in DI water), which further indicated that the RSHMs would be easy to restore and occupy a smaller space during the using process.
11
Figure 2. (A) The graphic symbols of (B), (C), (D), (E) and (H). (B) The FTIR spectra for the skeletons, RSHMs, and hydrogels. (C) The TGA curves for the skeletons. (D) The compressive stress-strain curves of the RSHMs and (E) HMs. (F) The digital photographs and schematics of the HMs under dried and wet states. (G) The digital photographs and schematics of the RSHMs (PES/PVP/Gel) under dried and wet states. (H) The digital photographs of the HMs and RSHMs (PES/PVP/Gel) under different states. (I) The swelling and drying schematic of the RSHMs (PES/PVP/Gel) and HMs. Table 1. The proportional relationship between the dry weight, wet weight and volume of the RSHMs and HMs. Sample
Dry weight (g)
Wet weight (g)
Volume (cm3)
PES/Gel
1
1.38
1.70
PES/VP/Gel
1
1.56
1.95
12
DMC Gel
1
56.25
129.98
The foremost property of the ion exchange materials is IEC, in order to explore whether the root-soil inspired structure would influence the IECs of PDMC hydrogels, we compared the practical and theoretical IECs of the RSHMs. Figure. 3A exhibited the practical and theoretical IECs of the RSHMs, as shown, the IECs of the RSHMs with hydrophobic skeletons were far below the theoretical values; interestingly, the IECs of the RSHMs with hydrophilic skeleton were similar to the theoretical values, indicating the RSHMs with hydrophilic skeletons exhibited fewer influence on the IECs of the PDMC hydrogels. What is more, the IEC of the PES/PVP/Gel was more than 3.8 mequiv/g, which is the highest IEC among all the anion-exchange materials at present [21-23]. Benefiting from the ultrahigh IEC, the RSHMs exhibited high adsorption capacities of anionic dyes (Figure. 3B). The adsorption capacities of MO, CR and AR for the PES/Gel were 1022, 483 and 107.6 mg/g (according to dry weight), respectively; while which for the PES/PVP/Gel were 1491, 1693 and 204.7 mg/g (according to dry weight); indicating the interactions between the dye molecules and adsorption sites were affected by the hydrophobic PES skeletons, resulting in the decrease in
the equilibrium adsorption amount. Additionally, the adsorption
capacities of the anionic dyes for the PES/PVP/Gel were one of the highest levels [24,25]. For closing to the actual condition, we calculated the adsorption capacity according to wet weight or volume, and the adsorption capacities of the MO, CR and AR for the PES/Gel were 956, 1085, 130.8 mg/g (according to wet weight) and 764.6, 868.2, 104.9 mg/cm3, respectively; which were much higher than those for other materials used as anion-exchange materials [26,27].
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Figure 3. (A) The practical and theoretical IECs of the RSHMs. (B) The adsorption capacities of MO, CR, and AR for the RSHMs. (C) Photos of the MO, CR and AR solutions before and after contacting with the RSHMs. (D) The adsorption amounts of MO for the RSHMs, HMs, and skeleton microspheres. (E) The adsorption amounts of MO for the PES/Gel and Gel/PES. (F) The adsorption amounts of MO for the PES/PVP/Gel and Gel/PES/PVP.
Furthermore, to investigate the adsorption mechanisms of the RSHMs, adsorption kinetics tests were performed. Figure. 3D exhibited the adsorption amounts of MO for the RSHMs, HMs, and skeleton microspheres, as shown, the PES/PVP/Gel showed the fastest adsorption rate and the highest adsorption amount among all the samples, while the PES/Gel showed slower adsorption rate and lower adsorption amount, and the skeletons had few adsorption amount of MO, indicating the PDMC hydrogel contributed to the adsorption capacities of the RSHMs mainly. Interestingly, the hydrogels showed lower adsorption rate and adsorption amount comparing with the RSHMs because the ammonium groups of the hydrogels were
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combined with MO during the adsorbing process, then the hydrogels shrinked and which further lead to the decrease in the porosity. However, the root-soil structure ensured that the hydrogels in the RSHMs would not shrink obviously, and which further resulted in the excellent adsorbing effect of MO for the RSHMs (Figure. 2I).
Figure 4. (A) The adsorption capacities of Cl-, SO42-, and CrO42- for the RSHMs (the concentrations of the Cl-, SO42-, and CrO42- solution were 500 μM). (B) The effects of temperature on adsorption amounts. (C) The effects of pH value on adsorption amounts. (D) The photos of the adsorption and desorption for the RSHMs. (E) The recyclability of the RSHMs. (Cycle number: 3)
Afterwards, the separation of inorganic anions by the RSHMs was also explored. As shown in Figure. 4A, the adsorption amounts of Cl-, SO42-, and CrO42- for the RSHMs were 72, 54, and 288 mg/g, respectively. Besides, the effects of temperature on adsorption amounts were explored, as shown in Figure. 4B, the adsorption amounts decreased slightly with the increase in temperature, because the adsorption process was a exothermic process, this phenomenon further indicated that the RSHMs might have a wide applicable range of temperature. After, the effects of pH value on adsorption amounts were also explored, as shown in Figure. 4C, the adsorption amounts decreased obviously with the increase in pH value, and the adsorption
15
amount for the RSHMs even decreased to 92 mg/g, which was meanly due to the deprotonation under an alkaline environment [16]. Since the adsorption amount of the RSHMs was very sensitive to the pH value of the environment, it was worthy to explore the recycling ability of the RSHMs by changing the pH value of the environment. As shown in Figure. 4D, after adsorption, the color of solution changed from orange to colourless, then the desorption process of the RSHMs was carried out by changing the pH of the solution, and the color of solution changed from colourless to orange. The adsorption and desorption ratios of the RSHMs reached more than 90% for three times (Figure. 4E). These results demonstrate that the RSHMs can be reused again by washing with NaOH solution. Considering the low cost of DMC and the high removal ratio of the RSHMs, the microspheres may be suitable for use in industrial wastewater treatment. For further exploring whether the RSHMs could be qualified for the practical water purification, the PES/PVP/Gel were filled into a chromatographic column (10 mL) for fabricating the adsorption column (column diameter = 1 cm, column height = 1.8 cm). As shown in Figure. 5A (Video S1), the adsorption column exhibited a high removal ratio (more than 95 %) for the MO solution (more than 2 L), then the removal ratios decreased with the increase in the volumes for the MO solutions because the adsorption of the RSHMs gradually became saturated with the increment of the solution volumes; the maximum adsorption capacity for MO of the adsorption column was 1151 mg/g, which was similar to the static adsorption capacity of the RSHMs (1491 mg/g). The breakthrough behavior of the adsorption column further indicated that the RSHMs might exhibit stable and efficient adsorption capacity for anionic dyes in actual industrial applications. In order to explore the clearance rate of MO for the RSHMs, goldfish were placed in the filtered water and original MO solution. As shown in Figure. 5C (Video S2), the filtered water was clean and colourless, and the goldfish placed in the filtered
16
water was alive even after a week, the UV-vis spectra of the MO solution before and after filtration also suggested that the MO dye was almost removed completely (nearly 100%). These phenomenons indicated that the adsorption column owned ultrahigh clearance rate for MO, which further demonstrated that the RSHMs exhibited great advantages in water remediation. For further expanding the application of the RSHMs in biomedicine application, bilirubin adsorption tests of the RSHMs were performed. Bilirubin is a pathogenic toxin in the blood of patients. Until now, heamoperfusion is the most common method for removing bilirubin from the blood, and high-preformanced bilirubin adsorbent is the key component in the heamoperfusion [28-30]. Many researches have been carried out to prepare the bilirubin adsorbents, but their adsorption capacities are still relatively low [31,32]. According previous reports, bilirubin could be removed via electrostatic interaction (Figure. 5D) [33-35]. As shown in Figure. 5E, the adsorption capacities could reach about 72.4 mg/g for the PES/Gel and 131.6 mg/g for the PES/PVP/Gel, respectively; and the skeletons showed no obviously adsorption capacities for bilirubin. Benefiting from the ultrahigh IEC of the PES/PVP/Gel, the PES/PVP/Gel exhibited a relatively high adsorption capacity for bilirubin. Thus the RSHMs may also show great advantages in heamoperfusion.
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Figure 5. (A) The breakthrough curves of adsorption column (MO concentration = 500 μM). (B) The filtering photo of the adsorption column. (C) Photos of goldfish lived in the filtered MO solutions and UV-vis spectra of the MO solution before and after filtration,. (D) The schematic of the bilirubin adsorption. (E) The bilirubin adsorption capacities of the skeletons and RSHMs.
Conclusion Hydrogel materials commonly exhibited poor dimensional stability. In this study, inspired by the root-soil structure, reinforced PDMC hydrogel microspheres were successfully fabricated via combining electrospray and phase conversion. Comparing with pure PDMC hydrogel, this novel structure effectively increased the compression strength and decreased the swelling ratios of the hydrogels. Additionally, the reinforced PDMC hydrogel microspheres owned the highest IEC comparing with other anion-exchange materials [21-23] and exhibited effective adsorption for anionic dyes. What is more, according to the breakthrough curves, the adsorption column filled with the microspheres also showed high clearance ratio and stable adsorption for anionic dyes. Also, the microspheres showed high adsorption capacities for bilirubin. The raw materials to prepare the reinforced PDMC hydrogel microspheres were cheap industrial chemicals. Thus, it is believed that the root-soil structure inspired PDMC hydrogel microspheres with ultra-high performances have great advantages in the application as anion-exchange materials. In our further study, we will fabricate the reinforced heparin-mimcking hydrogel microspheres with anticoagulation property, and we except they can be further used in heparin-free blood purification.
Acknowledgments This work was financially sponsored by the National Natural Science Foundation of China (No. 51673125 and 51773127), the State Key Research Development Programme of China (Grant Nos. 2016YFC1103000), State Key Laboratory for 18
Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1619) and State Key Laboratory of Polymer Materials Engineering (No. sklpme2017-3-07).
Supporting Information The
following
are
the
supplementary
data
related
to
this
article:
pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models, Figure.S1, and Video.S1-S2.
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