RSC Advances

PAPER

Tailored design o

aNational Institute for Materials Scienc

Ibaraki-ken 305-0047, Japan. E-mail: sher

nims.go.jp/waseda/en/labo.htmlbPetroleum Application Department, Egyptia

11727, Cairo, EgyptcGraduate School for Advanced Science an

Okubo, Shinjuku-ku, Tokyo 169-8555, Japa

http://www.nano.waseda.ac.jp/dChemistry Department, Faculty of Science,eProcesses Development Department, Egyptia

11727, Cairo, Egypt

† Electronic supplementary informationand crystallographic data in CIF or10.1039/c5ra01597a

Cite this: RSC Adv., 2015, 5, 19933

Received 27th January 2015Accepted 9th February 2015

DOI: 10.1039/c5ra01597a

www.rsc.org/advances

This journal is © The Royal Society of C

f Cu2O nanocube/siliconecomposites as efficient foul-release coatings†

Mohamed S. Selim,ab Sherif A. El-Safty,*ac Maher A. El-Sockary,b Ahmed I. Hashem,d

Ossama M. Abo Elenien,b Ashraf M. EL-Saeedb and Nesreen A. Fatthallahe

Environmental concerns about the use of toxic antifoulants have increased the demand to develop novel,

environmentally-friendly antifouling materials. Silicone coatings are currently the most effective non-toxic

alternatives. This study focused on developing a model for silicone foul-release nanocomposites that were

successfully designed, fabricated, characterized, and tailored toward foul-release (FR) coatings. A series of

elastomeric polydimethyl-siloxane (PDMS)/Cu2O nanocube composites with different nanofiller

concentrations was successfully synthesized, for the first time, as FR coatings via solution casting

technique. Emphasis was given to the study of the physicomechanical and surface properties, as well as

the easy release efficiency of the elastomer PDMS enriched with Cu2O nanocubes. The bulk properties

of the nanocomposites appeared unchanged after adding low amounts of nanofillers. By contrast,

surface properties such as contact angle and surface free energy were improved, and the settlement

resistance and easy release behavior of the nanocomposites were enhanced. The surfaces were further

proven to have reversible tunable properties and are thus renewable in water. The antifouling property of

the nanocomposites was investigated by laboratory assays involving microfoulants such as Gram-positive

and Gram-negative bacteria, as well as yeast organisms, for 30 days. Exposure tests showed that lower

surface energy and elastic modulus of coatings resulted in less adherence of marine microfouling. The

most profound effect recorded was the reduction of fouling settlement with nanofiller loadings of up to

0.1% Cu2O nanocubes. Thus, the good foul release and long-term durability confirmed that the present

strategy was an attractive nontoxic and environmentally-friendly alternative to the existing antifouling

systems.

1. Introduction

Marine fouling is an extensive natural phenomenon that causesserious problems in the marine environment and for the ship-ping industry.1,2 Shipping accounts for approximately 90% ofglobal trade, and seaborne trade has nearly quadrupled over thepast four decades.3 Once attached to the hull, fouling increasesfriction resistance because of surface roughness, therebyleading to an increase in hydrodynamic weight and subsequent

e (NIMS), 1-2-1 Sengen, Tsukubashi,

if.elsay@nims.go.jp; Web: http://www.

n Petroleum Research Institute, Nasr City

d Engineering, Waseda University, 3-4-1

n. E-mail: sherif@aoni.waseda.jp; Web:

Ain Shams University, Cairo, Egypt

n Petroleum Research Institute, Nasr City

(ESI) available. CCDC 73304. For ESIother electronic format see DOI:

hemistry 2015

top speed reduction and loss of maneuverability.4 Conse-quently, fouling increases fuel consumption, which in turnincreases emissions of harmful compounds such as CO2, NOx,and SOx to the atmosphere.5 The increase in fuel consumptioncan be up to 40%, and the overall voyage cost can increase by asmuch as 77%.6 The economic effects of hull fouling haveaccelerated the development of antifouling (AF) technologies, aglobal industry that has reached a worth of approximately US$ 4billion annually.7 Traditionally, fouling is prevented throughthe application of AF paints that release biocides, which aretoxic to marine organisms but may also affect non-targetspecies. The wide-spread use of toxicants has raised concernsabout their harmful effects on marine communities and led theInternational Maritime Organization in 2001 to the universalprohibition of further application of tributyltin compounds,which have been widely used before the complete phase-out oftheir use in 2008.8 Alternative tin-free AF coatings that employcopper and/or booster biocides are the principal replacementcoatings. Unfortunately, their effects have been found to extendto non-target species and present potential ecological risk to95% of organisms in the water column even at very lowconcentrations.9

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The substantial environmental toxicity issues that surroundthe use of biocidal AF coatings have driven research in anenvironment-friendly direction with a particular focus onnatural marine compounds and foul-release (FR) technology.10

Natural AF compounds also face regulatory hurdles with theestimated cost of assembling data packages on efficacy andenvironmental fate amounting to millions of dollars as well aswith the timeline for the approval process.11

Non-stick, silicone FR coatings present a feasible, cost-effective alternative to biocidal AF coatings. Silicone coatingsrely on a technology that acts in two ways: inhibiting thesettlement of colonizing species and weakening their adhesionstrength. By providing low-friction ultra-smooth surfaces,organisms that stick can be easily removed hydrodynamicallyideally by simply bringing the ship to speed. These coatings donot leach and could be more durable than tin-free AF paints forcertain vessel applications.12 Silicone polymers based onpolydimethyl-siloxane (PDMS) have been the most promisingFR coating system.13 The superior properties of PDMS FRcoatings are due to their low surface energy, low surfaceroughness, low porosity, and high molecular mobility.14 The O–Si–O linkage, which presents water repellency, causes the goodthermal stability, excellent resistance to oxygen, ozone, and UVlight, anti-stickiness, and low chemical reactivity of thecoating.15 Coatings based on silicone elastomers have inher-ently good FR properties; however, they require reinforcingadditives (usually mineral llers) to improve their specicproperties and reduce the costs. This reinforcement can beachieved by incorporating inorganic nanoparticles (NPs) andconventional macro- and micro-scale composites because of theincreased interaction at the polymer ller interface for thenanocomposites.16 The extent of nanocomposite propertyimprovement depends on ller properties, concentration,morphology, degree of dispersion, and degree of adhesion withpolymer chains.15,17

The use of NP-based metal oxide coatings represents apromising approach for the development of non-toxic controltechnologies for micro-fouling organisms. Surfaces can beengineered with low-surface energy coatings that minimizebiological adhesion strength and allow FR with modestbrushing/water spray pressures or with coatings that canprevent fouling through their photocatalytic activity. Metaloxide NPs are stable during contact with microorganisms.18

TiO2 NPs pose a greater potential than silica in minimizingbiofouling on optical surfaces. In addition, techniques likenano-metal oxide coatings seem to be an effective method forcombating fouling.19 Among them, Cu2O NPs are relatively easyto make, safe, and inexpensive, and the natural abundance ofits source materials favors the fundamental and practicalresearch on Cu2O.20

Cu2O NPs exhibit excellent antibacterial activity againstGram-positive and Gram-negative bacteria.18,21 A study hasproven that surface hydrophobicity/super-hydrophobicity canbe achieved on modied nano-Cu2O lms,22 which showedpotential for our eld of application. A variety of interestingCu2O nanostructures has been synthesized.23,24 Nanocubesrepresent one of the most important structural types of Cu2O

19934 | RSC Adv., 2015, 5, 19933–19943

because several other crystal morphologies of Cu2O (e.g.,nanocages, octahedral, and more complicated structures) canbe prepared through the shape transformation of Cu2O nano-cubes.25 Furthermore, the antibacterial activity of cubic Cu2ONPs against Escherichia coli is superior to that of octahedralCu2O NPs. The polar properties of the {100} crystal planes ofCu2O nanocubes are believed to perform an important functionin the increased antibacterial activity.26

In the present work, a series of hybrid PDMS/cubic Cu2Onanocomposites was fabricated via solution casting techniquefor use as FR coatings. The surface properties were discussedbased on the changes in water contact angle and surface freeenergy. New functions of Cu2O nanocubes were introduced herebased on increasing the easy cleaning phenomena throughraising hydrophobicity and lowering surface free energy thatresult in ultra-smooth surfaces with a mechanism that involvesphysical anti-adhesion. This research highlights the signi-cance of the extent of dispersion of CuO2 nanollers in deter-mining the improvement in the physicomechanical and surfaceproperties of the nanocomposites. Furthermore, AF perfor-mance was examined through biological assays to evaluate thenanocomposite FR behavior. The ndings in this context areattractive for their merits such as simplicity, safety, environ-mental benignancy, commercial feasibility, and good potentialfor easy-cleaning systems.

2. Experimental section2.1. Chemicals

Octamethylcyclotetrasiloxane (D4, 98%), which was used asPDMS source, tetramethyldivinyldisiloxane (C8H18OSi2, 97%),polymethylhydrosiloxane (PMHS; Mn ¼ 1700–3200), and plat-inum catalyst commonly known as Karstedt catalyst (plat-inum(0) and divinyltetramethyl-disiloxane in solution to controlcatalyst concentration, stability, viscosity, and inhibition, aswell as easy dosing and formulation; Pt content: 8–11%) wereobtained from Sigma-Aldrich Company Ltd., USA. Coppersulfate (CuSO4), which was used as copper source, and ascorbicacid were delivered from Acros Company (Belgium). Potassiumhydroxide, sodium hydroxide, orthophosphoric acid, trichloro-ethylene, toluene, and all solvents are analytical reagent gradeand were purchased from Merck, Mumbai, India and used asreceived.

2.2. Preparation of vinyl-terminated PDMS

In a three-neck round-bottom ask tted with a condenser, athermometer jacket, and a nitrogen inlet and outlet, a denitequantity of distilled D4 was introduced to remove Si–H- and Si–OH-containing species. Finely grinded potassium hydroxide(0.55%), which has the alkali metal counter ion K+, was thenadded. The temperature was gradually increased to 145 � 5 �Cand was kept constant for 3 h, during which the viscosity of thematerial was tremendously increased. Aerwards, tetrame-thyldivinylsiloxane (2 � 10�4 mol) was added, and the reactionwas continued for another 3 h. The temperature was then low-ered gradually to RT with stirring for 8 h to stop the reaction and

This journal is © The Royal Society of Chemistry 2015

Paper RSC Advances

complete the chain termination. The prepared polymer wasdissolved in toluene, and the unreacted KOH was neutralized byadding concentrated H3PO4 drop wise while stirring vigorouslyand detecting the pH of the resultant solution. The solution wasstirred overnight for complete neutralization and precipitationof the salt generated and then subjected to ltration andtoluene removal.

2.3. Preparation of cuprous oxide nanocubes

Cu2O nanocubes were prepared with copper sulfate as startingmaterial via a simple technique. Exactly 20 mL of NaOHaqueous solution (0.075 mol L�1) was added into 10 mL ofCuSO4 aqueous solution (0.5 mol L�1) with stirring (pH ¼ 10.5).Then 25 mL of ascorbic acid aqueous solution (0.1 mol L�1) wasadded dropwise into the above solution with vigorous stirring atRT. Aer 1 h, a yellow precipitate was obtained (pH ¼ 4–4.5).The particles were separated from the solution by centrifuga-tion (4233EC+ laboratory centrifuge, Italy) at 2000 rpm for 30min. The product was washed by distilled water and absoluteethanol. The nal product was dried in vacuum at 60 �C for 8 h.

2.4. Curing of the prepared vinyl-terminated PDMS

The preparation of unlled PDMS lm was easily employedthrough the addition curing system. It was carried out by theaddition reaction of the polyfunctional silicon hydride PMHSwith the unsaturated groups in polysiloxane chains, and thebond-forming reaction is called hydrosilation curing. To carryout hydrosilation curing, 10 g of the prepared polymers wasdissolved in 40 mL of toluene with continuous stirring until ahomogenous solution was obtained. Exactly 0.035 g of Karstedtcatalyst dissolved in trichloroethylene (10 mL) was then addedand stirred for 30 min. A homogenous solution of 0.3 g of PMHSin 10 mL of toluene was added drop wise under stirring. Theresulting solution was degassed and formed air bubbles for 15min to remove any dissolved gases from the solution. Thedegassed solution was used to coat cleaned surfaces and slides,which, upon the evaporation of the solvent, gave a smooth sheetof cured PDMS with uniform thickness. The PDMS wascompletely cured at RT for 16 h.

2.5. Preparation of PDMS/Cu2O nanocomposites

To prepare the PDMS/Cu2O nanocomposites, Cu2O NPs weredispersed in toluene by ultrasonication (Sonics & Materials,VCX-750, USA; at 20 kHz frequency and equipped with a 13 mm-diameter titanium probe) in an ice bath for 15min. A solution ofthe prepared vinyl-terminated PDMS resin in toluene was thenadded with stirring for 10 min and sonicated for additional 10min. The solution was subjected to hydrosilation curing asdescribed above.

2.6. Apparatus

Certain characterization methods for the prepared polymer,metal oxide, and their nanocomposites are discussed; however,the bulk of these methods provide information on the physi-cochemical and surface properties of the nanocomposites.

This journal is © The Royal Society of Chemistry 2015

The Fourier transform infrared (FTIR) spectra were recordedusing a Nicolet iS10 (Thermo Scientic, USA) with 1 cm�1

resolution and 4000–400 cm�1 range. The samples were cast onpotassium bromide (KBr) pellets (FTIR grade, Alfa Aesar,Karlsruhe, Germany). 1H NMR spectra were recorded on aVarian Mercury VXR-300 NMR spectrometer at 300 MHz (Var-ian, Inc., Palo Alto, CA, USA) using tetramethylsilane Me4Si(TMS) as internal standard and CDCl3 as the main solvent.

Particle size measurement based on the principles ofdynamic light scattering (DLS) was performed using a Broo-khaven Instruments 90Plus model nanoparticle size/zetapotential analyzer (USA). The accurate sizes of the NPs wereanalyzed by TEM because DLS gives hydrodynamic nanoparticlesize. High-resolution transmission electron microscopy(HRTEM) was conducted with an electron microscope (JEM2100LaB6, Japan) at 200 kV accelerated voltage and with 0.14 nmpoint–point resolution. In HRTEM, the solid sample wasdispersed in ethanol solution using an ultrasonicator and thendropped on a copper grid coated with carbon lm. Prior toinserting the samples in the HRTEM column, the grid wasvacuum dried for 10 min. The nanocomposite samples for TEManalysis were prepared by ultra-cryomicrotomy with a LeicaUltracut UCT (Leica Microsystems GmdH, Vienna, Austria).Freshly sharpened glass knives with 45� cutting edges were usedto obtain cryosections with approximately 100–150 nm thick-ness at �150 �C. The cross sections were collected individuallyin sucrose solution and directly supported on a 300-meshcopper grid.

X-ray diffraction (XRD) is a versatile and non-destructivetechnique that reveals detailed information about the chem-ical composition and crystallographic structure of natural andsynthetic materials. XRD patterns were measured using a Pan-lytical X'pent PRO (Netherlands) with monochromated CuKaradiation with scattering reections recorded for 2q anglebetween 10� and 80� corresponding to d-spacing between 1.47and 3.26 �A. To conrm the resolution of the diffraction peakswith standard reproducibility in 2q (�0.005), the samplemeasurement was recorded using a monochromator anddetector, which were used to generate focusing beam geometryand parallel primary beam. The standard diffraction data wereidentied according to the International Centre for DiffractionData (ICDD) soware with PDF-4 release 2011 database.

The optical micrographs of the samples obtained bymechanical mixing were recorded with an Olympus BH-2microscope (Japan) where the images were obtained usingImage J soware program. Scanning electron microscopyimages were obtained by a scanning electron microscope (JEOLJSM530). Before insertion into the chamber, the disk-likemonolith substrates were xed on the SEM stage using carbontapes. Gold (Au) lms were deposited on the substrates at RTusing an ion sputter (EDWARDS S150). The distance betweenthe target and the disk-like monoliths substrate was 5.0 cm. Thesputtering deposition system used for the experiments consistsof a stainless steel chamber, which was evacuated down to 8 �10�5 Pa with a turbo-molecular pump backed up by a rotarypump. Before sputtering deposition, the Au target (4 in. diam-eter, 99.95% purity) was sputter cleaned in pure Ar. The Ar

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working pressure (2.8� 10�1 Pa), the power supply (100W), andthe deposition rate were kept constant throughout the investi-gations. Moreover, to record the SEM images of the disk-likemonoliths well, the SEM micrographs were operated at 20 keV.

2.7. Test methods for the cured polymer and itsnanocomposites

2.7.1. Tensile modulus. The tensile properties of the modelFR coatings were obtained in accordance with ASTM D412

method. Dynamic mechanical analysis (DMA) was performed intension mode using a TTDMA (UK) from TA instruments.Rectangular-shaped (30 mm � 5 mm), free standing sampleswere cast from the solution. The tensile modulus was assessedat RT from stress–strain at 1 Hz single frequency, 2 N preload,and 0.5–27 mm amplitude.

2.7.2. Swelling tests. For the swelling tests, rectangularpieces of the synthesized unlled PDMS and PDMS/Cu2Onanocomposites (1 cm (l)� 1 cm (w)� 0.5 cm (h)) were weighedand then immersed in 100 mL of heptane for 24 h. The solutionwas renewed three times during the test, and aer the allottedtime, the nal swollen weight was determined. Each pointrecorded is the mean of three measurements. The swellingdegree at equilibrium, SD (%), is expressed as a percentage andwas calculated according to the literature27 and by using eqn (1).

SD (%) ¼ ((Wf � Wi)/Wi) � 100 (1)

where Wf was the nal swollen weight of the sample at t and Wi

is the initial weight of the dry sample. The sample measure-ments were determined at 25 �C.

2.8. Contact angle measurements

Static contact angle measurements were performed on thefabricated unlled and lled PDMS/Cu2O nanocomposites oncoated microscopic slides using a Tantec line of contact anglemeter apparatus (Germany) and the sessile drop technique. Thehydrophobic/hydrophilic character of the PDMS layer wasevaluated by measuring the contact angle between the surfaceof the coating and drops of test liquids. The results are themean of the minimum of three determinations. The test liquidswere water (JT Baker, HPLC grade), diiodomethane, andethylene glycol (Aldrich products of the highest purityavailable).

2.9. Wetting behavior and surface tension measurements

The measured values of contact angles were used to extract thesurface tension (gtotal

S ) of the cured polymer lms and nano-composites following the VOCG thermodynamic approach.28 Itrelies on the Fowkes's equation, which assumes the totalsurface energy to be the sum of different interaction compo-nents at the liquid–solid interface and postulates a geometricmean relationship for both the solid–liquid and liquid–liquidinterfacial tensions.29

The total surface tension of a solid gtotalS is composed of three

additive components: the Lifshitz–van der Waals dispersioncomponent, gLW

S , the polar electron-donor (Lewis base)

19936 | RSC Adv., 2015, 5, 19933–19943

component, gS�, and the polar electron-acceptor (Lewis acid)

component, gS+ (eqn (2) and (3)):

gtotalS ¼ gLW

S þ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigS

þgS�

p(2)

which results in the VOCG approach with the form

1þ cos q

2gL ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigLWS gLW

L

qþ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigS

þgL�

pþ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffigS

�gLþ

p(3)

Utilizing the surface tension gL of at least three differentliquids of known components, two polar and one nonpolar, arenecessary to obtain the three equations that can be solved forthe unknowns of the solid, gLW

S , gS+, and gS

�. This researchused diidomethane as the nonpolar liquid, and water andethylene glycol were chosen as the polar liquids.

2.10. Biological assays

2.10.1. Microorganisms' details. Representatives of micro-organisms that cause microbial fouling in cooling watersystems, cooling towers, and ship's hull were tested. The testedorganisms were the following:

(i) Gram positive bacteria: Staphylococcus aureus, NCTC-7447(Gram +Ve 1) and Bacillus subtilis, NCTC-1040 (Gram +Ve 2); (ii)Gram negative bacteria: Pseudomonas aeruginosa, NCTC-10662(Gram �Ve 1) and Escherichia coli, NCTC-10416 (Gram �Ve 2);and (iii) yeast: Candida albicans, IMRU 3669.

Nutrient broth media were used for the cultivation andmaintenance of the tested microorganisms. The nutrient brothcomposition (g L�1) was as follows: peptone, 5.0 g; NaCl, 5.0 g;yeast extract, 2.0 g; and beef extract, 1.0 g.30 Basal salt mediawere used for the weight loss and biodegradability tests. Theforegoing media broth composition (g L�1) was as follows:potassium dihydrogen orthophosphate, 2.44 g; sodium dihy-drogen orthophosphate, 5.57 g; ammonium chloride, 0.5 g;glycerol, 6.4 mL; magnesium chloride, 2.44 g; calcium chloride,5.57 g; ferrous sulfate, 2.00 g; yeast extract, 0.1 g; and distilledwater, 850 mL.31

2.10.2. Weight loss measurements. In weight loss experi-ments, 100 mL of fresh culture broth of each of the testedmicroorganisms was injected in 100 mL bottles that contain 30mL of basal salt media broth. Coated samples were hung in themedium using nylon threads. Weight loss was calculated usingeqn (4):32

Weight loss (mg cm�2) ¼ ((Wbefore � Wafter)/time) (4)

where time is the duration of sample immersion in days.2.10.3. Biodegradability test. The biodegradation study of

the prepared PDMS compounds (as painted glass slides) wasdone in 100 mL batch asks that contain 30 mL of basal saltsmedium with an initial pH of 7 prepared according to theliterature.33 The incubation period was 30 days at 30 �C in ashaking incubator (150 rpm). Aer the test period, the slideswere removed from the medium, washed with distilled water,and dried. The amount of degradation was determined bystudying the weight loss according to the literature.34,35 The

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Paper RSC Advances

biodegradable percentage (BD) was determined from the weightloss measurements using eqn (5).

%BD ¼ ([WC � WS]/WS) � 100 (5)

where WC and WS are the weight loss of the sheets in grams inboth control and sample conditions. Each value was the averageof three separate experiments.

Scheme 1 Preparation of Cu2O nanocubes (note: the picture on theright shows the crystal structure of Cu2O oriented to show {100}plane).

3. Results and discussion3.1. Prepared PDMS design characterization and curing

PDMS belongs to the water-insoluble matrix class and hasunique properties that distinguish it as a FR coating. PDMS hasmethyl (–CH3) side chains that cause its low surface energy (20–24mJm�2) and a exible inorganic –Si–O backbone linkage thatcauses its extremely low elastic modulus (z1 MPa), which areboth essential for the extremely low adhesion of fouling onsilicone coating surfaces. Thus, biolms can be easily removedfrom the surface by simple mechanical cleaning or duringvessel movement.36,37

In 2000, Wynne et al. evaluated two types of PDMS coatings,namely, the hydrosilation-cured and the condensation-curedPDMS, and found that the unlled hydrosilation-cured PDMShas superior properties such as hydrophobicity, roughness,stability in water, non-shrinkage, and lower adhesion ofbarnacles compared with lled condensation-cured PDMS.37

Vinyl-terminated PDMS was obtained via anionic ringopening polymerization of D4 tetramer (because it is a lessexpensive and more readily available monomer) using a strongbase catalyst (KOH), which is frequently used to ring open D4 atcommon polymerization temperatures of 140–160 �C (ref. 14and 38) (see ESI, Scheme S1†). Siloxane dimers (tetramethyldi-vinylsiloxane) are usually used as end-capping reagents tocontrol the molecular weight.39 Aer reaching equilibrium, thereaction is quenched by adding a strong acid (orthophosphoricacid). The conversion or polymerization rate of D4 is high at thebeginning and then attens out with time because of thedecrease in monomer concentration during polymerization andbecause the living centers are enclosed by polymer chains inbulk polymerization.

The FTIR spectrum of the prepared vinyl-terminated PDMSsample revealed absorption bands at 2963 and 2905 cm�1

ascribed to asymmetric –CH3 stretching, at 1411 cm�1 assignedto –CH3 symmetric deformation, and at 1595 cm�1 assigned toSi–CH]CH2 stretching absorption. The band at 1261 cm�1

corresponds to CH3 symmetric deformation, that at 1096 cm�1

to Si–O–Si asymmetric deformation, that at 866 cm�1 to Si–O–Siskeletal stretching, and that at 699 cm�1 to the symmetricstretching of the Si–C bond in –Si(CH3) group. The absence ofany absorption peak at 2060 cm�1 and 3000–3500 cm�1 indi-cates the absence of any hydrosilane (Si–H) or hydroxyl groups(Si–OH) in the prepared polymer (see ESI, Fig. S1†).39a

The 1H nuclear magnetic resonance (NMR) spectrumdistinguishes the signals of the chemical shi at 1.00 ppmcaused by (Si–CH3) from those at 5.94–6.2 and 5.71–5.92 ppm byCH2]CH–Si and CH2]CH–Si. The absence of a chemical shi

This journal is © The Royal Society of Chemistry 2015

at 4.6 ppm indicates the absence of Si–H and Si–OH linkages.The DSC sample was super-cooled at �130 �C and then heatedfrom �130 �C to 50 �C with a glass transition (Tg) at �122 �C,cold crystallization at �95 �C, and melting (Tm) at �46 �C. Acrystallization exothermal peak is observed during the coolingstep and a single melting endothermic peak during the secondheating step. The low Tg of silicones as reected in theirmolecular mobility may contribute to their superior FRcharacteristics.39b

The curing of the prepared vinyl-terminated PDMS followsthe hydrosilation curing mechanism where vinyl end-blockedpolymers react with the SiH groups carried by functional olig-omers. The addition occurs mainly on the terminal carbon andis catalyzed by organometallic compounds, preferably platinummetal complexes, to enhance their compatibility. This reactionhas no by-product. Molded pieces made with a product fromthis curing mechanism are very accurate (no shrinkage). Themechanism of platinum hydrosilation (see ESI, Scheme S2†)was proposed by Chalk and Harrod,40 and the catalytic cycle hasalso been reported before.14

3.2. Nanoller morphology and characterization

To regulate the shape and size in wet-chemical techniques,most synthetic strategies in preparing Cu2O NPs involvesurfactants or template reagents. However, these additives areusually expensive, toxic, and hard to wash, and thus may affectthe performance of the products. In this work, Cu2O nanocubeswere successfully prepared and controlled without using anytemplate or surfactant at room temperature (RT) (Scheme 1).The dominant factor that inuences the morphology and size ofthe particles is the concentration of the NaOH used. At lowNaOH concentration, the Cu2O crystal nuclei were ineffectivelycapped, remained in nanoscale, and grew randomly. In addi-tion, OH� ions affect the stability of {100}, leading to a cubicmorphology.25,41 However, continuous lowering of the NaOHconcentration may result in other forms of copper nanocubicmorphology.42 In this study, when the concentration of NaOHsolution was decreased to 0.075 M, small nanocubes with anaverage side length of 70–110 nm were obtained. On the

RSC Adv., 2015, 5, 19933–19943 | 19937

Fig. 1 XRD pattern of the as-synthesized Cu2O nanocubes, inside DLSof the as-synthesized Cu2O nanocubes, and schematic of a liquiddroplet on the surface of coated glass slide.

Fig. 2 (A), (B), and (C) are the TEM images of the prepared Cu2Onanocubes at low and high magnifications; (D) corresponding SEMimages of the as-synthesized Cu2O nanocubes; (E) correspondingSAED patterns of the as-synthesized nanocubic Cu2O; (F) corre-sponding crystal lattice, which is consitent with XRD results; and (G)and (H) are the TEM images of the PDMS/Cu2O nanocomposites (0.1%nanofillers) at low and high magnification powers.

RSC Advances Paper

contrary, the nanocube size increased with increasing concen-tration of NaOH solution. At small nanoscale, the number ofparticles per unit area increases, and thus antibacterial effectscan be maximized.42

The reaction mechanism can be summarized as follows:cupric sulfate could be dissolved in water and form a uniformionic solution. When NaOH is added to the solution, Cu2+ reactswith OH� and forms blue insoluble Cu(OH)2. At RT, Cu(OH)2 isdecomposed into cupric oxide and water. Ascorbic acid is thenadded as reducer to reduce cupric oxide (CuO) into Cu2O. In thisprocess, NaOH serves not only as a reagent, but also foradjusting the pH of the solution.

The FTIR spectrum of the prepared Cu2O NPs revealed astrong absorption band at 626 cm�1 attributed to the Cu–Olinkage of Cu2O, which agrees with previous literature.43

Therefore, the as-prepared products were pure Cu2O because noinfrared-active mode of CuO around 530 cm�1 appeared (seeESI, Fig. S2†).

The high morphological uniformity of these Cu2O crystalsis reected in their XRD patterns shown in Fig. 1. The strongand sharp diffraction peaks suggest that the resultant prod-ucts were well crystallized. The characteristic peaks for Cu2O(2q ¼ 29.69, 36.52, 42.41, 61.68, 73.61) marked by indices[(110), (111), (200), (220), (311)] showed that the resultingCu2O was essentially crystalline. All the peaks of the preparedCu2O NPs match well with that of standard Cu2O, and nodiffraction peaks from metal copper or cupric oxide appear inthe XRD patterns.

The size of the Cu2O crystallites was estimated from theDebye–Scherrer eqn (6):

D ¼ (Kƛ/b1/2)cos q (6)

where K is the Scherrer constant, which is related to the crys-tallite shape; ƛ and q are the radiation wavelength and Bragg'sangle, respectively; and b1/2 is the full width at half maximum ofthe diffraction peak. The crystal sizes of the product werecalculated and proven to be in the nanosize range.

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Fig. 3 Tensile modulus of the unfilled and filled PDMS/Cu2O nano-composites with different loadings.

Paper RSC Advances

DLS is a non-invasive technique that measures the size andsize distribution of NPs dispersed in a liquid, as shown in Fig. 1.The size distribution prole of the synthesized NPs showed anaverage particle size of 90 nm. The polydispersity index of Cu2ONPs was 0.236, which indicates that the particles are poly-dispersed in nature. These results matched with the resultsfrom the XRD analysis.

The transmission electron microscopy (TEM) analysis(Fig. 2A–C) and SEM analysis (Fig. 2D) of the prepared Cu2O NPsshowed that they have approximately uniform size, cubic shape,clean surface, and particle sizes between 70 and 110 nm. Theinset shows the selected area electron diffraction (SAED)patterns (Fig. 2E) obtained by directing the electron beamperpendicular to the square faces of the cube. The squaresymmetry of this pattern indicates that each Cu2O nanocubewas a single crystal bounded mainly by {100} facets. Further-more, it exhibits individual NPs and clear lattice fringes with dspacing of 0.25 and 0.30 nm, corresponding to the {111} and{110} reections of the cubic Cu2O structure, respectively(Fig. 2F).

3.3. Nanocomposite design and physicomechanicalcharacterization

For the fabrication of PDMS/Cu2O nanocomposites appropriatefor marine easy-release coatings, investigating various concen-trations of Cu2O nanocubes to be embedded into the PDMSmatrix is crucial. The detailed fabrication process of the PDMS/Cu2O nanocomposites is shown in Scheme 2. The TEM obser-vations of the PDMS/Cu2O nanocomposites (Fig. 2G and H)demonstrate that a complete disagglomeration of low concen-trations of Cu2O nanocubes is achieved in the nanocomposites,whereas high concentrations show a different trend. The brightbackground shows the polymer matrix, whereas the dark cubicstructures are the structure of Cu2O NPs, and their diameter isapproximately 90 nm. In terms of concentration (0.1% nano-llers), individual cubes are well dispersed and separated fromone another. Thus, the nest extent of dispersion is achieved,and the samples exhibit high-quality dispersion without anyremaining aggregate, thereby causing signicant improvementin the nanocomposite properties.

Scheme 2 Synthesis of PDMS/Cu2O nanocomposites.

This journal is © The Royal Society of Chemistry 2015

The FTIR spectra of the PDMS/Cu2O nanocompositesprovide evidence for the interaction between the polymer andthe NPs. The shis in the absorptions for Si–O–Si asymmetricdeformation and Si–O–Si skeletal deformation are less vivid forlow concentrations but are well observed when the nanollercontent was increased up to 5% loading. The peak for Si–O–Siasymmetric deformations shis from 1034 cm�1 for vinyl-terminated PDMS to 1020 cm�1 for the nanocomposites, andthe peak for Si–O–Si skeletal stretching shis from 805 cm�1 to792 cm�1 for the 5% loading sample. The increase in theintensity of the peak at 3500 cm�1, which is due to the OHstretching of the H-bond of the adsorbed water on the surface ofCu2O, did not appear at low concentrations but was observed onhigh ller loadings (see ESI, Fig. S3†).

The tensile modulus of the prepared nanocomposites isillustrated in Fig. 3. It is not affected by the incorporation of asmall amount of Cu2O NPs (up to 0.1%) in the nanocomposites

Fig. 4 Water contact angle of the unfilled and filled PDMS/Cu2Onanocomposites.

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with an average value remaining at 3.6 � 0.5 MPa. In otherwords, the stiffness of the silicone-based materials remainsconstant. A progressive increase of up to 8 MPa is observed forhigh concentrations (5% nanoller loadings), which may beexplained by aggregation and agglomerations that cause theincrease in stiffness. For the PDMS/Cu2O nanocomposites andwithin the limited content of nanollers (less than 0.5 wt%), thedynamic stress was unchanged with an average value of 1300 �185 Pa and then increased up to 3315.5 Pa (for the high contentin Cu2O at 5 wt% nanollers), which may be caused by thepresence of aggregates at high concentrations.

3.4. Surface and AF investigation of the nanocompositelms

Biofouling is a dynamic process that spans numerous lengthscales and involves a complex variety of molecules and organ-isms. Surface chemistry is a signicant factor in the formation,stability, and release of the fouling organisms' adhesion to

Table 1 Contact angle (q) measurements for various solvents (ethylechemically synthesized PDMS and filled PDMS/Cu2O nanocomposites coin various solvents according to VOCG equation; and the percentage de

Sample design

q Ethylene glycol q Diiodom

Dry Wet Dry

PDMS blank 84� � 2� 78� � 2� 75� � 1�

PDMS/Cu2O (0.01%) 95� � 2� 88� � 3� 80� � 2�

PDMS/Cu2O (0.05%) 104� � 3� 97� � 3� 84� � 2�

PDMS/Cu2O (0.1%) 120� � 2� 112� � 2� 89� � 1�

PDMS/Cu2O (0.5%) 106� � 1� 96� � 3� 85� � 1�

PDMS/Cu2O (1%) 98� � 3� 89� � 2� 81� � 2�

PDMS/Cu2O (3%) 91� � 2� 87� � 2� 76� � 2�

PDMS/Cu2O (5%) 83� � 4� 79� � 4� 71� � 1�

a Note: gtotalS calculated with van oss–Chaudhury–Good approach, Lifshitzacid component gS

+.

Fig. 5 (A) and (B) present the weight loss and biodegradability meascomposites with different microorganisms [Gram (+Ve 1), Gram (+Ve 2)

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surfaces.44 The hydrophobicity of the surface of hybrid nano-composites was evaluated using static contact angle measure-ments, as shown in Fig. 4. The measurements were performedbefore and aer immersion in demineralized water for 7days.15,45 The unlled PDMS contact angle obtained beforeimmersion was 102 � 2�, indicating a hydrophobic character,whereas the contact angle aer immersion dropped to 90�,indicating the decrease in surface hydrophobicity. Withdifferent loading concentrations of Cu2O nanollers before andaer immersion, the wettability of the coatings shows anincrease in the hydrophobic behavior with 0.1% nanollers.The water contact angle analysis (i.e., at 130�) shows a lmsurface that is simultaneously hydrophobic and lipophobic dueto the well dispersion of ller NPs, leading to (1) increasedsurface area and chemical bonding, and (2) reduced the surfaceroughness of polymer-NPs patterns, as shown in Table S1.†Results shows a decrease in the hydrophobicity at high NPsconcentrations for both the un-immersed and immersed

ne glycol and diiodomethane); total surface tensions (gtotalS ) of thentrols before (dry) and after (wet) immersion in distilled water for 7 daysgree of swelling SD (%) in n-heptane as a good solventa

ethane gtotalS (mN m�1)Swelling degreein n-heptane (SD (%))Wet Dry Wet

70� � 2� 20.23 23.06 93.6 � 1%75� � 3� 17.85 21.32 90.3 � 2%79� � 2� 16.22 19.6 88.5 � 1.5%85� � 2� 14.096 16.39 89.9 � 1%80� � 1� 15.47 18.55 88.6 � 2.6%76� � 2� 17.37 20.2 88.4 � 1.8%72� � 1� 20.38 23.11 87.5 � 1.2%67� � 2� 23.31 31.31 85.9 � 2.5%

–van der Waals component gLWS , lewise base component gS

� and lewise

urements, respectively, of the unfilled and filled PDMS/Cu2O nano-, Gram (�Ve 1), and Gram (�Ve 2) bacteria and yeast].

This journal is © The Royal Society of Chemistry 2015

Fig. 6 Optical microscope images (A), (B), (C), and (D) of the unfilledPDMS; (E), (F), (G), and (H) of the 0.01% nanofillers in the PDMS/Cu2Onanocomposites; (I), (J), (K), and (L) of the 0.05% nanofillers in thePDMS/Cu2O nanocomposites; (M), (N), (O), and (P) of the 0.1% nano-fillers in the PDMS/Cu2O nanocomposites; (Q), (R), (S), and (T) of the0.5% nanofillers in the PDMS/Cu2O nanocomposites; (U), (V), (W), and(X) of the 1% nanofillers in the PDMS/Cu2O nanocomposites; (Y), (Z),

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Paper RSC Advances

samples, indicating the effect of agglomeration and aggregationof NP nanollers. As a result, the absence of the chemicalboding interactions between the polymer and agglomeratedller NPs onto surface pattern may occur with the high-concentration of NP llers.46 Table S1† shows that with high-concentrations of NPs used, the ne Cu2O NPs may tend tocombine together and form strongly bonded aggregates ontosurfaces, leading to drastically decrease in surface area andhydrophobic character. The enhanced surface hydrophobicityincreases the surface roughness and facilitates the foulingadhesion. Our nding also shows that the contact angle reachesto a value close to that obtained before immersion under dryingcondition. Therefore, the unlled and lled silicone surfacesappear to have reversibly tunable properties.46

Surface energy measurements on the PDMS/Cu2O nano-composites were performed according to the van Oss–Chaud-hury–Good (VOCG) model by measuring the surface contactangle with both polar (water, as in Fig. 4, and ethylene glycol)and nonpolar (diiodomethane) liquids, as shown in Table 1.The calculated surface free energy values of the unlled andlled PDMS are summarized in Table 1. The results illustratelow surface free energy for the 0.1% Cu2O nanollers and thuslow adhesion of microorganisms. By contrast, the surface freeenergy gradually increases with increasing ller loadings of upto 5% because of the roughness caused by agglomerations andaggregation.

Swelling measurements represent a technique of choice forthe characterization of the polymer network. This test consistsof immersing a piece of composite in a good solvent (heptane)and monitoring the evolution of the swelling mass at regularintervals. The swelling degree of the lled PDMS was lower thanthat of the unlled, as shown also in Table 1. A low swellingdegree is typical of a more important crosslinking density.Thus, Cu2O NPs can be seen as additional (physical) cross-linking points, acting positively during network formationbecause of the excellent affinity between components.

Biodegradation can be dened as the process in whichsubstances are broken down by the action of microorganisms.The microorganism's growth in the material increases the sizeof pores and induces cracks. As a result, the structure of thematerial is destabilized.47 The biodegradation of PDMS innatural or living organisms has been poorly examined.39 PDMShas been treated as nonbiodegradable and inert. At the turn of1970s, the possibility of their biodegradation has beenproven.48,49 Only few studies that focus on the biodegradation ofvarious siloxanes have been reported. Nevertheless, the lack ofstudies did not affect the general assessment that polysiloxanesare a group of polymers that is difficult to biodegrade.49 In thiswork, AF and biodegradation analysis was performed on theprepared PDMS/Cu2O nanocomposites as follows.

Weight loss tests were carried out on the unlled and lledPDMS nanocomposites from 0.1% PDMS/Cu2O (which was

(AA), and (AB) of the 3% nanofillers in the PDMS/Cu2O nano-composites; (AC), (AD), (AE), and (AF) of the 5% nanofillers in the PDMS/Cu2O nanocomposites; all images were recorded before and afterimmersion in microorganisms for 30 days.

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proven to enhance characteristics with low nanoller loading of0.01% and 0.05%, as described in the aforementioned tests)and up to 5% llers. A comparison of the weight loss results ofthe unlled and lled PDMS nanocomposites at various load-ings were carried out and shown in Fig. 5A, which suggests thatthe weight loss is higher for the unlled PDMS and decreased tonearly zero with 0.1% loadings in PDMS/Cu2O. As shown inFig. 5B, the biodegradability percentages for 30 days werecalculated, where the blends with low nanoller concentrationsdegrade slowly, whereas those with high nanoller concentra-tions degrade rapidly in 30 days. This nding indicated that thesignicant FR characteristics were found with design patternsthat have well dispersion of Cu2O NP llers at loading amountup to 0.1%. In such patterns, the Cu2O NPs may lead to increasesurface area, chemical bonding, and surface smoothness ofpolymer-NPs surfaces (see Table S1†). Consequently, theenhancement in the surface properties and the adhesionresistance of microorganisms is achieved, thereby preventingsurface deterioration. On the contrary, with the surface designpatterns fabricated at high concentrations of up to 5% llerNPs, the gradual increase in the fouling adhesion is due to theagglomeration and aggregation of the NPs at high llerconcentrations.

Generally, the resistance of Gram-negative bacteria towardantibacterial substances is related to the hydrophobic surface oftheir outer membrane rich in lipopolysaccharide molecules.The membrane acts as barrier to the penetration of numerousantibiotic molecules associated with the enzymes in the peri-plasmic space, which are capable of breaking down the mole-cules introduced from outside.50

The biolm coverage on the surface of silicone was roughlydetermined through the biolm formation images in the opticalmicroscope for the unlled and lled PDMS specimens beforeand aer immersion in the used microorganisms, as shown inFig. 6. For the unlled PDMS, given that the silicone and glassslide are transparent, the picture background was white if nobacteria are attached to the silicone surface. The surfacecoverage of the dark area (fouled area) relative to the total areawas assumed to be the surface coverage of the bacteria. Thewell-dispersed NPs observed, which are related to the lowconcentrations of Cu2O NPs up to 0.1%, were the reason for thehomogeneity and immunity of the surface. By contrast, the non-homogeneity observed for the blank samples were due to thefouling settlement on its surface, which is observed for thecharacteristics achieved from the failure mechanism technique.With increasing nanoller concentrations of up to 5%, thespecimens were also densely fouled. The SEM micrographs forthe unlled and lled polymer nanocomposites (see ESI,Fig. S4†) were obtained and showed Cu2O NPs as white spots. Agood dispersion and homogeneity of NP distributions isobserved for concentrations up to 0.1%, thus affords improvedsurface characteristics and enhanced immunity against micro-organisms. By contrast, additional loadings up to 5% of Cu2ONPs lead to aggregation and agglomerations and consequentlyincreased surface roughness. Thus, air voids may be trappedbetween the agglomerates and reduce the surface propertiesand immunity of the coatings against fouling.

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4. Conclusions and outlook

A series of PDMS/cubic Cu2O nanocomposites with advancedsurface properties was successfully fabricated for use as FRcoatings. The XRD and TEM results of the prepared Cu2O NPsexhibit truncated nanocubes bounded by {100} facets with anaverage diameter of 90 nm. These nanocomposites weresynthesized with various ller concentrations, and conventionalhydrosilation curing mechanism was employed using platinumcatalyst and Si–H functional cross-linker. The inclusion of smallconcentrations (0.1%) of NPs can signicantly enhance thesurface properties and, to a lesser degree, the phys-icomechanical performance of the nanocomposites because ofwell dispersion. However, high NP concentrations (5%) tend tostrongly agglomerate (particle clustering) because the inter-molecular attraction forces of the NPs are high given their largesurface area, thus reducing the nanocomposite properties. Thesurface characteristics of the prepared nanocomposites werestudied from a mathematical viewpoint using appropriatemathematical tools. At low levels, an increase in hydrophobiccharacter and a decrease in surface tension were achievedbecause of the well dispersion of nanollers. The surface ishomogenous such that sessile drops of test liquids assume ahemispherical shape with maximum increase in contact anglefor 0.1% nanollers, which enhance the easy cleaningphenomena. The results obtained from the present investiga-tion revealed that the AF potential of PDMS/Cu2O graduallyincreased with increasing ller loadings up to 0.1% butdecreased at high loading levels because of agglomerations. Thepresence of Cu2O nanocubes in the PDMSmatrix endows it withvarious properties that make the polymer nanocomposite apromising candidate as an environment-friendly FR coating.

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