PEG-Grafted Polymers and Polymer

Article pubs.acs.org/Langmuir

Nanocomposite Films of Laponite/PEG-Grafted Polymers and Polymer Brushes with Nonfouling Properties Ekram Wassel,† Martha Es-Souni,†,‡ Nele Berger,† Dimitri Schopf,† Matthias Dietze,† Claus-Henning Solterbeck,† and Mohammed Es-Souni*,† †

Institute for Materials & Surface Technology, University of Applied Sciences, 24149 Kiel, Germany Cell Culture Laboratory, Clinic of Dentistry, University of Kiel, 24118 Kiel, Germany

‡

S Supporting Information *

ABSTRACT: We explore the suitability of nanocomposite thin films based on laponite nanomaterial and grafted antiadhesive polymers as transparent nonfouling surfaces. For this purpose, two polymers were chosen: a linear poly(ethylene glycol) (PEG) silane, 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane), and thermoresponsive poly(oligo ethylene glycol)-methyl ether-methacrylate (POEGMA) brushes. PEG silane was grafted on the laponite nanoparticles in solution yielding homogeneous and transparent thin films via a dip coating procedure on glass and silicon substrates. POEGMA was grafted on laponite-(3-Aminopropyl)trimethoxysilane (APTMS) nanocomposite films that were processed similarly to PEG-silane using atom transfer radical polymerization (ATRP). Film characterization with, among others, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM) attests to successful grafting of the polymers to the laponite nanoparticles. In particular, evidence of basal plane expansion of laponite with increasing silane concentration are obtained using XRD, while patent morphological changes are revealed with AFM. The results are discussed in terms of the different grafting sites on laponite and compared with literature. While LP-PEG-silane is easily applied to a surface from a precursor solution via a dip coating procedure LP-APTMS-OEGMA requires lots more chemicals, a thorough control of reaction parameters, and longer reaction time in order to generate films with the desirable properties. We therefore also addressed the antifouling properties of the films. These were tested together with control samples of bare glass and laponite thin films for 30 days in an algae container. More tests were conducted with fibroblast cell cultures. Our preliminary results show that grafting of PEG containing polymers and polymer brushes alters the properties of the laponite films from fouling to nonfouling surfaces. As a first estimate, the adhesion of particles (diatoms, algae, etc.) to surfaces is reduced by approximately 85% in the case of LP-PEG-silane and up to 92% in the case of LP-APTMS-POEGMA, in comparison to the control surfaces. Furthermore, practically no cell adhesion on such surfaces could be observed.

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brushes are perfect candidates for soft nanoscience.16,17 Polymer brushes possess another remarkable property that allows them to prevent nonspecific adsorption of biomolecules (proteins, cells, bacteria, organisms), and thereby makes them attractive candidates for surface modification of biomaterials. 8,18 In addition, these nonfouling and switchable polymeric systems exhibit also different surface-active characteristics such as wettability, friction, and motility.15,18 Antifouling polymers and polymer brushes containing poly(ethylene glycol) (PEG)19 are particularly attractive because they are easily soluble in water, nontoxic and possess low immunogenicity.20−22 Hence, these polymers21,23−25 and polymer brushes26−34 belong to the most widely investigated materials, and the mechanisms underlying their antiadhesion/nonfouling

INTRODUCTION Nonfouling materials based on polymers and polymer brushes have been garnering, and continue to attract, widespread research interest because of their potential applications in such areas covering medical, maritime, and water purification technology.1−4 Polymer brushes are generated by the grafting of macromolecular chains with one end onto a solid surface such as glass, SiO2 or gold. The high density of the grafted chains entails their overlapping and therefore their stretching away from the surface. The synthesis of these polymeric systems is well established, and various methods are reported in the literature that are based on physisorption5−7 or chemisorption subdivided in grafting to,8,9 grafting from,8,9 and grafting through.10,11 Because polymer brushes are known to change their properties, e.g., wetting, in response to external triggers such as temperature, the functional surfaces thus obtained are referred to as “smart”. Thereby reversible switching between different defined states of surface properties becomes viable12−15 and hence polymer © 2017 American Chemical Society

Received: February 15, 2017 Revised: May 23, 2017 Published: June 12, 2017 6739

DOI: 10.1021/acs.langmuir.7b00534 Langmuir 2017, 33, 6739−6750

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Langmuir properties have been discussed in some details.35−37 Some of the critical factors known to affect the antifouling properties of PEGbased polymers are the chain length of PEG material, the grafting density, the strength of the hydrogen bonds between the water molecules and the PEG-containing polymer chains as well as the conformation of the macromolecular chains. The higher the chain length and the grafting density the more resistant is the polymer to the adsorption of biomolecules, cells, etc.38,39 A higher chain length leads to the formation of hydrogen bonds between water molecules and polymer chains resulting in a strong hydration of the long macromolecular chains and the generation of a so-called “excluded volume”, which causes a higher conformational entropy and entails strong repulsion forces against biomolecules, thereby protecting the underlying substrate.40,41 A higher grafting density shields the surface more effectively from the adsorption and diffusion of biomolecules.30,38,42,43 Grafting of polymer brushes has been so far reported on various surfaces, among them silicon oxide (SiO2) and gold (Au) substrates as well as glass.27−34,44,45 Little is known about grafting on layered silicate materials, which may afford an attractive strategy to tune grafting density. Clay/polymer nanocomposites may be processed via dispersing the clay into an oligomer, followed by polymerization or via chemical grafting. Major improvements in materials properties have been reported, among them “high elastic moduli”,46 “increased strength and heat resistance”,47 “decreased gas permeability”48 and “flammability”,49 and “increased biodegradability of biodegradable polymers”.50 Of particular interest are nanocomposites based on the polymer Laponite (LP).51−53 LP is a synthetic clay mineral with a well-known structure54 and has many applications such as surface coating (textured coatings), general industrial applications (pigment suspension), binding products (plasters and fillers), household products (detergents, cleaners), personal care products (toothpaste, cosmetics), and polymer films (static and antistatic coatings, inert barrier films).55,56 Its chemical formula is given by Na0.7[(Si8Mg5.5Li0.3)O20(OH)4]. The material is commercially available as disk-shaped nanoparticles approximately 1 nm thick and 25 nm in diameter.55,56 The fabrication of LP-polymer nanocomposites can be performed in two ways: (i) via cationic exchange where the interlayer Na+ metal cations-located in the spacing between the negatively charged layers of the nanoclay are exchanged with organic cations such as long chain alkyl ammonium ions, and (ii) via reaction with Si−OH edge groups of the nanoclay and covalent attachment, e.g., of organic compounds such as alkoxysilane groups.57 Of particular interest for this work is the interaction between silane and LP. According to the literature, there are three different models for the grafting of silane into clay minerals: “interlayer”, “external surface”, and “broken” edge grafting. The “interlayer grafting” of silane occurs in the spacing between the layers of the clay material58−60 and causes a significant expansion of the basal plane. Grafting on “external surface”52 leads to no change of the basal plane, while grafting on “broken edges”56 can lead to a measurable increase of the basal plane spacing. Following ref 56, this increase is ascribed to the formation of silane polymers and their ability to penetrate the peripheral interlaminate space, thus pushing the clay sheets apart.56 Owing to the beneficial effects of laponite on the properties of polymers, we explore in the present study the feasibility of

LP-silane nanocomposites that should afford an easy soft chemistry method for surface modification with adherent and mechanically stable functionalized films. The LP nanoparticles are modified with the silane in solution, and a dip-coating method is applied to glass and silicon substrates for film processing. Two main surfaces are investigated: LP-2-[methoxy(polyethyleneoxy)propyl]-trimethoxysilane), designated hereafter as LP-PEG-silane and LP-(3-aminopropyl)trimethoxysilane)-poly(oligo ethylene glycol)-methyl ether-methacrylate brushes, hereafter designated as LP-APTMS-POEGMA. For the latter, The LP nanoparticles are first modified in solution with APTMS that serves as grafting linker for nonfouling PEG-based polymer brushes. The most widely investigated materials for the functionalization of clay minerals are trialkoxysilanes, which possess the aminofunctional groups acting as a coupling units for the attachment of other functional molecules61 such as atom transfer radical polymerization (ATRP) initiator (SI-ATRP) for the fabrication of brushes. SI-ATRP is, among others, a complementary method, which allows the preparation of PEG-containing polymer brushes on solid substrates with high surface densities and thus affords a suitable process for the generation of long-term stable and antifouling surfaces, which are indispensable for biological and marine applications. While LP-PEG-silane is easily applied to a surface from a precursor solution via a dip coating procedure, LP-APTMS-OEGMA require lots more chemicals, a thorough control of SI-ATRP reaction parameters and longer reaction time in order to generate films with the desirable properties. It should therefore be of interest to compare the antifouling properties of both films, which is also addressed in this work. A detailed materials characterization is provided by Fourier transform infrared spectroscopy (FT-IR), water contact angle (WCA), X-ray diffraction (XRD), and atomic force microscopy (AFM) techniques. Structure and properties are comparatively discussed for LP-PEG- silane and LP-APTMS-POEGMA nanocomposite films.

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EXPERIMENTAL SECTION

Materials. The following materials were purchased from the listed suppliers and were used as received unless otherwise noted: Silicon (100) wafers (P/Boron type): Silchem Handelgesellschaft mbH (Freiberg, Germany); oligo(ethylene glycol) methyl ether methacrylate (OEGMA), 2,2-bipyridine (bpy) ≥ 99%, Ethyl acetate 99.8%, molecular sieve 3 Å, copper(I)bromide ≥98%, (3-aminopropyl)trimethoxysilane (APTMS) 97%: Sigma-Aldrich; aluminum oxide 90 basic for column chromatography, triethylamine (TEA) ≥ 99.5%, dichlormethane ≥99.9%, methanol 99%, coverslips (76 × 26 mm, matt edge): Roth; 2[methoxy(polyethyleneoxy)propyl]trimethoxysilane (PEG-terminated silane) 90% 6−9 PE units: abcr GmbH; 2-bromoisobutyryl bromide: Merck; Laponite (Lp) RD: Southern Clay Products; algae (Nannochloropsis salina): Korralenzucht-Moers (Moers, Nordrein Westfallen, Germany); sea salt: Tropic Marin (Moutzek meeres a quaristik GmbH, Ritterhude, Germany). A Millipore system (Evoqua water technologies, Barbüttel, Germany) with a conductivity of ≤5 μS/cm and a resistivity of ≥0.2 MΩ·cm was used to produce deionized water for all experiments. Preparation of LP-Silane Nanocomposites (See Also Scheme 1). In order to clean glass substrates and thermally oxidized silicon wafers, they were immersed into a cuvette containing sulfuric acid (98%) and placed in an ultrasonic bath for 60 min, followed by a careful rinsing with deionized water. Subsequently, the substrates were immersed into a cuvette containing ethanol (99%) and sonicated for another 15 min. Finally the substrates plasma cleaned (plasma technology, Herrenberg-Gültstein, Germany) with a low process pressure (working pressure) of 0.10 mbar and a power of 300 W for 1 min. Argon was used as a nonreactive gas. 6740


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Langmuir Scheme 1. Initiator Attachment on Silicon Oxide and Glass

Preparation of Algae Solution. The algae solution consists of Nannochloropsis salina. To prepare the algae medium, 60 g of sea salt was dissolved in 2 L of deionized water in a beaker followed by stirring with a magnetic stir bar for 15 min to get 3% saltwater. After adding a special dung (100 μL), the solution containing algae was stirred for further 15 min. After the growth of algae for 2 weeks, the samples were placed into the algae solution. Fibroblast Cell Culture. Primary human fibroblasts from gingival explants were used in this study. Details of cell culture procedure are described in our previous work.62 In order to assess effects on cell adhesion, fibroblasts were seeded on different types of surfaces in 6-well tissue culture plates. Cultures on glass dip coated with LP and LP-APTMS were taken as negative controls. Cells were cultured at a density of 105 cells/well for 48 h (5% CO2, 95% humidity, 37 °C) in alpha-MEM (Sigma-Aldrich). At the end of the incubation period, cells were fixed in 3% glutaraldehyde (Sigma-Aldrich)/PBS (BioWest) and subsequently treated with Giemsa stain (Merck). Residual Copper Concentration Determination. As outlined above, the synthesis of POEGMA brushes requires CuBr. In order to check whether the films contain residual copper, which is known for its powerful antifouling properties, the POEGMA-coated samples were incubated in distilled water for 2.5/24 h. The eluates were subsequently tested for leached copper using a commercial copper assay kit (SigmaAldrich) with a linear detection range from 1 to 47 μM (see product information), and a spectrophotometer (PerkinElmer, Lambda950). Ellipsometry. The ellipsometric thickness, d, and the refractive index n of the LP/polymer layers prepared on silicon oxide wafers, were characterized using a single wavelength ellipsometer (EL X-02C, Dr. Riss Ellipsometerbau GmbH, Ratzeburg, Germany, 1995) with a He−Ne laser (632.8 nm). The measurements were performed at different azimuthal angles of 40, 45, 50, 55, 60, 65, 70, and 75° at four different positions. The layer thickness and the refractive index were obtained by using an optical layer model. Contact Angle Measurements. An OCA20 contact angle microscope (Data Physics Instruments GmbH, Germany) was used to measure the wettability of the surfaces with deionized water using the sessile drop technique. Two microliters of Milli-Q water was applied to the surface for each measurement. For each sample, the water contact angle (WCA) was measured at four different positions. The values are reported as the arithmetic mean and its standard deviation. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FT-IR). The chemical analysis of samples was investigated using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR). A PerkinElmer Frontier FT-IR spectrometer (Waltham, USA) was used to record FT-IR spectra (6 scans) in the transmission mode at a resolution of 4 cm−1 in the range from 4000−400 cm−1. X-ray Diffraction. X-ray diffraction (XRD) measurements were recorded on a Pananalytical X’Pert diffractometer (Eindhoven, Netherland) (Cu Kα radiation: λ = 1.5418 Å, 2Θ scan range: 2−30°, step size: 0.03°/1s) equipped with a X’celerator detector, and operating at a voltage of 40 kV and a beam current of 40 mA. The d-spacings were calculated from the angular 2Θ positions of the 001 reflection of LP using the Bragg’s equation. Atomic Force Microscopy. Morphological investigations of the thin films processed on oxidized silicon substrates, including LP, nanocomposite films of LP-PEG-silane, LP-APTMS, and LP-APTMSPOEGMA brushes, were carried out with an AFM (Nanowizard, JPK Instruments, Berlin, Germany) in intermittent contact mode in air.

The grafting reaction was carried out in a water/ethanol (70:30) mixture. LP was first suspended in the water/ethanol solution. In order to prepare the nanocomposite films, different amounts of PEG-silane (10%, 20%, 30% and 40%) (w/w) or APTMS (10%, 20% and 40%) (w/w) were added in each case to 50 mL of 1% LP. The resulting product was mixed for 3 h at room temperature. The films were deposited by dip-coating onto cleaned substrates (glass substrates and SiO2 wafers) with a withdrawal speed of 100 mm/min and a dwelling time of 300 s. The annealing of the dip-coated films was performed overnight at a temperature of 80 °C. Drying of Dichloromethane. Molecular sieves (pore size: 3 Å) were placed into a round-bottom Schlenk flask and dried at 140 °C for 12 h. The flask was cooled down to room temperature under nitrogen atmosphere, before dichloromethane (DCM) was filled in. A septum was attached to the neck and a long and a short cannula were added, the long one reaching into the DCM. Then nitrogen was bubbled through the DCM for at least 30 min. Preparation of Initiator Layer (See Also Scheme 1). The samples, which were already amino-functionalized, were placed in the glass cuvette. Dry DCM (60.0 mL) and triethylamine (TEA) (1000 μL) were filled into the cuvette and cooled down to 0 °C with the help of a water/ice bath. Afterward, a solution of dry DCM (3−5 mL) and 2-bromoisobutyryl bromide (500 μL) was added dropwise under constant nitrogen flow. Then the cuvette was closed and removed from the water/ice bath. The chemical reaction was allowed to continue at room temperature for 30 min. Afterward, the samples were dipped first in DCM and then in ethyl acetate, and finally washed with ethanol and dried in nitrogen. Synthesis of POEGMA Brushes (See Also Scheme 2). For the synthesis of polymer brushes on the modified surfaces covered with the initiator, a column chromatography method with alumina as a purification agent was used to eliminate the inhibitors present in the monomer oligo(ethylene glycol) methyl ether methacrylate (OEGMA300, n = 4−5) by filtration. The purified monomer OEGMA (19.08 mL), 2,2-bipyridine (1.2480 g), high purity water (12 mL), and methanol (48 mL) were mixed in a 100 mL three-neck round-bottom flask with a magnetic stir bar. The mixture was stirred under nitrogen stream for 45 min. After adding CuBr (0.572 g), the solution was purged and stirred for an additional 15 min. The mixture was then transferred via a metal cannula to a cuvette containing the initiatorcoated substrates (glass and oxidized silicon) under nitrogen stream at room temperature. The process of polymerization was interrupted by removing the substrates from the reaction solution, followed by washing with extensive amount of ethanol before drying with nitrogen gas.

Scheme 2. Polymerization of OEGMA after Initiator Attachment

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Figure 1. (A) FT-IR spectra of dip coated films on glass substrates: (a) 1% LP, (b−e) nanocomposites of 10%, 20%, 30%, and 40% PEG-silane in 1% LP. (B) FT-IR spectrum of PEG-silane measured as a bulk.

Figure 2. Observed changes with increasing PEG-silane concentration: (A) decrease of OH stretching vibrations; (B) increase of Mg−OH (at 3686 cm−1) stretching vibrations; (C) decrease of OH deformation vibrations; (D) increase of symmetric and asymmetric CH2 stretching vibrations; (E) increase of CH2 bending vibrations; (F) increase of wagging CH2 vibrations (see main text for discussion). x and y, with 256 samples/line ×256 lines and a scan rate of 0.4 lines s−1.

The scan ranges for all topographical and phase images were 5 μm (see Figure S2 in the Supporting Information) and 1 μm in 6742


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Figure 3. (A) FT-IR spectra of functionalized LP with (a) 10% APTMS, (b) 20% APTMS and (c) 40% APTMS; (B) FT-IR spectra of POEGMA brushes grown from an initiator layer anchored on (a) 10% APTMS in 1% LP, (b) 20% APTMS in 1% LP, and (c) 40% APTMS in 1% LP; (C) decreasing OH stretching vibrations with increasing APTMS concentration in 1% LP; (D) decreasing OH deformation vibrations with increasing APTMS concentration in 1% LP. When necessary, intermittent contact mode AFM images were processed by flattening in order to remove the background slope, and the contrast and brightness were adjusted. Section analyses and roughness measurements were performed with Gwyddion software. The root-mean-square (r.m.s.) roughness represents the standard deviation of the heights in the topographical image over a scan area of 1 μmx1 μm. Quantitative Analysis of Adsorbed Particles on Nonfouling Surfaces in Algae Solution. Five different samples, (1) a cleaned glass substrate, (2) a glass substrate dip coated with 1% LP, (3) a nanocomposite film of 1% LP and 40% PEG-silane coated on a glass substrate, (4) POEGMA brush grafted on the nanocomposite surface of 1% LP and 10% APTMS, and (5) POEGMA brushes grafted on a glass substrate that was first covered with APTMS via chemical vapor deposition (CVD), were placed in an algae tank for 30 days. Afterward, the samples were taken out from the algae tank, immersed in fresh deionized water three times and dried in air. For the data analysis of the adsorbed particle numbers, images were taken with an AX70 Research Microscope (Olympus Provis) and analyzed with a custommade algorithm programmed in Matlab.

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Figures 1 and 2. Figure 1A shows the FT-IR spectra of a series of samples with unmodified LP and PEG-silane functionalized LP-films containing different amounts of PEG-silane. For comparison, the FT-IR spectrum of the pure PEG-silane is shown in Figure 1B. The spectrum of LP, Figure 1A(a), shows a broad shoulder at around 3700−3400 cm−1, in the range of frequencies usually ascribed to surface hydroxyl groups and adsorbed water. The slightly resolved shoulder is composed of the overlapping of two components: Si−OH (≈ at 3628 cm−1) and Mg−OH (≈ at 3686 cm−1) stretching vibrations63 and the ν (O−H) stretching frequency at 3450 cm−1 due to physisorbed water. The δ (O−H) deformation band at 1638 cm−1 is a further indication of the presence of water.52 The spectra of the nanocomposite layers of LP and increasing amount of PEG-silane are shown in Figure 1A(b−e). These spectra indicate the same transmission peak positions but with different peak intensities. The intensity of the peaks at 3450 cm−1 (see Figure 2A), 3868 cm−1 (see Figure 2B), 1638 cm−1 (see Figure 2C), 2800−3000 cm−1 (see Figure 2D), 1400−1500 cm−1 (see Figure 2E), and 1352 cm−1 (see Figure 2F) may be correlated with the concentration of the silane in the nanocomposite. In the high energy range, 3600−3000 cm−1, the intensity of the broad band centered at 3450 cm−1 decreases (Figure 2A) and the shoulder at 3686 cm−1 becomes sharper (Figure 2B) with increasing amount of silane. In the low energy range, the intensity of the δ (O−H) deformation band at 1638 cm−1 decreases (Figure 2C) with increasing ratio of PEG-silane. This intensity decrease can be attributed to a reduction of the adsorbed water

RESULTS AND DISCUSSION

Characterization of LP-PEG-Silane Nanocomposites. In this section, the FT-IR results of the nanocomposites consisting of LP-PEG-silane with an ellipsometric thickness of 13−30 nm (see Figure S1A in the Supporting Information) and LP-APTMS grafted POEGMA brushes with a thickness of 37 nm (see Figure S1B in te Supporting Information), are presented. The FT-IR spectra of LP-PEG-silane nanocomposite films, produced via dip coating on cleaned glass surfaces, are shown in 6743


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Figure 4. (A) XRD patterns of 1% LP film and nanocomposite films of 1% LP and 10%, 20%, 30%, and 40% PEG-silane; (B) XRD patterns of 1% LP film and nanocomposite films of 1% LP and 5%, 10%, 15%, and 20% APTMS; (C) evolution of lattice spacing along the (001) plane (d001) with increasing amount of the silanes.

molecules onto LP clay and thereby attests to the covalent binding of the silane molecules to the LP edges and generation of new bonds of Si−O−Si−C.65 In addition, ν(OH) at 3450 cm−1 (see Figure 3C) and δ(O−H) at 1638 cm−1 (see Figure 3D) indicate an intensity decrease as a result of the physisorbed water.51 APTMS belongs to the trialkoxysilane group that can be grafted into LP by adsorption, intercalation, and condensation reaction as a final step. Among the important factors that can influence the grafting reaction is the surface energy of the solvent. For instance, while silane could not intercalate into the layered silicate in nonpolar solvents such as toluene,66 its incorporation was observed in polar solvents as ethanol, ethanol/water mixture,66 which also reflects the present finding (see Figure 4). The FT-IR spectra of LP-polymer brushes (see Figure 3B(a−c)) indicate all characteristic bands corresponding to the vibrations of POEGMA. The ν(CH) symmetric and asymmetric stretching vibrations belonging to the methyl groups appear at 2883 and 2983 cm−1, and stretching vibration of ν (CH) associated with the methylene groups are located at 2821 and 2927 cm−1. The symmetric vibrations of the carbonyl group (CO) are appear at 1732 cm−1, which is in agreement with previously published data.33,34 The symmetric deformation δ (CH) related to the methyl group occurs at 1450 cm−1. Major signals attribution of LP, PEG-silane, APTMS, and POEGMA brushes are given in Table 1. XRD Characterization of LP-Silane Nanocomposites. The X-ray diffraction patterns of LP and the nanocomposite films of LP/silane are shown in Figures 4A,B. The untreated LP film exhibits a broad 001 peak at approximately 6.16° (2Θ) as a result of low ordering and nanoscale particle size. This peak becomes sharper in the silylated samples because of the increase in the ordering of the disc-like structure,57 and is shifted toward smaller 2Θ values with increasing silane concentration. The initial basal

content, and a decrease in the intensity of the peak due to Si−OH. The latter outcome is a consequence of the covalent bonding of silane to the Si−OH groups via the formation of new Si−O−Si-C bonds.63 Further, according to the same reference as above, hydrolysis of the methoxyl groups of the organosilane to silanol groups by the adsorbed water on the clay, followed by condensation with the available surface hydroxyl groups located at the edges of the clay layers, leads to decreasing water content. The FT-IR spectrum of pure PEG-silane (Figure 1B) shows the symmetric νsy(CH2) and asymmetric νas (CH2) stretching vibration at 2861 and 2942 cm−1. The peaks at 1344 and 1455 cm−1 indicate wagging and bending vibration of CH2 groups. The peak positions for stretching vibration of Si−O−C and Si−C appear at 1084 and 819 cm−1. The symmetric stretching vibration of C−O−C is located at 942 cm−1.64 After modification, the LP-PEG-silane nanocomposite samples show the same characteristic vibrations of the aliphatic CH2 groups with a small displacement (2879 cm−1, 2945 cm−1 (Figure 2D) and 1352 cm−1 (Figure 2F)). The peak between 1400 and 1500 cm−1 is attributed to the ν(CH2) bending vibration (Figure 2E), thus indicating that the organic molecules were effectively grafted to the surface of silanol groups. In order to compare nonfouling surfaces fabricated by different grafting methods, POEGMA brushes were prepared via surface initiated-atom transfer radical polymerization (SI-ATRP) (see Figure S2 in theSupporting Information) on glass and oxidized silicon substrates covered with a nanocomposite of LP-APTMS (see also the Experimental Section). The FT-IR spectra of the LP-APTMS nanocomposites (Figure 3A(a−c)) show symmetric and asymmetric ν(CH) stretching vibrations in the range of 2850−2950 cm−1 as well as δ(CH) deformation vibrations at 1360 cm−1 (Figure 1A(c)), which is in agreement with spectra of grafted trifunctional silane 6744


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Langmuir Table 1. Frequencies (cm−1) and Assignment of the Different Vibrations in LP, PEG-Silane, APTMS, and POEGMA Brushes frequency [cm‑1]

assignment

type

layer

3686 3450 2983 2950 2927 2909 2883 2875 2850 2821 1732 1638 1456 1450 1360 1346 1084 942 819

Mg−OH, Si−OH OH CH3, CH CH CH3, CH OCH2, CH2 CH2, CH O−CH2, CH2 CH CH2, CH CO OH O−CH2, CH2 CH CH O−CH2, CH2 Si−O−C Si−C C−O−C

stretching stretching asymmetric stretching asymmetric stretching symmetric stretching stretching asymmetric stretching stretching symmetric stretching symmetric stretching stretching deformation bending symmetric deformation deformation wagging stretching stretching symmetric stretching

LP LP POEGMA APTMS POEGMA PEG-silane POEGMA PEG-silane APTMS POEGMA POEGMA LP PEG-silane POEGMA APTMS PEG-silane PEG-silane PEG-silane PEG-silane

the clay platelets in horizontal configuration for low concentration and intercalation for higher concentrations as a result of oligomer formation.51 In addition, according to former studies, the change of basal spacing in layered silicate such as LP can be influenced by the alkyl chain length of the silane,68,69 although their correlation was not explicitly investigated.70 Piscitelli et al. reported that a small shift of d001 in layered silicates is a consequence of long carbon chain length of alkoxysilane. Moreover, it was shown via molecular dynamics simulations of intermolecular interactions in silane molecules as well as the interactions between specific functional groups (of the silane) such as −NH2 and silicate surfaces can affect the basal spacing in clay minerals.70 Our results rather indicate that, depending on concentration, the long alkyl chain of PEG-silane can cause a substantial increase in d001 of LP clay, compared with APTMS (see Figure 4C). Based on the XRD and FT-IR results, we may state that a fraction of the organosilane is intercalated into the Laponite layer, probably following the mechanism reported by Herrera et al.,51 who suggest that for trifunctional silanes the intercalation of polysiloxane oligomers may start by grafting at the edge of Laponite and, depending on concentration and LP-disc arrangement, penetrates the interior of the particles, thus increasing the interlamellar spacing. A schematic of a possible grafting mechanism is depicted in Scheme 3. Scheme 3. (A) Schematic Structure of Synthetic LP Clay with Na+ Ions Present in the Intergallery Spacing of the Nanoclay; (B) Chemical Structure of APTMS and PEG-Silane; (C) Different Types of Nanocomposites Formed by Combining Polymer and Clay Particles

plane spacing of 14.35 Å corresponding to pure LP film increases to 18.32 Å with increasing concentration of PEG-silane, Figure 4A,C, with a similar behavior being observed for LP-APTMS-nanocomposites, Figure 4B,C. Similar results have been reported in the literature for the modification of two sorts of clay materials using APTMS in a mixture of water/ethanol (25/75) where a clear increase of d001 has been observed.67 In our case, it should be pointed out that the extent of d001 increase in the nanocomposite films is comparatively small. This could be indirectly attributed to the reaction medium used in the present work. According to previous work, the grafting method, the type of silane, and the number of the functional groups can affect the incorporation of organosilane into clay minerals. The water/ethanol mixture leads to hydrolysis of silanes such as APTMS, which entails the formation of polysiloxane oligomers that can hardly intercalate into the clay minerals and therefore result in a minimal increase of d001.66 By contrast, when the grafting reaction is conducted via vapor deposition, a large increase in d001 was observed. This was ascribed to the successive hydrolysis and condensation thereby enabling intercalation of silane into the interlayer cavities of the clay.66 Furthermore, according to Shen et al., grafting of trifunctional silanes such as APTMS and APTES onto layered silicate surfaces (montmorillonite) displayed a larger basal spacing compared to grafting of monofunctional silanes.66 Herrera et al. has shown that the number of functional groups affects d001 of LP (in anhydrous toluene). They observed that, while d001 remained almost unchanged in the case of monofunctional silane, trifunctional silane led to a progressive increase in the interlayer spacing. This was discussed in terms of polysiloxane oligomer formation, their interaction at the edge of the particles, and a progressive penetration toward the interlayer cavities.51 The case of LP-PEG-silane reveals a differentiated behavior. At low concentration d001 is marginally affected, but shows a sharp increase when the concentration increases above 20%. This suggests hydrolysis and adsorption of the silanol groups on

Morphological Characterization of Nanocomposite Films. AFM topographic images were obtained on nanocomposite films supported on glass substrates. Figure 5 illustrates how the LP film morphology evolves with chemistry of the nanocomposite. In all cases, the films are very smooth with an r.m.s (root-mean-square) roughness not exceeding 2.1 nm (see Table 2). The LP film surface morphology rather shows disordered particles with platelet stacks and edge-to-face arrangement (e.g., see arrows in the amplitude image Figure 5A). The equiaxed grains depicted in the phase image, Figure 5B, shows diameters in the range of 40 to 70 nm reflecting the arrangement of the 6745


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Figure 5. Intermittent contact mode AFM amplitude and phase images (1 μm × 1 μm), obtained in air, of (A,B) 1% LP film; (C,D) 1% LP-40% PEG-silane nanocomposite film; (E,F) 1% LP-10% APTMS nanocomposite film; (G,H) (2 μm × 2 μm) of POEGMA brushes grafted on 1% LP-10% APTMS nanocomposite film.

r.m.s. roughness of 0.4 nm obtained from a typical 1 μm × 1 μm scan size image. Figure 5C suggest that the LP particles are embedded in a continuous matrix, and this is corroborated by the phase image that shows a featureless matrix and bright LP particle

LP particles mentioned above (the particles are of the same chemistry). The surface morphology is drastically changed with the addition of PEG-silane. The films become very smooth with an 6746


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Langmuir Table 2. Roughness of LP, LP-PEG-Silane, LP-APTMS, and POEGMA Layers layer

r.m.s. roughness, nm

LP LP/PEG-silane LP/APTMS POEGMA

1.2 0.4 2.1 1.3

agglomerates (Figure 5D). We may surmise on the basis of these images that excess, nongrafted PEG-silane polymerizes via hydrolysis to form a continuous polymer matrix. PEG-silane grafted on the edges of the LP particles ensures in this respect continuity with the surrounding matrix also via polymerization (see Scheme 3). Figure 5E illustrates the effect of APTMS on film morphology, and clearly indicates a drastic decrease in the particle size, also well visible in the phase image (compare panels B and F in Figure 5). We suppose that preferential grafting of APTMS on the LP edges should prevent the edge to face arrangement of the particles that more likely tend to order with their (001) plane being perpendicular to the substrate surface. This is also suggested by the XRD patterns (Figure 4), which show sharpening of the 001 peak with increasing APTMS content. Surface modification with POEGMA brushes (Figure 5G) results in featureless morphology with scattered defects, probably pores, less than 8 nm deep. Outside these defects, the polymer film is rather smooth with an r.m.s roughness of ≈1.3 nm. At this stage, it is not possible to describe the fine brush structure of the polymer film, but efforts are under way to address this issue using AFM imaging in liquid medium and functionalized AFM tips. Wettability of the Nanocomposites. The contact angles of modified and unmodified clay layers were measured using the sessile drop approach at room temperature. The LP clay surface is highly hydrophilic with a static water contact angle (WCA) of 16.48 ± 0.35. The addition of silanes results in a slight increase of WCA to values that saturate at approximately 29° for PEG-silane and 36° for APTMS, Table 3. The increase of WCA may be

Figure 6. (A) Optical microscopy images of fibroblast cells that were seeded on POEGMA brushes synthesized on a nanocomposite film of LP-APTMS on glass, LP-APTMS nanocomposite film, and LP film dipcoated on glass. (B) Macrographs of wells containing a POEGMA brush samples and controls.

Table 3. Static Water Contact Angle Measurements Reported As the Arithmetic Mean Values and Its Standard Deviation silane concentration

LP/PEGsilane

0 10 20 30 40

16.48 ± 0.35 20.82 ± 0.25 27.35 ± 0.25 29.22 ± 0.26 29.52 ± 0.48

LP/APTMS

POEGMA on LP/APTMS

25.72 ± 1.02 28.55 ± 1.86

48.46 ± 0.32 49.16 ± 0.30

36.45 ± 1.30

48.70 ± 0.88

LP-APTMS-POEGMA surface in contrast to the control samples where cell proliferation is very clear. But this does not mean that the surface in question is biotoxic as illustrated in Figure 6B, which shows macrographs of wells containing a control and a POEGMA surface where the cells are seen to border the edges of the POEGMA sample (lower two panels in (B)) that in turn is an indication that no toxic substances are leaching from the aforementioned surface. This particularly holds for possible residual copper in the POEGMA films, because CuBr is required for the ATRP.72 Our measurements using a copper assay kit show that after 24 h incubation of the samples in distilled water, copper in the eluate, if present, is under the detection limit of the assay, which is 70 μg/L (see Supporting Information Figure S5). Even if copper is present at a concentration just below the detection limit, it is still far below the concentration limit of 2 mg/L recommended by the World Health Organization (WHO) for drinking water.73 In the second series of tests, five samples were tested in the algae solution container: (1) cleaned glass substrate, (2) LP film dip coated on glass, (3) a nanocomposite film of LP-PEG-silane on a glass substrate, (4) POEGMA brush grafted on the nanocomposite surface of LP-APTMS, and (5) POEGMA brushes grafted on a glass substrate only covered with APTMS via chemical vapor deposition (CVD). All the films were transparent

rationalized in terms of adsorbed water being decreased with increasing silane concentration, according to the FT-IR spectra (Figure 2A,C). POEGMA surfaces show WCA values of ∼49°, regardless of silane content (in the primer LP-silane layer) and polymerization time, which is in agreement with literature values.71 In general, PEG is a highly hydrophilic material. However, the hydrophobic character of the methacrylate backbone may explain the relatively high WCA values obtained for the POEGMA surfaces.21 Nonfouling Properties of LP-PEG Nanostructures. Two series of tests were conducted: the first one using fibroblast cell cultures, and the second one involved immersing samples for a duration of 30 days in an algae solution container. Figure 6A unambiguously shows that no fibroblasts are adherent on the 6747


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Figure 7. Optical microscope images of the different surfaces after 30 days dwell time in an algae container. (A,B) Control surfaces of bare glass (A) and LP-film (B); (C) particles on the LP-film at higher magnification showing adhering diatoms (scale bar:10 μm); (D) POEGMA brushes grafted on glass covered only with APTMS via chemical vapor deposition (CVD), (E) LP-APTMS-POEGMA brushes; (F) LP-PEG-silane film. Scale bar is 200 μm in all micrographs, apart from panel C.

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CONCLUSION The present work shows that nanocomposite films of Laponite, a synthetic layered silicate nanomaterial, and organosilanes can be processed on two different substrates. Homogeneous and transparent films with nanostructured morphology are obtained from precursor solutions of laponite-organosilanes, followed, in the case of POEGMA brushes, with SI-ATRP. Characterization by FT-IR and XRD show that the silylation process considerably reduces adsorbed water, and significantly affects the structure of the clay resulting in expansion of the basal plane that increases with increasing silane content. Further, the amphiphilic properties of the films are altered by the silylation process. The results obtained suggest that grafting starts at the edge of the LP particles and progressively penetrates the interlayer cavities with increasing organosilane content, thus resulting in larger interlayer expansion. Preliminary results of the antifouling properties of laponitePEG-silane and laponite-APTMS-POEGMA brushes were obtained and show promising results after 30 days in an algae solution. The performance of the polymer brushes was, however, with 92% reduction of adhering particles higher than that of LP-PEG-silane (85%). However, owing to the facts that the LP-PEG-silane films are easier to process, require substantially lesser chemicals and shorter reaction time, and are thereby costeffective, nontoxic, and environmentally friendly, it should be of interest to pursue their optimization.

with a transmittance of 94−100% with respect to bare glass (see Figure S4 in the Supporting Information). According to the present results, PEG-containing surfaces are significantly less prone to particle adhesion, including algae particles, diatoms and a protozoa type. The number of particles adhering on LP clay surfaces functionalized with PEG-silane (see Figure 7F) and grafted POEGMA brushes (see Figure 7E) was drastically reduced in comparison to bare glass (see Figure 7A) and pure LP surface (see Figure 7B). The LP-PEG silane surface shows a reduction in the number of adhering particles/unit surface area by 85%, while the POEGMA surface is slightly more efficient with 92% reduction with respect to the control surfaces. Thus, modification of LP with PEG-based polymers and polymer brushes alters its surface properties from a fouling to a nonfouling surface (compare panels B, E, and F in Figure 7). This significantly reduced tendency to particle adhesion can be attributed to the capability of PEG-silane and POEGMA surfaces to bind to the water molecules via hydrogen bonds. This entails the formation of a stable hydration layer that is thought to shield the surface from the adsorption of biomolecules, which are seen as precursors for the adhesion of higher living organisms. In addition to the grafting method, the type of PEG molecule and its grafting density as well as the chain length and conformation affect the resistance of the surface against the adsorption of biomolecules.74,75 In this respect, the functionalized surfaces with POEGMA brushes (see Figure 7D,E) are characterized by higher repelling forces against adsorbing biomolecules compared to linear PEG-silane (see Figure 7F), probably because of a higher grafting density. However, the (slightly) lower performance of LP-PEG-silane in comparison to LP-POEGMA brushes might be traded-off against a simpler processing route, lesser chemicals, and shorter processing time in comparison to LP-POEGMA that translate into cost-effectiveness and environmental friendliness.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00534. Ellipsometric thicknesses of LP-PEG-silane and POEGMA brushes; scheme of atom transfer radical polymerization reaction; tapping-mode AFM height and phase images (5 μm × 5 μm) of 1% LP film, 1% LP- 40% PEG-silane 6748


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nanocomposite film, and POEGMA brushes grafted on 1% LP- 10% APTMS nanocomposite film; absorption of LP-PEG-silane samples prepared on glass and LP-APTMSPOEGMA brushes grafted on glass measured by UV−vis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+49 431 210 2660; Fax: +49 431 2661; E-mail: [email protected]. ORCID

Mohammed Es-Souni: 0000-0003-4684-2900 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Financial funding of this work is provided by the German Federal Ministry of Economy and Energy, Project# 0325915E (FINO3). REFERENCES

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