Hydrophilic Polymer Brush Layers on Stainless Steel Using

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Hydrophilic Polymer Brush Layers on Stainless Steel Using Multilayered ATRP Initiator Layer Jakob Ege Friis,† Kaare Brøns,† Zakaria Salmi,‡ Kyoko Shimizu,○ Guruprakash Subbiahdoss,∥ Allan Hjarbæk Holm,# Olga Santos,◆ Steen Uttrup Pedersen,‡,∥ Rikke Louise Meyer,§,∥ Kim Daasbjerg,‡,⊥,∇,∥ and Joseph Iruthayaraj*,†,∇,∥ †

Department of Biological and Chemical Engineering, Aarhus University, Hangøvej 2, DK-8200 Aarhus N, Denmark Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark § Department of Bioscience, Aarhus University, Ny Munkegade 116, DK-8000 Aarhus C, Denmark ∥ Interdisciplinary Nanoscience Center, Aarhus Univeristy, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark ⊥ Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden ∇ Carbon Dioxide Activation Center, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark ○ SACHEM Japan GK 5-6-27 Mizuhai, Higashi Osaka 578-0921, Japan # Grundfos Holding A/S, Poul Due Jensens Vej 7, DK-8850 Bjerringbro, Denmark ◆ Materials and Chemistry Center, Alfa Laval Lund AB, P.O. Box 74, SE-22100 Lund, Sweden ‡

S Supporting Information *

ABSTRACT: Thin polymer coatings (in tens of nanometers to a micron thick) are desired on industrial surfaces such as stainless steel. In this thickness range coatings are difficult to produce using conventional methods. In this context, surface-initiated controlled polymerization method can offer a promising tool to produce thin polymer coatings via bottom-up approach. Furthermore, the industrial surfaces are chemically heterogeneous and exhibit surface features in the form of grain boundaries and grain surfaces. Therefore, the thin coatings must be equally effective on both the grain surfaces and the grain boundary regions. This study illustrates a novel “periodic rejuvenation of surface initiation” process using surface-initiated ATRP technique to amplify the graft density of poly(oligoethylene glycol)methacrylate (POEGMA) brush layers on stainless steel 316L surface. The optimized conditions demonstrate a controlled, macroscopically homogeneous, and stable POEGMA brush layer covering both the grain surface and the grain boundary region. Various relevant parameters surface cleaning methods, controllability of thickness, graft density, homogeneity and stabilitywere studied using techniques such as ellipsometer, X-ray photoelectron spectroscopy, scanning electron microscopy-energy-dispersive X-ray, surface zeta potential, and infrared reflection-adsorption spectroscopy. KEYWORDS: coating, surface Initiated ATRP, stainless steel 316L, graft density, poly(oxyethyleneglycol)methacrylate, diblock polymer brush, grain boundary, electrografting, aryldiazonium

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INTRODUCTION Polymer coatings on metals are used to modify interfacial properties such as adhesion, friction, wetting, thermal, electrical, and optical properties. Industrial coating methods are optimized to produce coatings of large thicknesses (tens of micrometers) that rely on properties such as wettability and/or physical adhesion of the coating onto metal substrates. In certain industrial applications ultrathin polymer coatings (several tens of nanometers to a micron) are sought to achieve high performance-to-energy ratio. Nevertheless, unlike conventional coating methods, control of ultrathin polymer coatings (in tens of nanometers) requires consideration of different set of criterianature of chemical affinity between polymer and the metal, the number and density of the surface anchoring points. © XXXX American Chemical Society

Surface-initiated controlled radical polymerization is a class of surface modification tools that utilizes graf ting-f rom technique to engineer surface properties.1 In surface-initiated atom transfer radical polymerization (SI-ATRP) the controllability is achieved via the regulation of the halogen atom between the active radical site and the inactive dormant state.2 Several types of polymer brushes including hydrophilic and superhydrophilic polymer brushes with imparted antifouling and anti-icing properties have been demonstrated using SI-ATRP method.3−5 Different Received: August 20, 2016 Accepted: October 11, 2016

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DOI: 10.1021/acsami.6b10466 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

4-(2-hydroxyethyl)benzenediazonium tetrafluoroborate (BD-OH) was synthesized according to the published procedure.19 The acylating agent, 2-bromoisobutyryl bromide (BIBB 98%), the ligand 2,2′-bipyridyl (BIPY ≥ 99.0%), the monomers, oligo(ethylene glycol) methacrylate, OEGMA (Mn ≈ 360 g mol−1 containing 500−800 ppm MEHQ as inhibitor), and [2-(methacryloyloxyethyl)trimethylammonium chloride], METAC (80 wt % in water), copper bromide (99.9%), and copper dibromide (99.9%) were used as received from Sigma-Aldrich without further purification. Chart 1 shows the chemical structure of the compounds employed in this work. Substrate. Stainless steel sheets type 316L (SS) of thickness 0.4 mm were obtained as flat sheets from Alfa Laval AB, Sweden. The sheets were precision cut to required size using a third-party laser cutting equipment (Jensen Metal A/S, Aarhus, Denmark). The laser cutting procedure ensured macroscopically a flat surface thereby rendering surface measurements feasible. Surface Cleaning of Stainless Steel Plates. First the SS plates were rinsed using acetone and dried. The dried plates were placed in individual glass tubes, and freshly prepared Piranha solution was distributed into the tubes. Piranha is a strong oxidative acidic mixture consisting of H2SO4 and 30% H2O2 (3:1 v/v). The plates were immersed in the Piranha solution for 10 min at 80 °C and rinsed thoroughly with Milli-Q water. Furthermore, the plates were left in Milli-Q water for 8 h at 70 °C, rinsed with ethanol, and dried using nitrogen gas. Scheme 1 illustrates the various chemical steps involved in the synthesis of the different types of polymer brushes on the SS substrate. Electrochemical Grafting. The precursor layer consisting of 2-hydroxyethylaryl units was prepared on the SS surface using electrografting method. A three-electrode setup consisting of a cleaned SS plate as working electrode, platinum wire as auxiliary electrode, and a pseudoreference electrode (Ag wire immersed in 2 mM of TBAI prepared in 0.1 M solution of TBABF4 in MeCN) was used for electrografting. First, a background cyclic voltammogram was recorded in 10 mL of 0.1 M TBABF4 in MeCN at a sweep rate of 0.1 V s−1 to check for any electrochemically active impurities. After ensuring the cleanliness of the system 5.3 mg of BD-OH (2 mM) was added under inert atmosphere by using continuous flow of argon gas. After complete dissolution of the compound, the first voltammogram was recorded at a sweep rate of 0.1 V s−1. The peak potential (Ep) of the reduction wave corresponding to the electrochemical reduction of BD-OH was noted. The second volatmmogram was recorded after the solution was stirred for sufficient time. In the subsequent step the SS plate was maintained at a constant reduction potential of Ep − 0.2 V for 300 s. During this time the current value was monitored, and the solution was gently kept under stirring. After 300 s of electrolysis the third voltammogram was recorded before removing the surface from the electrochemical cell and rinsed thoroughly using EtOH. The modified surfaces are denoted as SS-OH. The potential values measured with respect to the pseudoreference electrode were reported versus standard calomel electrode (SCE). By measuring the standard potential of ferrocenium/ferrocene redox couple (2 mM in 0.1 M solution of TBABF4 in MeCN) with respect to the pseudoreference electrode (EoFc+ vs pseudo reference = 0.79 V, glassy carbon as working electrode) the measured potential was referenced to SCE using the previously determined value for EoFc+ vs. SCE = +0.380 V.24 Acylation. SS-OH substrates were immersed in a solution mixture comprising of 8.7 mL of DCM, 0.62 mL of BIBB (0.5 M), and 0.7 mL of

chemical strategies have been used to covalently immobilize ATRP initiators on various model surfaces including silicon wafers, metal-coated silicon wafers, and mica.6−8 The need to protect industrial metal surfaces from scaling, fouling, and corrosion has initiated several research works in pursuit of robust polymer-based modification onto stainless steel surfaces. Techniques such as graf ting to, graf ting f rom, and layerby-layer deposition have been examined previously.9−11 Notably the covalent attachment of polymer chains via SI-ATRP technique have been reported on stainless steel surfaces using different types of ATRP initiator layer structuresmethoxysilanes, phosphonic acids, catechols, and electropolymerized layer.12−17 Alternatively, multilayered ATRP initiator layer was employed on conducting surfaces including carbon, stainless steel, and other metal oxides using electrochemical reduction of aryldiazonium tetrafluoroborate salts.18−21 All previous works have employed mechanical/ electrochemical polishing of stainless steel prior to polymer modification to eliminate grain boundary roughness. However, polishing methods are not always practically implementabledue to cost, sample geometry, and variation in surface chemical compositionnecessitating the investigation of chemical homogeneity of the coating method along the grain boundary regions. In this study, we systematically investigate various relevant parameterseffect of surface cleaning, graft density, thickness, homogeneity, and stabilitypertaining to surface-initiated polymerization of hydrophilic monomers on a multilayered ATRP initiator layer prepared via electrografting onto stainless steel 316L. Furthermore, a simple approachperiodic rejuvenation of surface initiationis successfully demonstrated to create mechanically stable and controlled thickness of poly(oligoethylene glycol methacrylate) (POEGMA) brush layers. Similar chemical approach has been used in a different context to synthesize surface-confined comb polymers.22,23 The chemical homogeneity of the POEGMA brush layer is investigated by measuring scanning electron microscopy (SEM)-energy-dispersive X-ray (EDX) across the grain boundary junctions. Finally the rejuvenation method is employed to synthesize diblock polymer brush layers comprising of an uncharged POEGMA brush layer followed by a layer of polycationic brush layer, poly[2-(-(methacryloyloxyethyl)trimethylammonium chloride], PMETAC.

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MATERIALS AND METHODS

Solvents and Reagents. Sulfuric acid (95.0−98.0%), hydrogen peroxide solution (30 wt % in H2O), acetone ≥99.0%, acetonitrile (MeCN ≥ 99.9%), ethanol (EtOH, 96%), isopropanol (i-PrOH ≥ 99.9%), dichloromethane (DCM ≥ 99.8%), and water (Millipore, Milli-Q , 18.2 MΩ cm−1). Chemicals. Tetrabutylammonium tetrafluoroborate (TBABF4 ≥ 99.0%), tetrabutylammonium iodide (TBAI ≥ 99.0%), and triethylamine (TEA) were used as received. The electrografting agent,

Chart 1. Chemical Structures of the Major Compounds Used in This Work

B

DOI: 10.1021/acsami.6b10466 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Chemical Illustration of Various Steps Involved in the Preparation of Surface Initiated Poly(OEGMA) Brush Layers and Poly(OEGMA-co-METAC) Block Copolymer Brush Layers on Stainless Steel Substratea

a (A) Electrografting step. (B) Acylation reaction. (C) Surface-initiated polymerization. (D) Repeat polymerization without initiator renewal and (E) repeat polymerization with initiator renewal. The superscript asterisk (*) is indicative of initiator renewal process between polymerization steps.

TEA (0.5M) at 0 °C for 2 h. After the reaction the substrates were rinsed thoroughly in DCM followed by sonication in DCM and EtOH. In some

cases the acylation reaction was conducted following the same protocol, except that no TEA was added. The brominated plates were kept C

DOI: 10.1021/acsami.6b10466 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces in freezer until further use. Typical storage time was maximum 24 h. The surfaces acylated with and without TEA are denoted as SS-Br+TEA and SS-Br−TEA, respectively. Synthesis of Polyoligo(ethylene glycol)methacrylate Brushes. In a typical SI-ATRP process 430 mg of the ligand BIPY (0.14 M) was placed in a clean Schlenk flask and dissolved using 20 mL of 1:1 i-PrOH/Milli-Q water mixture at 35 °C under argon atmosphere. Next, 70 mg of deactivator copper dibromide (0.017 M) was added to the solution and allowed to dissolve. Thereafter 6.6 mL of the monomer, OEGMA (1.02 M), was added, and the solution was allowed to stir under argon flow for 10−15 min before the addition of 86 mg of the activator, copper bromide (0.03 M; The copper bromide salt is only partially soluble in the ATRP mixture). The activator was allowed to dissolve under argon flow. While the solution was purged under argon, the surfaces (SS-Br+TEA/ SS-Br−TEA) were carefully clamped onto the stainless steel clamps that were previously mounted themselves through a rubber septum. Each batch comprised of five substrates. Next, the septum on the Schlenk flask was quickly replaced by the septum holding the substrates and sealed properly. The solution was then purged for 10 min before immersing the substrates. During the polymerization, the solution was purged gently. After the prescribed time, each sample was removed from the reaction medium, thoroughly rinsed and sonicated in Milli-Q water followed by EtOH, and dried under nitrogen. The modified surfaces are denoted as SS-POEGMA-Br+TEA (SS-POEGMA-Br−TEA). Synthesis of PMETAC Brushes. An initiator layer was prepared on top of the SS-POEGMA-Br−TEA surface following the same acylation procedure as in the case of SS-OH. The modified surfaces are denoted as SS-POEGMA-Br*−TEA (Scheme 1E). The superscript asterisk is indicative of the initiator renewal step conducted between successive polymerization steps. In a typical process, 430 mg of the ligand BIPY (0.14 M) dissolved in 20 mL of 1:1 i-PrOH and Milli-Q water at 35 °C under an argon atmosphere. Next, 70 mg of copper dibromide (0.017 M) was added to the solution and allowed to dissolve. 6.39 mL (1.7 M) of the METAC monomer was added to the reaction container and the SS-POEGMA-Br*−TEA surfaces mounted through a septum. Finally, 86 mg of copper bromide (0.03 M) was added to initiate the polymerization, and the reaction was stirred for 4 h at 35 °C under an argon atmosphere. Each sample was removed from the reaction medium and thoroughly rinsed and sonicated in Milli-Q water followed by EtOH. The modified surfaces are denoted as SS-POEGMA-PMETAC-Br*−TEA. Surface Analysis Techniques. Detailed description of the surface measurement techniquesellipsometry, infrared reflection-absorption spectroscopy (IRRAS) spectroscopy, Zeta potential, SEM, and X-ray photoelectron spectroscopy (XPS)are presented in the Supporting Information (SI-1).

Figure 1. Cyclic voltammograms of the electrografting agent (BD-OH) using SS (1 cm × 1 cm) as working electrode showing the (1) first cycle, (2) second cycle, and (3) third cycle recorded after electrolysis.

Figure 1 shows the current (i) versus potential (E) plot for a cyclic potential sweep between 0.0 and −1.2 V versus SCE at a sweep rate of 0.1 V s−1. The first cycle shows a characteristic reduction wave with Ep = −0.53 V versus SCE corresponding to the electrochemical reduction of BD-OH. After the solution was stirred, the second cycle exhibits a large decrease in the current signal indicating that the working electrode surface (SS) has been grafted with a thin organic insulating layer. The grafted layer blocks the further reduction of the diazonium compound, thereby giving rise to the decrease of the current signal. After the second cycle the SS plate was held at a constant potential of (Ep−0.2)V for 300 s to form a fully blocking film comprising of a multilayer of 2-hydroxyethylphenyl units (i.e., SS-OH). This is further confirmed by recording a third cyclic voltammogram showing the least current response (Figure 1). The average thickness of the dry electrografted layer as measured by ellipsometry is 2.9 ± 0.8 nm, which corresponds to 3−4 monolayers of 2-hydroxyethylphenyl units. The mechanism of multilayer formation in the electrografting of substituted aryl diazonium cations has been discussed in many previous works.25−27 One characteristic of this multilayer structure is that some parts of the structure are connected through azo (-NN-) groups, which are also evident herein from the presence of nitrogen signal in the XPS spectra (Table 1). This layer characteristic will be used later to estimate the relative content of the initiating groups using the XPS data. Effect of Triethylamine Concentration on Thickness and Polymerization Rate. The surface hydroxyl groups of SS-OH were acylated to attach the ATRP initiator units (i.e., the 2-bromoisobutyryl group) using BIBB and TEA as a tertiary base (Scheme 1B). Because of the surface confinement of the hydroxyl groups high concentration of HBr can accumulate in the close vicinity of the surface during the acylation process. Therefore, the functionary role of TEA is to neutralize HBr and thereby prevent it from reacting with and degrading the film. It can be anticipated that the rate of HBr removal could induce some indirect effects on the polymer brush layer. To investigate this aspect systematically several SS-Br surfaces were prepared by varying the concentration of TEA during the acylation reaction. Further, the SS-Br surfaces were subjected to polymerization using the OEGMA monomer to produce SS-POEGMA-Br surfaces (Scheme 1C). Figure 2 depicts the dry layer thickness obtained after 4 h of polymerization of OEGMA against the concentration of TEA used in the acylation step. The plot shows that the layer thickness

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RESULTS AND DISCUSSIONS Precleaning Step. The surface properties of SS substrate exhibited significant changes after the cleaning process using piranha. The surface became extremely hydrophilic (contact angle < 10°). The surface organic contaminant decreased by 37%, and the zeta potential (ζ) versus pH measurements featured a negatively charged surface wherein the ζ potential varied linearly between pH 3 and 6 at −8.7 mV/pH. Finally homogeneous POEGMA brush layers were obtained after this precleaning step. The XPS data comparing the C/O ratio pertaining to different surface-cleaning methods and the plot of ζ versus pH are shown in the Supporting Information (Table S1 and Figure S1). Other two surface cleaning methods using alkaline solution or rinsing with acetone resulted in nonhomogeneous polymer layer and therefore were not pursued further. The quality of the POEGMA layer on SS plates cleaned using different methods was assessed by measuring the thickness and the optical images. The results are shown in Supporting Information (Figure S2). Initiator Layer. As shown in Scheme 1A the first step toward the formation of the ATRP initiator started with the electrografting of the compound (BD-OH) onto SS in MeCN. D

DOI: 10.1021/acsami.6b10466 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Relative Surface Chemical Composition and Thickness of the Corresponding Films on SS-Br −TEA, SS-POEGMA-Br −TEA, SS-Br+TEA, and SS-POEGMA-Br+TEA Surfaces substrate

SS-Br−TEA

SS-POEGMABr−TEA

SS-Br+TEA

SS-POEGMABr+TEA

[TEA]/M thickness/ nm element

0 2.9 ± 0.8

0 13.8

0.5 2.9 ± 0.8

0.5 59.7

C 1s O 1s N 1s Br 3p Fe 2p Mn 2p Cr 2p Ni 2p Mo 3d Br/N Br/O C/O

atom %

atom %

atom %

atom %

64.7 22.7 4.5 1.1 1.2 0.63 2.7 0.18 0.31 0.24 − 2.9

67.9 31.9 − 0.08 − − 0.18 − − − 0.0025 2.1

63.2 23.9 4.7 2.3 1.6 0.78 3.0 0.13 0.36 0.49 10.4 2.6

68.1 28.6 − 3.4 − − − − − − 0.12 2.4

Figure 3. Dry layer thickness vs polymerization time for the polymerization of OEGMA initiated on the surfaces (■) SS-Br+TEA first batch, (□) SS-Br+TEA second batch, and (○) SS-Br−TEA. The error bars correspond to standard deviation on the same substrate.

SS-Br+TEA) than if no TEA had been used (rate = 4.2 nm h−1). In general, for the initiator layer prepared without TEA (denoted SS-Br−TEA) the resulting POEGMA layer on the SS-POEGMABr−TEA surface is uniform and thin (