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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 37461−37469
Smart, Piezo-Responsive Polyvinylidenefluoride/ Polymethylmethacrylate Surface with Triggerable Water/Oil Wettability and Adhesion Olga Guselnikova,†,‡ Roman Elashnikov,† Pavel Postnikov,‡ Vaclav Svorcik,† and Oleksiy Lyutakov*,†,‡ †
Department of Solid State Engineering, Institute of Chemical Technology, 16628 Prague, Czech Republic Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University, Tomsk 634050, Russian Federation
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S Supporting Information *
ABSTRACT: The design of smart surfaces with externally triggerable water/oil wettability and adhesion represents one of the most up-to-date challenges in the field of material science. In this work, the intelligent surface with electrically triggerable wettability and water/oil adhesion is presented. As a basic material background exhibiting electric field (EF) sensitivity, the piezoresponsive polymethylmethacrylate/polyvinylidenefluoride polymer fibers were used. To expand the available range of water/oil contact angles (CAs) and adhesion, the fibers were grafted with hydrophilic or hydrophobic functional groups using diazonium chemistry. The fiber functionality was evaluated using the static CA and wettability hysteresis measurements (increasing/decreasing drop volume and tilting angles), drops adhesion/repellence and graphite self-cleaning test performed with and without the application of EF. It was found that the proposed method enables tuning the surface wettability in the superhydrophobic/superoleophobic−hydrophilic/oleophilic range and changing of surface properties from low adhesive to high adhesive for water and oil. More convincing results were achieved in the case of fiber surface modification by ADT-C8F17, which may result from a rearrangement of the grated −C6H4C8F17 functional group under the application of EF triggering. Moreover, the triggering which can be performed in the extremely fast way (the surface responds to the EF switching on/off in seconds) was found to be fully reversible. Finally, the additional tests indicate the satisfactory stability of created fiber-based coating against the mechanical treatment. KEYWORDS: smart surface, polymer fibers, surface modification, piezo-responsivity, wettability switching, adhesion switching
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of addressable liquid CA on the conductive surface.30−32 An alternative method consists of modulation of the drop behavior through the EF-induced local molecular reorganization within the surface film.33,34 Additionally, the drop behavior on the conductive polymer surface can be controlled through the implementation of reduction/oxidation polymers transition.35 The methods reported so far are limited by their inability to control wettability of nonpolar liquids or by the use of the specific range of conductive substrates, restricting their application in some cases.9,30−32 Taking into account the main advantages of EF field switching, such as nondestructiveness, material quick response, reversibility, short modulation, and relaxation times,36,40,41 the development of alternative approaches for EF-based surface wettability and adhesion switching is strongly desirable. In this work, we propose an alternative way for switching of wettability and adhesion of water and oil drops. The proposed approach is fast, nondestructive, and makes it possible to control reliability of
INTRODUCTION One of the most popular and quickly developing trends in materials engineering, which has been extensively pursued during the last years,1−4 is the design and realization of the socalled smart surfaces. Such surfaces with triggerable wettability and adhesion properties are of immense importance for a wide variety of applications, for example, for the fabrication of micro- and nanofluidic devices, self-cleaning, antifogging, preparation of reconstructable surfaces, and many others.5−15 The functionality of such surfaces is governed not only by the contact angle (CA) of water or fatty liquid but also by their drop repellent/adhesion properties. Thus, even more interesting parameters for switching of surface properties are tuneable wettability and adhesion of water and oils.16,17 The reversible transitions of surface properties, including water/oil wettability and adhesion, can be triggered by chemical, physical, and biological stimuli.18−23 In particular, the most frequently used stimuli are temperature, pH, magnetic field, and light.24−29 Alternatively, the electrical field (EF) presents a versatile and easy-to-use method to switch the wettability.30,31 One of the more common examples of EF application for water drop behavior is related to electrowetting. This approach can be applied for the switching © 2018 American Chemical Society
Received: April 29, 2018 Accepted: September 18, 2018 Published: September 18, 2018 37461
DOI: 10.1021/acsami.8b06840 ACS Appl. Mater. Interfaces 2018, 10, 37461−37469
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic representation of (A) PVDF/PMMA fiber preparation by the electrospinning procedure and (B) PVDF/PMMA fiber modification via the covalent modification of PMMA using different diazonium salts. experiments, the drops were deposited immediately after EF switching on/off. CAs Hysteresis and Adhesion Switching Measurements. CA hysteresis was measured using following experimental procedures: (i) Gradual increasing followed by gradual decreasing of the applied EF intensity with the CA measurement of water/oil drops (2 μL) at each applied intensity. (ii) Slow tilting of the sample plane and the measurement of the difference between advancing and receding CAs and determination of changes in liquid adhesion (with and without application of EF). The volume of water/oil drops was 5 μL. (iii) Deposition of the water drop on the sample surface followed by increasing and decreasing of the water volume with simultaneous CA measurements at each volume. The water volume was varied in the 1.5−34.5 μL range at the volume increase/decrease speed of 0.03 mL/min. Self-Cleaning Experiment. The self-cleaning function of PVDF/ PMMA fibers was studied with graphite powder as contaminants. The samples were placed in contact with graphite powder under EF and then detached, the procedure of which results in the surface “contamination” by graphite particles. Then EF was turned off, water drops (4 drops, 8 μL) or EG drops (3 drops, 8 μL) were put on the contaminated surface and rolled down under a tilting angle of 5°, and surface cleaning was evaluated. Mechanical Stability Experiments. Bending Test on PVDF/ PMMA Fibers Modified by ADT-C8F17. The effect of bending on the PVDF/PMMA fibers modified by ADT-C8F17 was studied by confocal microscopy. The specimens were gently bended and kept in this position for 1 h. Then, their morphology on the place of bending was examined by confocal microscopy. Accelerated Test for Air Resistance on Fibers Modified by ADTC8F17 Using a Ventilator. The specimens of PVDF/PMMA fibers modified by ADT-C8F17 were blown by a ventilator CasaFan Speed 50-G CH (1305 rpm, 110 W, 5685 m3/h) for 1 h, and then their morphology was examined by confocal microscopy and wettability tests.
various liquids−surface interactions, through the application of EF.
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EXPERIMENTAL SECTION
Materials. Polymethylmethacrylate (PMMA, Mw = 1 500 000) and polyvinylidenefluoride (PVDF, Mw = 70 000) polymers were supplied by Goodfellow. 4-Aminobenzoic acid (≥99.0%), 4(heptadecafluorooctyl)aniline (≥95.0%), p-toluenesulfonic acid monohydrate (ACS reagent, ≥ 98.5%), tert-butyl nitrite (90%), high-purity water (EMD Millipore), acetic acid (≥99.5%), methanol (anhydrous, 99.8%), diethyl ether (≥99.7%), ethylene glycol (analytical standard), hexadecane (analytical standard), and glycerol (≥99.5%) were purchased from Sigma-Aldrich and used without further purification. Rapeseed oil was purchased from a local store. 4Carboxybenzenediazonium tosylate (ADT-COOH) and 4(heptadecafluorooctyl)benzenediazonium tosylate (ADT-C8F17) were prepared according to the published procedure.37−.39 Sample Preparation. PVDF/PMMA Fiber Preparation. PVDF (2.5 g) was dissolved in 95 g of DMF for 24 h at 70 °C. Then, 2.5 g of PMMA was added to the solution, and stirring was continued until the dissolution of PMMA. The nanofibers were prepared using a vertical electrospinning setup (feeding rate 1 mL/h; voltage 13 kV) on the glass substrate. PVDF/PMMA Modification. The modification procedure was based on the previously described procedure.40 Polymer fibers on a glass substrate were immersed into a stirred aqueous solution containing ADTs (−COOH or −C8F17, 20 mM) and stirred at 60 °C for 30 min. After the process, modified-PVDF/PMMA fibers were washed with water (3×) and methanol (3×). Measurement Techniques. Sample Characterization. X-ray photoelectron spectra were recorded using an Omicron NanoTechnology ESCAProbeP spectrometer fitted with an amonochromated Al Kα X-ray source working at 1486.6 eV. Fourier transform infrared (FTIR)−attenuated total reflection (ATR) spectra were recorded using a Nicolet 6700 spectrometer (Thermo Scientific, France) with a Smart ATR accessory device (1000 scans and 4 cm−1 resolution). Atomic force microscopy (AFM) measurements were performed with Icon (Bruker) setup on the areas of 0.5 × 0.5 μm2. The morphology and response and relaxation times of fibers were studied using a laser confocal microscope Olympus LEXT (405 nm laser wavelength). Static CA Measurements. Water, ethyleneglycol (EG), and hexadecane CAs were measured by Drop Shape Analyzer−DSA100 (Kruss, Germany) at 10 positions (drop volume2 μL) at room temperature. Additional experiments involve the utilization of glycerol and rapeseed oil as fatty probing liquids. Estimations of surface energy were performed according to the OWRK model on a surface energy evaluation system using water and EG CAs. Surface Triggering. For an application of external EF, two thin gold electrodes were sputtered on the sample surface and connected to a voltage source. The CAs were measured either during the dynamic triggering or in the case when the water/oil drops were deposited after the application of voltage. In the case of cyclic
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RESULTS AND DISCUSSION In Figure 1, the experimental concept used for the creation of an electrically switchable PVDF/PMMA fiber surface is shown. The first and most crucial step is the formation of stimulisensitive PVDF/PMMA fibers invariable of the polymer concentration ratio and apparent piezo-response. Previously, the fabrication of an EF-responsive PVDF/PMMA thin film surface was reported in refs 31 and 36 but in the present work, the additional nanostructuring of polymer blends in the nanofiber form was introduced with the aim to achieve greater functionality of the product. Then, the chemical modification via diazonium chemistry of the fiber structure was performed, followed by grafting of the hydrophilic (−COOH) or hydrophobic (−C8F17) functional groups to the PMMA surface within PVDF/PMMA fibers. The surface modification 37462
DOI: 10.1021/acsami.8b06840 ACS Appl. Mater. Interfaces 2018, 10, 37461−37469
Research Article
ACS Applied Materials & Interfaces
Figure 2. Dependence of the (A) water, (B) EG and hexadecane (on −C6H4−C8F17 grafted fibers) CA values on EF triggering.
was conducted by immersion of the fibers into the water solution of appropriate diazonium salts under heating to 60 °C and stirring. Conservation of fibers’ piezo-response was controlled by confocal microscopy of the samples subjected to electric field (EF) triggering (Figure S1). Application of EF immediately results in materials’ mechanical deformation with a response speed in milliseconds. Switching off results in the gradual relaxation of materials and returning to its initial state in ca 7 s (Figure S1). Successful chemical modification of PVDF/PMMA fibers, and the attachment of organic functional groups were confirmed using X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy (for a detailed description, see Table S1, Figures S2, and S3 and related discussion in the Supporting Information). Conservation of PVDF/PMMA fiber morphology during the chemical treatment/purification steps was proved using the scanning electron microscopy (SEM) measurements (see Figure S4). No disruption of the fiber structure after the grafting procedure was observed. Surface morphology and adhesion properties were also examined at the nanoscale level using nanomechanical AFM measurements (see Figure S5). The pristine fiber surface is rather rough with adhesion typical for PVDF/PMMA polymer blend.21 Modification by ADT-COOH slightly increases the surface roughness and transforms the surface to a more adhesive one. The adhesion increase can be attributed to changes in the fiber wettability and formation of water nanodrops between the AFM tip and the sample surface during the measurement. Even more pronounced changes in the fibers’ morphology were observed after ADT-C8F17 grafting when a quasi-ordered structure was formed on the surface. In this case, the adhesion between the AFM tip and the fiber surface apparently decreased because of the introduction of fluorinated functional groups. In the next step, triggering of the water or oil wettability and adhesion by application of EF to the prepared structure was studied in detail. In view of the abovementioned high speed of modulation (immediate response after the EF switching on and complete relaxation of materials during 7 s after the EF switching off), the surface response is expected to be extremely fast, compared with other available techniques and approaches.2−5 First, the water and fatty liquid wettability changes under EF triggering were investigated. The application of EF results in the apparent piezo-response of PVDF/PMMA, which in turn affects the fiber arrangement, orientation, density, and morphology.12,14 Results of water CA measurements are presented in the Figure 2A for pristine and modified polymer fibers. The pronounced effect of the grafting procedure on the
initial water CA is clearly visible. Awater CA of 135° for pristine PVDF/PMMA fibers decreases below 100° after the modification with hydrophilic ADT-COOH. The reaction of the surface with ADT-C8F17 results in an increase of CA up to a superhydrophobic value of 167°. Application of EF leads to a decrease of the water CA in all cases. Most significant changes of the CA were observed in the case of −C6H4−C8F17-grafted PVDF/PMMA, for which the CA decreased to 69° (difference by 96° after the EF application). Therefore, EF-triggered transition from the superhydrophobic to hydrophilic surfaces was achieved in this case. Because the polymer fibers with fluorinated functional groups reveal promising CA switching results, these samples were used for the further study of switchable oleophilic properties (Figure 2B). Two “fatty” liquids were used for the oleophilicity tests: EG and hexadecane. In the case of EG, −C6H4−C8F17-grafted PVDF/PMMA acquires a superoleophobic character (CA = 155°). The slightly lower initial value of CA in the case of hexadecane can be explained by a partial swelling of PVDF/PMMA fibers, during the measurements. Application of EF leads to an apparent decrease of the CA for both liquids tested. The most convincing changes were observed for EG, where the CA decreased by 58° under 1 V/μm EF intensity. Subsequent gradual decrease of the EF intensity leads to complete relaxation of the material, with almost identical CA values for each EF intensity. Our experiment shows that the hysteresis between the EF increase/decrease is negligible (
