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Antifouling Properties of Fluoropolymer Brushes toward Organic Polymers: The Influence of Composition, Thickness, Brush Architecture, and Annealing Zhanhua Wang†,‡ and Han Zuilhof*,‡,§ †
Material innovation institute (M2i), Mekelweg 2, P.O. Box 5008, 2600 GA Delft, The Netherlands Laboratory of Organic Chemistry, Wageningen University, Stippeneng 4, 6708 WE Wageningen, The Netherlands § Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: Fluoropolymer brushes are widely used to prevent nonspecific adsorption of commercial polymeric or biological materials due to their strongly hydrophobic character. Herein, a series of fluoropolymer brushes with different compositions, thicknesses and molecular architectures was prepared via surface-initiated atom transfer radical polymerization (ATRP). Subsequently, the antifouling properties of these fluoropolymer brushes against organic polymers were studied in detail using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements and polystyrene as a representative fouling polymer. Among all of the molecular architectures studied, homopolymerized methacrylate-based fluoropolymer brushes (PMAF17) show the best antifouling properties. Annealing the fluoropolymer brushes improves the antifouling property dramatically due to the reregulated surface composition. These fluoropolymer brushes can be combined with, e.g., micro- and nanostructuring and other advanced materials properties to yield even better long-term antifouling behavior under harsh environments. Recently, we reported the first systematic study of fouling by a wide range of polymers with variable molecular weight onto high-quality fluorinated monolayers on ultraflat Si surfaces.11 This choice was based on the high degree of control that is available in the construction of such monolayers,12,19 whereas the ultraflat substrate rigorously decouples the contributions of molecular structure and surface roughness. There we found that partially fluorinated monolayers, especially the monofluoro alkyne-derived Si−CHCH−C13H28CH2F monolayer F1, showed excellent antifouling behavior against a range of organic polymers with different molecular weights even on atomically flat Si surfaces. However, while highly successful for most of the tested polymers, in two regimes limitations were observed. First, for a variety of polymers, fouling was still observed in solvents of low polarity. Second, for two specific (pyridine moiety-containing) polymers P2VP and P4VP, heavy fouling was also found on the F1 monolayer. This was attributed to a strong interaction between the single, partially negatively charged F atom and protons on protonated N atoms in these polymers. Finally, the mechanical stability of monolayer-based systems is a facet to be improved, since
1. INTRODUCTION The fouling of surfaces by biological entities (from isolated proteins and other biopolymers to entire organisms) has been studied in great detail.1−3 This has led to a deep understanding of at least the initial stages of biofouling and the design of novel materials4 that successfully repel such fouling.5 Such progress has been nearly fully absent in the field of fouling by polymers in nonaqueous, organic media.6,7 Such fouling,8,9 especially around narrow orifices, is of significant industrial importance, e.g., in food processing, paper manufacturing, and highresolution 3D printing.10 Yet, there is only very little literature on this topic that probes the mechanisms behind it down to a molecular level.7,11 Basically, all reported studies use foulingrelease coatings, such as fluorine-containing monolayers11,12 or fluoropolymers,13,14 to prevent nonspecific adsorption of polymer/plastic materials from surfaces.15,16 These coatings are based on the concept that physical removal of fouling agents appears to be easiest from materials with low surface free energies.17,18 However, a common problem with such materials is their typically low substrate adhesion, as well as difficult processing due to low solubility in common organic solvents. The production of surfaces modified with covalently bound fluorine-containing monolayers19,20 or fluorinated polymer brushes21,22 would overcome all of above problems. © 2016 American Chemical Society
Received: February 22, 2016 Revised: May 27, 2016 Published: June 22, 2016 6571
DOI: 10.1021/acs.langmuir.6b00695 Langmuir 2016, 32, 6571−6581
Article
Langmuir Scheme 1. Schematic Representation of Surface Functionalization, Initiator Immobilization, and Preparation of the Fluoropolymer Brushes on Si(111) under Current Studya
a
All polymer brushes grown at 110 °C in TFT and catalyzed by CuBr.
these are relatively easily degraded under wear. All of these issues are overcome in a more recent study from our lab,14 by covalently grafting a 75 nm fluoropolymer brush onto a Si(111) surface. This polymer brush displayed even better antifouling properties against a range of polymers in different organic media than the monolayers. Apart from improved antifouling, we also found that upon damage in harsh environments (high/ low pH or UV) the antifouling property of the fluoropolymer brush can be repaired many times by a simple heat treatment. Given the highly promising self-healing and antifouling characteristics of this fluoropolymer brush, more detailed work on the factors governing this behavior is of significant interest. Specifically, the influence of the composition, brush thickness, molecular architecture,23 and effects of annealing on the antifouling property deserve to be systemically studied. Varying the thickness is really important given two opposing trends: On the one hand, ultrathin coatings will be most conformal to surface structures, and be useful on nanopatterned surfaces without effectively removing that structuring. On the other hand, we reasoned that a minimal thickness might be required for optimal antifouling and self-healing properties, as only for polymer coatings with a thickness greater than ca. 50 nm we hypothesize that the fluoropolymer brush possesses a roughly constant glass transition temperature (Tg) and this value is close to the corresponding bulk polymer.24,25 This means that above this threshold thickness the thermo-induced movement of the top of the polymer brush is independent of the substrate, which is favorited for the self-healing process. The role of the molecular architecture (structure and flexibility of the brush) can be of importance as they codetermine the interactions of the brush with both the solvent and any fouling polymeric solutes. This can be studied by co- or block-polymerizing two fluoro-monomers with the same length of fluorinated tails, but
different polymerized units (methacrylate and acrylate). Finally, annealing the fluoropolymer brush will induce the reregulation of the surface composition, which further influences the surface hydrophobicity and antifouling properties.26 To study the influence of all these parameters on the antipolymer-fouling characteristics, we prepared a series of fluoropolymer brushes (Scheme 1) with different thicknesses, molecular composition (acrylate- or methacrylate based), and brush architectures (homopolymer or block or randomly mixed polymer brushes) by surface-initiated atom transfer radical polymerization (ATRP).27 These freshly modified and dried surfaces were dipped overnight in a polystyrene (PS)-toluene solution, taken out, cleaned, and analyzed. PS was taken as a model fouling polymer, since it is a highly used polymer that also displayed rather severe fouling behavior in previous studies,11,14 and which thus allows to delineate the various factors involved. Static contact angle (SCA), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the fluoropolymer brushes and their antifouling properties. To allow a simple and generic way to analyze the fouling, a previously developed AFM−based analysis method11 was used to distinguish quantitatively between the fouled and non−fouled surface area (areas are considered to be fouled if height is above a threshold value). Finally, we provide a rationalization of the observed trends in polymer adsorption behavior on these different fluoropolymer brushes in terms of the polymer−solute−surface interactions and the ability of the polymer brush to rearrange itself in response to components in the medium.
2. EXPERIMENTAL SECTION 2.1. Materials. 10-Undecyn-1-ol (97%) was purchased from ABCR and used as received. Polystyrene (PS, Mn = 20 00 000 g/mol, PDI = 1.30) was bought from Alfa Aesar. α,α,α-Trifluorotoluene (TFT), 6572
DOI: 10.1021/acs.langmuir.6b00695 Langmuir 2016, 32, 6571−6581
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in 5 mL of TFT and kept at 110 °C for 30 min. Then the modified surface was taken out and sonicated in FC-40 for 30 min, followed by sonication in TFT for 10 min to remove any physisorbed reaction components, and dried under a stream of argon. This freshly prepared homopolymer brush was immediately used to carry out a follow-up ATRP reaction with the same procedure as used for the homopolymerization, and the resulting copolymer brush was then washed following the same procedure used for the homopolymer brush. 2.3.3. Annealing Polymer Brush. Freshly prepared and completely dried polymer brushes were put in a 120 °C oven [no vacuum] for 2 h, and then cooled down to room temperature gradually. 2.4. Polymer Adsorption Behavior onto These Fluoropolymer Brushes. Clean and well-characterized polymer brushes were used for our fouling studies. For all polymer fouling experiments, the concentration of the fouling PS polymer was 10 mg/mL in toluene. The well-cleaned surface was immersed into the polymer solution for 12 h, and then taken out, washed with toluene for 2 min in an autoshaker (at 50 rpm), taken out, and rinsed, and this washing cycle was performed in total three times. Next, the sample was dried with argon stream and put into vacuum for 2 h. Unfouled polymer brushes are not affected in any observed manner by this sonication and drying treatment. The extent of absorption (fouling) and the morphology of the adhered polymer on these monolayers were characterized by XPS and AFM measurements. 2.5. Static Contact Angle Measurements. The static contact angle measurements were conducted using a Krüss DSA 100 contact angle goniometer having an automated drop dispenser and an image/ video capture system. The static contact angles were measured at three different places on a modified surface by dispensing three small droplets (3.0 μL volume of deionized water or toluene) with the help of an automated drop dispenser. The tangent 1 fitting model was implemented for contact angle measurements with an accuracy of ±2°. 2.6. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra at ambient temperature were obtained using a JPS9200 photoelectron spectrometer (JEOL, Japan) for all 1 × 1 cm2 samples used in the study of the antifouling experiment. A monochromatic Al Kα X-ray source (hν = 1486.7 eV, 12 kV and 20 mA) with an analyzer pass energy of 10 eV was used. A base pressure of 3 × 10−7 Torr was maintained in the XPS chamber during measurements and the spectra were collected at room temperature. The X-ray incidence angle and the electron acceptance angle was 10° to the surface normal. The takeoff angle φ (angle between sample and detector) = 80° ± 1°. The intensity of the XPS core-level electrons was measured as the peak area after a linear baseline background subtraction. All XPS spectra were analyzed using the Casa XPS software (version 2.3.15), and for deconvolution a symmetrical GL(30) line shape was employed, which consists of a Gaussian (70%) and a Lorentzian (30%) component. The full width halfmaximum (fwhm) of each component was constrained to 1.0−1.1 eV. The relative areas of each component peak were fixed by the stoichiometry of the main hydrocarbon peak, which was assigned to aliphatic carbon (CH2) with a binding energy of 285.0 eV. 2.7. Ellipsometry. The ellipsometric thickness of the modified surfaces was measured using a rotating Sentech Instruments (Type SE400) ellipsometer, operating at 632.8 nm (He−Ne laser), and an angle of incidence of 70°. The optical constants of a freshly etched Hterminated Si(111) surface were taken as n = 3.850 and k = 0.057. The thicknesses of the polymer brushes were determined with a planar three layer (ambient, polymer brush, substrate) isotropic model, with assumed refractive index of 1.00 and 1.36 for ambient and the fluoropolymer brushes. The reported values are the average of at least 5 measurements. 2.8. AFM Characterization. AFM images (256 × 256 pixels) were obtained with an MFP3D AFM (Asylum Research, Santa Barbara, CA). The imaging was performed in tapping mode in air using OMCL-AC240 silicon cantilevers (Olympus Corporation, Japan) with a stiffness of 1.54 N/m. Images were flattened with a first-order flattening procedure using the MFP3D software. The root-meansquare (RMS) roughness was calculated from the fluctuations of the
dichloromethane (DCM), acetone (semiconductor grade VLSI PURANAL), 4,4′-dinonyl-2,2′-bipyridine (dNbpy), α-bromoisobutyryl bromide, copper(I) bromide (Cu(I)Br), Fluorinert FC-40, 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl methacrylate (MAF17), triethylamine, and 2-perfluorooctylethyl acrylate (AF17) were purchased from Sigma-Aldrich and used without further purification unless otherwise specified. Monomers MAF17 and AF17 were filtered through a basic alumina column to remove inhibitors before utilizing them for polymerization. Silicon wafers, with a 0.2° miscut angle along the ⟨112⟩ plane, were (111)-oriented, n-type, phosphorus-doped and with a specific resistance of 1−10 Ω cm−1, as purchased from Siltronix (France). Ammonium fluoride (Riedel-de Haën, 40%, semiconductor grade VLSI PURANAL Honeywell 17600), acetone (Aldrich, semiconductor grade VLSI PURANAL Honeywell 17617), and deionized water (resistivity 18.3 MΩ cm−1) were used as received. 2.2. Surface Modification and Initiator Immobilization. 2.2.1. Surface Functionalization. The surface functionalization and initiator immobilization were performed as reported previously.21 Briefly, a three-necked flask was charged with 2 mL of 10-undecyn-1-ol and purged with argon under reduced pressure for 30 min, while being heated to 80 °C. Si(111) wafers were cut into 1 × 1 cm2 pieces. The surfaces were sonicated for 5 min in pure acetone and subsequently cleaned using air plasma (Harrick Scientific Products, Inc. Pleasantville, NY) for 5 min and quickly transferred to freshly prepared, argonsaturated 40% aqueous ammonium fluoride solution for 15 min. The surfaces were again rinsed with water and dried with a stream of argon. These samples were then immediately transferred into the flask, which was immediately depressurized again. The reaction mixture was kept at 80 °C for 16 h. The sample was then removed from the flask and immediately extensively rinsed with CH2Cl2, sonicated for 5 min in CH2Cl2 to remove physisorbed molecules, and blown dry with a stream of dry argon. The resulting − OH terminated surfaces were directly used for surface characterization or stored in the glovebox until the initiator immobilization experiments. 2.2.2. Initiator Immobilization. A 25 mL vial containing 1 mL of dichloromethane and a clean hydroxyl-terminated surface was flushed using a stream of dry argon for 5 min. Initiator α-bromoisobutyryl bromide (2 mL) and triethylamine (5 drops) were added into the vial. The inert atmosphere was maintained by a continuous flow of argon for 5 min and followed by shaking at 80 rpm at 25 °C for 2 h. The resulting initiator-functionalized surface was washed using dichloromethane several times, sonicated for 5 min and dried with a stream of argon. 2.3. Synthesis of Fluoropolymer Brushes. 2.3.1. Preparation of Homopolymer Brush or Irregular Copolymer Brush. A single-neck 15 mL flask was filled with an initiator-immobilized Si(111) surface, 1 mL of monomer MAF17 or monomer AF17 or of a mixture of these monomers, 1 mL of TFT, and 25 mg of dNbpy. The mixture was put into liquid nitrogen, and after the mixture was completely frozen, 5 mg of CuBr was added. The flask was connected with a vacuum line and depressurized to 1 mbar, and then the frozen mixture was slowed thawed, after which the flask was filled again with argon. After repeating this freeze−pump−thaw cycle, the Ar-filled system was cooled in liquid nitrogen, placed under vacuum, and allowed to slowly warm up to room temperature. The reaction mixture (in closed flask) was then brought to 110 °C for a defined reaction time to obtain various thicknesses of the resulting polymer brushes. This procedure yielded faster and more reproducible brush thicknesses than performing the reaction under argon, as we reported previously.21 The polymerization was stopped after this fixed time by exposing the reaction mixture to air. The mixture was diluted with 5 mL of TFT and kept at 110 °C for 30 min. Then the modified surface was taken out and sonicated in FC-40 for 30 min, followed by sonication in TFT for 10 min to remove any physisorbed reaction components, and dried under a stream of argon. Finally, the surface was put under mild vacuum (∼10 mbar) overnight at room temperature to eliminate any left solvents. 2.3.2. Preparation of Block Copolymer Brush. To prepare block copolymer brushes, a freshly prepared homopolymer brush was placed 6573
DOI: 10.1021/acs.langmuir.6b00695 Langmuir 2016, 32, 6571−6581
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Figure 1. XPS wide (A) and carbon narrow (B) scan of PAF17 brushes.
Table 1. Ellipsometric Thickness, AFM-Determined RMS Surface Roughness, and Static Water and Toluene Contact Angle (±1°) of PMAF17 and PAF17 Brushes with Different Thicknesses PMAF17 sample name reaction time (h) thickness (nm) roughness (nm) water SCA (deg) toluene SCA (deg) a
S1 0.25 4.7 0.23 121 63a
S2 0.5 16.0 0.25 122 69a
PAF17 S3 1 30.3 0.35 121 75
S4 2 50.9 0.44 122 77
S1 16 6.9 0.27 121 74
S2 40 18.2 0.33 122 76
S3 64 26.9 0.54 121 77
S4 120 54.7 1.85 122 79
Note: for these surfaces, toluene SCA ± 2°.
surface height around the average height in the image. In this way, the RMS value describes the topography of the surface. To quantitatively determine the polymer surface coverage at low surface coverages, the measured surface height at a given pixel was compared to a predetermined threshold value. If the measured height exceeded the threshold, a polymer was considered to be adsorbed at that position. Analysis of the entire surface allowed the determination of the fraction of polymer surface coverage (i.e., the fraction of surface with a height above the threshold value).
carbons in a single monomeric unit of the polymer brushes. From a surface morphology survey of the brush by AFM a low root-mean-square (RMS) average roughness can be inferred (typically
