Superhydrophobic Hybrid Paper Sheets with Janus-Type Wettability

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Superhydrophobic Hybrid Paper Sheets with Janus-Type Wettability Ç ağla Koşak Söz,*,† Simon Trosien,‡ and Markus Biesalski*,‡ †

Faculty of Science, Material Science and Technologies, Turkish-German University, Sahinkaya Cad. No. 86, Beykoz, Istanbul 34820, Turkey ‡ Makromolekulare Chemie und Papierchemie, Technische Universität Darmstadt, Alarich-Weiss-Straße 8, Darmstadt 64287 , Germany

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S Supporting Information *

ABSTRACT: We introduce the design of Janus-type paper sheets where one side of the paper exhibits superhydrophobic properties, whereas the other side of the sheet remains hydrophilic and therefore can take up aqueous solutions by capillary wicking. Such papers are being prepared by chemically immobilizing a thin hybrid coating on paper sheets that consists of cross-linked poly(dimethylsiloxane) (PDMS) and inorganic particles of various sizes ranging from nanometers to several tens of micrometers. Both commercially available Whatman No. 1 filter paper and lab-engineered cotton linters-based paper substrates were treated with this approach. The hybrid paper sheets have high chemical durability, mechanical stability, and flexibility because of a covalent attachment of the particles to paper fibers and the inherent elasticity of PDMS chains. In spite of the superhydrophobicity of the coating, the untreated side of the paper substrates preserved its hydrophilicity, resulting in Janustype wetting and wicking properties, respectively. The functionalized paper samples remained porous and permeable to gases, while possessing a gradual change in chemistry between the two sides exhibiting a dramatic wetting contrast. Such two-sided properties open up new applications for such hybrid paper materials, such as in wound dressings and/or bandages with a liquid directing and confinement ability. KEYWORDS: hybrid materials, superhydrophobic surfaces, lotus effect, hydrophilic surfaces, Janus interface materials, Janus membranes, poly(dimethylsiloxane)

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superoleophobic.7,8 This is called a Janus interface property and has been mimicked by researchers for approximately 30 years. Generally, a Janus material has two faces with different properties, and its name stems from an ancient, two-faced Roman god, Janus. Several years after the first paper about Janus-type materials and the famous Nobel lecture of de Gennes in 1991 mentioning Janus grains, the emergence of a new concept called “superwettability integration” has caused researchers of various fields to refocus on Janus-type materials.9,10 This concept relies on the principle of combining the functions of two superwetting surfaces on the same material to better mimic nature and solve complicated realworld problems.11,12 Four types of superwettability integration are reported in the literature: (i) surfaces with microscopic wetting patterns, (ii) surfaces with macroscopic wetting patterns, (iii) Janus interface materials with asymmetric wettability, and (iv) surfaces with a specific wettability boundary.11 Among those, Janus interface materials are

INTRODUCTION Superhydrophobic surfaces have attracted the attention of industry and researchers because of their self-cleaning, antifouling, antifogging, anti-icing, and corrosion-resistant properties, which widen their potential applications to areas such as self-cleaning coatings, waterproof textiles, windshields, roofing, oil/water separating membranes, and microfluidic devices.1−3 Although superhydrophobicity was discussed by Cassie and Baxter several decades ago, this concept gained interest especially after the studies of Barthlott and Neinhuis on the water repellency of different plant species and insect wings.4−6 Particularly, the superhydrophobic and waterrepellent leaves of the lotus plant (Nelumbo nucifera) were investigated by these two botanists, and scanning electron microscopy (SEM) studies revealed that the hierarchical micro−nano surface topography combined with a hydrophobic wax layer imparts the plant with its superhydrophobic behavior, yielding water contact angles (CAs) above 150° and water CA hysteresis values below 10°.4−6 Another important characteristic of the lotus leaf is its wetting contrast: the upper side of the lotus leaf is superhydrophobic, whereas the lower side is hydrophilic and © XXXX American Chemical Society

Received: July 18, 2018 Accepted: October 2, 2018

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DOI: 10.1021/acsami.8b12116 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

the preparation of Janus-type paper substrates with superhydrophobic and hydrophilic properties on opposite sides, with the substrates being achieved through the inkjet printing of a hydrophobic precursor containing tetraethyl orthosilicate and a fluorinated ethoxysilane compound.51 Here, the superhydrophobic site of the substrate was reported to be in a “Wenzel” state of wetting, meaning that high tilt angles (TAs) are required for the droplets to move on the surface, which may limit possible application areas of the material. Knowledge on the preparation as well as on structure− property profile of Janus-type paper substrates is still limited, despite a wide range of possible applications of such sheets. Therefore, we were interested in the design of further novel and alternative approaches to already existing procedures that describe the preparation of chemically and mechanically stable Janus-type paper. Frequently, composite materials of inorganic particles surrounded by a polymer matrix in thin-film configurations are used to modify the wetting properties of paper substrates.30,35 However, to our knowledge, in these studies, inorganic particles are not chemically attached to the paper surface. The latter is of utmost importance to achieve chemical and mechanical durability of the final sheets. Other studies report more sophisticated approaches for the formation of inorganic particles on the substrate surface by silane−silane reactions during surface modification processes.52 However, control of size and geometry variation of the particles and its impact on water and water vapor permeation was not reported. Because the latter is of importance for any final application such hybrid materials will be used for, we were interested in finding an inexpensive and easy to perform procedure suitable for attaching various types of inorganic particles with different shapes and sizes, as well as to learn about the impact of the latter on the final structure−property profile. Besides using inorganic particles being coated on paper sheets, a number of studies exist, where a hydrophobic paper surface is being designed using silicon-based hydrophobic polymers and polymer precursors.53,54 Some of the used techniques are based on a covalent attachment of the silicon reactants directly to the cellulose fibers.54 However, these methods typically require −OH functionalized silicon products for covalent attachment to the fiber surface. Galembeck et al. as well as McCarthy et al. claim that poly(dimethylsiloxane) (PDMS), which is generally considered to be a nonreactive polymer, can be chemically linked with hydroxylated surfaces when heat-treated.55−57 In their studies, various inorganic surfaces (silicon wafers, metal oxides, and glass) were thermally hydrophobized by simple PDMS coatings. However, a similar strategy has yet not been applied to paper as a solid substrate. To explain the hydrophobization process, two possible reaction mechanisms for the covalent attachment of PDMS chains onto various substrate surfaces were hypothesized: (i) the hydrolysis of PDMS followed by condensation with a surface silanol in the presence of a minor amount of water and (ii) the direct or acid-catalyzed silanolysis of PDMS by a surface silanol.57 Because of the hydrophilic and slightly acidic nature of our starting materials (cellulose-based filter paper, glass spheres, and silica particles), both of these mechanisms could be applicable for the attachment of PDMS.58 Thus, PDMS can act as a binder between the paper substrate and inorganic particles on the surface as well as impart the surface with a superhydrophobic behavior.

fabricated in the forms of particles, rods, sheets, and membranes.7,9,12 However, the most studied form is that of Janus membranes with the asymmetric wettability on different sides because such smart membranes have many application areas such as oil/water separation,11 unidirectional water flow,13,14 fog collection,15 and bubble aeration.16 Recently, natural cellulosic materials such as textiles17 and paper18 with high porosities and low densities have garnered interest as membranes, despite their synthetic counterparts. Additionally, paper, which is a nonwoven mat mainly composed of cellulose fibers, is favored over fabric in the design of new sophisticated membrane-type materials, especially because of its highly optimized industrial process and simple production route compared to that of woven textiles.19 Moreover, the facile adjustment of morphological properties such as pore thickness, grammage, and fiber density enables the tailoring of paper substrates according to the requirements of the end-use product. Paper is also thin, lightweight, and flexible, and it does not fracture when bent.20 Therefore, a paper-based membrane can be readily shaped into different structures, which is impossible, or at least, not easy to perform with other materials such as silicon wafers, glass slides, metal oxides, and most of polymeric materials.21 In addition, paper is mainly composed of cellulose fibers, and being the most abundant polysaccharide in nature, cellulose is inexpensive, biodegradable, renewable, and disposable. Furthermore, cellulose-based fiber mats show nearly no thermal expansion. Finally, because of the presence of a large number of functionalizable hydroxyl groups on cellulose fibers, paper can be easily modified through the covalent attachment of various types of chemicals.21,22 An abundance of articles in the literature describe the preparation of superhydrophobic surfaces.23,24 Typically, silicon wafers,25 glass slides,26 polymeric materials,27,28 and metal-oxide surfaces29 are modified to achieve superhydrophobicity. However, research on the preparation of superhydrophobic paper surfaces30−34 remains limited in number despite superhydrophobic paper having significant potential applications in such areas as microfluidic devices, oil/water separating membranes, packaging materials, or bioactive substrates.31−34 Some methods for developing superhydrophobic paper surfaces include the use of dip-coating,31 doctor blade-coating,35 and spray-coating,32 as well as plasma processing,36,37 layer-by-layer deposition,38 or liquid flame spray method.34 Breedveld and co-workers have introduced highly practical methods that enable full control on surface hydrolysis through the usage of trimethylmethoxysilane units, which are especially suitable for the commercialization of highscale products.39,40 Other approaches include the hydrophobization of the cellulose fibers using small reactive molecules such as alkyl ketene dimers (AKDs) or alkenyl succinic anhydrides (ASAs).41−43 In addition, Janus interface materials or membranes with the superhydrophobic behavior at one side have infrequently been reported; usually, Janus-type natural and synthetic fabrics44−47 or metal oxides48 with superhydrophobic/(super)hydrophilic properties are fabricated. To our knowledge, just few studies have been published on the preparation of Janus-type paper substrates.49−51 Kehr and Motealleh, for example, reported the preparation of Janustype paper sheets as a three-dimensional (3D) cell culture system. The focus of this study was to fabricate composite papers as sophisticated scaffolds for cell growth. Thus, no data about CA studies were reported.49 He and co-workers reported B


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SEM Studies. SEM images were obtained on a Philips XL30 FEG high-resolution scanning electron microscope (SEMTech Solutions, Inc., North Billerica, MA). Prior to the SEM studies, the samples were coated with a thin gold layer of 12 nm to prevent/minimize charging on the surface. Surface Roughness Analysis. Atomic force microscopy (AFM) studies were performed by using a Bruker Dimension Icon atomic force microscope with ScanAsyst. All images were taken in standard tapping mode in air. Nanoscope software was used, and the analyses of images were performed by using NanoScope Analysis software. Images (5 × 5 μm2) of the sample surfaces were obtained by using a Bruker TESPA V2 tip with a force constant of 42 N/m and a resonance frequency of 320 kHz. At least three AFM images were taken for each sample. More details can be found in the Supporting Information. Thermogravimetric Analysis (TGA). TGA was performed with a Mettler Toledo TGA instrument. The experiments were carried out in a N2 atmosphere. The flow rate of N2 was 30 mL/min. The mass of each sample was recorded against the temperature as the sample was heated up to 800 °C at a rate of 10 °C/min. Three measurements were performed per sample, and the results were averaged. Water Imbibition Test. Samples were cut to 2 × 2 cm2 pieces and were conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for 24 h. After recording the thickness and initial mass, the samples were entirely immersed into Milli-Q water for 24 h to monitor the swelling performance. At the end of 24 h, the samples were removed from the water, wiped with a clean delicate tissue, and weighed again. The amount of water absorbed was calculated from the increase in mass of the specimen, and the water absorption was recorded as the % increase in mass. Three tests were performed for each sample, and the average value of the data was taken. The immersion step was repeated, and the samples were weighed at the end of 48 h. The water absorption values did not change, meaning that at the end of 24 h, the saturation values were achieved. Mercury Porosimetry Measurements. The measurements were made with a PoreMaster GT (33/60) from Quantachrome Instruments. The sample chamber was filled with elemental mercury. After applying an external pressure of 1−50 000 PSI, the large pore spaces of the samples were filled with the increasing pressure. Duplicate determinations were carried out in each case. Water Vapor Permeation Studies. BYK-Gardner Permeability Cups equipped with a flat retaining ring and threaded ring cover (BYK Additives & Instruments, USA) were used for water vapor permeability studies. Tests were conducted in climate room with a temperature of 23 ± 2 °C and a relative humidity of 50 ± 5%. After being conditioned for 24 h before the measurements, samples were clamped to the cups filled with 20 mL Milli-Q water each. A value of permeability calculated per sample by measuring the weight loss of each cup by an analytical balance, and the results were recorded in terms of the number of grams of water which permeates through square meter of film in 1 day. At least three tests were performed for each sample. Sand Abrasion Test. Untreated sides of the samples were first attached onto the glass slide with the help of a two-sided tape for easy handling. The samples were then put face-down onto the sandpaper (grid no. 220) with a 100 g weight on top. The samples were moved for 10 cm forth and 10 cm back along the ruler on the sandpaper (Supporting Information). CA and TA values were recorded at every 2 cm of movement within a standard deviation of ±2°. Tensile Tests. Dry tensile strength and wet tensile strength values of the paper strips were determined by using a Zwick Roell Z1 stress− strain instrument equipped with the testXpert II program. Paper test strips (15 mm wide) were prepared and conditioned for 24 h at 23 ± 2 °C and 50 ± 5% relative humidity prior to the test. At least five wet or dry paper strips were clamped to the clamps of the instrument, and the width-related breaking force was determined according to the ISO 1924-1/1 testing standard. The wet strength (Sw) and dry strength values were recorded, and the relative wet strength was calculated according to eq 1

In our study, we make use of the versatile reactivity of PDMS for the immobilization of inorganic silicon oxide particles within a hybrid coating on the surface of a paper substrate. Therefore, both the surface chemistry and the roughness of the material could be modified and an anisotropic Janus-type profile with respect to wettability was created, where one side of the paper possesses superhydrophobic properties, whereas the opposite side is still hydrophilic. The particle sizes progressed from nanometers to several tens of micrometers, and the impact of particle morphology on wetting as well as water and water vapor permeation was studied.

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MATERIALS AND METHODS

Filter paper no. 1 was purchased from Whatman International, Ltd. (Maidstone, England). Glass spheres with a particle size of 9−13 μm were kindly supplied by Sigma-Aldrich, Germany. Aeroperl 300 Pharma colloidal silica with an average particle size of 20−60 μm and Aerosil 300 Pharma fumed silica were kindly supplied by Evonik (Essen, Germany). Stearoyl chloride (90%) and microcrystalline cellulose with a granule size of 50 μm were bought from SigmaAldrich, Germany. Trimethylsiloxy-terminated PDMS (silicone oil) with a molecular weight of 5970 g/mol was kindly supplied by Gelest (Morrisville PA, USA). Reagent grade tetrahydrofuran (THF) was purchased from Fisher Chemical GmbH (Germany) and used as received. Reagent grade n-hexane was purchased from Berd Kraft GmbH (Duisburg, Germany) and used as received. Preparation of the Lab-Engineered Paper Sheets. Bleached, dry cotton linters pulp was used for the preparation of the paper substrates. The cotton linters pulp was prepared and then refined in a Voith LR 40 laboratory refiner with 75 000 rotations. A series of paper sheets with a diameter of 200 mm and grammages of 120, 100, 80, 60, and 40 g/m2 were fabricated on a conventional Rapid-Koethen handsheet maker according to DIN 54358 and ISO 5269/2 in the absence of additives and fillers. The sheets were dried at 93 °C for approximately 10 min (up to a constant weight). Preparation of Superhydrophobic Sample Surfaces. A dilute solution of PDMS oil in THF (0.75 wt %) was prepared by magnetic stirring. The solution was added to a mixture of 0.5 g glass spheres, 0.5 g colloidal silica, and 0.1 g fumed silica. The resulting mixture was stirred and sonicated until a stable dispersion occurred. The resulting dispersion mixture was applied through spin-coating or spray-coating onto Whatman No. 1 filter paper and cotton linters paper substrates. After the spin- or spray-coating process, the samples were left in the hood overnight for the evaporation of the THF solvent. The samples were then heat-treated at 120 °C for 36 h. After heat treatment, the samples were left to cool to 23 °C. Soxhlet extraction with hexane, THF, or the mixed solvent was performed to get rid of the excess/ unreacted PDMS. The coating on the paper was strongly resistant to Soxhlet extraction with THF or hexane. A superhydrophobic behavior was achieved after solvent extraction for 3 h. The preparation of superhydrophobic Whatman No. 1 filter paper-based samples coated with nanostructured cellulose stearoyl ester particles (SH-CSE-filter paper) is explained elsewhere.22 Dynamic and Static CA Measurements. All CA measurements were performed with a DataPhysics OCA 35 goniometer equipped with SCA 20 Software. For static water CA measurements, 10 μL Milli-Q water was dropped onto the sample surface, and the CA was measured at a relative humidity 50 ± 5% and a temperature of 23 ± 2 °C. At least five measurements were performed for each surface so that an average static water CA could be determined. TA studies were performed by automatically tilting the Dataphysics OCA35 instrument up to 90° with a speed of 2.74°/s. The samples were first fixed onto the stage of the instrument. A 10 μL Milli-Q water drop was put onto the sample surface before the goniometer was tilted automatically with the parameters written above. The angle was determined at the point the water droplet on the surface started to move. At least three measurements were performed for each sample. C


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Figure 1. (A) Preparation route for our Janus-type hybrid paper sheets. (B) Lower unreacted side preserves the fibrous morphology and inherent hydrophilic behavior, whereas the coated upper side gains a different multiscale topography and becomes superhydrophobic. Wet strength (rel. ) in % =

wet tensile strength × 100% dry tensile strength

cotton linters-based Whatman No. 1 filter paper was subjected to chemical treatment to optimize the procedure. The Whatman No. 1 filter paper-based samples that had a chemically treated and superhydrophobic side are denoted as SH-filter paper for ease of explanation. In this study, we regard samples with a water CA value above 150° and a TA value close to or less than 10° as superhydrophobic.36 The spincoated surfaces had a CA value of 163.1 ± 1.2° and a TA value of 5.5 ± 2.7°. The spray-coated surfaces had similar properties with a CA of 162.3 ± 1.8° and a TA of 6.3 ± 1.6°. On the other hand, the untreated sides preserved their inherent hydrophilic behavior. A mixture of three different inorganic particles with size ranges of around 20−60, 9−13 μm, and several nanometers was required to achieve the desired roughness pattern for superhydrophobicity on paper surfaces. Control samples which were prepared through incorporation of only one or two of the above-mentioned inorganic particles had CA values in the range of only 120−140° and were far from being superhydrophobic. Particles with sizes of around 20−60 μm were mainly deposited in the large pore openings of the paper substrate making the paper surface relatively less heterogeneous. These large particles have a similar effect similar to that of clay particles applied prior to the sizing procedure of paper. Glass spheres with 9−13 μm provide the micron-sized structures and fumed silica particles provide the nanoscale roughness required for hierarchical lotus-like behavior (Figures S6 and S7). To detect the superhydrophobic coating thickness, uncoated and hybrid paper sheets were subjected to TGA studies. The residual masses of the sheet and coating were determined, and the thickness of the coating layer was estimated by the given densities of the coating ingredients. Nevertheless, we report only the applied/surface area: the SH-filter paper samples had coatings of 3.7 ± 0.6 g/m2. Furthermore, the coating had a dramatically low amount of the PDMS layer of 0.3 ± 0.1 g/m2 (Figures S1−S4). We also prepared cotton linters sheets of different paper grammages to prove that we could achieve superhydrophobicity on paper substrates irrespective of their physical

(1)

The tensile index values for each sample were automatically calculated by the software, and the average of at least five measurements was taken.

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RESULTS AND DISCUSSION Several different paper grades are present to work with, and the paper substrate can be selected based on the intended production method and end-use application. Here, we decided to use a cotton linters paper because it consists of pure cellulose fibers and thus has the highest number of reactive hydroxyl groups on its surface. We modified one side of both Whatman No. 1 and lab-engineered cotton linters papers to achieve Janus-type hybrid materials. Conventional papermaking laboratory instruments were used to produce paper substrates of different grammages, with the aim of controlling the final topography and morphology, as explained in the experimental part. To prepare superhydrophobic paper surfaces, first, a mixture of PDMS, silica particles, and glass spheres was prepared and applied to the paper substrates through spin- or spray-coating. The samples were then heat-treated at 120 °C so that covalent attachment of PDMS chains occurred at the hydroxylproviding surfaces. This temperature is known to be nondestructive to paper substrates and thus suitable for paper modification processes.59,60 In this way, a highly stable, superhydrophobic coating was achieved on the paper sheets, which were strongly resistant to the exposure of organic solvents. Additionally, the nontreated side preserved its inherent hydrophilicity, as shown in Figure 1. Superhydrophobicity could be successfully achieved on both Whatman No. 1 filter paper and cotton linters paper sheets upon modification, irrespective of the morphological structure of the paper substrate. Therefore, the approach of McCarthy and Galembeck, invented to modify inorganic surfaces, was applied for the first time to modify an organic material. Hybrid Paper Sheets with Contrast Surface Wetting Properties on Opposite Sides. Commercially available D


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Table 1. Comparison of Static Water CA and TA Values on SH-Filter Paper and SH-Cotton Linter Paper Substrates paper substrate

paper grammage (g/m2)

SH-filter paper SH-cotton linters paper

80 120 100 80 60 40

substrate thickness (mm) 0.152 0.240 0.231 0.195 0.140 0.104

± ± ± ± ± ±

0.016 0.007 0.015 0.008 0.016 0.003

CA (deg) 162.3 162.2 161.3 162.7 162.3 159.9

± ± ± ± ± ±

1.8 2.0 1.1 0.8 0.4 2.1

TA (deg) 6.3 4.4 6.7 6.2 8.0 10.1

± ± ± ± ± ±

1.6 1.5 1.3 0.1 3.1 1.6

Figure 2. (A) SEM images of the Whatman No. 1 filter paper substrate before and after coating, SH-filter paper, and the control sample. (B) Height, 3D height, and phase AFM images of SH-filter paper revealing the nanosized structures on the sample surface.

properties.19 A series of cotton linters paper sheets with different paper grammages and thickness values were fabricated and treated, as revealed in Table 1. Superhydrophobicity could be achieved on the cotton linters paper substrates irrespective of the initial paper grammage or the thickness of the starting material. Only a slight change in the TA value was observed for the sample with a paper grammage of 40 g/m2 mainly because of the great distance between the fibers, which slightly disrupted the homogeneous distribution of micro- and nanosized domains. Typical SEM images of the cotton linters paper sheets before and after modification are shown in Figure 1. More images can be found in the Supporting Information. Influence of Surface Topography and Chemistry on Water−Paper Interactions. A hierarchical micro−nano surface roughness is required for the superhydrophobic behavior.3−6,11,23 To understand the correlation between the wetting behavior of our samples and the corresponding surface topography, we performed SEM studies. The SEM images reveal that the surface topography of Whatman No. 1 filter paper significantly changed after the modification process, as revealed in Figure 1B. Colloidal silica particles and glass spheres were irregularly and densely distributed on the surface and had diameters of 10−60 μm. The presence of a nanosized secondary layer composed of primary fumed silica particles was also proven by SEM investigations at higher magnification levels and AFM studies, as revealed in Figure 2. Such a hierarchical topography composed of micro- and nanosized structures highly resembled that of a lotus leaf surface and resulted in the superhydrophobic behavior of the sample surface. SEM studies on the untreated and superhydrophobized surfaces of the lab-engineered SH-cotton linters paper sheets revealed the fibrous and lotus-leaf-like topographies of

the hydrophilic and superhydrophobic sides, respectively (Supporting Information). To prove the covalent attachment between the coating and fibers of the paper sheets, control samples were prepared, which were not heat-treated, which is necessary for the reaction between −OH groups on both the fiber and particle surfaces and PDMS. These control samples were then subjected to Soxhlet extraction, and we observed that the coating was nearly completely removed at the end of 3 h. This finding reveals that there was no covalent attachment of the coating to the control sample surfaces. On the other hand, the heat-treated samples preserved the coating layer during Soxhlet extraction and were superhydrophobic. It is also important to point out that even for these samples, an excess of unbound coating was initially present but was removed through Soxhlet extraction so that the hierarchical surface topography, buried in the excess coating layer, emerged, as revealed in Figure 2A. The removal of the coating layer, and thus the PDMS units on the surface, was also investigated by energy-dispersive X-ray spectroscopy (EDS) studies. Figure 3 reveals the EDS spectra of the control sample, which was not heat-treated at 120° but left at 23 °C for 36 h, before and after Soxhlet extraction. Before Soxhlet extraction, an intense Si K peak appeared at the EDS spectrum. After extraction, on the other hand, the net intensity of the Si K peak decreased from 81.49 to 8.77%, which clearly shows that the PDMS units were washed off the surface. Moreover, Raman studies were performed to monitor the covalent bonding of PDMS units onto cellulose, as shown in the Supporting Information. Furthermore, high-resolution (258 × 258 μm2) images of the sample surfaces were taken with a laser scanning microscope, and the arithmetic average roughness (Ra) values were calculated for each sample (Supporting Information). E


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Figure 3. EDS maps of the control sample surface, which is not heattreated at 120 °C but left at RT for 26 h (A) before and (B) after Soxhlet extraction.

The neat filter paper surface with a fibrous structure had an average Ra value of 6.66 ± 0.44 μm. On the other hand, the superhydrophobic sample surfaces had roughness values, with an Ra value of 14.21 ± 0.59 μm, higher than that of the neat filter paper, as expected. Influence of Sample Morphology/Porosity on Water− Paper Interactions. For Janus-type materials in general, the interface between the two sides with different characteristics is as important as the surface topography because this affects the behavior of water inside Janus-type membrane materials.7,8,12−16 Additionally, the characteristics of their interface are used to categorize Janus-type materials: when there is a sharp change in properties at the interface, the material is called A-and-B (sides or layers with different characteristics are denoted as A and B for ease of explanation). When there is a gradient of properties between two different sides, we talk about an A-to-B-type material. If one layer is substantially thinner than the other one, the material is named as A-on-B or B-on-A, as shown in Figure 4A−C.7 To gain insights about the interface characteristics of our treated hybrid paper sheets, SEM cross-sectional images were taken and EDS element mapping studies were performed. Tests to determine the morphology/porosity and the mechanical properties of our samples were mainly performed on the spray-coated SH-filter paper because the spray-coating method is easily applied to both small and large paper surfaces, whereas the spin-coating procedure limits the substrate dimensions. The SEM images D, E, and F in Figure 4 reveal the cross section of the SH-filter paper at different magnification levels. The fibrillar structure of the paper substrate was clearly visible, whereas there was a distinct coating layer of PDMS and inorganic particles, which was mainly deposited on the paper substrate. The EDS map (Figure 4G,H) of the same sample reveals that the Si atoms were mainly located on the coating but were also slightly diffuse throughout the substrate. This result shows that not only the surface but also the inside of the paper substrate was modified, leading to a gradual change in substrate chemistry and thus in wettability. The same behavior was also observed for all SH-cotton linters paper samples (Supporting Information). These data correlate well with A-to-B Janus materials.

Figure 4. (A) A-and-B, (B) A-to-B, and (C) A-on-B Janus membranes. (Green side is the modified side, which is denoted as A, whereas the gray side is the nonmodified paper substrate, which is denoted as B.) SEM images of the (D) cross section of our A-to-B SH-filter paper sample at 800×, (E) interface between the coating and paper substrate at 5000×, and (F) fibrous mat at a 5000× magnification level with the (G) EDS map of Si along the cross section revealing the gradual decrease of Si content from side A to side B and (H) corresponding Si content along the Z-axis. This special type of cross-sectional morphology led to our (I) Janus-type samples, which preserved their superhydrophobicity even after wetting of the back side. (The water was dyed with betanin for ease of visualization.)

The special characteristic of our samples resulted in a very distinct type of wetting: an immediate wicking and swelling of the hydrophilic side occurred upon application of water droplets, whereas the treated side preserved its superhydrophobic behavior, as revealed in Figure 4I. This result demonstrates that the elastic nature of the covalently attached PDMS chains prevented the deterioration of the surface topography of the coating and the superhydrophobic behavior of the upper side of the paper substrate, even when the cellulosic fibers of the paper substrate swelled upon application of water droplets. All functionalized paper sheets preserved the hydrophilicity of their nontreated side, irrespective of being lab-engineered or commercially sold. Full penetration of PDMS was not detected in a paper grammage range from 40 to 120 g/m2. This finding reveals that in our procedure, the Janus- type wettability does not depend on the paper grammage, which also have an impact on porosity. To understand the effect of the different chemical and physical characteristics both on the surface and throughout the cross section of our samples on their water imbibition capability, water absorption tests were performed. This property is important in determining the final application F


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breathability value of 282.6 ± 18.8 g/m2 d was detected; this value was very similar to that of Whatman No. 1 filter paper (287.6 ± 21.8 g/m2 d). The findings of this test correlate well with those of the mercury porosimeter experiments claiming that the treated samples still possessed a porous structure (Supporting Information). The results of the water vapor permeation study imply that our samples could breathe and shield from liquid water at the same time, which is a property similar to that of Gore-Tex fabric membranes.64 Because of this discrimination, the membranes were superhydrophobic at one side yet were permeable to water vapor.64 Mechanical Stability of Our Functional Paper Samples. Because mechanical durability determines the application areas of a product, we performed a series of mechanical tests. First, bending tests were performed to gain insights into the flexibility of our SH-filter paper samples. Briefly, a 2.5 mm wide sample was bent so that the distance between the two sides was 0.8 mm (Scheme S1). The CA of the SH-filter papers was measured before bending and after bending, and the interaction of water with the substrate surface in the bent state was monitored, as shown in Figure 5. The SHfilter paper was superhydrophobic in the bent state and after its release to the initial position. The covalent attachment and the elastic nature of the PDMS material had a positive impact on both the chemical stability and mechanical durability of our samples and enabled the preservation of superhydrophobicity during and after bending. For further testing of the mechanical resistance of our samples, a sand abrasion study was performed, as shown in Figure 6.65 The CA and TA values of the superhydrophobic sample surfaces were measured at every 2 cm after movement of the sandpaper with a 100 g weight on top. Superhydrophobic Whatman No. 1 filter paper-based samples coated with nanostructured cellulose stearoyl ester particles (SH-CSE-filter paper) were used as a benchmark material.24 The SH-filter paper samples remained superhydrophobic after sand abrasion for 10 cm, whereas the SH-CSE-filter paper samples preserved their superhydrophobic behavior up to 4 cm. These results again demonstrate the positive impact of the covalently attached coating on the mechanical durability. However, we should mention that the TA values for the CSEcoated samples reached a plateau of approximately 74° after exposure to the sandpaper for 18 cm, whereas water droplets that hung on to the SH-filter paper samples did not move upon tilting and were absorbed by the sample after a distance of 24 cm was traveled (recorded as a CA of 0° in Figure 6E). This finding reveals that bare sites of cellulose fibers were exposed upon sand abrasion. Because the covalent attachment of PDMS should also work as a cross-linking agent between the fibers, the (wet-) tensile strength of our paper material should be significantly improved

areas of porous samples, such as water/fluid storage materials. A simple test procedure based on a single continuous immersion in water was developed and performed. The water absorption test results for each sample at the end of 24 h are tabulated as the % increase in weight in Table 2. Table 2. Water Absorption Test Data for Neat Paper Substrates and Their Modified Janus-Type Counterparts water absorption at 24 h (% increase in weight) type of paper substrate Whatman No. 1 filter paper sheets cotton linters paper sheets

grammage (g/m2)

neat paper substrate

Janus-type substrate

80

160.6 ± 19.2

108.2 ± 2.3

120 100 80 60 40

254.4 244.6 231.1 251.9 222.2

± ± ± ± ±

5.9 24.8 12.2 28.9 18.9

182.1 168.2 141.0 144.9 127.5

± ± ± ± ±

9.8 6.5 13.4 6.6 11.7

All samples exhibited high water absorption because of the hydrophilic nature of cellulose and its strong affinity toward water. The treated paper substrates had lower water imbibition capabilities because of the gradual hydrophobization throughout the cross section. Therefore, the amount of water absorption increased with increasing paper thickness, as expected. Our Janus-type hybrid paper sheets could imbibe a significant amount of water, and this property places them in a special classification among superhydrophobic materials, which nearly completely lose their interaction with water after modification. We also point out that the Whatman No. 1 filter paper- and cotton linters paper-based samples exhibited different water imbibition capabilities, though both featured similar grammages of approximately 80 g/m2. The results suggest the presence of hydrophobizing additives, for example, wet-strengthening agents, in the commercialized product. Another important property, which affects the water imbibition capability and the breathability of membrane-type materials, is porosity. Porosity also affects the final selectivity of a material toward different types of liquids. Additionally, to investigate the effect of surface treatment on the final porosity of our samples, mercury porosimeter studies were performed on untreated Whatman No. 1 filter paper and SH-filter paper samples, revealing that filter paper and SH-filter paper had similar pore diameters of 5.9 ± 1.7 and 4.7 ± 1.1 μm and porosity values of 0.31 and 0.30, respectively (Supporting Information). Because the porous nature of the paper substrates was preserved after the surface treatment, we performed the water permeation test to investigate the breathing capability of our samples.61−63 For our hybrid paper sheets, an average

Figure 5. Static and dynamic water CA studies on SH-filter papers: (A) static water CA before bending, (B) behavior of a 10 μL Milli-Q water droplet on the bent surface (the droplet immediately bounced back within seconds of hitting the superhydrophobic surface), and (C) static water CA after bending. G


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Figure 6. (A) Forward movement and (B) backward movement of SH-filter papers on the sandpaper. Water CAs on the same sample (C) before the test and (D) after a 10 cm exposure to external damage. (E) CA and (F) TA values of the SH-filter paper (red) and SH-CSE-filter paper (blue) samples with respect to the distance traveled by the sandpaper.

Therefore, the water droplets on the superhydrophobic surface were absorbed and transferred by means of capillary forces through this spot to the hydrophilic side of our Janus-type membrane material, which acted as a small reservoir for water or aqueous solutions. As revealed in the previous paragraphs, one side of our samples was superhydrophobic and could not be wetted by water, whereas the other side was hydrophilic and readily absorbed and stored. Furthermore, although our samples were modified with a composite, they remained porous and could breathe. They were mechanically and chemically stable as well as bendable. All of these properties make our samples potential candidates as a wound dressing and/or bandage, which could allow the wounded/burned skin to breathe and also protect it from wetting under water pressure, as revealed by hydrotesting experiments. When used as a wound dressing, our Janus-type device could enable the incorporation of aqueous drug solutions without the need to open the wounded area, which would reduce the risk of infection. Moreover, because of the ease of fabrication, a large number of our devices could be produced on demand. In the literature, there are examples of fabrics in which one side was made hydrophobic to be used as a wound dressing or bandage.44,69 Some bacterial cellulosebased wound dressings are also commercially available.70 However, as a nonwoven material, the grammage, and thus the blood/body liquid/drug absorption capability, could be tailored according to the severity or type of wounded area. Our paper-based wound dressings could be very efficient and inexpensive alternatives for treatment purposes.

by the modification. Thus, the influence of the PDMS coating on the dry and wet tensile strengths of the hybrid paper sheets was also investigated. The dry tensile strength (Sd) and wet tensile strength (Sw) values for Whatman No. 1 filter paper were recorded as 1.21 ± 0.05 and 0.07 ± 0.00 kN/m, respectively. Furthermore, both the Sd and Sw values increased for the SH-filter paper samples. An Sd value of 1.33 ± 0.05 kN/ m and an Sw value of 0.13 ± 0.02 kN/m were recorded for SHfilter papers, which means that the relative wet strength of the SH-filter paper (9.4%) was nearly twofold the wet strength value for Whatman No. 1 filter paper (5.9%). Note that a typical paper sheet having no wet strength additives showed a mean relative wet strength of 1−3%. This result suggests that Whatman No. 1 filter paper already possessed some wet strength additives. Although the observed relative wet strength enhancement was below the limit for the polymeric coating to act as a wet-strengthening agent, it remains significant to obtain such an improvement in the wet strength value with a very thin coating.38,66−68 In addition, the hybrid papers showed enhanced burst pressures (Supporting Information) in contrast to that of the nonmodified paper. Applicability of Our Functionalized Janus-Type Hybrid Materials as Wound Dressings. Because of their Janus membrane properties, our SH-filter paper samples can act as water transport and storage devices. For this, we fabricated substrates that possessed a circular-shaped, noncoated hydrophilic region on the superhydrophobic side, the schematic representation of which is shown in Figure 7A.

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CONCLUSIONS In the present study, we demonstrated a new and easy to perform method for the preparation of Janus-type hybrid paper sheets with chemically stable superhydrophobic surface coatings through the use of inexpensive starting materials. Covalent attachment of both nano- and micrometer-sized inorganic particles to the hybrid materials was achieved, enabling a robust and hierarchical micro−nano surface topography of the treated side, which is required for the superhydrophobic behavior. In contrast, the hydrophilic side preserved the inherent fibrous structure. Apart from the change

Figure 7. (A) Schematic representation of the Janus-type membrane device for the manipulation of droplet movement and droplet storage. (B) Water droplets were captured and stored with our device. H


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support from the LOEWE program of excellence through the project initiative “BAMP!”.

in surface topography, the gradual change in chemical properties between the two sides was clearly demonstrated. Both commercially available Whatman No. 1 filter paper and lab-engineered cotton linters paper sheets were modified to enable comparative studies. The chemical stability and mechanical durability of the samples were proved by Soxhlet extraction studies and mechanical tests. Because our procedure enables covalent attachment of inorganic particles with different size scales to the hybrid sheets, it has the potential for the adjustment of both the density of inorganics and surface roughness of the functionalized paper sheets, which also opens up the possibility to fabricate diversified end products for varied purposes with controlled wettabilities, water, and water vapor permeation rates. Because of their unique chemical, physical, and mechanical properties, our paper-based Janus-type paper sheets may find applications in areas, such as smart wound dressings and/or bandages.

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ABBREVIATIONS SH, superhydrophobic; SH-filter paper, superhydrophobic Whatman No. 1 filter paper; SH-cotton linters paper, superhydrophobic cotton linters-based paper; SH-CSE-filter paper, superhydrophobic Whatman No. 1 filter paper coated with cellulose stearoyl ester particles; PDMS, poly(dimethylsiloxane); CA, contact angle; TA, tilting angle; SEM, scanning electron microscopy; AFM, atomic force microscopy; TGA, thermogravimetric analysis; AKD, alkyl ketene dimer; ASA, alkenyl succinic anhydride

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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/acsami.8b12116. TGA of neat hybrid paper sheets; profilometer studies; SEM pictures with corresponding EDS maps of SHcotton linters paper samples; results of mercury porosimetry; and details about water vapor permeation studies and hydrostatic pressure tests (PDF) Tilt angle study for spray-coated SH-filter paper and behavior of water droplets on bent SH-WFP samples (ZIP) TA studies on superhydrophobic surfaces; behavior of water droplets on bent sheets; and sand abrasion test (AVI)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Ç .K.S.). *E-mail: [email protected] (M.B.). ORCID

Simon Trosien: 0000-0001-5081-5265 Markus Biesalski: 0000-0001-6662-0673 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This research was financially supported by German Academic Exchange Service (DAAD). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by German Academic Exchange Service (DAAD). We thank Prof. Halil Akkanat from Türkish-German University and Prof. Lambert Alff from Darmstadt Technical University who made the exchange between these two universities happen and greatly assisted the research. We are also immensely grateful to Dr. Marcus Schulze for his help in AFM investigations and Sabine Hesse for Raman studies. Furthermore, we acknowledge financial I


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Superhydrophobic Hybrid Paper Sheets with Janus-Type Wettability

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