Ageing of atactic and isotactic polystyrene thin films treated by oxygen DC pulsed plasma

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Surface & Coatings Technolog

Ageing of atactic and isotactic polystyrene thin films treated by oxygen

DC pulsed plasma

J. Larrieua,*, B. Helda, H. Martinezb, Y. Tisonb

aIPREM-FR CNRS 2606, Laboratoire d’Electronique, des Gaz et des Plasmas (EA 750), Universite de Pau et des Pays de l’Adour, UFR Sciences,

BP 1155, 64013 Pau Cedex, FrancebLaboratoire de Chimie Theorique et Physicochimie Moleculaire, UMR 5624 CNRS, Universite de Pau et des Pays de l’Adour, 64000 Pau Cedex, France

Received 5 May 2004; accepted in revised form 29 June 2004

Available online 11 August 2004

Abstract

This work deals with the study of atactic and isotactic polystyrene (aPS and iPS, respectively) films treated by oxygen DC pulsed plasma

of very low duty factor (1%). The main surface modifications were investigated using water contact angle measurements, X-ray

photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). By using a btemporal afterglowQ, our plasma treatment leads to the

grafting of hydrophilic moieties such as C–O, CMO and O–CMO, minimising the degradation and mechanical processes at the surface.

Ageing studies showed a better stability of the treatment under our experimental conditions, compared to published studies made under radio

frequency or microwave plasma conditions.

Ageing studies have also highlighted a surface relaxation with time, depending on the degree of crystallinity of the polymer. The

hydrophobic recovery commonly observed is restricted by an orderly packed structure of the crystallites in iPS, reducing the polymer chains

motion towards the polymer bulk. This reorientation of polar group kinetics has also been studied with increasing the storage temperature in

order to have a thermodynamic signature of the ageing phenomena.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Polystyrene; Plasma treatment; Wettability; XPS; AFM; Ageing

1. Introduction

The plasma modification of polystyrene surface is a well-

known method in order to improve its surface hydrophilicity

[1–8]. Owing to its low cost and good surface properties,

there have been many applications of this plasma-treated

polymer such as biomaterials, packaging, adhesion or

printability.

By modifying the physical and chemical structures of a

few monolayers (about 10 nm) on the polymer surface,

reactive plasmas enable the polymer wettability to be

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.06.032

* Corresponding author. Physique Department, Universite de Pau, LEGP,

Avenue de l Universite, 64000 Pau, France. Tel.: +33 559407657; fax: +33

559407634.

E-mail address: [email protected] (J. Larrieu).

increased without any change in bulk properties. According

to the nature of gas used, the surface wettability is correlated

to chemical changes, which can be characterised by

measurements of the contact angle before and after plasma

treatment [9–13]. On the other hand, studies of plasma-

treated surfaces are still running in order to have a better

understanding of its behaviour with time of storage.

One commonly observed phenomenon is a progressive

hydrophilic surface properties loss through ageing time.

This hydrophobic recovery is commonly observed with the

water drop contact angle increase with time of storage [14–

19]. This phenomenon is mainly due to the combination of

two processes. The first one consists in a minimisation of

the free surface energy linked to the reorientation of polar

chemical functions into the polymer bulk; the other one is

mainly due to polymer chains diffusion into the matrix

y 200 (2005) 2310–2316

J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–2316 2311

[14,15]. These processes can be limited with increasing

polymer crystallinity, where chains mobility is reduced

because of an orderly packed structure occurring in

crystallites [14,18]. Therefore, there are more polar groups

on surface after long ageing times with crystalline polymers.

We have investigated the changes in surface properties of

atactic and isotactic polystyrene (aPS and iPS, respectively)

after oxygen plasma treatment. Materials were characterised

by size exclusion chromatography (SEC) and differential

scanning calorimetry (DSC) in order to determine their

macromolecular dimensions (Mw, Mn and Ip), glass tran-

sition temperature (Tg) and degree of crystallinity.

Ageing behaviour of oxygen plasma-treated aPS and iPS

was investigated using water contact angle measurements.

Chemical changes on surface were investigated using X-ray

photoelectron spectroscopy (XPS) and atomic force micro-

scopy (AFM) was used to see morphological changes before

and after treatment. The surface stability has also been

studied with increasing the storage temperature at atmos-

pheric pressure, in order to have a better understanding of

the thermodynamical aspect of ageing.

2. Experimental

2.1. Preparation of polymer thin films

aPS and iPS were dissolved in toluene (ca. 0.5% v/w)

during 1 h at room temperature and at 75 8C, respectively.Some drops of this solution are deposited on glasses slides

(dimension: 1.5�3.5�0.1 cm) cleaned beforehand with

toluene and dried with acetone. These slides are stored in a

dessicator, in which the evaporation of solvent is carried out

under controlled atmosphere, in order to obtain homogeneous

PS films, with a thickness of approximately 10 Am.

Fig. 1. Experimen

2.2. Plasma treatment

Fig. 1 presents the experimental set-up used for plasma

treatment of aPS and iPS samples. The plasma reactor

consisted of a stainless steel vacuum chamber with electro-

des (10 cm in diameter) in a symmetrical plane to plane

configuration. The gap between electrodes was fixed at 1

cm. The upper electrode was grounded and the other was

negatively polarised using a high-power pulsed generator.

The generator was used at a frequency of 500 Hz with a

pulse duration time of 20 ms. The duty factor (Cr), defined

as the ratio between pulse duration time (t) and pulse

voltage period (T), was set at Cr=1% for all plasma

treatments. The load resistor value (200 V) and a maximum

duty factor of 1.5% were imposed by the manufacturer.

Interelectrode current and voltage waveforms were recorded

via a TDS 620B Tektronix oscilloscope (bandwidth and

acquisition rate equal to 500 MHz and 2.5 GS/s, respec-

tively) at the negative polarised electrode through a 25 V

resistor. Oxygen (purityN99.99%) was introduced in the

reactor. After reactor cleaning, the gas pressure was fixed at

4 mbar. As shown in Fig. 1, PS samples were placed on the

negative polarised electrode.

All plasma treatments were performed in glow discharge

conditions, with a 6 W average power (P) value and a

treatment duration time of 60 s corresponding to the best

running conditions, in order to ensure that PS surface is

saturated with hydrophilic functions [20,23]. Taking into

account the duty cycle (1%), the effective time of discharge

is of 0.6 s, and the total mean energy which is injected in

plasma is approximately of 360 J. These first results ensure

a good efficiency of treatment with a low cost for the

experiment.

Taking into account the very low duty factor, samples

are in a first approximation mainly exposed to a

tal set-up.

J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–23162312

btemporal afterglowQ of a duration time tag (Fig. 1). These

experimental conditions are of great interest because they

can permit to highlight long-lived species responsible for

the treatment, as it has been shown in preceding papers

[20–23].

2.3. Contact angle measurements

The treatment was characterised by measurement of the

contact angle of a deionised water drop with surface.

Measurement is made immediately after the treatment

(waiting time lower than 5 min). Four drops are deposited

at various places on polymer with a syringe (volume of 5 Al)in order to obtain an average value of the contact angle. The

error of measurement is estimated to be 58. The results of

wettability will be presented from the relative variation of

the contact angle:

Dh=hi ¼ hi � hf Þ=hið ð1Þ

where hi and hf are the initial and final contact angles,

respectively.

2.4. Polymer characterisation

Molecular weight determinations by size exclusion

chromatography relative to aPS standards were carried out

using a bank of four columns (HR 0.5, 2, 4 and 6R) of 300mm�5 Am Styragel at 40 8C, with THF eluent at a flow rate

of 1.0 ml min�1, controlled by a WatersR 2690 pump

equipped with an ERCR INC 7515A refractive index (RI)

detector and a WatersR 996 multiple wavelength UV–

visible photodiode array detector.

DSC analyses were performed on polymers using a

differential scanning calorimeter in order to determine their

glass transition temperature and the degree of crystallinity of

iPS. DSC trace was recorded using a temperature program

in the 50 to 300 8C range, with a rate of 20 8C/min. The

degree of crystallinity was calculated from the area of the

melting temperature peak.

2.5. X-ray photoelectron spectroscopy (XPS)

XPS spectra were recorded using an SSI M-Probe

spectrometer at room temperature. A monochromatic AlKa

X-ray (1486.6 eV) was used for the excitation. The analysis

chamber pressure was of 5�10�10 mbar. Survey spectra

were recorded at constant pass energy of 150 and 50 eV for

high-resolution analysis. A 5 eV flood gun was used in

order to prevent charge effects. The take-off angle was set at

358 in order to have a constant depth analysis.

Experimental and theoretical bands were fitted (80%

Gaussian and 20% Lorentzian) using a nonlinear baseline

with a least-square algorithm. Quantitative analyses were

calculated using Scofield factors [24] and binding energies

were determined using the C (1s) binding energy of

contamination carbon (284.6 eV) as the reference with an

experimental error of F0.2 eV.

2.6. Atomic force microscopy (AFM)

We imaged the samples in ambient conditions using

commercial (CP from Park Scientific Instrument) atomic

force microscopy (AFM) head, controlled by feedback

electronics and software of conventional design. In this

study, an AFMwith a laser beam deflection sensor is applied.

Cantilever-type Si3N4 springs with integrated tips are used as

force sensors. Typical tip radii of curvature are 20 nm and

spring constant are 0.07 N/m. Images were recorded in the

constant force mode, in the range 10–20 nN with a low scan

frequency (1.0 Hz). One micrometer grilled and mica used as

calibration samples always gave the correct periodicity.

For a line containing n data points, the root-mean-

squared (rms) roughness is given by the average deviation

of the data, determined using the standard definition:

rms ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPNn¼1ðzn � zÞ2

N � 1

s; where z ¼ mean z height

ð2Þ

The rms has been calculated on the total image sample

(dimensions 5�5 Am); this variable will give us the

signature of morphological changes occurring with plasma

treatment. Images have been recorded on different zones in

order to be representative of the total sample surface state.

2.7. Ageing conditions

Polystyrene samples treated in oxygen were stored at

atmospheric pressure in dark PVC boxes immediately after

plasma treatment. Samples were aged at room temperature

(22F2) 8C with an average relative humidity value of

(45F7)%.

2.8. Effect of the storage temperature

In order to determine the temperature stability of treated

surface, polystyrene samples were placed in an oven at the

desired temperature for 1 h. Samples were removed from the

oven, cooled to room temperature, and water contact angle

was then measured.

3. Results and discussions

3.1. Materials characterisation

SEC and DSC results of untreated aPS and iPS are given

in Table 1. aPS prepared by radical polymerisation had

weight average molecular weight 327000 g mol�1 and

polydispersity, Mw/Mn, of 1.9 by SEC.

Table 1

Macromolecular data dimensions of aPS and iPS

SEC DSC

Mw (g/mol) Mn (g/mol) Ip Tg (8C) Percent crystallinity

aPS 327000 172000 1.9 105.6 –

iPS 536000 372000 1.44 104.6 20


SEC indicated the degraded iPS to have a weight average

molecular weight of 536000 g mol�1 and polydispersity of

1.44.

DSC indicated aPS and iPS to have glass transition

temperature of 105.6 and 104.6 8C, respectively. The degreeof crystallinity of iPS was found to be approximately of

20%. The aPS is a 100% amorphous polymer with a very

low rate of crystallinity (b0.5%).

In a first approximation, these results ensure the two

polymers to have comparative macromolecular dimensions,

and only to differ by their degree of crystallinity. These

considerations are important in order to have a better

understanding of the ageing phenomenon through the study

of the surface reorganisation.

3.2. Oxygen plasma treatment

3.2.1. Surface chemical changes

Before treatment, we ensured that polymers were totally

free of oxygen contamination in order to be sure that the

functionalisation surface is only linked to the reactive

species created into the plasma bulk. After oxygen-plasma

treatment, contact angle of water decreased considerably for

both polymers and oxygen concentration increased (Table

2). These results suggest an introduction of hydrophilic

moieties at the surface.

Fig. 2 shows the high-resolution XPS spectra in the C

(1s) region of untreated and O2-plasma-treated aPS.

Untreated sample presents a symmetrical peak at 284.6 eV

corresponding to C–C and CMC bonds. The phenyl rings

are characterised by a peak at 291.2 eV corresponding to the

pYp* shake-up satellite. After plasma treatment, The C

(1s) spectrum of aPS presents a dissymmetry with higher

binding energies. As it has been previously reported [17,20],

these changes are attributed to the formation of hydroxyl

(C–O), carbonyl (CMO) and carboxyl (O–CMO) bonds.

It can be noticed that the intensity of pYp* hake-up

satellite peak at 291.2 eV decreases for both aPS and iPS

corresponding to a loss of polymer aromaticity. This

Table 2

Contact angle, XPS and AFM measurements before and immediately after treatm

Untreated

Contact angle (8) O/C ratioa rmsb (

Atactic PS 85 – 6.01

Isotactic PS 90 – 108

a XPS measurements.b AFM measurements.

observations indicate a possibility for the ring sites to be

major sites of attack from the plasma and/or a loss of phenyl

rings by breakdown of the C–C bond between aliphatic

chain and the hanging groups.

Oxygen concentration of aPS is a little higher than the

iPS one, reflecting a greater chemical reactivity of amor-

phous zones from the plasma. Preceding studies concerning

the plasma treatment of aPS and iPS showed similar results,

showing a sensitivity of treatment to the polymer structure

[23]. These results are in agreement with the literature,

notably with Morra et al. [25] who have reported that the

highest reactivity of amorphous polymers is linked to their

greater freedom of motion.

3.2.2. Surface morphology changes

The analysis of untreated and plasma-treated samples by

AFM has permitted us to see the physical modifications

occurring during our plasma treatment (Fig. 3). The first

AFM result concerns the rms roughness of untreated

samples. aPS and iPS films have a rms of 6.01 and 108

2, respectively (see Table 2), involving that aPS film

surface is very smooth compared to the iPS one. This

difference is mainly linked to the presence of an orderly

packed structure of the crystallites in iPS embossing the

surface.

After oxygen-plasma treatment, the rms is increased by a

factor of 4.9 and 1.4 for aPS and iPS, respectively. This

result is of a great interest because it underscores a principal

phenomenon: it seems that there is a preferential attack of

the plasma on the amorphous zones of aPS. After oxygen-

plasma treatment, crystalline regions are etched more slowly

than amorphous regions, because of the kind of interactions

between phenyl groups occurring in crystallites of iPS [23].

Nevertheless, preceding studies showed that if a surface

roughness is below than 100 nm, there was no influence on

contact angle [26]. Hence, according to our preceding

results, it can be considered that physical modifications

observed do not have any influence on the surface properties

and would not affect ageing phenomena.

ent

O2-plasma-treated

2) Contact angle (8) O/C ratioa rmsb (2)

15 0.296 29.4

25 0.264 146

Fig. 3. AFM images of untreated (a) aPS and (b) iPS, and O2-plasma-treated (c) aPS and (d) iPS (scale analysis=5�5 Am).

Fig. 2. Example of C (1s) core level spectra obtained for (a) untreated aPS and (b) O2-plasma-treated aPS.


Fig. 5. Surface composition variation as a function of time of storage in

ambient air.


3.3. Study of the ageing phenomena

Ageing of treated samples has been studied through

measurements of the surface wettability. Fig. 4 shows the

relative contact angle variation versus storage time for

treated aPS and iPS samples. It is observed that ageing

process is characterised by a quick decrease of Dh/hi in the

very first hours of storage time, decreases more slowly for

longer times, to reach finally a plateau value (Dh/hic0.5

and 0.58 for aPS and iPS, respectively). The plateau is more

quickly reached in the iPS case (400 h of storage) than in the

aPS one (c1400 h). It is important to notice that after 2

months (i.e., 1400 h) of storage, a good surface wettability is

kept, with a loss of the treatment efficiency equal to 40%

and 25% for aPS and iPS, respectively.

The decrease in wettability is less marked for the sample

having the highest rate of crystallinity (i.e., iPS), involving

that this hydrophobic recovery can be limited by the ordered

packed structure of the crystallites.

Ageing has also been observed by XPS, in order to find a

correlation between chemical changes at the surface and

wettability results. Surface compositions (O/C ratios)

calculated from XPS peak intensities have been plotted as

a function of storage time, as shown in Fig. 5. The main

behaviour observed for both aPS and iPS is an O/C ratio

decrease in the first times of storage, being less drastic for

longer times. It is important to notice that after 200 h of

storage time, the O/C ratio seems to be stabilised for iPS,

whereas it still decreases in the case of aPS.

Taking into account the constant analysis depth, the

surface atomic ratio decrease observed for both polymers

involves a reorientation of the oxidised grafted functions

towards the polystyrene bulk. This oxygen concentration

lowering at the extreme surface can explain the hydrophobic

recovery observed in Fig. 4, having the same variations.

Thus, we can suppose that in our experimental conditions,

ageing phenomenon can be correlated to the modifications

of the surface chemistry.

Fig. 4. Relative contact angle variation as a function of time of storage in

ambient air.

In a general way, the hydrophobic recovery over time of

storage observed is mainly linked to the diffusion of

hydrophilic groups, tending to minimise the surface free

energy after plasma treatment and depending on the ageing

media [17]. There are surface relaxation phenomena with

kinetics depending on various parameters, notably the

physical properties of the polymer such as crystallinity.

This principal mechanism of chain polymers migration

has been highlighted with increasing the storage temper-

ature. Fig. 6 presents the relative contact angle variations as

a function of the storage temperature, for oxygen-plasma-

treated aPS and iPS. We ensured that for untreated samples

there were no variations of contact angle with increasing the

temperature in the 20 to 120 8C range.

The main behaviour observed is a linear Dh/hi decrease(i.e., h increase) with increasing the storage temperature in

the 20 to 110 8C range, and it seems that this hydrophobic

recovery is less marked with iPS, with a slower kinetic.

Above 110 8C (corresponding approximately to the Tg for

both polymers), the Dh/hi value is stabilised under the 0

critical value (untreated samples), indicating that the surface

Fig. 6. Relative contact angle variation as a function of storage temperature.


is not totally hydrophobically recovered with high temper-

atures. By increasing the temperature, the polymer chains

mobility is increased, with the diffusion of polar groups to

the bulk phenomenon. The difference observed is probably

due to the crystallinity in iPS slowing down the oxidised

polymer chains diffusion compared to aPS.

4. Conclusion

A study of treatment of atactic (aPS) and isotactic (iPS)

polystyrene thin films under pulsed plasma conditions was

carried out. O2-plasma treatment leads to the grafting of

polar moieties at the extreme surface, increasing hydro-

philicity characterised by contact angle and XPS measure-

ments. By varying the degree of polymer crystallinity, AFM

and XPS measurements showed a selectivity of the plasma

for the treatment.

The study on ageing has highlighted surface relaxation

phenomena. The functional groups introduced diffuse from

the surface to the polymer bulk with storage time, and with

kinetics depending on the storage temperature and polymer

physical properties such as crystallinity and tacticity.

By using a DC pulsed discharge of very low duty factor

(1%), plasma treatment leads mainly to chemical modifica-

tions and the degradation process is limited using a temporal

afterglow. These experimental conditions ensure a good

efficiency of the treatment with a low energy spent and good

treatment stability even after 2 months compared to

published studies of polymer treatment under radio fre-

quency or microwave conditions [14,27].

By taking into account these results, it will be interesting

to have a complementary study of ageing phenomena,

notably the surface stability towards washing and ageing

media. This question is in progress.

Acknowledgements

L. Billon and R.C. Hiorns (LPCP, Pau University) are

thanked for DSC analysis and iPS synthesis. C. Guimon

(LCTPCM, Pau University) is thanked for XPS analysis.

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Ageing of atactic and isotactic polystyrene thin films treated by oxygen DC pulsed plasma

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