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Materials and Interfaces

Superhydrophobic Melamine Sponge Modified by Crosslinked Urea Network as Recyclable Oil Absorbent Materials Chih-Hsiang Chung, Wan-Chen Liu, and Jin-Long Hong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01595 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Industrial & Engineering Chemistry Research

Superhydrophobic Melamine Sponge Modified by Crosslinked Urea Network as Recyclable Oil Absorbent Materials

Chih-Hsiang Chung, Wan-Chen Liu and Jin-Long Hong* Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan.

ABSTRACT: A novel “urea crosslinking reaction”, involving the facile reaction between urea amide and isocyanate groups of an isocyanate-terminated poly(dimethyl siloxane) (iPD), was employed to fabricate superhydrophobic melamine sponge (SMS) as efficient oils- and organic solvents-absorbent material. By heating, isocyanate terminals of iPD can react with the secondary amine groups of melamine sponge (MS), rendering covalent urea bonds over the surface of MS skeleton. The resultant SMS exhibits a high water contact angle (WCA) of 153.4o, excellent volumetric absorption capacity (1.163 ~ 1.661 m3/m3), superior selectivity and high absorption capacity retention (85.1 ~ 98.7 %) over 30 sorption-squeezing cycles for different oils and organic solvents. The fabrication procedure of SMS is simple and cost-effective and the robust, recyclable SMS is efficient in the separation of various oils and organic solvent/water mixtures. Therefore, a readily available process for the environmental cleanups and remediation was provided in this study.

Keywords: Superhydrophobicity, urea chemistry, MS, oil absorbent materials.

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INTRODUCTION A variety of technologies, including in situ burning,1,2 chemical dispersant,3 skimming,4,5 bioremediation6,7 and use of oil absorbent materials8−15 had been used for the separation of oils from water. Among them, use of oil absorbent materials belongs to one of the effective technologies due to its propensity for oil collection and separation. Moreover, by the use of recyclable oil absorbent materials16−18 with high efficiency, the cost and the chance of secondary pollution can be reduced. In this respect, 3D porous oil absorbent materials19, such as foams, sponges, aerogels, and xerogels, and those which use microparticles (MPs) and nanoparticles (NPs), with combined superhydrophobic and superhydrophilic wetting properties have become one effective approach for removing oils from water surfaces. Polyurethane (PU),20 poly(melamine-formaldehyde),21−23 poly(dimethyl siloxane) (PDMS),24 poly(vinylidene fluoride),25 and carbon/graphene-based sponges,11,26 have been used to modify sponges, rendering absorbent materials for oil absorption applications. Varieties of methods, such as chemical vapor deposition (CVD),27 sol-gel,28 dip-coating29 and addition of NPs and MPs were also applied for the functionalization of porous materials. All the study suggested that the recyclability and cost-effectiveness are crucial issues remained to be further improved.30 Recent studies of novel oil absorbent materials have been focused on superhydrophobic sponges and sponge-like materials. PDMS-based oil absorbent with high selectively, fast absorption, and excellent reusability can be prepared by a sugar-template method. However, its absorption capacities for oils and organic solvents are relatively on (4−11 g/g).24 Carbon nanotube (CNT)-based sponges from CVD are light in weight (bulk density of 5−10 mg/cm3), highly porous, superhydrophobic with a WCA of 156o and with high gravimetric absorption capacity ~180 g/g.11 For the ultralight, nitrogen-doped graphene frameworks their absorption 2


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capacities for common pollutants, organic solvents, and oils are excellent 200−600 g/g.14 Graphene was further combined with CNT to produce ultralight, superelastic aerogels with excellent absorption capacity,31 too. However, all these CNT and graphene-based absorbent systems involved the high cost, expensive CVD process, extremely high processing temperature, associated complicate preparation and existing challenges in scaling their fabrication, which becomes issue to be solved. The surface modification of commercial polymer sponges and natural materials have tuned the surface properties of the sponges from hydrophilicity to superhydrophobicity.13,26,32−38 Modified polymer sponges have several superior advantages, such as low density, high porous, good elasticity, compression, recyclability, availability, nontoxicity and flexibility and most importantly, excellent absorptions for oils. Herein, a reduced graphene oxide was synthesized via a cost-effective, green thermal reduction process and was used for the modification of MS to generate material for sewage treatment.39 Hydrophobic bacterial cellulose aerogels had been modified through a trimethylsilylation process followed by freeze-drying, which provided alternative route for multi-functionalization of cellulose aerogel without the CVD method.40 However, the nature cellulose has the inherent drawback that it is not stable in acidic or alkali environment. Integration of 3D porous graphene foam with pH‐responsive block copolymer resulted in smart material with switchable superoleophilicity and superoleophobicity dependent on the medium pH.41 MS was also treated with an octadecylsiloxane coupling agent to create hydrophobic sponge with a ~70 g/g absorption capacity and good recyclability.42 In addition, a two-step grafting process was conducted on PU sponge which generating modified sponge with a ~40 g/g absorption capacity but excellent recyclability for 50 cycles.43 These chemically modified sponges exhibited quite good oil absorption capacity. However, their recyclability by the convenient mechanical 3



sorption-squeezing process seems to be worth considerate. During the sorption-squeezing cycle, the hydrophobic coating easily detached from the surface of the sponge skeleton due to the weak adhesion between coating and sponge. Therefore, fabrication of robust superhydrophobic sponges that can withstand multiple cycles of sorption-squeezing continues to be challenge. In this study, a facile urea chemistry between an amino-terminated polydimethylsiloxane, aPD (Scheme 1) and methylenediphenyl diisocyanate (MDI, 2eq.) was developed for the preparations of novel crosslinked urea network and SMS. The key intermediate iPD, derived from the facile reaction of aPD and MDI, can be either heated alone to form crosslinked urea network cPD (Route a, Scheme 1) or react with the secondary amino groups of MS to generate SMS (Route b, Scheme 1) as absorbent material for oils and organic solvents. Unlike previous attempts44−46 that the involved isocyanate-terminated polymers were always used for the preparation of linear PU, the intermolecular crosslinking reaction between the urea amide groups and the isocayante terminals of iPD actually provides facile route for the preparation of homogenous cPD films with the desired hydrophobicity. Moreover, the isocyanate terminals of iPD provide convenient reaction sites for the secondary amino groups of MS, which form covalent urea bonds on the surface of the resultant SMS skeleton. The cPD-modified SMS is superhydrophobic and is incremental advance and was found to be efficient absorbent material for various oils and organic solvents. Unlike the physically-attached coatings of sponges, the stronger covalently-bonded urea linkages of SMS can tolerate repeated sorption-squeezing cycles and increase the absorption capacity. The SMS product can absorb a wide range of organic solvents and oils with excellent volumetric absorption capacities (1.214 ~ 1.661 m3/m3) and exhibit high absorption capacity retentions (85.1 ~ 98.6 %) after 30 sorption-squeezing cycles. The urea chemistry investigated here is rather simple and 4


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is readily scalable. Therefore, we hope that a facile, selective and cost-effective methodology for large-scale removal of oils and organic pollutants is provided in this study.

EXPERIMETAL SECTION Materials. The aPD5K (Mw ~ 5,000 g/mol, Gelest), aPD27K (Mn ~ 27,000 g/mol, Sigma-Aldrich), MDI, Span 80 as a surfactant (Acros), MS (BASF), motor oil (Castrol), lubricant oil (MR-100, Morescro), edible oil (Morescro) were used directly. Tetrahydrofuran (THF), used for the preparation of iPD, was refluxed with benzophenone and sodium before distillation for use. Preparations of iPD and the crosslinked cPD film. For the preparation of cPD film, iPD was required to be prepared first by the reported procedures.47 Before reaction, aPD needed to be dried under vacuum at 80 oC for 1 hr. Then, into an argon-blanketed, vigorously-stirred solution of aPD (0.5 g, 0.1 mmol) in THF (10 mL), solution of MDI (55 mg, 0.2 mmol) in THF (5 mL) was added dropwise. The resultant mixture was then heated at 60 oC for 5 hr under argon atmosphere. The reaction mixtures were then subjected to vacuum distillation at 70 oC to remove most of the residual THF. The resultant viscous slurry was then placed in a Teflon container and dried in an oven at 120 oC for 12 hr to afford the crosslinked cPD film. Preparation of SMS. The unmodified MS (2 × 1 × 1 cm3) was subjected to ultrasonic cleaning in hexane/ethanol before being heated to dry at 60 oC for 3 hr. The cleaned sponge was then immersed into a solution of iPD (~ 0.001 g/mL) in THF and the whole solution mixtures were allowed to stir for another 2 hr for maximizing the absorption of iPD. Then, the iPD-coated sponge was heated to cure at 120 oC for 3 hr to obtain the desired SMS (~18 mg/cm3). Preparation of water-in-oil emulsions. Surfactant-free water-in-oil emulsions 5



were prepared by mixing water with an oil (n-hexane, n-hexadecane, or toluene) in 1/9 v/v ratio and then subjected to ultrasonic agitation for 1 hr to produce milky emulsion solution. To demonstrate, surfactant-stabilized water-in-oil emulsion was also prepared. Span 80 (0.04 g) was added into mixture solution of oil (100 mL) and water (2 mL) and the resultant solution was then stirred for 3 hr to prepare surfactant-stabilized emulsion for separation. Characterizations. The 1H NMR spectra were recorded by a Varian Unity VXR-500 MHz instrument. FTIR was obtained from a Bruker Tensor 27 FTIR spectrophotometer, with 32 scans collected at a spectral resolution of 4 cm-1. The UV-vis absorption spectra were recorded with an Ocean Optics DT 1000 CE 376 spectrophotometer. A Krüss GH-100 goniometer interfaced to image-capture software was used to measure the static WCA. For the dynamic sliding angle and hysteresis angle of water droplet (5 uL) over surface of SMS were measured using an FDSA MagicDroplet-100; each reported contact angle represents the average of six measurements. For sliding angle measurement, SMS27K was immersed in hexane and during the long immersion time of 6 hr, SMS27K was removed every 30 minutes and dried before performing sliding angle tests. Atomic force microscopy (AFM) was performed in dynamic force mode using a Hitachi AFM5300E scanning probe microscope to examine the surface morphology of the film. Root-mean-square (RMS) roughness was calculated over scan area of 5 µm × 5 µm. The elemental composition of the surface of the sponges was determined by X-ray photoelectron spectroscopy (XPS), conducted in an ultrahigh vacuum chamber equipped with a Phoibos 100 MCD analyzer. The thermal decomposition temperatures and char yields were determined using a TA Q-50 thermogravimetric analyzer (TGA) operated under a N2 atmosphere with a heating rate of 20 °C min-1. Scanning electron microscope (SEM) images were recorded using a Jeol JSM-6700F microscope operated at 10 kV. Optical 6


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microscopy images were recorded using an Olympus BX51M instrument after placing a drop of an emulsion solution onto a biological counting board. Absorption capacity and recyclability of SMS. SMS was immersed into pools of various oils or solvents until they were saturated. The gravimetric absorption capacity G (m/m), gravimetric/volumetric absorption capacity M (m/v), and volumetric absorption capacity V (v/v) were calculated according to the following equations: Gm/m =

m1 − m0 m0

Mm/v = Gm/mds = V m/v =

m1 − m0 ds m0

M m/v m1 − m0 ds = d0 m0 d0

in which, m0 is the weight of the pristine SMS and m1 is the weight of the saturated SMS and ds and do are the bulk density of the sponge and the density of absorbed oils (or organic solvents), respectively. The sorption−squeezing was performed by immersing the SMS into oils (or organic solvents), waiting until the sponge became saturated with the oils (or organic solvents), and then manually squeezing the sponge using a clamp to extract the absorbed oils.48 To assess the absorption recyclability, the sorption-squeezing process were repeated for 30 cycles and the absorption capacities were evaluated by the above equations.

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Scheme 1. Synthesis of iPD as key intermediate for the preparations of (a) crosslinked cPD and (b) SMS absorbent material.

RESULT AND DISCUSSION Crosslinking reaction of iPD to obtain cPD film. As illustrated in Scheme 1, the key intermediate iPD needed to be prepared from the facile reaction between aPD and MDI (2 eq.). The reaction brought one of the two isocyanate groups of MDI into urea linking groups of iPD. At a higher temperature of 120 oC (Route a), interchain crosslinking reaction between the urea -NH groups in one iPD chain and the isocyanate terminals in another iPD chain occurred to result in crosslinked network structure of cPD. The crosslinking reaction was previously treated as undesired side reaction.44 The isocyanate-terminated polymers was used as precursors for the preparation of linear PU. To secure the PU with the desired linear chain architecture, the isocyanate-terminated polymers were refrained from heating at high temperatures to void the accompanied crosslinking reaction. Nevertheless, in our study, this 8


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undesired but facile crosslinking reaction at 120 oC was used for the preparation of high-quality, flexible and transparent crosslinked films of cPD. The key to high-quality crosslinked cPD film (Figure S2) is the preparation of homogenous iPD slurry from heating the solution of aPD and MDI in THF at two different temperatures. At 60 oC, facile reaction between aPD and MDI afforded the key intermediate of iPD. At 70 oC, most of THF solvent was removed to generate homogeneous iPD slurry for further curing reaction. Therefore, heating this iPD slurry at 120 oC conveniently afforded homogeneous cPD film, cPD5K and cPD27K with high flexibility, 21.4% and 74.8% (strain) respectively, and good transparency (Figure S2, S3). A low MW (Mn = 5,000 g/mol) aPD5K was used to produce iPD5K with detectable 1H NMR signals (Figure 1a) for analysis. For aPD5K, its terminal amino protons a resonate atδ1.5. Reaction with MDI converted all amino groups of aPD5K into urea amide linkages of iPD5K. The corresponding urea amide protons a and i appear atδ5.3 and 6.2, respectively. These urea amide protons can be used to calculate the exact number of urea linkages per iPD chain, simply by comparing their integration intensity to that of the methyl protons e of the methylsiloxane main chain. The result suggests that 97 % of the iPD chains are end-capped with MDI units, which is not perfect but reasonably correlated with the reaction condition. The multiple resonance peaks atδ7.0-7.2 and 7.5 are due to the aromatic protons g, h and k of the incorporated MDI terminals.

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Figure 1. (a) 1H-NMR spectra of aPD5K and iPD5K (CDCl3) and (b) FTIR spectra of iPD5K and cPD5K (panel: magnified amide carbonyl bands). To characterize the crosslinking reaction and the hydrogen bond (H-bond) interactions involved in urea linkages,49 FTIR spectra of iPD5K and cPD5K were compared in Figure 1b. Isocyanate –NCO groups of iPD5K exhibit the characteristic stretching band at 2265 cm-1. Completeness of the crosslinking reaction was confirmed by the absence of the –NCO peak in the spectrum of cPD5K. For iPD5K, the ordered, bidentate H-bond interactions, between the donating –NH in one urea linkage and the accepting –C=O group in another urea linkage,50 resulted in the amide I and II absorption peaks at 1638 cm-1 and 1571 cm-1 (Figure 1b inset), respectively. In contrast, spectrum of cPD5K contains only broad, multiple absorption peaks in the corresponding range from 1500 cm-1 to 1700 cm-1. Here, the urea carbonyl –C=O stretching of cPD5K appears as a tiny peak at 1645 cm-1 over the broad absorption background. For cPD5K, the broad amide stretchings reflect the amorphous nature of the cured cPD5K. Steric constraint imposed by the crosslinked network inhibited formation of ordered H-bonds between bidendate H-bonds of the terminal urea groups. 10


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Free and H-bonded −NH of the urea linkages absorb at 3448 cm-1 and 3363 cm-1,

respectively. A quick comparison of the spectra suggests that linear iPD5K is higher in the fraction of H-bonded −NH compared to crosslinked cPD5K. This observation is correlated with the above results from the amide absorption peaks, that is, crosslinked network of cPD is liable to form disordered H bonds. The cPD27K derived from high MW aPD27K (Mn = 27,000 g/mol) was prepared for the sake of evaluating the role of siloxane unit in the hydrophobicity of the cured cPD films. Comparatively, cPD27K film is superior in hydrophobicity to cPD5K film according to the resolved water contact angles (WCAs) of 126.5o and 118.5° (Figure 2a), respectively. The hydrophobicity of the cPD films is mainly determined by the hydrophobic siloxane units of the samples. With a higher siloxane content (~ 98.2 wt%), cPD27K film is therefore superior in hydrophobicity to cPD5K containing lower fraction of siloxane unit (~ 90.9%) even for swelling properties (Figure S4). High-resolution images from AFM (Figure 2b) provide the 2D and 3D surface microphotography of the cPD5K and cPD27K films, respectively. Suggestively, the resolved surface morphology should be determined by the majority of siloxane units of the cPD films. Therefore, the particles over the surface of cPD films should be attributed to the siloxane units of the samples. The images suggest that surface of cPD5K film contains more siloxane particles with the resolved particle sizes larger than the small amounts of siloxane particles over the surface of cPD27K film. The calculated root-mean-square (RMS) surface roughness value (Rq) (10.97 nm) of cPD5K is also higher than that (6.52 nm) of cPD27K.52 It is assumed that the surface particles can overlap with each other and at the moment they overlap, only the edge parts of the siloxane particles can merge together. Therefore, the resolved surface morphology became the true reflection of the unmerged parts of the siloxane particles.53 The cPD27K film displays a smooth surface because it contains more 11



siloxane units and shows fewer unmerged parts of siloxane groups than cPD5K film. The high surface siloxane content of cPD27K film therefore contributes to its superior hydrophobicity to cPD5K with lower surface siloxane units.

Figure 2. (a) Water droplets on the surfaces of cPD5K and cPD27K films and the resolved WCAs, and (b) the 2D and 3D AFM images of cPD5K and cPD27K films.

Modified SMS and its absorption properties. Unmodified MS have low density (4−12 mg/cm3), high porosity (> 99%), and excellent elasticity even compression, all of which are ideal characteristics for absorbent materials. Therefore, commercialized MS was used as based material for the modified SMS. Production of SMS was conducted by a simple dipping and heating process that MS was immersed in the solution of iPD in THF for 2 hr before heated to cure at 120 oC for 3 hr. During the cure step, two potential reactions are responsible for the transformation of the unmodified MS to the modified SMS: (i) inter-reaction between the isocyanate terminals of iPD and the secondary amino groups of the MS skeleton (Route b, Scheme 1). This reaction form covalent urea bonds as the junction linkage between MS and the crosslinked cPD network, (ii) the self-interaction between the urea amide 12


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group and isocyanate group of iPD, which resulted in crosslinked cPD network as the outer coating layer (Figure S1). The outer crosslinked cPD layer and the covalent urea linkages in the interphase region are factors leading to the high stability and absorption capacity of SMS product, which can sustain repeated sorption-squeezing cycles without noticeable reduction on its performance. XPS and TGA were used to characterize the SMS27K product. Primarily, the chemical compositions of unmodified MS and modified SMS27K were analyzed by XPS (Figure 3a). For the unmodified MS, the three peaks detected at 283, 403, and 531 eV are attributed to bonding energies of C 1s, N 1s, and O 1s, which are due to the chemical components of MS. The N 1s peak here is attributed to the secondary amino groups of MS, which is no longer seen in the spectrum of modified SMS. Suggestively, after reaction with the isocyanate groups of iPD, the secondary amino groups over the surface of MS skeleton were almost converted into urea linkages buried by the surrounding crosslinked cPD matrix and are difficult limitedly to be detected by XPS. For SMS, two new peaks at 101 and 152 eV are attributed to Si 2p and Si 2s of the siloxane units. The primary element components of Si, C, and O are therefore attributed to the main dimethylsiloxane units in the outer cPD layer. We also evaluated the coating adhesion robustness. The SMS was manually compressed flat and released for 30 cycles and the resultant SMS was further analyzed by XPS. The compression cycles caused no damage on SMS since all peaks and the resolved intensities are essentially the same with the pristine one without mechanical compression. The covalent urea bridge bonds and the crosslinked cPD layer are therefore strong enough to sustain the repeated compression cycles.

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Figure 3. (a) XPS spectra of the unmodified MS, SMS27K and SMS27K after 30 compression cycles and (b) TGA curves of MS, SMS27K and cPD27K (heating rate = 10 oC/min). Figure 3b shows TGA curves of MS, SMS27K, cPD27K characterized in the nitrogen atmosphere. Basically, TGA curve of MS can be divided into four temperature ranges of 30−100, 230−370, 370−405, and 405−600 oC, respectively.54 The ~5 wt% mass loss in the 30−100oC range can be attributed to evaporation of the moisture water absorbed by the hydrophilic MS. The main mass loss of ~30.0 wt % that occurred in temperature range of 370−405oC can be ascribed to the breakdown of the methylene linkages of MS. Mass losses at > 405 oC were attributed to thermal decomposition of the triazine ring. Decomposition behavior of SMS27K basically resembles to that of MS except some differences: (i) the mass loss of SMS27K in the first temperature region was only ~1.5 wt % because the superhydrophobic SMS27K is less liable to absorb moisture water, (ii) there are less mass loss in the temperature range of 370−550 oC, which may attribute to the high thermal stability of the incorporated cPD27K coating of SMS. This deduction can be confirmed by the less mass loss of pure cPD27K in the temperature range of 370−550 oC when compared to MS. Thus, the incorporated cPD27K layer acted to raises the thermal stability of SMS27K. 14


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SEM was used to identify the morphological difference between MS and SMS27K. Unmodified MS exhibits a 3D porous structure (Figure 4a) with pore diameters ranging from tens to hundreds micrometers, which is important for the absorption and preservation of adsorbate. The magnified image of MS (Figure 4b) shows that the surface of the sponge skeleton is rather smooth. After coated with cPD27K, SMS still maintains the 3D porous structure (Figure 4c) and is packed more compactly like spider mesh coating, which suggests that the basic structure of MS was not damaged with cPD27K coating. However, the rough sponge skeletons covered or wrapped by crosslinked cPD27K could be clearly observed. The magnified image (Figure 4d) further reveals the presence of rough, folding edges over the surface of sponge skeletons. The results confirm that SMS27K was successfully prepared through thermal curing of iPD on the sponge skeletons. The rough, wrinkled surface of SMS is of great importance for preparation of superhydrophobic materials. Such a morphology raised the surface roughness dramatically and provided a composite interface in which air became trapped within the grooves beneath the liquid, thereby inducing superhydrophobicity.21

Figure 4. SEM images of (a, b) unmodified MS and (c, d) SMS27K. 15



For comparison, SMS5K product was also fabricated through the same dipping and heating process. As cPD27K is superior in hydrophobicity to cPD5K, SMS27K derived from iPD27K is also superior in hydrophobicity to SMS5K derived from iPD5K according to their respective WCAs (Figure S5) of 153.4 o and 144.2 o. The resolved hydrophobicity of the modified sponges is due to the hydrophobic cPD coating over the surface of SMS skeleton. Derived from the high MW iPD27K, SMS27K contains higher fraction of hydrophobic siloxane units and therefore, is higher in hydrophobicity than SMS5K with lower siloxane content. The superior SMS27K product was therefore used as absorbent materials for a variety of oils and organic solvent. Figure 5a shows a photograph with a pure MS (white color) sank to the bottom of beaker and a modified SMS27K (light orange color) floated on the water surface. When SMS27K was forced into water by an external force, it floated on the top of the water surface immediately after the force was released, and no water uptake was found by examining the sample’s weight afterward. The silver mirror-like appearance of the immersed SMS27K (inset) is attributed to a uniform air layer trapped between the water and the superhydrophobic surfaces of SMS27K, which refers to the non-wetting Cassie–Baxter surfaces.55 SMS27K also shows superior stability towards different environmental variables. The acidic, alkaline and salty water droplets (Figure 5b) still attained in the spherical shapes (with the resolved WCAs all greater than 150 o) after standing on the surface of SMS27K for more than 12 hr. Moreover, water droplets of different temperatures (Figure 5c) are also in spherical shapes on the surface of SMS27K. In contrast, when the lubricant oil was dropped on the surface of SMS, it was immediately absorbed by the sponge (marked by red circle). Therefore, SMS27K is stable and durable to different environments, which is attributed to its stable crosslinked cPD coating.

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Figure 5. (a) Unmodified MS (white color) and modified SMS27K (light orange color) after being forced into contact with water (inset: partially-immersed SMS27K with silver mirror-like appearance), (b) acidic, salty and alkali water droplets on the surface of SMS27K (all water droplets had been placed on SMS27K for over 12 hr) and (c) water droplets of different temperatures and trace of lubricant oil on the surface of SMS27K.

Water sliding angle and WCA hysteresis of SMS27K were also measured to demonstrate the stability of the modified sponge. For the water sliding angle measurement, SMS27K is rather stable and can be sustained for long-term immersion in hexane. During a long period of 6 hr, SMS was removed every 30 minutes and dried before performing sliding angle tests. The water sliding angle (Figure S6) for SMS27K was determined to be 10.0°, but the water sliding angle of MS could not be measured since the added water droplet penetrated quickly into the sponge at the contact moment. We also conducted WCA hysteresis of SMS27K. The results (Figure S7 and Table S1) show an advancing contact angle of 153.5 + 1.9°, a receding contact angle of 148.0 + 2.4° and a hysteresis of 5.5°, respectively. As shown in Figure 6a and b, when the SMS27k was held to approach oils (dyed in red) in water, the oils droplet 17



was rapidly sucked up by the sponge upon contact for few second. SMS27K also can selectively absorbed oils or organic solvents from water in Figure S8. To demonstrate, SMS27K was held to absorb oils in water and the excess absorbed liquids squeezed out from SMS27K contain only pure oils (dyed in red, Figure 6c). In contrast, excess absorbed liquids absorbed by unmodified MS contain both water droplets and oils (Figure 6d). The selective oil absorption capability of SMS27K is thus demonstrated.

Figure 6. (a, b) Snapshots showing the absorption of oil (dyed in red) from water, the extruded liquids from (c) SMS27K and (d) MS saturated with absorbed oil/water. SMS27K can be also applied in a continuous separation process for the hexane in water. To begin with, both surfactant-stabilized and surfactant-free water-in-hexane emulsions were primarily prepared. The resultant emulsion solutions were then placed over the top of a SMS27K filter inserted in a vacuum suction system (Figure 7a). When the vacuum system was on, the emulsion solution was continuously filtered through the SMS filter and the collected filtrate was analyzed. We may tell the separation efficiency by the optical difference between the feed emulsion solution and the filtrate (Figure 7b, c). The feed emulsion is turbid; in contrast, no water droplet is 18


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evident in the collected clear filtrate. The high purity of the collected hexane filtrate suggests the high efficiency of SMS27K for continuously separation of hexane from both surfactant-stabilized and surfactant-free water-in-hexane emulsions. As no droplet was present in the collect filtrate over the whole view, the water should have been removed from both emulsion solutions. Transmittance measurements were further conducted. The results (Figure S8) suggest that the filtrates are highly transparent, in contrast to the opaque staring emulsions.

Figure 7. (a) SMS27K as filter for continuous separation of hexane/water emulsion in a vacuum suction apparatus. Photographs of (b) surfactant-stabilized and (c) surfactant-free water-in-hexane emulsions. The distinct appearance between the feed emulsion and the hexane filtrate under normal and optical images.

SMS27K exhibits high gravimetric absorption capacities, ranging from 54 to 100 g/g time its own weight (Figure 8a), for different kinds of oils and organic solvents. Here, the resolved absorption capacity for chloroform is exceptionally high (~100 g/g), however, this is due to the high density of chloroform solvent. Use of gravimetric absorption capacity may not provide practical evaluation on the real performance of 19



Page 20 of 46

absorbent materials, which will be discussed later. The most important point, the crosslinking nanostructures can increase the amount of absorption capacity by its swelling inherence different from others absorbent materials. Besides absorption capacity, the recyclability of modified sponge is also an important property in oil/chemical cleanup applications. Herein, the absorbed oils and organic solvents were harvested by manually squeezing the sponges. Figure 8b shows the resolved gravimetric absorption capacities during recyclable uses of the SMS27K for a variety of oils and organic solvents. The recyclable absorption capacity curves for all tested oils and organic solvents follow similar pattern that there are some minor decreases of the absorption capacities during the initial 7 cycles but afterwards, the resolved absorption capacities remain almost the same for the rest of 23 cycles. The calculated absorption capacity retentions of SMS27K for different oils and organic solvents are high, ranging from 85% to 99 %, after 30 recycled uses (Table 1). The initial decrease of the absorption capacity is due to the residual oils (or organic solvents) inside the sponges, which cannot be removed by manual squeeze.

Table 1. Absorption capacity and absorption capacity retention after 30 cycles of sorption-squeezing of SMS27K for various oils and organic solvents. Solvent/Oil

Density 3

[g/cm ]

G [g/g]

M

V 3

[kg/m ]

3

Absorption capacity 3

[m /m ]

retention after 30 cycles [%]

chloroform

1.48

100.1

1795.9

1.214

98.6

hexane

0.66

60.4

1095.8

1.661

87.0

hexadecane

0.77

54.1

980.0

1.267

85.1

toluene

0.87

61.2

1110.7

1.276

98.7

lubricant oil

0.86

55.1

1000.2

1.163

86.2

motor oil

0.86

61.4

1113.3

1.294

91.1

20


Page 21 of 46 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


edible oil

0.88

59.4

1077.2

1.224

87.3

Considering of those studies of the oil absorbent materials, the gravimetric absorption capacity density greatly depends on the bulk density, much more than on the porosity of the absorbent materials. Therefore, the absorbent materials that have low or ultralow bulk densities typically exhibit very high gravimetric absorption capacities. However, using volumetric absorption capacity (m3/m3) should be more appropriate than gravimetric absorption capacity (g/g) in assessing the real capability of the absorbent materials. As illustrated in Table 1, the gravimetric absorption capacity of SMS27K for chloroform is higher than for motor oil (100 g/g vs. 61 g/g). However, the calculated volumetric absorption capacity for chloroform is 1.214 m3/m3, which is lower than the resolved value of 1.294 m3/m3 for motor oil. Absorption capacities, including G, M and V, of SMS27K were then compared with previous results in Table 2. Among them, carbon absorbents, such as carbon fiber areogel,56 graphene-CNT aerogel,57 carbon microbelts,58 and nanobacterial fibers,59 are the representatives of novel superabsorbents, which show ultra-high G values. Absorbents made from cellulose nanofibrils60 also exhibited high G value (up to102 g/g) for various oils and organic solvents. Comparatively, the resolved G values (from 62 g/g to 100 g/g) of our SMS27K product are lower than those of the carbon absorbents mentioned above. However, the resolved V values (from 1.163 m3/m3 to 1.294 m3/m3) of SMS27K are higher than most of the absorbents listed in Table 2. Although mono-layered graphene-coated MS26 exhibits a higher V value of 1.259 m3/m3 for chloroform, its recyclability is rather poor considering the weak interaction force between the physically-coated graphenes and sponge skeleton of MS. Theoretically, the resolved G value greatly depends on the bulk density, which is strongly related to the porosity of the absorbent materials.61 In other words, absorbent 21



materials that have low or ultralow bulk density generally exhibit very high G value. However, their V values may be lower than other absorbent materials that have low gravimetric absorption capacity. For example, graphene-CNT aerogel has a high G (up to 568 g/g) for chloroform but the calculated V is only 0.537 m3/m3, which is significantly lower than that (= 1.135 m3/m3) of marshmallow-like macroporous gel.62 In other words, the marshmallow-like macroporous gel can retain a larger volume of chloroform than the volume that is encapsulated by the exterior surface of the gel itself (by a factor of 1.135). With this respect, use of V as index of absorption capability is practically more reasonable than using G. Our SMS27K exhibits high V values for motor oil and organic solvents. The fabrication of SMS27K involves inexpensive staring chemicals of aPD and MDI and a rather simple dipping and heating process. In addition, SMS also possesses markedly enhanced recyclability by the simple sorption-squeezing process (Figure S10). All these superior properties make SMS27K a very promising material for oils/pollutants spill remediation applications.

Figure 8. (a) Gravimetric absorption capacities (g/g) of SMS27K for different oils and organic solvents and (b) the corresponding gravimetric absorption capacities at different sorption-squeezing cycles.

22


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Table 2. Comparison of sponge and sponge-like absorbent materials. Absorbent materials

Solvent / oil

G [g/g]

M

Recyclability by

V 3

[kg/m ]

3

3

[m /m ]

Ref

sorption-squeezing process

CNT PU sponge by

chloroform

176

1038

0.701

diesel oil

144

849

0.998

chloroform

490

1029

0.695

olive oil

480

1008

1.095

chloroform

195

-

0.90

pump oil

100

-

0.80

soybean oil

105

-

0.83

toluene

125

-

0.98

chloroform

165

1864

1.259

pump oil

92

1040

1.155

Reduced mono-layered

chloroform

160

1408

0.951

graphene oxide-coated

pump oil

100

880

0.978

PU sponge by chemical

diesel oil

85

748

0.87

soybean oil

110

968

1.125

CNT/PDMS-coated PU

diesel oil

25

875

1.017

sponge

motor oil

23

805

0.936

chloroform

140

-

-

graphene foam by

pump oil

80

-

-

thermal reduction

toluene

78

-

-

Smart surface graphene

chloroform

196

-

-

foam modified by block

pump oil

72

-

-

toluene

68

-

-

chloroform

115

1380

0.932

olive oil

85

1020

1.133

chloroform

568

795

0.537

motor oil

341

477

0.542

chloroform

150

870

0.588

pump oil

188

1090

1.267

chloroform

280

1400

0.946

aerogel from bacteria

pump oil

140

700

0.814

cellulose

diesel oil

165

825

0.960

chloroform

205

615

0.416

motor oil

102

306

0.356

CVD N-doped mono-layered graphene framework Fluorinated mercapto-functionalized polydopamine coated MS Mono-layered graphene-coated MS

reduciton

Reduced mono-layered

copolymer Carbon fiber aerogel from raw cotton Graphene-CNT aerogel Carbon microbelts from waste paper Carbon nanofiber

Cellulose nanofibrils by silylation

23


excellent

11

poor

14

outstanding

23

poor

26

good

33

excellent

37

good

39

-

41

fair

56

excellent

57

poor

58

poor

59

good

60


sMS by silanization

chloroform

163

1337

0.903

mineral oil

101

862

0.938

Marshmallow-like

chloroform

14

1680

1.135

macroporous gel

mineral oil

8

960

1.090

SMS27K

chloroform

100

1796

1.214

toluene

62

1111

1.276

motor oil

61

1113

1.294

Page 24 of 46

outstanding

61

-

62

good

*

* present work

CONCLUSIONS In summary, the novel hydrophobic siloxane/urea cross-linked cPD network can be conveniently prepared by simple heating and cure process. By heating the solution mixture of aPD and MDI in THF at 60 oC and at 70 oC, isocyanate-terminated iPD slurry can be readily prepared. Cure of the iPD slurry at 120 oC afforded flexible, crosslinked cPD films with high transparency and hydrophobicity. Use of high MW aPD27K resulted in cPD27K film with higher hydrophobicity than cPD5K film derived from low MW aPD5K. It is interesting to note that SMS product can be readily obtained by heating the iPD-coated MS. Favorable reaction between secondary amines of MS and isocyanate terminals of iPD turned the hydrophilic MS into superhydrophobic material with high absorption capacity for a variety of oils and organic solvents without extremely high processing temperature. The resultant SMS27K shows a WCA 153.4 o and exhibits excellent volumetric absorption capacities ranging from 1.163 to 1.661 m3/m3, high selective absorption, highly recyclability with absorption capacity retentions ranging from 85.1 to 98.7 % after 30 cycles of sorption-squeezing process, lightweight, robustness, and inertness towards corrosive environment. The simple and low-cost process and wide availability here might be a promising absorbent material makes SMS27K attractive for oils/chemicals spill remediation applications in the ocean. 24


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ASSOCIATED CONTENT Supporting information Mechanism of the chemical structures of the unmodified MS and modified SMS; photographs of iPD5K slurry, cPD5K, cPD27K and UV-vis spectra and Stress/Strain curve of cPD5K and cPD27K; gravimetric absorption capacities of SMS5K and SMS27K for different oils and organic solvents; photographs and WCAs of modified SMS5K and SMS27K films; snapshots showing the process of selective absorption of chloroform from water; UV-vis spectra of separation of surfactant-stabilized and surfactant-free water-in-hexane emulsions; photographs of droplets of oils and organic solvents on the surfaces of SMS27K. Movie S1 showing the water sliding angle (=10o); Movie S2 clips showing the selective absorption of oil in water by a simple suction system; Movie S3 snapping the selective absorption of chloroform in water by SMS27K; Movie S4 separation of chloroform from water by a stirred SMS27K; and Movie S5 continuous filtration and separation of hexane-water emulsion by the vacuum suction system.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Tel: +886-7-5252000-4065. ORCID Jin-Long Hong: 0000-0003-1066-5818 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes 25



The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the financial supports of the project sponsored by the Ministry of Science and Technology, Taiwan, under the contract MOST105-2221-E-110-091-MY2.

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33



We provided “urea chemistry” potential application and incremental advance between film to sponge they exhibited superhydrophobic and cost-effective route as oil absorbent materials.

34


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Table 1. Absorption capacity and absorption capacity retention after 30 cycles of sorption-squeezing of SMS27K for various oils and organic solvents. Solvent/Oil

Density [g/cm ]

G [g/g]

[kg/m ]

[m /m ]

Absorption capacity retention after 30 cycles [%]

chloroform

1.48

100.1

1795.9

1.214

98.6

hexane

0.66

60.4

1095.8

1.661

87.0

hexadecane

0.77

54.1

980.0

1.267

85.1

toluene

0.87

61.2

1110.7

1.276

98.7

lubricant oil

0.86

55.1

1000.2

1.163

86.2

motor oil

0.86

61.4

1113.3

1.294

91.1

edible oil

0.88

59.4

1077.2

1.224

87.3

3

a

Gravimetric absorption capacity [g/g]

b

Gravimetric/volumetric absorption capacity [kg/m3]

c

Volumetric absorption capacity [m3/m3]

M

V 3


3

3


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Table 2. Comparison of sponge and sponge-like sorbent materials. Absorbent materials

Solvent / oil

G [g/g]

M [kg/m3]

V [m3/m3]

Recyclability by sorption-squeezing process

Ref

CNT PU sponge by CVD

chloroform diesel oil

176 144

1038 849

0.701 0.998

excellent

11

N-doped mono-layered graphene framework

chloroform olive oil

490 480

1029 1008

0.695 1.095

poor

14

Fluorinated

chloroform

195

-

0.90

outstanding

23

mercapto-functionalized polydopamine coated MS

pump oil soybean oil toluene

100 105 125

-

0.80 0.83 0.98

Mono-layered graphene-coated MS

chloroform pump oil

165 92

1864 1040

1.259 1.155

poor

26

Reduced mono-layered graphene oxide-coated PU sponge by chemical reduciton

chloroform pump oil diesel oil soybean oil

160 100 85 110

1408 880 748 968

0.951 0.978 0.87 1.125

good

33

CNT/PDMS-coated PU sponge

diesel oil motor oil

25 23

875 805

1.017 0.936

excellent

37

Reduced mono-layered graphene foam by thermal reduction

chloroform pump oil toluene

140 80 78

-

-

good

39

Smart surface graphene foam modified by block copolymer

chloroform pump oil toluene

196 72 68

-

-

-

41

Carbon fiber aerogel

chloroform

115

1380

0.932

fair

56

olive oil

85

1020

1.133

Graphene-CNT aerogel

chloroform motor oil

568 341

795 477

0.537 0.542

excellent

57

Carbon microbelts from waste paper

chloroform pump oil

150 188

870 1090

0.588 1.267

poor

58

Carbon nanofiber aerogel from bacteria cellulose

chloroform pump oil diesel oil

280 140 165

1400 700 825

0.946 0.814 0.960

poor

59

Cellulose nanofibrils by

chloroform

205

615

0.416

good

60

motor oil

102

306

0.356

chloroform

163

1337

0.903

outstanding

61

from raw cotton

silylation sMS by silanization


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mineral oil

101

862

0.938

Marshmallow-like macroporous gel

chloroform mineral oil

14 8

1680 960

1.135 1.090

-

62

SMS27K

chloroform toluene motor oil

100 62 61

1796 1111 1113

1.214 1.276 1.294

good

*

* present work



Scheme 1. Synthesis of iPD as key intermediate for the preparations of (a) crosslinked cPD and (b) SMS absorbent material.


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Figure 1. (a) 1H-NMR spectra of aPD5K and iPD5K (CDCl3) and (b) FTIR spectra of iPD5K and cPD5K (panel: magnified amide carbonyl bands).



Figure 2. (a) Water droplets on the surfaces of cPD5K and cPD27K films and the resolved WCAs, and (b) the 2D and 3D AFM images of cPD5K and cPD27K films.


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Figure 3. (a) XPS spectra of the unmodified MS, SMS27K and SMS27K after 30 compression cycles and (b) TGA curves of MS, SMS27K and cPD27K (heating rate = 10 oC/min).



Figure 4. SEM images of (a, b) unmodified MS and (c, d) SMS27K.


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Figure 5. (a) Unmodified MS (white color) and modified SMS27K (light orange color) after being forced into contact with water (inset: partially-immersed SMS27K with silver mirror-like appearance), (b) acidic, salty and alkali water droplets on the surface of SMS27K (all water droplets had been placed on SMS27K for over 12 hr) and (c) water droplets of different temperatures and trace of lubricant oil on the surface of SMS27K.



Figure 6. (a, b) Snapshots showing the absorption of oil (dyed in red) from water, the extruded liquids from (c) SMS27K and (d) MS saturated with absorbed oil/water.


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Figure 7. (a) SMS27K as filter for continuous separation of hexane/water emulsion in a vacuum suction apparatus. Photographs of (b) surfactant-stabilized and (c) surfactant-free water-in-hexane emulsions. The distinct appearance between the feed emulsion and the hexane filtrate under normal and optical images.



Figure 8. (a) Gravimetric absorption capacities (g/g) of SMS27K for different oils and organic solvents and (b) the corresponding gravimetric absorption capacities at different sorption-squeezing cycles.


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Superhydrophobic Melamine Sponge Modified by ... - ACS Publications

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