EFFECTS OF VARIOUS TREATMENTS ON SILICONE RUBBER SURFACE
ABSTRACT
Hydroxy groups were generated on a poly(dimethylsiloxane) (PDMS) surface using different methods. Attempts were made to prepare a homogeneous film on the PDMS surface and eliminate microwrinkles from the surface. Because the hydroxylation process changes the chemical composition of the PDMS surface, resulting in a cracked surface, selecting the best method for surface treatment with minimized surface microwrinkles and cracks was attempted. The results obtained from scanning electron microscopy showed that using the pulsed ultraviolet-ozone (UVO) radiation method with a controlled duration time, ozone treatment, continuous UVO treatment using a glass filter, and water media in UVO treatment was more effective than other methods evaluated in this study to prevent microwrinkles. The results obtained from contact angle measurements and attenuated total reflectance Fourier transform infrared spectroscopy revealed that the UVO treatment in the presence of a water medium created more hydroxy groups compared with other methods.
INTRODUCTION
Poly(dimethylsiloxane) (PDMS) consists of Si–O groups in its backbone and Si(CH3)2O repeating units.1 The main properties of silicone elastomers include low surface energy, excellent oxidative stability, chain flexibility, rotational mobility, large free volume, low glass transition temperature, easy modification of solubility characteristics by substitution of different organic groups on silicon, and the use of simple chemistry alterations allowing a range of molecular weights for making high- and low-consistency compositions that can be cross-linked either at elevated or room temperature conditions by a variety of reactions.2−6
Polymers containing siloxane (Si–O) groups in their backbones have more efficiency in applications such as microfluidic devices, ophthalmologic biomaterials, artificial lungs, artificial finger joints, membranes, electrical transistors, sensors,4,7 and polymer electrolyte membrane fuel cells.8,9 In applications such as gas permeability, one method to increase the selectivity of a membrane for gas separation is to make selective polymer brushes on the surface.5 Selecting a defect-free surface treatment technique6 is an important challenge in synthesizing polymer brushes on a surface and other surface modifications.
On the other hand, PDMS is a highly hydrophobic polymer, and its tendency for biofouling can limit its usage in some applications, since these properties lead to nonspecific adsorption of biomolecules.10,11 Therefore, important challenges in its surface are the channel filling and surface wetting. Because silicones do not normally possess appropriate surface functional groups that could be used, for example, to immobilize initiators for surface-initiated atom transfer radical polymerization, some methods were used for treatment or hydrophilization of the PDMS surface. Different ways to hydroxylate the PDMS surface include piranha treatment,12 treatment with HCl/H2O2/H2O solution,13 ultraviolet treatment,14,15 ozone treatment,16 ultraviolet-ozone (UVO) treatment,14 and plasma treatment.16−18 Using acidic solutions mixed with H2O2 (such as piranha solution and HCl/H2O2/H2O solution) creates a hydroxy group on the silicone surface. Such surfaces are hydrophilic in nature and are easily wetted by aqueous solutions.19 It was shown that when the surface was exposed to UV light or a combination of UV light and ozone, silicone rubbers underwent drastic surface chemical changes. However, the UV- and UVO-based modification processes were much slower (by about an order of magnitude) than the modification by plasma-based techniques.20 All of the treatment methods changed the surface structure by different intensities, which can result in crack and microwrinkle formation. Some researchers investigated the degradation and durability of silicone rubber for sealing and gasketing on proton exchange membrane fuel cell.8,9 The mechanism for migration of cyclic siloxanes to the proton exchange membrane interface, causing it to fail in a brittle fashion, was presented.8 The results suggested that the degradation of silicone rubbers resulted in weight loss and voids or cracks on the surface through the chemical decomposition of silicon-based backbone and the leaching of fillers.9 Surface wrinkling has also been recognized for controlling surface topography on micrometer and nanometer scales and has been extensively studied by many groups.21−24 On the other hand, other groups25,26 attempted to prevent wrinkling and cracking of polymer surfaces. This work deals with evaluation and comparison of some methods used to generate hydroxy groups on the PDMS surface without damaging or cracking the PDMS surface. The defect-free surface could be used to create a selective thin film of polymer brush for example by atom transfer radical polymerization on PDMS surface. These methods included piranha solution and HCl/H2O2/H2O solution treatment. It was observed that piranha solution could etch the silicone surface that was easily observed by naked eyes. However, the etching effect of the HCl/H2O2/H2O solution was poorer than that of the piranha solution. Other methods were ozone and UVO treatments, in which ozone treatment created fewer hydroxy groups on the defect-free surface. On the contrary, UVO treatment created enough −OH groups, for example, for immobilizing macromolecules with a higher tethering density on the surface, but it created a crack on the surface.
In the present study, different methods were examined to prevent the damaging effect of UVO on the silicone surface. By these methods, despite a significant reduction in the contact angle, no crack was created on the surface. In addition, different solution treatments including piranha treatment and HCl/H2O2/H2O treatment were compared with UVO and ozone treatments. To our knowledge, employing the pulsed UVO radiation and continuous UVO treatment using a glass filter introduced in this work has not been used before to treat the silicone rubber surface.
EXPERIMENTAL
Materials
Liquid silicone rubber (LSR 40) was purchased from Quail Court (Santa Paula, CA, USA) and contained two parts: a liquid silicone rubber (oligomer) base and a curing agent. Sulfuric acid (>95%) and hydrochloric acid (37%) were purchased from Merck (Kenilworth, NJ, USA). Methanol (>99.9%), ethanol (>99.5%), and toluene (>99.0%) were supplied by Merck.
Substrate preparation
The oligomer base of PDMS and curing agent (10 wt. %) were mixed with a mechanical stirrer for 5 min. After mixing the silicone and its curing agent, the trapped air was removed from the mixture by applying a vacuum and breaking it several times. About 8 g of prepolymer mixture was molded on a polished silicon disk that was located inside a mold to form a PDMS film of about 500-μm thickness. After air removal, the mold was placed in an oven at 70 °C and was then cured at 100 °C for 24 h. After completion of the curing, the PDMS film was removed from the disk surface, and the sample was sliced to 1 × 1 cm2. The migration of low-molecular-weight (LMW) components has been reported to be an important factor influencing the hydrophobic recovery in PDMS materials.14,27,28 To minimize this effect, the film was Soxhlet extracted by toluene for 48 h to remove the LMW and un-cross-linked polymer chains. Then the silicone rubber films were dried in a vacuum oven. The cleaned and dried samples were stored under nitrogen atmosphere.
Pretreatment and activation of PDMS surface
Treatment of PDMS was done by two different methods: the first method was solution treatment and the other was UV (or UVO) treatment. Both methods create hydroxy groups on the surface but by different mechanisms. For example, the piranha treatment creates hydroxy groups by generating hydronium ions, bisulphate ions, and a reactive atomic oxygen species. The formation of a reactive atomic oxygen species leads to the formation of an oxidized PDMS surface because it can attack the Si–CH3 bonds in PDMS to form a silanol (Si–OH) group.29 On the other hand, the UVO treatment is a photosensitized oxidation in which the molecules being exposed to UV radiation typically react with the atomic oxygen arising from the continuous dissociation of oxygen molecules and the generation of ozone molecules. Polymer substrates are oxidized by atomic oxygen, molecular oxygen, and ozone through the abstraction of hydrogen atoms from the polymer chains, producing carbon radical sites. The hydroxy and carboxy groups have been detected on the polymer surface after the ozone treatment.30
In this work, the cleaned PDMS films were treated by piranha solution (H2SO4/H2O2), HCl/H2O2/H2O solution, UV radiation, ozone radiation, and UVO radiation according to Table I.
Characterization and analysis
The dried samples were sputter-coated with gold and observed by scanning electron microscopy (SEM) that was carried out by a Cam Scan, MV 2300 SEM (Czech Republic). Contact angle measurements were done by a contact angle measuring system (Sahand University of Technology, Tabriz, Iran) equipped with a DV-3000 binocular (Bel, Italy). When choosing a liquid for contact angle studies, several criteria must be considered. It should be a nonsolvent for rubber, have low volatility and low viscosity, and have a contact angle that changes significantly with changing surface characteristics.31 Thus, water was selected for contact angle studying. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was carried out on a Tensor 27 spectrometer (Bruker, Germany).
RESULTS AND DISCUSSION
Since various surface treatments significantly affect surface roughness,32 and, according to Wenzel law, roughness increases wetting characteristics of solid surfaces, enhancement of roughness causes a decrease in water contact angle.33 Therefore, for the surfaces obtained by different pretreatment techniques, water contact angle measurements were carried out. The water contact angle for the untreated cross-linked PDMS surface molded in contact with silicon wafer was 110.6°.
Piranha treatment
The PDMS surface was treated by piranha solution in samples 1, 2, and 3, as well as the sample prepared by the combined method (sample 18) in Table I. The functionalized samples by piranha solution were sonicated in water for 5 min immediately after the treatment and then were dried by a nitrogen flow. Because of severe corrosion of sample in strong solution (H2SO4/H2O2 = 7/3 v/v), the tests were conducted with H2SO4/H2O2 (= 3/1 v/v) solution. As shown in Figure 1, this H2SO4/H2O2 ratio resulted in microcracks on the surface.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
By decreasing the temperature from 70 to 40 °C while other parameters were kept constant, the electron microscope image exhibited a little decrease in corrosion, but it was not completely eliminated (Figure 2). These observations indicated that the H2SO4/H2O2 solution was not a suitable choice for surface treatment of PDMS.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
The prior study dealing with the effect of piranha treatment on the topography of the PDMS surface showed that the surface was oxidized, hardened, and wrinkled due to compressive stress developed in the hardened PDMS layer owing to buckling instability.21 Another mechanism is that the swelling of the hardened layer can also create wrinkles under different experimental conditions. If the oxidized layer does not get tension, the nonoxidized PDMS becomes stretched, but this layer is attached to the PDMS substrate and PDMS is prevented from elongation of it, because the bands are strongly linked on the silica-like layer. Consequently, the swelling layer can be folded.21 The reason for swelling may be the hydrophilic nature of the silica-like layer, which is capable of adsorbing humidity or moisture from the acidic mixture.
Mechanical stresses can cause cracks, due to thermal stresses induced by mismatch in the coefficient of thermal expansion between the underlying bulk PDMS and the silica-like layer.34 In addition, wrinkles can induce cracking because the stress component normal to the film (silica-like layer)–substrate (PDMS substrate) interface varies from a maximum tensile value to zero at the crests and from a maximum compressive value to zero at the valleys. At the crests, they are compressive near the film–substrate interface but can turn into tensile ones close to the free surface after a critical value of wrinkle curvature is exceeded. Film cracking can be observed in the boundary regions between crests and valleys of wrinkles, where the normal stresses change sign (i.e., from tensile into compressive ones).35
Treatment by HCl/H2O2/H2O solution
Figure 3 indicates the electron microscope image of sample 4 in Table I. Because this solution was weaker than prior ones, it did not cause corrosion; instead, it caused a wrinkle and wave off on the surface. Increasing the solution temperature from 70 to 100 °C also resulted in a wrinkled surface. When the surface treatment with HCl/H2O2/H2O (5/1/1 v/v/v) solution was associated with exerting a shear force, applied by injecting the solution onto the surface (sample 6 in Table I), cracks formed on the PDMS (Figure 4).
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Figure 5 shows the electron microscopy image for the sample treated in the acid mixture that was cooled for 30 min at room temperature after removing it from the reaction media and then washing with water. This micrograph shows that the treatment in acid mixture, due to washing immediately after the reaction, did not cause any damage on the surface or surface wrinkle.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Continuous UVO treatment
As shown in Figure 6a, continuous UVO treatment resulted in wrinkle formation on the surface. This effect was due to the fact that the PDMS chains on the surface underwent chain scission of both the main chain backbone and the side groups in the network when it was exposed to the UV radiation. The radicals formed during this process recombined and formed a network whose contact angle was 94.3°. Contrary to the UV radiation, the UVO treatment caused very significant changes in the surface and near-surface structure. Specifically, the molecular oxygen and ozone created during the UVO radiation interacted with the UV-modified specimen. As a result of these interactions, the surface of the sample contained a large number of hydrophilic (mainly –OH) groups, providing a more wettable surface with water contact angle equal to 72.7°. The UVO treatment of PDMS may cause uncontrollable changes to the surfaces. In particular, long-time exposure of PDMS to UVO creates almost a rigid silica-like layer on the surface of the PDMS.14 Therefore, decreasing the effect of UVO treatment was attempted, using different methods such as pulsed UVO, a water medium, and a glass filter.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Pulsed UVO treatment
While the continuous UVO method resulted in the wrinkle on the surface, pulsed UVO treatment exhibited surfaces that did not have any defect (Figure 6b). In pulsed UVO treatment, the sample was exposed to UV radiation for a short time. In the meantime, the functionalization because of the presence of ozone on the surface continued without UV radiation. Therefore, the PDMS surface could be made hydrophilic by increasing hydroxy groups on the surface, but the surface defects were prevented by decreasing the diffusion depth of radicals.
The effects of continuous and pulsed UVO treatments on the hydrophilic characteristics of PDMS films (samples 8 and 9 in Table I) were additionally evidenced by contact angle data. Water contact angles of 72.7° and 86.9° were measured for samples 8 and 9, respectively.
Figures 7a, 7b, and 8, the electron microscope images of samples 10, 11, and 15, respectively, in Table I, indicate the effects of relaxation time (i.e., the time interval between the UVO pulses) and duration of irradiation between two intervals. The results showed that the continuous UVO radiation caused crack formation on the surface; however, the pulsed UVO radiation with ton = 2 min and toff = 3 min (Figure 7a) and ton = 1 min and toff = 2 min (Figure 7b) for total radiation time of 30 min prevented the crack formation on the surface.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Effect of UVO treatment using water as a shielding environment
Sample 14 (in Table I) was submerged in water to avoid the destructive effects of UV radiation on PDMS. This sample showed a significant reduction in the depth and density of cracks (Figure 9) compared with sample 15, in which water was not used to protect the sample (Figure 8). In addition, sample 14 indicated a lower water contact angle compared with sample 15. This phenomenon might have resulted from hydrogen bonding between water and the siloxane linkages. Such multiple hydrogen bonding would minimize the interfacial tension in the presence of PDMS chains but would pin the chains at the interface and prevent them from rediffusing into the bulk.36 Therefore, in water, the state of PDMS chains was oriented with their polar siloxane backbones to the water and their methyl groups away from the interface. According to ATR-FTIR spectra recorded around 3400 cm−1, the characteristic wave number of hydroxy groups (Figure 10), more hydroxy groups were created compared with the sample that was treated by UVO for 1 h without water. For this sample, the contact angle was 45.2°.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Effect of UVO treatment using glass filter as a shielding environment
In addition to the presence of water media in UVO treatment and its protective effect, a glass filter can also avoid the damaging effect of UV radiation. Therefore, as shown in Figure 11, the sample was put under a glass filter and exposed to continuous UVO radiation. Figure 12 shows that no defect was created on the surface, and the water contact angle for this sample was 66.4°, which indicated an effective oxidation by this method.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
Ozone treatment in water as a shielding environment
Ozonation is a relatively simple and inexpensive method for uniform surface modification of polymers by imprinting peroxides on the polymer surface.37 Because of the ability of ozone radiation to penetrate the polymer film, oxidation takes place not only on the surface but also inside the sample. According to Figure 13, no defect was observed on the surface when it was treated by ozone in water media. The water contact angle value for this sample was 90.1°, indicating that ozone treatment produced fewer −OH groups than UVO treatment.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
In summary, ozone treatment, pulsed UVO treatment, continuous UVO treatment using a glass filter, and continuous UVO treatment in water media were the effective methods to prevent the damaging effects on the surface. Thus, the ATR test was done to compare hydroxy groups qualitatively in these methods. According to Figure 14, the sample exposed to UV radiation passing water for 2 h (e) had the most hydroxy groups, then UV radiation passing through a glass filter for 30 min (d), pulsed UVO radiation for 15 min (c), and finally the sample treated with ozone for 90 min had the lowest hydroxy group among the treated specimens.
Citation: Rubber Chemistry and Technology 90, 1; 10.5254/rct.16.83782
CONCLUSIONS
Investigation of the effect of different solutions on PDMS surface treatment showed that because of severe corrosion, piranha solution was not suitable for hydroxylation of the PDMS surface. On the other hand, other weaker solutions caused wrinkling on the surface. Among the other investigated methods including ozone and UVO treatments, ozone treatment created a defect-free surface but with lower hydroxy groups. On the contrary, UVO treatment created enough −OH groups for subsequent surface modifications, for example, synthesizing polymer brushes on the surface. However, this method created a crack on the surface. In addition, the intensity of UV and ozone was lowered by pulsed UVO radiation, using water media or a glass filter. SEM results showed that these methods created crack-free surfaces. The water contact angle measurements also showed UVO treatment in water media and under a glass filter had suitable hydroxy groups.
SEM image of a treated PDMS sample with H2SO4/H2O2 (=3/1 v/v) solution (temperature = 70 °C and time = 30 s).
SEM image of a treated PDMS sample with H2SO4/H2O2 (=3/1 v/v) solution (temperature = 40 °C and time = 30 s).
SEM image of a treated PDMS sample with (HCl/H2O2/H2O) (=5/1/1 v/v/v) solution (temperature = 75 °C and time = 5 min).
SEM image of a treated PDMS sample with injecting (HCl/H2O2/H2O) (=5/1/1 v/v/v) solution (temperature = 40 °C and time = 5 min).
SEM image of a treated PDMS sample by HCl/H2O2/H2O (=5/1/1 v/v/v) at 70 °C for 5 min. The sample was washed by water after cooling.
SEM images of UVO-treated PDMS with (a) continuous and (b) pulsed (ton = 2 min and toff = 3 min) radiation for 15 min.
SEM micrographs of treated PDMS samples with UVO treatment; (a) pulsed radiation (ton= 2 min, toff = 3 min) and (b) pulsed radiation (ton =1 min, toff = 2 min) for total radiation time of 30 min.
SEM image of UVO irradiated PDMS for 1 h.
SEM image of UVO-treated PDMS for 2 h after submerging in water.
Normalized ATR-FTIR spectra for untreated PDMS (a), UVO treated PDMS for 1 h (b), and UVO treated in water media for 2 h (c).
The glass filter used to avoid the damaging effect of UVO radiation on PDMS surface.
SEM image of UVO-treated PDMS for 30 min under a glass filter.
Electron microscope image of treated PDMS by ozone radiation in water media.
Normalized ATR-FTIR spectra for unmodified PDMS (a), ozone treatment (b), pulsed UVO treatment (ton= 2 min, toff = 3 min) (c), continuous UVO treatment by using a glass filter (d), and continuous UVO treatment in water media for 2 h (e).
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