COLLOIDS A AND

Colloids and Surfaces SURFACES E L S E V I E R A: Physicochemical and Engineering Aspects 105 (1995) 277 289

A physicochemical study of oxygen plasma-modified polypropylene

N. Shahidzadeh-Ahmadi a, M.M. Chehimi b, F. Arefi-Khonsari a,., N. Foulon-Belkacemi c, j. Amouroux a M. Delamar b

a Laboratoire de GOnie des ProckdOs Plasmas, ENSCP-UniversitO Pierre et Marie Curie, 11 rue Pierre et Marie Curie, 75231 cedex 05 Paris, France

b lnstitut de Topologie et Dynamique des Syst~mes de l'Universit~ Denis Diderot (Paris 7), associd au C N R S ( U R A 34), 1 rue Guy de la Brosse, 75005 Paris, France

c Laboratoire de Physique des Ddcharges, kquipe N ° 114 du CNRS, Ecole Sup&ieure d'Electricitd, Plateau du Moulon, 91190 Gif-Sur- Yvette, France

Received 10 April 1995; accepted 12 May 1995

Abstract

The physicochemical properties of oxygen treated polypropylene (PP) films have been studied using a series of analytical techniques with different degrees of surface sensivity. The treated polymer surfaces were found to be acidic by X-ray photoelectron spectroscopy (XPS) in conjunction with the molecular probe technique, a result in agreement with previous wettability studies. Dimethyl sulfoxide (DMSO) has been used for the first time as a molecular probe to investigate the surface acidity of O2 plasma treated PP. The retention of DMSO increased with plasma treatment time and is thus governed by surface modification which permits acidic groups to be grafted onto the host polymer. Such retention is depicted in the binding energy of S2p from DMSO. DMSO contact angle measurements showed that the treated surfaces exhibit acid-base character from the initial stage of plasma treatment as judged by the acid- base contribution of the total work of adhesion t~W aboMso_PPj~ which was found to readily reach a plateau at 14 mJ m -2. We have used capillary electrophoresis ion analysis (CIA) to identify the acidic low molecular weight fragments formed at the plasma-treated surface and found that they are mainly oxalic, malonic and fumaric acids, a result which is confirmed by ATR-IR analyses.

Keywords: Oxygen plasma; Polypropylene

1. Introduction

The plasma technique is a dry process which modifies polymer surfaces by grafting polar functional groups [ 1,2]. This modification usually results in an improvement of e.g. metallization, wettability and adhesion properties of polymer surfaces [3 -5 ] . For a long time intermolecular

* Corresponding author.

0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03314-9

interactions have been classified as "polar" or "nonpolar" and it has become usual to discuss dipole-dipole interactions and the dipole-induced dipole interactions for molecules in the liquid or solid states, or at liquid/solid interfaces. Only recently, it has been found that in condensed phases there is no evidence for any measurable dipole interaction energies of cohesion and adhe- sion [6] . Instead, the importance of acid-base interactions between the so-called "polar" groups

278 N. Shahidzadeh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289

in liquids and solids is now well recognised and it is found that these interactions are quite indepen- dent of "polarity" as measured by dipole moments [7-10]. Therefore, it is concluded that dipole- dipole interactions, in those cases, are negligibly small compared to acid-base and dispersion force interactions [ 8 ]. However, to validate this concept it is necessary to characterize such properties by adequate techniques and methods [11-15]. In previous work, we have carried out a comparative study on the acid-base properties acquired by polypropylene (PP) films treated in a reducing or an oxidizing atmosphere using contact angle meas- urements and X-ray photoelectron spectroscopy (XPS) in conjunction with the molecular probe technique [3,16]. We have used chloroform (TCM), a reference Lewis acid [15] to probe the Lewis basicity of ammonia-plasma treated PP surfaces [ 16].

The objective of this paper is to survey in more depth the physicochemical properties of Oz-treated PP for different treatment times. That is to anal- yze the surface with different techniques having different degrees of surface sensivity. These tech- niques include contact angle measurements which respond to the functional groups in direct contact with the liquid phase, and capillary electrophoresis ion analysis (CIA) which allows analysis of the droplet of water after deposition on the treated surfaces. This technique permits identification of the existing ionic organic species dissolved in water uptake by the droplet. XPS in conjunction with molecular probe technique characterizes the acid- base properties of the treated and untreated poly- mer surfaces (analysis d e p t h s 5 nm); and finally ATR-IR, a vibrational technique, to identify func- tional groups in a relatively thick surface layer (~ 10 3 nm), introduced by the plasma treatment.

2. Theory

Generally, compounds which are referred to as "polar" have both acidic and basic sites, as in water and acetone [8]. It should be noted that "polar" or rather acid-base interactions occur when one phase has basic sites and the other has acidic sites. For this reason, in compounds such as

water, molecules self-associate through acid-base bonds, to line up the dipole moments thereby resulting in a high dielectric constant and an anomalously high boiling point [6]. In the case of complex materials such as polymers and metal oxides, the determination of acid-base parameters is an important step to understanding and pre- dicting their adhesion, mixing and mechanical properties.

The theory of acid-base interactions in relation to adhesion science has been discussed at length elsewhere [7,14,16]. However some issues of fundamental and practical importance follow:

2.1. Fundamental aspects

The aci~base interactions are considered to be of Lewis type and occur between electron pair acceptors and electron pair donors [ 17].

The reversible work of adhesion between a solid (s) surface and a liquid (1) indicates the degree of intermolecular interactions. It is defined by [ 15]:

W = = y 1 ( l + c o s O ) = W d + W ab (1)

Where W a is the dispersion or London forces and W ab is the non dispersive or acid-base interactions, ~,l is the liquid surface energy and 0 represents the contact angle of the liquid at the surface.

= ( 2 )

W ~b = W ~ - W d (3)

7~ and 7~ define the dispersive part of the liquid and solid surface energy respectively.

2.2. Practical aspect

2.2.1. Wettability The dispersive component of the solid surface

energy is obtained using a non-polar liquid such as bromonaphthalene with no acidobasic character [ 18,19].

7b( 1 + COS 0b) z (4) 7d-- 4

Where 7b and 0b stand for the surface energy and the contact angle of bromonaphthalene, respectively.

N. Shahidzadeh et al./Colloids Surfaces A. Physicochem. Eng. Aspects 105 (1995) 277-289 279

Once the dispersive properties have been deter- mined, one can rely on the so-called "contact angle titration method" - - using unbuffered aqueous acidic and basic solutions - - to gain information on the acid-base interactions due to different functional groups at the surface [ 11,12].

grade. Dimethyl sulfoxide (DMSO) and HC1 were Prolabo products, bromonaphtalene was from Janssen Chimica and N a O H was from R.P. Normapur A.R. Doubly distilled water was pre- pared at the laboratory in a glass distillation apparatus.

2.2.2. XPS: a tool for evaluating the acid-base properties of polymers

XPS permits determination of the acid-base properties of polymers by monitoring the concen- tration and the chemical shifts experienced by a Lewis acid or base sorbed in the host polymer [20-21] . The polymer film is exposed to liquid vapours of known acid-base properties. The film is then allowed to outgas the excess of solute and is transferred into the XPS equipment for surface characterization. If the polymer-solute acid-base interactions are strong enough, then a residual amount of solute is detected and the molar ratio of solute per polymer repeat unit is evaluated. The binding energies (BEs) of core electrons photoemit- ted by label elements from the solutes are investi- gated. F ls and C1 2p are target core holes for fluorinated and chlorinated acidic solutes used to characterize the basicity of polymers, whereas N ls from pyridine is usually monitored to determine the surface acidity of catalysts and polymers [14,20]. In the present work, we make use of dimethyl sulfoxide (DMSO) for the very first time as a molecular probe to investigate the surface acidity of O2 plasma treated PP. S 2p is the target core hole, the relative intensity and chemical shift of which will be related to the surface treatment.

3. Experimental

3.1. Materials

Isotactic polypropylene films, 8 lam thick, were used in this work. In all cases, experiments were performed on the side of film facing the inside of the stock roll. XPS analyses prior to plasma treat- ment have shown a negligible amount of oxygen on the surface of PP.

Solvents, reagents and oxygen gas were com- mercially obtained and were of a high purity

3.2. Reactor for plasma surface treatments

A low pressure plasma reactor with non symmet- rical configuration of electrodes (high voltage (HV) hollow electrode-earthed cylinder) was used in this study [22]. The film was cut into 23 × 23cm squares and was rolled on the earthed cylinder which rotated in front of the HV electrode. A base pressure of 5 x 10 - 4 Pa was established with the help of a primary pump and a turbomolecular pump. The working pressure was about 100 Pa and the oxygen discharge was established with the help of a 70 kHz excitation source. The oxygen gas was introduced through a MKS mass flow control- ler and the pressure was monitored with an MKS capacitive gauge. The electrical characteristics of the discharge were measured with a Lecroy 9400 digital oscilloscope with a sampling frequency of 100 MHz. The power of the discharge was approxi- mately 30 W. The treatment time of the polymer films was measured by multiplying the rotation time of the cylinder by the ratio of the plasma width on the film over the perimeter of the cylinder.

3.3. X P S

XP spectra were recorded using a VG Scientific ESCALAB MKI system, with A1 K~ radiation (1486.6eV), operated in the constant analyzer energy mode. The pass energy was set at 50 eV to detect properly the S 2p signal from the retained DMSO and to obtain an adequate signal-to-noise ratio. Contrary to the previous paper [16] using chloroform as a molecular probe, DMSO contains only one sulfurated atom per molecule whereas chloroform contains three chlorine atoms per molecule. The pressure in the analysis chamber was 5 x 10 -8 mbar. Digital acquisition was achieved with a Cybernetix system and the data collected with a personal computer. The data pro- cessing software allowed smoothing, linear or shir- ley background removal, static charge referencing,

280 N. Shahidzadeh et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 277-289

¢q

160

140

120

100

80

60

40

30 sec

non t rea ted PP

0 5 10 15 pH

Fig. 1. Variation of the total work of adhesion with different test liquids for different treatment times. Treatment conditions: gas: 02, P = 150 200 Pa, Q = 100 sccm, Pw = 30W, dintere lee t rode =

1 cm. (D) Untreated PP, treatment time (s) (11) 0.046; (~I,) 1; (A) 5; (O) 30 I-3].

peak fitting and quantification. Charge referencing was determined by setting the main C ls compo- nent at 285 eV. The surface composition (at.%) of the various oxygen-treated PP samples was deter- mined by considering the integrated peak areas of C is, S 2p, O Is, and their respective experimental sensitivity factors. A(%), the fractional concen- tration of a particular element A is computed using:

A = IA /SA /~ ( I . / Sn ) × 100 (5)

where I and s are the integrated peak area and the sensitivity, respectively, for a given element, and n stands for the nth element considered in the quantitative analysis [23] . This equat ion was subsequently used to determine O/C and S/O atomic ratios shown in Fig. 7.

3.4. Contact angle measurements

The apparatus developed in our laboratory is based on an image processing system. It allows

v

I/1 t-.

.E

t l l 0 Q .

282B

225t

1692

1121 /

/. 1

/ $6~ / ;

~ ~ ° ° . . ° o ° ° . . ° ° ° o

261

~~ C-C/C-H / t.

' i . .

I "'..C-O

"-.. C=O

• . . . . .

2~ 2b~

Cls binding energy (eV)

Fig. 2. XPS C ls photoelectrons peaks of oxygen-treated PP (30 s).

N. Shahidzadeh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277 289 281

measurement of the contact angle of a drop of liquid on a flat surface, and of modification of the contact angle measurements as a function of time. The apparatus has been described in detail else- where [24]. A monochromatic camera equipped with a zoom lens and a double focal extender produces a video image of the object (in this case the drop of the liquid on the surface). With the help of an analogue-digital interface the video image is digitized. Homemade software is used to obtain the contour of the drop and secondly to measure the contact angle. This system offers the advantage of being completely objective, reproduc- ible (mean square deviation: 0.3 for 100 responses) and rapid (0.6-1 s) as compared to the goniometer or photographic measurements. The advancing angle is then measured each 0.6 s, so there is no need to extrapolate to time zero for the determin- ation of the advancing contact angles. The humid- ity and the temperature of the chamber were not controlled. The latter varied between 20 and 25 ° C. The drop was considered as part of a sphere and the volume of the drop used was always 2 ~tl. Therefore, the deviation from spherical contour due to gravitational perturbations is negligible.

Samples were cut to 0.5 × 4 cm and attached by the back of the sample to a flat support using a two sided Scotch tape to keep the sample flat. All reported values are an average of seven measure- ments, taken at different locations on the films, and have a maximum error of _+2 °. The solid/ liquid/vapor system is considered to be at thermodynamic equilibrium.

3.5. A T R - I R measurements

Transmission spectra were obtained on IFS 48 BRUKER Spectrometer and converted directly to absorption spectra by a computer for quantifica- tion. The films were cut to the size of (KRS5, 45 °) crystal faces (4.6 × 1.5 cm) and were equilibrated at pH 13 (double distilled water + NaOH) for a few minutes (~ 1 min) [24]. Then, the solutions were blotted dry on filter paper and dried in air for 30min prior to contact with the crystal to prevent crystal damage and to eliminate peaks due to excess of water from the spectrum. Afterwards, the films were pressed against the crystal faces with a sample holder. Each experiment was carried out three times.

Table 1 Standard solutions used as references to identify by CIA the polymeric fragments soluble in water

Standard solutions Conductivity '2' (anions) (10 -4 m 2 S mol 1)

Concentrat ion of anions in prepared solutions as references (ppm)

DL-Malic acid (disodium 58.8 salts) C4H405Na2

Sodium acetate trihydrate 40.9 Na2C2H3Oz.3HzO

Sodium oxalate C204Na2 74.11 Sodium formate H C O O N a 54 Fumaric acid (disodium 61.8

salts) C4H2OaNa2 Malonic acid (disodium 63.5

salts (C3H204Na2 NaC1 3% Acetonitrile 76.31

l g L -1

N a N O 3 3% Acetonitrile 71.42 l g L -~

N a / S O 4 3% Acetonitrile 80 l g L -1

N a H C O 3 44.5

0.74

0.81

0.66 0.66 0.71

Not available

1

1

1

1

282 N. Shahidzadeh et al. 'Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 277 289

o2oii -0.4Oil

-0.60

- O. 8 O- } /,

1ooi[ -1.20-

-1.40

-1.60

-1.80

-2 .00 - -

1 2 3 4

/ - - \ j - . / ' ~ -.. ~ ' x / ' v ~ t . . . . / N . \ / .

\~ ." C

5 6 7 '~- '~ . . . . "~'-- b

9 ~ ' ~ ~ d

2.60 2.80 '3 .'00' 3 . 2 0 3 .40 '3.~60' '3 .'80' 4.~00' '4 .20 4 . 4 0

Minutes

Fig. 3. Capillary electrophoresis analysis of anions in (a) pure water; and water taken from (b) untreated PP, (c) 02 treated PP (20 s), (d) 02 treated PP (30 s). The peaks 1, 2, 3, 8 correspond to CI-, SO~-, NO;- and HCO;- respectively. These anions are present in water prior to deposition. Peaks 7 and 9 correspond to formate and acetate, contaminants at the surface of untreated PP. Peaks 4, 5, 6 appear after treatment and correspond to oxalate, malonate and fumarate, respectively.

3.6. Capillary electrophoresis ion analysis

The analysis were carried out by a Waters capil- lary ion analysis (CIA) apparatus. This technique permits analysis of small ionic species. Separations are performed by applying an electrical field to the sample in a capillary filled with an electrolyte. Indirect UV detections were carried out by using a mercury lamp line at 185 nm. Electromigration injection was chosen in order to enrich the target analytes prior to analysis and to increase detection levels by more than two orders of magnitude. A very pure drop (20 gl) of water - - obtained by Millipore apparatus - - was placed at the surface of the sample (oxygen-treated PP) for a few seconds

(20 s). The drop was then picked up and analyzed. No organic ionic species were detected in the pure water prior to deposition.

4. Results and discussion

The PP films were treated by oxygen plasma for different treatment times. It is important to underline the strong plant-like odor of the 02 treated surface after long treatment times (> 10 s) which is probably due to the presence of esters or aromatic compounds at the surface.

The contact angle of liquids, i.e. aqueous solu- tions with different pH and DMSO, at the surface

N. Shahidzadeh et aL /Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289 283

were measured immediately after treatment. In the case of aqueous solutions, it was demonstrated that the contact angles on nonpolar surfaces were independent of p H [ 13,25,26].

The results obtained with aqueous unbuffered solutions on the oxygen-treated PP surfaces were discussed in detail in a previous paper [3] , and showed the acidic character of the treated surfaces (Fig. 1). This property was enhanced with treat- ment times due to an increase of grafted acidic groups at the surfaces as estimated by XPS. Indeed, Fig. 2 shows the peak fitting for oxygen-treated PP. The high-binding-energy components (at 286.6, 288, 289 eV) can be attributed to hydroxyl, car- bonyl and carboxylate and/or carboxylic acid groups respectively.

d~

g~

8

;.)

250 •

200 •

1 5 0 "

1 0 0 •

5 0 "

0 o x a l a t e

• F u m a r a t e

o

. ~ • ÷ • T , , . , • , , j 0 1 7 15 20 30 35 40 Treatment times (s)

(a)

800 * Malonate

600

4002000 __~ ~ ~ . ~ (b)

o 1 7 15 20 30 35 40 Treatment times (s)

Fig. 4. Variation of ionised organic acids: (a) oxalate, fumarate and (b) malonate at the surface with plasma treatment times.

To have more information about the surface modifications, i.e. the nature of soluble acidic groups in water and the degradation effect of oxygen plasma leading to low molecular weight fragments, the droplets of very pure water (V = 20 gl, pH = 5) deposited on the surfaces were ana- lyzed by CIA. Since the application of this tech- nique to surface treatments is very recent [27], the main problem is the identification of ionic species which vary with plasma gas and the nature of the surface studied. The analysis obtained in the case of PP treated for 20 s with oxygen plasma (Fig. 3) shows, indeed, the presence of ionic species, in the analyzed volume. To identify these ionic species, we based our investigation on XPS results which indicate the possible presence of organic acids at the surface ( - C O 0 - , C ls component at 289 eV). A range of standard solutions (known conductivity and therefore a known migration time) was chosen close to the unknown peaks. Organic standard solutions came from Sigma and mineral solutions from standard Analys SA. Their structures, conduc- tivity and the concentration prepared for the refer- ence solutions are detailed in Table 1. For each treatment time, the peaks were identified with the help of their migration times and by preliminary calibrations with standard solutions with known conductivity. However, the migration times of some of these organic solutions (such as fumarate and malonate) are too close, so that the peaks overlap and the identification is not straight- forward in those cases. This problem can be solved by adding different standard solutions to each sample prior to CIA. This allows us to see either the increase of the peak area of the unknown species (if it corresponds to the standard solution) or the appearance of a new peak. In the latter case, we start again the doping procedure with another standard solution and so on. Thus, we have identified the presence of oxalate, fumarate and malonate species the structures of which are dis- played below:

O O II II

Oxalate - O- - C - - C - - O -

Malonate

O O II II

- O - - C - - C H 2 - - C - - O -

284 N. Shahidzadeh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289

O O II II

Fumarate - O - - C - - C H = C H - - C - - O -

Fig. 4 shows the variation of these species with plasma treatment times. It must be noted that the concentration of malonate is expressed as uV s (area of the peak) because the standard solution was not available to calculate the concentration in p.p.b. However it was identified by the migration time of malonate which depends on its molar conductivity at finite dilutions (for 1 malonate 2- ' 2=63.510 -4 m 2 S mol-1). Note in Fig. 4 that oxa- late and malonate were formed with a short treat- ment time of t = 5 s. Oxalate concentration levels off around 100 p.p.b, for ( t--7 s) and does not change with treatment time, whereas, malonate concentration decreases with long treatment times (t > 15 s). Actually, the latter is likely to be replaced by fumarate which begins to be formed with treat- ments longer than 15 s (Fig. 4(a)).

Oxygen plasma brings about degradation leading to low-molecular-weight fragments and volatilization of these by decarboxylation. So, the organic acids mentioned above are formed by oxi- dative cleavage of PP chains and it seems that with longer treatment times the surface is enriched with conjugated double bonds due to the oxalate

and fumarate species. The presence of these kind of structures could explain the increase of the surface conductivity with treatment time [5].

Moreover, to have a direct measure of acid-base interactions between acidic groups at the surfaces and a basic liquid, we have calculated also the work of adhesion between DMSO, polar liquid with basic tendency (•1=43.5 mJ m -2, 3;ab = 14.5 mJ m -2, ?a=29 mJ m -2 [8]), and the treated polymers using wettability measurements. In addi- tion, the contact angle measurements of DMSO will be related to the XPS study of P P - D M S O interactions reported below. The total work of adhesion was calculated using Eq. 1. The dispersive component, W a, was determined by Eqs. (2) and (4), whereas the acid-base contribution W ab was obtained using Eq. 3. Fig. 5 shows that the total work of adhesion increases with the treatment time and levels off for a treatment time of around 5 s ( w T = 8 6 m J m - 2 ) . The treated surface exhibits acid-base character from the initial stage of plasma treatment (t = 0.023 s), as judged by the W ab which is found to reach readily a plateau at 14 mJ m -2. These results underline the fact that the strength of po lymer -DMSO acid-base interactions does not vary with treatment times although the O/C ratio reaches a plateau for longer treatment time,

100

80

e,l 60

40.

20.

• WTDMsO.Pp

WdDMsO_Pp

[] WaBDMsO_P P

0 . . . . . . . . u . . . . . . . . I . . . . . . . . I . . . . . . . . I . . . . . . .

.00l .01 .1 1 10 100

Treatment t ime (s)

Fig. 5. Variation of the work of adhesion: WTDMsO-PP, wdDMso-PP and W abDMso.Pv with oxygen plasma treatment times.

N. Shahidzadeh et al. rColloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289 285

c

e -

d g

¢n

• " . E

about 5s. That would mean that the type of interactions are always the same in spite of the increase of the acidic groups and the morphological modification of the surfaces with treatment times.

To confirm the results obtained by wettability measurements (direct contact between solutions and samples), we have used the XPS in conjunction with the molecular probe which is sensitive to all of the grafted functional groups over a thickness of 5 nm. The untreated and oxygen-treated surfaces were exposed to DMSO (Lewis base) vapours for 30min and were then analyzed by XPS. If the oxygen-treated surfaces present an acidic character, then acid-base interactions should occur between DMSO molecules and acid groups and therefore a residual amount of solute should be detected. No elemental information could be gained from the S 2p region in the XPS wide scans. On the contrary, the high resolution S 2p region exhibits a feature attributed to sorbed DMSO. Fig. 6 shows S 2p signals from DMSO vs. the treatment time. The intensity of the S 2p signal from DMSO increases with the treatment time. No S 2p is detected for treatments shorter than 1 s. As, the treatment time increases, S 2p is readily detected with a better signal to noise ratio. Actually, since oxygen-treated PP has Lewis acidic sites, it is likely that acid-base interactions govern the retention of DMSO by the surface. This is supported by the S 2p BE, which was in the range of 166.5-168 eV, even higher than the values indicated in the litera- ture for (C6H4)2S=O and CH3C6H4S=O, 165.8 eV [28], therefore indicating a stronger acid-base bond. This positive shift undergone by basic species has been mentioned elsewhere for the interaction of pyridine with polymers 1-20] and zeolite surfaces 1-29]. For each treatment time, the number of DMSO molecules sorbed per oxygen atom (S) were calculated:

DMSO (Is2p/1 × Ss2p) S - - - - × 100 (6)

0 (Iols/sols)

I and s were defined in Eq. (5).

163 168 173

S2p binding energy (eV) Fig. 6. S2p signals of DMSO sorbed in oxygen plasma treated polypropylene vs. the treatment time. (A) untreated PP; treatment time/s: (B) 3; (C) 8; (D) 15; (E) 30.

286 N. Shahidzadeh et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 277-289

The results in Fig. 7 reveal an increase of the O/C ratio (calculated using Eq. 5) for very short treatment times, which levels off after 5 s. Although,

100.

~" 10 =

..~

• ~ A_ •

O/C

m

~ " ~ - - - - D/V~SO/O

• J , i • i • i • i , i * J ,

5 10 15 20 25 30 35 40

Treatment times (s)

Fig. 7. O/C (in %) and DMSO/O (%S) vs. the treatment time.

the O/C ratio reaches a plateau very quickly (5 s), the S value does not vary in the same manner. Indeed, S also increases with treatment time, but reaches its asymptotic value at much longer treat- ment times (15 s). Actually, S reaches its maximum value i.e. 1.17% once the surface becomes enriched with conjugated double bonds as judged from CIA plots (Fig. 4). These structures are in fact very stable and the carboxylate/carbonyl groups impli- cated in structures with conjugated double bonds are more acidic due to the electronic delocalization effect [30]. One should however emphasize that in the case of CIA, only low molecular weight fragments dissolved in the water are analyzed, whereas by XPS in conjunction with the molecular probe technique, the global acid-base interactions of the treated surfaces are measured.

To complement the CIA, contact angle measure- ments and XPS results, we have used ATR-IR which probes a depth of about 103 nm. Fig. 8 shows ATR-IR spectra of untreated and O 2 treated

i

I I I f I I I I

4000 3750 3500 3250 3000 2750 2500 2250 2000

,...-, o , 0 o

4

I I I ] I I I • I I I

18b0 17bo 16bo

I I I I I I

1750 I500 1250 1000 700

Wavenumber cm- 1

Fig. 8. ATR-IR spectra of untreated PP and Oz treated PP (30 s). In the treated case, two weak signals centered at 1640 and 1710 cm -1 are detected•

N. Shahidzadeh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289 287

PP. Note that in this figure no peaks were detected in the 1550-1800cm -~ region of untreated PP whereas two weak signals centered at 1715 and 1640cm -~ were detected for the oxygen-treated surface. The absorption at 1715 cm-1 can be attrib- uted to acids (carboxylic acids) and esters (such as formate and fumarate and possibly benzoate), whereas absorption at 1640 cm -1 is attributed to (aliphatic and possibly aromatic) aldehydes and ketones, or the presence of double bonds. Whilst 1715 cm- a is a standard wavelength for carboxylic acids, 1640 cm-1 is lower than the reference values for aldehydes and ketones, and is likely to be due to a conjugation of double bonds. Indeed, the con-

jugation effect lessens the double-bond character and lowers the frequency 1-31].

To confirm the presence of acids and esters (1715 cm -1) after oxygen plasma-treatment, and the increase of the acidic character of the surface with treatment times, the oxygen plasma-treated PP surfaces were washed with a basic solution (pH 13). This reaction results in the appearance of new peaks centered at 1520 and 1425cm -1 as shown in Fig. 9. The intensity of these vibrational peaks increases with the plasma treatment time and no absorption remains at 1715 cm -~ (Fig. 10). Therefore, Fig. 9 confirms that the peak at 1715 cm -1 is almost totally due to R C O O H which

o

o

o

I r I I I I I I I

k,/ I

I I I I

', i t I

I I I I I I

I I L I I I I I I I I I ' 1 ' I I I I I I

~7~[ 1675 t650 1625 1600 1575 1550 i525 LSO0 1475 1453 1425 1400 1375 1350 !325 13[}0 ',275 1250 i . _ I,,Javenunber cm -I

Fig. 9. ATR-IR spectra of (A) untreated PP and 0 2 treated PP films (B) 1 s; (C) 10 s; (D) 30 s, after washing with a basic solutions (pH 3). Appearance of new peaks centered at 1520 and 1425 cm -1, the antisymmetrical and symmetrical vibrations of COO respectively.

288 N. Shahidzadeh et aL/Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 277-289

L~

t~

8 ~

LO

0

I I I I I I I

A

I I I I I I I I I

i

I

J I

2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 I~00 1350 1300 1250 1200 I[50 Ii00

N a v e n u m b e r cm -t

Fig. 10. Carbonyl region of (A) oxygen-treated PP (20 s); (B) 02 treated PP (20 s) washed with a basic solution (pH 13). No absorption remains at 1715 cm -1.

is converted into RCOO-, the signals of which are centered at 1520 and 1425 cm -1. These fre- quencies correspond to the antisymmetrical and symmetrical vibrations of carboxylate anions respectively [31,32]. Considering that no esters are present at the surfaces, the strong plant odor of the O2 treated PP seems to be due to aromatic, ketones and aldehydes which appear with long treatment times. It should be noted that these peaks have not been detected when washed with neutral or acid solutions•

This ATR-IR study confirms the results drawn from the CIA measurements and moreover is in agreement with the work of Whitesides and co-workers on the reactivity of carboxylic acid and ester groups in the functionalized interfacial region

of polyethylene carboxylic acid and its derivatives, using ATR-IR spectroscopy and contact angle measurements as probes [12,22,33]•

Conclusion

Polypropylene surfaces have been modified by oxygen-plasma treatment and characterized by XPS, contact angle measurements, capillary elec- trophoresis (CIA) and ATR-IR. The main points are summarized below:

1. Oxygen plasma modifies the surface by graft- ing especially hydroxyl, carbonyl and carboxylate groups at the surface• The surface becomes hydro- philic with a dominant acidic character for short and long treatment times.

N. Shahidzadeh et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 277-289 289

2. The acidity of the surface was confirmed by XPS in conjunction with molecular probe technique. DMSO has been chosen for the first time as Lewis basic probe. DMSO adsorbs on oxygen-treated PP and not on untreated PP. This result is interpreted in terms of acid-base interactions between DMSO and acidic groups grafted at the surface.

3. The strength of the acid-base interactions does not vary with treatment time, in spite of the increase of concentration of acidic groups and the morphological modification of the surfaces.

4. Low molecular weight fragments which are identified to be organic acids (oxalic, malonic and fumaric) are formed at the polymer surface following oxygen-plasma treatments. These species appear for treatments above 1 s and were identified by capillary electrophoresis analysis.

5. For long treatment times (> 15 s), the surface is enriched with double conjugated bonds. These structures reinforce the acidic character and could allow the formation of aromatic compounds at the surface. Oxygen plasma gives an acidic character to the surface and favors bond cleavage followed by decarboxylation and a probable cyclisation of polymeric fragments for long treatment times.

6. A vibrational study of the treated PP sur- face by means of ATR-IR confirmed the results obtained by CIA.

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