Fluorocarbon Coatings Via Plasma Enhanced Chemical Vapor Deposition of 1H,1H,2H,2H-perfluorodecyl Acrylate - 2, Morphology, Wettability and Antifouling Characterization

Fluorocarbon Coatings Via Plasma EnhancedChemical Vapor Deposition of 1H,1H,2H,2H-perfluorodecyl Acrylate - 2, Morphology,Wettability and Antifouling Characterizationa

Virendra Kumar,* Jerome Pulpytel, Hubert Rauscher, Ilaria Mannelli,Francois Rossi, Farzaneh Arefi-Khonsari

Introduction

Low surface energy surfaces having hydrophobic andoleophobic characteristics are highly desirable for variousapplications, such as biocompatible surfaces, antifoulingcoatings, durable, waterproof and stain resistant paper,textiles and wood, low dielectric constant material formicroelectronics, etc.[1–3] Fluorocarbon polymers are thesuitable candidates for these applications due to theirunique properties, such as high thermal stability, excellent

Full Paper

V. Kumar, J. Pulpytel, F. Arefi-KhonsariLaboratoire de Genie des Procedes Plasmas et Traitement deSurface, Universite Pierre et Marie Curie, ENSCP,11 rue Pierre etMarie Curie, 75231 Paris cedex 05, FranceFax: (þ33) 1 44276813; E-mail: [email protected]. KumarRadiation Technology Development Division, BARC, Trombay,Mumbai 400085, IndiaH. Rauscher, I. Mannelli, F. RossiInstitute for Health and Consumer Protection, EuropeanCommission Joint Research Centre, Ispra, Italy

a Part 1: cf. ref. [34]

Low surface energy fluorocarbon polymer coatings were prepared via plasma enhanced chemicalvapor deposition (PECVD) of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) in a low pressureinductively excited RF plasma. The influence of plasma parameters, such as, deposition time,continuous wave (CW) and pulse modulated (PM) plasma mode, plasma power and plasma dutycycle (DC) on morphology and wettablity of the PFDA was investigated using field emissionscanning electron microscopy (FESEM) and contact angle (CA) measurement techniques. Theplasma mode, plasma power and pulse duty cycle played a pivotal role in tailoring the surfacemorphology, wettability and the surface energy of the PFDA coating. The water CA hysteresisvalues for PFDA coatings suggested the wetting characteristics of the coating satisfying Wenzelmodel of nanostructured solid-water wetted contact. Athin conformal PFDA coating transformed a super-hydro-philic Whatman filter paper into a super-hydrophobicand oleophobic surface, which has industrial appli-cations for development of durable, stain resistanceand liquid repellent papers. The antifouling propertyof PFDA coatings investigated by quartz crystal micro-balance (QCM) exhibited the protein repellent behavioragainst three model proteins namely, ovalbumin (OVA),human serum albumin (HSA) and fibrinogen (FGN).

926Plasma Process. Polym. 2010, 7, 926–938

! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com DOI: 10.1002/ppap.201000038

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chemical resistance, low friction coefficient, superiorweatherability, oil andwater repellence, low flammability,low dielectric constant, etc.[4–7]

Non-specific adsorption of proteins and associatedbiofouling is one of the most significant limitations tothe end point utility of several biomaterial and marinedevices.[8] A large number of existing and emergingbiotechnological applications suffer from biofouling pro-cess; including contact lenses, catheter tubes, bloodcontacting devices, implant devices, biosensors, microflui-dics and drug delivery systems. The adsorption of proteinsonto a biomaterial surface initiates a cascade of eventsinvolved in biofouling process, including the biofilmformation, that can ultimately result in inflammation,infection and rejection of the material/implant causingextra pain and medical cost to the patient.[8,9] In fact, theprimary mechanism in the attachment of microorganismsto surfaces involves conditioning of thematerial surface byprotein adsorption from the biological fluid or protein andglycoprotein secreted by the microorganism.[10–12] Marinedevices also suffer from the bio-fouling resulted from theundesired accumulation of colonizing organisms, e.g.,bacteria, cyanobacteria, algae, etc. The undesirable accu-mulation of biomass on a ship hull leads to increasedweight and higher hydrodynamic drag resulting in loweroperational speeds and increased fuel consumption.[13]

Therefore, the development of efficient protein-resistantcoatings has great industrial relevance to the production ofantibiofouling surfaces.[14–19] The surface-free energy,wettability and mechanical properties of the coating playa vital role in defining the extent to which a surface canresist biofouling or facilitate fouling release.[20–22] In thisregard, the potency of fluoropolymers[17,23,24] and siliconeelastomers,[25] which have emerged as the most promisingcandidates, has been explained in terms of their inert-lowsurface energy and interestingmechanical properties.[21,26]

In general, the conventional wet deposition methods offluoropolymer coatings involve various chemicals andsolvents and suffer from processing difficulties, such aspoor adhesion of coatings to the substrate due to the inertnature of fluorocarbons and poor control over the thick-ness.[27–33] On the other hand, the dry and solvent freeplasma enhanced chemical vapour deposition (PECVD) orplasma polymerization process does not suffer from theprocessing difficultiesmentionedbefore, andoffers a betterand effective process to deposit fluorocarbon coatings onany substrate with good adhesion and better control overthickness, chemistry, morphology and final properties ofthe coatings.[34–40] Majority of the literature on thefluorocarbon polymer coatings by PECVD process targetedfor different applications involve saturated low molecularweight precursors, namely CF4, C2F6, C3F8, C3F6O, CH2F2,CHF3, CF3CHF2 and C4F8.

[41–48] For example, Vaswani et al.have reported the applications of plasma polymerized

fluorocarbon coatings fromCF3CHF2 andC4F8precursors forenhancement of barrier properties and hydrophobiccharacteristics of paper and regenerated cellulose sur-faces.[48] However, recently, great attention has been paidto produce fluorocarbon polymer coatings by plasmapolymerization of highmolecularweight organic precursormolecules containing polymerizable unsaturated acrylategroup with perfluoroalkyl pendant chain due to theirexceptional properties suchas fast polymerization, require-ment of mild processing conditions and hydrophobic aswell as oleophobic (i.e., liquid repellent) characteristics. Theextraordinary properties of these precursors are attributedto the presence of acrylate group and the peculiar chemicalarchitecture of the perfluorocarbon polymer chains.[29,49,50]

Some of the examples of these precursors include1H,1H,2H,2H-perfluorodecyl acrylate (PFDA),[34,51,52]

1H,1H,2H,2H-perfluorooctyl acrylate[53] and 1H,1H,2H-per-fluoro-1-dodecene.[54,55] Themajority of the applications ofthefluorocarboncoatingsdependonthesurfaceenergyandwettability of the surface, which is governed by thetopographyandthemorphology inaddition to thechemicalcomposition of the coating. Surfaceswith regularly alignedandcloselypackedCF3groupshavebeen reported toexhibitsurface energy of 6.7mJ "m#2, which is well below the 18–20mJ "m#2 value for polytetrafluoroethylene (PTFE).[28,36]

The main challenge is to preserve the chemical archi-tecture of the precursor molecule in the plasma polymer-ized coating to the maximum extent possible with tailorcapability of the surfacemorphology andwettability of thecoating.[37,51,54] In our earlier study, we have shown byspectroscopic techniques that plasma polymerized PFDAcoating with wide range of chemistries could be producedby varying plasma process parameters.[34] In the presentwork, we have investigated the influence of plasmaparameters on the morphological and surface wettabilityof the plasma polymerized PFDA coatings. Most of theresearch works, carried out on plasma deposited perfluor-oalkyl acrylate coating, have been targeted mainly toachieve the low surface energy surfaces with liquidrepellent properties.[51–57] To the best of our knowledge,no study has been aimed to explore the protein repellentcharacteristics of plasma polymerized PFDA coatings.Therefore, the objective of the present study has beenfocused on the antifouling (i.e., protein repellent) aswell asthe liquid repellent applications of the plasmapolymerizedPFDA coatings.

Experimental Part

Materials

1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) precursor

(CH2¼CH#COO#CH2CH2(CF2)7CF3,purity¼ 97%,Molecularweight

(M)¼518.17, Sigma–Aldrich, France)was used as receivedwithout

Plasma Process. Polym. 2010, 7, 926–938

! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plasma-polymers.org 927

Fluorocarbon Coatings Via Plasma Enhanced Chemical Vapor Deposition of 1H,1H,2H,2H-perfluorodecyl Acrylate - 2 . . .

further purification. Argon gas (Air Liquide, France, purity> 99.9%)

was used as a carrier gas. Thermanox cover slips (Nalge Nunc

International, Thermo Fisher Scientific, USA) and polished silicon

wafers (100) (Siltronix) were used as coating substrates. Whatman

quantitativefilter papergrade50 (Pore size 2.7mm,97g "m#2)were

procured from Sigma–Aldrich France. Three test proteins, namely,

ovalbumin (OVA,Sigma–A5503), humanserumalbumin (HSA,30%

solution, Sigma-A6909) andfibrinogen (FGN, lyophilized Fraction I,

Type I, human,Sigma-F3879)werediluted in10mMPBS (pH7.4) toa

final concentration of 50mg "mL#1 and used for quartz crystal

microbalance (QCM) analysis.

Plasma Deposition Setup

1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) polymer coatings

were fabricated in a low pressure inductively excited radio

frequency-tubular quartz plasma reactor system (5 cm diameter,

40 cm length, base pressure of 3% 10#2mbar). The schematics of

plasma deposition setup and technical details of the process have

been provided in our earlierwork.[34] The flow rate of the precursor

was controlled by flow rate of carrier gas (i.e., Ar), which was

regulated and measured by electronic mass flow controllers (MKS

instruments). Prior to each experimental run, the reactor was

scrubbed and cleaned with detergent, organic solvents and dried

using a compressed air. The plasma reactor system was

reassembled and cleaned further with 20W Ar plasma discharge

at 0.5mbar pressure for 30min. PFDA coatings were deposited at

working pressure of 0.5mbar, precursor temperature of 65 8C,precursor flow rate of 0.44 sccm, Argon flow rate of 20 sccm, unless

otherwise mentioned.

Coating Characterization Techniques

Field Emission Scanning Electron Microscopy (FESEM)Themorphologyandthicknessof theplasmapolymerizedcoatings,

prepared under different plasma process conditions, were inves-

tigatedbyusingFESEM.FESEMimageswere takenusingZeissUltra

55 FEGSEMwith GEMINI Column on goldmetallized PFDA coating

surfaces by sputter coating (Cressinton sputter coater-108 auto).

Electrons with accelerating voltage of 5 and 6kV were used to

obtain the FESEM images. The thicknesses of the coatings were

measured from the cross-sectional images of fracturing PFDA

coatings deposited on silicon wafers.

Atomic Force Microscopy (AFM)An atomic force microscope NTEGRA PRIMA from NT-MDT

was used to examine the topography and roughness of the

plasma polymer coatings deposited onto silicon wafer substrates.

The microscope was operated in semi-contact mode. All of the

AFM images were acquired at room temperature (&24 8C) in air

and are presented as unfiltered data. ‘NOVA’ SPM Software

was used for data acquisition and data processing. The average

surface roughness (Ra) values were derived from (10mm%10mm)

AFM images.

Contact Angle (CA) MeasurementSessile drop CA values were measured using a video capture CA

apparatus (Digidrop GBX-3S system, France). For each measure-

ment, a6mLdropletwasdispensedonto thecoatingsurfaceand the

CAwasmeasured inastaticaswell as inadynamicmode.Thestatic

water contactangle (WCA)weremeasured just after thedispensing

the liquid drop on the surface and reported as an average (mean)

value' s (standard deviation), estimated from five individual CA

valuesmeasured per each sample. Surface energy of PFDA coatings

were estimatedby static CAvaluesmeasured for twoprobe liquids,

namely water and diiodomethane using Owen and Wendt

geometric mean Equation (1)[58]

1þ cosuð Þg l ¼ 2 gdsgdl

! "1=2 þ gpsgpl

! "1=2h i(1)

and

gs ¼ gps þ gds (2)

where u is the CA, gs and g l are the surface free energies of the solid

and liquid, respectively. The superscripts d and p refer to the

dispersive and polar components of surface energy, respectively.

Advancing CAs were measured by lowering the liquid droplet

towards the underlying surface until it just touched (without any

distortion of the circular shape) and measuring the CA while

increasing the volume of liquid drop until the CA remained

constant. Receding angle was measured by decreasing the

volume of the drop until the CA value remained constant and

the solid/liquid interface started to decrease.[59] The values

reported are the average value' s, estimated from five individual

measurements per sample.Water contact angle hysteresis (WCAH)

is defined as the difference between the advancing and receding

CAs. WCAH is originated by the presence of topographical and

chemical heterogeneities of the surface. The CA hysteresis is an

indication of the tendency of a liquid droplet to roll off across the

surface (i.e., in the case of lowCAHvalue the droplet easily rolls off,

and vise versa). Two descriptions have been proposed for the

dependency of the wetting behavior on surface heterogeneity or

surface roughness: A very low CAH value for a rough surface

indicates the slippery behavior of composite rough surfaces (air

trapped in the grooves) satisfying Cassie–Baxter model, whereas,

high CAH value for a rough surface indicates the non-slippery

behavior of wetted rough surfaces satisfying the Wenzel

model.[53,60] Comparison of the Cassie–Baxter equation with the

Wenzel equation shows that the main difference between

composite (air trapped in the grooves) and rough surfaces is that

formost offiniteCAvalues; the formerpredicts greaterCAs relative

to the corresponding smooth surface, whereas this only holds true

in the latter case when CAs exceed 908CA on the flat substrate (i.e.,

less than 908 on a smooth surface gives rise to a smaller CA upon

roughening).[61]

Quartz Crystal Microbalance (QCM)Recent studies have proven that the QCM-dissipation (QCM-D)

technique is very useful for the evaluation of surface-related

processes in liquids, including protein adsorption.[62–67] The QCM

instrumentmonitors resonating frequencychangeof theAucoated


! 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ppap.201000038

V. Kumar, J. Pulpytel, H. Rauscher, I. Mannelli, F. Rossi, F. Arefi-Khonsari

sensorquartzdiscduetotheadsorptionofproteinmoleculesonit in

a time resolved manner. The change in the resonating frequency

(Df) of the sensor disc could be related to the mass change (Dm) of

the sensor disc due to adsorption or desorption of material using

Sauerbrey Equation (1).[68]

Dm ¼ # C=n½ +: Dfð Þ (3)

where n is the overtone number and C is a mass sensitivity

constant. At 5MHz, a frequency shift of 1Hz corresponds to amass

change of&18ng " cm#2. Sauerbrey relation (1) is applicable to only

sufficiently thin and rigid adsorbed films.[67] However, the QCM

analysis has been widely used to obtain the adsorption behavior

and approximate mass of adsorbed protein on to various

substrates.[62,66] However, the trapped solvent molecules may

make theadsorbedprotein layer soft,which influence thedamping

of the quartz oscillation and the sensitivity constant of the sensor,

resulting in the error in the estimation of the deposited mass.

Therefore, we had already examined this aspect and found a low

dissipationshift (<0.1E-5 forHSAandOVA;<0.6E-5 forFGN)during

the protein adsorption process,which pointed out the formation of

a rigid protein layer where the viscoelastic effect was not

significant; allowing us to estimate approximatemass of absorbed

proteins using Sauerbrey equation. PFDA coatings were analysed

for their antifouling properties by measuring protein adsorption

using three different proteins, as a function of time in a continuous

flow mode using a Quartz crystal microbalance QCM-D E4

instrument (Q-Sense AB, Gothenburg, Sweden). All data presented

in this studycorrespondto thefifthovertone,which is less sensitive

to variations of the mounting conditions of different crystals.

Polished AT-cut and Au coated sensor quartz discs (14mm in

diameter, 0.30mm thick) with a fundamental frequency of 5MHz

(Q-SenseAB,Gothenburg, Sweden)wereusedas substrate for PFDA

coatings for the QCM experiments. After assembling the sensor

quartz disc into the QCM, the plasma polymerized PFDA coated

surfaceswere first exposed to 10% 10#3M PBS solution at pH 7.4, in

order to stabilize the systemandobtain thebase line.After that, the

protein solutionwith50mg "mL#1 concentration in10%10#3MPBS

waspumpedcontinuously through themeasurement chamberat a

flow rate of 20mL "min#1. The frequency changes due to adsorption

of proteins were monitored as a function of time. All QCM

adsorption experiments were performed at 25 8C' 0.02 8C in

duplicate.

Results and Discussion

Plasma Deposition TimePlasma deposition time is an important parameter thatgoverns the properties of plasma polymerized coatingssuch as, coating thickness, morphology and topography ofthe coating. PFDA coatings were deposited for variousdeposition times and their surface morphology andwettability were investigated by FESEM and WCA mea-surement techniques, respectively. We have reportedearlier that the coating thickness increases linearly withthe deposition time, whereas, the surface chemistry ofPFDA coating was not influenced by plasma depositiontime.[34] Figure 1(a–c) shows the FESEM micro-images ofsurfaces of PFDA coatings prepared at three differentdeposition times that revealed the influence of depositiontime on the surface morphology of PFDA coating. At lowerdeposition time the PFDA coating showed less prominentwith less number of nano-particulate structures appearedonthecoatingsurfaces. Theparticulate surfacemorphologygradually becamemore prominentwith the increase in thedeposition time. The WCA of the PFDA coatings increasedwith the increase in deposition time (Figure 1 caption).PFDA coating prepared at 10min deposition time (thick-ness¼ 120' 15nm) exhibited WCA¼ 1168, whichincreased to 1218 and 1268 for deposition time of 20min(thickness¼ 290' 45nm) and 40min (thickness¼ 580'50nm), respectively. The increase in the hydrophobicity ofPFDA coatings with the increase in plasma deposition timewasmainly attributed to the increase in the roughness andthe change in the surface morphology to rougher withnanostructured particulate features [Figure 1 (a–c)]. Thedependency of particulate size on the plasma depositiontime has also been reported in literature.[69,70] Effect ofdeposition time on surface structures, morphology andwettability of plasmafluorocarbon coatings produced fromtetrafluoroethylene precursor have also been reportedearlier by other research groups.[71,72] They have foundthat the surface morphology alters from nanostructured

Figure 1. Field emission scanning electron microscopy (FESEM) images of PFDA coating surfaces prepared at different deposition time(a) 10min (WCA¼ 1168), (b) 20min (WCA¼ 1218) and (c) 40min (WCA¼ 1268) (Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms).




particulate to complex micro/nano sized ribbon likestructures with the increase of thickness of coating andplasmadeposition times,whichwasalsomanifestedby thesignificant increase in the WCA values. These observationsshowed that the plasma deposition time influenced thecoating thickness, surface morphology and wettability butnot the chemical composition of the PFDA coatings.[34]

PM Versus CW Plasma ModeInfluence of the plasma operation modes i.e., CW and PMplasma discharge modes on the composition, surfacestructures and morphology of different plasma fluorocar-bon coatings have been reported by other researchgroups.[71,72] Figure 2 shows the FESEM images of PFDAcoatingspreparedunder PMandCWplasmamodes. It couldbe seen that PFDA coatings deposited under PM plasmamode exhibit particulate nanostructured morphology(Figure 2a), whereas, the PFDA coatings prepared in CWplasma condition exhibit smooth surface morphology(Figure 2b). AFM analysis also revealed that the PFDAcoating prepared under PMmode exhibit higher roughness(Ra¼ 22.3 nm) as compared to that prepared under CWmode (Ra¼ 0.5 nm) (AFM images not shown). Cicala et al.havealso reported thedifference in the surfacemorphologyand roughness of fluorocarbon films obtained by CW andPM plasmas fed with C2F4 precursor.[71] The difference inthe surface morphology and roughness of fluorocarboncoatings produced in CW and PM mode was attributed tothe difference in the growth mechanism.[51,71] CW plasmaprocess deposits coatings under more energetic condition,as theenergy is continuouslybeing fed to thesystem,whichgives rise to high plasma density generating energeticspecies through out the treatment time that causefragmentation, polymerization initiation of precursors,generation of reactive sites on the coating and surfacemodification of coating via etching/ablation processesgiving rise to smooth coatings surface. On the other hand,PMplasmaoperatesat lowtotalpower input i.e., equivalent

powerPeq conditions.[34,73] In PMplasmaconditions, amore

ordered growth process occurs during plasma ‘off’ times(toff), relative to complex processes during the highlyenergetic plasma ‘on’ periods (ton). The lifetime of neutrals(responsible for deposition/polymerization) ismuchhigherthan those of the ions and electrons (responsible for surfacemodifications by ablation/etching).[74] Therefore, PMplasma process provides the control over the extent ofprecursor fragmentation, coating degradation or surfacemodification by varying ton/toff ratio, which regulates therelative concentration of the ions and the neutrals duringtheplasmaoff time. In PMplasmamode, theneutral radicalcould stay longer and take part in the polymerization/deposition process during plasma toff withminimal surfacemodifications of coatings via etching/ablation caused byions, which was manifested by nanostructured roughPFDA coatings.[75,76] In PMmode, upon reaching to a criticalnumber density of the radical species, rapid agglomerationis triggered leading to the particulate morphology of thecoating.[77] However, in CW mode excessive secondaryproducts cause the partial pressure of precursor to drop toan extent that precludes exclusive powder or particulateformation. Moreover, in CW mode, the polymerization,fragmentation and ablation processes take place simulta-neously throughout the plasma discharge time leading tosmooth coating surface by competitive ablation andpolymerization (CAP) principle.[34] It could be concludedthat in CW plasma mode, the uniform and smooth filmgrowth resulted from the synergistic effect of depositionand surface modification by etching/ablation of coatings.On the contrary, the film growth in PM plasmas is mainlygoverned by the growth of coalesced nuclei producedduring plasma off time, resulting in the rough andparticulate morphology of coatings.[72]

Table 1 shows static CA values and the surface freeenergy for PFDA coatings on Thermanox, preparedunder PM and CW plasma modes using two test liquidsi.e., water and diiodomethane. The static CA valuesare significantly higher for PFDA coatings prepared on

smooth Thermonox surfaces under PMmode (PM-1W: ton/toff¼ 10ms/90ms,WCA¼ 126.58; CA of diiodomethane¼1058) as compared to that preparedunderCW mode (CW-1W: WCA¼ 107.08; CA ofdiiodomethane¼ 93.08). The lower WCAvalues (i.e., lower hydrophobicity) forPFDA coatings prepared in CW mode ascompared to that prepared under PMmode was attributed to de-fluorinationand lower CF2 and CF3, and incorporationofoxygenatedgroup inthePFDAcoatingsprepared under CW plasma mode,[34] inaddition to the lack of surface roughness(a smoother surface) in CWmode (FESEM

Figure 2. Field emission scanning electron microscopy (FESEM) images of the PFDAcoatings prepared under CW and PM plasma mode (ton¼ 25ms, toff¼ 75ms) (a) PM-1W,(b) CW-1W.




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and AFM analysis). The similar trend of static CA wasexhibited for PFDA coatings prepared on rough Whatmanpaper (PM-1W: ton/toff¼ 10ms/90ms, WCA¼ 1518;CA of diiodomethane¼ 1228) as compared to thatprepared under CW mode (CW-1W: WCA¼ 1358; CA ofdiiodomethane¼ 1178). However, the CA values weresignificantly higher for PFDA coatings on Whatman paperas compared to that on Thermanox surfaces, due to themicro-roughness of paper surface. The high CA values forwater and diiodomethane clearly revealed the waterrepellent (hydrophobic) and oil-repellent (oleophobic)characteristic of PFDA coatings. The surface energy of PFDAcoating was found to be greatly influenced by theplasma operation mode (Table 1). The surface energy ofPFDA coating prepared under PM mode exhibits very lowsurface free energy (PM-1W, ton/toff¼ 10ms/90ms:gs¼ 7.3mN "m#1, gp

s ¼ 0.2mN "m#1, gds ¼ 7.1mN "m#1) ascompared to that prepared under CW mode (CW-1W:gs¼ 13.4mN "m#1, gp

s ¼ 2.0mN "m#1, gds ¼ 11.4mN "m#1).

There was an increase in both the polar as well as thedispersive component of the surfaceenergy when PM mode is changed toCW plasma mode, which is explained byde-fluorination leading to the decrease inCF2 groups in perfluoroalkyl chain, andincorporation of polar groups in CWplasma operation mode.[34]

Plasma PowerThe influence of plasma power was alsoreflected in the surface morphology,hydrophobicity and surface energy ofthe PFDA coatings. FESEM images shown

in Figure 2 and Figure 3 clearly reveal that plasmapower (inboth CW and PMmode) affect the surface morphology andthe distribution of nanostructures of PFDA coatingssurfaces. The surface morphologies of coatings alter fromrough morphology with dense and overlapped prominentparticulate nanostructures for PM-1W (Figure 2a) tocomparatively lesser rough morphology with very fewand less prominent particulate structures for PM-4W; itseems the particles were embedded inside the polymercoating film (Figure 3a). On the contrary, PFDA coatingprepared under CW at 1W power (CW-1W) exhibited asmooth surface with no particulate nanostructures(Figure 2b), however, surprisingly, the increase in powerresulted in the formation of very tiny and well separatednanoparticles (dust) on the surface for CW-5W (Figure 3b).The deposition rate of plasma polymerized of PFDA coatinghave been reported to decreasewith the increase in plasmapower[51] due to the fact that at higher power conditionsspecies responsible for fragmentation and ablation (UVradiation, ions or electron bombardment) dominate over

Table 1. Static CA of PFDA coatings on Thermanox coverslips andWhatman paper-50, and surface energy of PFDA coating surface depositedon Thermanox coverslips under different experimental conditions. (DC10: ton¼ 10ms, toff¼90ms, DC25: ton¼ 25ms, toff¼ 75ms, DC50:ton¼ 50ms, toff¼ 50ms, DC75: ton¼ 75ms, toff¼ 25ms).

Sample Static contact angle (degree) Surface energy

Water Diiodomethane mN"m#1

Thermonox Paper Thermanox paper Total Polar Dispersive

(gs) (gps ) (gd

s )

PM-1W-DC25 123.5' 1.5 144.0' 1.5 100.0' 1.5 122.0' 2.0 8.6 0.1 8.5

PM-4W-DC25 111.5' 1.8 136.0' 1.8 93.5' 1.2 111.0' 1.5 12.3 1.1 11.2

CW-1W 107.0' 1.5 135.0' 2.0 93.0' 1.5 117.0' 1.5 13.4 2.0 11.4

CW-5W 102.0' 1.5 129.0' 1.7 84.5' 1.9 102.0' 1.3 17.6 2.3 15.3

PM-1W-DC10 126.5' 1.5 151.0' 1.5 105.0' 1.4 122.0' 1.9 7.3 0.2 7.1

PM-1W-DC50 113.5' 2.0 – 98.5' 1.5 – 10.4 1.2 9.2

PM-1W-DC75 110.0' 2.6 – 96.5' 1.6 – 11.6 1.7 10.0

Figure 3. Effect of plasma power on the surface morphology of the coatings preparedunder CW and PM plasma mode (ton¼ 25ms, toff¼ 75ms) (a) PM-4W, (b) CW-5W.




those responsible for polymerization/deposition (i.e., radi-cals). In PM mode, at higher plasma power condition,combinations of processes: (i) the overlapping of coalescednuclei, (ii) the entrapment of agglomerated particles in tothe polymer coatings and (iii) the simultaneous occurrenceof a secondary reaction process (degradation/ablationprocess) along with polymerization process, were respon-sible for the lessprominentparticulate surfacemorphology.The degradation and ablation of PFDA coating at higherpower condition has been manifested by the decrease inF/C ratio and CF2 group concentration,[34] and decrease inthe thickness of the coating with the increase in plasmapower (PM-1W: thickness¼ 600' 50nm; PM¼ 4W: thick-ness¼ 400' 25nm; CW-1W: thickness¼ 190' 25nm; CW-5W: thickness¼ 50' 15nm). InCWmodethe formationofvery tiny nanoparticles on the coating surface at higherpower condition could be attributed to the high plasmaenergy density that leads to generation of large concentra-tions of reactive species creating a favorable condition forthe gas-phase reactions between radicals and otherspecies.[77,78] The variation in the coating thickness onlycannot justify the difference in the surfacemorphologies ofPFDA coatings because the chemical composition was alsofound to be different for coatings prepared under varyingpower conditions.[34] Therefore, the difference in thesurface morphology may be attributed to the differentgrowth mechanism of PFDA coating prepared underdifferent plasma power conditions.

Table 1 shows the effect of plasmapower on the static CAand the surface free energy for PFDA coatings preparedunder PM and CW plasma modes. The static CA decreaseddrastically with the increase in the plasma power for PFDAcoating deposited on Thermanox as well as on Whatmanfilter paper prepared under CW and PM plasma mode. Thetotal surface free energy, polar component and dispersivecomponent of surface energy increasedwith the increase in

the plasma power. The long perfluoroalkyl chains in PFDAare responsible for showing very low polar and dispersiveinteractions with polar and nonpolar liquids.[51] We havereported that higher plasma power caused de-fluorinationofperfluoalkyl chainand incorporationofoxygenatedpolargroups in the PFDA coating,[34] which was reflected here ashigher polar anddispersive surface energy.Moreover, PFDAcoatings prepared under PM plasmamode exhibited lowersurface energy values than those prepared under CWconditions, which is attributed to higher chemical resem-blance of coating to the monomer, higher F/C ratio andhigher roughness caused by particulate surface morphol-ogy of PFDA coating prepared in PM plasma mode.However, PFDA coatings prepared even at strong plasmaconditions (CW-5W) exhibited lower surface energy(15.3mJ "m#2) thanthatofPTFEcoatings (18mJ "m#2).[56,57]

Table 2 shows the advancing and receding WCAs andwater contact angle hysteresis (WCAH) of PFDA coatings onThermanox coverslip and Whatman paper prepared underdifferent plasma conditions. Similar to static CA, advancingand receding CA showed lower values for higher power andCWplasmaconditions. TheobservedhighWCAHvalue (358)for PFDA coating with highest roughness value(Ra¼ 22.3 nm) prepared at PM-1W indicated that the levelof roughness is not sufficient for there to be a trulycomposite interface (air-trapped) and so ruled out theslippery-composite interface postulated by Cassie–Baxtertheory.[53] Thewetting behavior satisfies theWenzelmodelof solid-water wetted contact, which predicts that surfaceroughening decreases/increases the repellency of liquidsmaking a CA of less/greater than 908 on the correspondingflat surface.[3,79,80] In PM plasma mode, the WCAHdecreased from 358 to 268 when the power is increasedfrom 1 to 4W, whereas, in CW plasma mode, the WCAHincreased from438 to 508whenpower is increased from1to5W. The particulate and rough morphology (for PM-1W)

Table 2. Advancing (ua) and receding (ur) WCA and WCAH of PFDA coatings on Thermanox coverslips and Whatman filter paper, depositedunder different experimental conditions.

Experimental condition Substrate ua ur WCAH

PM-1W-DC25 Thermanox 137.0' 1.6 102.0' 1.8 35

PM-4W-DC25 Thermanox 124.0' 1.4 98.0' 1.6 26

CW-1W Thermanox 120.0' 2.1 77.0' 1.4 43

CW-5W Thermanox 117.0' 1.0 67.0' 1.5 50

PM-1W-DC50 Thermanox 128.0' 1.6 94.0' 1.4 34

PM-1W-DC75 Thermanox 126.0' 1.9 85.0' 1.8 41

PM-1W-DC25 Whatman paper 157.0' 2.0 140.0' 1.5 17

PM-4W-DC25 Whatman paper 148.0' 2.1 122.0' 1.3 26

CW-1W Whatman paper 150.0' 1.6 122.0' 1.7 28

CW-5W Whatman paper 148.0' 1.8 111.0' 1.5 37




increased the actual surface area ofthe coating surface in contact with theliquid and, therefore, manifested as theincreased advancing CA (according tothe Wenzel theory) as compared to thesmooth surface in PM-4W and CW-1Wcoatings. The decrease in theWCAHwiththe increase in the power in PM mode isattributed to the decrease in the surfaceroughness that otherwise acts as ahurdlefor liquid to slide on the surface. More-over, the higher WCAH for coatingsprepared in CW plasma mode is attrib-uted to the polar interaction between thewater and the oxygenated polar groupson coatings. The higher is the plasmapower in CW mode, the higher is theconcentration of polar groups generatedthat eventually lead to higher WCAHvalues.

Pulse Plasma Duty Cycle (DC)In PM plasma mode, at a fixed pulsefrequency, pulse DC governs the plasmapulse characteristics such as ton/toff ratioand equivalent power (Peq), and conse-quently influences coating growth pro-cesses (e.g., deposition, surface modifica-tion), which is manifested by thevariation of chemical and morphologicalcharacteristics of the polymer coat-ings.[34,71,72,81] The influence of the DCof the pulse plasma on surface morphol-ogy of the PFDA coatingswas revealed byFESEM images of PFDA coating surfaces shown in Figure 4.The size, density and distribution of the particulatenanostructures of the PFDA coating surface altered withthe DC. At 10% DC small particulate structures in over-lapped formappeared, but at 25%DCwell separated, biggerand prominent particulate surface morphology wasobtained. At 50% DC few particles and widely separatedbut bigger globular structure with decreased particledensity appeared, and with further increase in DC, particlesize started decreasing and finally disappeared in CW (i.e.,100% DC) conditions. In principle, increase in the DC ofplasma pulse leads the soft PM regime towards the strongCW plasma regime (i.e., DC¼ 100%). At higher DC, higherconcentration of reactive species will be produced durington resulting higher probability of agglomeration ofcoalescednuclei, andhigher fragmentationof theprecursormolecule and surface modification of the coatings, leadingtosmoothsurfaces.[34] Thedecrease in the coating thicknesswith the increase in the DC of plasma pulse suggested the

contribution of ablation leading to the depletion of PFDAcoatings at higher DC (DC¼ 10%: thickness¼ 700' 40nm;DC¼ 25%: thickness¼ 600' 50nm; DC¼ 50%: thick-ness¼ 450' 50nm; DC¼ 75%: thickness¼ 205' 25nm;CW (DC¼ 100%: thickness¼ 190' 25nm). Therefore, thegradual change in the surfacemorphologyof PFDAcoatingsmay be attributed to the alteration of relative contributionof deposition and ablation/etching under different DCconditions.[45]

In order to rule out the effect of thickness on the surfacemorphology of PFDA coatings prepared under CW and PMmode, we have taken the cross-sectional FESEM images ofthe PFDA films (Figure 5) and compared the surfacemorphology of two films prepared under CW and PMmode having almost comparable thickness values(DC¼ 75%: thickness¼ 205' 25nm;CW(DC¼ 100%: thick-ness¼ 190' 25nm). A clear difference can be seen not onlyin the surfacemorphology but also in the bulkmorphologyof the two films prepared under PM and CW plasma mode.

Figure 4. Field emission scanning electron microscopy (FESEM) images of PFDA coatingsprepared under different DC of PM mode (Ppk¼ 1W) (a) DC¼ 10%, (b) DC¼ 25%,(c) DC¼ 50%, (d) DC¼ 75%, (e) CW (DC¼ 100%), (f) untreated silicone.




Moreover, we have shown in our earlier study that thechemical compositionof thePFDAcoatingsalsovariedwiththe change in plasma parameters i.e., CW and PM mode,plasma power, DC (other experimental parameters are thesame).[34] These observations clearly suggest that thecoating thickness is not the parameter that governs thesurface morphology of the plasma polymerized PFDAcoating. Therefore, it can be concluded that the differencein the surfacemorphology of PFDA coatings is attributed tothe difference in the growth processes involved in thedifferent plasma conditions.

The influence of DC on the surface wettability andmorphology of the PFDA coatings was revealed by gradualdecrease in the CA of water (from 126.58 for DC¼ 10% to107.08 for DC¼ 100%) and diiodomethane (from 105.08for DC¼ 10% to 93.08 for DC¼ 100%) with the increase inthe DC of the plasma discharge (Table 1). It can be seenthat there is an increase in the total surface energy (from7.3mN "m#1 for DC¼ 10% to 13.4mN "m#1 for DC¼ 100%)as well as polar and dispersive component of surfaceenergy of the PFDA coating with the increase in the DCof the plasma discharge (Table 1). The advancing andreceding CAs for water decreased and WCAH increasedwith the increase in the DC of the plasma discharge(Table 2), which is attributed to the increase in the polarsurface energy component and changein the surface morphology.

Super-Hydrophobic Property of1H,1H,2H,2H-perfluorodecyl acrylate(PFDA) Coating on PaperFigure 6 shows FESEM images of theuntreated and plasma polymerizedPFDA coated Whatman filter paper-50,showing no structural damage to thefibrous microstructure of paper. In addi-tion, the ribbon like micro-fibrous mor-phology of the paper was maintained

and even became more prominentlyvisible after the thin coating of PFDA,which leads to quite higher CAs(ua¼ 1578) (Table 2). The conformal thincoating of PFDA on the super-hydrophilicpaper transformed it into a super-hydro-phobic surface, due to the combinedeffect of the presence ofmicro-roughnessof intrinsic fibrous structured of paperand high degree of fluorination. Thesuperhydrophobic property of plasmapolymerized fluorocarbon coatings withribbon like nanostructured morphologyproduced from C2F4 precursor gas havebeen reported by d’Agostino and cow-

orkers.[73] Moreover, the static CA of 1228 for diiodo-methane on PFDA coated paper revealed the oleophobicproperty of the PFDA coatings, which is attributedto the long perfluorinated carbon chain length contain-ing CF2 and terminal CF3 groups present in the PFDAcoatings.[51,55]

The PFDA coated Whatman paper (PM-1W-DC25) exhi-bits higher advancing and receding CA and lower WCAHvalue (ua¼ 1578, uc¼ 1408, WCAH¼ 178) as compared toPFDA coated smooth Thermanox substrates (ua¼ 1378,uc¼ 1028, WCAH¼ 358) (Table 2). However, the lowestWCAH value of 178 found for Whatman paper at PM-1W-DC25 conditions is quite high and therefore, theCA theoriessuggest that the super-hydrophobic character of thesesurfaces follow the Wenzel model of solid-water wettedcontact leading to sticking (non-slippery) behavior ofPFDA coated paper surfaces.[60,82] In principle, mere rough-ness is not directly correlated to slippery super-hydro-phobicity; rather the height and distribution of thestructures are more important factors for slippery super-hydrophobicity. In general, structures with greaterheight and lower density result in reduction of hysteresis,which is an essential requirement to reach a Cassie–Baxterregime showing a slippery character for self cleaningapplications.[82]

Figure 5. Field emission scanning electron microscopy (FESEM) images of PFDA coatingsprepared under (a) DC¼ 75%, thickness¼ 205' 25 nm and (b) CW (DC¼ 100%), thick-ness¼ 190' 25 nm (Ppk¼ 1W).

Figure 6. Field emission scanning electron microscopy (FESEM) images of Whatmanpaper (a) untreated (b) PFDA coated (PM mode; ton¼ 25ms, toff¼ 75ms, Ppk¼ 1W).




Protein Repellent Property of 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) CoatingsLow-surface-energy polymeric coatings, such as fluorocar-bon coatings offer a potential means of inhibiting thebiofouling process by presenting a non-stick surface tobiofouling species.[83,84] Increased interest in this approachwas motivated by the observation that the well-knowngorgonian corals with low surface energy natural antifoul-ing surfaces do not allow the colonization bymarinemicro-organism.[85]

Theprotein repellent properties of the PFDA surfaces (PMmode; ton¼ 25ms, toff¼ 75ms, Ppk¼ 1W, thickness &50nm) were analyzed by QCM in a continuous flowmode.Three different proteins, namely, OVA, HSA and FGN in PBSsolution were injected individually and their adsorptionbehavior was studied by monitoring frequency as afunction of time. QCM has been widely utilized as an easytechnique for online investigation of surface-relatedprocesses in liquids, including protein adsorption.[62–67]

In the present study, we have used decrease in thefrequency as an approximate measure of the adsorbedprotein for making a comparison among the adsorptionlevels of three different proteins on a same PFDA surfaceunder the identical conditions. Moreover, the dissipationshiftduring theproteinadsorptionprocesswere foundtobelow (<0.1E-5 for HSA andOVA;<0.6E-5 for FGN), indicatingthe formationofa rigid layerofproteins,which rulesout thesignificant influence of viscoelastic effect; allowing us tocompare the adsorption behavior of different proteinmolecules on PFDA coatings and untreated Au surfaces.Thehigher dissipation shift in case of FGNwasattributed toits higher adsorbed amount leading to thicker layers,whichcan imbibe water molecules that make the protein layersoft with a resulting damping of the quartz oscillation. Infact, several reports are available on the estimation ofamount of proteins adsorbed by QCM using Sauerbreyrelation and the adsorption behavior of proteins on varioushydrophilic and hydrophobic surfaces, using Sauerbreyequation.[62,86,87] Rickert have reported that despite thehigh hydration of the protein layers, viscosity-inducedeffects play a negligible role and that the frequencydecrease reflects primarily mass changes at the surface.[66]

Quartz crystal microbalance (QCM) frequency responsesof untreated Au coated quartz crystal and PFDA coated Aucoatedquartz crystal forOVA,HSAandFGNarepresented inFigure 7a, b and c, respectively. For untreated as well asPFDA coatedQCMcrystal surfaces, the frequency decreasedsharply immediatelyafter injectionofproteinsolutionsdueto adsorption of proteinmolecules, after that the frequencyalmost leveled off indicating the saturation of proteinadsorption. The initial fast decrease in resonance frequencyfollowed by second slower adsorption process indicated anirreversible protein adsorption process that depends on the

120001000080006000400020000-25

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Figure 7. Time-dependent QCM frequency responses of untreatedAu coated QCM crystal sensor and Au coated QCM crystal sensorcoated with plasma polymerized PFDA film (PM mode, DC¼ 25%,Ppk¼ 1W) when contacted with protein solutions (a) OVA, (b) HSAand (c) FGN. The arrow shows the point of injection of proteinsolution.




type of protein, number of available binding sites andsurface energy of substrates, which determines the finalcoverage. PlasmapolymerizedPFDAcoatings showed loweradsorption of proteins as compared to gold surfaces, whichindicated the inert and protein repellent, i.e., anti-biofoul-ing characteristics of PFDA coatings towards adsorption ofprotein biomolecule. The extent of frequency change ofQCM sensor due to the adsorption of protein moleculesvaried for different proteins, indicating different adsorp-tion behavior of different proteins. From the QCM analysis,itwasobserved that coatingofPFDAonAusurfaces reducedthe protein adsorptionby&50%, 70%and17% forOVA,HSAandFGN, respectively.Another interestingaspectofproteinrepellent characteristics of surface is the kinetics ofadsorption. The kinetics of protein adsorption on untreatedAucoatedQCMcrystalwas found tobe faster thanonPFDA-coated Au coated QCM crystal. The adsorption kinetics ofOVA andHSA on PFDA coatingswas slower as compared toFGN. The variation in the adsorption kinetics and theantifouling characteristics of PFDA coatings against differ-ent proteinmolecules is attributed to thevarying affinity ofthe coating surfaces towards protein molecules, due todifference in the surface-chemical properties of differentproteinmolecules. Another factor is the number of contactsbetween the macromolecule and the substrate surface;multiple contact formation leads to higher and strongeradsorption. FGN being a hydrophobic and bigger proteinmolecule (MW& 340 kDa) as compared to HSA(MW& 66 kDa) and OVA (MW& 45 kDa) interacted withsubstrate through hydrophobic interaction and offershigher numbers of contact sites for adhesion, which leadsto faster and higher adsorption as compared to otherproteins.[88] Evans-Nguen and Schoenfisch have reportedthat protein (FGN) adsorption could be substantially higheron hydrophilic surface provided that the surface ispositively or negatively charged.[89] Similar results havebeen reported by Choukourov et al. where they have alsofound that the FGN adsorbs faster and in higher amount ongold surface than on CF3-terminated SAM, which wasattributed to the influence of the surface charge present ongold surface.[86]

Conclusion

The deposition time, CW and PM plasma mode, plasmapower and pulseDCplay crucial role in tailoring the surfacemorphology, CA and surface free energy characteristics ofthe plasma polymerized PFDA coating. The plasma poly-merized PFDA coating has been successfully utilized totransform a super-hydrophilic surface of Whatman filterpaper into a super-hydrophobic and oleophobic surface dueto the conformal lowsurface energy thinPFDAcoatingwithhigh degree of fluorination complemented by the micro-

rough and fibrous microstructured morphology of paper.Low-surface-energy PFDA coatings offer possible applica-tions in paper industry for the development of waterrepellent, oil repellent, stain resistance papers. The proteinadsorption study showed that PFDA coating exhibitedsignificant protein repellent behavior against three modelproteinsnamely,OVA,HSAandFGN, suggestingapotentialmeansof inhibiting thebiofoulingofmaterial substratesbypresenting a non-stick surface.

Acknowledgements: ACTECO-EC project (515859-2) is acknowl-edged for financially supporting thework. Authors sincerely thankto Dr. D. Mataras, Dr. Ergina Farsari, PTL, University of Patras,Greece for AFM analysis, Dr. Reza Jafari from ENSCP, Paris,Dr. G. Giudetti from JRC, Italy and other partners of the project fortheir active support and discussion.

Received: March 20, 2010; Revised: June 11, 2010; Accepted: July8, 2010; DOI: 10.1002/ppap.201000038

Keywords: antifouling; fluorocarbon coating; 1H,1H,2H,2H-per-fluorodecyl acrylate (PFDA); hydrophobic; morphology; oleopho-bic; plasma enhanced chemical vapor deposition (PECVD); proteinadsorption; surfaces

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