Romanian Reports in Physics, Vol. 64, Supplement, P. 1227–1242, 2012

Dedicated to Professor Ioan-Iovitz Popescu’s 80th Anniversary

REVIEW OF THE PROCESSES IDENTIFIED DURING THE POLYMERIZATION OF ORGANIC AND ORGANOSILICON

LIQUID FILMS IN ATMOSPHERIC PRESSURE AIR CORONA DISCHARGES

A. GROZA

National Institute of Lasers, Plasma and Radiation Physics, 077125 Bucharest- Magurele, Romania E-mail: andreea@infim.ro

Received July 13, 2012

Abstract. The paper presents the recent studies on the processes responsible for polymerization of organic and organosilicon liquid films placed on the surface of the plane electrode in corona discharges produced in air at atmospheric pressure.

Key words: corona discharges, polymerization processes, oxide layers.

1. INTRODUCTION

It is well known that organic and organosilicon polymer layers are produced especially for protective coatings [1–3]. For example, organic polymers, like linseed oil based polymers, are widely used in protective coating of metallic surfaces. In [4], it was showed that using a corona discharge in air at atmospheric pressure instead of chemical dryers, a high reaction rate polymerization process of linseed oil can be initiated.

Layers of polysiloxane deposited on various substrates by different methods have proved their utility as corrosion protective layers towards heat, ozone and chemicals. Most oxidizing chemicals do not affect these remarkable thermal stable coatings due to the bonding strength of the Si-O network. The presence of the Si–OH groups in the layers can perform strong bonds with the substrate surfaces, this kind of polymers being used as adhesion promoters [1, 2].

The polymerization mechanism of polydimethylsiloxane (PDMS) liquid films under atmospheric pressure air corona discharges, allows the formation of silicon oxides structures on the free surface of the polymers and of different oxides at the polymer/substrate interface, depending on the substrate material and

A. Groza 2 1228

on the end groups of PDMS liquid precursors [5, 6]. In [7] it was demonstrated that the coupling between negative and positive corona discharges in air at atmospheric pressure can be used also for generation of SiO2 thin films, having an organosilicon compound as liquid precursor.

Generally, for an improved adherence of polymeric layers to substrate materials, the substrate surfaces are usually anodized, especially the aluminum ones. Thus, these polymers and anodized layers could act as an effective barrier preventing the contact of water based corrosive media with the underlying metallic surfaces [1, 2]. The anodized oxide layers generated at polymer/substrate interface could increase the adherence of the polymer to the substrate. Therefore, the corrosion resistant properties of the system formed by polymer and interfacial oxide layer could be improved.

The main advantage of this physical method is that the polymer layers obtained under positive and negative corona charge injection may uniformly coat any polished or unpolished complex metallic surfaces by penetrating the electrode surface pores.

In this paper will be presented the results obtained after the investigation, by different complementary methods, of the organic and PDMS layers produced in corona discharges in air at atmospheric pressure, in order to evidence the processes that take place at polymer surfaces, polymers bulk and at polymer/substrate interface.

2. EXPERIMENTAL SET-UP

The experimental set-up used is presented in Fig. 1 [4–7].

Fig. 1 – Experimental set-up.

It consists in a point to plane corona discharge electrode configuration. The wire electrode of 40 mm in length was placed perpendicular to the center of disk electrodes of 10–20 mm in diameter under air at atmospheric pressure. The

3 Polymerization of organic and organosilicon liquid films 1229

interelectrode gap was varied between 8–30 mm function of the liquid precursor type and it quantity. In order to avoid any fluctuation of ambient air in the interelectrode gap, the electrodes were introduced in a glass cage. The DC high voltage was also varied between 7 to 24 kV. As working regime for corona discharges we used the Trichel pulse regime of negative corona discharge and the glow discharge regime of positive corona discharge.

The current pulses duration depend on the interelectrode gap as it was observed in the waveforms of the intensity of the discharge current obtained in Trichel pulse regime and presented in [4–7]. The pulsating nature of the current which assure a uniform and periodically charge transport between electrodes can be explained as follows. When a high DC negative voltage is applied to the sharp electrode, the surrounding air becomes ionized creating charged particles that form a cloud drifting along the field lines. Due to both the geometric configuration and the local presence of these clouds, the electric field between the electrodes is strongly non-uniform and varies with time as successive clouds drift from one electrode to the other.

The current of the positive corona discharge in a glow regime consists only in a DC component having the same value as that of the permanent current of the negative corona discharge within the given electrical and geometrical conditions [4, 7].

Another reason for running corona discharges in Trichel pulsed regime of negative corona discharge respectively in the glow regime of the positive corona discharge is the uniformity of current density distribution on the plane electrode in the point to plane corona discharge electrode configuration, which was experimentally determined for the first time by Warbung in 1899. Lately, Sigmond [8] established that the uniformity of the current density distribution on the plane electrode can be obtained on a circular surface with the radius given by the relation r = 3 d, where d is the interelectrode gap.

The injection of ions produced in corona discharges on the free surface of the liquid precursors of different quantities (0.1–10 µl) induces electrohydro-dynamic instabilities with the formation of some stationary convective structures of Benard cell type [9]. The size of a Benard cell depends on the quantity of the liquid placed on the plane electrode. The corona charged particles accumulated on the free surface of the liquid induce a charge gradient and consequently a convective liquid movement with the formation of the convective structures of Marangoni-Benard cells type [10]. These kinds of structure were obtained for the first time by Benard in 1900 when a dielectric liquid was heated from below [11]. Surface tension driven thermal convection is also known as Marangoni-Benard convection [12].

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The spatial distribution of these convective structures on a plane electrode can give information about the current density uniformity. In the experimental conditions presented above, the ions produced in the sharp electrode region travel through the drift region of the corona discharge attain the free surface of the liquid film and continue to move through the fluid until they reach the plane electrode. The convective movement of the liquid associated to the charge movement through the liquid determines the formation of convective structures like those presented in Fig. 2 [9, 13].

Fig. 2 – Convective structures of Marangoni - Benard cells obtained

in PDMS liquid precursors.

Thus, after 2 hours of corona charge injection solid polymer layers of organic (polymers generated from linseed oil) respectively organosilicon layers (polymers generated from PDMS liquid precursors) were obtained on the surface of the plane electrode.

3. RESULTS AND DISCUSSIONS

3.1. IR SPECTRAL INVESTIGATIONS OF POLYMERS GENERATED FROM LINSEED OIL

The linseed oil, the richest oil in linolenic acids, CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH, introduced in negative

5 Polymerization of organic and organosilicon liquid films 1231

respectively positive corona discharge, suffers chemical changes, namely the generation of a radical chain polymerization process. The radical chain mechanism of the polymerization process involves three steps: initiation, propagation and termination.

In the negative corona discharge in air at atmospheric pressure, the initiation of the linseed oil polymerization which is the key of the whole process involves the negative ions like O2

−, O3−, O4

−, CO2− , CO3

− generated on the free surface of the dielectric fluid introduced in the discharge [14]. The convective movement of the liquid allows the propagation of the initiation process in the linseed oil. These negative ions injected in the linseed oil, attack like a polar solvent the polarized carboxylic functional group –COOH [15] and extra activate the α-methylene groups. Thus, the supplementary infusion of oxygen, forcefully introduced by convective movement in the liquid bulk, favors the initiation of the common auto-oxidation process with a high reaction rate and the formation of the peroxides, hydroperoxides and alcohols radicals. In [4] it was evidenced the presence in the liquid bulk of the hydro-peroxide radicals, the main chemical products in the propagation step of the polymerization process of linseed oil. They were identified in the IR spectrum of the linseed oil (25 ml) subjected to negative corona charge injection for 50 hours at around 3400 cm−1 (hydro-peroxide and/or alcohol vibrations), [3, 16]. The hydro-peroxides are not stable, cleave in time and form free radicals such as peroxyl (ROO•) and alkoxy (RO•) radicals that can attack a nearby double bond. In this way, the corresponding fatty acid is chemically linked to the fatty acid where the radical was present. This reaction consumes the attacked double bond seen at 1650 cm−1 (νC=C) and the free radicals are available and can propagate the polymerization reaction [3, 4, 16]. The bands of the C=C bonds at 1650 cm−1 are diminished and the generation of the oxidation products is identified by the appearance of the absorption band of C-O from around 950 cm−1. The C-O-C vibrational bands are also formed [3, 4, 16, 17].

In the positive corona discharge, the positive ions like N2+, O2

+, generated at the free surface of the linseed oil introduced in the discharge, affect the carboxylic functional group –COOH. The hydro-peroxide radicals formation, the C=C bonds consumption and the C-O-C bonds formation are revealed in the IR absorption spectrum presented in [4], just as in the negative-corona-discharge case.

In Fig. 3 are presented the IR spectra of the solid polymers generated by stretching a drop (10 µl) of linseed oil in a thin layer on a germanium support. The polymer layer obtained after two hours of charge injection in corona discharge cover the entire surface of the substrate (20 mm). Compared to the linseed oil dried in air after ten days [4], the IR spectra of the polymers present an increase in the intensity of the polymeric band at 1050–1150 cm−1 (νC-O-C) [17].

A. Groza 6 1232

4000 3500 3000 2500 2000 1500 1000 5000

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Tran

smitt

ance

[%]

w avenum ber [cm -1]

a)

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Tran

smita

nce

[%]

wavenum ber [cm -1]

b)

Fig. 3 – IR spectrum of linseed oil polymer layer obtained on a germanium support: a) after two hours

of negative corona charge injection; b) after two hours of positive corona charge injection [4].

3.2. OPTICAL INVESTIGATIONS OF POLYMER LAYERS

The linseed oil polymer layers obtained in negative respectively positive corona discharges, in the experimental conditions presented above, was investigated by an optical microscope with a 500× magnifications. In Fig. 4 can be observed that the polymer layer cover uniformly even the non uniform surface of the substrate [18].

Fig. 4 – Image of the linseed oil polymer layer [18].

3.3. IR SPECTRAL INVESTIGATIONS OF POLYMERS GENERATED FROM PDMS LIQUID PRECURSOR FILMS ON DIFFERENT SUBSTRATES

The processes involved in the polymerization of PDMS liquid precursors and those responsible for oxides generation on polymer surfaces and at the polymer/substrate interface in corona discharges have been studied by recording IR spectra of the polymer layers with an IR Perkin Elmer spectrometer equipped with a variable angle specular reflectance accessory. The angle of incidence light

7 Polymerization of organic and organosilicon liquid films 1233

on the sample (equal but opposite to the angle of reflected light) can be varied from 30° to 60° in order to perform depth profiling studies of the polymeric layers. The penetration depth of the light into the layer can thus be changed depending on the angle of incidence light on the sample. For an angle of reflection of 30°, the geometrical position of the variable angle specular reflectance accessory inside the IR spectrometer allows the investigation of the polymer/substrate interface. As the angle of reflection is increased up to 60° we can get information about molecules bonds arrangement in the polymer bulk and as close as possible to the layer surface. The depth profiling studies performed on polymer layers by IR spectroscopy, varying the angle of incident light on the sample from 40° to 60°, has been presented in [13]. It was shown that in the PDMS layers bulk and at their surfaces are formed some silicon oxides structures.

In order to investigate the interface zone of the PDMS with the substrate we recorded IR spectra at reflection angle of 300 for the liquid precursor of hydroxyl - terminated PDMS (0.1 µl), Fig. 5a and for the polymer obtained in negative corona discharges, using titanium as substrate material Fig. 5b [13]. As it can be seen in the figures, there are some additional absorption bands in the polymer, at 3300 cm–1

and 1660 cm–1 characteristic to O-H groups [19].

Fig. 5 – IR spectra of hydroxyl terminated polydimetylsiloxane: a) liquid precursor; b) polymer [13].

The presence of the hydroxyl groups in the polymer layers could be explained if we consider the injection of negative ions [14] such as – – –

2 3 4O , O , O , – –2 3CO , CO produced in corona discharges and of the water vapor from atmosphere

[20] at the free surface of the liquid precursor which acts as a virtual electrode.

A. Groza 8 1234

Also, some water molecules can be delivered to the polymer bulk due to the following polymerization mechanism in which the end groups of the PDMS liquid precursor are involved: -Si-O-H + -Si-O-H = Si-O-Si + H2O. No Ti –O or Si-O-Ti IR bands were observed in the IR spectra from Fig. 5.

By using as substrate material germanium (Fig. 6) [6], the IR, spectra of the PDMS layers revealed besides the formation of OH groups some new IR bands at 970 cm-1 characteristic to Si–OH groups [19], bands at 860 cm-1, 750 cm-1 and 570 cm-1 which are assigned to stretching and bending of Ge-O-Ge bonds [21], and band at 660 cm-1, which evidence the Si-O-Ge bonds formation [22]. The Si-OH groups present in the polymer layers obtained after 2 hours of negative corona charge injection can perform strong bonds to the substrate surface and favor the Si-O-Ge bond formation.

In the IR spectra of the PDMS layer obtained in positive corona discharge, Fig. 6 (red line) there are no OH or Si-OH vibrational bands. The germanium oxide formation is olso indicated by the Ge-O-Ge vibrational band observed at 570 cm-1.

The IR spectral investigations showed that the germanium oxides present in the polymer layers generated both in positive and negative corona discharges do not affect the polymerization process [6].

4000 3500 3000 2500 2000 1500 1000 50050

60

70

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Ref

lect

ance

[a.u

]

wavenumber [cm -1]

δGe-O-Ge

νGe-O-Ge

Fig. 6 – IR spectra of: PDMS layers obtained after 2 hours of charge injection in negative corona

discharge (black line) and positive corona discharge (red line) for a 300 reflection angle on a germanium substrate starting from a hydroxyl terminated liquid film precursor [6].

When the polymers were generated from hydroxyl – terminated PDMS precursors on Al substrates in negative corona discharges, in their IR reflectance spectra are identified besides OH groups, new IR bands at around 900 cm-1 and 642 cm-1 which can be assigned to the formation of Al-O and Si-O-Al bonds

9 Polymerization of organic and organosilicon liquid films 1235

respectively [23]. Intensities of these bands increase with the intensity of the corona discharge current as can be observed in Fig.7. For a corona discharge current intensity of 10 µA, the first peak is very weak and the second one is not visible.

It means that during the polymerization process of PDMS at polymer/substrate interface the oxidation of the aluminum substrate occurs too. The formation of Si-O-Al bonds at polymer/substrate interface can increase the adherence of the layers to the substrate.

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Al-O-Si bondsformation

Al-O bonds formation

642 cm-1

900 cm-1

Ref

lect

ance

[%]

wavenumber [cm-1]

I = 10 µA I = 20 µA I = 30 µA I = 40 µA

Fig. 7 – IR spectrum of polymers obtains at r = 30°: function of negative corona

discharge current intensities [13].

In both cases of Ge and Al substrates, the water present in the polymers as a result of the polymerization process of PDMS or obtained from the atmosphere water vapor can be decomposed under the influence of corona electric fields and associated charges injection generating OH radicals in the liquid and consequently in the polymer bulk. So, ions produced in negative corona discharges and OH radicals present in the material bulk can oxidize the substrate surface and generate germanium respectively aluminum oxides by a process similar to anodization [24].

It is well known that in the classical electrochemical anodization process of Al, the OH radicals and negative ions of oxygen are responsible for the oxidation of the metal/oxide interface and the generation of aluminum oxides on Al substrates. The current density passing across the oxide film depends on electric field intensity [20]. So, the Al-O and Si-O-Al IR bands identified in Fig. 7 and their intensity current dependence can be assigned to the initiation of an anodization process of aluminum and to the formation of aluminum oxides at polymer/substrate

A. Groza 10 1236

interface. This process involves not only the charged species generated in corona discharges but also the Si-OH groups of PDMS precursor which can perform strong bonds to the substrate generating Si-O-Al [13].

3.4. GD-OES INVESTIGATIONS OF POLYMERS GENERATED FROM PDMS LIQUID PRECURSOR FILMS ON DIFFERENT SUBSTRATES

The IR spectral results regarding the formation of oxides on the free surface of the PDMS layers and at the polymer/substrate interface in negative corona discharges were confirmed by distribution of the Si, O, C, H, Al, Ti, Ge in their elemental depth profiles done with Glow Discharge Optical Emission Spectrometry (GD OES) [6, 13]. In [25] in the GDOES depth profile of the organic and PDMS based polymer layers it was identified the presence of nitrogen atoms. They are not involved in any bond generation of the polymer structure.

In this technique a representative area of the polymer layer (typically 4 mm in diameter) is sputtered by a pulsed RF Ar plasma. The sputtered atoms from the polymer are then excited by inelastic collisions in the plasma and the emitted light is monitored in real time providing the elemental composition of the polymer sample as a function of the depth in layer. The GD OES used in the experiment was a GD Profiler 2 from Horiba Jobin Yvon. The operating conditions were: specified in [6, 13].

In Fig. 8 a, b, c the peaks on Si and O depth profiles show the silicon oxides structures formation at polymer surface. The fact that C and H present similar depth profiles means that C and H atoms are linked to silicon oxides structures. So, the Si, C, O, H depth profiles indicates that the formation of silicon oxides structures on the surfaces of PDMS layers take place all the time and the substrates nature do not count when the polymers are generated from hydroxyl terminated liquid precursors.

The Si, C, O, H depth profiles differ for Ti, Ge and Al substrates in the zone of the polymer/substrate interface.

In Ti case, Fig. 8a, due to its reduced interaction with oxygen, hydroxyl radicals and water at room temperature in air, similar Si, C, O, H profiles curves are obtained for polymer components on it free surface, in the bulk and at substrate interface. The steep rise in the aluminum depth profile curve and the decreasing of the Si, O, C, and H signals indicates the polymer/substrate interface.

In Ge case, Fig. 8b, at polymer- Ge interface can be observed a hump on Si, C, Ge and also a peak on the O depth profile curve. There are no humps on the H depth profile. These humps on Si, C and Ge depth profiles indicate a dominant polymer bonds accumulation at the polymer/Ge interface and a germanium oxide formation.

In Fig. 8c, at the polymer/Al interface, one can observe a hump on the O depth profile curve. There are not such humps on the Si, C, and H depth profiles.

11 Polymerization of organic and organosilicon liquid films 1237

The change of sputtering rates from layer to layer characteristics of GD- OES multilayer analysis cannot simply explain this phenomenon which can on the other hand be an indicator of the presence of Al oxides at the polymer/Al substrate interface respectively of Ge oxides at polymer/Ge interface.

In Fig. 8c, it can also be observed a simultaneous increase of the oxygen and aluminum signals during the decreasing of the Si depth profile curve. Due to the roughness of the aluminum oxide layer formed at the polymer substrate interface the ending point of the polymer layer cannot be exactly established. The roughness of the Al oxide layer usually results in the surface ridges which are sputtered more rapidly than the valleys [26]. The aluminum substrate is then removed markedly faster at the peaks of the surface than the surrounding oxide present over the surface valleys, due to the difference of sputtering yield between aluminum and aluminum oxides, therefore contributing to selective material removal [26].

The variation in sputtering rate during elemental depth profiling of PDMS polymers generated on Ti, Ge and Al substrates does not allow an accurate conversion of sputtering time into sputtered depth [13].

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.)

time (s)

Si O C H Ti

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.)

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I (a.

u.)

time (s)

Si O C H Al

a) b) c)

Fig. 8 – Elemental depth profile of a polymer layer deposed in negative corona discharges at I = 10 µA on: a) Ti; b) Ge; c) Al substrates starting from a hydroxyl terminated polydimethylsiloxane liquid precursor

[6, 13].

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3.5. SEM INVESTIGATIONS OF PDMS LAYERS

The features of the samples surface morphology of PDMS layers generated on Ti and Al substrates was examined using the scanning electron microscopy (SEM). Images were acquired with a FEI Inspect S scanning electron microscope in both high- and low-vacuum modes

Fig. 9 shows images of the surfaces of the polymer layers deposited under negative corona discharge currents of 10 µA on Ti respectively Al substrates, using as liquid precursors hydroxyl terminated PDMS. The layer surfaces do not present any Benard cells. They show the presence of silicon oxides structures due to their preponderant generation on polymer surfaces as it was evidenced by IR and GDOES spectral measurements. The density of these structures depends on the substrate material. It is higher on polymer surfaces generated on titanium substrates, Fig. 9a, than on polymers deposited on aluminum ones (Fig. 9b). This can be simply explained if we consider anodization as a concurrent process to silicon oxides formation [13].

a) b)

Fig. 9 – SEM images of polymer surfaces generated from hydroxyl terminated PDMS liquid precursor on: a) Ti; b) Al substrate [13].

After the chemical removal of PDMS layers generated on Al substrates for two different time periods in negative corona discharges from the hydroxyl terminated liquid precursors, the surfaces of Al oxide layers were investigated by SEM. In Fig.10 a, b can be observed that the aluminum oxide layers are porous, the pores size increasing with the duration of the running of the corona discharge [27].

13 Polymerization of organic and organosilicon liquid films 1239

a) b)

Fig. 10 – SEM images of anodized aluminum layers obtained in negative corona discharges after the removal of PDMS layers from their surfaces. The duration of the running of the corona discharge was:

a) 2h; b) 6h [27].

3.6. EDS INVESTIGATIONS

In order to evidence the importance of the end groups (vinyl respectively hydroxyl end group) of PDMS liquid precursor in the anodization process of Al substrate, we measured by energy dispersion X-ray spectroscopy (EDS) the spectra of PDMS layers and their atomic percentages (Fig. 11). The measurements were

0.0 0.5 1.0 1.5 2.0 2.5 3.0

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14000

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C K

O KAl K

Inte

nsity

(a.u

.)

E(keV) Fig. 11 – EDS spectra of polymer layers generated in negative corona discharges from a) vinyl

terminated PDMS liquid precursor; b) hydroxyl terminated PDMS liquid precursors; c) EDS spectrum of the anodized Al layer [13, 27].

A. Groza 14 1240

performed at low voltage (5 kV) in order to avoid the substantial presence of metal atoms in the spectrum. Although, due to the diffusion of the aluminum oxides formed at polymer/substrate interface in the layer bulk, the C, O and Si atoms lines intensity distribution in the spectrum Fig. 11a is different from that observed in Fig. 11b. In Fig. 11c it is presented the EDS spectrum of an anodized layer obtained after the removal of polymer layer formed on it surface [13, 27].

4. CONCLUSIONS

In this paper, it was demonstrated that the negative and positive corona discharge in air at atmospheric pressure is a convenient physical method for the generation of organic and organosilicon polymeric layers.

Unlike the common low-rate oxidation process, the corona-induced polymerization process of linseed oil is very fast because of the extra-activation of the groups involved in the peroxydation reactions. The linseed oil polymers obtained under corona charge injection in both electrode polarities were in solid state and characterized by the presence of C-O-C bands in the IR spectra [4, 18].

The polymerization of PDMS in atmospheric pressure air corona discharges were investigated as a function of electrode polarities, substrate material (Al, Ti, Ge) and of the liquid precursor end groups (vinyl or hydroxyl terminated PDMS) [6, 13, 27].

The anodizing of Al substrates take place only during the polymerization time duration of PDMS with hydroxyl end groups due to H2O and OH radicals generated in the polymer layer bulk, the Si-OH groups being responsible of Si-O-Al bond formation at polymer/substrate interface. The formation of Al2O3 and of possible aluminosilicates type compounds at polymer/substrate interface and its linkage to the polymer chains by Si-O-Al bonds were evidenced by IR and GDOES spectral investigation. The chemical composition of the polymers derived from EDS analysis shows also the formation of aluminum oxides [13, 27].

The fact that Ti substrate it is not anodized during the polymerization time period of PDMS with hydroxyl end groups indicates the failure of negative oxygen ions to penetrate the substrate, thus they being involved only in the polymerization process [13, 27].

The IR, GDOES, and SEM measurements showed also the presence of some silicon oxides structures in the polymer bulk preponderantly on the free surface of the PDMS layer [13, 27].

Acknowledgements. This work is supported by UEFISCDI projects with the following codes: PN II –ID-PCE-2011-3-0958 and PT-PCCA-2011-3.1-1136.5.

15 Polymerization of organic and organosilicon liquid films 1241

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