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An Underwater Superoleophobic Surface Which can be Activated/Deactivated via External Triggers Gary James Dunderdale, Chihiro Urata, and Atsushi Hozumi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la503492e • Publication Date (Web): 15 Oct 2014 Downloaded from http://pubs.acs.org on October 22, 2014
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An Underwater Superoleophobic Surface Which can be Activated/Deactivated via External Triggers Gary J. Dunderdale, Chihiro Urata and Atsushi Hozumi* Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan
Keywords: Underwater oleophobicity, Superoleophobicity, Stimulus responsive, Polymer brush, Poly[2(dimethylamino)ethyl methacrylate]
Abstract Poly[(2-dimethylamino)ethyl methacrylate] (pDMAEMA) brush surfaces were prepared using a facile aqueous Activators ReGenerated by Electron Transfer Atom Transfer Radical Polymerization (ARGETATRP) protocol at ambient temperature without any need to purge reaction solutions of oxygen. This produced underwater superoleophobic surfaces, which exhibited high advancing (θA, 164-166o) and receding (θR, 153-165°) contact angles (CAs), and low CA hysteresis (1-11o) with a variety of oils. Both in-situ spectroscopic ellipsometry and dynamic CA measurements confirmed that pDMAEMA brush surfaces responded to three different external stimuli (pH, ionic strength, and temperature) by changing their thicknesses, degree of hydration and/or their chemical composition. Increasing pH resulted in the largest decrease in hydration, followed by increasing ionic strength, and increasing temperature gave only a small change in hydration. Coincident with these structural changes, stimulus-responsive dynamic dewetting behavior with various oils was observed. Increasing pH or ionic strength drastically reduced the θR values of oil drops and increased CA hysteresis, resulting in a sticky surface on which oil drops were pinned. No noticeable changes in dynamic oleophobicity were observed with increasing temperature. In addition, when oil drops impacted onto the brush surface instead of being gently placed, surfaces did not exhibit stimulusresponsive dewetting properties, being oleophobic under all conditions.
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Introduction Polymer brush surfaces have been shown to have superior surface properties because of the high density of stretched polymer chains grafted to the substrate,1-4 and many stimuli-responsive polymer brushes have been reported to date.3, 5 These responsive surfaces are able to change their surface properties in response to a wide range of stimuli such as solution pH,6-8 temperature,9,
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charged ions,11,
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acidic vapor,13 carbon
dioxide,14, 15 glucose16 or electrical stimuli17-20 to name a few. These stimuli-responsive surfaces have been proposed as useful in a wide range of applications,21 such as controlled drug release22, 23 and lubrication,24, 25 which make use of their responsive nature to change the observed behavior. Polymer brush coatings have also recently attracted attention as a new class of material because of their excellent oil-repellent (superoleophobic) properties when submerged underwater.26-28 Common among these underwater superoleophobic surfaces are polymer chains that contain hydrophilic (usually ionically charged) repeat units, which are highly solvated by water, leading to highly hydrophilic and so highly oleophobic surfaces. More recently, stimulus-responsive functionalities have been incorporated into these polymer surfaces, resulting in tunable or environment-responsive oleophobicity. So far several stimulus-responsive oilrepellent polymer brush surfaces have been reported. For example, Cheng et al29 reported poly(acrylic acid) brush surfaces which can switch adhesion force to oil drops depending on the solution pH from 22 µN at pH 1 to 0 µN at pH 12. The thermally responsive nature of poly(N-isopropylacrylamide) gels to oil drops was reported by Chen et al30. They confirmed that static contact angle (CA, θS ) of dichloroethane decreased from 152o at 23 oC to 127o at 40 °C. Mingjie et al31 also showed electrically responsive properties of a polypyrrole nanotube functionalized surface. By adding a negative or positive electrical potential, surface dewettability towards dichloroethane changed markedly from oleophobic (θS = 158o) to oleophilic (θS = ~0o), respectively. Zheng et al32 reported on a polyelectrolyte multilayer complex which responds to the presence of different counterions, enabling it to be fluorinated and superoleophobic in air (θS = 151.0o with 1,2-dichloroethane), and non-fluorinated and superoleophobic in seawater (θS = 163.6o with 1,2-dichloroethane). As can be seen, although the underwater oleophobicity of such surfaces has been commonly measured by θS values, dynamic oleophobicity measured by CA hysteresis (Δθ, difference between the advancing (θA) and receding (θR) CAs) or substrate tilt angle (TA) measurements have been rarely reported. Some of the authors have previously reported that dynamic oleophobicity not static oleophobicity is the key to generating practical “oil-repellent” surfaces,33 because dynamic dewettability is known to be independent of magnitude of θS values. Unfortunately, the relationship between the physical/chemical states of polymer brushes (degree of protonation/hydration, chemical species present, or polymer-segment density distribution) and underwater dynamic oleophobic properties has not yet been clearly identified. Thus, in this study, we have particularly focused on the changes in both physical/chemical natures of polymer brushes induced by specific external stimuli and underwater dynamic oleophobicity. Among various polymers, we selected poly[2-(dimethylamino) ethyl methacrylate] (pDMAEMA) as a model polymer, because it has been shown to respond to three different stimuli. Firstly, because of the
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existence of tertiary amine groups, this polymer is protonated at low pH values, and changes from a thin neutral brush layer at high pH, to water-swollen thick cationic polymer layer at low pH values.34 Secondly, when protonated, the polymer chain is also responsive to changes in the ionic strength of the surrounding solution, as ions in solution can screen cationic charges from each other, resulting in a collapse in the swollen polymer brush.35, 36 Thirdly, because of hydrogen bonding interactions between polymer segments and water molecules, pDMAEMA homopolymer or copolymers have been reported to be thermoresponsive,37, 38 similar to the more widely studied poly(N-isopropylacrylamide) (pNIPAM).39, 40 However, there have only been a limited number of studies on the thermo-responsive nature of pDMAEMA brush surfaces.38, 41 Although changes in physical/chemical properties of pDMAEMA polymer chains against these three different stimuli have been studied extensively, how these physical/chemical changes affect the dynamic dewettability of the surface toward various low surface tension liquids have not been clearly identified. As will be shown, in our present study, pDMAEMA brush surfaces showed limited stimuli-responsive dymanic oleophobicity underwater, only responding to pH or ionic strength. No marked changes in underwater dynamic oleophobicity were observed with increasing solution temperature, contrary to our expectations. We find that physical/chemical changes induced by external stimuli do not always directly translate to responses in the underwater dynamic dewetting behavior. Experimental Materials. (3-aminopropyl)triethoxysilane (APTES), n-hexadecane, 1,4-dioxane, 2-(dimethylamino)ethyl methacrylate (DMAEMA), and ascorbic acid (AA) were purchased from Wako Chemicals Japan. Copper (II) chloride, pentamethyldiethylenetriamine, and α-bromoisobutyryl bromide (BiBB) were purchased from Sigma-Aldrich. Preparation of Silane Initiator Monolayers. N-type silicon (100) wafers used as the polymer brush substrate were cut into small pieces ~ 2 x 4 cm2. They were sonicated in ethanol for 5 minutes to remove dust and grease, and dried in a stream of nitrogen (N2) gas. Next, these samples were cleaned by an UV-ozone treatment for 30 minutes at 103 Pa. They were then placed in a large glass vial along with a smaller glass vial containing ~100 µL of APTES, the large vial sealed using a screw top, and then heated to 100 oC for 60 minutes. This procedure functionalized the silicon surface with APTES via a condensation reaction of triethoxysilane groups with silanol groups on the silicon surface, to generate surface bound amine groups. After which the silicon substrates were removed from the glass vials while still hot, any excess APTES liquid allowed to evaporate, rinsed with toluene and blown dry with a stream of N2 gas. The APTES-modified substrates were then functionalized with the ATRP initiator BiBB by immersion in a 0.1 M solution of BiBB in 1,4-dioxane overnight, Finally, they were rinsed with 1,4dioxane, dried and immediately used in the following experiments in order to avoid contamination.
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ARGET-ATRP of 2-(dimethylamino)ethyl methacrylate from Silane Initiator Monolayers. DMAEMA (8 mL, 48 µmol), water (7 mL, 390 µmol), copper (II) chloride (2.8 mg, 21 µmol), and pentamethyldiethylenetriamine (5 µL, 24 µmol) were added to a glass vial (20 mL, 2 cm diameter). AA (1 mg, 5.7 µmol in 1mL water) was then added to this solution, and the solution was stirred for 3 minutes. After which an initiator substrate was inserted to the reaction solution (polished side upwards at the base of the vial) and the glass vial sealed with a screw-top lid. Reaction solutions were not degassed, and glass vials contained ~4 mL of ambient air. Polymerizations were allowed to proceed at room temperature (23-28 oC) without stirring for 2 hours. After polymerization, the substrates were removed from the vial and extensively rinsed with water. Typically, these reaction conditions generated polymer brush thicknesses of 70-90 nm. Spectroscopic Ellipsometry Measurements. Ellipsometric measurements were carried out using a Horiba Jobin-Yvon MM-16 instrument, at an incident angle of 70o, at wavelengths 500-1000 nm at 2 nm intervals. In the dry state, data was modeled using a Cauchy layer with constants A = 1.49 µm, B = 0.005 µm2. For determination of brush layer thicknesses under different wet conditions, a substrate was placed in a sample cell with quartz windows inclined at 70o. The cell was then filled with water adjusted to a particular pH value and ionic strength using HCl/NaOH/NaCl, and a particular temperature using a hotplate (Kyowa Interface Science) underneath the sample cell. The obtained data was fitted using an estimated medium approximation (EMA) (Bruggeman type) model, consisting of a Cauchy material (A=1.49 µm, B=0.005 µm2) and water. Quality of fit was assessed through the χ2 parameter, and by multiplying the measured wet thickness and polymer content together, which in all cases approximately equaled the dry thickness of the brush layer. Contact Angle Measurements.
θA and θR were measured using a CA goniometer (Kyowa Interface Science, CA-V150). Briefly, a small drop of liquid (~5-10 µL) was dispersed from a syringe needle onto the surface to be measured, and then the volume increased until the three phase contact line was observed to advance across the surface. A photograph was then captured using the supplied software and the θA deduced using the tangent angle algorithm. The volume of the drop was then decreased until the three phase contact line was observed to recede, and the θR measured in the same manner as before. For CA measurements underwater, a sample cell with inverted sample holder, along with a hooked syringe needle was used. Particular care was taken when measuring the θR of oils underwater, because these angles were >150o, and movement of the three phase contact line was difficult to judge. This issue has been highlighted in another publication.42 The pH and ionic strength were adjusted using HCl/NaOH/NaCl, and the temperature maintained using an identical Kyowa hotplate underneath the sample cell, in a manner similar to the ellipsometric measurements mentioned above.
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Results and Discussion
Figure 1 Kinetics of polymer brushes grown by the ARGET-ATRP protocol. Insert is a typical AFM image of the resulting polymer brush surface taken in air (Rrms = 0.74 nm).
Table 1 θA and θR values of pDMAEMA brush surfaces with 4 different probe liquids when submerged in aqueous solution at either pH 2 or 10. Interfacial tensions are from references.43-46 pDMAEMA brush surfaces were prepared using an ARGET-ATRP protocol according to our previously reported procedure.47 It has numerous advantages over preparation of polymer brushes by conventional ATRP as follows. First, water is used as the solvent which avoids the use of toxic and environmentally damaging organic solvents. Second, water significantly increases the rate of polymerization, meaning the reaction can be performed at room temperature. Third, because of the ARGET mechanism, the reaction solution does not need to be purged of oxygen beforehand. For these reasons, this polymerization is extremely facile and an easy
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route to create water-soluble polymer brushes, compared to conventional ATRP reactions. As shown in Figure 1, this ARGET-ATRP protocol gave a fast increase in brush thickness against reaction time, which quickly reached a plateau of 70 nm after around 20 minutes reaction time. This curvature to the plot indicates that the polymerization was a moderately poorly controlled polymerization, and that terminal halogen atoms were lost from the ends of growing polymer chains over the course of the reaction. Therefore, this polymerization is considered to be more similar to an uncontrolled surface-initiated free-radical polymerization than the wellcontrolled living polymerizations generally associated with ATRP. Nevertheless, this method is much easier to perform and yields polymer brushes, with reasonable thickness (~70 nm). As shown in the insert of Figure 1, the silicon substrate is completely covered with the polymer brush and free from holes or particulate defects. The surface is fairly smooth with a root mean square roughness, Rrms = 0.74 nm). Thus, surface morphology had little if any influence on the dynamic dewettability of sample surfaces. As we have previously reported,47 the pDMAEMA brush thickness can be tailored by adjusting the concentration of reducing agent (ascorbic acid) in the reactions, and the underwater superoleophobic properties are independent of the degree of control in the ARGET-ATRP reaction. In the next sections, we investigate the changes in dynamic dewetting behavior of pDMAEMA brush surfaces against external stimuli such as solution pH, salt concentration, and solution temperature, in terms of their surface physical/chemical properties, including their thickness, chemical composition, and degree of protonation and hydration.
pH-Responsive Behavior It is widely known that pDMAEMA is a pH-responsive polymer. In this section, we studied the changes in the physical/chemical properties which take place on the pDMAEMA brush surfaces at different solution pH values in more detail using in-situ spectroscopic ellipsometry. Figure 2 shows the ellipsometrically measured thickness of a pDMAEMA brush surface as it was titrated from high to low pH (indicated by the black open circles). As shown, a brush of 58 nm dry thickness swelled to around 80 nm when submerged in water at pH 10 by the uptake of water and ions. From the refractive index of the hydrated layer, we estimated its composition to be ~55 % v/v polymer at high pH, with the remaining 45 % being water and ions. This thickness and composition data is consistent with the expected nature of the polymer brush at high pH values, where tertiary amine groups within the polymer chains are mostly neutral (deprotonated), and so the polymer layer is only moderately swollen by water. Upon decreasing solution pH, a high degree of tertiary amine groups become ionized (protonated). This ionization leads to repulsion between the neighboring polymer chains and an increase in osmotic pressure. This pressure is readily relieved by an uptake of water and ions into the brush layer, which gives an increase in thickness. Subsequently, the pDMAEMA brush layer is swollen to about 3 times its dry thickness at low pH (~160 nm). Because of this increase in volume, the composition of the brush layer became richer in water (~70 % at low pH) and contained less polymer (~30 % at low pH). At intermediate pH values the polymer brush had a thickness and composition proportional to the calculated fraction of ionization48 based on a pKb of 7.5
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(indicated by blue open circles in Figure 2), consistent with other literature reports of pKb = 7.7.49 Transitions in thickness induced by changes in pH appeared to occur very quickly, faster than we were able to measure using ellipsometry (
