Synthesis of Polyurethane Foams Loaded with TiO2 Nanoparticles

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Materials and Interfaces

Synthesis of Polyurethane Foams Loaded with TiO2 Nanoparticles and Their Modification for Enhanced Performance in Oil Spill Cleanup Qian Wei, Oluwasola Oribayo, Xianshe Feng, Garry L. Rempel, and Qinmin Pan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01037 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Industrial & Engineering Chemistry Research

Synthesis of

Polyurethane Foams

Nanoparticles

and

Their

Loaded

Modification

for

with TiO2 Enhanced

Performance in Oil Spill Cleanup Qian Wei†, Oluwasola Oribayo†, Xianshe Feng‡, Garry L. Rempel‡, Qinmin Pan*,† †

Green Polymer and Catalysis Technology Laboratory, College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou 215123, Jiangsu Province, People’s Republic of China

‡

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada

ABSTRACT:

Sponge-like superhydrophobic and superoleophilic materials are

attracting significant attention for effective clean-up and recovery of spilled oils from water. We report on the synthesis of TiO2 nanoparticle/polyurethane (TPU) composite foam substrate and its surface modification with tetradecylamine (TDA)-amidated graphene oxide (GO-TDA), thereby forming superhydrophobic and superoleophilic TPU-GO-TDA sorbent for oil spill clean-ups. Spectroscopic analyses confirmed that TDA was successfully grafted onto GO and the reduced GO by TDA was successfully grafted onto the TPU foam. Sorption experiments with engine oil, crude oil, silicone oil and chloroform demonstrated that the TPU-GO-TDA foam was an effective sorbent with sorption capacity of 20.2-62.4 times its own weight. The absorbed oils in the sorbent could be recovered simply by squeezing the oil-laden foam. The much higher selectivity

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to oils and better reusability of the TPU-GO-TDA foam than commercial sorbents make the TPU-GO-TDA foam promising for the separation and recovery of spilled oils and organic solvents from water. Key words: polyurethane foam, TiO2 nanoparticle, graphene oxide, tetradecylamine, oil-water separation

1. INTRODUCTION In recent years, the increasing number of oil spill incidents has become a devastating ecological challenge to the aquatic ecosystem. They have posed great threat to the marine wildlife habitats, with serious consequences on human health, needless to say that frequent occurrence of oil spills represents a direct significant economic loss.1-3 For instance, the oil spill incident at Dalian Xingang Oil Port on July 16, 2010 caused by the damage of a 200-m oil pipeline resulted in a loss of more than 10,000 m3 crude oil, and such a catastrophic oil spill caused significant water pollution.4, 5 Therefore, there is a high demand for developing efficient and environmental friendly cleanup materials. Various oil spill clean-up methods, including mechanical clean-up, bioremediation, in situ burning of oil in water, and the uses of absorbents, solidifiers and dispersants, have been employed to contain the oil spills. Especially, numerous sorbents and dispersants based on mineral products,6 chemical dispersants,7 polymers,8 textiles/fibers,9, 10 metallic meshes,11 and carbon materials12 have been used to separate oil from water. However, some of these materials have the disadvantages of low sorption capacity, complex fabrication process, secondary pollution, poor mechanical property and poor oil-water

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selectivity.13-16 At present, oil absorbents have received significant attention for efficient clean-up and recovery of the spilled oil because they are not only capable of absorbing and holding oil pollutants in place due to their matrix structures, but also making it possible to recover the oil subsequently from their semi-solid phase. Therefore, porous foam materials are particularly used as a substrate in developing cost-effective and high-performance oil sorbents.17-19 Polyurethane foam (PU) is a three-dimensional (3D) material, having urethane moieties formed by reacting polyol with polyisocyanate.20 PU foam has become a conventional substrate used in fabricating oil spill cleanup absorbent materials and many efforts have been made to fabricate superhydrophobic and superoleophilic absorbent foams for oil spill clean-up and recovery. For instance, Oribayo et al.21 synthesized lignin-based polyurethane (LPU) and used it as a substrate for fabricating lignin-based polyurethane/reduced graphene oxide/octadecylamine foam absorbent, which was shown to be effective to separate oils or organic solvents from water. Zhang et al.22 investigated the synthesis of fibers and polyurethane foams for separating oil from water, and absorbent exhibited a water contact angle of 146.6° with an absorption capacity in the range of 5.1-44.8 times its own weight. Li et al.23 incorporated activated carbon into polyurethane foam during polyurethane synthesis, and the composite foam displayed increased surface roughness and other improved mechanical, physical and chemical properties.24-27 Wang et al.28 fabricated a superhydrophobic carbon nanotube/poly (dimethyl siloxane)-polyurethane foam by the dipping coating method, and the foam

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sorbent was effectively used together with a vacuum system for continuous removal of oils or organic solvents from water. Liu et al.29 fabricated a magnetic polyurethane foam absorbent by anchoring Fe3O4-nanoparticles onto the sponge via dopamine. Although this absorbent was reported to exhibit excellent repellence to water, its absorption capacity for gasoline was not very high (18 g/g). Nikkhah et al.30 integrated nanoclay into a polyurethane foam and the absorbent exhibited good oil clean-up attribute, but the procedure used to prepare nanoclay/polyurethane foam was rather complicated. Graphene oxide with carboxyl, epoxy, hydroxyl groups in a two-dimensional structure have also been utilized in the development oil spill cleanup materials, because of its huge specific surface area and excellent sorption capability.31-34 Li et al.35 coated polyurethane foam with graphene oxide modified with γ-methacryloxypropyl trimethoxy silane. The absorbent exhibited excellent superhydrophobicity with a water contact angle of up to 161°, and its absorption capacity was up to 39 times its own weight. Zhu et al.36 coated reduced graphene oxide on the skeletons of polyurethane foam to increase the hydrophobicity of the foam absorbent for effective oil/water separation. The feasibility of using graphene oxide-tetradecylamine (GO-TDA) to modify TiO2 nanoparticle/polyurethane (TPU) foam as an oil clean-up absorbent was studied in this work. Oleophilic TiO2 nanoparticles, which are widely used as a functional material with good chemical stability, were utilized in synthesizing TPU foam substrate to enhance the absorption performance for oil spill clean-up. Unlike the previously work on coating nanoparticles onto the skeleton of the PU foam,37-39 in this work the TiO2 nanoparticles

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were incorporated into the PU foam substrate, followed by surface modification with tetradecylamine to take advantage of its long hydrophobic -CH2 chains in an effort to enhance the hydrophobicity of the foam absorbent. In the TPU-GO-TDA foam absorbent synthesized, the incorporated TiO2 nanoparticles and graphene oxide nanosheets produced rough structures, and tetradecylamine was responsible for the desired oil wettability. The difference in the wettability between oil and water on the porous TPU-GO-TDA sponge surface forms the basis of effective oil/water separation for oil spill clean-up. The properties and performance of the fabricated TPU-GO-TDA foam absorbent as an effective oil spill remediation material were evaluated.

2. EXPERIMENT 2.1. Materials Poly(ethylene glycol) (PEG 400, average molecular weight 400), 1, 4-dioxiane, sulfuric acid and acetone were purchased from Chinasun Specialty Products Co. Polyphenylmethane polyisocyanate (PMDI) was purchased from Wanhua Chemical Group, China. Titanium oxide (particle size 100 nm, lipophilic), tetradecylamine, graphite, dibutyltin dilaurate (catalyst) and polymethylphenylsiloxane (surfactant) and silicone oil were all supplied by Aladdin Shanghai. Chloroform and hydrogen peroxide (30%) were purchased from Yonghua Chemical Technology Co. Engine oil was purchased from China Petroleum & Chemical Corporation (Sinopec), and crude oil was purchased from Dongying Haike Group, China. Methylene blue was purchased from Energy Chemical Shanghai. Ethanol, tetrahydrofuran (THF), cyclohexane, toluene,

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hydrochloric acid, hexane, dichloromethane, Sudan red G and potassium permanganate were all supplied by Sinopharm Chemical Reagent Co. All the reagents were used directly without further treatment. 2.2. Synthesis of TPU foam 0.33 g of TiO2 (1 wt.% based on the TPU foam weight) nanoparticles was added to 13.8 g of PMDI in a 50-mL beaker, followed by ultrasonic dispersion for 1 h to form a uniform suspension. The TPU foam was synthesized according to the formulation shown in Table S1. The various components (PEG 400, catalyst, surfactant, 1, 4-dioxane, water) were premixed in sequence under agitation for 2 min, and then a predetermined amount of PMDI was added and stirred for 22 s. The resultant mixture was immediately transferred into a mold and was allowed to rise freely at ambient temperature. Finally, the foam so produced was allowed to cure at ambient conditions for a minimum of 2 days to ensure it was fully cured. 2.3. Preparation of graphene oxide The modified Hummers method was used to prepare graphene oxide.40 2.0 g of graphite powder and 1 g of NaNO3 were mixed with 100 mL of concentrated H2SO4 (98%) in a three-neck flask at 0 °C. Then, 8.0 g of KMnO4 was added gradually at a temperature maintained below 10 °C under magnetic stirring. The mixture was stirred for 2 h below 10 °C before its temperature was raised to 35 ºC, at which temperature the mixture was stirred for 1 h. Then, 100 mL of deionized water was added to the mixture and the temperature of the mixture was increased to 80 ºC. Afterwards, 20 mL of H2O2

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(concentration 30% (volume ratio)) was added to the suspension and the color of the mixture became brilliant yellow. Finally, the reaction mixture was washed with 600 ml of 5% HCl solution and 1000 mL of deionized water several times to obtain graphene oxide. The yellow-brown graphene oxide obtained was then dried in air. 2.4. Fabrication of TPU-GO-TDA foam 150 mg of GO was dispersed into 75 ml of deionized water under ultrasonication in a three-necked flask for 1 h, followed by addition of 75 mL of ethanol containing 300 mg of TDA. The suspension was refluxed at 90 °C for 20 h with mechanical stirring. After washing with 500 mL of ethanol, the product (GO-TDA) was separated by filtration and dried at 50 °C overnight in an oven. Then, 150 mg of the amidated GO-TDA was dispersed into 120 mL of ethanol under ultrasonication for 30 min, and the TPU foam was immersed into the dispersed solution to induce grafting reaction at 75 °C for 5 h under continuous stirring respectively. Finally, the resultant TPU-GO-TDA foam was washed with ethanol and distilled water repeatedly and then dried overnight in air. 2.5. Oil absorption in TPU-GO-TDA foam Experiments for oil absorption of the TPU-GO-TDA foam sorbent were carried out gravimetrically using model oil and organic solvents. The sorbent sample with a known initial dry weight was dipped into a mixture of water and various types of oils (or other organic solvents) until it became saturated with the liquid. Then, the sorbent foam was removed using a pair of tweezers, drained for 1 min and squeezed to remove oil (or other

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organic solvents). The absorption capacity (QC) and recovery were calculated using the following equations: QC = R% =

(1)

(2)

×100

where Mo and Mt are the weights of the TPU-GO-TDA foam sorbent before and after absorption, respectively. Mc is the weight of the recovered oil (or organic solvent). Qc measures the weight of the liquid absorbed relative to the weight of the foam sorbent, and R measures the oil content in the liquid taken up in the foam. The sorption-desorption experiments were repeated for 20 times to test the reusability of the TPU-GO-TDA foam sorbent. 2.6. Measurements and Characterization Fourier transform infrared spectrometer (Thermo Scientific Nicolet IS10) was used to confirm the functionalization reaction that occurred during the foam sorbent preparation. The FTIR spectrum was recorded at wavenumbers in the range of 400-4000 cm-1. The surface morphology of the foam was evaluated using scanning electron microscopy (SEM) (Quanta 200 FEG, FEI Company, USA), and the foam specimen were sputter coated with gold for the SEM analysis. To investigate the hydrophobicity of the foams, the contact angle of distilled water on the foam was determined at room temperature using the Dataphysics OCA 20 contact angle instrument (Stuttgart, Germany). Thermal analysis on the foams was conducted using TA Instruments SDT 2960 TGA/DSC analyzer. The TGA analysis was performed with about 2-3 mg of the foam samples

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placed in a ceramic crucible in the temperature range of 25-600 °C at a heating rate of 20 °C/min with nitrogen purge at a flow rate of 80 ml/min. The elemental compositions of the TPU and TPU-GO-TDA foams were confirmed by energy dispersive X-ray spectra (EDS) using a FEI Quanta 200 FEG system.

3. RESULTS AND DISCUSSION This work aimed at synthesis of TPU foam substrates and then transforming the substrates to TPU-GO-TDA foams as oil spill clean-up absorbent. Figure 1 illustrates the process for fabricating the TPU-GO-TDA foam sorbent. When amidated graphene oxide sheets (GO-TDA) was grafted onto the TPU foam, the foam became dark black. GO, with protruding structural hydrophilic groups (hydroxyl, carboxyl and epoxy), has excellent ion exchange, swelling and intercalating properties. In this work, the hydrophilic GO was amidated with TDA to form hydrophobic GO-TDA, which was then covalently attached onto the TPU foam skeleton to produce the TPU-GO-TDA foam sorbent. The TiO2 nanoparticles and amidated GO nanosheets, which contribute to the surface roughness, and the TDA functional groups, which has a low surface energy, are expected to yield the proper wettability properties as an oil clean-up sorbent when they are incorporated into foam skeleton. This resulted in a transformation of the hydrophobic TPU foam to a superoleophilic and superhydrophobic sponge ideal for use in oil spill clean-up and recovery.

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Figure 1. Schematic illustration for the fabrication of TPU-GO-TDA foam sorbent. Photographic images of TPU foam (a) and TPU-GO-TDA foam (b). 3.1. FTIR analysis FTIR was used to investigate the chemical reaction between graphene oxide, tetradecylamine and the TPU foam. Figure 2a shows the FTIR spectra of GO and GO-TDA, where the characteristic peak of GO at 1732 cm-1 is associated with C=O stretching, the peak of GO at 3373 cm-1 is due to OH stretching, the peaks at 1621 cm-1, 1223 cm-1, 1056 cm-1 correspond to the C=C in aromatic ring, C-O-C in epoxide and alkoxy C-O stretching,41 respectively. GO-TDA showed two peaks at 2919 cm-1 and 2857 cm-1 in the FTIR spectra, which could be assigned to the -CH3 and -CH2 stretching of the TDA. In GO-TDA, there was no peak at 1732 cm-1, and a new peak at 1660 cm-1 appeared as a result of NHCO stretching, indicating that the epoxy groups reacted with the NH2 groups.42 The FTIR spectra confirm the chemical reaction between GO and TDA.

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The FTIR spectra of the PU, TPU, PU-GO-TDA, and TPU-GO-TDA foams are shown in Figure 2b. The FTIR spectrum of the PU and TPU foam confirmed the presence of urethane linkages as represented by the protruding peak at 2870 cm-1 is due to -CH2 stretching, and the broad peak at 3308 cm-1 is associated with -NH stretching. The peak at 1726 cm-1 is ascribed to C=O stretching, while the peaks at 1606 and 1224 cm-1 may be attributed to the urea and ether groups, respectively. After grafting with GO-TDA, TPU-GO-TDA sorbent displayed a new peak at 2922 cm-1 and the intensity of the 2870 cm-1 peak was strengthened which are ascribed to the -CH stretching of -CH3 and -CH2 of the TDA monomer21. Compared to the PU and TPU foam, the intensified peaks of the PU-GO-TDA and TPU-GO-TDA foam suggest that the GO-TDA was indeed incorporated into the foam skeleton.

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Figure 2. FTIR spectra for (a) GO and GO-TDA; (b) PU, TPU, PU-GO-TDA and TPU-GO-TDA foams; (c) TPU foam and TPU-GO-TDA foam; (d) PU foam and PU-GO-TDA foam. 3.2. Surface morphology of the foam The surface morphology of the foam substrate and the foam sorbent are depicted in Figure 3. These foams exhibit large surface area and void space for absorbing oil or other organic solvents. The PU foam exhibited smooth skeletons as shown in Figure 3a. It was clearly seen that the introduction of TiO2 nanoparticles in the foam formulation increased the surface roughness of the foam, as shown in Figure 3 (b). This introduced roughness to the foam affects its interaction with a non-wetting or wetting liquid, due to an amplification or enlargement of the solid-liquid interactions. The foam sorbent retained its spherical shape after modification, and the cell structure of the foam remained uniform, indicating that incorporating GO-TDA into the foam did not destroy the cell structure of the foam skeletons. Figure 3 also shows that the skeleton of the PU-GO-TDA and TPU-GO-TDA foam had a rougher surface than the PU and TPU foam. The protrusions of the PU-GO-TDA and TPU-GO-TDA foam indicated adherence of GO-TDA onto the PU and TPU foam skeletons. The morphology of the TPU-GO-TDA foam is much rougher than that of the PU-GO-TDA foam, which is attributed to the TiO2 nanoparticles and GO-TDA. This further illustrates the contribution effect of TiO2 nanoparticles and GO-TDA to the surface roughness. The EDS spectra (shown in Figure S1) show that the substrate TPU foam (Figure S1a) displayed C, O, N and Ti elements,

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confirming the incorporation of TiO2. The carbon content was shown to have increased after the modification with GO-TDA (Figure S2b). After modification, the carbon element content of the TPU-GO-TDA foam sorbent was estimated as 69.1%, while the pristine TPU foam is 64.8%. This analysis indicated the surface modification was successfully.

Figure 3. Morphology of PU foam (a1, a2, a3, a4), TPU foam (b1, b2, b3, b4), PU-GO-TDA foam (c1, c2, c3, c4) and TPU-GO-TDA foam (d1, d2, d3, d4) at different magnifications. 3.3. Surface wettability of the foam The surface wettability of the foam is illustrated in Figure 4. For ease of observation, water was dyed with methylene blue and engine oil with Sudan red. When a water droplet

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placed on the PU foam substrate, the water totally immersed into its matrix quickly (as shown in Figure 4a1), while for TPU foam, the water droplet did not penetrate into its matrix completely because the TiO2 nanoparticles incorporated increased the hydrophobicity of the foam. On the other hand, the engine oil droplet did not penetrate easily into the TPU foam either, indicating that the oleophilicity of the foam was rather limited (as shown in Figure 4a2). In contrast, when the foam was modified with GO-TDA, the water droplet remained on the surface of the PU-GO-TDA and TPU-GO-TDA foam sorbent, with TPU-GO-TDA foam exhibiting a higher water contact angle than PU-GO-TDA foam. It was also observed that the engine oil could penetrate quickly into the TPU-GO-TDA and PU-GO-TDA foam sorbent as shown in Figure 4 (a3 and a4). It becomes clear that the TPU and the PU foam became superhydrophobic and oleophilic after grafting with GO-TDA. The contact angle of water was employed to further confirm the hydrophobicity of the foams. Based on five measurements at different locations on the foam surface, the water contact angle was shown to have an average value of 84.7° on the PU foam (see Figure 4c1), 114.9º on the TPU foam (see Figure 4c2), 127.8º on the PU-GO-TDA foam (see Figure 4c3) and 140.2º on the TPU-GO-TDA foam sorbent (see Figure 4c4). After 20 times of repeated uses for oil/water separation, the TPU-GO-TDA foam sorbent still exhibited a contact angle of 138.4º (see Figure 4c5). This indicates that although the hydrophobicity of the foam sorbent experienced a slight reduction over a period of repeated uses, its hydrophobicity was essentially retained. When the foams were placed on water, the PU foam sunk into the water quickly as shown in Figure 4c1.

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After modified with GO-TDA, the PU-GO-TDA foam floated on the water surface but slightly submerged into the water after a time period as shown in Figure 4c2. the TPU foam initially floated on the water surface and then gradually sunk over time, whereas the TPU-GO-TDA foam always floated on the water surface, as shown in Figure 4c3. In fact, an external force would be needed for the TPU-GO-TDA foam sorbent to enter water (as shown in Figure 4c4), and a mirror-like surface was observed. Upon releasing the external force applied to the TPU-GO-TDA foam, it immediately floated back to the water surface with little water absorbed, demonstrating an excellent water repellence property of the foam.

Figure 4. Photographic images of water droplets and engine oil droplets on the foams. For ease of observation, water and engine oil were dyed with methylene blue and Sudan red,

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respectively, water droplets and engine oil on the PU foam (a1), TPU foam (a2), PU-GO-TDA foam (a3) and TPU-GO-TDA foam (a4); Images of water droplets on the PU foam (b1), TPU foam (b2), PU-GO-TDA foam (b3), TPU-GO-TDA foam (b4) and TPU-GO-TDA foam after repeated uses for oil/water separation (b5); PU foam sink in water (c1), PU foam sink in water while the PU-GO-TDA remained floating on the water surface (c2), Unmodified TPU foam began to sink in water while the TPU-GO-TDA foam remained floating on water surface (c3); when forced to immerse in water, the TPU-GO-TDA foam showed a mirror-like surface (c4). 3.4. TGA analysis TGA analyses on the foams were conducted to investigate their thermal stability, and the results are shown in Figure 5 and Table 1. The TPU-GO-TDA foam experienced a mass loss in the temperature range of 312-399 °C, and its thermogram was similar to that of the TPU foam. For these foams, TPU-GO-TDA foam shows the decomposition temperature where there is an initial 5% weight loss (Td, 5%) is 274.9 °C, the next TPU foam is 260.8 °C , while the PU foam is 241.2 °C. The TPU-GO-TDA foam has the highest Td, 10% (304.8 °C), which is similar with the TPU foam (304.4 °C), the initial decomposition temperature was increased by 27°C after incorporation of TiO2 nanoparticles in the TPU foam substrate.43 The TPU-GO-TDA foam shows the lowest residue weight44 (Rw) 16.41%, the TPU foam is 17.59%, the PU foam is 27.2%. The mass loss of TPU-GO-TDA foam was a slightly higher than that of the TPU foam due to the GO-TDA coating on the foam skeletons. This is an indication that the TiO2

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nanoparticles and GO-TDA improved the thermal stability of the foam.

Figure 5. TGA thermograms of the PU foam, GO-TDA, TPU foam and TPU-GO-TDA foam. Table 1. Thermal properties of PU, TPU and TPU-GO-TDA foams. Sample PU foam TPU foam TPU-GO-TDA foam

Td, 5% (°C) 241.2 260.8 274.9

Td, 10% (°C) 277.8 304.4 304.8

Rw (wt %) 27.20 17.59 16.41

3.5 Organic solvents-water separation The TPU-GO-TDA foam repelled water from oil or organic solvents effectively due to its hydrophobic properties. To demonstrate its potential for uses as an oil spill clean-up sorbent, the foam samples were fitted in a glass funnel, as shown in Figure 6, and a mixture of the solvent/water was poured into the glass funnel to observe the performance of the foam for water/organic solvent separation. For ease of observation, water and the organic solvent were dyed with methylene blue and Sudan red respectively. When a

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mixture of chloroform/water was allowed to contact the PU foam, both chloroform and water passed through the foam quickly, as shown in Figures 6 (a1, a2, a3, and a4). Unlike the PU foam, chloroform flowed through the foam quickly but water passed through the TPU foam over a time period Figure 6 (b1, b2, b3 and b4). This was due to the added TiO2 nanoparticles, which made microstructured roughness on the foam surface and enhanced hydrophobicity. In contrast, when the TPU-GO-TDA foam was fitted in the glass funnel, chloroform flowed down quickly, while water was essentially remained in the glass funnel, as shown in Figures 6 (c1, c2, c3, c4). The significant difference in the performance between the PU foam and the TPU-GO-TDA foam sorbent may be attributed to the grafted nucleophilic groups on the superhydrophobic and oleophilic TPU-GO-TDA foam sorbent. (See Supporting information, Movie S1 and S2).

Figure 6. Pictures showing chloroform/water separation using PU foam (a1, a2, a3, a4), TPU foam (b1, b2, b3, b4) and TPU-GO-TDA foam (c1, c2, c3, c4).

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Figure 7 shows the selective absorption of engine oil and chloroform from water surface by the TPU-GO-TDA foam, further illustrating the effectiveness of this foam sorbent in capturing oil or organic solvent from water. The incorporated TiO2 nanoparticles and grafted GO-TDA contributed rough structures in the production of the superhydrophobic foam sorbent, which exhibited different wettability between oil/organic solvent and water. When the TPU-GO-TDA foam was placed on the water surface as shown in Figure (a3, b3), the engine oil and chloroform were quickly taken up by the TPU-GO-TDA foam. After 1 min, most of the engine oil was absorbed, and the foam sorbent was taken out using tweezers. For the chloroform separation from water, chloroform was absorbed by the TPU-GO-TDA foam in only a few seconds, and a complete removal chloroform from water was accomplished in about 10s. This confirms that the TPU-GO-TDA foam is a promising absorbent for oil or organic solvents spill remediation.

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Figure 7. Pictures illustrating the TPU-GO-TDA foam absorption of engine oil from water surface (a1, a2, a3, a4, a5, a6) and TPU-GO-TDA foam absorption of chloroform from water. (b1, b2, b3, b4) 3.6 Oil absorption capacity Engine oil, silicone oil, crude oil and some other organic solvents were used to evaluate the absorption capacity of the foam. The sorption experiments were conducted at room temperature, and it was found that it only took a few minutes for the TPU-GO-TDA foam sorbent to reach maximum absorption capacity of the oils tested, while it took a few seconds to reach maximum absorption of the organic solvents. The absorption capacities of the TPU-GO-TDA foam for engine oil, silicone oil and crude oil were determined to be 26.6±1, 28.5±1 and 25.6±1 g/g, respectively. The sorption capacity of the TPU-GO-TDA foam for chloroform, THF, toluene, acetone, cyclohexane, and hexane were 62.4±1, 43.6±1, 28.5±1, 22.9±1, 21.7±1 and 20.2±1 g/g, respectively. Before and after each scorpion test, the water contact angle of the sorbent was measured and there was no noticeable change in the surface hydrophobicity of the foam. Furthermore, the sorption capacity of PU-GO-TDA foam for engine oil, silicone oil, crude oil, chloroform, THF, toluene, acetone, cyclohexane and hexane were determined to be 24.3±1, 26.8±1, 23.2±1, 57.6±1, 39.8±1, 26.3±1 20.0±1, 20.2±1and 19.1±1 g/g. The TPU-GO-TDA foam showed slightly higher absorption capacity than PU-GO-TDA foam. For comparison purposes, the sorption capacity of commercial polypropylene (PP) clean-up pad was also determined and compared with the TPU-GO-TDA foam sorbent, and the

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results are shown in Figure 8a. The sorption capacity of commercial PP pad for engine oil, crude oil, silicone oil, chloroform, THF, cyclohexane, toluene, hexane and acetone were determined to be 13.6±1, 13.2±1, 14.2±1, 15.1±1, 8.9±1, 10±1, 8.7±1, 7.5±1 and 7.7±1 g/g. respectively. These values are much lower than those obtained with the TPU-GO-TDA foam sorbent. Reusability is another important requirement for oil spill cleanup sorbent from an application point of view. The TPU-GO-TDA foam sorbent was also subjected to absorption-desorption experiments for at least 20 cycles to evaluate the reusability of the sorbent foam, and the results are presented in Figure 8 (b, c). The desorption was carried out by squeezing the oil or organic solvent-loaded sorbent foams. It was shown that the absorption capacity of the foam was very stable and over 90% of the original sorption capacity was maintained for at least 20 cycles of the sorption-desorption tests. In addition, the foam sorbent was not deformed after repeated squeezes in the absorption-desorption experiments. As shown in Figure 8d, the TPU-GO-TDA foam also displayed a much higher recovery rate than commercial PP sorbent. For example, the recovery rate for engine oil, silicone oil, crude oil, hexane, cyclohexane, acetone, tetrahydrofuran, chloroform and toluene was shown to be 92.1%, 90.5%, 94.5%, 92.1%, 91.4%, 90.0%, 91.8%, 90.2 and 91.3%, whereas the commercial PP sorbent had a recovery rate of 51.2-70.3%. It was also observed that there was a considerably large amount of residual oil or organic solvents trapped in the PP sorbent after squeezing. Based on the absorption capacity, reusability and recovery rate, it is clear that the TPU-GO-TDA foam is a

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promising sorbent for practical uses in the clean-up of oil spills from water bodies.

Figure 8. Absorption capacities (a), reusability (b, c) and recovery rates (d) of the TPU-GO-TDA foam, PU-GO-TDA foam and the commercial PP sorbent for separating oil and organic solvents from water.

4. CONCLUSION A facile method to fabricate absorbent foams for oil spill clean-up was developed, which involved synthesis and modification of polyurethane foam substrate loaded with TiO2 nanoparticles, followed by anchoring graphene oxide-tetradecylamine onto the TPU foam substrate, thereby producing a superoleophilic and superhydrophobic 3D structure. The foam sorbent displayed a high sorption capacity and high recovery rate for the model oils and organic solvents tested. The foam also displayed excellent reusability for at least

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20 sorption-desorption cycles. The experimental results demonstrated that the TPU-GO-TDA foam absorbent holds great promise for potential applications in cleanup and recovery of spilled oils in water bodies. Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Formulation for PU foam and TPU foam synthesis. Fabrication of PU-GO-TDA foam. The EDS of TPU foam and TPU-GO-TDA foam sorbent. Surface morphology of TPU foam. Reusability of PU-GO-TDA foam sorbent.(PDF) Movie showing chloroform/water separation using PU foam (Movie S1) (AVI); Movie showing chloroform/water separation using TPU-GO-TDA foam (Movie S2) (AVI). AUTHOR INFORMATION Corresponding Authors

E-mail: [email protected] ORCID Qinmin Pan: 0000-0001-5410-6845

ACKNOWLEDGEMENTS Financial support from the National Natural Science Foundation of China (No. 21176163; No. 21576174), Suzhou Industrial Park, the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Program of Innovative Research Team of Soochow University are gratefully acknowledged.

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Table of Contents

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Figure 1. Schematic illustration for the fabrication of TPU-GO-TDA foam sorbent. Photographic images of TPU foam (a) and TPU-GO-TDA foam (b). 254x160mm (300 x 300 DPI)


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Figure 2. FTIR spectra for (a) GO and GO-TDA; (b) PU, TPU, PU-GO-TDA and TPU-GO-TDA foams. (c) TPU foam and TPU-GO-TDA foam; (d) PU foam and PU-GO-TDA foam. 131x106mm (300 x 300 DPI)



Figure 3. Morphology of PU foam (a1, a2, a3, a4), TPU foam (b1, b2, b3, b4), PU-GO-TDA foam (c1, c2, c3, c4) and TPU-GO-TDA foam (d1, d2, d3, d4) at different magnifications. 162x114mm (300 x 300 DPI)


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Figure 4. Photographic images of water droplets and engine oil droplets on the foams. For ease of observation, water and engine oil were dyed with methylene blue and Sudan red, respectively, water droplets and engine oil on the PU foam (a1), TPU foam (a2), PU-GO-TDA foam (a3) and TPU-GO-TDA foam (a4); Images of water droplets on the PU foam (b1), TPU foam (b2), PU-GO-TDA foam (b3), TPU-GO-TDA foam (b4) and TPU-GO-TDA foam after repeated uses for oil/water separation (b5); PU foam sink in water (c1), PU foam sink in water while the PU-GO-TDA remained floating on the water surface (c2), Unmodified TPU foam began to sink in water while the TPU-GO-TDA foam remained floating on water surface (c3); when forced to immerse in water, the TPU-GO-TDA foam showed a mirror-like surface (c4). 102x82mm (300 x 300 DPI)



Figure 5. TGA thermograms of the PU foam, GO-TDA, TPU foam and TPU-GO-TDA foam. 120x89mm (300 x 300 DPI)


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Figure 6. Pictures showing chloroform/water separation using PU foam (a1, a2, a3, a4), TPU foam (b1, b2, b3, b4) and TPU-GO-TDA foam (c1, c2, c3, c4). 109x109mm (300 x 300 DPI)



Figure 7. Pictures illustrating the TPU-GO-TDA foam absorption of engine oil from water surface (a1, a2, a3, a4, a5, a6) and TPU-GO-TDA foam absorption of chloroform from water. (b1, b2, b3, b4) 100x86mm (300 x 300 DPI)


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Figure 8. Absorption capacities (a), reusability (b, c) and recovery rates (d) of the TPU-GO-TDA foam, PUGO-TDA foam and the commercial PP sorbent for separating oil and organic solvents from water. 144x115mm (300 x 300 DPI)


Synthesis of Polyurethane Foams Loaded with TiO2 Nanoparticles

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