Journal ofMaterials Chemistry A

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A magnetic and

aDipartimento di Scienze e Tecnologie B

Universita del Salento, Via Monteroni, 731

unisalento.it; elisabetta.mazzotta@unisalenbDipartimento di Ingegneria dell'Informazio

56122, Pisa, ItalycDipartimento di Ingegneria dell'Innovazion

73100, Lecce, Italy

† Electronic supplementary information10.1039/c5ta04353k

Cite this: J. Mater. Chem. A, 2015, 3,17685

Received 15th June 2015Accepted 23rd July 2015

DOI: 10.1039/c5ta04353k

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

highly reusable macroporoussuperhydrophobic/superoleophilic PDMS/MWNTnanocomposite for oil sorption from water†

A. Turco,*a C. Malitesta,a G. Barillaro,b A. Greco,c A. Maffezzolic and E. Mazzotta*a

Oil/water separation is a worldwide challenge to prevent serious environmental pollution. The development

of sorbent materials with high selectivity, sorption capacity, easy collection and recyclability is of high

demand for spilled oil recovery. In this field, magnetic controllable materials have received wide

attention due to the possibility of easily being driven to polluted areas and recovered by simple magnetic

interaction. However, most of them exhibited low reusability, low oil uptake ability and low mechanical

properties. Moreover, their synthesis is complex and expensive. Here, we propose for the first time the

fabrication of a porous reusable magnetic nanocomposite based on polydimethylsiloxane (PDMS) and

multiwalled carbon nanotubes (MWNTs) via a low cost approach. The material can selectively collect oil

from water reaching equilibrium in less than two minutes, evidencing a higher volume sorption capacity

with respect to other already proposed materials for oil sorption from water. Furthermore, the material

evidenced excellent mechanical properties with a stress at 60% strain at least 10 times higher with

respect to other proposed similar materials and maintained its characteristics after 50 cycles at 90%

strain, along with high thermal and chemical stability, making them useful as high-performance systems

for plugging oil leakage.

Introduction

In recent years, oil-spill accidents and leakage of organic liquidshave caused ecological problems, including nearly irreversibledamage to ecological systems with major consequences ineconomic, social and health elds.1–3 Different methods havebeen proposed and used to recover organic liquids dispersed inwater such as physical sorption by sorbent materials, mechan-ical recovery by oil skimmers,4,5 in situ burning,6 physicaldiffusion,7 ltration membranes8–10 and biodegradation.11

However, low separation efficiency, generation of secondarypollutants and cumbersome instrumentation have causeddifficulties in their practical use. To overcome these problems,the synthesis of lightweight porous oil absorbent materials isone of the most promising strategies.12–15 Porous absorbentscan concentrate oil liquid inside the pores making their trans-port, storage and removal from water easier. An ideal sorbentmaterial should be (super)hydrophobic and (super)oleophilic in

iologiche ed Ambientali (Di.S.Te.B.A.),

00 Lecce, Italy. E-mail: antonio.turco@

to.it

ne, Universita di Pisa, Via G. Caruso 16,

e, Universita del Salento, Via Monteroni,

(ESI) available: Videos. See DOI:

hemistry 2015

order to be oil selective and with a high oil sorption rate andcapacity.16 Practical application also needs easy and economicalfabrication and, related to this, the possibility of the sorbentmaterial to be readily reusable.16 The high recyclability of thesorbent could in fact dramatically reduce the cost of oil spillrecovery. Recently, various materials have been proposed for oil/water separation such as carbon nanotube (CNT) sponges,17,18

polydimethylsiloxane (PDMS) sponges,19,20 polystyrenebers,21,22 and graphene sponges.23 Among them, carbon andsilane based materials showed higher sorption capacity andreusability. Besides the chemical affinity, the porosity of thesematerials plays a key role for the achievement of high oilsorption capacity. However, as the porosity is increased, silaneand carbon based sorbents with high sorption capacity usuallyevidenced low mechanical strength (less than 1 kPa at 60%strain). This highly limits their practical application due to thefact that the viable stack height of the sorbent aer adsorptionis only a few centimeters.17 Moreover, the preparation process ofthese materials oen requires complex synthetic steps andcumbersome and expensive instrumentation (vacuum appa-ratus, centrifuges.), which limit their practical application.PDMS sponges with fast oil absorption using sugar particles astemplates have been fabricated. Although satisfactory oil uptakeperformances were reported, the use of vacuum apparatus orcentrifuges complicates the large-scale production of thematerial. Moreover, the sponges evidenced low stress strain(�0.05 kPa at 70% strain).

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Choi et al.20 fabricated reusable PDMS sponges with fast oilabsorption using sugar particles as templates. Although satis-factory oil uptake performances were reported, the syntheticsteps included shaping of sugar templates and the use ofvacuum apparatus that complicate the large-scale production ofthe material. Zhang et al.24 prepared sponges with excellent oilmass absorption by diluting the PDMS prepolymer in p-xyleneand using sugar particles as the template; however, thecompressive strain of sponges remains to be improved. Zhaoet al.19 synthesized sponges with high oil absorption by dilutingPDMS in dimethicone and using sugar particles as the template.Also in this case, the large-scale production of the material wasprevented by the required use of centrifuges; in addition, thesponges with higher absorption capacity evidenced low stressstrain (�0.05 kPa at 70% strain).

A recent trend in the eld of water/oil separation sorbentmaterials is represented by the fabrication of “smart absor-bents”16 on which the absorption properties can be controlledby pH, electrical, photonic, thermal or magnetic input. Amongthem, magnetic-controllable materials have received wideattention in recent years due to the possibility of being easilydriven to polluted areas and recovered by simply exploitingmagnetic interaction. To this aim, various hydrophobic mate-rials17,25–28 were combined with magnetic materials. Mainly, twostrategies have been adopted to prepare magnetic composites.The rst approach consists of depositing a magnetic coating onthe surface of porous materials. This approach could presenttwo main disadvantages: the volume of the pores can bereduced affecting the sorption capacity of the sponge, especiallywith magnetic particles of size larger than microns29 andmagnetic layers are not stable, typically requiring new deposi-tion and new synthetic steps to recycle the sponge.25 The secondapproach consists of polymerizing superhydrophobic spongesin the presence of magnetic nanoparticles. However, most ofthe reported materials collapse or easily fracture in compres-sion or stretch tests.26 In addition, most of the magneticsuperhydrophobic spongy materials involved complex syntheticprocedures that make them scarcely suitable for practicalapplications.

Among the developed porous magnetic materials, PDMShas not been proposed until now due to the difficulties inpreserving the mechanical properties of the sponge structureaer modications required for conferring it magneticproperties.19

Here, we report for the rst time the fabrication of a porousmagnetic reusable nanocomposite integrating PDMS spongeswith multi-walled CNTs (PDMS–MWNTs) via a facile hardtemplate approach that does not require the use of complexinstrumentation. The PDMS–MWNT sponges show excellentsuperhydrophobicity and superoleophilicity and, importantly,higher compressibility, enhanced mechanical strength andimproved thermal stability compared with PDMS alone andwith other proposed porous systems for oil uptake. Themagnetic sponges swell in various organic liquids with a highand fast absorbency rate. Moreover, the organic liquids can beeasily recovered by simply squeezing the sponges that can bethus reused without loss of function.

17686 | J. Mater. Chem. A, 2015, 3, 17685–17696

Experimental sectionChemicals

The PDMS prepolymer (Sylgard 184) and a curing agent werepurchased from Dow Corning. Magnetic MWNTs (10–15 nm indiameter, 1–10 micrometers in length) were provided byNanoledge Inc. Hexane, dichloromethane, chloroform, toluene,tetrahydrofuran, petroleum ether, ether, and glucose werepurchased from Sigma Aldrich and used as received. Gasolinewas purchased from Kuwait Petroleum Corporation.

Preparation of PDMS–MWNT sponges

Porous magnetic PDMS–MWNT sponges were preparedaccording to the following procedure. MWNTs were mixedovernight with 1 g of glucose particles (GMps) with the sizeranging from 68 � 29 to 620 � 100 mm under dry conditions atroom temperature. The mixture was then packed in a syringetube equipped with a porous septum. The PDMS prepolymerand the thermal curing agent in a ratio of 10 : 1 by weight wereplaced into a glass dish and were diluted with an appropriateamount of different solvents. Aer being intensively stirred for5 minutes, 10 mL of the mixture were poured into the syringetube and inltrated into the sugar templates with the help of apiston. The as-obtained composite was then cured in an oven at80 �C overnight to accomplish polymerization. The curedsamples were placed in a beaker with distilled boiling water for24 hours to dissolve the GMps and then sonicated with warmwater and ethanol to remove residual sugar microparticles andnot entrapped MWNTs. Different PDMS–MWNT sponges wereprepared by changing different parameters such as theconcentration of the prepolymer, solvent to dilute the pre-polymer, hard template average size, and the amount ofMWNTs. If not differently indicated, results discussed below arereferred to sponges prepared by diluting the PDMS pre-polymerin hexane with a ratio equal to 4 : 6, with the hard templateaverage size equal to 620 � 100 mm, with 3% (w/w) of MWNTswith respect to the template and aer a contact time with oilsolution of 20 minutes.

Oil absorbency and reusability of PDMS–MWNT sponges

A piece of sample was immersed in a bath containing only oil atroom temperature for 20 minutes. Aer that, the sample wasremoved from oil, wiped with lter paper in order to removeexcess oil and weighed. The oil mass sorption capacity (Mabs)and volume absorption (Vabs) capacity were evaluated by usingthe following equation:

Mabs ¼ (m � m0)/m0

Vabs ¼ (m � m0)r0/rm0

where m is the weight of the sample aer sorption, m0 is theinitial weight of the sample, r0 and r are density values of theabsorbent material and absorbed oil, respectively, and V0 indi-cates the volume of the oil absorbent before saturation by anorganic solvent or oil. Repeated absorption–desorption cycles of

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oils were performed to evaluate sponge reusability. For thispurpose, the sample was immersed in oil until the absorptionequilibrium was reached (i.e. 20 min), and then weighed tocalculate the oil mass sorption capacity. The sample was thensqueezed and washed with ethanol three times and dried in anoven at 60 �C. The absorption–desorption procedure wasrepeated 10 times for each tested oil.

Characterization of the PDMS–MWNT sponges

The dynamic and static contact angle measurements werecarried out using an OCA 15 Plus instrument (DataPhysicsInstruments, Filderstadt, Germany) equipped with a high-resolution camera and an automated liquid dispenser. SCA 20soware was employed to obtain additional information on thesamples: the soware uses an algorithm based on the Young–Laplace equation and it allows to correlate the shape of the dropwith its surface tension and to measure the contact anglebetween the liquid and the analyzed surface.

XPS measurements were performed with an AXIS ULTRADLD (Kratos Analytical) electron spectrometer using a mono-AlKa source (1486.6 eV) operating at 225 W (15 kV, 15 mA). Surveyscan spectra were recorded using a pass energy of 160 eV and a 1eV step. Narrow scans of single regions were acquired using apass energy of 20 eV and a 0.1 eV step. The hybrid lens modewas used for all measurements. In each case, the area of anal-ysis was about 700 � 300 mm. The base pressure in the analysischamber was 2� 10�9 torr. During the acquisitions, a system ofneutralization of the charge was used. Processing of the spectrawas accomplished with CasaXPS Release 2.3.16 soware.

Morphological characterization of PDMS and PDMS–MWNTsamples at the micro- and nanoscale was carried out using ascanning electron microscope (SEM) JEOL JSM-6390 at anacceleration voltage of 3 kV.

Quasi-static hysteresis compression tests were performed ona LLOYD LR5K dynamometer equipped with 50 mm diameterparallel plate tools and a 1 kN load cell. Tests were performed byloading the samples at different strain levels, equal to 60 and90%. The loading and unloading stages were performed at thesame rate of deformation of 2 mm min�1. Each test reportedwas obtained as the average of three samples.

Thermogravimetric analysis (TGA) was performed on aMettler Toledo DSC/TGA1 instrument by heating 20mg samplesbetween 30 and 700 �C at a 10 �C min�1 heating rate in anitrogen atmosphere.

Results and discussionPreparation of PDMS–MWNT sponges

Porous PDMS–MWNT sponges were fabricated by polymeriza-tion of the prepolymer in different solvents in the presence ofglucose microparticles (GMps) covered with MWNTs. Anappropriate amount of MWNT powder was allowed to adsorb onthe surface of glucose crystals by simply mixing the two powdersunder dry conditions overnight. In this way, the impact of sugarcrystals on MWNTs could mechanically break the p–p stackingbetween the nanotubes, favoring their absorption on the GMp

This journal is © The Royal Society of Chemistry 2015

surface.30 The as-prepared MWNT–GMp nanocomposites werepacked in a syringe and then pre-polymerized PDMS was addedinto the syringe and inltrated into GMp–MWNTmicroparticleswith the help of a piston in order to compact the materials andconsequently to make them uniform. Aer curing, the porousPDMS–MWNT nanocomposites were sonicated in warm waterand ethanol until the washing solutions became clear (Fig. 1a).These steps are crucial in order to completely remove GMps andnot entrapped MWNTs. As shown in Fig. 1b, the as-preparedPDMS–MWNT sponges are magnetic and the simple prepara-tion method makes their fabrication suitable for large-scaleproduction without the need for complex and expensiveequipment.

Performance of the PDMS–MWNT oil absorbent

The porous PDMS–MWNT sponges have the ability to quicklytake up oil. Although porous PDMS materials with these abili-ties have recently been synthesized,19,20,24 none of them evi-denced magnetic properties. Moreover, the inuence ofdifferent solvents for prepolymer dilution on the stability andsorption capacity of the developed material was not evaluated inany case.

As detailed before, here, the sponges are easily prepared indifferent shapes and dimensions without the use of cumber-some and expensive instruments. Furthermore, differentsolvents and different ratios between the pre-cured PDMS andsolvent were used for preparing the PDMS–MWNT sponges andtheir oil mass absorption was evaluated with the aim to selectthe optimal conditions, determining the most satisfactory oiluptake properties.

For the tests, hexane was used as the solvent to dilute thepre-polymer and dichloromethane uptake was evaluated. Asreported in Fig. 2, an oil mass absorption higher than 7 g g�1 fordichloromethane was obtained for the PDMS–MWNT spongesprepared with a ratio of 4 : 6 PDMS/hexane by weight, by using atemplate particle size of 68 � 29 mm and 3% (by weight) ofMWNTs with respect to the template. Sponges prepared withlower concentration of the pre-polymer exhibited lowermechanical stability resulting in a collapse of the porousstructure. From Fig. 2, it is clearly evident that the oil masssorption increases when the PDMS prepolymer concentrationdecreases. This could be due to a lower density of the mixturethat facilitates the penetration of the reagents across thetemplate microparticles producing a more porous structurewith interconnected pores.19

The impact of solvents on PDMS–MWNT sponge formationwas evaluated by preparing different sponges with differentsolvents. We tested chloroform, ether, hexane and dichloro-methane; 4 g of the pre-polymer were diluted in 6 mL of eachsolvent, and the uptake of dichloromethane was evaluated. Thetemplate particle size (68 � 29 mm) and the amount of MWNTs(3% w/w) were maintained equal in all samples. As evidenced inFig. 3, the solvent used to dilute the pre-polymer has a signi-cant effect on the oil uptake process. In particular, similar towhat was observed above for the PDMS pre-polymer dilution,the decrease of the density of the solvent increases the

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Fig. 1 (a) Schematic illustration of the preparation of the PDMS–MWNT sponge; (b) photograph of the PDMS–MWNT sponge anchored to amagnet.

Fig. 2 Dichloromethane mass absorption (g g�1) of the PDMS–MWNTsponges with different mPDMS/mhexane ratios (the size of GMps is 68 �29 mm and 3% (w/w) of MWNTs with respect to GMps is used). Fig. 3 Variation of mass absorption of the PDMS/MWNT sponges for

dichloromethane with different solvents used to dilute the PDMS pre-polymer (the size of GMps is 68� 29 mm and 3% (w/w) of MWNTs withrespect to GMps is used).

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absorption capacity, conrming the need to favour the pene-tration of the reagents across the template microparticles toachieve the formation of a highly porous structure with inter-connected pores allowing an efficient sorption mechanism.

Also the size and the kind of template can signicantlyinuence the porosity and thus the oil uptake performances.19,20

For template-assisted synthesis of porous materials, a commonstrategy to modify the dimension of pores is to use micropar-ticles with different sizes as the template. To study theseaspects, we prepared different PDMS–MWNT sponges bymaintaining the ratio of mPDMS/mhexane equal to 4 : 6, theamount of MWNTs equal to 3% (w/w) with respect to thetemplate and using them as hard template GMps with differentaverage sizes, such as 68 � 29 mm, 230 � 85 mm, 620 � 100 mmand 3638 � 252 mm. By only using templates with an averageparticle size lower than about 700 mm, we were able to producestable and well-shapedmaterials. Scanning electronmicroscopy(SEM) images of sections of the sponges prepared in the pres-ence of particles with an average size of 68� 29 mm (Fig. 4a) and620� 100 mm (Fig. 4b and c) evidenced an interconnected threedimensional (3D) framework with a macropore size consistentwith the size of the used hard template. Comparing highermagnication SEM images of PDMS–MWNT (Fig. S1†) and

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porous PDMS (Fig. S2†) sections, it is evident that MWNTs arehomogenously dispersed inside the polymer matrix withrandom orientation. The formation of a homogeneous networkof MWNTs inside the polymer is further demonstrated by theconductive behaviour of thematerial (evidenced by contacting itto a multimeter; data not shown), suggesting the presence of acontinuous network of MWNTs partly emerging from the poly-mer bulk. Evaluating the uptake of dichloromethane of thesponges prepared with different porosities (Fig. 4e), weobserved that by increasing the template dimensions from 68 �29 mm up to 230 � 85 mm, the oil mass absorption remainsconstant (�7.6 g g�1), while using ten times bigger GMps (620�100 mm) oil mass absorption increases up to 12 g g�1.

The collected results suggest that bigger pores can provide alarger volume for oil storage. In addition, sponges prepared inthe presence of MWNTs evidenced a rougher surface withrespect to porous PDMS alone (Fig. 4d), suggesting the presenceof MWNTs on and/or close to the pore surface.

The presence of MWNTs in the fabricated porous materialshas a dual key role. On the one hand, they are responsible for

This journal is © The Royal Society of Chemistry 2015

Fig. 4 SEM images of porous PDMS–MWNTs with the size of GMps equal to (a) 68� 29 mm and (b) 620� 100 mm (with the relative zoom in (c)).(d) Porous PDMS with the size of GMps equal to that in (a). Scale bar is 200 mm. (e) Oil mass absorption of the PDMS–MWNT sponges preparedfrom different templates.

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the magnetic properties of PDMS–MWNT sponges and, on theother hand, they possess oleophilic and hydrophobic proper-ties,17 large specic surface area and rougher surface, thusdetermining the enhancement of the absorption properties ofPDMS–MWNT nanocomposite sponges. With the aim to verifythe contribution of MWNTs in the oil uptake mechanism,different samples were prepared with different ratios of MWNTswith respect to GMps by maintaining a mPDMS/mhexane ratioequal to 4 : 6 and using them as hard template GMps with anaverage size of 620 � 100 mm. We found that by increasing theamount of MWNTs from zero to 3%, the average dichloro-methane uptake slightly increases from (11.8 � 0.38) g g�1 to(12.28 � 0.45) g g�1, while the use of a lower amount of MWNTs(i.e. 1%) determined the same dichloromethane uptake as inthe case of PDMS alone, evidencing the lack of the contributionof MWNTs in oil mass absorption in this case. We also observedthat with the further increase of the MWNT amount (i.e. up to

Fig. 5 Oil mass absorption of the PDMS/MWNT sponges preparedfrom different ratios of MWNTs with respect to GMps.

This journal is © The Royal Society of Chemistry 2015

6%), the material lost consistency and stability and dichloro-methane uptake decreased to 10.8 g g�1 (Fig. 5).

The dichloromethane sorption of the sponges is plotted as afunction of time in Fig. 6. The equilibrium is reached in fewminutes as shown by the oil mass absorption remainingconstant without any signicant variation, at least for theanalyzed time (60 min), evidencing the fast oil absorptionproperties of the material. Such an absorption rate is signi-cantly higher with respect to previously reported PDMSsystems,19 for which a time higher than 60 minutes wasrequired to saturate pores with dichloromethane, thusevidencing that the proposed system can dramatically speed upthe oil uptake process.

Fig. 7 shows the XPS wide scan spectra of sponges made ofPDMS only (green line) and PDMS–MWNTs (red line). In bothsamples, the peaks of Si 2p (103 eV), C 1s (285 eV) and O 1s

Fig. 6 Oil mass absorption for different contact times ofPDMS–MWNTs (the size of GMps is 620 � 100 mm and 3% (w/w) ofMWNTs with respect to GMps is used).

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Fig. 7 XPS wide scan spectra of PDMS (green) and PDMS–MWNT sponges (red).

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(530 eV) are present. The Si/C atomic ratio was calculated asequal to 0.45 in the PDMS sample, in good agreement with thetheoretical ratio of 0.5, and to 0.25 in PDMS–MWNT samples,conrming the presence of MWNTs not only in the bulk of theporous PDMS but also on its surface as already evidenced bySEM images.

Mechanical properties of PDMS–MWNT sponges

PDMS is known to be a material with excellent compressionproperties both in bulk and porous forms.19 Carbon nanotubesalso display superlative mechanical properties and they canstrengthen polymers if used as the reinforcing ller.31,32

Fig. 8a evidences that the stress of PDMS–MWNT sponges at60% strain increases with respect to the PDMS alone byincreasing the amount of MWNTs. In particular, PDMS spongesprepared in the absence of MWNTs (red curve) reach a stress of0.03 MPa at 60% strain, in good agreement with previouslyreported results.19 The stress increases to 0.15 MPa at 60%strain for the sample prepared in the presence of 1% w/w ofMWNTs (green curve) and up to 0.33 MPa at 60% strain for thesample prepared with 3% w/w of MWNTs (blue curve).

The stress of the PDMS–MWNT 3% (w/w) sponges at 60%strain evidently increases with increasing PDMS prepolymer/hexane ratio, as shown in Fig. 8b, indicating that the intro-duction of hexane dilutes the PDMS prepolymer and the curingagent, which leads to a decrease of the mechanical strength.Nevertheless, it should be pointed out that the value of 0.33MPaat 60% strain for the sponge with higher dichloromethaneuptake (mPDMS/mhexane of 4 : 6) is appropriate for oil removalapplications and it is much higher than that of the otherreported oil-uptake systems such as metallic MWNT sponges(66 kPa),17 carbon aerogels (0.3 kPa),33 3D graphene framework

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(4 kPa),34 and PDMS (12 kPa).19 This could be due to the MWNTcontinuous network that may act as homogeneous reinforcingllers supporting the polymer matrices.

Another important aspect is that there is no signicantchange in the compressive stress–strain even aer 50 cycles at90% strain, for which a stress of 1.8 MPa was still observed(Fig. 8c). Interestingly, the materials did not show any fractureor collapse during the mechanical tests conrming their highmechanical stability, which makes these sponges suitable forrepeated use and recycling. The hysteresis loops indicatesubstantial energy dissipation due to the viscoelastic nature ofthe rubbery PDMS and to the expulsion of air from the openpore structure of the sponges.35

Wettability and stability of PDMS–MWNT sponges

The PDMS–MWNT sponge hydrophobicity was evaluated bycontact angle measurements (CA). In particular, PDMS–3%MWNT sponges evidenced superhydrophobic behavior withCAwater � 153.4� 6.9� (Fig. 9c), much higher than that observedon PDMS alone prepared under the same conditions (CAwater �131.4 � 6.5�, Fig. 9a) and PDMS–1% MWNT porous nano-composites (CAwater � 146.6 � 4.6�, Fig. 9b). This could be dueto the presence of MWNTs on the pore surface and/or to theincreased roughness of the PDMS–MWNT surface (Fig. S1†)with respect to PDMS alone (Fig. S2†). Thanks to lightweight,porous structure and superhydrophobic properties,PDMS–MWNT sponges can oat on the surface of water. Whensponges were immersed in water with the help of an externalforce, it was possible to observe the formation of an air cushionaround them, maintaining the sponges’ dryness (Fig. 9d). Thisis conrmed by the weight of the sponges remaining constantbefore and aer immersion in clean water (data not shown).

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Fig. 8 Compressive stress–strain curves at 60% strain of (a) PDMS (red curve), PDMS–1% MWNTs (green curve) and PDMS–3% MWNTs (bluecurve); (b) PDMS–3% MWNTs at the ratio of mPDMS/mhexane of 4 : 6 (blue curve), 6 : 4 (green curve), 8 : 2 (red curve), and 10 : 0 (black curve);(c) compressive stress–strain curve at 90% strain after one and fifty cycles at the ratio ofmPDMS/mhexane of 4 : 6. In all the cases, the size of GMpsis 620 � 100 mm.

Fig. 9 Optical images of a drop of water deposited on (a) PDMS, (b) PDMS–1% MWNTs, and (c) PDMS–3% MWNT sponges. (d) PDMS–3%MWNTsponge during immersion in water. (e) Time lapse of dichloromethane absorption for the PDMS–MWNT 3% w/w sponge (size of GMps is620 � 100 mm).

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Fig. 12 Different oil uptakes of the PDMS–3% MWNT sponge (size ofGMps is 620 � 100 mm).Fig. 10 TGA profiles for PDMS (red curve) and PDMS–3% MWNTs

(blue curve) (the size of GMps is 620 � 100 mm).

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The sponge also exhibited superoleophilic properties(CAoil � 0�), thus being capable of adsorbing 10 mL of oil in lessthan 90 ms, more than two times faster than other PDMSbased porous materials.19 Interestingly, the sponges increasedtheir volume during the absorption process, indicating theoccurrence of the swelling process due to oil absorption(Fig. 9d, movie S3, ESI†). All these characteristics clearlyevidence the superhydrophobic/superoleophilic propertiesand selective oil absorption capabilities of the sponge. The

Fig. 11 Removal of (a) oil red O colored hexane on the water surface anGMps is 620 � 100 mm).

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superhydrophobic/superoleophilic properties of PDMS-MWNTs sponges could be due to a combined effect involvingmethyl groups of PDMS and MWNTs. Methyl groups, being onthe surface of the sponge, can act decreasing the surfaceenergy of the sponge. MWNTs can further enhance this effectincreasing the surface roughness of the sponges (as revealedby SEM images, Fig. 4 and S1†).

The superhydrophobicity of the PDMS–MWNT sponge ismaintained also towards corrosive aqueous liquids including1 M HCl (CAwater � 153�) and 1 M NaOH (CAwater � 158�).

d (b) oil red O colored chloroform with PDMS–3% MWNTs (the size of

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Table 1 Comparison of various oil absorbents

Oil absorbentOil Absorbentdensity (g cm�3) Oil Mabs (g g�1) Vabs (cm

3 cm�3) Stress at 60% strain (kPa) Reference

Swellable porous PDMS/MWNTs 0.308 Dichloromethane 11.9 2.77 330 This workPetroleum ether 8.8 4.07Hexane 15.05 4.01Chloroform 20.5 4.24Tetrahydrofuran 10 3.46Toluene 12.4 4.39Gasoline 11.1 5.63

Swellable porous PDMS 0.18 Gasoline 22 5.35 5 24Diesel 12 2.57Crude oil 9 2.19Chloroform 34 4.14Toluene 18.7 3.86

PDMS sponge 0.18 Chloroform 11 1.34 12 20Toluene 5 1.03Transformer oil 4.3 0.89

Polyurethane–graphene–PDMS 0.0113 Chloroform 160 1.2 — 37DMF 95 1.13Hexane 50 0.85

Polyurethane–CNT–PDMS 0.035 Hexane 15 0.79 — 38Gasoline 18 0.95Diesel oil 19 0.80

Me-CNT sponge 0.015 Diesel 56 1 66 17

Carbon nanober aerogel 0.006 Diesel 170 1.21 8 39

CNT/GGO aerogel 0.00045 Crude oil 350 0.18 0.3 33

CNT solids 0.024 Hexane 26 0.96 350 (250 at equilibrium) 36

CNT sponge 0.0075 Diesel 143 1.28 30 35

Cotton towel 0.24 Diesel 5 1.43 — 35

PU sponge 0.03 Diesel 42 1.5 5 35

MTMS–DMDMS gels 0.12 Hexane 6 1.09 10 40Chloroform 14 2.54Petroleum ether 6 1.07Toluene 8 1.1

Graphene aerogels 0.015 Hexane 25 0.56 130 41DMF 25 0.39Toluene 42 0.72

3D graphene framework 0.0021 Chloroform 470 0.66 4 42Toluene 200 0.48THF 250 0.59

Carbon soot 0.003 4-Methyl-2-pentanone 40 0.15 — 43Cyclohexane 33 0.13N-Methyl-2-pyrrollidone 33 0.01Dimethylformamide 30 0.01Methanol 35 0.13Ethanol 33 0.12Acetone 45 0.17Hexane 25 0.12Tetrahydrofuran 58 0.2

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Table 1 (Contd. )

Oil absorbentOil Absorbentdensity (g cm�3) Oil Mabs (g g�1) Vabs (cm

3 cm�3) Stress at 60% strain (kPa) Reference

Benzene 50 0.17Dichloroethane 78 0.18Toluene 50 0.17Crude oil 30 0.11Soybean oil 35 0.12Used pump oil 34 0.12Engine oil 32 0.11Pump oil 35 0.12

Silanized melamine sponge 0.008 Chloroform 163 1.94 — 44Acetone 83.6 0.84Butanol 82.00 0.81Toluene 86.2 0.79THF 91.1 0.82DMF 97.4 0.83Diesel 93.6 0.9Motor oil 94 0.87Machine oil 99.8 0.93Biodiesel 98.2 0.89Mineral oil 100.7 0.91

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Remarkably, the presence of MWNTs also has a signicanteffect in improving sponge stability, as shown in Fig. 10, whichshows the comparison between TGA curves of the PDMS–3%MWNT and PDMS alone sponges (blue and red curves, respec-tively). The presence of MWNTs increases the thermal stabilityat about 110 �C, with no weight loss being detected at temper-ature below �300 �C for the PDMS–3% MWNT sponge andbelow �190 �C for PDMS alone.

Application of PDMS–MWNT sponges in selective oilabsorption

The veried superhydrophobic/superoleophilic properties of thePDMS–MWNT sponges as well as their 3D structure with inter-connected pores certainly prompt their application in oil/water

Fig. 13 Recovery of adsorbed oil from the PDMS–3% MWNT sponge bycolored chloroform).

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separation and oil absorption processes. Fig. 11a (movie S2 ESI†)shows the ability of the PDMS–MWNT sponges to selectivelyadsorb oils from the water surface. The oating oil could beadsorbed in a short time, the porous structure allowing thestorage of the collected oil inside the 3D structure of the sponge.As shown in Fig. 11a (movie S6 ESI†), besides their ideal wettingproperties and oil absorption capacity, the PDMS–MWNTsponges can also be magnetically actuated in order to be drivenwith minimum friction onto polluted water areas by means of amagnetic eld. More in detail, we observed that a magnetic eldof �90 Gauss is enough to easily move a PDMS–MWNT spongeof 0.2 g on the water surface during the oil uptake processmaking such systems very useful for practical applications. Aeroil sorption, no evidence of MWNTs was noticed on the surfaceof water indicating the good stability of the material. The

squeezing (the colorless solvent is ethanol and the red one is oil red O

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PDMS–MWNT sponge is also able to adsorb heavy oils underwater if driven by an external force (Fig. 11b, movie S5 ESI†).

The PDMS–MWNT sponge oil mass absorption capacity wastested against different oils and organic solvents. As illustratedin Fig. 12, by changing the oil type, the oil mass absorption ofthe PDMS–MWNT sponge is in the range of 8.5 to 20 g g�1

depending on density, viscosity and surface tension of theorganics. Moreover, different PDMS–MWNT sponges exhibitedthe same dichloromethane oil uptake (RSD 7.5% n ¼ 3), sug-gesting good reproducibility and homogeneity of the material.

The collected results are reported in Table 1, comparing theproposed PDMS/MWNT nanocomposite with other sorbents, interms of absorption capability and mechanical properties, whichboth represent fundamental aspects for application in oil spillrecovery. Since mass-based absorption capacity is stronglyaffected by the density of oils and absorbing materials, we alsoconsidered volume absorption capacity to characterize theabsorptive capability of the sponges, dened as (m � m0)r0/rm0

(where r0 and r are density values of the absorbent material andabsorbed oil, respectively34), as it is not a function of the solventand absorbent density and may better describe the absorptioncapacity of the material. As shown in Table 1, the volumeabsorption capacity values of the present sponges for oils andorganic solvents are higher than those of other porous oilabsorbents. This could be possibly due to the swelling of PDMS–MWNTs aer immersion in oil, which signicantly increases theavailable volume for oil storage (Fig. 9e). Also the mechanicalproperties of sorbent materials are of utmost importance inpractical application since in materials with lower mechanicalstrength the viable stack height of the sorbent aer sorption isonly a few centimeters. As shown in Table 1, the mechanicalproperties of the here proposed PDMS–MWNT sponges are higherwith respect to all other reportedmaterials, taking in account thatin spite of the CNT solid36 evidencing an initial comparable value(�350 kPa), their mechanical strength decreased quickly whenrepeatedly pressed at 60% strain, reaching an equilibrium aer 50cycles for which a stress of 250 kPa was observed.

Finally, a key point in practical oil clean up application is therecyclability of the material and the recoverability of oil. Aerthe uptake, the oils are stored in the large pores of the spongesand can be easily recovered by simple squeezing, as shown inFig. 13 (movie S6 ESI†) for chloroform as an example.

PDMS–MWNT sponges with different adsorbed solventswere squeezed and dried 10 times and their weight wasmeasured before and aer drying. It is interesting to highlightthat no signicant variation of weight of the sponge and oiluptake ability aer each cycle was observed, independentlyfrom the solvent used for the uptake. Also the water contactangle remained the same aer ten cycles, suggesting that thecharacteristics of the material were not altered during therepeated tests (data not shown), thus conrming the goodrecyclability and stability of the material.

Conclusions

In this study, a new functional nanocomposite material basedon PDMS and MWNTs is developed and its water/oil separation

This journal is © The Royal Society of Chemistry 2015

and oil absorption properties are demonstrated. In particular, a3D PDMS porous structure embedding magnetic MWNTs wasfabricated by polymerizing the PDMS prepolymer in the pres-ence of a hard template covered with magnetic MWNTs. Theproposed fabrication technique is simple and easy to be scaledup and the obtained materials are reusable. It was demon-strated that the presence of MWNTs in polymer matrices, hereproposed for the rst time as a way for preparing magneticporous absorbent nanocomposites, not only provides magneticproperties to the sponges, but has also a great impact onimproving their mechanical properties, thermal stability and oiluptake ability. The sponges exhibited excellent mechanicalperformance with respect to other proposed oil-uptake systems,as well as higher superhydrophobicity/superoleophilicityallowing two times faster oil sorption with respect to otherPDMS based porous materials. It is worth mentioning that themagnetic properties of the developed materials can be exploitedin order to drive the water-repellent, oil adsorbing sponge topolluted areas adopting non-contact methods. Furthermore,aer collection of organics the sponge can be driven back torecover spilled oil by simply squeezing it. The achieved volumeand mass absorption capacity values are fully appropriate forpractical applications. PDMS–MWNT sponges exhibited highervolume absorption capacity with respect to other alreadyproposed materials, together with density properties allowingeasy transport and storage. As an example, for the absorption of1 ton of petroleum (density � 0.88 g cm�3), only �96 kg of thematerials are estimated to be required, that correspond to asponge volume of only 0.31 m3 (approximately equal to thevolume occupied by�4 persons). All these characteristics, alongwith their high chemical and thermal stability, make thedeveloped materials very attractive for real applications inenvironmental remediation procedures.

Acknowledgements

We acknowledge G. De Benedetto for contact angle measure-ments. Financial support from the Italian Ministry of Universityand Research (MIUR) Futuro in ricerca (FIR) grant Sens4Bio No.RBFR122KL1_003 is gratefully acknowledged.

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