Amphiphilic Copolymer Coatings via Plasma Polymerisation Process: Switching and Anti-Biofouling Characteristics

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Amphiphilic Copolymer Coatings via PlasmaPolymerisation Process: Switching andAnti-Biofouling Characteristics

Virendra Kumar,* Jerome Pulpytel, Guido Giudetti, Hubert Rauscher,Francois Rossi, Farzaneh Arefi-Khonsari*

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

Environmentally benign-solvent free plasma process is employed to produce nanostructuredPFDA-co-DEGDME amphiphilic coatings via plasma co-polymerisation of 1H,1H,2H,2H-per-fluorodecyl acrylate (PFDA) and diethyleneglycol dimethyl ether (DEGDME) precursors in a lowpressure-RF-inductively excited tubular plasma reactor using argon as a carrier gas. Theplasma-polymerised coatings are characterised by Fourier transform infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy(FESEM), atomic force microscopy (AFM) and contact angle measurements. PFDA-co-DEGDMEcoatings with varying chemical environments and morphologies are achieved by varying theplasma parameters such as, continuous wave (CW) and pulse modulated (PM) plasma mode,and plasma deposition time. Plasma polymerised PFDA-co-DEGDME coatings are found to exhibit a switchingproperty in terms of wettability, i.e., from hydrophobic tohydrophilic and vice versa, in response to the contactingenvironment. Quartz crystal microbalance (QCM) is usedto study the adhesion of two model proteins, namely,human serum albumin (HSA) and fibrinogen (FGN) incontinuous flow conditions, which reveals the proteinrepellent, i.e., anti-biofouling characteristics of the PFDA-co-DEGDME amphiphilic coatings.

120001000080006000400020000-30

-25

-20

-15

-10

-5

0

Adsorption of HSA

PFDA-co-DEGDME

PFDA

Untreated Au

∆f/Hz

Time/s

Plasma Process. Polym. 2011, 8, 373–385

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlin

Introduction

‘Biofouling’ is an unwanted adhesion and growth of bio-

molecules, such as proteins and micro-organism on the

surfaces of devices and tools that poses an adverse effect on

the life, activity, performance and efficiency of the devices.

The adsorption of proteins onto a biomaterial surface

initiates a cascade of events, right from the conditioning of

surface up to the biofilm formation that can ultimately

result in inflammation, infection and rejection of the

implant and medical devices.[1–4] In fact, the non-specific

adsorption of proteins and associated bio-adhesion is one of

the most significant limitations to the end point utility of

elibrary.com DOI: 10.1002/ppap.201000109 373

374

V. Kumar, J. Pulpytel, G. Giudetti, H. Rauscher, F. Rossi, F. Arefi-Khonsari

many biomaterial devices, e.g., catheters, blood contacting

devices, implant materials, biosensors, microfluidics and

drug delivery systems.[5] Moreover, marine applications

also suffer from the biofouling resulted from the adsorption

and accumulation of proteins, bacteria, cyanobacteria,

algae, etc. The undesirable accumulation of biomass on a

ship hull leads to increased weight and higher hydro-

dynamic drag resulting in lower operational speed, which

consequently increases the fuel consumption and operational

cost.[6] Therefore, the modification of a material surface to

impart protein repellent property has a great technological

and industrial relevance. Two strategies based on surface

modification have been mostly used to reduce the biofouling

of material surfaces. First approach is based on the surfaces

with high surface energy, i.e., hydrophilic surfaces, e.g.,

polyethylene glycol (PEG) like coatings;[4,7,8] and second

approach is based on the inert surfaces having low surface

free energy, i.e., hydrophobic surfaces, e.g., fluorocarbon and

silicon based inert coatings.[9–14]

In the present work, we have used a relatively recent

approach of fabricating anti-biofouling surfaces via plasma

process, which is based on the nanostructured PFDA-co-

DEGDME amphiphilic polymer coatings, prepared by

copolymerisation of hydrophilic (i.e. DEGDME) and hydro-

phobic polymer (i.e. PFDA) domains in a single step. In the

amphiphilic polymer coating, the intrinsic incompatibility

between the hydrophilic and hydrophobic components

leads to phase segregation and consequently results into

surfaces with compositional, morphological and topogra-

phical surface heterogeneity, which makes the interaction

between biomolecule and substrate energetically unfa-

vourable.[15] Amphiphilic surfaces with nanoscale com-

plexities, prepared by conventional deposition methods,

have been reported to exhibit anti-biofouling character-

istics.[15,16] Furthermore, coupling of well-known protein

resistance of PEG like coating from DEGDME surfaces with

inert or anti-adhesive perfluorocarbon PFDA coatings is

expected to exhibit synergistic effect towards protein

repellent properties of the PFDA-co-DEGDME copolymer

coatings.[4,7–9,13,17,18] Different amphiphilic polymer net-

works have been prepared by different research groups

using different combinations of hydrophilic and hydro-

phobic polymers, e.g., hyperbranched fluoropolymers

crosslinked with polyethylene glycol (PEG),[15] polyiso-

butylene and poly(2-hydroxyethyl methacrylate) or poly-

(N,N-dimethylacrylamide),[19,20] polystyrene and poly-

urethane,[21] poly(ethoxylated fluoroalkyl acrylate)-

polystyrene amphiphilic block copolymer,[22,23] etc. These

amphiphilic polymer networks have been reported to

exhibit reduced adhesion of various biomolecules including

protein, cell, platelet, UlVa sporelings and cells of the

diatom NaVicula. Most of the methods used for preparation

of the amphiphilic coatings involved solvent casting and

spin coating using volatile organic solvents, or involved a


� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

multi-step synthesis protocol, including photo-curing

process, which suffer with environmental and processi-

bility issues in addition to the poor adhesion to the

substrates.[16,24,25] On the other hand, plasma-polymerised

coatings offer processing, environmental and device

performance related advantages over the conventional

polymer coatings.[26–31] Plasma deposition process being a

clean, dry and environmental benign process, provides

coatings with better stability and adhesion to substrate

through covalent bonding. Pin hole free coatings having

extreme range of surface chemistries with thickness control

can be deposited onto uneven shaped inorganic as well as

organic substrates using plasma enhanced chemical vapour

deposition (PECVD) or plasma polymerisation process.

Amphiphilic copolymers have been utilised to create

smart surfaces, also called switchable or responsive

surfaces that are capable of undergoing reversible changes

of properties in response to surrounding environmental

conditions or stimuli. The polymer surfaces cannot be

considered as rigid and unchangeable, since polymer chains

tend to respond to the environment or applied stimuli by

changing the conformation and location of their backbones,

side chains, segments, pendant groups or end groups.[32]

Different studies have been carried out to develop

responsive smart surfaces by coating or grafting a variety

of amphiphilic environment sensitive copolymers onto

substrates.[33–35] Water contact angle measurement has

been found to be an easy and fast technique to study the

switching property of amphiphilic polymer surfaces in

different environmental conditions.[35]

To the best of our knowledge, no work has been reported

in the literature so far, on the application of the single step,

solvent free-dry-plasma process to fabricate amphiphilic

coatings exhibiting switching and anti-biofouling char-

acteristics. We report the investigation of chemistry as well

as the surface and bulk morphology of the plasma

polymerised amphiphilic polymer coatings deposited

under different experimental conditions. Herein for the

first time, we report the plasma polymerisation process for

fabrication of nanostructured amphiphilic PFDA-co-

DEGDME coatings exhibiting switching property in terms

of wettability, i.e., from hydrophobic to hydrophilic and vice

versa, in response to the contacting environment; and anti-

biofouling characteristics against protein adsorption. The

amphiphilic PFDA-co-DEGDME coatings were found to

exhibit better protein repellent characteristics as compared

to that of hydrophobic PFDA coatings.[9]

Experimental Part

Materials

A fluorocarbon precursor, namely, 1H,1H,2H,2H-perfluorodecyl

acrylate (CH2¼CH�CO2CH2CH2(CF2)7CF3, purity¼ 97%, mol.

DOI: 10.1002/ppap.201000109

Amphiphilic Copolymer Coatings via Plasma Polymerisation Process: Switching and Anti-Biofouling Characteristics

wt.¼518.17 g �mol�1, Sigma–Aldrich, France) and diethylenegly-

col dimethyl ether (CH3�(OCH2CH2)2�O�CH3, purity¼ 99.5%, mol

wt.¼134.18 g �mol�1, Sigma–Aldrich, France), were used without

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

was used as carrier gas. Thermanox1 coverslips (13 mm diameter,

Nalge Nunc International, Thermo Fisher Scientific, USA) and

polished silicon wafers (100) (Siltronix, France) were used as

coating substrates. For QCM analysis, human serum albumin

(A6909, Sigma) and fibrinogen (F3879, Sigma) were diluted

in 10�3M PBS, pH7.4, to a final concentration of 50mg �mL�1.

Plasma Deposition Set up-Deposition of

PFDA-co-DEGDME Coatings

A low pressure inductively excited radio frequency-tubular quartz

plasma reactor system was used for deposition of plasma

polymerised PFDA-co-DEGDME amphiphilic coatings (Figure 1).

The plasma setup consists of an electrodeless cylindrical quartz

reactor (4 cm inner diameter, 50 cm length, base pressure of

3�10�2 mbar) with an externally wound copper coil for RF power

coupling. The RF power was delivered from a Dressler Cesar RF

generator through a matching L–C unit to the copper coil wound

around the cylindrical quartz reactor. The substrate is placed 9.0 cm

below the coil, so that the sample remains in the after glow region.

The plasma reactor was connected to a vacuum pump (Pfeiffer

vacuum) attached with inlet chemical filter trap (Edwards high

vacuum, Britain). Argon gas passes through the precursor bubbling

systems via two feed lines in order to transport the precursor

vapours to the plasma reactor. The flow rates of the carrier gas and

precursors were regulated 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 blow. The plasma reactor system was

reassembled and cleaned further with 20 W Ar-plasma at 0.5 mbar

Figure 1. Schematics of a low pressure inductively excited RF-tubular quartz plasma reactor setup with two feed lines.



pressure for 30 min. Two precursor-feed lines in the experimental

set up provides us the option to deposit polymer coating from single

precursor using one feed line at a time or to deposit a copolymer

coating from two precursors by using both the feed lines

simultaneously. Plasma polymer coatings were deposited at

working pressure of 0.5 mbar for 20 min deposition time, unless

otherwise mentioned.

Coating Characterisation

Fourier Transform Infrared (FT-IR) Spectroscopy

The bulk chemical environment of plasma polymerised coatings

were analysed by a Fourier transform infrared spectrometer

(Bruker-Tensor 27 FT-IR spectrophotometer) in the transmission

mode. Silicon wafers (i-type, 100) were used as deposition

substrates for FT-IR analysis. The FT-IR spectra were recorded with

the resolution of 4 cm�1 and averaged with 100 scans. The FTIR

spectra presented in the manuscript were corrected for a sloping

baseline.

X-Ray Photoelectron Spectroscopy (XPS)

Surface chemical analysis of the coatings was carried out by XPS,

which was performed on a Kratos Axis Ultra instrument equipped

with a monochromatic aluminium Ka X-rays source (1 486.6 eV).

The emitted photoelectrons were detected by a multichannel

detector at pass energy of 20 eV for recording the high resolution

C1s spectra. A photoelectron take-off angle of 908 relative to the

surface was used, which corresponds to a sampling depth of

approximately 5–8 nm for the C(1s) envelope. The base pressure in

the analysis chamber was �3� 10�9 torr, however, after loading

the samples the pressure during XPS measurement was

�1�10�8 torr. For the high-resolution C1s spectra, CF2 at

292.4 eV was used as reference for binding energy calibration.

High-resolution C1s spectra were deconvoluted into different

chemical groups using a Gaussian curve-fitting technique, taking

FWHM equal for all peaks. XPS peaks were fitted using Casa XPS

fitting software assuming linear background subtraction. Two

analysis areas (700mm�300mm) of the each sample were used for

XPS analysis.

Field Emission Scanning Electron Microscopy (FESEM)

Field emission scanning electron microscopy (FESEM) images were

taken using Zeiss Ultra 55 FEG SEM with GEMINI Column on gold

metallised coating surfaces by sputter coating (Cressinton sputter

coater-108 auto). The bulk morphology and thickness of the

coatings were investigated by fracturing coatings deposited on Si

wafers and then taking cross-sectional images of fractured

samples. Electrons with accelerating voltage of 6 kV were used

to obtain the FESEM images.

Atomic Force Microscopy (AFM)

Atomic force microscope (AFM) NTEGRA PRIMA from NT-MDT was

used to examine the topography and roughness of the plasma

polymerised coatings. The instrument 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.

www.plasma-polymers.org 375

5 10 15 20 25 30 35 40 450.0

0.5

1.0

1.5

2.0

Pre

curs

or fl

ow ra

te/s

ccm

Argon gas flow rate/sccm

(a)

(b)

Figure 2. Flow rates of precursor molecules in the plasma reactoras a function of Ar flow rate at 0.5mbar pressure. (a) DEGDME(25 8C) and (b) PFDA (65 8C).

376


‘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) Measurements

Sessile drop contact angle values were measured in a kinetics mode

using a video capture apparatus (Digidrop GBX-3S system, France).

For each measurement, a 6mL droplet was dispensed onto the

coating surface and images of water droplets were captured

continuously to measure the time resolved water contact angle

values.

Quartz Crystal Microbalance (QCM)

Plasma polymerised coatings were analysed for anti-biofouling

characteristics by measuring adsorption of two different proteins,

namely, HSA and FGN as a function of time in a continuous flow

mode, using a Quartz crystal microbalance QCM-D E4 instrument

(Q-Sense AB, Gothenburg, Sweden).[25,36] All data presented in this

study correspond to the fifth overtone, which is less sensitive to

variations of the mounting conditions of different crystals. Polished

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

0.30 mm thick) with a fundamental frequency of 5 MHz (Q-Sense

AB, Gothenburg, Sweden) were used for QCM experiments. After

assembling the sensor quartz disc into the QCM, the plasma

polymer surfaces were first exposed to 10 mM PBS solution at pH

7.4, in order to stabilise the system and obtain the base line. After

that, the protein solution with 50mg �mL�1 concentration in 10 mM

PBS was pumped continuously through the measurement chamber

at a flow rate of 20ml �min�1. The frequency changes due to

adsorption of proteins were monitored as a function of time. The

change in the resonating frequency (Df) of the sensor disc can be

related to the approximated mass change (Dm) of the sensor disc

due to adsorption or desorption of material using Sauerbrey

Equation (1), which is restricted to the sufficiently thin and rigid

adsorbed films.[37,38]

Prec

Plasma

� 2011

Dm ¼ � C=n½ � � Dfð Þ (1)

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

constant.

Results and Discussion

We have prepared amphiphilic PFDA-co-DEGDME coatings

via plasma copolymerisation of PFDA and DEGDME

precursors, under different experimental conditions, such

as CW plasma mode, PM plasma mode, deposition time and

precursor feed ratio. The precursor flow rate was controlled

by the flow of carrier gas, i.e., Ar gas. The flow rates of

precursors were estimated using following relation (2).

ursor flow rate ðsccmÞ ¼ ðDm=Dt � MÞ ðRT=PÞ (2)

where T¼ 298 K, P¼ 1 atm, R¼ 0.082 L � atm �K�1 �mol�1,

M¼mol. wt. of the precursor, Dm the mass of precursor

Process. Polym. 2011, 8, 373–385

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

consumed in Dt time period under a fixed argon flow.

Unlike DEGDME, the vapour pressure of PDFA was quite

low and was not sufficient enough to provide plasma

polymerised PFDA coatings at room temperature (�25 8C)

(confirmed by WCA and XPS analysis). Therefore, for all the

experiments, temperatures of precursors were optimised

as 65 8C and 25 8C for PFDA and DEGDME, respectively.

PFDA and DEGDME precursor flow rates exhibited linear

correlations with the flow rate of argon gas, as shown in

Figure 2. The Ar flow rate was used to regulate precursor

flow rate in order to prepare amphiphilic copolymer

coatings with varying chemical compositions.

Characterisation of Coatings

FT-IR spectroscopy in transmission mode provides the

information about the chemical environment of the overall

polymer coating film; whereas, XPS analysis gives the

information about the surface chemical composition of the

outermost layer film up to 10 nm depth depending upon the

take off angle. Figure 3 shows the FT-IR spectra of PFDA, PEG

like coating from DEGDME and PFDA-co-DEGDME copoly-

mer coatings prepared under PM and CW plasma modes.

The FTIR spectra of pure PFDA coating (Figure 3a) exhibits a

sharp and intense peak at 1 741 cm�1, which corresponds to

the C¼O stretching of acrylate group, and IR bands between

1 400 and 1 000 cm�1 region are attributed to the CFx (x¼ 1–3)

vibrations representing the main feature of plasma

polymerised fluorocarbon coatings.[39–43] The sharp and

intense peaks at 1 205 and 1 149 cm�1 are attributed to the

asymmetric stretching and symmetric stretching of the

�CF2 groups, respectively. The IR peak at 1 234 cm�1

represents to the C�F stretching vibration overlapped with

the CH2 wagging.[44–46] A small absorption peak at

DOI: 10.1002/ppap.201000109

1800 1700 1400 1300 1200 1100 1000 900

1107 cm -1

1119 cm -1

1114 cm -1

1114 cm -1

(c)

(a)

(b)Abs

orba

nce/

a.u.

wavenumber/cm-1

(d)

Figure 3. FT-IR spectra of (a) PFDA coating prepared under PMplasma mode (Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms), (b) PFDA-co-DEGDME coating in PM mode (PFDA:DEGDME¼0.3 sccm:1.5 sccm; Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms), (c) PFDA-co-DEGDMEcoating in CW mode (PFDA:DEGDME¼0.3 sccm:1.5 sccm,Ppk¼ 1W) and (d) PEG like coating from DEGDME in PM mode(Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms).


1 119 cm�1 is attributed to the C�O stretching overlapped

with the tail of the CFx band. PEG like coating from DEGDME

exhibited a sharp but low intensity IR absorbance peak at

1 107 cm�1 corresponding to the C�O ether bond[47]

(Figure 3d). The plasma polymerised PFDA-co-DEGDME

coatings (Figure 3b and c) showed all the IR peaks of PFDA in

addition to a clear and more significant peak at 1 114 cm�1,

which represents the C�O ether bonds (1 107 cm�1 for PEG

in Figure 3d), blue shifted due to presence of C�F vibrations.

FTIR spectra of amphiphilic PFDA-co-DEGDME coatings

show the signature of PEG segments, i.e., ether linkages

(hydrophilic component) along with PFDA segments, i.e.,

CFx groups (hydrophobic component); although peak

intensities of fluorinated groups of PFDA component are

more prominent as compared to that of ether bonds of PEG

component. The high intensity band of fluorocarbon groups

from PFDA are attributed to (i) higher extinction coefficient

of CFx (x¼ 1–3) groups as compared to ether linkages, as

clearly understood from the absorption bands of pure PEG

and PFDA spectra in Figure 3; and (ii) the high reactivity of

acrylate groups in PFDA leads to higher deposition of PFDA

as compared to DEGDME.

Figure 4a–d shows the high-resolution XPS scan of the

C1s peak of the plasma polymerised PFDA, PEG like coatings

from DEGDME, and PFDA-co-DEGDME coating prepared

under PM and CW plasma conditions. Figure 4a shows the

C1s XPS spectra of plasma polymerised PFDA coatings,

where peaks for different chemical groups are assigned as

CF3 (C1: 294.6 eV), CF2 (C2: 292.4 eV), CF/O�C¼O (C3:

290.0 eV), C�O(CF) (C4: 287.7 eV) and C�CF/C�O (C5:

286.6 eV).[10] The well resolved peaks corresponding to



CF2 and CF3 groups signify a low degree of fragmentation

and rearrangement of perfluorocarbon chain of precursor

molecule in the plasma polymerised PFDA coatings.

Figure 4b represents the C1s XPS spectra of plasma

polymerised PEG like coatings from DEGDME precursor,

which is fitted with four components attributed to the

O�C¼O (C6: 289.3 eV), C¼O (C7: 288.0 eV), C�O (C8:

286.5 eV) and C�C (C9: 285.1 eV) groups. The high intensity

of the peak at 286.5 eV attributed to the ether function

(C�O) indicates the formation of PEG like coatings by

DEGDME system.[47] Figure 4c and d shows the C1s XPS

spectra of PFDA-co-DEGDME coatings prepared under PM

and CW plasma modes, respectively, showing the peaks of

all the chemical groups present in PFDA. However, the C1s

spectrum of PFDA-co-DEGDME coating prepared in PM

plasma mode (Figure 4c) quite resembles to that of PFDA

coating (Figure 4a), whereas, coatings prepared under CW

mode (Figure 4d) shows different C1s XPS spectrum as

compared to that of PFDA coating (Figure 4a).

CW Versus PM Plasma Mode

Influence of the plasma operation modes, i.e., CW and PM

plasma discharge modes on the composition, surface

topography and morphology of different plasma deposited

fluorocarbon coatings have been reported in the litera-

ture.[9,10,48,49] It was interesting to observe that the CW and

PM plasma modes showed large influence not only on the

chemical composition but also on the morphology and

topography of the amphiphilic PFDA-co-DEGDME coating.

FTIR spectra shown in Figure 3b and c indicate the

difference in the overall chemical composition of PFDA-

co-DEGDME coating prepared under PM and CW plasma

mode, using constant precursor feed ratio. The A1114/A1149

peak ratio was found to be higher in case of CW mode as

compared to that in case of PM mode, suggesting the higher

content of PEG segments in the copolymer coating in CW

plasma mode. In fact, mild PM plasma is more suitable for

plasma polymerisation of PFDA as compared to energetic

CW mode from chemical group retention point of view

because PFDA being a large molecule with unsaturated

acrylate group needs radicals just to initiate the polymer-

isation reaction.[10] In CW mode, energy is continuously

being fed to the system, which generates reactive species

continuously causing more fragmentation of the PFDA

molecules and ablation/damage to the coatings. On the

contrary, DEGDME being a saturated organic precursor

needs comparatively energetic plasma condition that can

fragment the DEGDME precursor molecule to give rise PEG-

like coatings. So there is a difference in the plasma process

requirement for deposition of PFDA and DEGDME due to the

basic difference in the deposition mechanism for the

two precursors. In CW mode, there is higher possibility

of de-fluorination of PFDA polymer chains, which is


296 294 292 290 288 286 284

C5

C4C3

C2

C1

(a)In

tens

ity/a

.u.

B.E./eV296 294 292 290 288 286 284

C9

C8

C6C7

(b)

Inte

nsity

/a.u

.

B.E./eV

296 294 292 290 288 286 284

(c)

Inte

nsity

/a.u

.

B.E./eV296 294 292 290 288 286 284

(d)

Inte

nsity

/a.u

.

B.E./eV

Figure 4. High-resolution C1s XPS spectra of plasma polymerised coatings prepared under CW (Ppk¼ 1W, pressure¼0.5mbar) and PMplasma conditions (1W, pressure¼0.5mbar, ton¼ 25ms, toff ¼ 75ms). (a) PFDA coating, prepared under PM plasma conditions, (b) PEG likecoating from DEGDME in PM mode, (c) PFDA-co-DEGDME coating in PM mode (PFDA:DEGDME¼0.3 sccm:1.5 sccm) (d) PFDA-co-DEGDMEcoating in CW mode (PFDA:DEGDME¼0.3 sccm:1.5 sccm).

378


manifested by lower intensities of CFx absorption bands.

On the contrary, in CW mode, there is a higher extent of

deposition of PEG like coating due to higher fragmentation

of DEGDME precursor, which is manifested by higher

intensity of IR absorption band at 1 114 cm�1.

Interestingly, FT-IR analysis shows the difference

between the bulk chemistry of PFDA and PFDA-co-DEGDME

prepared in both PM and CW plasma mode (Figure 3). The

presence of the IR peak at 1 114 cm�1 in PFDA-co-DEGDME

coatings indicated the incorporation of PEG, however, the

peak is more prominent in case of CW plasma mode as

compared to PM mode (Figure 3). On the contrary, XPS

analysis shows that unlike in CW mode, the PFDA-co-

DEGDME prepared in PM mode exhibited almost similar

surface chemistry of PFDA coatings (Figure 4a, c and d).

From the XPS analysis, the concentrations of C�O(CF)

groups, which is formed due to the fluorination of ether

(C�O) linkage, were found to be 31.0, 12.5 and 11.5 at.-% in

PFDA-co-DEGDME prepared in CW, PFDA-co-DEGDME

prepared in PM, and PFDA coatings, respectively. The

higher concentration of C�O(CF) in CW mode revealed by

XPS analysis indicates the presence of significant amount of



PEG components in outer surface layers of PFDA-co-

DEGDME coating. The FT-IR and XPS analysis suggested

that in CW mode, significant amount of PEG is present in the

bulk as well as at the outermost surface layers of the PFDA-

co-DEGDME coatings, because the strong CW plasma

condition is more suitable for the deposition of the

saturated DEGDME precursor. On the other hand, in mild

PM plasma condition, comparatively lesser amount of PEG

like component was incorporated in the PFDA-co-DEGDME

coatings. Moreover, the PFDA and PEG components were

reorganised in such a way that hydrophobic PFDA segments

were present on the outer layers of the copolymer coating

due to the hydrophobic interaction between hydrophobic

PFDA segments and air. As a consequence, in PM mode,

PFDA chains migrate to the outer most layer of the coating

and showed almost similar chemical structure of PFDA, as

indicated by XPS results (Figure 4a and c).

The reorganisation of chemical groups in PM plasma

mode is further supported by the FT-IR analysis, which

showed that in PM mode, the bulk chemical environments

of coatings changes with the precursor feed ratios (Figure 5),

whereas, no significant effect of precursor feed ratio was

DOI: 10.1002/ppap.201000109

1000110012001300140017201740176017801800

(d )

(c )

(b )

Abs

orba

nce.

/a.u

.

Wavenumber/cm-1

(a )

Figure 5. FT-IR spectra of PFDA-co-DEGDME coatings preparedunder PM plasma conditions (Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms)at varying precursor feed ratio, i.e., PFDA (sccm):DEGDME (sccm):(a) 0.4:1.3, (b) 0.3:1.5, (c) 0.2:1.6 and (d) 0:0.6.

Figure 6. FESEM images showing bulk and surface morphologiesof PFDA-co-DEGDME copolymers prepared under (a) PM mode(PFDA:DEGDME¼0.3 sccm:1.5 sccm, Ppk¼ 1W, ton¼ 25ms,toff¼ 75ms) (b) CW plasma mode (PFDA:DEGDME¼0.3 sccm:1.5sccm, Ppk¼ 1W).


observed on the surface chemical environments of the

PFDA-co-DEGDME coatings (XPS analysis). As shown in

Figure 5, there is a decrease in the IR band intensity of CFx

groups with the decrease in the [PFDA]/[DEGDME] feed

ratio, revealing the decrease in the concentration of PFDA

component in the amphiphilic coating. The intensity of

1 114 cm�1 band for ether functionality remained almost

constant for all the feed ratios, might be because of the fact

that the tail of the CFx band is overlapped with C�O band at

1 114 cm�1, therefore, the increase in the FT-IR C�O

absorption band would be compensated, to some extent,

by the decrease in CFx absorption band. However, the ratio

A1114/A1149 was found to increase with the increase in the

[DEGDME]/[PFDA] feed ratio, which clearly suggested that

at higher [DEGDME]/[PFDA] feed ratio a higher concentra-

tion of PEG component is incorporated in the copolymer

coating.

Figure 6 shows the FESEM images of amphiphilic PFDA-

co-DEGDME coatings exhibiting marked difference in the

bulk morphology as well as surface morphologies of the

coatings prepared in PM and CW mode. The difference in the

surface morphology of fluorocarbon coatings produced in

CW and PM mode have been reported earlier and attributed

to the difference in the growth mechanism.[9,40,48] PFDA-co-

DEGDME coating prepared in PM mode exhibited a

nanostructured surface morphology with entangled-worm

like bulk morphology (Figure 6a), which is attributed to the

thermodynamically driven phase segregation of two

intrinsically non-compatible polymer segments, i.e., hydro-

philic (PEG) and hydrophobic (PFDA) domains, during the

plasma deposition process. On the other hand, coatings

prepared in CW mode exhibited smooth, featureless surface

morphology with compact and homogeneous bulk mor-

phology (Figure 6b). These observations were further

supported by AFM analysis. Figure 7 shows the 2D and



3D AFM images of PFDA-co-DEGDME copolymers, exhibit-

ing different surface topography of the coatings prepared

under PM and CW plasma modes. The coating prepared

under PM plasma mode has rough and complex domains

resulted from the phase segregation of incompatible PFDA

and PEG segments, which was manifested by the high

roughness values (Ra ¼ 26.0 nm). However, the coating

prepared in CW mode showed very low surface roughness

(Ra¼ 2.0 nm).

In CW plasma mode, the energy is continuously fed into

the system that results in high plasma density, generating

energetic species throughout the treatment time that cause

polymerisation, fragmentation, generation of reactive sites

on the coating causing surface modification and higher

crosslinking extent. Furthermore, in energetic CW mode the

fragmented precursor molecules will quickly copolymerise

and crosslink and polymer chains will have little freedom

for reorientation and phase segregation. Therefore, the


Figure 7. 2D and 3D AFM images of PFDA-co-DEGDME copolymers prepared under PM plasma mode (PFDA:DEGDME¼0.3 sccm:1.5 sccm,Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms) and CW plasmamodes (PFDA:DEGDME¼0.3 sccm:1.5 sccm, Ppk¼ 1W). (a) PM-2D image, (b) PM-3D image,(c) CW-2D image and (d) CW-3D image.

380


smooth film growth with a compact and homogeneous bulk

morphology of the coating prepared in CW mode are

attributed to the combined effect of deposition, surface

modifications via etching/ablation and crosslinking of

coatings, taking place simultaneously. On the other hand,

PM plasma process offers an opportunity to generate

plasma films under mild or low power input condition, i.e.,

equivalent power Peq conditions; Peq ¼ [ton/(ton þ toff)]Ppk,

where ton and toff are the ‘plasma on’ and ‘plasma off’ times

and Ppk is the plasma peak power.[50] In mild PM plasma

process, a more relaxed, ordered and selective polymerisa-

tion reaction occurs during ‘plasma off’ times, relative to

that occurring during the highly energetic ‘plasma on’

periods. The lifetimes of neutrals are much higher than

those of the ions, as electron–ion recombination reactions

are much faster than the radical recombination reaction.[51]

In PM plasma mode, the neutral radical could stay longer

and take part in the polymerisation/deposition process

during plasma off period with minimal surface modifica-

tions of coatings.[52,53] Therefore, by regulating the ton/toff,

PM plasma process provides a control over the extent of

precursor fragmentation and coating modification by

controlling the selectivity of neutrals, i.e., radicals (respon-



sible for deposition/polymerisation) over the ions (respon-

sible for ablation/etching) during the ‘plasma off’ time.

Moreover, in PM mode, the lower extent of crosslinking

of polymers allows the growing polymer chains to

relax and to reorient during ‘plasma off’ time that facilitate

the phase segregation process, manifested by the

complex (nanostructured and worm like) morphology

and topography of PFDA-co-DEGDME coating. Moreover,

under similar experimental conditions, the thickness of the

copolymer coatings prepared in CW mode is less, as

compared to that prepared in PM mode (Figure 6), which

signifies the presence of etching/ablation process as a

competing process with polymerisation in CW plasma

mode.

Deposition Time

The FT-IR analysis of PFDA-co-DEGDME coatings showed

the increase in the IR peak intensities with the increase in

the deposition time, which was attributed mainly to the

increase in the coating thickness only, as XPS analysis

showed no significant difference in the chemical composi-

tion of the coatings prepared under different deposition

DOI: 10.1002/ppap.201000109

Figure 8. FESEM images of PFDA-co-DEGDME copolymers prepared under PMmode as a function of deposition time (a) 10min, (b) 20min, (c)30min and (d) 40min (PFDA:DEGDME¼0.3 sccm:1.5 sccm, Ppk¼ 1W, ton¼ 25ms, toff¼ 75ms).


times (data not shown). The coating thickness (estimated by

FESEM analysis) showed almost linear increase with the

plasma deposition time from which the deposition rate

was estimated as �20 nm �min�1, which is nearly twice

the deposition rate of plasma polymerised pure PFDA

coating.[9]

Figure 8 shows the FESEM images of PFDA-co-DEGDME

coatings prepared under varying deposition time, clearly

showing the influence of deposition time on the surface

morphologies. At deposition time of 10 min, unevenly

distributed nano-particulate structures appeared on the

coating surface, whereas, at 20 min deposition time, a well

ordered and evenly distributed mountain like micro-

structured features with the nano-structure on the top of

the microstructures were observed. However, at 30 min, the

microstructures started disappearing and nano-structures

were still present along with few microstructures, but at

40 min, only nano-structures with almost no prominent

microstructures were seen. These observations suggested

the important role of plasma deposition time for controlling

the surface morphology of the PFDA-co-DEGDME amphi-

philic coatings.



Switching Characteristics of PFDA-co-DEGDMECoatings

The design and fabrication of surfaces with switchable

polymeric structures is quite important for sensing and

actuation applications on the micro- and nano-scales. The

switching characteristics of polymer brushes have been

reported by other research groups for tissue engineering

applications.[54,55] We have observed a responsive beha-

viour of plasma polymerised PFDA-co-DEGDME amphiphi-

lic coatings in terms of wettability, i.e., switching from

hydrophobic to hydrophilic and vice versa in response to the

surrounding contacting media. Figure 9a and b shows the

reversible change of WCA of PFDA-co-DEGDME coatings

deposited under PM and CW plasma mode, respectively, as a

function of time. Plots shown in Figure 9a,b consist of two

parts; the first part shows the variation of WCA of PFDA-co-

DEGDME coating just after the plasma deposition, which

was cleaned with dry air blow to remove the dust particles

from the coating surface if any; and the second part shows

variation of WCA of the coating, which was dipped in water

for 30 min and then dried quickly by air blow. The WCA


0 100 200 1900 2000 2100 2200 2300

60

70

80

90

100

110

120 air dried coating after dipped in water

II partI part

WC

A / d

egre

e

Time / s

(a)

30 min

0 100 200 1900 2000 2100 2200 2300100

105

110

115

120

air dried coating after dipped in water

II part

(b)

WC

A/de

gree

I part

30 min

Figure 9. Switching properties of PFDA-co-DEGDME coatings:WCA as a function of time (a) CW plasma mode(PFDA:DEGDME¼0.3 sccm:1.5 sccm, Ppk¼ 1W) and (b) PM plasmamode (PFDA:DEGDME¼0.3 sccm:1.5 sccm, Ppk¼ 1W, ton¼ 25ms,toff¼ 75ms).

382


values of plasma deposited PFDA-co-DEGDME amphiphilic

coatings were found to decrease gradually with the time.

The initial WCA value for PFDA-co-DEGDME coatings

prepared in CW and PM mode were 1188 and 1218,respectively, which clearly indicated the enrichment of

PFDA segments on the surface due to the hydrophobic

interaction with air. The WCA values of coatings prepared in

CW and PM mode decreased to 608and 1108, respectively, in

4 min, which suggested the gradual reorientation of

hydrophilic-PEG segments on to the coating–water inter-

face due to the polar interaction between the PEG segments

and water molecules. In order to test reversibility of the

switching behaviour, we have contacted the coating

surfaces with water for 30 min to insure the restructuring

of hydrophilic segments on the top of the coating surface

and quickly dried by air blow before measuring the WCA.

We observed that during the air drying the coatings quickly

regained almost the initial WCA values (1188 for CW mode

and 1218 for PM mode) and again started decreasing with



time and reached to the values of 638 and 1058 in case of CW

and PM mode, respectively (Figure 9). The lower WCA value

in case of CW mode as compared to PM mode indicates the

presence of PEG segments along with the PFDA segments on

the topmost layers, which is consistent with XPS results.

The switching behaviour of amphiphilic polymer coating

is attributed to the restructuring of hydrophilic and

hydrophobic polymer domains at the coating surface in

response to the contacting media. After exposing the

amphiphilic coating to air, the hydrophobic PFDA segments

reorient towards the coating–air interface and the topmost

surface layer is enriched with PFDA segments and the

coating surface behaves as hydrophobic, manifested by

high WCA values. On the contrary, when the amphiphilic

coating comes in contact with water drop during CA

measurement, the hydrophilic PEG segments start reor-

ienting towards the coating-water interface and coating

surface behaves as hydrophilic, which is manifested by the

decrease in WCA. In case of PM mode, the responsive range

was relatively narrow due to the fact that the concentration

of PEG segment in the copolymer coating was lower in case

of PM mode (FTIR analysis) and, moreover, in PM mode the

top surface layer of PFDA-co-DEGDME coating was mainly

dominated by the PFDA segments (XPS analysis). Therefore,

in PM mode, even after the restructuring of the PEG

segments on the surface, the WCA of the PFDA-co-DEGDME

coating was dominated by the PFDA segments due to the

lower concentration of PEG. Nevertheless, even the low PEG

content was sufficient to give rise to the phase separation of

two incompatible segments leading to complex nanos-

tructured morphology of PFDA-co-DEGDME coating (FESEM

and AFM images). It can be seen from the Figure 9 that for

coating prepared in CW plasma mode exhibited wider

switching range and more systematic responsive beha-

viour as compared to that prepared in PM plasma mode.

Antifouling Characteristics of PFDA-co-DEGDMECoatings

Quartz crystal microbalance (QCM) has been reported as a

very useful technique for evaluation of surface-related

processes in liquids, including protein adsorption beha-

viour and mass of adsorbed protein on to various

substrates.[25,36–38,56–58] We have analysed the protein

repellent properties of the nanostructured PFDA-co-

DEGDME amphiphilic surfaces by QCM in a continuous

flow mode and compared with that of plasma deposited

PFDA coatings.[9] HSA and FGN protein solutions prepared

in PBS solution were injected individually in the QCM

sample cell and their adsorption behaviour on to the QCM

sensor surfaces was investigated by monitoring resonance

frequency as a function of time. In the present study, the

dissipation shifts during the protein adsorption processes

were found to be low (<0.1 E�5 for HSA; <0.6 E�5 for

DOI: 10.1002/ppap.201000109


fibrinogen) indicating the formation of a rigid layer of

proteins, which rules out the significant influence of

viscoelastic effect; allowing us to use Sauerbrey

equation in order to compare the protein adsorption

behaviour of the polymer coatings with that of the

untreated Au surfaces.[25,36,59–61] Moreover, the changes

of the mass sensitivity constant of the sensors due to the

acoustical impedance of the thin deposited plasma-

polymerised coatings are assumed to be negligibly small,

in agreement with earlier measurements of protein uptake

on thin polymeric Teflon-AF coated quartz sensors.[61]

QCM frequency responses of untreated Au quartz crystal,

PFDA coated Au quartz crystal and PFDA-co-DEGDME

coated Au quartz crystal for HSA and FGN are presented

in Figure 10a and b, respectively. For untreated as well as

PFDA and PFDA-co-DEGDME coated QCM crystal surfaces

the frequency decreased sharply immediately after injec-

0 2000 4000 6000 8000 10000 12000-30

-25

-20

-15

-10

-5

0(a)

PFDA-co-DEGDME

PFDA

Untreated Au

∆f/H

z

Time/s

0 2000 4000 6000 8000 10000 12000-100

-80

-60

-40

-20

0

Untreated Au

PFDA

PFDA-co-DEGDME

(b)

∆ f/H

z

Time/s

Figure 10. Time-resolved QCM frequency responses of untreatedAu QCM crystal sensor, Au QCM crystal sensor coated with PFDAand Au QCM crystal sensor coated with PFDA-co-DEGDME film(PM mode, DC¼ 25%, Ppk¼ 1W) when contacted with proteinsolutions (a) HSA (b) FGN. The arrow shows the point of injectionof protein solution.



tion of protein solutions due to adsorption of protein

molecules, after that the frequency almost levelled off

indicating the saturation of protein adsorption. The initial

fast decrease in resonance frequency followed by slower

decrease indicated an irreversible protein adsorption

process that depends on the type of protein, number of

available binding sites and surface energy of the substrates,

and determines the final coverage. From the QCM analysis,

the adsorption of the protein molecules was observed to

follow the order PFDA-co-DEGDME coating< PFDA coat-

ing<untreated Au surfaces. The PFDA-co-DEGDME coating

exhibited �91 and �37% reductions of adsorption of HSA

and FGN, respectively, as compared to Au surfaces.

Whereas, PFDA coating exhibited�70 and�17% reductions

of adsorption of HSA and FGN, respectively, as compared to

Au surfaces.[9]

The superior protein repellent property of PFDA-co-

DEGDME amphiphilic surfaces is consistent with the earlier

studies where it was reported that the compositional,

topological and morphological complexities of the amphi-

philic surfaces make the hydrophobic or the hydrophilic

interactions between the biomolecule and the substrate

energetically unfavourable, thereby weaken the adhesion

strength of the biomolecule with the coating surface.[15,16]

Furthermore, the improved protein repellent property of

PFDA-co-DEGDME coating may be further attributed to the

synergistic effect of well known protein resistant PEG like

surface[4,7,8,47] and the inert and anti-adhesive nature of

perfluorocarbon coatings.[9,11–15]

The variation in the protein repellent, i.e., anti-biofouling

characteristics of plasma polymer coatings against differ-

ent protein molecules (Figure 10) is attributed to the

varying affinity of the coating surfaces towards protein

molecules due to difference in the surface-chemical

properties of different protein molecules. Another factor

is the number of contacts between the proteins and the

substrate surface; multiple contact formation leads to

higher extent of adsorption. Fibrinogen being a hydro-

phobic and bigger protein molecule (mol. weight�340 kDa)

as compared to HSA (mol. weight �66 kDa), adsorbs on

substrates through hydrophobic interaction and offers

higher numbers of contact sites for adhesion that lead to

faster and higher adsorption as compared to other

proteins.[62] It has been reported that the adsorption of

fibrinogen could be substantially higher even on hydro-

philic surfaces if the surface is positively or negatively

charged, e.g., Au surfaces.[63]

Conclusion

PFDA-co-DEGDME amphiphilic coatings were generated for

the first time by a single step-plasma co-polymerisation

process. PFDA-co-DEGDME coatings with varying chemical


384


environments, surface as well as bulk morphologies and

topographies could be achieved by regulating the plasma

process parameters such as, CW and PM plasma mode, and

plasma deposition time. PFDA-co-DEGDME coatings were

found to exhibit a fast reversible switching property in

terms of wettability, i.e., from hydrophobic to hydrophilic

and vice versa, in response to the contacting environment.

The switching behaviour of amphiphilic polymer coating is

attributed to the restructuring of hydrophilic and hydro-

phobic polymer segments at the coating surface due to the

interaction of polymer segments with the different

contacting environment. Plasma polymerised amphiphilic

PFDA-co-DEGDME coatings were found to show better anti-

biofouling (i.e., protein repellent) characteristics as com-

pared to the hydrophobic PFDA coatings. The improved

protein repellent property of PFDA-co-DEGDME coating is

attributed to (i) the synergistic effect of well known protein

resistant property of PEG and the inert and anti-adhesive

property of PFDA coatings, and (ii) the morphological and

compositional complexities of nanostructured amphiphilic

surfaces, generated due to the phase segregation of two

incompatible monomer precursors, which weaken the

interactions between the biomolecule surface and the

substrates. The present study shows that plasma copoly-

merisation process offers an easy method to fabricate

amphiphilc polymer coatings having swtiching behaviour

and anti-biofouling characteristics.

Acknowledgements: ACTECO-EC project (515859-2) is acknowl-edged for financially supporting the work. Authors sincerely thankDr. Alan Paul, CSMA for XPS analysis. Dr. D. Mataras, Dr. L.Amanatides’ and Dr. Ergina Farsari, PTL, University of Patras,Greece for AFM analysis, Dr. Ilaria Mannelli and Dr. GiacomoCeccone from JRC, Italy, and partners of ACTECO-EC project aresincerely acknowledged for their active support and discussion.

Received: August 15, 2010; Revised: December 11, 2010;Accepted: December 16, 2010; DOI: 10.1002/ppap.201000109

Keywords: amphiphilic; anti-biofouling; FT-IR; plasma copoly-merisation; protein; switching

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Documents

Amphiphilic Copolymer Coatings via Plasma Polymerisation Process: Switching and Anti-Biofouling Characteristics