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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
J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–2316 2313
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.
J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–23162314
Fig. 5. Surface composition variation as a function of time of storage in
ambient air.
J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–2316 2315
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.
J. Larrieu et al. / Surface & Coatings Technology 200 (2005) 2310–23162316
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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