4He+ Ion Beam Irradiation Induced Modification of Poly

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Heþ Ion Beam Irradiation Induced Modification of Poly(dimethylsiloxane). Characterization by Infrared Spectroscopy and Ion Beam Analytical Techniques 4

R. Huszank,*,† D. Szikra,‡ A. Simon,† S. Z. Szilasi,† and I. P. Nagy‡ † ‡

Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary Department of Physical Chemistry, University of Debrecen, P.O. Box 7, H-4010 Debrecen, Hungary ABSTRACT: In this study we investigated the chemical and surface wettability changes of poly(dimethylsiloxane) (PDMS) induced by a 2.0 MeV Heþ beam irradiation. The chemical changes created in PDMS were characterized by universal attenuated total reflectance infrared (UATR-FTIR) spectroscopy, while the changes of the wettability were determined by contact angle measurements. In a separate analysis, hydrogen depletion was also investigated with a 1.6 MeV Heþ beam by applying the elastic recoil detection analysis (ERDA) and Rutherford backscattering spectrometry techniques simultaneously. The ERDA results showed that the hydrogen content of PDMS decreased irreversibly, which means that volatile products were formed under radiolysis, such as hydrogen or methane. The results were completed with UATR-FTIR measurements. We propose a complete reaction mechanism for the processes taking place in PDMS. These ion beam induced processes, such as chain scissions, crosslinking, and depletion of small molecular weight fragments, lead to the formation of a silica-like final product (SiOx). The significant chemical changes at the surface influence the wettability of PDMS, making it considerably more hydrophilic. The penetration depth of the 2.0 MeV Heþ ions is significantly higher compared to that of other surface modification techniques, which makes the modified layer thick and homogeneous; on the other hand, it is easily controllable by the energy of the incident ions.

1. INTRODUCTION Poly(dimethylsiloxane) (PDMS) is a promising polymer base material for several applications nowadays. This material is a silicon-based, cross-linkable, flexible polymer; it has attractive properties, such as low surface energy, constant and high ductility over a wide range of temperatures, high electrical resistance, long-term endurance, thermal curing property, etc. It is optically clear, chemically inert, stable, nontoxic, and biocompatible. It has been widely used to fabricate micro-electromechanical systems (MEMSs), microoptical and microfluidic devices,13 biosensors, biochips or microstamps,4,5 and antifouling coatings.6 Because of its remarkable gas permeability and biocompatibility, it can be particularly suitable for the integration of biological and biomedical applications.7 Usually, when applying a polymer, its surfaces needs to be modified to improve its properties, such as wettability or adhesion. Thus, the modification of technically relevant polymers such as PDMS is a growing research field. Surface modification can be achieved by the conversion of the polymer surface from hydrophobic to hydrophilic, without any etching or physical structuring. The surface energy of PDMS against water is high, so in MEMSs or microfluidic applications surface wettability has to be enhanced to allow fluids to flow smoothly in small channels.8 Another important field of application is controlled cell growth on polymer surfaces;9 hence, the understanding of cellsubstrate interactions is essential for biomedical and bioanalytical r 2011 American Chemical Society

applications. The hydrophobicity of silicone rubbers provides a high surface resistivity and water repellence. The initially hydrophobic surface becomes hydrophilic after exposure to electrical discharges, and it then loses its water repellence. There are large numbers of studies describing modifications of the inert and hydrophobic PDMS surfaces. Several techniques are available for the modification of the surface energy, such as laser beam, electron beam, or γ-ray irradiations,10 corona discharges,11 and UV irradiation12 in combination with ozone1315 and oxygen plasma16 treatments. It was found that the hydrophilic surfaces of modified PDMS are not stable in time. After exposure to plasma or UV treatment, a hydrophobic recovery occurs gradually.17,18 This recovery is caused by relaxation processes of polar groups at low degrees of oxidation. Another proposed explanation of this hydrophobic recovery is the gradual diffusion of free low molar mass siloxanes to the surface that migrate from the bulk.19 These free siloxanes are always present in the polymer network and can be formed by chain scission reactions as well. The main goal of this work is the detailed investigation of the effect of 2.0 MeV energy 4Heþ irradiations on the PDMS polymer. We focused on the characterization of the chemical Received: July 16, 2010 Revised: February 18, 2011 Published: March 14, 2011 3842

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Figure 1. Schematic view of the irradiation chamber.

changes in the polymer chain and the subsequent changes of the surface wettability. As a unique feature, we have investigated the hydrogen loss during the irradiation in situ. There are radiolysis studies on PDMS by γ- or e-beam irradiations and only a very few studies about ion beam irradiation.10,2022 The effect of ion beam irradiation on PDMS is still a developing topic; hence, it is very important for space or medical applications. On the other hand, compared to other surface modification techniques, for example, oxygen plasma or corona treatment, high- or medium-energy ions have deep penetration in materials (from several micrometers to several tens of micrometers) and the ions travel in an almost straight path, which allows the creation of homogeneous modified layers with controllable thickness (varying the energy of the incident ions). Furthermore, with a focused ion beam, it is possible to modify the surface of the polymers selectively, by creating microstructured modifications or micropatterns, as well.2,23 The chemical changes created in PDMS were characterized by universal attenuated total reflectance infrared (UATRFTIR) spectroscopy, while the changes of the wettability were determined by contact angle measurements. To follow the hydrogen content, elastic recoil detection analysis (ERDA) was applied.

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. The PDMS polymer samples were made by the commonly used Sylgard 184 elastomer kit from Dow Corning24 (base polymer, dimethylsiloxane oligomers with vinyl-terminated end groups, platinum catalyst, dimethylvinylated and trimethylated silica; curing agent (vulcanizer), dimethyl methyl hydrogen siloxane cross-linker and a tetramethyl tetravinyl cyclotetrasiloxane inhibitor). The base polymer and the curing agent were used in the recommended ratio of 10:1. The two components were thoroughly mixed and placed in an ultrasound bath for 5 min to remove the formed bubbles. Then the prepolymer was poured into a glass Petri dish, cured for 30 min at a temperature of 125 °C, and allowed to finish curing overnight at room temperature. During the curing process, the components undergo a hydrosilylation reaction in the presence of the platinum catalyst, resulting in cross-linking of the polymer chains. After curing, the PDMS polymer substrate (about 2 mm thick) was cut into 3 cm diameter disks. The disks were extracted in chloroform (CH3Cl) solvent twice to remove the unreacted, un-cross-linked monomers and curing agent from the network. The samples were dried in ambient conditions for 48 h before the irradiations. The density of the cured PDMS network was obtained by measuring the mass and volume of several pieces of PDMS by a pycnometer, and it was found to be 0.9836 g cm3. For the quantitative determination of the hydroxyl groups, hydroxy-terminated

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poly(dimethylsiloxane) was used as the standard material, obtained from Aldrich (viscosity 90150 cSt). 2.2. 4Heþ Ion Irradiation. All 2.0 MeV Heþ irradiations in this work have been performed in the PIXE chamber of HAS-ATOMKI, Debrecen, Hungary.25 Figure 1 shows the schematic view of the irradiation chamber. The target chamber is isolated,26 allowing the measurement of absolute current and charge. Charging was eliminated by using a carbon filament electron source. The energetic ions are produced by a 5 MV single-ended Van de Graaff accelerator. The beam was 5 mm in diameter, and it was made homogeneous using a collimator system and a 0.51 μm thin Ni foil. The irradiations were done in an ultrahigh-vacuum environment; the beam current was 20 nA. 2.3. Contact Angle Measurements. The contact angle measurements were done using the static sessile drop method with means of measuring the angles formed between the liquid/solid and the liquid/ vapor interfaces at the surface of the samples. This method is efficient and requires a low-cost, simple apparatus. In our experiments, a selfmade contact angle measurement setup was applied with a long focal length microscope attached with a SONY Hyper HAD (SSC-DC38P) camera. Deionized, Milli-Q water (∼18 MΩ cm1) was dropped (10 μL) on the original and the irradiated PDMS surfaces by a Finnpipette, and the resulting angle between the droplet and surface was then measured using a digital camera. The static contact angles were measured on both sides of the drops immediately after the irradiations.

2.4. Ion Beam Analysis with Simultaneous Elastic Recoil Detection and Rutherford Backscattering Spectrometry. For the hydrogen content measurements elastic ERDA and Rutherford backscattering spectrometry (RBS) methods were used with a focused microbeam in an Oxford-type nuclear microprobe facility at ATOMKI, Debrecen, Hungary.2729 The energy of the 4Heþ ion beam was 1600 keV, and the beam spot size was 2  2 μm2. The applied tilt angle was 80°; thus, the effective scan size was 2879  500 μm2. For the calculation of the total deposited fluence, the effective scan size was taken. The ERDA detector was placed at a recoil angle of 30° (IBM geometry) mounted with a 6 μm Mylar absorber and 1.1 mm wide vertical aperture. Two detectors collected the micro-RBS data; one of them was placed at a scattering angle of 165° at Cornell geometry, and the other one was set to 135° at IBM geometry.29 Data were collected by the OM_DAQ 2004 system30 in list mode, the files of which contain the entire events collected by the detectors during the measurement. To follow the decrease in the hydrogen content during the irradiation, the list mode files were further processed. One count (or signal) corresponds to one recoiled proton. At first, the numbers of the RBS and the ERDA counts were extracted from each data block together with the measurement parameters, such as deposited charge and elapsed time. The count rates of the ERDA and RBS signals were calculated using the time stamps of each data block. The charge was collected on the sample while the absolute total deposited charge value was determined from the RBS spectra as well for better accuracy. To be independent of any possible beam current fluctuation, the accumulated charge rate distribution was also calculated using the time stamps and the deposited charge in each data block. Finally, the calculated ERDA and RBS count rate distributions were normalized using the calculated charge rate distributions, which eliminates the error coming from the fluctuation of the beam current.

2.5. Universal Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. To determine the chemical changes of the functional groups at the surface of the polymer, infrared spectroscopic measurements were carried out with a diamond head PerkinElmer Spectrum One type UATR-FTIR spectrometer equipped with a DTGS detector (4 cm1 resolution). Equal amounts of force were applied to all samples during the measurements. The analyzing depth is around ∼6 μm. Before the measurements, the samples were covered and 3843

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Figure 2. Energy loss and the ion range distribution during stopping of 2.0 MeV Heþ ions in the PDMS polymer, calculated by the SRIM code.

Figure 3. Penetration depth of the Heþ ions (i.e., modified layer thickness) in PDMS as a function of the energy, calculated by the SRIM code. stored in Petri dishes at ambient conditions. The data were collected and analyzed using Spectrum ES 5.0 software.

2.6. Determination of the Penetration Depth of the Ions (Thickness of the Modified Layer). During ionmatter interactions, fast charged particles ionize the atoms or molecules while gradually losing their energy in many small steps. The stopping power is defined as the average energy loss of the particle per unit path length, and it depends on the type and energy of the particle and on the properties of the material it passes. In the beginning of the slowing process at high energies, the ion is slowed mainly by electronic stopping, and it moves almost in a straight path. The linear energy transport (LET) has a local maximum value around the end of the range region (Bragg peak). Computer simulation methods were developed a long time ago to calculate the behavior of ions in a material, and nowadays, this is the most common way of treating the stopping power. To simulate the interactions between the incident ions and a medium, we used the SRIM software package,3133 which concerns the stopping and range of ions in matter. This code calculates ion ranges on the basis of the binary collision approximation (BCA).34

3. RESULTS Figure 2 shows the ion range distribution and the energy loss profile during stopping of 2.0 MeV Heþ ions in PDMS calculated by the SRIM code. The results of the calculation of the ion range distribution show that the penetrating ions will stop around 11.9 μm deep in the polymer. The energy deposition profile slightly ascends until it reaches the maximum at about 8.4 μm depth (Bragg peak), and after that it decreases fast until reaching zero (where the ions stop). The starting energy deposition is already high enough to break any bond in the polymer, and its increase until the maximum is not significant, so the chemical changes in

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Figure 4. Surface contact angle versus the irradiation dose of 2.0 MeV Heþ.

the polymer caused by 2 MeV Heþ irradiation can be considered homogeneous until ∼10 μm depth (see Figure 2). The advantages of this ion beam induced modification method are that the modified layer thickness and the level of oxidation of the modified layer can be easily controlled with the energy and the dose of the incident ions. Figure 3 shows the penetration depth of the Heþ ions (i.e., modified layer thickness) in PDMS as a function of the energy, calculated by the SRIM code over a 1004000 keV range. 3.1. Measurements of the Surface Contact angle. Contact angle measurement is an easy, quick, and direct method for examining the hydrophilicity of the sample surface. We have used the static sessile drop method to determine the wettability of the Sylgard-184 polymer before and after the Heþ beam treatment. Figure 4 shows the contact angles of deionized water on Sylgard184 samples irradiated with a Heþ beam as a function of the irradiation dose (ions/cm2). The data show that with an increasing irradiation dose the contact angle decreases, so the initially hydrophobic substrate becomes more hydrophilic. A significant decrease occurred in the contact angle already at a low irradiation dose. After that initial decrease, the contact angle did not change significantly over a high range of doses, only after about 1.24  1015 ions/cm2 when it reached a plateau. This indicates that there is a dose limit for the modification; there is no further change in wettability with a dose greater than about 1.87  1015 ions/cm2. A very similar type of drastic decrease in contact angle was observed by other researchers upon exposure of PDMS to 254 nm excimer radiation.14 As has been described before by others,16,35,14 the exposure of PDMS to UV radiation, corona discharges, or oxygen plasma leads to the formation of hydroxyl groups and an inorganic, silicalike final product, which consists of SiOx (silicon atom bonded to three or four oxygen atoms). The initial significant increase in hydrophilicity of irradiated samples can be related to the formation of hydroxyl groups, which further increases until it reaches the stationary state, when the SiOx formation is more advanced. However, in our experiment, without the presence of molecular oxygen, the mechanism of oxidation is different (see Scheme 1), but the initial drastic increase in hydrophilicity is also related to the formation of a significant amount of hydroxyl groups. Furthermore, the stationary state in the contact angle correlates with the formation of the final SiOx product (which forms by leaving the methyl groups and breaking the main polymer chain during the irradiation). Theoretically, if this oxidized layer is continuous and compact enough and not cracked, it hinders the recovery of hydrophobicity.11 The available free volume for diffusion of 3844

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Langmuir low molar mass PDMS species is significantly reduced in the compact, oxidized layer; thus, the diffusion rate of these species from the bulk to the surface, and therefore the hydrophobicity recovery rate, is greatly hindered.16 While other surface modification methods, for example, oxygen plasma or corona treatment, led to the formation of an oxidized surface layer of only about several hundred nanometers, in our case the Heþ ion beam irradiation causes an orders of magnitude thicker, around 9 μm Scheme 1. Proposed Mechanism for the Reactions Taking Place in the PDMS Polymer Induced by a 2.0 MeV Heþ Ion Irradiation

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thick, homogeneous oxidized layer. This thick silica-like layer was compact, smooth, and without micrometer-scaled cracks. Some macroscopic cracks appeared on samples which received more than 6.24  1014 ions/cm2 after several days. Consequently, these results suggest that the hydrophobic recovery should be very limited on PDMS samples treated with a megaelectronvolt Heþ ion beam. 3.2. Infrared Spectroscopic Measurements. Under ion beam irradiation, besides the different physical effects, complex chemical processes can take place in organic materials. For initial steps, loss of the kinetic energy of the penetrating ions occurs by inelastic collisions, resulting in ionization and excitation of the target material. The excited-state molecules may return to the ground state through radiationless decay or undergo homolytic dissociation reactions to form free radicals. These free radicals are very reactive and cause a number of chemical reactions in polymers. The irradiation-induced chemical changes in PDMS caused by irradiation of different fluences of the 2.0 MeV Heþ beam were studied by UATR-FTIR spectroscopy. The initial PDMS polymer has a series of characteristic IR bands; for the recognized bands see Table 1 and Figure 5 (black line). The most intense bands are the CH3 rocking and SiC stretching (790 cm1), asymmetric SiOSi stretching (1060 cm1), symmetric CH3 deformations (1258 cm1), asymmetric SiCH3 stretching (2960 cm1), and SiOH stretching (∼3400 cm1). The IR spectrum of the initial PDMS polymer is in good accordance with previously published data.36,37 After the irradiation of the PDMS with different fluences of the 2.0 MeV Heþ beam, the irradiated areas were studied by UATRFTIR spectroscopy. Figure 5 shows the infrared spectra of the nonirradiated and the ion beam irradiated PDMS samples. The intensity of the absorption bands of the methyl (2960 cm1) and the SiCH3 (1258 cm1) groups decreased significantly. A new high-intensity, broad band appeared in the 32003500 and 1625 cm1 regions, and several less intensive bands appeared around 2928, 2876, and 1720 cm1. The broad absorption peak around 3400 cm1 indicates the formation of hydroxyl groups. The intensity of the OH stretch increased with the irradiation dose almost linearly. On the basis of the results of previous investigations, the majority of hydroxyl groups formed were bonded to Si atoms and not to carbon

Figure 5. ATR-FTIR spectra of the initial and irradiated PDMS polymer (Sylgard-184). 3845

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Table 1. Assignment of IR Spectra of the Irradiated PDMS Samples Shown in Figure 5 IR bands (cm1)

description

∼3400

OH stretching in SiOH and COH

2928, 2904 2960, 2876

asymmetric CH3 stretching in SiCH3 CH2 stretching in SiCH2

1720

CdO stretching

1625, 1357

CdC stretching, trans-alkene

1407

asymmetric CH3 deformation in SiCH3

1455

CH bending

1258

symmetric CH3 deformation in SiCH3

1060

asymmetric SiOSi stretching in [(CH3)2SiO]

900 790

SiO stretching in SiOH CH3 rocking and SiC stretching in SiCH3

Figure 6. Concentration of the formed hydroxyl groups as a function of the irradiation dose of the 2.0 MeV Heþ beam.

atoms.11 The very high intensity of the SiOH band, compared to that from other surface modification techniques (UVO, corona discharge, etc.), can be attributed to the high penetration depth of the 2.0 MeV Heþ ions causing a thick modified layer and the relatively high value of the LET during the stopping of Heþ ions (about 16 eV/Å) causing the breakage of the SiO main chain. The oxygen atom in the OH group could originate from the main chain only if the irradiations took place in a high vacuum, without the presence of molecular oxygen. In this case, when the main SiO chain broke, oxygen radicals were formed, which can react with the methyl groups, producing hydroxyl groups, while the formed methyl radicals will further react (see Scheme 1). On the other hand, the absorption band of SiOSi has decreased and broadened significantly, also confirming this. The raw estimation of the concentration of the formed hydroxyl groups was determined using silanol-terminated PDMS as a standard with a known OH group concentration. The concentration of the hydroxyl groups as a function of the irradiation dose can be seen in Figure 6. The concentration of the OH groups increases considerably, almost by 2 orders of magnitude, with increasing irradiation dose. In addition to the intensity shifts and broadening of the SiOSi absorption bands between 900 and 1200 cm1, a shoulder appears at 1150 cm1. On the basis of the theoretical calculation of the infrared spectrum of SiO2, this absorption band is the asymmetric stretching vibrations of the oxygen atoms in SiO2,36,38 which suggests the formation of the final irradiationinduced product, the inorganic silica-like SiOx species. The band around 1720 cm1 indicates the presence of a carbonyl group as is reported by using different techniques, such

as corona discharge, UV/ozone treatment, or laser irradiation.14,39,40 The mechanism of carbonyl group formation in the presence of molecular oxygen was mainly assumed to be the reaction of the oxygen with a methylene group, which previously formed by cross-linking of methyl groups,40 but without the presence of molecular oxygen, this mechanism cannot be realized in a high-vacuum environment. A possible formation mechanism of the carbonyl group is the recombination reaction of two previously formed radicals, such as an oxygen radical formed by the main chain scission and a methylene radical. After the recombination, bond rearrangement can result in the carbonyl group and silane derivative (see Scheme 1). The absorption bands of the CH3 and the SiCH3 groups decreased significantly with an increasing ion dose, which suggests a cross-linking and/or gas-yielding reaction mechanism. This result was also confirmed by the ERDA measurements indirectly (see later), with the detection of a significant hydrogen loss. It is well-known that cross-linking reactions also happened in PDMS by high-energy radiation. The bands appearing at about 2928 and 2904 cm1 correspond to the methylene group, which confirms that cross-linking reaction takes place. A high-intensity absorption band appeared at around 1625 cm1 and a lower intensity band at 1360 cm1 as well, which correspond to the carboncarbon double bond stretching vibration and to the trans-alkene deformation, respectively. This means that, after the cross-linking reaction, the irradiation causes further hydrogen loss, leading to the formation of double bonds. In conclusion, on the basis of the appearance and disappearance of bands, the most probable mechanism of the 2.0 MeV He ion induced degradation of PDMS is the formation of a significant amount of hydroxyl groups via the scission of the main chain and the reaction of the formed oxygen radical with the methyl groups and, furthermore, the formation of ethylene groups via the cross-linking reaction and of course many kinds of random recombinations. Finally, these processes and the leaving methyl groups lead to the formation of the inorganic, silica-like SiOx. For the suggested mechanism of the main processes taking place in PDMS by 2 MeV Heþ irradiation, using the mechanisms already proposed earlier,41,12 see Scheme 1. Periodic monitoring of the surface by infrared spectroscopy after the ion beam irradiations did not show any change in the spectra for up to 5 months. 3.3. Elastic Recoil Detection Analysis Study: Hydrogen Loss Determination. We are able to determine the absolute hydrogen concentration of PDMS with the elastic recoil detection analysis technique. The ion beam irradiation and the hydrogen measurement were implemented at the same time in situ. We followed the change of the hydrogen content of PDMS by measuring the rate of the recoiled hydrogen atoms, which were extracted from the list mode files of the ERDA measurements. This count rate distribution can be seen in Figure 7. The H count rate is in a linear correlation with the hydrogen content. Please note that a constant H rate distribution means the same detected count numbers in each time interval, i. e., a constant hydrogen content. The hydrogen loss takes place by an irreversible process of homolytic scission of CH bonds. As can be seen in the section “Infrared Spectroscopic Measurements”, the mechanism of the 2.0 MeV Heþ irradiation induced chemical processes consists of more reaction steps which lead to the formation of hydrogen; afterward it can leave from the bulk to the vacuum. The hydrogen 3846

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’ ACKNOWLEDGMENT We acknowledge Dr. Mihaly Novak (ATOMKI-HAS) for his valuable contribution to the H-loss calculations. This work was supported by the European Union cofunded Economic Competitiveness Operative Programme (Grant GVOP-3.2.1.-2004-040402/3.0). ’ REFERENCES

Figure 7. Normalized hydrogen count rate distributions as a function of the deposited ion beam fluence.

content decreases with increasing ion fluence. The irradiation at fluences below 2  1013 ions/cm2 leads to desorption of less than 10% of the hydrogen. For ion fluences above 1  1014 ions/cm2 the amount of hydrogen falls to ∼30% of its initial value in untreated PDMS. It is clearly seen in Figure 7 that the H loss is already significant at relatively low ion beam fluence. The distribution is fitted well with an exponential function, which suggests quasi-first-order reaction kinetics. We can see that the slope of the hydrogen loss rate decreases to 4  1014 fluence, which is in correlation with the decrease of the methyl groups (see Figure 5). We can see that the formation rate of the OH groups is lower than the H loss rate or the degradation rate of the methyl groups. On one hand, the methyl group degradation happens both directly by irradiation and through the formation of the OH group, while the OH group formation is a two-step process (see Scheme 1). On the other hand, the SiO bond energy is higher than the SiC or CH bond energy, so the bond scissions of SiO take place less efficiently, even if the energy deposition of the stopping ions is more than enough to break any bond in the polymer (average ∼16 eV/Å).

4. CONCLUSIONS The chemical changes of the PDMS polymer caused by the 2.0 MeV Heþ ion beam were investigated. Irradiation of PDMS by the 2 MeV Heþ ion beam causes a rapid and controllable surface oxidation of the polymer. By the ATR-FTIR measurements we found that the absorption bands of the CH3 and the SiCH3 groups are decreased irreversibly. The absorption band of SiOSi has decreased, as well, which means that the scission of the main chain of PDMS has occurred, processes which lead to the formation of a significant amount of hydroxyl groups, cross-linking, and even carbonyl groups. The ERDA measurements showed that the hydrogen content decreased irreversibly, which means that volatile products formed under radiolysis, such as hydrogen or methane. These ion beam induced oxidation processes cause the formation of a silicalike, inorganic final product (SiOx). The formation of a significant amount of OH groups and the degradation of the methyl groups influenced the wettability of PDMS, making it considerably more hydrophilic. Due to the deep penetration depth of the accelerated ions, the oxidized layer is thick and homogeneous compared to that from other surface modification techniques. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: þ36 (52) 509200, ext 11360. Fax: þ36 (52) 416181.

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dx.doi.org/10.1021/la200202u |Langmuir 2011, 27, 3842–3848