Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Fast Hydrophobicity Recovery of the Surface-Hydrophilic Poly(dimethylsiloxane) Films Caused by Rechemisorption of Dimethylsiloxane Derivatives Takahiro Senzai†,‡,∥ and Shigenori Fujikawa*,†,§,∥
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Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Tokyo Ohka Kogyo Co. Ltd., 1590, Tabata, Samukawa-machi, Koza-gun, Kanagawa 253-0114, Japan § International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ABSTRACT: Long-term stability of the surface hydrophilicity of poly(dimethylsiloxane) (PDMS) remains a critical issue for a wide range of applications including, e.g., biomedical materials, biochip devices, and microfluidics. Although several mechanisms for recovering hydrophobicity have been proposed, none has been proven unequivocally. We discovered that the hydrophobic recovery of surface-oxidized PDMS films was accelerated when the films were stored in a closed chamber under an atmosphere containing dimethylsiloxane derivatives such as octamethylcyclotetrasiloxane, which had evaporated from the films and were detected by gas chromatography−mass spectrometry measurements. X-ray photoelectron spectroscopy and quartz crystal microbalance measurements were used to monitor the chemical deposition of the derivatives on the surface-oxidized substrates, and the formation of a hydrophobic dimethyl silicone layer rendered the oxidized surfaces hydrophobic again. In the absence of superficial hydrophilic functional groups, volatile species did not react with the surface, and the hydrophobic dimethyl silicone layer did not form. The results show that the formation of a thin layer of dimethylsiloxane derivatives by chemisorption is a new mechanism for recovering the surface hydrophobicity of PDMS films.
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molecules,17 alternation of surface roughness,18 and environmental surface contaminations19 have been discussed as possibilities. Recently, dimethylsiloxane volatiles such as octamethylcyclotetrasiloxane (i.e., (dimethylsiloxane)4 or D4), which evaporate from PDMS films, have been suggested to contribute to the hydrophobic recovery of silicone films.20 Although volatiles evaporated from silicone films have not been directly detected yet, commercially available silicone materials such as Sylgard 184an MQ (Me3SiO/SiO4)-based silicone containing no D unitsand an additive-free D-based silicone were judiciously chosen to compare and study the effects of D-unit-containing volatiles on the surface hydrophobic recovery, which have not been experimentally well investigated yet. Solving the problem of volatile species causing hydrophobic recovery in widely used silicone materials may lead to the development of silicone films showing long-term durable hydrophilic surfaces. Herein, we revealed a detailed mechanism for recovering the surface hydrophobicity in PDMS films. Volatile molecules that had evaporated from PDMS films were directly detected and were chemisorbed on the PDMS surfaces by hydrolysis and
INTRODUCTION Poly(dimethylsiloxane)s (PDMSs) are silicon-based nonflammable elastic polymers that are inert,1,2 nontoxic,3 and biocompatible.4 PDMS polymers have found a wide range of applications, e.g., in medical devices, as antifoaming agents in food, as caulking materials, and in lubricants and heat-resistant materials.5−7 PDMS-based biochip devices in particular have recently attracted much attention owing to their attractive shape-molding properties and elasticities, similar to those of biomaterials,8 as well as their biocompatibilities.4 However, the PDMS surface is highly hydrophobic, causing serious problems in aqueous environments and preventing the introduction of aqueous solutions into small channels of PDMS-based microfluidic devices. Substantial efforts have been devoted to rendering the PDMS polymer surfaces hydrophilic, including surface oxidation by dry processes such as oxygen-plasma treatment,9 vacuum ultraviolet irradiation,10 corona-discharge treatment,11 surface coating, and graft polymerization.12,13 However, long-term stability of PDMS-surface hydrophilicity remains insufficient. When PDMS is treated using the aforementioned strategies, the surface becomes hydrophobic within 24 h to a few days.14 Although the specific mechanism for hydrophobic recovery remains an issue of debate, reorientation of hydrophilic functional groups,15 condensation of surface silanols,16 migration of low-weight hydrophobic © XXXX American Chemical Society
Received: May 16, 2019 Revised: June 20, 2019 Published: July 9, 2019 A
DOI: 10.1021/acs.langmuir.9b01448 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. (a) Schematic illustrating sample setup. (b) Time dependences of static (WCAs) and (c) advancing and receding WCAs of surfaceoxidized PDMS films aged in closed (sample I) and open (sample II) chambers. Volatile Species Characterization and Redeposition on Substrates. To identify volatile species that may have evaporated from the PDMS films, the PDMS samples were deposited into headspace vials and heated for 15 min to 60, 120, or 180 °C. Gaseous samples were collected from the vials by a syringe and analyzed by a gas chromatography−mass spectrometry system (GC−MS, 7890B5977A; Agilent Technologies, Inc.). Rechemisorption of the volatile species on PS substrates was monitored using a QCM. A PS solution (1 wt % PS dissolved in toluene) was initially spin-coated at 2000 rpm for 60 s onto an Aucoated QCM electrode whose surface was then oxidized by oxygen plasma. The QCM electrodes showing surface-oxidized and nonoxidized PS layers were then deposited into closed glass vials, and the electrode frequencies were monitored for 300 s. Slices of PDMS films (0.5 g) were then inserted into the vials, and the changes in electrode frequencies were monitored for 2000 s.
condensation, thereby rendering the surfaces hydrophobic again, as confirmed by X-ray photoelectron spectroscopy (XPS) and quartz crystal microbalance (QCM) measurements.
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EXPERIMENTAL SECTION
Materials. A commercial PDMS kit (Sylgard 184, Dow Corning Toray Co., Ltd.) was used for the preparation of PDMS films. As a reference material for Sylgard 184, vinyl-terminated poly(dimethylsiloxane) (V-PDMS, Sigma-Aldrich; Mw = 25 000) and trimethylsilyl-terminated poly(dimethylsiloxane-co-methylhydrosiloxane) (H-PDMS, Sigma-Aldrich; Mw = 13 000) were cross-linked using the catalyst platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Pt complex, Sigma-Aldrich). Polystyrene (Mw = 35 000) was purchased from Sigma-Aldrich. Silicon (Si) wafers and polystyrene (PS) plates were purchased from SUMCO Corp. and Hikari Co., Ltd., PS2035-1, respectively, and used as received. Preparation and Surface Oxidization of PDMS Films. Atmospheric organic species contaminate the PDMS film surfaces. To avoid such external contamination, all PDMS films were prepared in a semiconductor-grade clean room (class: 1000) according to two methods: (i) components A and B of the Sylgard 184 kit were mixed in a 10/1 ratio (A/B; w/w), poured into in a perfluoroalkoxy ethylene case and heated to 150 °C for 30 min, yielding ∼1 mm thick PDMS films. Because the contents of the Sylgard 184 kit were not disclosed and to clarify the uniqueness of the Sylgard 184 kit toward the recovery of surface hydrophobicity, PDMS films were also prepared by (ii) cross-linking V-PDMS and H-PDMS, which were mixed with the Pt-based catalyst in a 10/6/0.01 ratio (w/w) and heated to 150 °C for 30 min, yielding PDMS films of similar thickness. Oxygen plasma (TCA-3822, Tokyo Ohka Kogyo Co., Ltd.; RF power: 50 W; oxygen flow rate: 200 mL/min; chamber pressure: 40 Pa) was used to render the substrate surfaces hydrophilic. Surface Characterization. Elemental compositions of the film surfaces were analyzed by XPS (K-Alpha XPS, Thermo Fisher Scientific, Inc.) with monochromated X-rays (Al Kα radiation; 15 kV, 300 W). The surface wettability of the prepared PDMS films was evaluated by the static and dynamic contact angle (CA) methods (Drop Master DM300, Kyowa Interface Science Co., Ltd.).
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RESULTS AND DISCUSSION Two PDMS films obtained from the Sylgard 184 kit were exposed to oxygen plasma to render their surfaces hydrophilic. Initially, both samples showed similar water contact angles (WCAs) (i.e., >30°). Samples I and II subsequently were stored in covered and opened plastic chambers, respectively, at room temperature (Figure 1a), and the time dependencies of the WCAs of both samples were monitored (Figure 1b). During the first 60 min, the WCA of sample I increased sharply, while that of sample II remained constant. After 60 min, the WCAs of samples I and II gradually increased to 60 and 40°, respectively. After 6 h, the WCA of sample I remained stable, even when the sample was exposed to air for several hours, suggesting that the surface properties of sample I had been irreversibly altered. The dynamic contact angles of samples I and II are plotted as functions of time in Figure 1c. The intrinsic WCAs of samples I and II advanced and receded differently owing to different surface wettabilities obtained before and after water contact. Although static contact angles of the samples were between advancing and receding ones, corresponding samples aged for the same amount of time B
DOI: 10.1021/acs.langmuir.9b01448 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. (a) Schematic illustration of the sample setup of silicon substrates and (b) the corresponding WCAs plotted as functions of aging time. Plasma-oxidized Si substrates were stored (i) in closed chamber without PDMS film, (ii) in open chamber with PDMS film, (iii) in closed chamber with PDMS film, and (iv) in closed chamber with PDMS film wrapped in aluminum lami-zipper bag; and (v) plasma-oxidized PS substrate was stored in closed chamber with PDMS film.
obviously showed different contact angles, suggesting that storing PDMS samples in closed chambers accelerated hydrophobic recovery. Air contamination, reorientation of hydrophilic groups, and self-condensation of the neighboring hydrophilic functional groups have been proposed as mechanisms for surfacehydrophobic recovery of surface-oxidized PDMS films. In this case, however, although air contamination was negligible given that sample I had been isolated from the ambient atmosphere in a closed chamber, sample I still showed a WCA that increased more rapidly than that of sample II, which had been exposed to ambient atmosphere. Reorientation and selfcondensation of surface-hydrophilic groups, on the other hand, are essentially independent of the surroundings. However, surface accumulation of low-molecular-weight siloxane molecules can hardly be considered a possible mechanism in this case because molecular diffusion should be similar in both PDMS films subjected to same experimental conditions, especially considering that the WCA of sample I remained stable after the sample had been exposed to air. Therefore, the results imply that the atmosphere within the closed chamber may have altered the surface properties of the enclosed PDMS films. Our hypothesis at this point was that volatile species may have been released from the PDMS films. To confirm this hypothesis, we used the sample setup shown in Figure 2 to investigate the redeposition of volatile species, which may have evaporated from the PDMS films, on Si wafer substrates. The Si wafers were initially treated with oxygen plasma to render their surfaces hydrophilic and were then stored in closed chambers without (sample (i)) and with (sample (iii)) a PDMS film. An open chamber was used as a reference (sample ii). Sample (iv) consisted of a surfaceoxidized Si substrate stored in a closed chamber with a PDMS film wrapped in an aluminum lami-zipper bag. The WCAs of the samples were measured after 1, 3, and 7 days. Samples (i), (ii), and (iv) retained low WCAs (
