Preparation, characterization, and properties of fluorine-free superhydrophobic paper based on layer-by-layer assembly

Accepted Manuscript Title: Preparation, characterization, and properties of fluorine-free superhydrophobic paper based on layer-by-layer assembly Authors: Jin Yang, Hui Li, Tianqing Lan, Lincai Peng, Rongqi Cui, Hao Yang PII: DOI: Reference:

S0144-8617(17)31065-2 http://dx.doi.org/10.1016/j.carbpol.2017.09.040 CARP 12785

To appear in: Received date: Revised date: Accepted date:

29-6-2017 25-8-2017 11-9-2017

Please cite this article as: {http://dx.doi.org/ This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation, characterization, and properties of fluorine-free superhydrophobic paper based on layer-by-layer assembly Jin Yanga, Hui Lia*, Tianqing Lana, Lincai Pengb, Rongqi Cuia, Hao Yangc a

Research Institute of Food Safety, Kunming University of Science and Technology, Kunming 650600, China b

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China

c

School of Chemistry and Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430060, China *Corresponding author. Tel: +86087165920293, E-mail: [email protected]

Highlights     

1. A facile approach to fabricate fluorine-free superhydrophobic paper was proposed. 2. The method relied on combination of layer-by-layer assembly and silane post-treatment. 3. Tensile strength of obtained superhydrophobic paper was enhanced. 4. Superhydrophobic paper was low bacterial adhesion, self-cleaning and high durability. 5. Superhydrophobic paper exhibited moisture-proofing property.

Abstract: A fluorine-free superhydrophobic paper was prepared by a facile method involving layer-by-layer deposition of cationic starch and sodium alginate together with subsequent modification of trichloromethylsilane has been reported in this article. The surface chemical compositions, potentials and surface morphologies of the modified papers were characterized, respectively. The wetting abilities and physical strength properties of the modified papers were investigated. After 4-time deposition of cationic starch/sodium alginate bilayer followed by trichloromethylsilane treatment, the water contact angle of modified paper reached up to 161.7°, and the tensile strength increased by 6.8% in comparison to that of pristine paper. This as-prepared superhydrophobic paper not only showed low bacterial adhesion property, self-cleaning behavior, water repellency, as well as high durability against deformation, chemical and time, but also kept a high strength property under high relative humidity condition, which might has a great application potential in the liquid paper packaging industry. Keywords: Layer-by-layer; Superhydrophobic paper; Moisture resistance; Cationic starch; Alginate 1. Introduction Plastics is one of indispensable packaging materials in modern society because of its unique characters, such as durability, low cost, proper mechanical strength and barrier properties against water (Hu, Gao, Chen, & Chen, 2014). However, the usage of plastic products not only consumes non-renewable petrochemicals, 1

but also results in serious environmental pollution (Wu et al., 2016). As an alternative choice, paper made from wood fibers, which is widely used as packaging materials for food and consumer products, has attracted tremendous attentions due to its several favorable properties such as inexpensiveness, biodegradability, environmental friendliness and renewability (Khwaldia, Arab-Tehrany, & Desobry, 2010; Thuo et al., 2014; Oyola-Reynoso et al., 2016). However, paper is naturally hydrophilic because hydrophilic hydroxyl groups present on the fibrous cellulose (Ogihara, Xie, & Saji, 2013), which have been the main limitation in industrial applications demanding high resistance to water, such as liquid drink packaging materials, microfluidic devices. If paper has superhydrophobicity, the waterproof paper would be useful (Glavan et al., 2014; Oyola-Reynoso et al., 2016). A superhydrophobic paper is defined as a paper surface with a water contact angle >150°, which means that a water droplet that settles on the superhydrophobic paper surface is almost a sphere and easily roll off (Ogihara, Xie, Okagaki, & Saji, 2012). It is well known that surface energy (chemical property) and surface roughness (geometric property) are important properties that are essential to control the wettability of solid surface. Based on this fact, the superhydrophobic paper fabricated by combining of increased surface roughness with low surface energy materials such as fluorosilanes, fatty acids, paraffin wax and alkylthiols. To date, a variety of treatment methods have been employed for roughness construction over on cellulose fiber-based products (such as paper, cotton fabric) surface, include sol-gel method (Vasiljevic et al., 2014; Mahltig & Böttcher, 2003; Wang, Zheng, & Wang, 2012), immobilization of the nanoparticles by heat treatment (Bae et al., 2009), chemical vapor deposition (Li, Xie, Zhang, & Wang, 2007), rapid expansion of supercritical CO2 (Werner, Quan, Turner, Pettersson, & Wågberg, 2010), plasma etching (Balu, Breedveld, & Hess, 2008) and phase separation (Zhang, Lu, Qian, & Xiao, 2014). Despite the great technological advancements on fabrication of superhydrophobic paper with the above-mentioned methods, several strategies still require special and costly equipment, complex processes under harsh conditions and expensive ingredients, or likely cause shrinkage of the paper and poor paper recyclability (Suryaprabha & Sethuraman, 2017), which could restrict the large-scale industrial applications. Thus, the development of a low cost and simple method to confer superhydrophobicity to paper is highly necessary. The layer-by-layer (LBL) assembly technique is known as a simple and versatile method to fabricate thin multilayers with tunable composition, thickness and morphology for targeted functionalization of solid surface (Decher & Hong, 1991; Decher 1997; Zhang et al., 2004). The LBL assembly usually involves the procedures where the building blocks are alternatively deposited on the solid surface followed by solution rinse to remove the physically adsorbed materials. The driving force for LBL assembly is diverse (Borges & Mano, 2014), including electrostatic interaction (Decher & Hong, 1991), hydrogen bond interaction (Kim, Park, & Hammond, 2008), coordination interaction (Wang et al., 2012), hydrophobic interaction (Zhao et al., 2013), covalent bonds (Liu et al., 2017) and so on. The LBL multilayers modified cellulose fiber-based products can achieve different functional properties, such as biocatalytic activity (Lu & Hsieh, 2010), flame 2

retardancy (Köklükaya, Carosio, Grunlan, & Wågberg, 2015; Maluceli et al., 2014a; Maluceli et al., 2014b; Alongi, Carosio & Malucelli, 2014; Alongi & Malucelli, 2015; Alongi, Han, & Bourbigot, 2015), antibacterial ability (Gomes, Mano, Queiroz, & Gouveia, 2013), conductivity (Agarwal, Lvov, &Varahramyan, 2006), photochromic property (Tian, Wang, Li, Zeng, & Chen, 2017). Recently, there are several reported studies about fabrication of superhydrophobic paper or cellulose fiber-based products using LBL assembly (Ogawa, Ding, Sone, & Shiratori, 2007; Yang & Deng, 2008; Zhao, Tang, Wang, & Lin, 2010; Zhang, Wang, Wang, & Li, 2012; Zhao, Xu, Wang, & Lin, 2012; Zhao, Xu, Wang, & Lin, 2013), two steps emerged to obtain artificial surfaces with superhydrophobicity: (1) creating micro-scale rough structure on paper or cellulose fiber surface through LBL deposition of synthetic polyelectrolytes (poly(acrylic acid), polydiallyldimethylammonium chloride, poly(allylamine hydrochloride)) and inorganic particle (SiO2, TiO2, layered double hydroxides); (2) lowering the surface energy of paper or cellulose fiber through deposition of a final fluoroalkylsilane layer on top of the LBL stack. One of main limitations of the above-mentioned reported studies is the use of petroleum-based synthetic polymers, these polymers are produced from nonrenewable resources and are not biodegradable. In this sense, the bio-based alternative polymers are preferable to use as LBL building blocks for preparing superhydrophobic paper (Iwata 2015). Furthermore, these reported approaches relying on an expensive fluorinated compounds, which enables facile realization of superhydrophobicity with the help of their extremely low surface energies, have drawn more and more attentions as detrimental methods because of their adverse effects on human beings and environment (Choi, Yoo, Park, & Kim, 2017). Therefore, the development of a cost-effective method for fabricating fluorinefree superhydrophobic paper has essential application significance. In addition to the superhydrophobicity, the durability of superhydrophobic paper is also very important in practical applications, but is rarely considered in the above-mentioned reported studies (Chen et al., 2017). Starch is one of the most abundant natural polysaccharides in the world, and a large number of hydroxyl groups on starch molecules provides active sites for chemical modification (Granö, YliKauhaluoma, Suortti, Käki, & Nurmi, 2000). As an important derivative of starch, cationic starch (CS) is positively charged due to introduction of cationic groups such as amino, imino, ammonium, etc. It is nontoxic, easily biodegradable and can be used as wet-end additive in the paper industry in order to improve retention, drainage and strength (Gulsoy 2014). Alginate is a natural anionic polysaccharide. It occurs as the major structural polysaccharide from brown marine algae (Phaeophyceae) and as extracellular mucilage secreted by certain species of bacteria. It is a linear chain structure of (1→4)-β-D-mannuronic acid and α-Lguluronic acid residues and usually presents as sodium salt called sodium alginate (SA) (Yang, Xie, & He, 2011). SA has shown promise as a strength enhancing additive for paper industry (Song, Yao, & Jin, 2012; Bai et al., 2017). In this study, we presented a simple method for the fabrication of fluorine-free superhydrophobic paper through LBL assembly of CS and SA together with post-treatment with low surface energy material. A 3

commercially available the simplest fluorine-free silane, trichloromethylsilane (TCMS), was chosen as low surface energy material in this work. The surface chemical compositions, functional groups, surface potentials and surface morphologies of modified papers were studied in detail. The wetting behaviors and physical strength properties of the modified papers were investigated. Moreover, the anti-bacterial adhesion, self-cleaning effect, water repellency property and durability (including mechanical stability, chemical stability and long-term stability), as well as moisture-resistance property of obtained superhydrophobic paper were evaluated, respectively. 2. Materials and Methods 2.1. Materials Cationic starch (CS) was purchased from Kang Pu Hui Wei Technology Co., Ltd. (Beijing, China), sodium alginate (SA) was purchased from Bomei Biotechnology Reagent Co., Ltd (Hefei, China). Trichloromethylsilane (TCMS, 99% purity) was obtained from Sigma-Aldrich and was used as received. Cellulose fibers were obtained from Xinhua No.1 filter papers (Hangzhou, China). The deionized water was used throughout all experiments, and all other chemicals were analytical grade and used without further purification. 2.2. Preparation of superhydrophobic paper The preparation process of superhydrophobic paper was carried out as illustrated in Scheme 1. Cellulose fibers were firstly treated consecutively with CS and SA to form CS/SA multilayers on their surfaces. Amount of 200 mg/g fibers of the CS was added to a 5 g/L fibers suspension for adsorption 20 min under mechanical stirring (Step 1), then rinsed thoroughly three times using ultrapure water remove excess CS (Step 2). Subsequently, amounts of 200 mg/g fibers of the SA was added to 5g/L fibers suspension for adsorption 20 min under mechanical stirring (Step 3), followed by rinsing thoroughly three times with ultrapure water to remove the excess SA (Step 4). No exact pH control was used throughout the whole assembly process, but due to the buffering capacity of the fiber suspensions, the pH was close to 6.8. The step (1)-(4) was repeated until the desired number of bilayers was reached. After the LBL deposition, the CS/SA modified cellulose fibers were made into handsheets with a grammage of 80 g/m2 using a semiautomatic sheet former equipped with circulation water. Finally, the handsheets were immersed into 50 mL of TCMS-toluene solution (0.05 mol/L) for 20 min with a relatively humidity of 60%, then successively washed with toluene, 50% ethanol and water, dried under 60 oC for 30 min. 2.3. Characterization The surface chemical composition of paper samples were investigated by X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd., UK) with an Al Kα X-ray source. The functional groups of paper samples were studied by Attenuated total reflectance-Fourier transform infrared spectra (ATR-FTIR) 4

(Magna-IR 750 spectrometer, Nicolet, USA). The surface charges of paper samples were monitored by the zeta potential measurements using a Mütek SZP-10 zeta potential tester (BTG Group, Germany) based on streaming potential method. The surface morphologies of paper samples were observed by field-emitting scanning electron microscopy (SEM) (FESEM, Hitachi S-4800, Japan) and tapping mode AFM (Nanoscope IIIa Multimode, Veeco Co., Santa Barbara, CA, USA). Software version 5.12r3 (Veeco Co.) was used for AFM image offline data analysis. No image processing except flattening was made for the AFM image. At least five different spots from the same sample were scanned, and only images with reproducible features were reported. The contact angles of paper samples were measured using a contact angle goniometer (OCA20, Dataphysics, Germany) equipped with video capture at room temperature. Liquid droplets of 3.0 μL were carefully dropped onto the paper sample surface through a syringe, the contact angles were recorded at a contact times of 60 s, and presented contact angles values were averages from measurements of five different positions for each sample. The tensile strength of paper samples were tested using a tensile tester (DCP-KZ300, Sichuan Changjiang Paper Instrument Co., Yibin, China), and the values presented were the average from at least five measurements for each sample. The zero-span tensile strength was measured by a Pulmac zero-span tensile tester (Pulmac International Inc., Middlesex, CT, USA). 2.4. Properties of as-prepared superhydrophobic paper 2.4.1. Anti-bacterial adhesion test Escherichia coli (E.coli) and Staphylococcus aureus (S. aureus) are the most prevalent species of Gram-negative and Gram-positive bacteria, respectively, we chose these two bacteria as model bacteria to estimate the anti-bacterial adhesion property of obtained superhydrophobic papers. The degree of the bacteria adhering onto the sample surfaces was investigated according to the method reported by Hizal et al. (2017) with some modification. Briefly, concentration of bacterial suspensions in NaCl medium were adjusted to 107 CFU/mL and verified by enumeration of cultivable cells on Luria–Bertani (LB) agar plates. The bacterial suspensions (30 μL) were placed onto the tested paper samples surfaces for a contact time of 4 h. After that, the tested samples were rinsed thoroughly three times by ultrapure water to remove nonadhered or weakly adhered bacteria, then, the samples were immerged into 5 mL of LB medium and shaken at 200 rpm at 37 oC for 4 h. After shaking, 100 μL of resulting suspension was serially diluted and dilutions were plated in triplicate on LB agar, and incubated at 37 oC for 24 h. The number of bacteria adhered onto paper sample surface was determined using the counting technique of colony forming units. 2.4.2. Self-cleaning behavior test To determine the self-cleaning property of obtained superhydrophobic paper, the model contamination (methylene blue) was placed onto the paper samples surfaces, subsequently, the water droplets were dripped, and the paper samples were inclined 10° to remove contamination with water droplets, the self-cleaning 5

performance of paper sample was recorded using a digital camera. For comparison, the pristine paper was used as control. 2.4.3. Durability test The durability tests of as-prepared superhydrophobic paper include three parts: mechanical stability, chemical stability and long-term stability. (1) The mechanical stability was tested by deformation test according to Wang et al. (2016), the paper samples were bent forwards and backwards, from -90° to 90°, this process was defined as 1 cycle. (2) The chemical stability was measured by immersing the paper samples into the corrosive aqueous solutions with different pH (2‒12) for 24 h at room temperature. After the immersion, the paper samples were subsequently washed with water and dry in air. (3) The long-term stability was tested by keeping the paper samples in constant temperature and humidity room (23oC, 60%RH) for several days. After above-mentioned treatment, the water contact angle (WCA) and tensile strength of paper samples were determined. 2.4.4. Moisture resistance test A moisture-resistance tests were conducted according to the method of Yang & Deng (2008). Briefly, the paper samples were placed in the sealed oven under different relative humidity for 24 h at room temperature. The different relative humidity in the sealed oven was obtained by controlling the amount of total water in the oven for 48 h before testing. Subsequently, the moisture content and tensile strength of paper samples were measured. At least five repeats were performed for each sample. 3. Results and discussion 3.1. Formation of LBL structured multilayers on paper surface XPS was performed to analyze the surface chemical compositions of the pristine and modified paper samples, Fig. 1A gives the XPS survey spectra of the paper samples. The pristine paper surface generated two expected peaks, the binding energies are 286.5 eV for C 1s and 532.9 eV for O 1s. Compared with the spectrum of the pristine paper sample, additional two peaks of N 1s and Na 1s at 400.2 and 1072.1 eV appeared in the XPS spectra of paper samples modified with CS/SA multilayers. The nitrogen element was assigned to CS molecule, and the sodium element was from the SA molecule. Hence, the nitrogen and sodium could be regarded as the representative elements for characterization of CS/SA multilayers formation on paper surface. Insets in Fig. 1A show the high-resolution N 1s spectra and Na 1s spectra of the modified paper samples with different bilayers number, respectively. The peaks intensities of N 1s and Na 1s were correspondingly increased with the increasing bilayers number, indicating the successful assembly of CS/SA multilayers on paper surface. After the post-treatment with TCMS, new Si 2p peak at 103.1 eV was observed in the XPS spectrum, and the N 1s peak and Na 1s peak are not detected (Fig. 1A), suggesting that a TCMS layer was effectively developed on top of the CS/SA multilayers. 6

ATR-FTIR analysis was applied to provide further evidence for the LBL assembly of CS and SA on paper surface. The ATR-FTIR spectra obtained from the pristine and modified paper samples are shown in Fig. 1B. The spectrum of the pristine paper exhibited the characteristic absorption peaks of cellulose. It contained the O-H absorption peak at 3320 cm-1, C-H absorption peaks at 2910, 1369 and 1312cm-1, and a C-O-C absorption band from 950 to 1200 cm-1 (Chung, Lee, & Choe, 2004). With the increase of bilayers number, these characteristic peaks intensities were enhanced as a result of sugar unit structure of assembly polymers. Besides, an obvious signal at 1640 cm-1 was observed in the spectra of the modified paper samples, which was ascribed to the N-H bend vibration in CS molecules (Ma, Yang, Yao, Xu, & Tang, 2017). These results further revealed the growth of CS/SA multilayers on paper surface. After the immersion treatment with TCMS, it was observed that a new peak at 769 cm-1 assigned to a network Si-O-Si symmetric stretching vibration appeared (Shajesh, Smitha, Aravind, &Warrier, 2009), and characteristic absorption peaks intensities (O-H, C-H, and C-O-C) were attenuated because of the shielding action of the formed TCMS coating. The change in surface charge is important for the LBL assembly process, which can provide a qualitative indication as to whether multilayers are deposited (Abdullayev, Shchukin, & Lvov, 2008). Fig. 1C demonstrates the evolution of the surface charges of cellulose fibers during LBL deposition of CS and SA. As expected, the pristine cellulose fibers showed a negative potential of ‒130.9 mV. The first deposition of a positvely charged CS layers changed the surface potential with a value of ‒94.6 mV, and subsequent deposition of a negatively charged SA switched the surface potential to a value of ‒120.9 mV. Further polymers deposition caused the alternately and regularly changes in zeta potential, depending on whether the outermost layer was positively charged CS or negatively charged SA. It was worth mentioning that the phenomenon of zeta potential reversal did not appeared as observed in regular electrostatic LBL assembly (Zheng et al., 2006). A similar appearance was also reported by Bellanger et al. (2017), who demonstrated that the multilayers assembly could take place even without surface charge reversal, importance was a change in surface charge but not necessarily its reversal. One possible explanation of our findings is that deposition of CS/SA not only depended on electrostatic interaction but also on hydrogen bonds. Overall, the alternative change in zeta potential qualitatively indicated a successful deposition of each charged polysaccharide layers on fiber surface, and suggested the stepwise layer growth occuring during the LBL assembly process. For each layer deposited, the underneath layer could have a different molecular distribution and conformation, which may explain the slight variations of the measured zeta potential values for each layer (Xing, Eadula, & Lvov, 2007). SEM was employed to observe the surface morphologies of the pristine and modified paper samples, the images are shown in Fig. 2 (A‒E). It can be seen that the pristine paper showed a looser sheet structure. As the increase in bilayers number, the CS/SA multilayers appeared and resulted in more bridging and bonding between fibers. This observation was consistent with the results reported by Wu & Farnood (2014). 7

who studied a carboxymethyl cellulose/chitosan system on cellulose fibers surfaces. In addition, highmagnification SEM images showed no qualitative increase in fiber surface roughness at nano-scale, and the 3D-morphological features of fiber surfaces were further investigated by AFM analysis for more clear recognition, as illustrated in Fig. 2 (A′‒E′). The values of root-mean-square (RMS) roughness were obtained by the analysis of software version 5.12r3 (Veeco Co.) and are listed in Fig. 2F. The RMS roughness of a (CS/SA)1 multilayer modified fiber was lower than that of pristine fiber. This may be explained by the reason that the compact CS/SA multilayers covered on the fiber surface and filled the low-lying parts of fiber surface, thus resulted in decrease in RMS roughness. With more bilayers of CS/SA multilayers deposited on fiber surface, the RMS roughness increased. When a (CS/SA)5 multilayer modified fibers were further treated by TCMS, a large number of flocculent TCMS presented on the fiber surface, and resulted in more rough surface character (RMS roughness 68.52 nm), implying the formation of a TCMS layer with a low surface energy, which was crucial to generate a superhydrophobic surface.

3.2. Wettability of the superhyrophobic paper The pristine cellulose fibers have relative smooth surface and high surface energy, and thus have strong affinity with water, resulting in rapid absorption of water by paper. The CS/SA multilayers modified cellulose fibers still kept highly hydrophilic characters since the presence of abundant hydroxyl groups in building blocks. After the paper samples were immersed into TCMS solution, TCMS can react not only with hydroxyl groups on the surface of the fibers to give ether bridges through a condensation reaction but also with itself, leading to the formation of a polymethylsilsesquioxane coating (Li, Xie, Zhang, & Wang, 2008; Li, Zhang, & Wang, 2008). Briefly, the Cl atoms were substituted by hydroxyl (thus the silane is converted into a silanol) by reaction with tiny amounts of water existing either in the air or on the surfaces of the fibers. Some of those silanol groups reacted with hydroxyl groups on fiber surfaces, but some may remain unreacted at the end of the treatment (Gamelas, Salvador, Hidalgo, Ferreira, & Tejado, 2017). Thus, the hydrophobicity of both pristine and CS/SA multilayers modified paper samples were significantly improved (as described in Table 1) and superhydrophobic papers were obtained, which was ultimately due to two separate effects: (1) hydrophilic hydroxyl groups previously presented on fiber surfaces were no longer available; (2) the fiber surfaces were coated with a Si-O-Si network decorated with CH3 chain ends. Furthermore, it can be found from Table 1 that the WCA of the modified paper samples was strongly dependent on the number of CS/SA bilayers. When a (CS/SA)1 multilayer was deposited on the fiber surface followed with post-treatment with TCMS, the WCA is 155.3°, which was lower than that of pristine paper with a WCA of 156.6°. This is mainly because of the lower surface roughness of a (CS/SA)1 multilayer modified fiber as mentioned above. The WCA increased with a further increase in the bilayers number. It 8

was thought that these observation were quantitatively supported by results from AFM analysis and surface energy calculation. That is, on the one hand, the surface RMS roughness of fibers increased with the increasing bilayers number as analyzed by AFM. On the other hand, the calculated surface energies declined with the increase in bilayers number, which can be explained by the fact that more hydroxyl groups were available to react with TCMS, resulting in lower surface energy. 3.3. Tensile strength of the superhydrophobic paper The tensile strength measurements were performed for paper samples modified with CS/SA multilayers and TCMS-treated CS/SA multilayers to investigate the effect of LBL multilayers and post-treatment with TCMS on the physical strength properties of papers, and the results are showed in Fig. 3A. It can be observed that the CS/SA multilayers had positively effect on the paper physical strength since the tensile strength index continuously and significantly increased until the bilayers reached to 4. When a (CS/SA)4 multilayer was deposited on paper surface, the tensile strength index of modified paper increased by 39.2% than that of pristine paper. Nevertheless, the tensile strength index of a (CS/SA)5 multilayer modified paper exhibited an obvious decrease, which could be explained as a result of reduction of positive effect from CS/SA multilayers with the LBL proceeding. In addition, post-treatment with TCMS partly removed the positive effect from CS/SA multilayers and resulted in recession in paper physical strength, but the CS/SA multilayers modified paper samples still showed strength property higher or comparable to that of the pristine paper. There still was an increase by 6.8% in tensile strength for paper modified with a TCMStreated (CS/SA)4 multilayer in comparison with pristine paper. This result was consistent with the report by Gustafsson et al. (2012) who prepared superhydrophobic paper by combining the LBL technique with the adsorption of a colloidal paraffin wax onto the multilayer structure. In fact, a superhydrophobic paper with enhanced strength property has a great value for practical application in the field of packaging during storage and transportation. Therefore, integrate into account the wetting property and strength property, a (CS/SA)4 multilayer followed by TCMS treatment is well-suited to fabricate a superhydrophobic paper with improved strength property. It is well known that the paper strength mainly influences by individual fiber strength and bonding strength between fibers (Li, Fu, Peng, & Zhan, 2012). In order to further explore the underlying strengthening mechanism of CS/SA multilayers on paper strength property, zero-span tensile strength (an evaluation index for the average strength of individual fiber) and relative bonded area (RBA) between fibers (an evaluation index for the bonding strength between fibers) were measured according to our previous work (Peng, Meng, & Li, 2016), and the results are shown in Fig. 3B‒D. The zero-span tensile strength index consecutively increased with the growth of CS/SA multilayers due to the increase in both fiber wall thickness and hydrogen bonding between layers. The post-treatment with TCMS obviously decreased the zero-span tensile strength index of paper samples, the reason for explanation is that the hydrogen chloride 9

generated when the paper was treated with TCMS, and the acidic solution caused the degradation of the cellulose in paper, as reported by Fadeev & McCarthy (2000). (Fig. 3B). As shown in Fig. 3C, the RBA gradually increased until the 4 bilayer, then decreased when 5 bilayer deposited. This result was in good agreement with the SEM observation as discussed earlier, and this phenomenon can be explained by the reason that CS and SA formed gels-like substances which are flexible enough to reach adjacent fibers and bridge between them. With the increased bilayers number, gels effect from CS/SA increased, resulting in more bridging and bonding between fibers. A similar mechanism was suggested by Wu & Farnood (2014). However, when the bilayers number exceeded 4, the gels effect from CS/SA is attenuated or eliminated, thus resulting in 15.3% decrease in RBA between fibers, which was similar as reported by Peng et al. (2016). Moreover, it can be observed from Fig. 3D that post-treatment with TCMS caused reduction in RBA compared with that of CS/SA multilayers modified paper samples, which may due to the fact that gels effect from CS/SA deteriorated by TCMS since the consumption of hydroxyl groups, resulting in a reduction in flexibility of gels from CS/SA and hence weakening of the RBA between fibers. Overall, the change trends of RBA between fibers were consistent with those of tensile strength index as shown in above, demonstrating that the increase in paper strength can be predominantly attributed to the increased RBA between fibers. 3.4. Anti-bacterial adhesion effect Bacterial adhesion and biofilm formation on surfaces are significant issues in many industrial processes, causing a decrease in efficiency and thus an increase in operating costs. In particular, bacterial adhesion can cause serious health hazards when on food packaging materials and medical devices (Lorenzetti et al., 2015; Sezer, Sanko, Yuksekdag, Uzundağ, & Sezer, 2016). Superhydrophobic surfaces have been found to be able to reduce bacterial adhesion and have an easy removal capability of bacteria before a thick biofilm formed on the surface (Zhang, Wang, & Levänen, 2013). The bacterial adhesion assays were performed with E.coli and S. aureus on surfaces of pristine paper (PP) and as-prepared superhydrophobic paper (SHP) in this study (i.e. paper modified with a TCMS-treated (CS/SA)4 multilayer), respectively. Fig. 4A shows the numbers of colony forming units of E.coli and S. aureus after the paper samples contacted with bacterial suspension for 4 h. The results showed that the PP was seriously contaminated by E.coli or S. aureus, indicating that PP had high bacterial adhesion. However, improvement in hydrophobicity of paper samples very effectively reduced bacterial adhesion on the paper surface. Compared with PP sample, the SHP sample showed a 98.3% reduction in E.coli adhesion and 98.9% reduction in S. aureus adhesion after 24 h culture, indicating low bacterial adhesion property of SHP sample. 3.5. Self-cleaning property The self-cleaning property, one of the most attractive properties of superhydrophobic surfaces, is an effective tool to get rid of contamination from solid surfaces in the presence of water droplets. The self10

cleaning property of the as-prepared SHP was examined by using methylene blue dye as contaminant, and the self-cleaning test process is illustrated in Fig. 4B. The tested paper samples were initially covered with dye power (Fig. 4B1 and 4B2). When the water droplets were dropped onto the SHP surface, the dye immediately absorbed on the surface of water droplets due to the low adhesion force of the contaminants to the paper surface, the water droplets rolled easily and almost maintained spherical shapes. In comparison, PP surface was completely wetted by water droplet and contaminated by dye (Fig. 4B3). After the SHP was inclined 10°, the water droplets taken away the dye contaminants completely and left a clean paper surface (Fig. 4B4), which indicated that the obtained superhydrophobic paper in this work would be potential for self-cleaning application, such as self-cleaning wallpaper, packaging carton. 3.6. Water repellency property and application in our daily life Water repellency property is significant for paper in practical application. To further explore the water repellency of the as-prepared superhydrophobic paper in this work and to clarify its potential application in our daily life, the immersing tests were represented in this study. As shown in Fig. 4C1, a silver mirror phenomenon can be seen at a certain angle of view when the SHP was immersed into the water, which was a signature of the trapped air between the interface of liquid and solid (Wen, Guo, Yang, & Guo, 2017). Owing to the property of air trapping in the grooves of SHP surface to insulate from liquid phase, the SHP exhibited great water repellency function. Moreover, the SHP samples were immersed into three non-alcoholic beverage (tea, coffee, and cola) for 60 s, respectively. The PP also was used as comparison. The immersion processes and images were recorded by digital camera (See Fig. 4C2). We can see that the PP samples were wetted obviously and exhibited the color of the beverage after immersion, whileas the SHP samples still maintained nearly clear as before immersion. In addition, the optical top-view image of different liquid droplets (tea, cola, coffee, milk, water, soy sauce, and vinegar) on SHP sample surface is also shown in Fig. 4C3. It can be observed that these droplets exhibited typical spherical shapes on the SHP surface, indicating that superhydrophobic paper prepared in this study have great potential application in the field of liquid packaging, such as paper cups, beverage container, liquid flavoring packaging, etc.

3.7. Durability Although plenty of studies have been carried out for development methods to create superhydrophobic surfaces, challenges still remain in applying them in practical applications. They generally suffer from weak durability due to the mechanically fragile micro/nano structure on the surface and the fast degradation of surface chemistry (Zhang, Wang, & Levänen, 2013). In this work, the durability of the fabricated SHP

11

samples were investigated from three following aspects: mechanical stability, chemical stability and longterm stability. The poor mechanical stability of superhydrophobic materials severely hindered their large-scale practical applications (Su & Yao, 2014). Compared with superhydrophobic surface that fabricated on rigid substrate (e.g., metal, glass, wafer, etc.), the superhydrophobic coatings on paper surfaces mainly suffer the bending, folding or deformation, rather than the abrasion (Wang et al., 2016). Fig. 5A shows the changes in WCA and tensile strength index of SHP samples during the multiple bending cycles. After being bent 100 cycles, the paper sample surface still showed excellent superhydrophobic character with a WCA of 155.7°, and the tensile strength index of paper sample decreased by 29.4% compared with unbent sample, suggesting good mechanical stability of fabricated SHP in our work. The Fig. 5B shows the WCA and tensile strength index of the fabricated SHP as the function of pH value. The WCA were still larger than 155° for not only in water but also in corrosive liquids, such as acidic and basic aqueous solutions. The tensile strength index decreased obviously under extreme pH conditions, but still was comparable to that of PP. These results indicated the chemical resistance of fabricated SHP samples and the potential application of SHP in harsh environmental conditions. The long-term stability in humid conditions is of great significance for the storage of superhydrophobic paper products. We tested the long-term stability of SHP samples under a 60% RH condition at room temperature. The WCA and tensile strength index of tested samples during storage process are showed in Fig. 5C. It can be observed that the WCA stayed above 155° throughout the entire testing process despite observing a slight declining tendency, and tensile strength index decreased slowly, demonstrating a remarkable storage stability of fabricated SHP samples. 3.8. Moisture resistance property In industrial application, superhydrophobic paper should not only be durable, but also adapt to humid environments. To broaden the future application of our obtained superhydrophobic paper, moisture resistance property of SHP was investigated under different humidity conditions at room temperature. The relative moisture content and relative tensile strength of tested paper samples with varying relative humidity are demonstrated in Fig. 6. For the PP samples, the relative moisture content obviously increased and the relative tensile strength easily reduced when exposed relative humidity from 25% to 95% RH because of the hygroscopic cellulose and the porous fiber network. In comparison, the TCMS-treated LBL multilayers formed on the fiber surface protected the hydrogen bonding between neighboring fibers from moisture penetration, thus resulted in that the relative moisture content gently risen and the relative tensile strength largely maintains its original level, showing remarkable moisture resistance ability. The prepared SHP in this study possessing excellent moisture resistance property will have potential application in the area of moisture proof packaging, such as moisture-resistance container for food to prolong shelf life and maintain good food quality. 12

4. Conclusions A facile method was proposed to fabricate fluorine-free superhydrophobic paper with improved physical strength, which is layer-by-layer assembly of CS and SA followed by silanization with TCMS. With the LBL deposition of CS/SA multilayers on paper surface, two characteristic element (i.e. N and Na) contents and characteristic peaks intensities increased, the zeta potential alternately and regularly changed, the paper surface gradually covered by CS/SA multilayers and resulted in more bonding between fibers and more surface RMS roughness. The post-treatment with TCMS significantly decreased the surface energy of modified paper, and the paper made from a TCMS-treated (CS/SA)4 multilayers modified fibers can obtain a superhydrophobic surface with a WCA of 161.7°. However, the post-treament with TCMS imparied the paper physical properties compared with those of CS/SA mulitilayer modified paper, there was still a 6.8% increase in tensile strength for TCMS-treated paper made from a (CS/SA)4 multilayer modified fibers compared with that for pristine paper. As a result, the as-prepared superhydrophobic paper not only exhibited a outstanding anti-bacterial adhesion ability, self-cleaning property as well as extreme water repellency, but also presented an enhanced durability against deformation, chemical and time. Moreover, the as-prepared superhydrophobic paper can remain high tensile strength under high relative humidity condtions, showing excellent mositure resistance property. In summary, the superhydrophobic paper prepared by a facile fabrication method in our work may have promising industrial applications in different fields, such as food packaging, microfluidic system and paper industry. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 21566020, No. 21766014), the Applied Basic Research Program of Yunnan Province (No. 2014FD008), and the Talent Training Program of Yunnan Province (No. KKSY201305002). Reference Abdullayev, E., Shchukin, D., & Lvov, Y. M. (2008). Halloysite clay nanotubes as a reservoir for corrosion inhibitors and template for layer-by-layer encapsulation. Polymeric Materials: Science and Engineering, 99: 331‒332. Agarwal, M., Lvov, Y., & Varahramyan, K. (2006). Conductive wood microfibers for smart paper through layer-by-layer nanocoating. Nanotechnology, 17(21): 5319‒5325. Alongi, J., Carosio, F., & Malucelli, G. (2014). Current emerging techniques to impart flame retardancy to fabrics: An overview. Polymer Degradation and Stability, 106: 138‒149. Alongi, J., Han, Z. D., & Bourbigot, S. (2015). Intumescence: Tradition versus novelty. A comprehensive review. Progress in Polymer Science, 51: 28‒73. 13

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19

(A)

(B)

399

Si 2p

402

405

1065

Na 1s

0-bilayer 5-bilayer 200

1075

C 1s N 1s

0

1070

Absorbance(a.u)

O 1s 396

400

600

0-bilayer 1-bilayer 3-bilayer 5-bilayer TCMS-5-bilayer

1369

1312

1640

769 3320

2910

1800

1600

1400

1200

1-bilayer 3-bilayer TCMS-5-bilayer 800

1000

1200

3600

1400

3000

2400

1800

1200

600

Wavenumbers (cm-1)

Binding Energy (eV)

(C) Zeta Potential (mV)

-80

-100

-120

-140 0

2

4

6

8

10

Number of Layers

Fig. 1. (A) X-ray photoelectron spectroscopy (XPS) spectra of pristine and modified paper samples, insets showed high-resolution XPS spectra of N 1s and Na 1s region, respectively. (B) ATR-FTIR spectra for pristine and modified paper samples. (C) Zeta potential of pristine and modified cellulose fiber as a function of layers number.

(A′)

20

(B′)

(C′)

(D′)

(E′)

21

(F)

Fig. 2. SEM and 3D-AFM morphological images for different paper samples: (A, A′) pristine paper. (B, B′)‒(D, D′) paper modified with a (CS/SA)1, (CS/SA)3 and (CS/SA)5 multilayer, respectively. (E, E′) paper modified with a TCMS-treated (CS/SA)5 multilayer. (F) RMS rougheness for different fiber samples.

(B)140 CS/SA multilayers TCMS-treated CS/SA multilayers

16

CS/SA multilayers TCMS-treated CS/SA multilayers

120

Zero-span Tensile Strength Index (Nm/g)

Tensile Strength Index (Nm/g)

(A)18 14 12 10 8 6

100 80 60 40 20

4 0

1

2

3

4

5

0

1

2

Number of Bilayers

3

4

5

Number of Bilayers

(C)

(D)

0.36 0.24

0.24

26 24

0.20 28

Tensile Strength Index (Nm/g)

0.28

RBA (%)

Tensile Strength Index (Nm/g)

RBA (%)

0.32

26

0.16

24

22 10

12

0.12

142

Light Scattering Coefficient (m /kg)

0

1

2

3

4

6

8

10

Light Scattering Coefficient (m /kg)

5

0

Number of Bilayers

1

2

3

4

2

12

5

Number of Bilayers

Fig. 3. (A) Tensile strength index as a function of bilayers number for different paper samples. (B) Zerospan tensile strength index as a function of bilayers number for different paper samples. (C) The RBA as a 22

function of bilayers number for CS/SA multilayers modified paper. (D) The RBA as a function of bilayers number for TCMS-treated CS/SA multilayers modified paper.

(A)200.0k Colony Forming Units of Bacteria

o

0

E.coli S.aureus

160.0k

120.0k

80.0k 161.7 40.0k

0.0

PP

SHP

(B) (1)

(2)

(4)

(3)

23

o

(C) (2) (1)

(3)

Fig. 4. (A) Bacteria culture on pristine paper (PP) and as-prepared superhydrophobic paper (SHP) samples. (B) Digital images of self-cleaning process of SHP samples. (C) silver mirror phenomenon of SHP sample immersed into the water (1); photographs of PP and SHP samples before and after immersion into the tea, cola and coffee for 60 s (2); optical top-view image of different liquid droplets on SHP sample surface (3).

(B)

O

18 140 12 130

Tensile strength index

6

WCA

16 160

12

155

150 8

Tensile strength index

WCA

120 0

20

40

60

80

145

100

2

Bending Cycle

4

6

8

pH

24

10

12

O

150

165

WCA ( )

160 24

WCA ( ) Tensile Strength Index (Nm/g)

Tensile Strength Index (Nm/g)

(A)

Tensile Strength Index (Nm/g)

(C) 18

165

16 160

O

WCA ( )

14 155

12 10

150

8

WCA

Tensile strength index

145 0

4

8

12

16

20

Storage Time (Day)

Fig. 5. (A) The tensile strength index and WCA of SHP samples as a function of bending cycles, inset illustrated the bending test for the SHP samples. (B) The tensile strength index and WCA of SHP samples as a function of pH value. (C) The tensile strength index and WCA of SHP samples as a function of storage time.

(A)

(B) 100

PP SHP

Relative Tensile Strength (%)

Relative Moisture Content (%)

1000 800 600 400 200 0

80

60

PP SHP

40

20 20

30

40

50

60

70

80

90

100

20

Relative Humidity (%)

30

40

50

60

70

80

90

100

Relative Humidity (%)

Fig. 6. Relative moisture content (A) and relative tensile strength (B) vs. relative humidity for different paper samples.

25

Scheme 1 Schematic representation of the preparation process of superhydrophobic paper.

Table 1 Contact angle and surface energy components for different paper samples. Water

Diiodomethane

Contact

Contact

Angle (°)

Angle( °)

(mJ/m2)

(mJ/m2)

(mJ/m2)

0

156.6±0.3

112.9±0.2

6.21±0.08

1.47±0.01

7.68±0.08

1

155.3±0.2

111.0±0.3

6.81±0.10

1.54±0.01

8.35±0.10

2

157.2±0.2

113.9±0.2

5.90±0.06

1.42±0.01

7.31±0.06

3

160.4±0.1

116.1±0.3

5.32±0.07

1.47±0.01

6.79±0.09

4

161.7±0.2

116.8±0.2

5.15±0.06

1.50±0.01

6.65±0.07

5

164.1±0.3

117.5±0.2

5.01±0.05

1.61±0.01

6.63±0.05

Number of Bilayers

Dispersion

Polar

Component Component

26

Surface Energy