Porous hydrogels embedded with hydrated ferric oxide nanoparticles

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Porous hydrogels embedded with hydrated ferric oxide nanoparticles for arsenate removal Ryan Zowada, and Reza Foudazi ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00047 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Polymer Materials

Porous Hydrogels Embedded with Hydrated Ferric Oxide Nanoparticles for Arsenate Removal Ryan Zowadaa, Reza Foudazia* a. Department of Chemical and Materials Engineering, New Mexico State University, Las Cruces, NM, 88003, USA *Corresponding author, email: [email protected]

KEYWORDS Emulsion-templating, polyHIPE, arsenic, hydrated ferric oxide (HFO), nanoparticle, hydrogel. ABSTRACT The aim of this work is to assess the capabilities of porous hydrogels functionalized with hydrated ferric oxide nanoparticles to remove arsenate from aqueous media. Iron oxide nanoparticles can effectively remove arsenic through ionic interactions, binding the arsenic species to the nanoparticle. The use of polymeric hosts to these nanoparticles enhances the recovery of adsorbent after removal. The hydrogels were synthesized by high internal phase emulsion (HIPE) templating with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as the base monomer. The porous hydrogel has a pore diameter range of 9 – 30 m, and a maximum water holding capacity of over 4000 wt%. The removal efficiency of the material is up to 60% for high concentrations of 4.5 ppm in a batch removal process.

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1. INTRODUCTION Due to the adverse health effects of heavy metals, developing solutions for their removal from water resources has received considerable research attention.1 Heavy metals are also a source of membrane fouling in nanofiltration and reverse osmosis systems, a significant concern in advanced water treatment.2 Arsenic in particular, commonly found in ground water, has human carcinogenic effects on the skin, lungs, urinary bladder, liver, and kidney.2 The most common inorganic species of arsenic are As(III) and As(V), (arsenite and arsenate, respectively). The species of arsenate at pH = 7 is HAsO42- and the species of arsenite is H3AsO3 with a neutral charge which makes it difficult to be removed. Common methods for removing As(III) from water include an oxidization step to convert As(III) to As(V),3 thus this work solely focuses on As(V) removal. Heavy metals have been treated by different methods such as membrane separation,4,5 flocculation,1 ion-exchange (or chelation),6 adsorption5,7–10 and bio-adsorption.11–14 This study focuses on adsorption due to several advantages, e.g. simple operation, easy scalability, and generally lower operating cost.15 Polymer composites synthesized with nanoparticles have metal ion removal capabilities for wastewater treatment.16 While nanoparticles have enhanced functionally, they can survive excessive pressure drops in a fixed bed or other flowing systems if hosted in a polymer matrix.17 Nanoparticles supported by polymeric hosts have been widely used for water treatment. Wang et al. produced a polymeric fiber composite where polycaprolactone/polyethylene oxide supported polydopamine nanoparticles. This composite successfully removes dyes such as methylene blue and methyl orange from water in a batch removal process.18 Huang et al. developed a water treatment material using poly(vinyl alcohol)/poly(acrylic acid) as a host for Fe3O4 nanoparticles and MXene nanosheets embedded with silver nanoparticles that reacts with nitro compounds in


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solution.19 Nanoparticles have also been used to specifically remove heavy metals from water for example, Urbano et al. used TiO2 nanoparticles embedded in 4-(vinylbenzyl) trimethylammonium chloride film to remove arsenic in a batch process,20 and An et al. used strong-base anion resin support with Cu2+ ions that interact with arsenate ions through a Lewis acid-base interaction.7 Ferric oxide minerals in particular have been utilized for arsenic removal in previous works.21 Liu et al. used activated carbon beads as a support for Fe3O4 nanoparticles to remove arsenate while using a magnetic field to remove the carbon beads after treatment.22 Biopolymers have been specifically targeted for their biodegradability, e.g., Singh et al. used alginate beads crosslinked with iron chloride23 and Gupta et al. used chitosan beads embedded with iron nanoparticle.24 Kumar et al. used porous polyacrylamide as a polymeric host for iron-aluminum hydroxide particles showing success in removing both As(III) and As(V) ions.25 Functionalized polystyrene beads have been commonly used as a support for iron-based nanoparticles. DeMarco et al. used sulfonated polystyrene beads embedded with hydrated ferric oxide (HFO) nanoparticles in a packed bed,26 whereas Li et al. used chloromethylated polystyrene beads with HFO nanoparticles in both a packed bed and isothermal batch system.27 In this study, a polymeric host for incorporation of hydrated ferric oxide (HFO) nanoparticles is used. The polymeric host is created through high internal phase emulsion (HIPE) templating to produce a porous media with tunable pore size. PolyHIPE is an emulsion-based polymerization technique that creates an interconnected porous network. The HIPE templating process requires two immiscible liquids – usually denoted as water and oil phases – that form continuous and disperse phases (Figure 1). The disperse phase is added to the continuous phase dropwise and is stabilized by added emulsifiers that lower the interfacial tension. Once the disperse volume of the system surpasses 74% of the total volume, the droplets undergo deformation.28 This deformation


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will create flattened areas of contact between droplets, and a packed configuration that induces mechanical interference between droplets, thus, prohibiting their free movement.29 Polymerizing the continuous phase followed by removing the disperse phase leaves behind an interconnected porous polymer network.30 The polyHIPE is an economical choice due to the fact that over 90% of the volume can be made up of void space.31 The polyHIPE can also be formed to a custom shape by molding before polymerization. Therefore, polyHIPEs can be produced as packed bed column, beads, and monoliths for removal applications. PolyHIPEs can be functionalized and have been used for a wide variety of applications including remediation of heavy metal contamination. For example, Krajnc et al. produced polyHIPEs for silver ion removal using glycidyl methacrylate and methyl methacrylate crosslinked by ethylene glycol dimethacrylate functionalized by different thiol molecules.32 Macroporous polyHIPEs are beneficial for arsenic removal, as previous studies reported limited ion diffusion in the polymeric hosts containing HFO nanoparticles due to the Donnan exclusion effect. Cumbal et al. showed this effect was dominant when the pore sizes of sulfonated polystyrene bead hosts were too small, and the charge from the sulfonic groups prevented the arsenic ions reaching the embedded HFO nanoparticles.33 There are also reported results that the binding of arsenic species can build up blocking pores and prevent further adsorption.34 Macroporous polyHIPE structures have shown to be an effective host for a removing agent in comparison to non-porous materials. Feng et al. compared the anionic dye removal capability of functionalized polyHIPEs to non-porous functionalized silica nanoparticles.35 They synthesized a monomer using branched and linear polyethyleneimine that was alkylated with glycidyl-capped poly(styrene-co-2-ethylhexyl acrylate), resulting in a dendritic amphiphile. They concluded the polyHIPE method could remove 104-fold more than the coated non-porous silica particles.35 Katsoyiannis et al. embedded


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sulfonated polystyrene polyHIPE beads with HFO nanoparticles, finding success in removing high levels of arsenate (50 – 200 ppm) through a packed bed system.36 This work assesses the advantages of using a hydrophilic porous polymer as a host for embedding hydrated ferric oxide nanoparticles to remove arsenate ions from water. The hydrophilic nature of the polymer should increase the loading efficiency of the nanoparticles into the polymer matrix. The arsenate ion adsorption potential will increase due to a high surface area and interconnectivity increasing the number of loading sites and accessibility, respectively.


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2. EXPERIMENTAL 2.1 MATERIALS Anhydrous iron(III) chloride (FeCl3, purchased from Sigma-Aldrich 97%) was used for the synthesis of iron nanoparticles along with sodium chloride (NaCl, purchased from Cynmar Corporation 99%), sodium hydroxide (NaOH, purchased from Sigma Aldrich >97%) and ethanol (Parmco-Aaper 95%). PolyHIPE monoliths were synthesized with the continuous phase containing 2-acrylamidopropanesulfonic acid (AMPS, purchased from Sigma-Aldrich, 99%) as the monomer, N’,N-methylene-bis-acrylamide (MBA, purchased from Sigma-Aldrich, 99%) as the crosslinker, Pluronic F68 (provided by BASF) as the surfactant, and potassium persulfate (KPS, purchased from Arcos-Organics, >99%) as the thermal initiator. Cyclohexane (purchased from Pharmco-Aaper, >99%) was used as the disperse phase. The arsenic stock solution was prepared by dissolving arsenic pentoxide anhydride (purchased from Spectrum, >97%). Hydrochloric acid (HCl, purchased from Sigma Aldrich, 0.1 M) and nitric acid (purchased from Sigma Aldrich, 70%) were used during the arsenate removal study. Deionized water was nanopurified through EMD Millipore water purification system.

2.2 POLYHIPE SYNTHESIS Samples of polyHIPE monoliths were prepared by mechanical mixing oil-in-water emulsions (o/w) using an overhead mixer (Talboys Model 4136 Stirrer, manufactured by Henry Troemner, LLC). The aqueous phase was mixed first at 250 RPM until complete dissolution of ingredients. The disperse phase, cyclohexane, was added by an automated syringe pump (KD Scientific 120 Push/Pull Syringe Pump). To ensure the correct volume of cyclohexane was added, the emulsion was weighed before and after the dispersion accounting for evaporated cyclohexane during


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addition and emulsification. The samples were then polymerized at 70oC for four hours in an enclosed container and then opened to evaporate off the cyclohexane under fume hood at 50oC for 24 hours (Figure 1). The monomer to crosslinker weight ratio (M:C) and porosity of the polyHIPEs were varied as summarized in Table 1. Control samples (non-emulsions) with same monomer to crosslinker ratio in water were polymerized to make non-porous hydrogels for comparison. In sample coding, PH and P represent polyHIPE and polymer (non-porous), respectively. The first number in sample coding represents the ratio of AMPS to MBA on a weight basis, and the second number refers to the resulting porosity of the sample. Omission of a second number refers to a non-porous sample.

Figure 1 Schematic representation of the polyHIPE synthesis process. Table 1 Composition of samples on volume percent basis.

Water

AMPS

MBA


PF68

KPS

7


PH6_85 PH5_85 PH4_85 PH5_80 PH5_75 P6 P5 P4

55.0% 55.0% 55.0% 55.0% 55.0% 55.0% 55.0% 55.0%

20.6% 20.0% 19.2% 20.0% 20.0% 38.6% 37.5% 36.0%

3.4% 4.0% 4.8% 4.0% 4.0% 6.4% 7.5% 9.0%

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20.0% 20.0% 20.0% 20.0% 20.0% 0% 0% 0%

1.0% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0% 1.0%

2.3 CHEMORHEOLOGY As developed in Foudazi et al.,11 the polymerization of HIPEs was studied with a TA Instrument Discovery HR-3 rheometer through measuring the viscoelastic moduli using oscillatory shear in the linear viscoelastic regime with fixed frequency (10 rad/s) and strain (5%) under time sweep condition at 70oC.

2.4 HYDRATED FERRIC OXIDE NANOPARTICLES SYNTHESIS The synthesis of hydrated ferric oxide (HFO) nanoparticles in the polymeric host was adopted from Cumbal et al.33 As illustrated in Figure 2, there are four steps to the process: binding, precipitation, agglomeration and crystallization. The process begins with soaking a polyHIPE monolith in an excess of 4 wt% FeCl3 solution with a pH of < 2, to prevent oxidation of chloride ions, to bind the iron cations to the sulfonic groups on the polymer matrix (Figure 2A). This is followed by immersing the swollen polyHIPE in an excess of 50-50 NaOH and NaCl solution at a 5% w/v concentration in order to precipitate the Fe3+ into the polymer matrix and react with the hydroxide ions to form Fe(OH)3 (Figure 2B). The duration of these first two steps was 6 hours to ensure the reactions were carried out. PolyHIPEs were washed by an immersion in an excess of 50-50 (v/v) ethanol-water solution to remove sodium and allow Fe(OH)3 agglomeration (Figure 2C). To test the samples had been properly washed, the ionic conductivity of the solution was


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measured (with a Hach HQ40d portable pH and ion conductivity meter). The formation of the nanoparticles through aggregation of reduced iron is dependent on the relative concentration of water to ethanol, where the higher the water concentration, the bigger particles will form 37. The final step was a mild thermal treatment for one hour at 50oC to facilitate crystallization of Fe(OH)3 to FeOOH (Figure 2D). Samples that have been functionalized with HFO nanoparticles are denoted with a “T” rather than “PH” or “P”. The HFO nanoparticles that were embedded into the polymer matrix interact with the arsenate ions though a coulombic interaction that would bind the arsenate ions to the surface. By adding these nanoparticles into a macroscopic hydrogel, the arsenate ions can effectively diffuse through the porous structure and bind to the surface of the polymer.


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Figure 2 Simplified illustration of synthesis process of hydrated ferric oxide (HFO) nanoparticle in the polyHIPE network: (A) binding, (B) precipitation, (C) agglomeration, and (D) crystallization.

2.5 STRUCTURE AND MORPHOLOGY ANALYSIS The morphology in terms of pore and window sizes of samples was characterized using an S3400N II Scanning electron microscope (SEM) from Hitachi High-Technologies Corp. Samples were first sputter coated by a gold filament in a Denton Desk IV Sputter Coater. The diameter of the pores and windows were measured through image analysis using AMScope software. The atomic species of the polymer and embedded nanoparticles were characterized by energy


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dispersive x-ray spectroscopy (EDS) using a Noran System Six 300 from Thermo Electron Corp. attached to the SEM instrument. The characterization was analyzed using NSS EDS software from Thermo Electron Corp. The data collected from this analysis is included in supporting information. The BET surface area of both treated and untreated samples were measured through nitrogen gas adsorption using an ASAP 2050 from Micrometrics and calculated using ASAP 2050 software. To study the formation and shape of HFO nanoparticles, functionalized polyHIPEs were embedded in Embed 812 resin (purchased from Electron Microscopy Sciences) to infiltrate the pores for microtomy. Thin sections of samples were analyzed by an H-7650 Transmission Electron Microscope (TEM) from Hitachi High-Technologies Corp.

2.6 THERMOGRAVIMETRIC ANALYSIS The thermogravimetric analysis was carried out using a TGA Q500 thermogravimetric analyzer and the data was analyzed using TA Universal Analysis software. The cell was heated to 500oC at a rate of 10oC per minute under nitrogen gas to induce decomposition. The differential thermal analysis was calculated using TA Universal Analysis software. The samples with and without HFO nanoparticles were tested. In addition, functionalized samples were tested after being immersed in DI water for 72 h and dried to investigate if there is a wash out of HFO nanoparticles during the remediation process.

2.7 WATER UPTAKE KINETICS Small sections (less than 0.1g) of polyHIPE samples were immersed in excess deionized water at ambient temperature for 72 h to study the influence of porosity and crosslink density (i.e., M:C)


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on water uptake. Non-porous control samples were prepared, by polymerization of monomer and crosslinker without emulsification (non-porous) and studied under the same conditions.

2.8 ARSENIC REMOVAL EFFICIENCY The arsenic sorption behavior of the monoliths was studied at ambient conditions using a batch process. The embedded monoliths (0.1 g) were immersed in a solution with 4.5 mg/L concentration of arsenic pentoxide (4.5 ppm). The pH of the solution was adjusted at a pH of 7.2 (i.e., drinking water) using 0.1 M NaOH and HCl. The functionalized polyHIPEs soaked in the arsenic solution for 24 h with occasional shaking. Aliquots of 15 ml of the treated arsenate solutions were removed and nitric acid was added at 1 wt% for sample preservation. Elemental analysis by a Perkin Elmer 4300 ICP-OES was carried out with the preserved samples and the stock solution to measure the removal of arsenate.

3 RESULTS AND DISCUSSION 3.1 CHEMORHEOLOGY The rheology of highly concentrated emulsions and their transition to polymers are related to their colloidal properties and degree of polymerization of the continuous phase.38,39 Figure 3 shows a typical polymerization of HIPEs under rheometer. Three stages can be identified: the first stage occurring (within the first 200 s) shows a decrease in moduli readings in the induction period, an indication of droplet coalescence at the first stage of HIPEs polymerization. If the system is stable at elevated temperature as in Ref. 11, the storage modulus, G', shows no change. Eventually the increase in viscosity of continuous phase becomes dominant upon polymerization, where there is a steep increase in the storage modulus (occurring between 200 and 400 s in Figure 3). The curing


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is completed when the storage modulus plateaus (occurring after 400 s in Figure 3).11 The dynamic moduli of the plateau indicate the mechanical properties of the polyHIPEs.

10

10

10

4

G" G' Max. Moduli (Pa)

Dynamic Moduli (Pa)

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


3

2

0

10

4

(6:1) (5:1) (4:1)

400 800 1200 Time (sec)

Figure 3 Typical chemorheology plot of polyHIPE samples. The inset shows the maximum dynamic moduli of each crosslinked system

There is a direct correlation between the crosslink density and the maximum moduli (plateau) in a fixed volume fraction of droplets, where the more crosslinked the polymer the higher the modulus. The sample with the lowest crosslinker weight ratio (M:C = 6:1) is very soft as it has close storage and loss moduli, while the most crosslinked sample (i.e. PH4_85) is more robust. The shear modulus of samples in the range of 10 KPa indicates that these hydrogels can be classified as super-soft monoliths. It should be noted that the dynamic moduli are also dependent on the droplet size of HIPEs,38 and thus, on the pore size of polyHIPEs, which is not the scope of this work.

3.2 STRUCTURE AND MORPHOLOGY The calculations of De Brouckere mean diameter, D[4,3], and Sauter mean diameter, D[3,2], are adopted from Maibelle et al. where Di is the diameter of droplet i:40 ∑𝑖𝐷3𝑖

𝐷[3,2] = ∑ 𝐷2 𝑖 𝑖


(1)

13


∑𝑖𝐷4𝑖

𝐷[4,3] = ∑ 𝐷3 𝑖 𝑖

𝐷[4,3]

PDI = 𝐷[3,2]

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(2)

(3)

The physical meaning behind the De Brouckere and Sauter mean diameter are the diameter of a particle with the same volume and surface area as the actual particles, respectively. These two values have been chosen for analysis since surface area and bulk material properties are measured (adsorption and water uptake, respectively). The polydispersity index (PDI) can be calculated by ratio of the DeBrouckere and Sauter diameters. A PDI equal to 1 means a monodisperse system and as the PDI becomes greater than one, the size distribution becomes broader. The analysis of the SEM images (typical images shown in Figure 4) are presented in Table 2 showing that the average size, polydispersity, and frequency of the windows and pores are primarily influenced by the disperse phase volume fraction (𝜙) and the monomer-to-crosslinker weight ratio (M:C). The average and PDI of pore size decrease with an increase in 𝜙, which is due to the enhanced shear transport for refining droplets during emulsification; thus, a higher pore area frequency (Ap: number of pores per unit area) is observed. The M:C ratio also affects the average and PDI of pore size to a lesser degree by influencing the wall thickness and stability of the forming polyHIPE, so as the crosslink density increases the pore diameter increases. In other words, as MBA (which has a higher tendency toward oil phase than AMPS) content increases, the emulsions become less stable. The controlling factor of the window diameter is the contact between droplets influenced by the surfactant concentration and interdroplet film thickness (both affected by the volume fraction). The surfactant concentration was kept constant among all samples therefore the concentration of surfactant present in the continuous phase is influenced by the droplet size. In other words, since


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higher amount of surfactant is adsorbed to the droplets surface of emulsions with smaller droplet size (i.e., higher interfacial area), less surfactant molecules remain in the continuous phase to induce depletion attraction.38 The driving force for window formation diminishes when depletion attraction weakens. The nature of continuous phase can affect the surfactant adsorption at the interface and resulting depletion attraction. In addition, as we increase the volume fraction, the interdroplet film becomes thinner, increasing the possibility of windows formation. Therefore, as we decrease droplet size, both depletion attraction and interdroplet film thickness decrease. These competing factors are the sources of the window size variation with respect to M:C or 𝜙. As the crosslink density increases the window diameters have higher variation (higher PDI), and as the 𝜙 increases the window size distribution becomes narrower (lower PDI). The M:C ratio mainly affects the frequency of windows per unit area (Aw). As the crosslinking density increases, the frequency of windows decreases due to stronger pore walls (resisting the shrinkage process, responsible for window formation during polymerization28,41), reducing the rate of window formation and the average frequency of windows per pore (W:P). By increasing the disperse phase, Aw increases due to a higher packing factor of droplets (thinner pore walls), enhancing the window formations.


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Figure 4 Typical SEM of polyHIPEs after drying: (a) scale bar of 10 µm, and (b) scale bar of 50 µm. The arrow in (a) indicates a typical thin wall between pores (i.e., an immature window).

The average frequency of windows per pore (W:P) is also influenced by 𝜙. We observe that as 𝜙 increases, there is a lower W:P ratio due to the smaller pore surfaces available for window formation. In other words, since the window diameter range is similar across all samples, there will be less surface available for window sites on smaller pores. Table 2 Comparison of the effects caused to the pore diameter (Pd) and window diameter (Wd) by varying monomer to crosslinker weight ratio and varying porosity. The standard error is equal to the standard deviation.

Pores

Windows Frequency

D[3,2] (𝜇m) D[4,3] (𝜇m) PDI D[3,2] (𝜇m) D[4,3] (𝜇m) PDI W:P Ap (mm-2) Aw (mm-2)

PH6_85 8.41 9.09 1.11 2.23 2.54 1.14 1.7 2.2E+04 3.7E+04

PH5_85 9.26 10.61 1.14 3.94 4.50 1.14 1.1 2.6E+04 2.8E+04

PH4_85 9.60 12.27 1.28 3.21 5.06 1.58 0.6 3.1E+04 1.7E+04

PH5_80 9.97 12.20 1.22 2.54 2.96 1.17 1.3 1.9E+04 2.4E+04

PH5_75 29.38 47.26 1.61 3.98 4.86 1.22 1.7 4.3E+03 7.3E+03

The BET analysis showed the surface area of the polyHIPEs reaches up to 0.54 m2/g with the highest porosity (i.e. PH6_85), and when functionalized with HFO nanoparticles the surface area of the corresponding material (i.e., T6_85) increases to 2.65 m2/g. The increase in surface area can


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be attributed to the presence of HFO nanoparticles at or near the pore surface increasing the roughness.

3.3 TRANSMISSION ELECTRON MICROSCOPE The TEM images (Figure 5) show dark nanoparticles inside the polymer matrix that appear similar in size and shape to previous literature,37 confirming the functionalization was successful. The HFO nanoparticles synthesized had two major morphologies: spindle-like (amorphous) and cubic (crystalline) in agreement with the literature.37 The cubic morphology of the nanoparticles becomes more prevalent as the degree of crosslinking decreases, which may be a result of the structure’s more flexible network allowing larger particles to form.

Figure 5 Typical TEM images of polyHIPEs functionalized with HFO nanoparticles with: (a) amorphous morphology, and (b) cubic morphology. The scale bar in both micrographs is 200 nm.

3.4 THERMAL GRAVIMETRIC ANALYSIS The TGA results show the decomposition of the polyHIPEs occurrs in three steps: dehydration (

Porous hydrogels embedded with hydrated ferric oxide nanoparticles

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