An XPS study of pulsed plasma polymerised allyl alcohol film growth on polyurethane

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 8203–8211

An XPS study of pulsed plasma polymerised allyl alcohol film

growth on polyurethane

Lucy Watkins a, Alexander Bismarck b, Adam F. Lee a,*, Darren Wilson c,Karen Wilson a,*

a Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdomb Department of Chemical Engineering, Polymer & Composite Engineering (PaCE) Group, Imperial College London,

London SW7 2AZ, United Kingdomc Smith & Nephew Research Centre, York Science Park, Heslington, York YO10 5DF, United Kingdom

Received 25 July 2005; received in revised form 23 August 2005; accepted 24 October 2005

Available online 7 December 2005

Abstract

The growth of highly functionalised poly allyl alcohol films by pulsed plasma polymerisation of CH2 CHCH2OH on biomedical grade

polyurethane has been followed by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Film thickness is observed to

increase approximately linearly with plasma modification time, suggesting a layer-by-layer growth mode of poly allyl alcohol. Water contact angle

measurements reveal the change in the surface free energy of wetting decreases linearly with plasma modification up to the monolayer point after

which a constant limiting value of �24 mJ m�2 was attained. Films prepared at 20 W plasma power with a duty cycle of 10 ms:500 ms exhibit a

high degree of hydroxyl (–OH) retention with minimal fragmentation of the monomer observed. Increasing the plasma power up to 125 W is found

to improve –OH retention at the expense of ether formation generating films close to the monomer stoichiometry. Duty cycle plays an important

role in controlling both film composition and thickness, with longer off times increasing –OH retention, while longer on times enhance allyl alcohol

film growth.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Pulsed plasma polymerisation; Allyl alcohol; Biomaterials; Thin films; Polyurethane; XPS; Contact angle

1. Introduction

Many polymeric materials while possessing excellent bulk

properties such as chemical inertness, tensile strength, elasticity

or density for a particular application may not exhibit ideal

adhesive or wetting interactions at the surface. Surface

modification is often employed to tune the surface functionality

and improve interactions in the interfacial region [1,2] thereby

optimising the performance of the polymer. Plasma surface

modification is a convenient technique which can achieve

profound changes in the surface chemical functionality of a

material by enabling control over key factors such as wetting or

adhesive properties without changing the bulk properties of the

solid [3]. A plasma is a gaseous mixture of electrons, radicals,

ions and excited molecular states which are created by inelastic

* Corresponding authors. Tel.: +44 1904 432586; fax: +44 1904 432516.

E-mail address: [email protected] (K. Wilson).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2005.10.045

collisions between high energy electrons and ground state

atoms or molecules [4]. In recent years modification of surfaces

by plasma treatment has been the subject of extensive research

in both industry and academia with coatings developed for

application in the fabrics [5], biomaterials [6,7], electronics [8]

and optics [9] sectors.

In contrast to many other types of wet chemical treatments,

plasmas have a unique ability to initiate free radical and ionic

reactions without the need to apply heat or catalysts. There is also

greater freedom in the choice of the reacting gas and a wide range

of functional monomers have been investigated including

alcohols [10,11], amines [12–14], cyano groups [15], carboxylic

acids [16], anhydrides [17], epoxides [18], fluorocarbons [19,20],

ferrocenes [21] and aromatic compounds [22]. As a result the

potential of plasma modification and polymerisation for enabling

surface properties to be tuned to a particular application has

attracted great interest from the biomaterials community where

both surface and bulk structural properties play an important role

in material performance.

L. Watkins et al. / Applied Surface Science 252 (2006) 8203–82118204

Plasma polymerisation can be achieved using either a

continuous wave (CW) process or by pulse mode. There is a

wealth of literature concerned with the application of CW

plasma polymerisation processes [3]. The presence of high

energy electrons and ions in the plasma generate a high density

of activated gaseous monomer species which polymerise to

generate films possessing a high degree of cross-linking.

Although CW plasma polymerisation is effective in altering

surface properties there is little control over surface functional

group density due to decomposition of the monomer under

plasma excitation which results in poor retention of functional

groups. To address this issue, in recent years there has been

interest in polymerisation occurring from pulsed plasma

methodologies. Pulsed plasma systems offer processes advan-

tages by permitting formation of high quality films under much

lower power inputs than are usable under CW conditions [23]. By

utilising a pulsed system, polymerisation takes place during both

the ‘on’ and the ‘off’ periods, with conventional polymerisation

reactions such as radical additions or cationic chain transfer

taking place in the off cycle leading to higher retention of the

monomer functionality.

Here we investigate the application of pulsed plasma

polymerisation for coating biomedical grade polyurethane

(PU) with polymeric allyl alcohol films. PU was selected due to

its desirable bulk properties for biomedical applications such

as elasticity, tensile strength, durability, tear resistance and

ease of fabrication. The versatility of PU’s originates from their

composition of alternating rigid ‘hard’ diisocyanate and

flexible ‘soft’ aliphatic polyether or polyester segments [24]

which can be varied depending on the desired application.

Currently PU is employed in a range of applications including

wound dressings, catheters, pacemakers, artificial heart and

arterial grafts [25]. The ability to control their wettability

through the generation of surface films with controlled

surface –OH density would be desirable to extending their

range of biomedical applications [26]. Allyl alcohol

(CH2 CHCH2OH) is a desirable monomer to introduce

hydroxyl functionality to a surface, however it particularly

susceptible to C–O cleavage and as a result is unstable under

high power CW plasma conditions [27]. It is also of interest to

note that allyl alcohol polymer films cannot be readily

synthesized by wet chemical radical polymerisation due to

degradative chain transfer processes which prevent high

molecular weight polymers forming [28].

Previous investigations of plasma polymerisation of allyl

alcohol have employed continuous wave conditions where

relatively high powers �20 W [29] or 60 W [30] have been

utilised during film preparation. A detailed study of allyl

alcohol polymerisation over aluminium substrates revealed

powers as low as 1 W were necessary under CW conditions to

retain film functionality [31]. A subsequent pulsed plasma

investigation over silicon [32] also revealed good –OH

retention could be achieved using 300 W plasma power and

duty cycles of 1 ms on and 30 ms off to generate films of

thickness 100–300 nm. Here we report on the growth, surface

morphology and composition of pulsed plasma polymerised

allyl alcohol films synthesised under low power (10–125 W)

conditions on biomedical grade PU. Using low power and

slower deposition rates we aim to retain –OH functionality and

generate ultra thin films with tuneable wettability.

2. Experimental

2.1. Materials and plasma polymerisation

Allyl alcohol (AA) (Aldrich > 99%) was subjected to

several freeze–thaw cycles prior to use. Thin films (1.5 mm) of

PU (Estane, 58245 NAT 031, Noveon) were prepared by

compression moulding of beads between sheets of release film

(Tygaflor) using a Fontyne Press (Mackey Bowley) at 220 8Cand 50 kN m�2 pressure. Prior to compression, beads were

dried overnight in a vacuum oven at 80 8C to remove excess

water. Films were cut into 1 cm � 3 cm strips after pressing,

and then sonicated in distilled water for 10 min. They were

finally washed in ethanol before use.

A schematic of the plasma reactor is shown in Scheme 1. It

consists of a large tubular glass reactor 5 cm in diameter and

25 cm length into which samples were centred and the reaction

chamber evacuated to base pressure of �3.0 � 10�3 Torr by a

rotary pump (Trivac B, Leybold). All vacuum seals were

grease-free to minimise contamination. Allyl alcohol vapour

was dosed into the chamber via a needle valve at a constant

pressure of 1.1 � 10�1 Torr. Pressures were measured with a

thermocouple gauge, calibrated to have a linear response over

the operation range of the reactor. The reaction chamber was

purged with monomer vapour for 5 min prior to and after

experiments to ensure the flow was stable, and subsequently to

allow time for passivation of the surface.

The pulsed plasma was produced via an RF generator

(RFG150 Coaxial Power Systems) operating at 13.56 MHz,

inductively coupled to the reactor through an externally wound

copper coil via an L–C matching network (MN150 Coaxial

Power Systems). The matching network was adjusted to

minimise reflected power. These were in turn connected to an

oscilloscope (Farnell) and 10 MHz pulse generator (Thurlby

Thandar Instruments). The pulse generator was used to set

differing duty-cycles (ton/(ton + toff)) which were monitored via

the oscilloscope.

The effect of peak power was investigated over the range

10–125 W whilst employing a duty cycle of 10 ms:500 ms and

treatment time of 400 s. The effect of duty cycle on film

deposition rate and composition was studied by variation of

the on or off time duration at 20 W peak power. The total

treatment time was varied in each instance to maintain a

constant number of cycles in each experiment. Treated

samples were stored under vacuum in a dessicator prior XPS

and contact angle analysis which were performed within 3 h

of their preparation.

2.2. X-ray photoelectron spectroscopy (XPS)

XP spectra were obtained using a Kratos AXIS HSi

photoelectron spectrometer with a charge neutraliser and Mg

Ka excitation source (1253.6 eV). Energy referencing was

L. Watkins et al. / Applied Surface Science 252 (2006) 8203–8211 8205

Scheme 1. Schematic of inductively coupled pulsed plasma reactor.

employed using the carbon C 1s (C–C/C–H) 285 eV peak and

valence band. Wide-scan spectra were acquired for each

sample, together with high-resolution C, N and O 1s spectra at

pass energies of 160 eV and 20 eV respectively. Peak analysis

was performed using Casa-XPS Version 2.1.9 software with all

spectra Shirley-background subtracted prior to fitting. A

common lineshape was adopted for each element, based on

a Gaussian–Lorentzian mix, with FWHM = 1.23 (C 1s), 1.44

(N 1s), 1.76 (O 1s), and percentage Lorentzian convolu-

tion = 30% (C), 30% (N), 70% (O) respectively. Elemental

compositions were determined using the respective atomic

sensitivity factors for C (0.318), O (0.736) and N (0.505).

2.3. Contact angle measurements

Water contact angles on films were measured using the

sessile drop method applied in the Drop Shape Analysis System

(DSA 10 MK2, Kruss GmbH, Hamburg, Germany) at room

temperature. Water droplets were placed from above on the film

surface using a motorized microsyringe. A 0.5 ml water drop

was placed on the surface of the samples via a needle and water

was continuously added at a constant rate. The contact angles

were measured during the advancing movement of the water

droplet over dry surface. At least 10 readings were taken for

different drops placed on several spots of the surface. Tangent

method 1 was used for computation of the contact angles in all

cases. Deionised water from a Nanopure (Barnstead, Dubuque,

IA, conductivity 1.8 mV cm�1) was used as the test liquid.

3. Results and discussion

3.1. Surface composition

The growth mode of polymerised allyl alcohol films was

initially studied at 20 W peak power with a duty cycle of 10 ms

on:500 ms off. Fig. 1a shows representative C 1s spectra for

untreated PU and samples exposed to 1 and 10 min of AA

plasma. The parent PU exhibits four peaks at 285, 286.5, 288

and 289.5 eV characteristic of the aliphatic carbon (CHx), ether

(C–O–C), carbonyl (C O) and carbamide (HN–C(O)O)

functionalities respectively [33] as shown in Scheme 2.

The intensity of the 286.5 eV peak increased following

plasma treatment with AA, consistent with enhanced alcohol

(C–OH) surface functionalisation. A slight rise in the 288 eV

component was also observed, suggesting there is also an

increase in the surface C O content which may reflect either

some degree of fragmentation or rearrangement of the AA

monomer during the treatment process, or post oxidation of

the hydroxyl groups [16,31]. Fig. 1b shows the integrated

intensities of the deconvoluted C 1s components, which reveal

the surface composition evolves rapidly with treatment time

over the first 200 s. The corresponding O 1s spectra in Fig. 2a

show that PU is composed of a single broad feature centred

around 532.5 eV which shifts to 532.7 eV following plasma

treatment with AA. Deconvolution of this feature, shown in

Fig. 2b, reveals the progressive emergence of a new com-

ponent attributable to C–OH surface groups, centred at

532.7 eV. These carbon and oxygen surface environments

formed by longer treatment times are consistent with

fingerprint spectra expected for AA [31,32]. XPS on films

aged in air for 12 months revealed no change in the surface

C:O ratio suggesting that the surface is not susceptible to

post oxidation.

The build-up of this AA-like film is clear in Fig. 2c, wherein

the hydroxyl component grows at the expense of PU features.

The N 1s region, shown in Fig. 3a, exhibits a single peak at

400.1 eV characteristic of carbamide (HN–C(O)O) moeities

arising from the PU substrate. This nitrogen feature was fully

attenuated following 600 s of plasma modification, consistent

with the formation of a thick capping overlayer, enabling the


Fig. 1. (a) Deconvoluted C 1s XP spectra following plasma deposition of allyl alcohol at 20 W power and a duty cycle 10 ms on:500 ms off; (b) deconvoluted C 1s

peak intensities as a function of AA plasma treatment time at 20 W power and a duty cycle 10 ms on:500 ms off.

final surface stoichiometry to be estimated. The O:C ratio for

this plasma treated surface was 0.4, which is close to the

theoretical value for pure AA of 0.33, confirming successful

genesis of an encapsulating polymeric AA film. The higher

oxygen content suggested by our experimental ratio is

consistent with an oxygen (–OH) terminated surface as

expected.

3.2. Allyl alcohol film growth mode

The attenuation of the surface carbamide peak, unique to the

substrate, allows accurate determination of the AA polymer

film thickness and growth mode, as shown in Fig. 3b. Film

thicknesses were calculated using Eq. (1) below [34], wherein I

and I0 are the intensities of the N 1s signal from the treated and

untreated PU samples respectively, t the film thickness and a

calculated value of the inelastic mean free path (l) of the N 1s

photoelectron of 1.44 nm [35]:

I ¼ I0 exp

�� t

l

�(1)

The resulting AA film thickness increased approximately lin-

early with plasma modification time, suggesting a layer-by-

layer growth mode, reaching a value of �35 A after 600 s

(equivalent continuous wave on time = 11.8 s).

Scheme 2. Structure of po

Table 1 shows the results of contact angle measurements

performed on the series of AA plasma treated PU samples, from

which it can be seen that PU exhibits a contact angle of 888which decreases progressively with PAA treatment time

reaching a plateau of 308 after 600 s. The surface free energy

of wetting (Dga) was subsequently calculated from the

advancing contact angle (ua) and the molar surface area of

the polymer film (A) according to Eq. (2) [36]:

Dga ¼1

3

�RT

A

�ln½ð1� cos uaÞ2ð2þ cos uaÞ�

4(2)

The molar surface area, A, was in turn calculated according to

Eq. (3) from the molecular weight of the monomer (M) of

58.08 g mol�1, Avagadro’s number (N) and assuming a bulk

PAA density (r) of 1.28 g cm�3 [37]:

A ¼�

M

r

�2=3

N1=3 (3)

The molar surface area of the untreated PU substrate was

likewise calculated assuming a bulk density of 1.22 gcm�3

and monomer molecular weight for x = 5 for the structure

shown in Scheme 2, which equates to 1308 g mol�1.

Dga represents the change in the surface free energy of the

system as the surface wets with water [38], and thus

lyurethane (x = 5–10).


Fig. 2. (a) O 1s XP spectra as a function of 0–600 s AA plasma treatment at 20 W power and a duty cycle 10 ms on:500 ms off; (b) deconvoluted O 1s XP spectra

following plasma treatment of allyl alcohol; (c) deconvoluted O 1s peak intensities as a function of AA plasma treatment time at 20 W power and a duty cycle 10 ms

on:500 ms off.

quantifies the strength of interactions driving water sprea-

ding over the treated PU surfaces. Fig. 4 shows the change in

Dga relative to the untreated PU surface as a function of AA

plasma treatment. In all cases surface modification resulted in

a more negative surface free energy of wetting, indicating

more favourable surface wetting by water. The change in

the surface free energy of wetting decreased linearly

with plasma modification up to �600 s treatment time, at

which point a constant limiting value of �24 mJ m�2 was

attained.

The general variation in the surface free energies of

wetting is consistent with the formation of an increasingly

thick and polar AA film, with Dga continuing to evolve for

treatment times up to 600 s, which were required to fully

encapsulate the PU substrate and generate a continuous

AA overlayer.

3.3. Polymer purity/degradation

Fragmentation of the monomer species can occur in the gas

phase during plasma excitiation, particularly when high powers

are used. It is therefore important to determine the extent of

functional group retention in the final film. Mass spectrometry

studies of plasma excited AA have demonstrated that one of the

main decomposition pathways is dehydration of the monomer

to a cyclopropyl fragment, which results in oxygen deficient

films [31,39]. The low power pulsed conditions employed in

our experiments, yielded O:C ratios close to the initial

monomer value of 0.33 (Table 2), suggesting that minimal

monomer dehydration occurs. Considering a simple reaction

scheme involving minimal fragmentation, plasma polymerisa-

tion of AA can occur in a number of ways as illustrated in

Scheme 3a and b. If only Scheme 3a operated then a pure –OH


Fig. 3. (a) N 1s XP spectra as a function of AA plasma treatment time at 20 W power and a duty cycle 10 ms on:500 ms off; (b) Attenuation of N 1s signal and

calculated film thickness of AA as a function of plasma treatment time; 20 W power and a duty cycle 10 ms on:500 ms off.

terminated film should arise, resulting in a surface composition

with the –CHx (285 eV) and –CH2OH (286.5 eV) C 1s XP

components in a 2:1 ratio.

Under low power plasma polymerisation conditions ether

formation can occur either by the reaction of the –OH

Table 1

Water contact angles measured for AA films prepared at 20 W and duty cycle

10 ms on:500 ms off as a function of treatment time

Treatment time (min) Advancing contact angle (8)

0 88

60 84

180 83

420 58

600 31

1800 30

Fig. 4. Calculated change in surface free energy of wetting (Dg) relative to

untreated polyurethane as a function of AA plasma treatment time; plasma

ignited at 20 W power with a duty cycle 10 ms on:500 ms off.

terminated polymer with a gas phase activated AA monomer, or

via alcohol dimerisation in the gas phase to form the ether

which subsequently polymerises as shown in Scheme 3b. Ether

formation would also contribute to a C 1s state at 286.5 eV,

however the theoretical CHx:COC ratio for a pure poly ether

film would now be of 1:2. The ratio of these low:high C 1s states

can therefore be used to estimate the extent of –OH retention, as

outlined by O’Toole and Short [31], assuming a simple

fragmentation scheme whereby each C–O–C group formed

removes one CHx equivalent during the polymerisation process.

Conversely the degree of etherification can be estimated using

Eq. (4):

�1� ð1=2Þð2� actual CHx : COHðRÞÞ

2

�� 100 (4)

Higher plasma powers are expected to increase cross-linking

and stability of the polymer films, but may compromise film

functionality due to monomer fragmentation. The effect of

plasma power on film composition was therefore also

Scheme 3. (a) Poly allyl alcohol formation. (b) Ether formation during AA

polymer film growth.


Table 2

Effect of plasma power on AA film composition using a duty cycle of 10 ms

on:500 ms off and 600 s treatment time

Plasma power (W) C (at.%) O (at.%) N (at.%) O/C ratio

0 82.1 16.4 1.5 0.20

10 70.9 28.9 0.2 0.41

20 70.3 29.7 0 0.42

30 71.2 28.8 0 0.40

50 71.5 28.5 0 0.40

100 72.1 27.9 0 0.39

125 74.2 25.8 0 0.35

Table 3

AA film composition as a function of plasma power using a duty cycle of 10 ms

on:500 ms off and 600 s treatment time; average power = peak power � duty

cycle

Peak plasma power

(average) (W)

CHx

(%)

COH(R)

(%)

CHx:COH(R)

ratio

OH retention

(%)

10 (0.2) 56.3 37.3 1.5 87.5

20 (0.4) 57.6 35.5 1.6 90

30 (0.6) 57.6 35.5 1.6 90

50 (1.0) 60.2 33.5 1.8 95

100 (2.0) 59.2 34.1 1.7 92.5

125 (2.5) 63.2 30.2 2.1 102.5

Table 4

Variation of surface composition with plasma off time for AA films prepared at

20 W peak power with a total on time of 400 s

Duty cycle on:off (ms) Average power (W) C (at.%) O (at.%) O/C ratio

1:1 10 84.6 15.4 0.2

1:5 3.3 74.6 25.4 0.3

1:10 1.8 71.1 28.9 0.4

investigated by varying the peak plasma power over the

range 10–125 W while maintaining a constant duty cycle of

10 ms on:500 ms off. Table 2 shows the film composition as

a function of plasma power from which it can be seen that

the gross O:C film composition remains constant at �0.4

showing that our polymeric films remain essentially AA-

like irrespective of power.

The surface functionality of the plasma polymer was also

determined by XPS as a function of peak power, the results of

which are shown for the same fixed duty cycle of 10 ms

on:500 ms off in Table 3. As the plasma power was increased

from 20 to 100 W the ratio of CHx:COH(R) remains

essentially constant at around 1.6–1.7 indicative of an OH-

rich terminating layer, but with some ether formation. At

higher powers (125 W) this ratio increases to 2.1 consistent

with a higher degree of OH termination with a limiting value

close to the expected CHx:COH theoretical value for pure

PAA.

The increase in –OH retention with plasma power can be

accounted for by considering possible intermediates formed

following activation of AA in the plasma. Previous mass

spectrometry studies [39] have identified a C3H5O+ carbocation

intermediate which may exist in the resonance forms A and B

shown in Scheme 4. Under the lowest power conditions

investigated in this study we assume that the lifetime of isomer

A is sufficiently high to favour ether formation. As the plasma

power is increased, isomerisation to structure B, which

undergoes polymerisation to PAA, is favoured over ether

formation. The role of such resonance structures, and of isomer

A in particular, has been previously invoked to rationalise the

formation of oxygen deficient ether containing films from

Scheme 4. Mechanism for ether

cyclopropyl cations via cyclisation of A [31]. It is also

interesting to note that the stabilility of the keto form (C) of

resonance structure B, should result in C O termination of all

growing polymer chains, in good accord with the XPS

observations in Fig. 1a.

The effect of duty cycle on film composition was

investigated by preparing films at 20 W peak power using a

constant on time of 1 ms and a variable off time. In all

experiments the total on-time was kept constant at 400 s.

Table 4 shows the C:O ratio as a function of the off-time. The

duty cycle had a marked effect on the film composition, with the

observed O/C ratio increasing from 0.2 to 0.4 as the off time is

increased from 1 to 10 ms. Fig. 5 shows the deconvoluted C 1s

spectra as a function of duty cycle from which it is interesting to

note that the COH(R):C O ratio also increased with off time, as

determined from the corresponding component intensities

shown in Table 5. Such an observation is consistent with the

keto form, resonance structure C being involved in the initiation

of plasma polymer chains [31]. The plasma excited resonance

cation is only formed during the on cycle and its relative partial

pressure will depend on the duration of the duty cycle. During

the off-cycle ‘cationic like’ polymerisation can occur between

and AA polymer formation.


Fig. 5. Deconvoluted C 1s XP spectra showing the effect of duty cycle on

plasma treated AA at 20 W with a total pulse on time of 400 s.

Fig. 6. AA film thickness as a function of on or off pulse time: variable off time

utilising constant pulse on time of 1 ms; variable on time utilising constant off

time of 10 ms. All films prepared at 20 W with 3600 cycles.

the plasma excited resonance cation and the monomer, thus

with longer off periods greater chain growth will occur

accounting for the higher OH:C O ratios observed under these

conditions.

The extent of polymerisation may also be expected to vary as

a function of pulse on and off time, with the latter factor

expected to depend upon the gas exchange rate and associated

lifetime of activated species within the plasma reactor. To

investigate this behaviour, the early stages of film deposition

were followed with the on time varied from 1 to 5 ms for a fixed

off time of 10 ms, and subsequently the off time varied from 1

to 20 ms while employing a constant on time pulse of 1 ms. A

measure of the film thickness (in ML) was obtained by

monitoring the attenuation of the N 1s XP signal for these films

as compared to an untreated sample with the coverage

calibration determined from Fig. 3b. In all cases the treatment

time was varied to maintain a constant number of treatment

cycles (3600 cycles). Fig. 6 shows that the film thickness was

essentially independent of the cycle off time. In contrast the

film thickness was directly proportional to the pulse on time,

revealing this as the crucial parameter for controlling film

deposition rate.

Table 5

Variation of surface composition with plasma off time for AA films prepared at

20 W with a total on time of 400 s

Duty cycle

on:off (ms)

CHx

(%)

COH(R)

(%)

C O

(%)

COH(R):C O

ratio

CHx:COH

(R) ratio

1:1 75.5 18.9 4.8 3.9 4.0

1:5 65.4 28.5 5 5.7 2.3

1:10 59.8 34.0 5.2 11.5 1.8

Fig. 5 showed that changing the off time had a major impact

on the resulting surface functionality CHx:CO. However, such

variations in film composition were not observed with variable

pulse on time. Fig. 7 shows deconvoluted C 1s spectra as a

function of on time, which demonstrate a high degree of C–OH

and C O retention in all cases. The increase in the relative

proportion of C O component at 288 eV is again consistent

with the keto form of AA being involved in polymer chain

initiation, the proportion of which depends on the duration of

excitation pulse.

Fig. 7. Deconvoluted C 1s XP spectra showing the effect of pulse on time

during plasma treatment of allyl alcohol at 20 W. All films prepared with

3600 cycles.


4. Conclusions

Pulsed plasma polymerisation has been successfully

employed to deposit highly functionalised allyl alcohol films

onto a PU substrate. The three process variables of total

treatment time, peak power and duty cycle all influence the

surface chemistry. XPS reveals that films prepared at 20 W with

using a duty cycle of 10 ms:500 ms exhibit a high degree of –

OH retention with minimal fragmentation of the monomer.

Detailed XPS characterisation of allyl alcohol growth under

these conditions reveals that film thickness increases linearly

with plasma modification time, suggesting a layer-by-layer

growth mode, with a film thickness of �35 A obtained after

600 s treatment time. Contact angle measurements reveal the

surface free energy of wetting decreases continuously with

plasma modification, consistent with the greater surface

polarity of allyl alcohol, reaching a limiting value of

�24 mJ m�2.

Under these pulsed conditions, increasing plasma power

over the range 10–125 W actually helps to improve surface –

OH retention at the expense of ether formation, resulting in

films more akin to pure allyl alcohol. Duty cycle also has a

significant effect on film properties, with pulse off times

influencing film composition, while pulse on times alter the

polymer growth rate.

Acknowledgements

Financial support is gratefully acknowledged from Smith &

Nephew and the EPSRC for the provision of a DTA to LMW.

KW also acknowledges the Royal Society for an Equipment

Grant (2004/R2).

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An XPS study of pulsed plasma polymerised allyl alcohol film growth on polyurethane

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