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Singularities in hydrophobic recovery of plasma treated polydimethylsiloxane surfaces under non-contaminant atmosphere
Sensors and Actuators A 197 (2013) 25–29
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Singularities in hydrophobic recovery of plasma treated polydimethylsiloxane surfaces under non-contaminant atmosphere Jalal Bacharouche, Hamidou Haidara, Philippe Kunemann, Marie-France Vallat, Vincent Roucoules ∗ Institut de Sciences des Materiaux de Mulhouse, IS2M – C.N.R.S. – UMR 7361 – UHA, 15, Rue Jean Starcky, 68057 Mulhouse Cedex, France
a r t i c l e
i n f o
Article history: Received 7 December 2012 Received in revised form 19 March 2013 Accepted 2 April 2013 Available online 13 April 2013 Keywords: Argon plasma Polydimethylsiloxane Hydrophobic recovery Contact angles Surface dynamics
a b s t r a c t Herein, the hydrophobic recovery of argon plasma treated polydimethylsiloxane surfaces is explored. In contrast to previous works, environmental contamination is here taken into account. We find that diffusion and reorientation strongly dominate the hydrophobic recovery under high thermal activation (60 ◦ C), no matter the surrounding environment during storage. However, at lower temperature (24 ◦ C and below), we find that contamination plays a major role in lab atmosphere environment. By working at low temperature and under inert nitrogen atmosphere to slow down the diffusion and reorientation dynamics and to avoid contamination, we identify two different temperature-dependent regimes in the kinetics of the hydrophobic recovery. One is fast and involves exclusively relaxation processes occurring in the surface region. The second, much slower, concerns diffusion phenomena in the sub-surface region. Thereby, the specific impact of bulk diffusion and surface reorientation processes can be distinguished during aging by slowing down the surface dynamics and by eliminating all possible sources of contamination. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Polydimethylsiloxane (PDMS) substrates possess many advantages like biocompatibility [1,2], thermal stability [3,4], low toxicity [5,6] and high optical transparency [7], which make it an attractive material. This substrate is used for adhesives, lubricants, as well as in defoaming agents, damping fluids, implants, cosmetics and other applications [8]. The inherently low surface energy and chemical inertness have generally required elaborate surface modification schemes to optimize favorable bonding interactions. PDMS surfaces can be directly and easily activated by plasma, resulting in the introduction of polar functional groups on the surface [9–15]. The ease of bonding of oxidized PDMS to other substrates has simplified the fabrication of biomedical or microfluidic devices. But among the interesting phenomena with plasma treatments are the aging and the relaxation processes of the induced surface modifications and still not well known [16]. Probably the most extensively studied aspect in this field is the hydrophobic recovery of plasma treated PDMS surfaces evaluated by water contact angle (CA) measurements [17,18]. During plasma treatment, the surface is driven away from its thermodynamic equilibrium by developing concentration gradients of polar groups in the surface region or in the sub-surface region. After treatment, the modified surface reconstructs in order to minimize its surface energy and to return to an equilibrium state
∗ Corresponding author. Tel.: +33 0389608782. E-mail address: [email protected] (V. Roucoules). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.04.003
[18]. This entails migration of non-modified low molecular weight species from the bulk to the near surface region, the migration of oxidized low molecular weight from the top most layer inside the bulk, the migration of unmodified low molecular weight macromolecules towards the surface, the reorientation of the bulk polar chemical groups especially that near the surface and even the relaxation of the surface roughness [18,19]. The kinetic of this surface dynamics is affected by many factors including plasma parameters (pressure, gas, time and power), molecular structure (mobility, glass transition, degrees of crystallinity and crosslinking) and storage conditions [17] (temperature, humidity and dielectric constant of environment). Surprisingly, the role of environment contamination during aging is rarely discussed whereas this can dominate the hydrophobic kinetics, especially when the plasma treatment results in the formation of highly reactive surfaces. In this case, surface deactivation (decrease of hydrophilicity) proceeds through the combined effect of the relaxation processes and organic contamination. Besides, although temperature-dependent relaxation processes (mainly reorientation and diffusion) have been generally proposed to account for the hydrophobic recovery of plasma treated surfaces, the characteristic time scales associated to these processes and their assessment through standard CA measurements still remain poorly studied and understood. In this report, the impact of bulk diffusion and surface reorientation is investigated and both impacts decoupled using experimental conditions that significantly reduce all possible sources of environment contamination. PDMS specimens were exposed to radio-frequency (r.f.) argon plasma at 100 Pa pressure
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J. Bacharouche et al. / Sensors and Actuators A 197 (2013) 25–29
Fig. 1. (a) SEM image and (b) AFM image of the plasma treated PDMS surface at rest. A large fraction of the surface was composed of silica fillers or more likely of silica aggregates. In the AFM image the bright areas correspond to the aggregates-rich regions while the dark areas are related to the aggregates-poor regions. (c) AFM image of the plasma treated under elongation.
for 60 s. Then, we examine the competition between relaxation processes and surrounding contamination at three different temperatures (i.e. 4 ◦ C, 25 ◦ C and 60 ◦ C) by comparing the hydrophobic recovery in air and under inert nitrogen atmosphere. By working at low temperature and under nitrogen, two distinct temperaturedependent regimes in the kinetics of hydrophobic recovery were identified. The first and fast regime involves essentially processes which are confined to the near surface region. The second and more slower regime concerns the sub-surface region and involves the diffusion process that brings low molecular weight species at the surface.
2. Experimental 2.1. Materials Substrates of silicone (PDMS) were molded by Statice Santé SAS (France) using MED-4750 from NuSil Silicone Technology LCC (Carpinteria, CA 93013, USA).
2.2. Plasma treatment Plasma surface modifications were conducted using a radiofrequency (r.f.) inductive coupling plasma reactor (Plassys MDS 130) consisting in a cylindrical glass chamber (14.5 cm diameter and 5.5 L volume) enclosed in a Faraday cage. In the Ar plasma reactions the PDMS (with approximate dimensions of 15 mm × 45 mm × 0.3 mm) was placed into the plasma chamber at 27 cm from the gas inlet. The reactor was evacuated to 0.1 Pa, following by purging with Ar gas to the desired experimental pressure, typically 100 Pa. At this point, r.f. radiations were turned on (60 W) to induce plasma reactions during 1 min. Upon completion, the r.f. generator was switched off. Then, the system was vented up to atmospheric pressure and the sample was immediately removed from the reactor (the exposure time with air was less than 15 s) and stored under air or under nitrogen at three different temperatures (4 ◦ C, 24 ◦ C and 60 ◦ C).
2.3. Contact angle measurements Static contact angle measurements were carried out using a video capture apparatus (DSA100, Germany) with 2-l high-purity liquid drops (water). Measurements were made on both sides of the drop and were averaged. Each result is the average of five experiments.
2.4. Electronic microscopy The Scanning Electron Microscopy (SEM) observations were performed by using a FEI environmental microscope (Quanta 400 model, The Netherlands) working at 30 keV. The films were observed using low vacuum mode without metallization. 2.5. Atomic force microscopy AFM images were realized with a Dimension 3000 scanning probe microscope (Digital Instrument, United States). Silicon cantilevers (ARROW-NC from Nanoworld) were used for all measurements. The spring constant was 48 N/m. Typically the surface morphology of 10 m × 10 m areas near the center of each sample were observed in the tapping mode of the scanning probe microscope. 3. Results and discussion PDMS samples are exposed to a low pressure argon plasma leading mainly to chain scissions and formation of silyl free radicals [18,20]. All experimental details are presented in supporting information. In a previous study, we have shown the silyl radicals react mainly with oxygen of the atmosphere (after removal of the samples from the plasma chamber) to form an oxidized and hydrophilic layer whose structure is somewhere between Si(O,CH3 )O and SiO2 species [21]. SEM image (Fig. 1a) and AFM image (Fig. 1b) did not show any defects (i.e. buckling or cracks) at the plasma treated surface when the PDMS substrate was kept at rest. However, because of the brittleness of the plasma treated layer, mechanical deformations readily underwent fragmentations at the PDMS surface (Fig. 1c). This point has been largely described in the literature. The aging of the plasma treated surface is followed here by contact angle measurements ONLY on the samples kept at rest meaning free of cracks. No change has been observed during aging by SEM and AFM analysis. Fig. 2 shows the contact angle values of water drops on plasma treated PDMS surfaces which were stored in air (Fig. 2a) or under inert nitrogen atmosphere (Fig. 2b) at three different temperatures, 4 ◦ C, 24 ◦ C and 60 ◦ C respectively. The initial contact angle value is measured between 1 and 3 min after plasma treatment and this time is considered as the time t0 for the aging on the graph. The contact angle values of the water drop which initially wets completely the surface ( 0 ∼ 5◦ ) increase during aging to a plateau value which depends on the storage temperature. This hydrophobic recovery is usually attributed to diffusion and molecular reorientation as depicted in Scheme 1a [22,23].
J. Bacharouche et al. / Sensors and Actuators A 197 (2013) 25–29
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Fig. 2. Contact angle values of water drops on plasma treated PDMS surfaces as a function of the aging time (a) under ambient conditions (air) and (b) under inert atmosphere (nitrogen).
The kinetics of this recovery can be described by an exponential function (t)∼(0 − ∞ ) exp(−t/) + ∞ where 0 is the initial contact angle at t0 , ∞ the contact angle value on the plateau of the hydrophobic recovery and the characteristic time of the hydrophobic recovery. It is worth noting here that ∞ ∼ virgin where virgin is the contact angle value measured on the native PDMS surface. To take an example, the fitting parameters estimated from the experimental water contact angle values obtained at 60 ◦ C and under nitrogen are 0 = 0◦ , ∞ = 102◦ and = 3 h. In conditions of aging, the contact angle values and thus the kinetics of the hydrophobic recovery depends strongly on the temperature (see Fig. 3) and the differences between ∞ and virgin are due to permanent surface modifications induced by the plasma treatment. These observations agree with previous findings [24,25]. At high temperature, the system gains energy simply by thermal activation which is proportional to ∼kB T (where kB is the Boltzmann constant and T is the temperature in Kelvin). The result is a temperature-dependent dynamic which impacts strongly the migration and angular reorientation of low molecular weight
chains and surface polar groups through the temperature dependence of the diffusion coefficient (both translational and angular), D(T ) ∼D0 exp(−Ea /kB T ) where Ea is the self-diffusion activation energy. At 60 ◦ C in particular, both aging atmospheres lead to regular, high magnitude and fast recovery kinetics. At this temperature, diffusion and reorientation dynamics strongly dominate the hydrophobic recovery and the contribution of contamination on the overall hydrophobic recovery can be considered as negligible, even in the case of contaminant air environment. On the other hand at lower temperatures (24 ◦ C and 4 ◦ C), not only the magnitude of the hydrophobic recovery decreases but the rate of diffusion and reorientation dynamics also decreases as D(T) while the contribution of the contamination in the overall recovery process increases. Besides this strong sensitivity of the hydrophobic recovery to temperature, our results clearly show the influence of the surrounding atmosphere on both the kinetics and magnitude of the recovery. In particular, this influence of the surrounding environment is well demonstrated in Fig. 2b by the occurrence of singularities under nitrogen (versus air) for the two lowest aging temperatures. These singularities show up as lag times in the
Scheme 1. Schematic representation of (a) the dynamic processes involved during aging in the surface region and in the sub-surface region, (b) the equilibrium state of the surface after aging in air and (c) the equilibrium state of the surface after aging in nitrogen.
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θVirgin
Contact Angle (°)
Under inert nitrogen atmosphere In air
Fig. 3. Contact angle values at the maximum of hydrophobic recovery for the three different aging temperatures in air and under inert nitrogen atmosphere.
recovery kinetics at 4 ◦ C and 24 ◦ C under nitrogen. These lag times have characteristic values of 24 ◦ C ∼10 h and 4 ◦ C ∼19 h respectively, as determined by extrapolating the experimental data. How to account for these characteristic lag times which are otherwise experimentally reproducible for these two low temperatures? We have shown previously that argon plasma lead to free radical formation developing a gradient of crosslinking in the sub-surface region [20]. As a rough assumption, we can consider the gradient to be composed of two regions: a highly crosslinked and oxidized topmost layer of thickness e, and a second one softer and less crosslinked sub-region. In the former topmost layer the low molecular weight chains are almost completely involved and immobilized in the highly crosslinking process resulting in a very low density of these free molecular weight chains. At the opposite, in the softer sub-region the low crosslinking rate leaves more free molecular weight chains which can therefore freely move in this sub-surface region. As a result, for these low molecular weight chains to contribute to the hydrophobic recovery (surface free energy minimization process), they have to cross the distance corresponding to the thickness e of the highly crosslinked topmost layer, from the soft sub-surface region. This directional diffusion over the distance e is associated to a characteristic time lag which scales following the Einstein relation, as c ∼e2 /D(T ) . Although strictly phenomenological, this relationship effectively shows first that a lag time for the occurrence of contribution of the low molecular weight chains to the hydrophobic recovery is required. In a second step, this lag time −1 increases as the inverse of D(T or equivalently as the inverse of the ) temperature. Our results are well supported by this phenomenological picture, the lag times for the two low aging temperatures being, respectively, 24 ◦ C ∼10 h and 4 ◦ C ∼19 h, while being experimentally too low to be observed for the higher aging temperature of 60 ◦ C. Aside this relaxation processes arising from the diffusion of low molecular weight chains, a second and simultaneous relaxation process involving mostly the reorientation of polar groups at the surface. At the difference of the aforementioned diffusion of short chains, the reorientation processes of surface polar groups occur within much shorter time scales, contributing to the hydrophobic recovery during the lag times and even reaching its plateau value over that lag time. As a result, the occurrence of the contribution of the low molecular chains coming from the sub-surface region, at the end of the lag time creates these singularities in the hydrophobic recovery kinetics.
During aging in air the contribution of the contamination can not be avoid and its effect on the hydrophobic recovery become predominant. At low temperatures (24 ◦ C and 4 ◦ C), the rate of diffusion and reorientation dynamics decreases while the contribution of the contamination in the overall recovery process increases. The effect of the contamination in the hydrophobic recovery occurs within shorter time scale compare to the effect of relaxation processes and finally the singularities (i.e. lag times) cannot be observed. 4. Conclusion In this work, the surface of PDMS is modified using low pressure argon plasma and the modified surfaces of PDMS are analyzed using contact angle measurements. In the case of aging in air or under inert nitrogen atmosphere, contact angle measurements show short-term hydrophilic stability upon temperature. Besides this strong sensitivity of the hydrophobic recovery to temperature, our results clearly show the influence of the surrounding atmosphere on both the kinetics and magnitude of the recovery. We have shown the occurrence of singularities under nitrogen for low aging temperatures. These singularities account for different population of relaxation processes occurring in two different regions in the plasma modified layer (a highly crosslinked and oxidized topmost layer and a second one softer and less crosslinked subregion) and at different scale times. The faster process concerns exclusively reorientation processes of polar groups at the surface of the highly crosslinked layer. The second, much slower, concerns diffusion processes of low molecular weight chains in the sub-surface region. References [1] J.H. Park, K.D. Park, Y.H. Bae, PDMS-based polyurethanes with MPEG grafts: synthesis, characterization and platelet adhesion study, Biomaterials 20 (1999) 943–953. [2] M.A. Sherman, J.P. Kenned, D.L. Ely, D. Smith, Novelpolyisobutylene/polydimethylsiloxane bicomponent networks: III. Tissue compatibility, J. Biomater. Sci. Polym. Ed. 10 (1999) 259–269. [3] L.V. Interrante, Q.H. Shen, J. Li, Poly(dimethylsilylenemethyleneA regularly alternating copolymer of co-dimethylsiloxane). poly(dimethylsiloxane) and poly(dimethylsilylenemethylene), Macromolecules 34 (2001) 1545–1547. [4] M. Dahrouch, A. Schmidt, L. Leemans, H. Linssen, H. Goetz, Synthesis and properties of poly(butylene terephthalate)-poly(ethylene oxide)poly(dimethylsiloxane) block copolymers, Macromol. Symp. 199 (2003) 147–162. [5] S.I. Ertel, B.D. Ratner, A. Kaul, M.B. Schway, T.A. Horbett, In vitro study of the intrinsic toxicity of synthetic surfaces to cells, J. Biomed. Mater. Res. 28 (1994) 667–678. [6] T.G. van Kooten, J.F. Whitesides, A.F. von Recum, Influence of silicone (PDMS) surface texture on human skin fibroblast proliferation as determined by cell cycle analysis, J. Biomed. Mater. Res. 43 (1998) 1–14. [7] W.C. Sung, C.C. Chang, H. Makamba, S.H. Chen, Long-term affinity modification on poly(dimethylsiloxane) substrate and its application for ELISA analysis, Anal. Chem. 80 (2008) 1529–1535. [8] G.M. Whitesides, E. Ostuni, S. Takayama, X. Jiang, D.E. Ingber, Soft lithography in biology and biochemistry, Ann. Rev. Biomed. Eng. 3 (2001) 335–373. [9] S. Befahy, P. Lipnik, T. Pardoen, C. Nascimento, B. Patris, P. Bertrand, S. Yunus, Thickness and elastic modulus of plasma treated PDMS silica-like surface layer, Langmuir 26 (2010) 3372–3375. [10] B. Olander, A. Wirsén, A.C. Albertsson, Silicone elastomers with controlled surface composition using argon or hydrogen plasma treatment, J. Appl. Polym. Sci. 90 (2003) 1378–1383. [11] S. Hemmilä, J.V. Gauich-Rodríguez, J. Kreutzer, P. Kallio, Rapid, simple, and costeffective treatments to achieve long-term hydrophilic PDMS surfaces, Appl. Surf. Sci. 258 (2012) 9864–9875. [12] D. Bodas, C. Khan-Malek, Hydrophilization and hydrophobic recovery of PDMS by oxygen plasma and chemical treatment – an SEM investigation, Sens. Actuat. B 123 (2007) 368–373. [13] L. Gengembre, N. De Geyter, C. Leys, R. Morent, F. Axisa, N. De Smet, L. De Leersnyder, J. Vanfleteren, M. Rymarczyk-Machal, E. Schacht, E. Payen, Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics, J. Phys. D: Appl. Phys. 40 (2007) 7392–7401. [14] N. De Geyter, R. Morent, T. Jacobs, F. Axisa, L. Gengembre, C. Leys, J. Vanfleteren, E. Payen, Remote atmospheric pressure DC glow discharge treatment for adhesion improvement of PDMS, Plasma Process. Polym. 6 (2009) S406–S411.
J. Bacharouche et al. / Sensors and Actuators A 197 (2013) 25–29 [15] S. Bhattacharya, A. Datta, J.M. Berg, S. Gangopadhyay, Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength, J. Microelectronic Syst. 14 (2005) 590–597. [16] J. Kim, M.K. Chaudhury, M.J. Owen, Modeling hydrophobic recovery of electrically discharged polydimethylsiloxane elastomers, J. Colloid Interface Sci. 293 (2006) 364–375. [17] I.J. Chen, E. Linder, The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces, Langmuir 23 (2007) 3118–3122. [18] S. Pinto, P. Alves, C.M. Matos, A.C. Santos, L.R. Rodrigues, J.A. Teixeira, M.H. Gil, Poly(dimethyl siloxane) surface modification by low pressure plasma to improve its characteristics towards biomedical applications, Colloids Surf. B 81 (2010) 20–26. [19] J. Kim, M.K. Chaudhury, M.J. Owen, Hydrophobic recovery of polydimethylsiloxane elastomers exposed to partial electrical discharge, J. Colloid Interface Sci. 293 (2006) 364–375. [20] V. Roucoules, A. Ponche, A. Geissler, F. Siffer, L. Vidal, O. Soppera, M.F. Vallat, P. Marie, J.C. Voegel, P. Schaaf, J. Hemmerlé, Changes in silicon elastomeric surface properties under stretching induced by three surface treatments, Langmuir 23 (2007) 13136–13145. [21] J. Bacharouche, P. Kunemann, P. Fioux, M.F. Vallat, J. Lalevée, J. Hemmerlé, V. Roucoules, A simple method for fabrication of filler-free stretchable polydimethylsiloxane surfaces, Appl. Surf. Sci. 270 (2013) 64–76. [22] H. Hillborg, M. Sandelin, U.W. Gedde, Hydrophobic recovery of polydimethylsiloxane after exposure to partial discharges as a function of crosslink density, Polymer 42 (2001) 7349–7362. [23] H. Hillborg, U.W. Gedde, Hydrophobic recovery of polydimethylsiloxane after exposure to corona discharges, Polymer 39 (1998) 1991–1998. [24] I.J. Chen, E. Lindner, The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces, Langmuir 23 (2007) 3118–3122. [25] N. De Geyter, R. Morent, C. Leys, Influence of ambient conditions on the ageing behaviour of plasma-treated PET surfaces, Nucl. Instrum. Methods Phys. Res. Sect. B 266 (2008) 3086–3090.
Biographies Dr. Jalal Bacharouche obtained his PhD degree in Chemistry of Materials from Haute-Alsace University, France in 2012. Subsequently he joined the National Centre of Scientific Research of Mulhouse. His research interests are smart material and structures, plasma polymerization, cell adhesion and protein adsorption.
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Dr. Hamidou Haidara has received a Master Degree in Solid State PhysicsCondensed Matter from Université L. Pasteur, Strasbourg-France in 1982, and a PhD in Physical-Chemistry from Université de Haute Alsace (UHA) Mulhouse-France in 1985, on a work on the “cohesion and adhesion of protective mineral and polymer coatings on steel”. After his PhD, he successively worked within a European (BRITE) project as a Postdoctoral Research Fellow on “thin metal coatings-on-polymer films”, and at the Dow Corning Corp. Research Centre in Midland-Michigan (USA), on “SelfAssembly Monolayers in Wetting and Adhesion”, working with Pr. M.K. Chaudhury. Since 1990 he works at the CNRS (Centre National de la Recherche Scientifique)France, where he currently has a senior research scientist position and leads the group on “Wetting and Self-Assembly Phenomena” at the Institut de Science des Matériaux de Mulhouse (IS2M)-CNRS/UHA, in Mulhouse. Dr. Haidara’s major fields of interest are on physics of interface phenomena, with special emphasis on wetting of complex fluids and media, wetting-mediated structure formation, self-organization and self-assembly of molecules, micro- and nanoobjects. Mr. Philippe Kunemann Degree of Chemical Engineer speciality analytical chemistry (Conservatoire National des Arts et Métiers de Paris). Surface analysis applied to the wetting study on self assembled monolayer and polymers, by wetting, infrared, X-ray diffraction, differential scanning calorimetry, AFM, Ellipsometry Analytical method development by wetting with development of experimental methods – existing relationships between adherence properties from copolymers and their surface properties. Dr. Marie-France Vallat obtained her PhD degree in Physical Science from the University of Haute-Alsace in Mulhouse (France) in 1983. She joined Professor Jacques Schultz’s team at the Centre de Recherches sur la Physico-Chimie des Surfaces Solides (CNRS) in Mulhouse where she started to study the relationship between surface and bulk properties of polymers and their adhesive behavior of assemblies. She has a special interest for elastomers and the creation of interphases that are the consequence of gradients of composition during joint formation. Dr. Vincent Roucoules is active in materials science for the past 10 years, mainly in polymers and surface treatments. After obtaining his doctoral degree in PhysicalChemistry from France, He worked as a post-doctoral researcher at Durham University in the Surface Science groups of Professor J.S.P. Badyal, England. During this period he was able to significantly increase his proficiency and knowledge of plasma chemistry. In consideration of his achievements in this field, he was offered the position of Assistant Professor at the Science Material Institute of Mulhouse, CNRS, France in 2002. His current area of research focuses on developing new responsive materials using plasma chemistry.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Singularities in hydrophobic recovery of plasma treated polydimethylsiloxane surfaces under non-contaminant atmosphere Jalal Bacharouche, Hamidou Haidara, Philippe Kunemann, Marie-France Vallat, Vincent Roucoules ∗ Institut de Sciences des Materiaux de Mulhouse, IS2M – C.N.R.S. – UMR 7361 – UHA, 15, Rue Jean Starcky, 68057 Mulhouse Cedex, France
a r t i c l e
i n f o
Article history: Received 7 December 2012 Received in revised form 19 March 2013 Accepted 2 April 2013 Available online 13 April 2013 Keywords: Argon plasma Polydimethylsiloxane Hydrophobic recovery Contact angles Surface dynamics
a b s t r a c t Herein, the hydrophobic recovery of argon plasma treated polydimethylsiloxane surfaces is explored. In contrast to previous works, environmental contamination is here taken into account. We find that diffusion and reorientation strongly dominate the hydrophobic recovery under high thermal activation (60 ◦ C), no matter the surrounding environment during storage. However, at lower temperature (24 ◦ C and below), we find that contamination plays a major role in lab atmosphere environment. By working at low temperature and under inert nitrogen atmosphere to slow down the diffusion and reorientation dynamics and to avoid contamination, we identify two different temperature-dependent regimes in the kinetics of the hydrophobic recovery. One is fast and involves exclusively relaxation processes occurring in the surface region. The second, much slower, concerns diffusion phenomena in the sub-surface region. Thereby, the specific impact of bulk diffusion and surface reorientation processes can be distinguished during aging by slowing down the surface dynamics and by eliminating all possible sources of contamination. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Polydimethylsiloxane (PDMS) substrates possess many advantages like biocompatibility [1,2], thermal stability [3,4], low toxicity [5,6] and high optical transparency [7], which make it an attractive material. This substrate is used for adhesives, lubricants, as well as in defoaming agents, damping fluids, implants, cosmetics and other applications [8]. The inherently low surface energy and chemical inertness have generally required elaborate surface modification schemes to optimize favorable bonding interactions. PDMS surfaces can be directly and easily activated by plasma, resulting in the introduction of polar functional groups on the surface [9–15]. The ease of bonding of oxidized PDMS to other substrates has simplified the fabrication of biomedical or microfluidic devices. But among the interesting phenomena with plasma treatments are the aging and the relaxation processes of the induced surface modifications and still not well known [16]. Probably the most extensively studied aspect in this field is the hydrophobic recovery of plasma treated PDMS surfaces evaluated by water contact angle (CA) measurements [17,18]. During plasma treatment, the surface is driven away from its thermodynamic equilibrium by developing concentration gradients of polar groups in the surface region or in the sub-surface region. After treatment, the modified surface reconstructs in order to minimize its surface energy and to return to an equilibrium state
∗ Corresponding author. Tel.: +33 0389608782. E-mail address: [email protected] (V. Roucoules). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.04.003
[18]. This entails migration of non-modified low molecular weight species from the bulk to the near surface region, the migration of oxidized low molecular weight from the top most layer inside the bulk, the migration of unmodified low molecular weight macromolecules towards the surface, the reorientation of the bulk polar chemical groups especially that near the surface and even the relaxation of the surface roughness [18,19]. The kinetic of this surface dynamics is affected by many factors including plasma parameters (pressure, gas, time and power), molecular structure (mobility, glass transition, degrees of crystallinity and crosslinking) and storage conditions [17] (temperature, humidity and dielectric constant of environment). Surprisingly, the role of environment contamination during aging is rarely discussed whereas this can dominate the hydrophobic kinetics, especially when the plasma treatment results in the formation of highly reactive surfaces. In this case, surface deactivation (decrease of hydrophilicity) proceeds through the combined effect of the relaxation processes and organic contamination. Besides, although temperature-dependent relaxation processes (mainly reorientation and diffusion) have been generally proposed to account for the hydrophobic recovery of plasma treated surfaces, the characteristic time scales associated to these processes and their assessment through standard CA measurements still remain poorly studied and understood. In this report, the impact of bulk diffusion and surface reorientation is investigated and both impacts decoupled using experimental conditions that significantly reduce all possible sources of environment contamination. PDMS specimens were exposed to radio-frequency (r.f.) argon plasma at 100 Pa pressure
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J. Bacharouche et al. / Sensors and Actuators A 197 (2013) 25–29
Fig. 1. (a) SEM image and (b) AFM image of the plasma treated PDMS surface at rest. A large fraction of the surface was composed of silica fillers or more likely of silica aggregates. In the AFM image the bright areas correspond to the aggregates-rich regions while the dark areas are related to the aggregates-poor regions. (c) AFM image of the plasma treated under elongation.
for 60 s. Then, we examine the competition between relaxation processes and surrounding contamination at three different temperatures (i.e. 4 ◦ C, 25 ◦ C and 60 ◦ C) by comparing the hydrophobic recovery in air and under inert nitrogen atmosphere. By working at low temperature and under nitrogen, two distinct temperaturedependent regimes in the kinetics of hydrophobic recovery were identified. The first and fast regime involves essentially processes which are confined to the near surface region. The second and more slower regime concerns the sub-surface region and involves the diffusion process that brings low molecular weight species at the surface.
2. Experimental 2.1. Materials Substrates of silicone (PDMS) were molded by Statice Santé SAS (France) using MED-4750 from NuSil Silicone Technology LCC (Carpinteria, CA 93013, USA).
2.2. Plasma treatment Plasma surface modifications were conducted using a radiofrequency (r.f.) inductive coupling plasma reactor (Plassys MDS 130) consisting in a cylindrical glass chamber (14.5 cm diameter and 5.5 L volume) enclosed in a Faraday cage. In the Ar plasma reactions the PDMS (with approximate dimensions of 15 mm × 45 mm × 0.3 mm) was placed into the plasma chamber at 27 cm from the gas inlet. The reactor was evacuated to 0.1 Pa, following by purging with Ar gas to the desired experimental pressure, typically 100 Pa. At this point, r.f. radiations were turned on (60 W) to induce plasma reactions during 1 min. Upon completion, the r.f. generator was switched off. Then, the system was vented up to atmospheric pressure and the sample was immediately removed from the reactor (the exposure time with air was less than 15 s) and stored under air or under nitrogen at three different temperatures (4 ◦ C, 24 ◦ C and 60 ◦ C).
2.3. Contact angle measurements Static contact angle measurements were carried out using a video capture apparatus (DSA100, Germany) with 2-l high-purity liquid drops (water). Measurements were made on both sides of the drop and were averaged. Each result is the average of five experiments.
2.4. Electronic microscopy The Scanning Electron Microscopy (SEM) observations were performed by using a FEI environmental microscope (Quanta 400 model, The Netherlands) working at 30 keV. The films were observed using low vacuum mode without metallization. 2.5. Atomic force microscopy AFM images were realized with a Dimension 3000 scanning probe microscope (Digital Instrument, United States). Silicon cantilevers (ARROW-NC from Nanoworld) were used for all measurements. The spring constant was 48 N/m. Typically the surface morphology of 10 m × 10 m areas near the center of each sample were observed in the tapping mode of the scanning probe microscope. 3. Results and discussion PDMS samples are exposed to a low pressure argon plasma leading mainly to chain scissions and formation of silyl free radicals [18,20]. All experimental details are presented in supporting information. In a previous study, we have shown the silyl radicals react mainly with oxygen of the atmosphere (after removal of the samples from the plasma chamber) to form an oxidized and hydrophilic layer whose structure is somewhere between Si(O,CH3 )O and SiO2 species [21]. SEM image (Fig. 1a) and AFM image (Fig. 1b) did not show any defects (i.e. buckling or cracks) at the plasma treated surface when the PDMS substrate was kept at rest. However, because of the brittleness of the plasma treated layer, mechanical deformations readily underwent fragmentations at the PDMS surface (Fig. 1c). This point has been largely described in the literature. The aging of the plasma treated surface is followed here by contact angle measurements ONLY on the samples kept at rest meaning free of cracks. No change has been observed during aging by SEM and AFM analysis. Fig. 2 shows the contact angle values of water drops on plasma treated PDMS surfaces which were stored in air (Fig. 2a) or under inert nitrogen atmosphere (Fig. 2b) at three different temperatures, 4 ◦ C, 24 ◦ C and 60 ◦ C respectively. The initial contact angle value is measured between 1 and 3 min after plasma treatment and this time is considered as the time t0 for the aging on the graph. The contact angle values of the water drop which initially wets completely the surface ( 0 ∼ 5◦ ) increase during aging to a plateau value which depends on the storage temperature. This hydrophobic recovery is usually attributed to diffusion and molecular reorientation as depicted in Scheme 1a [22,23].
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Fig. 2. Contact angle values of water drops on plasma treated PDMS surfaces as a function of the aging time (a) under ambient conditions (air) and (b) under inert atmosphere (nitrogen).
The kinetics of this recovery can be described by an exponential function (t)∼(0 − ∞ ) exp(−t/) + ∞ where 0 is the initial contact angle at t0 , ∞ the contact angle value on the plateau of the hydrophobic recovery and the characteristic time of the hydrophobic recovery. It is worth noting here that ∞ ∼ virgin where virgin is the contact angle value measured on the native PDMS surface. To take an example, the fitting parameters estimated from the experimental water contact angle values obtained at 60 ◦ C and under nitrogen are 0 = 0◦ , ∞ = 102◦ and = 3 h. In conditions of aging, the contact angle values and thus the kinetics of the hydrophobic recovery depends strongly on the temperature (see Fig. 3) and the differences between ∞ and virgin are due to permanent surface modifications induced by the plasma treatment. These observations agree with previous findings [24,25]. At high temperature, the system gains energy simply by thermal activation which is proportional to ∼kB T (where kB is the Boltzmann constant and T is the temperature in Kelvin). The result is a temperature-dependent dynamic which impacts strongly the migration and angular reorientation of low molecular weight
chains and surface polar groups through the temperature dependence of the diffusion coefficient (both translational and angular), D(T ) ∼D0 exp(−Ea /kB T ) where Ea is the self-diffusion activation energy. At 60 ◦ C in particular, both aging atmospheres lead to regular, high magnitude and fast recovery kinetics. At this temperature, diffusion and reorientation dynamics strongly dominate the hydrophobic recovery and the contribution of contamination on the overall hydrophobic recovery can be considered as negligible, even in the case of contaminant air environment. On the other hand at lower temperatures (24 ◦ C and 4 ◦ C), not only the magnitude of the hydrophobic recovery decreases but the rate of diffusion and reorientation dynamics also decreases as D(T) while the contribution of the contamination in the overall recovery process increases. Besides this strong sensitivity of the hydrophobic recovery to temperature, our results clearly show the influence of the surrounding atmosphere on both the kinetics and magnitude of the recovery. In particular, this influence of the surrounding environment is well demonstrated in Fig. 2b by the occurrence of singularities under nitrogen (versus air) for the two lowest aging temperatures. These singularities show up as lag times in the
Scheme 1. Schematic representation of (a) the dynamic processes involved during aging in the surface region and in the sub-surface region, (b) the equilibrium state of the surface after aging in air and (c) the equilibrium state of the surface after aging in nitrogen.
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θVirgin
Contact Angle (°)
Under inert nitrogen atmosphere In air
Fig. 3. Contact angle values at the maximum of hydrophobic recovery for the three different aging temperatures in air and under inert nitrogen atmosphere.
recovery kinetics at 4 ◦ C and 24 ◦ C under nitrogen. These lag times have characteristic values of 24 ◦ C ∼10 h and 4 ◦ C ∼19 h respectively, as determined by extrapolating the experimental data. How to account for these characteristic lag times which are otherwise experimentally reproducible for these two low temperatures? We have shown previously that argon plasma lead to free radical formation developing a gradient of crosslinking in the sub-surface region [20]. As a rough assumption, we can consider the gradient to be composed of two regions: a highly crosslinked and oxidized topmost layer of thickness e, and a second one softer and less crosslinked sub-region. In the former topmost layer the low molecular weight chains are almost completely involved and immobilized in the highly crosslinking process resulting in a very low density of these free molecular weight chains. At the opposite, in the softer sub-region the low crosslinking rate leaves more free molecular weight chains which can therefore freely move in this sub-surface region. As a result, for these low molecular weight chains to contribute to the hydrophobic recovery (surface free energy minimization process), they have to cross the distance corresponding to the thickness e of the highly crosslinked topmost layer, from the soft sub-surface region. This directional diffusion over the distance e is associated to a characteristic time lag which scales following the Einstein relation, as c ∼e2 /D(T ) . Although strictly phenomenological, this relationship effectively shows first that a lag time for the occurrence of contribution of the low molecular weight chains to the hydrophobic recovery is required. In a second step, this lag time −1 increases as the inverse of D(T or equivalently as the inverse of the ) temperature. Our results are well supported by this phenomenological picture, the lag times for the two low aging temperatures being, respectively, 24 ◦ C ∼10 h and 4 ◦ C ∼19 h, while being experimentally too low to be observed for the higher aging temperature of 60 ◦ C. Aside this relaxation processes arising from the diffusion of low molecular weight chains, a second and simultaneous relaxation process involving mostly the reorientation of polar groups at the surface. At the difference of the aforementioned diffusion of short chains, the reorientation processes of surface polar groups occur within much shorter time scales, contributing to the hydrophobic recovery during the lag times and even reaching its plateau value over that lag time. As a result, the occurrence of the contribution of the low molecular chains coming from the sub-surface region, at the end of the lag time creates these singularities in the hydrophobic recovery kinetics.
During aging in air the contribution of the contamination can not be avoid and its effect on the hydrophobic recovery become predominant. At low temperatures (24 ◦ C and 4 ◦ C), the rate of diffusion and reorientation dynamics decreases while the contribution of the contamination in the overall recovery process increases. The effect of the contamination in the hydrophobic recovery occurs within shorter time scale compare to the effect of relaxation processes and finally the singularities (i.e. lag times) cannot be observed. 4. Conclusion In this work, the surface of PDMS is modified using low pressure argon plasma and the modified surfaces of PDMS are analyzed using contact angle measurements. In the case of aging in air or under inert nitrogen atmosphere, contact angle measurements show short-term hydrophilic stability upon temperature. Besides this strong sensitivity of the hydrophobic recovery to temperature, our results clearly show the influence of the surrounding atmosphere on both the kinetics and magnitude of the recovery. We have shown the occurrence of singularities under nitrogen for low aging temperatures. These singularities account for different population of relaxation processes occurring in two different regions in the plasma modified layer (a highly crosslinked and oxidized topmost layer and a second one softer and less crosslinked subregion) and at different scale times. The faster process concerns exclusively reorientation processes of polar groups at the surface of the highly crosslinked layer. The second, much slower, concerns diffusion processes of low molecular weight chains in the sub-surface region. References [1] J.H. Park, K.D. Park, Y.H. Bae, PDMS-based polyurethanes with MPEG grafts: synthesis, characterization and platelet adhesion study, Biomaterials 20 (1999) 943–953. [2] M.A. Sherman, J.P. Kenned, D.L. Ely, D. Smith, Novelpolyisobutylene/polydimethylsiloxane bicomponent networks: III. Tissue compatibility, J. Biomater. Sci. Polym. Ed. 10 (1999) 259–269. [3] L.V. Interrante, Q.H. Shen, J. Li, Poly(dimethylsilylenemethyleneA regularly alternating copolymer of co-dimethylsiloxane). poly(dimethylsiloxane) and poly(dimethylsilylenemethylene), Macromolecules 34 (2001) 1545–1547. [4] M. Dahrouch, A. Schmidt, L. Leemans, H. Linssen, H. Goetz, Synthesis and properties of poly(butylene terephthalate)-poly(ethylene oxide)poly(dimethylsiloxane) block copolymers, Macromol. Symp. 199 (2003) 147–162. [5] S.I. Ertel, B.D. Ratner, A. Kaul, M.B. Schway, T.A. Horbett, In vitro study of the intrinsic toxicity of synthetic surfaces to cells, J. Biomed. Mater. Res. 28 (1994) 667–678. [6] T.G. van Kooten, J.F. Whitesides, A.F. von Recum, Influence of silicone (PDMS) surface texture on human skin fibroblast proliferation as determined by cell cycle analysis, J. Biomed. Mater. Res. 43 (1998) 1–14. [7] W.C. Sung, C.C. Chang, H. Makamba, S.H. Chen, Long-term affinity modification on poly(dimethylsiloxane) substrate and its application for ELISA analysis, Anal. Chem. 80 (2008) 1529–1535. [8] G.M. Whitesides, E. Ostuni, S. Takayama, X. Jiang, D.E. Ingber, Soft lithography in biology and biochemistry, Ann. Rev. Biomed. Eng. 3 (2001) 335–373. [9] S. Befahy, P. Lipnik, T. Pardoen, C. Nascimento, B. Patris, P. Bertrand, S. Yunus, Thickness and elastic modulus of plasma treated PDMS silica-like surface layer, Langmuir 26 (2010) 3372–3375. [10] B. Olander, A. Wirsén, A.C. Albertsson, Silicone elastomers with controlled surface composition using argon or hydrogen plasma treatment, J. Appl. Polym. Sci. 90 (2003) 1378–1383. [11] S. Hemmilä, J.V. Gauich-Rodríguez, J. Kreutzer, P. Kallio, Rapid, simple, and costeffective treatments to achieve long-term hydrophilic PDMS surfaces, Appl. Surf. Sci. 258 (2012) 9864–9875. [12] D. Bodas, C. Khan-Malek, Hydrophilization and hydrophobic recovery of PDMS by oxygen plasma and chemical treatment – an SEM investigation, Sens. Actuat. B 123 (2007) 368–373. [13] L. Gengembre, N. De Geyter, C. Leys, R. Morent, F. Axisa, N. De Smet, L. De Leersnyder, J. Vanfleteren, M. Rymarczyk-Machal, E. Schacht, E. Payen, Adhesion enhancement by a dielectric barrier discharge of PDMS used for flexible and stretchable electronics, J. Phys. D: Appl. Phys. 40 (2007) 7392–7401. [14] N. De Geyter, R. Morent, T. Jacobs, F. Axisa, L. Gengembre, C. Leys, J. Vanfleteren, E. Payen, Remote atmospheric pressure DC glow discharge treatment for adhesion improvement of PDMS, Plasma Process. Polym. 6 (2009) S406–S411.
J. Bacharouche et al. / Sensors and Actuators A 197 (2013) 25–29 [15] S. Bhattacharya, A. Datta, J.M. Berg, S. Gangopadhyay, Studies on surface wettability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasma treatment and correlation with bond strength, J. Microelectronic Syst. 14 (2005) 590–597. [16] J. Kim, M.K. Chaudhury, M.J. Owen, Modeling hydrophobic recovery of electrically discharged polydimethylsiloxane elastomers, J. Colloid Interface Sci. 293 (2006) 364–375. [17] I.J. Chen, E. Linder, The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces, Langmuir 23 (2007) 3118–3122. [18] S. Pinto, P. Alves, C.M. Matos, A.C. Santos, L.R. Rodrigues, J.A. Teixeira, M.H. Gil, Poly(dimethyl siloxane) surface modification by low pressure plasma to improve its characteristics towards biomedical applications, Colloids Surf. B 81 (2010) 20–26. [19] J. Kim, M.K. Chaudhury, M.J. Owen, Hydrophobic recovery of polydimethylsiloxane elastomers exposed to partial electrical discharge, J. Colloid Interface Sci. 293 (2006) 364–375. [20] V. Roucoules, A. Ponche, A. Geissler, F. Siffer, L. Vidal, O. Soppera, M.F. Vallat, P. Marie, J.C. Voegel, P. Schaaf, J. Hemmerlé, Changes in silicon elastomeric surface properties under stretching induced by three surface treatments, Langmuir 23 (2007) 13136–13145. [21] J. Bacharouche, P. Kunemann, P. Fioux, M.F. Vallat, J. Lalevée, J. Hemmerlé, V. Roucoules, A simple method for fabrication of filler-free stretchable polydimethylsiloxane surfaces, Appl. Surf. Sci. 270 (2013) 64–76. [22] H. Hillborg, M. Sandelin, U.W. Gedde, Hydrophobic recovery of polydimethylsiloxane after exposure to partial discharges as a function of crosslink density, Polymer 42 (2001) 7349–7362. [23] H. Hillborg, U.W. Gedde, Hydrophobic recovery of polydimethylsiloxane after exposure to corona discharges, Polymer 39 (1998) 1991–1998. [24] I.J. Chen, E. Lindner, The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces, Langmuir 23 (2007) 3118–3122. [25] N. De Geyter, R. Morent, C. Leys, Influence of ambient conditions on the ageing behaviour of plasma-treated PET surfaces, Nucl. Instrum. Methods Phys. Res. Sect. B 266 (2008) 3086–3090.
Biographies Dr. Jalal Bacharouche obtained his PhD degree in Chemistry of Materials from Haute-Alsace University, France in 2012. Subsequently he joined the National Centre of Scientific Research of Mulhouse. His research interests are smart material and structures, plasma polymerization, cell adhesion and protein adsorption.
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Dr. Hamidou Haidara has received a Master Degree in Solid State PhysicsCondensed Matter from Université L. Pasteur, Strasbourg-France in 1982, and a PhD in Physical-Chemistry from Université de Haute Alsace (UHA) Mulhouse-France in 1985, on a work on the “cohesion and adhesion of protective mineral and polymer coatings on steel”. After his PhD, he successively worked within a European (BRITE) project as a Postdoctoral Research Fellow on “thin metal coatings-on-polymer films”, and at the Dow Corning Corp. Research Centre in Midland-Michigan (USA), on “SelfAssembly Monolayers in Wetting and Adhesion”, working with Pr. M.K. Chaudhury. Since 1990 he works at the CNRS (Centre National de la Recherche Scientifique)France, where he currently has a senior research scientist position and leads the group on “Wetting and Self-Assembly Phenomena” at the Institut de Science des Matériaux de Mulhouse (IS2M)-CNRS/UHA, in Mulhouse. Dr. Haidara’s major fields of interest are on physics of interface phenomena, with special emphasis on wetting of complex fluids and media, wetting-mediated structure formation, self-organization and self-assembly of molecules, micro- and nanoobjects. Mr. Philippe Kunemann Degree of Chemical Engineer speciality analytical chemistry (Conservatoire National des Arts et Métiers de Paris). Surface analysis applied to the wetting study on self assembled monolayer and polymers, by wetting, infrared, X-ray diffraction, differential scanning calorimetry, AFM, Ellipsometry Analytical method development by wetting with development of experimental methods – existing relationships between adherence properties from copolymers and their surface properties. Dr. Marie-France Vallat obtained her PhD degree in Physical Science from the University of Haute-Alsace in Mulhouse (France) in 1983. She joined Professor Jacques Schultz’s team at the Centre de Recherches sur la Physico-Chimie des Surfaces Solides (CNRS) in Mulhouse where she started to study the relationship between surface and bulk properties of polymers and their adhesive behavior of assemblies. She has a special interest for elastomers and the creation of interphases that are the consequence of gradients of composition during joint formation. Dr. Vincent Roucoules is active in materials science for the past 10 years, mainly in polymers and surface treatments. After obtaining his doctoral degree in PhysicalChemistry from France, He worked as a post-doctoral researcher at Durham University in the Surface Science groups of Professor J.S.P. Badyal, England. During this period he was able to significantly increase his proficiency and knowledge of plasma chemistry. In consideration of his achievements in this field, he was offered the position of Assistant Professor at the Science Material Institute of Mulhouse, CNRS, France in 2002. His current area of research focuses on developing new responsive materials using plasma chemistry.