Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics

Transcription

1 IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 40 (2007) doi: / /40/23/021 Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics R Morent 1, N De Geyter 1, F Axisa 2, N De Smet 3, L Gengembre 4, E De Leersnyder 2, C Leys 1, J Vanfleteren 2, M Rymarczyk-Machal 3, E Schacht 3 and E Payen 4 1 Department of Applied Physics, Research Unit Plasma Technology, Faculty of Engineering, Ghent University, Jozef Plateaustraat 22, 9000 Ghent, Belgium 2 Department of Electronics and Information Systems, TFCG Microsystems, Technologiepark 914, Grote Steenweg Noord, 9052 Ghent, Belgium 3 Department of Organic Chemistry, Biomaterials Research Group, Faculty of Sciences, Ghent University, Krijgslaan 281 S4, 9000 Ghent, Belgium 4 Unité de Catalyse et Chimie du Solide, UMR CNRS 8181, Université des Sciences et Technologies de Lille, Bât. C3, Cité Scientifique, Villeneuve d Ascq, France Rino.Morent@ugent.be Received 11 September 2007, in final form 15 October 2007 Published 16 November 2007 Online at stacks.iop.org/jphysd/40/7392 Abstract Currently, there is a strong tendency to replace rigid electronic assemblies by mechanically flexible and stretchable equivalents. This emerging technology can be applied for biomedical electronics, such as implantable devices and electronics on skin. In the first step of the production process of stretchable electronics, electronic interconnections and components are encapsulated into a thin layer of polydimethylsiloxane (PDMS). Afterwards, the electronic structures are completely embedded by placing another PDMS layer on top. It is very important that the metals inside the electronic circuit do not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, resulting in poor adhesion properties. Therefore, in this paper, PDMS films are plasma treated with a dielectric barrier discharge (DBD) operating in air at medium pressure (5.0 kpa). Contact angle and XPS measurements reveal that plasma treatment increases the hydrophilicity of the PDMS films due to the incorporation of silanol groups at the expense of methyl groups. T-peel tests show that plasma treatment rapidly imparts adhesion enhancement, but only when both PDMS layers are plasma treated. Results also reveal that it is very important to bond the plasma-treated PDMS films immediately after treatment. In this case, an excellent adhesion is maintained several days after treatment. The ageing behaviour of the plasma-treated PDMS films is also studied in detail: contact angle measurements show that the contact angle increases during storage in air and angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS species to the surface. 1. Introduction It is believed that, in the near future, many electronic assemblies on rigid substrates will be replaced by mechanically flexible or even stretchable alternatives. This is a consequence of the ambient intelligence vision where citizens carry along more and more electronic systems near the body, on the body or even inside the body. These systems should not lead to a decrease in comfort for the user and should therefore be compact and light-weight [1]. Moreover, the electronic assemblies should be soft, stretchable and elastic, so that they can take the shape of the object in which they are integrated to guarantee maximal comfort. Typical examples are implants, intelligent textiles, portable electronic equipment (e.g. mobile phones, navigation systems, etc), robotics, etc [2]. Stretchable electronics consist of a stretchable substrate material with embedded electronic components connected by electronic interconnections. Since little or no stretchable electronic components are available, it is vital to engineer electronic interconnections which are stretchable. Today /07/ $ IOP Publishing Ltd Printed in the UK 7392

2 Adhesion enhancement by a dielectric barrier discharge of PDMS Figure 1. Process sequence for the fabrication of stretchable electronics. metals (copper, nickel and gold) are the best option to realize the interconnections with high performance and low cost. However, metals are not intrinsically stretchable; therefore, a suitable design such as a meander-shaped horseshoe structure is necessary. This structure can be reproducibly stretched and released many times without breaking [2]. Among the identified stretchable substrate materials appropriate for the fabrication of implantable electronic systems, polydimethylsiloxane (PDMS) is one of the most promising due to its specific mechanical properties (compliance, softness) and its good biocompatibility [3]. The general production process of the stretchable electronics is shown in figure 1. A photoresist is spin coated on a copper foil and patterned with the desired conductor shape. In the next step, a nickel seed layer followed by a 4 µm thick gold layer and a nickel-gold finish are electroplated and the photoresist is dissolved. The components are then assembled and connected to the interconnections by gluing or soldering. Then, the sample is overmoulded with viscous PDMS and thermally cured. The copper foil is removed, resulting in a thin film of PDMS with the interconnections and components at the surface. Finally, the electronic structures are encapsulated by placing another PDMS layer on top [1,4]. This technology has been developed in the frame of the Bioflex programme (IWT SBO-Bioflex (contract 04101)). The metals inside the electronic circuit should not leak out in order to obtain a highly biocompatible system. Therefore, an excellent adhesion between the 2 PDMS layers is of great importance. However, PDMS has a very low surface energy, which makes the formation of strong and permanent bonds to this material impossible without some kind of surface treatment [5]. Adhesive bonding of materials is being frequently utilized; however, this technique has some important limitations, such as a long bonding time and a low resistance to heat and chemicals [6]. Recent studies [7,8] have shown that bonding of polymers to polymers can be achieved without the necessity of any adhesive by treating the involved surfaces with a non-thermal plasma. Therefore, in this paper, PDMS films are treated with a dielectric barrier discharge (DBD) to enhance their adhesion. Over the last two decades, DBDs have received much attention on account of their many potential industrial applications. These discharges demonstrate great flexibility with respect to their geometric shape, working gas composition and operational parameters (power input, frequency of the driving voltage, pressure, gas flow, exposure time, etc) [9]. Usually, a DBD operates in the filamentary mode: the breakdown starts at many points, followed by the development of independent current filaments (named microdischarges). These microdischarges are of nanosecond duration and are randomly distributed over the dielectric surface [10 13]. With respect to a uniform surface treatment, a homogeneous discharge condition is very desirable and it has been demonstrated that a homogeneous DBD can be obtained under special, quite restrictive conditions [14 16]. However, in practice, it is generally believed to be difficult and tricky to reliably control such homogeneous DBDs. For many industrial applications, this latter behaviour is a severe disadvantage compared with the ready production of random filamentary discharges. In this work, an air DBD operating in the filamentary mode at medium pressure (5.0 kpa) is used to enhance the adhesion between two PDMS films. DBDs operating in air at medium pressure are shown to be very convenient for the modification of polymers and non-wovens, since they provide desirable surface characteristics without causing physical degradation [17, 18]. Moreover, it has been shown that the apparent inhomogeneity of the filamentary discharge does not lead to an operational disadvantage [9]. In this paper, the effects of the air DBD treatment at medium pressure on a PDMS film are studied using contact angle measurements and x-ray photoelectron spectroscopy (XPS). This work also investigates whether the changes induced by plasma treatment contribute to an improvement of the adhesion properties of the PDMS surfaces. In addition, the ageing behaviour of the plasma-treated PDMS films is elucidated and its effect on the adhesion behaviour is studied in detail. 2. Experimental 2.1. Materials The PDMS films are manufactured from a biomedical-grade silicone rubber kit (Silastic -MDX Dow Corning), consisting of an elastomer component and a curing agent. The elastomer component consist of a dimethylsiloxane polymer, a reinforcing silica and a platinum catalyst, while the curing agent component consists of a dimethylsiloxane polymer, an inhibitor and a siloxane crosslinker. The elastomer component and the curing agent from the kit are thoroughly blended in a 10 : 1 ratio after which the air bubbles are removed by vacuum. The silicone mix is placed into a mould (to produce 250 µm thick sheets) and afterwards, the silicone is cured in air by 7393

3 R Morent et al using a digital oscilloscope (Tektronix TDS MHz) and the required calculations are then performed Contact angle measurements Figure 2. Chemical structure of PDMS. The contact angles before and after plasma treatment are obtained using a commercial Krüss Easy Drop optical system (Krüss GmbH, Germany). This system is equipped with a software operated high-precision liquid dispenser to precisely control the drop size of the used liquid. The drop image is then stored, via a monochrome interline CCD video camera, using PC-based acquisition and data processing. Using the computer software provided with the instrument, measurement of the static contact angles is fully automated. In this work, distilled water drops of 2 µl are used as test liquid. The values of the static contact angles, shown in this paper, are obtained using Laplace Young curve fitting based on the imaged sessile water drop profile and are the average of eight values measured over an extended area of the treated samples XPS analysis Figure 3. Experimental set-up of the DBD (1 gas cylinder, 2 mass-flow controller, 3 plasma chamber, 4 pressure gauge, 5 needle valve, 6 pump). placing the mould in an oven of 150 (30 min). PDMS consists of an inorganic backbone of alternating silicon and oxygen atoms and methyl groups are attached to the silicon atoms forming the repeating unit in the polymer (see figure 2) DBD set-up A schematic diagram of the plasma configuration is depicted in figure 3. Two circular copper electrodes (diameter = 7 cm) are placed within a cylindrical enclosure. Both electrodes are covered with a glass plate (thickness = 2 mm) and the gas gap between the glass plates is 3 mm. The upper electrode is connected to an ac power source with a frequency of 50 khz and the lower electrode to earth through a resistor of 100. Between the electrodes, air (Air Liquide, Alphagaz 1) is fed into the system at a rate of 200 sccm. A rotary vane pump is attached to the gas outlet and the pressure in the chamber is maintained at 5.0 kpa by the use of a valve. During the experiments, a PDMS film is placed on the lower glass plate and plasma treatment is carried out for varying treatment times. The voltage applied to the electrodes is measured using a high voltage probe (Tektronix P6015A), whereas the discharge current is obtained by measuring the voltage over the resistor of 100, connected in series to ground. This resistor can be replaced by a capacitor of 100 nf; the voltage across this capacitor is then proportional to the charge stored on the electrodes. This latter measurement is widely used to obtain voltage-versus-charge plots, which form Lissajous figures [9]. From these figures, the electrical energy consumed per voltage cycle E el can be estimated since this value is equal to the area enclosed by the Lissajous figure. The electrical power P el can be obtained by multiplying the electrical energy with the frequency of the feeding voltage, which is 50 khz in this work [11]. All the above mentioned parameters are recorded XPS analysis is used to determine the chemical changes on the PDMS surfaces introduced by the plasma treatment. XPS measurements are carried out on a VG Escalab 220 XL system (Thermo Fisher Scientific, USA), using non-monochromatic Mg K α radiation (hν = ev) operated at 15 kv and 20 ma. The pressure in the analysing chamber is maintained at 10 7 Pa or lower during analysis and the size of the analysed area is 8 mm 8 mm. The high-resolution spectra are taken in the constant analyser energy mode with a 40 ev pass energy and the value of ev of the C1s core level is used for calibration of the energy scale. Depth profiling can be obtained using XPS by measuring the surface at different angles towards the photoelectron analyzer. The total sampling depth of the XPS measurement is given by the following equation [19]: d = 3λ sin θ, (1) where λ is the attenuation length of electrons in the polymer film and θ the angle between the photoelectron emission direction and the plane of the sample. The attenuation length is kinetic energy dependent and the kinetic energy of the Si2p photoelectrons is ±1384 ev. Using the TTP-2M equation [20], the attenuation length in this case is ±3.6 nm. Using equation (1), the sampling depth ranges between 0.94 nm (θ = 5 ) and 10.8 nm (θ = 90 ) Adhesion test: T-peel test Polymer-to-polymer peel test species are prepared according to the international standard ISO [21]. PDMS samples are carefully cut into specimens with a width equal to 10 mm and a length of 60 mm. In the next step, two PDMS films are bonded together without any adhesive over a length of 50 mm. The two unbounded ends of the flexible films are then bent in opposite directions until each end is perpendicular to the bonded assembly to form a T-shaped specimen, as shown in figure 4. After this, the two unbounded ends are mounted into the jaws of an Instron 5543 mechanical tester. The mechanical 7394

4 Adhesion enhancement by a dielectric barrier discharge of PDMS tester is set in motion with a separation rate of 1 mm min 1 and the applied force versus the distance of the grip separation is recorded. 3. Results 3.1. Electrical characterization of the DBD As previously mentioned, the most common electrical diagnostic of a DBD consists of the measurement of the voltage applied to the electrodes and the resultant discharge current. Figure 5 shows the waveforms of the high voltage used to drive the discharge and of the discharge current, obtained with the PDMS film placed on the lower glass plate. Numerous short peaks superimposed on the capacitive current can be seen in figure 5. These peaks are an indication of the microdischarge activity and every peak corresponds to a series of microdischarges [10 13]. Therefore, one can conclude that the used DBD operates in the filamentary mode. Figure 6 depicts the voltage-versus-charge plot or the so-called Lissajous figure. As stated before, this Lissajous figure can be used to calculate the electrical power, which in this case is equal to 6.6 W. Figure 4. The T-peel test sample geometry (1 bounded area, 2 direction of pull) [adapted from [21]] Plasma treatment: contact angle measurements Contact angle measurements represent the easiest and quickest method for examining the properties of surfaces. In figure 7, the evolution of the water contact angle on the PDMS films is presented as a function of plasma treatment time. As seen in figure 7, the contact angle of the PDMS film changes from for the untreated sample to the lowest value 14.4 after 3 s of plasma treatment. However, when the PDMS film is treated longer than 3 s, the contact angle does not change anymore as a function of treatment time. This shows that there is a saturation of the plasma effect on the PDMS film. An important fact to highlight is that the contact angles, which are measured over an extended area of the surface, show a standard deviation of about 2, which is within experimental error. This implies that the surface treatment is quite uniform at micrometre scales, despite the fact that the discharge operates in the filamentary mode. The large decrease in the contact angle after plasma treatment demonstrates the strongly increased wettability induced by the air DBD treatment, suggesting that the surfaces contain an increased number of hydrophilic groups after treatment Plasma treatment: XPS results XPS measurements can be used to elucidate information about the various chemical functionalities present in the unmodified and the modified PDMS samples. The XPS data shown in this section are acquired with an angle of 90 between the photoelectron emission direction and the plane of the sample, which means that the sampling depth is approximately 10.8 nm. The plasma-treated PDMS films are placed in the XPS system within 2 min after treatment and the XPS measurements are performed in air within 1 h after treatment. XPS analysis can reveal information about the chemical composition of the PDMS surfaces before and after plasma treatment. Table 1 shows the atomic composition of an untreated PDMS film and a PDMS film after 1 s and 5 s of plasma treatment. As shown in table 1, the oxygen content gradually increases with increasing exposure time, while the carbon content decreases. The silicon content remains essentially constant at 30 31%. Figure 5. High voltage and current waveforms of the air DBD at medium pressure. 7395

5 R Morent et al Figure 6. Lissajous figure of the air DBD at medium pressure. Figure 7. Evolution of the contact angle on the PDSM film as a function of treatment time. Table 1. Atomic composition of an untreated PDMS film and plasma-treated PDMS films after 1 and 5 s of plasma treatment. Treatment time (s) O (at.%) C (at.%) Si (at.%) To obtain further insight into the molecular changes taking place during plasma treatment, curve resolution of the Si2p peak is performed. The full width at half maximum (FWHM) of all the fitted Si2p components is kept constant at ±1.6 ev. Figure 8 shows the Si2p peaks of the untreated PDMS sample (a), the PDMS film after 1 s of plasma treatment (b) and the PDMS film after 5 s of plasma treatment (c). The Si2p peak of the untreated sample can be resolved into 2 peaks according to the method developed by O Hare et al [22]: a peak at ev (C1), which can be associated with silicon bound to two oxygen atoms and a peak at ev (C3) due to silicon bound to four oxygen atoms. This latter peak is present in the spectrum of the unexposed sample due to the reinforcing silica present in the elastomer component of the rubber kit (see section 2.1). After 1 second of plasma treatment, one can see that the peak at ev decreases, while the peak at ev increases. Moreover, a new peak at ev (C2) appears, which can be attributed to silicon bound to three oxygen atoms. After 5 s of plasma treatment, the peak at ev decreases even more, while the peak at ev further increases. Moreover, the peak at ev disappears after 5 s of plasma treatment. The concentration of the different silicon bounds before and after plasma treatment can be seen in table 2. Within the first second of treatment, the number of oxygen atoms bounded to silicon is increased from two to three and even four. With increasing treatment time, the oxidation process proceeds resulting in the presence of a high concentration of silicon bound to four oxygen atoms. The above results suggest the formation of linkages between Si and O atoms on the plasma treated PDMS samples: the plasma treatment leads to the incorporation of silanol (Si-OH) groups at the expense of methyl (CH 3 ) groups. These silanol groups make the exposed surface highly hydrophilic since they are polar in nature [8]. 7396

6 Adhesion enhancement by a dielectric barrier discharge of PDMS Figure 8. Si2p peaks of the untreated PDMS sample (a), the PDMS film after 1 s of plasma treatment (b) and the PDMS film after 5 s of plasma treatment (c). Table 2. Concentration of the different silicon bounds on PDMS before and after plasma treatment. Treatment Time (s) (%) (%) (%) Plasma treatment: adhesion testing To test the adhesion behaviour of the plasma-treated PDMS samples, a T-peel test is performed, as discussed in detail in section 2.5. To perform the T-peel test, PDMS films are cut into test specimens with a width of 10 mm and a length of 60 mm. Afterwards, different samples are prepared by bonding two PDMS films without any adhesive over a length of 50 mm: a sample where two untreated PDMS films are bonded together (sample A), a sample where 1 untreated and 1 plasma-treated (treatment time = 5 s) PDMS film is bonded together (sample B), a sample where 2 plasma-treated films (treatment time = 1 s) are bonded together (sample C) and a sample where 2 plasma-treated films (treatment time = 5 s) are bonded together (sample D). For the plasma-treated PDMS films, bonding occurs within 30 s after treatment. Figure 9 shows the load displacement curves of these prepared samples: when the load remains constant with increasing displacement, peeling of the two PDMS layers occurs. This constant load F(N)can be used to calculate the peel strength (N m 1 ) using the following equation [23]: peel strength = 2F/W, (2) where W is the specimen width in metres. The calculated peel strengths are shown in table 3 for the samples A, B, C and D, suggesting that the peel strength is not enhanced when only one PDMS layer is plasma treated. In contrast, plasma treating both PDMS layers results in a highly increased peel strength of up to 840 N m 1 after 5 s of plasma treatment and this without the use of any adhesive Ageing effect: contact angle measurements Different authors [5, 24 26] have stated that the increase in surface hydrophilicity is only temporary: if a PDMS surface which has become hydrophilic after plasma treatment is left under suitable conditions, the surface can regain its original hydrophobicity. This process is referred to as hydrophobic recovery or ageing process. Therefore, the ageing behaviour of plasma-treated PDMS films is investigated in detail in this paper. After 5 s of plasma treatment, PDMS samples are stored in air at room temperature and at a relative humidity of (60 ± 2)%. Figure 10 shows the evolution of the water contact angle on the plasma-treated PDMS films as a function of storage time. It is observed that the ageing process is characterized by a quick increase in contact angle during the first hours of storage. At longer storage times, the contact angle increases more slowly and finally reaches a plateau value after 5 days of ageing. It should be highlighted that this plateau value (82 ) is lower than the for an untreated sample, which means that a part of the surface wettability is kept after 6 days of storage in air Ageing effect: XPS analysis To investigate what chemical changes occur on the PDMS surfaces during the ageing process, XPS analysis is performed on aged samples. Table 4 shows the elemental composition of the plasma-treated PDMS films (treatment time = 5s) immediately after treatment and after 5, 20 and 48 h of storage in air. These data are acquired with an angle of 90 between the photoelectron emission direction and the plane of the 7397

7 R Morent et al Figure 9. Load displacement plots for a sample where both PDMS layers are untreated (sample A), a sample where only one layer is plasma treated during 5 s (sample B), a sample where both layers are plasma treated during 1 s (sample C) and a sample where both layers are plasma treated during 5 s (sample D). Table 3. Calculated peel strengths of different PDMS samples (a sample where both PDMS layers are untreated (sample A), a sample where only one layer is plasma treated during 5 s (sample B), a sample where both layers are plasma treated during 1 s (sample C) and a sample where both layers are plasma treated during 5 s (sample D)). Sample Peel strength (N m 1 ) A 0 B 0 C 668 D 840 sample, which means that the sampling depth is approximately 10.8 nm. As seen in table 4, the elemental composition of the top 10.8 nm remains practically constant after storage in air. Only after 48 h of ageing a small decrease in oxygen content and a small increase in carbon content can be found. Despite XPS measurements indicating that a large fraction of the oxidized groups is still present in the plasma-treated samples after storage in air, contact angle measurements suggested otherwise. This apparent anomaly can be explained by the relative surface sensitivity of both techniques; in the former technique, elemental information is obtained from a depth of 10.8 nm, while in the latter case, surface sensitivity is of the order of 1 nm [3]. In order to correlate the XPS and contact angle measurements, angle-resolved XPS was performed, by changing the angle between the photoelectron emission direction and the plane of the sample (take-off angle). By decreasing this angle, the penetration depth of the XPS experiments can be decreased, as stated before in section 2.5. Table 5 shows the used take-off angles and the according penetration depths. The elemental composition of the plasma-treated PDMS films after 5 and 48 h of storage in air is shown in table 6 for the different take-off angles. These results show that after 5 h of storage in air, the top 0.94 nm has an elemental composition almost equal to the untreated PDMS, while after 48 h of storage, the top 1.88 nm has an elemental composition equal to the untreated PDMS film. These results suggest that during the ageing behaviour migration of free PDMS chains through the oxidized layer occurs, resulting in the formation of a thin untreated PDMS layer at the top which is responsible for the increased hydrophobicity. This migration process was also observed by Hillborg and Gedde [25] who studied the hydrophobic recovery of PDMS after exposure to corona discharges in dry air Ageing effect: adhesion testing As mentioned in section 3.5, hydrophobic recovery of the PDMS films occurs after storage in air. Therefore, it is necessary to investigate whether the adhesion between two plasma-treated PDMS layers is also subjected to ageing. Different test specimens are prepared where both PDMS layers are plasma treated during 5 s. Both layers are immediately bounded after plasma treatment and the adhesion behaviour is tested immediately after treatment and after 2, 4 and 6 days. (samples A, B, C and D, respectively). Figure 11 shows the load displacement curves for the different samples, which can be used to calculate the peel strength, as shown in table 7. This table shows that the peel strength remains constant at approximately 800 N m 1, even after 6 days. This reveals that an excellent adhesion between the two PDMS layers remains for several days after treatment. The adhesion behaviour of 2 plasma-treated PDMS films stored in air is also studied in detail. Therefore, different PDMS films are plasma treated during 5 s and are stored in air during 1, 2, 4 and 6 days. After storage, two aged PDMS films are bonded together to perform the T-peel test. Results show that even after 1 day of ageing in air, no adhesion (peel strength = 0Nm 1 ) occurs between two aged PDMS films. Therefore, one can conclude that is it very important to bond the two PDMS films immediately after plasma treatment. 7398

8 Adhesion enhancement by a dielectric barrier discharge of PDMS Figure 10. Evolution of the water contact angle on the plasma-treated PDMS films as a function of storage time. Table 4. Elemental composition of the plasma-treated PDMS films (treatment time = 5 s) immediately after treatment and after 5, 20 and 48 h of storage in air. Ageing time (h) O (at.%) C (at.%) Si (at.%) Discussion The objectives of this research were to study the surface modifications of PDMS induced by air DBD treatment and to investigate the adhesion properties of PDMS after plasma treatment. In this study, several techniques have been used to investigate the consequences of plasma treatment on PDMS films. From the obtained results, it has emerged that plasma treatment can strongly enhance the surface wettability of PDMS: the contact angle can be decreased from for the untreated sample to 14.4 after 3 s of plasma treatment. The principal chemical changes induced by air plasma treatment is the replacement of pendant CH 3 groups by silanol (Si-OH) groups [8]. These silanol groups are polar in nature and make the surface highly hydrophilic, as observed by contact angle measurements. T-peel tests have shown that the peel strength between two untreated PDMS films is equal to 0 N m 1 and this peel strength is not increased when only one PDMS layer is plasma treated. However, when both PDMS layers are plasma treated, the peel strength can be increased to 840 N m 1 without the use of any adhesive. This can be explained as follows: when two PDMS layers are brought in contact, the silanol groups condense with those on another surface, resulting in the formation of Si O Si bonds. These covalent bonds form the basis of a tight irreversible seal between two PDMS layers [8]. When only one PDMS layer is plasma treated, the formation of these Si O Si bonds is impossible, explaining the poor adhesion. Ageing of plasma-treated polymers has been investigated by several authors [27 29] and the chemical changes are often found to be either partially or totally reversible as the time Table 5. XPS take-off angles and according XPS penetration depths. Take-off angle ( ) Penetration depth (nm) Table 6. Elemental composition of the plasma-treated PDMS films after 5 and 48 h of storage in air for different take-off angles. Ageing time Take-off angle O C Si (h) ( ) (at.%) (at.%) (at.%) from plasma treatment increases. It has been established that during the ageing process the induced polar chemical groups re-orientate into the bulk of the material to reduce the surface energy [30]. Contact angle measurements have shown that the contact angle of the plasma-treated PDMS films gradually increases during storage in air. However, XPS results with a take-off angle of 90 indicate that a large fraction of the polar silanol groups is still present in the modified region of the aged PDMS samples. This apparent anomaly can be explained by the relative surface sensitivity of the two techniques: using XPS with a take-off angle equal to 90, elemental information was obtained from a depth of about 10.8 nm, while with contact angle measurements the surface sensitivity is of the order of 1 nm [3]. Angle-resolved XPS revealed that the ageing behaviour is not due to the re-orientation of the induced polar silanol groups, but due to the migration of free PDMS chains through the oxidized layer. The oxidized surface layer, which gradually recovered hydrophobicity, was gradually coated with a thin layer of low molar mass PDMS. T-peel tests have shown that plasma-treated PDMS films stored in air and bounded 7399

9 R Morent et al Figure 11. Load-displacement curves of two plasma-treated PDMS films bonded immediately after treatment and tested immediately after treatment and after 2, 4 and 6 days (samples A, B, C and D, respectively). Table 7. Calculated peel strengths of different PDMS samples immediately bonded after plasma treatment and tested immediately after treatment and after 2, 4 and 6 days (samples A, B, C and D respectively). Sample Peel strength (N m 1 ) A 840 B 784 C 774 D 821 together after several days of storage have a peel strength close to 0 N m 1. Therefore, it is very important that the plasma-treated PDMS layers are bonded together immediately after plasma treatment. Moreover, it is shown in this paper that an excellent adhesion between two plasma-treated PDMS layers bonded together immediately after treatment is sustained several days after plasma treatment. 5. Conclusion In this work, it is shown that an air DBD treatment can greatly enhance the hydrophilicity of PDMS, since plasma treatment replaces the hydrophobic CH 3 groups by hydrophilic OH groups. T-peel tests reveal that plasma treatment of two PDMS layers significantly increases the adhesion between them due to the formation of strong covalent Si O Si bonds. When only one PDMS layer is plasma treated, the adhesion is not enhanced due to the impossibility to form the covalent Si O Si bonds. This work also shows that the surface hydrophilicity of the plasma-treated PDMS films is lost with time after plasma treatment. Angle-resolved XPS reveals that this hydrophobic recovery is due to the migration of low molar mass PDMS to the surface. Results also show that it is very important to bond the plasma-treated PDMS films immediately after treatment, since PDMS films bonded after 1 day of storage in air show no adhesion. Taking into account the above results, one can conclude that the air DBD is a very efficient tool to enhance the adhesion between two PDMS layers. Currently, it is being investigated if this adhesion enhancement is sufficient to prevent leakage of metals into the human body. References [1] Axisa F, Brosteaux D, De Leersnijder E, Bossuyt F, Gonzalez M, De Smet N and Vanfleteren J 2007 Proc. IEEE Polytronic 2007/6th Int. IEEE Conf. on Polymers and Adhesives in Microelectronics and Photonics (Tokyo, Japan) pp [2] Brosteaux D, Axisa F, Gonzalez M and Vanfleteren J 2007 IEEE Electron. Device Lett [3] Williams R L, Wilson D J and Rhodes N P 2004 Biomaterials [4] Brosteaux D, Axisa F, Vanfleteren J, Carchon N and Gonzalez M 2006 Proc. MRS Spring Meeting (San Fransisco, USA) [5] Fritz J L and Owen M J 1995 J. Adhes [6] Katzenberg F 2005 e-polymers no 060 [7] Jo B H, Van Lerberghe L M, Motsegood K M and Beebe D J 2000 J. Microelectromech. Syst [8] Bhattarcharya S, Datta A, Berg J M and Gangopadhyay S 2005 J. Microelectromech. Syst [9] Borcia G, Anderson CAandBrownNMD2005 Plasma Sources Sci. Technol [10] Pietsch G J 2001 Contrib. Plasma Phys [11] Wagner H-E, Brandenburg R, Kozlov K V, Sonnenfeld A, Michel P and Behnke J F 2003 Vacuum [12] Kogelschatz U 2003 Plasma Chem. Plasma Process [13] Kogelschatz U, Eliasson B and Egli W 1997 J. Physique IV France [14] Kanazawa S, Kogoma M, Moriwaki T and Okazaki S 1988 J. Phys. D: Appl. Phys [15] Okazaki S, Kogoma M, Uehara M and Kimura Y 1993 J. Phys. D: Appl. Phys [16] Trunec D, Brablec A and Stastny F 1998 Contrib. Plasma Phys [17] De Geyter N, Morent R and Leys C 2006 Surf. Coat. Technol [18] De Geyter N, Morent R, Leys C, Gengembre L and Payen E 2007 Surf. Coat. Technol [19] Hillborg H and Gedde U W 1999 IEEE Trans. Dielectr. Electr. Insul [20] Tamuna S, Powell C J and Penn D R 1994 Surf. Interface Anal

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