One-Way Water Transport Fabrics Based on Roughness Gradient

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Surfaces, Interfaces, and Applications

One-way Water Transport Fabrics based on Roughness Gradient Structure with No Low Surface Energy Substances Hongjie Wang, Wenyu Wang, He Wang, Xin Jin, Jialu Li, and Zhengtao Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08277 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

One-way Water Transport Fabrics based on Roughness Gradient Structure with No Low Surface Energy Substances

Hongjie Wang1, Wenyu Wang1*, He Wang1, Xin Jin2*, Jialu Li1, Zhengtao Zhu1,3

1

School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China.

2

School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387,

China.

3

Department of Chemistry and Applied Biological Sciences, South Dakota School of Mines and

Technology, Rapid City, SD 57701, USA.

* Corresponding authors: [email protected]; [email protected]

Keywords: one-way water transport, fabric, electrospray, roughness gradient

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Despite great recent progress in one-way water transport (OWT) fabrics, the development of these fabrics based on roughness gradient without low surface energy materials has yet to be achieved. In this work, we prepared OWT fabrics using five polymers with hydrophobic or hydrophilic groups by constructing a roughness gradient structure along the fabric thickness. Electrospraying was used to deposit a rough layer on fabric’s single side. The surface energy gradient change across the fabric thickness derived from roughness gradient structure played a major part in determining the OWT performance. With the roughness gradient structure, even polymers with hydrophilic groups, such as PAN and PA6, could become OWT fabrics. Besides, the layer deposited on the surface of the fabric showed no effects on the air permeability of the fabric. These novel results provided an opportunity for more polymers, especially for hydrophilic polymers, to be used to prepare OWT fabrics by designing a roughness gradient along the thickness of the fabric. The method would be applied in designing of OWT fabrics with high performance.

1. Introduction OWT fabrics have become a topic research in recent years, which have potential application for designing sportswear, summer clothing, medical and defense fabrics with high performance. For OWT fabrics, water can transport spontaneously from one side to the other side of the fabric, but it can’t transport in the opposite direction.1 OWT property directed by the wettability or structural difference of the surface was observed in nature on insects and plants.2-5 One example is the water capture capability of the desert beetle, which mainly relies on the alternating hydrophobic and hydrophilic regions which were coated and non-coated by wax on its bumpy surface.2 The alternating hydrophobicity and hydrophilicity forms the surface energy gradient change (denoted as SEGC) on the surface, contributing to the water capture capability.2 Therefore, this hydrophobicity/hydrophilicity gradient-generated-SEGC may help to endow the function of OWT to the fabrics. Inspired by the OWT mechanism of the desert beetle, researchers 2


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prepared OWT fabrics by hydrophobicity modification on the surface of the fabric to create the SEGC generated from the wettability gradient along the thickness of the fabric. For example, polymers with low surface energy (e.g., polymers containing fluorine or silicon or nanoparticles) were used to prepare hydrophobic surface on hydrophilic fabrics to generate SEGC across the fabric thickness.

Tian

et

al

prepared

OWT

fabric

by

depositing

1H,1H,2H,2H-perfluorooctyltrichlorosilane on one side of the fabric through chemical vapor deposition.6 Sun et al prepared OWT fabrics through single side of plasma treatment including a final hexamethyldisiloxane polymerization.7 Kong et al prepared OWT fabric using TiO2 nanosol through a dip-pad-dry process followed by one-side UV irradiation to increase the hydrophilicity of the fabric.8 Liu et al prepared fabrics with water and oil one-way transport property by electrospraying of the solution consisted of fluoroalkyl silane, silica nanoparticles, and heptadecafluorononanoic on one side of the fabric.9 Our group also prepared OWT fabrics through hydrophobic treatment followed by one-side UV irradiation or electrospraying a hydrophobic layer on a hydrophilic surface.10-12 At present, most OWT fabrics are prepared by using low surface energy substances containing fluorine or silicon to construct hydrophobicity/hydrophilicity gradient-generated-SEGC,6, 9, 10, 12-16 but the safety of fluorine and silicon may limit the application of the OWT fabrics for clothing and medical care. In a different approach, some plants or insects in nature, such as wet spider silk,3 cactus,4 and Nepenthes alata,5 form the SEGC structures with OWT properties not using the low surface energy substances but using the roughness gradient across the surface. As a successful example, the wet spider silk, in which the roughness gradient structure builds SEGC on the surface without fluorine or silicon contained substances, has the ability of OWT. Bai et al prepared artificial spider silk with directional water collection ability by immersing a nylon fiber in polymer solutions such as PMMA, PVAc, PS, and PVDF solutions and drawing it out horizontally to form rough knots.17 The prepared fiber was with rough knot and possessed directional water collection ability, which was attributed to the SEGC formed only by designing a structure of roughness gradient. 3



Inspired by the relationship of the roughness gradient structure-generated-SEGC and the wet spider silk’s water collection capacity, we suggest that fabrics with the roughness gradient structure-generated-SEGC containing no low surface energy substance may have excellent OWT performance. In this work, five polymers containing no fluorine and silicon, i.e. three hydrophobic polymers (Polyethylene terephthalate, Ethyl cellulose and Polysulfone) and two hydrophilic polymers (Polyamide 6 and Polyacrylonitrile), were chosen to design a rough layer on fabric’s single side. The OWT fabric was prepared by using the five electrospray precursors, and the water transport process of the five electrosprayed fabrics was studied. Electrospray technique was used to construct a rough layer on fabric’s single side, which was composed of micro-to-nano particles and/or beaded nanofibers. It was found that with the similar morphology, all five electrosprayed fabrics showed OWT property. This unexpected OWT ability could be mainly attributed to the electrosprayed rough layer on fabric’s single side, which formed a roughness gradient across the thickness of the fabric. The roughness gradient structure-generated-SEGC played an important part in affacting the OWT ability. Besides, the electrosprayed layer showed almost no effects on the air permeability of the electrosprayed fabric. The results suggested that OWT fabrics could be prepared by depositing a nanoscale rough layer on one side of the hydrophilic fabric by using polymers not necessarily with low surface energy, even using hydrophilic polymers. We demonstrated that it was possible to design fabrics with property of OWT based on the mechanism of roughness gradient structure-generated-SEGC along the fabric thickness and this design could be realized using no low surface energy substances, even using hydrophilic polymers. The novel results can be used to design various OWT fabrics applied in clothing, medical care and defense. The results of the directional water transport may also be used for designing filter materials with high efficiency and low resistance, enhancing oil-water separation efficiency, and improving the efficiency of solar thermal-driven desalination.

2. Experimental Methods 4


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2.1 Materials The cotton fabric (240 µm, 104 g m-2) was kindly supplied by the fabric market. N,N-Dimethylacetamide (DMAc), Rhodamine B, and Dimethyl sulfoxide (DMSO) were provided by Tianjin Kemiou Chemical Regent Co., Ltd. Formic acid (FA) and N, N-dimethylformamide (DMF)

were

from

Tianjin

Fengchuan

Chemical

Reagent

Technology

Co.,

Ltd.

1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was kindly provide by Jinan Hui Feng Da Chemical Co., Ltd. Ethyl cellulose (EC) was from Aladdin. Polyacrylonitrile (PAN) was from Spectrum. Polyethylene terephthalate (PET) was form Tianjin Guangfu Fine Chemical Research Institute. Polyamide 6 (PA6) was from Ube Industries, Ltd. Polysulfone (PSF) was kindly provided by Solvay. All the chemical regents were used without further treatment. 2.2 Preparation of solutions 4% Rhodamine B was prepared by adding Rhodamine B into the DMF to form the indicator solution. 10 wt% EC/DMF solution, 0.5 wt% PAN/DMF solution and 5 wt% PSF/DMSO solution were prepared by stirring for 12h at the temperature of 50 °C. 0.3 wt% PET/HFIP solution and 3 wt% PA6/FA solutions were stirred for 12 h at ambient temperature. The prepared Rhodamine B solution was mixed with electrospun precursor to form a homogenous solution. The above homogenous solutions were prepared for electrospraying. 2.3 Single-side electrospraying The plastic syringe with electrospun solution was mounted on an electrospinning set up. The parameters such as rotation speed of the collector, the flow rate, the spinning distance and the voltage were shown in Table 1. The cotton fabric was fixed onto the rotating drum covered by an aluminum foil. After electrospraying, the functional electrosprayed layer was formed on fabric’s single side. The prepared fabric was put in the oven for one hour at 100 °C. 2.4 Preparation of cast film

5



The solutions used for electrospraying were also used to prepare cast films. 3 g solution was poured into a pan made of aluminum foil and stored in ambient environment for 48 h to evaporate the solvent. 2.5 Characterizations Electron microscopic images were observed through the scanning electron microscope (S4800, Hitachi). The surface chemical composition of fabrics was tested by XPS (K-alpha, Thermofisher). The characteristic groups of fabrics were measured by FTIR (iS50, Nicolet). The thickness of the electrosprayed rough layer was characterized through confocal microscope (TCS SP5, Leica). Surface roughness was tested by the True Color Confocal Microscope (Axio CSM 700, Zeiss). Water contact angle (WCA) of the samples was measured through the contact angle tester (Kruss, DSA225). The process of the water transport was recorded through a camera. The breakthrough pressure was tested through a device including a pressure sensor. The flow rate of the deionized water was 20 ml h-1, and the pressure when water started to transport through the fabric was the critical breakthrough pressure. Air permeability of the fabrics was tested by the air permeability tester (Textest, FX 3300).

3. Results and Discussion 3.1 Preparation and morphology The one-side electrospraying procedure and the SEM images of control and electrosprayed fabrics before and after the electrospraying treatment is shown in Figure 1a. The control fabric (i. e., the cotton fabric without the electosprayed layer) had smooth surface. After PET was electrosprayed on fabric’s single side (denoted as E-side), the surface of E-side became rough with a layer of micro-to-nano particles connected through nanofibers. Similar morphologies were observed in the fabrics electrosprayed with PSF, EC, PA6, and PAN (Figure S1). The rough PET deposition formed a rough and porous layer on the E-side, whereas the other side of the fabric without electrospraying (denoted as U-side) remained smooth. Thus, a roughness gradient structure 6


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was formed across the fabric thickness. Figure 1b shows the schematic of the OWT property of the fabrics. When dropped on the U-side, water droplet could permeate into the relatively smooth and hydrophilic cotton matrix while was stuck by the electrosprayed rough layer. However, when dropped on the E-side, water droplet could pass through the electrosprayed rough layer and penetrate into the cotton fabric. Such an OWT ability could be mainly attributed to the roughness gradient structure-generated-SEGC derived from the roughness gradient generated by single side electrospraying on a smooth fabric, as discussed later.

3.2 Water transport process The images of dropping water on the control and electrosprayed fabrics and the changes of the water contact angle (denoted as WCA) with time were shown in Figure 2a. As Figure 2a shows, when 10 µl water was dropped on control fabric, it quickly spread and permeate into the hydrophilic cotton matrix. WCA changes of the control sample over time were shown in Figure 2b. WCA changed from 85°to 0°within 1 s. However, water droplet showed different behaviors when it was dropped on both E-side and U-side of the PET electrosprayed fabrics. As Figure 2c shows, when dropped on the E-side, water droplet can transport through the electrosprayed rough layer and penetrate into the U-side as shown schematically in Figure 1b. The whole water droplet transfer process took about 2.3 s. On the contrary, when water was on the U-side, it just spread on hydrophilic side and can’t permeate into the E-side; and the process was similar to the control fabric, which took 2.5 s to permeate into the fabric. The WCA changes of dropping water on both sides were shown in Figure 2d. On the E-side, the WCA changed from 101 ° to 0° within 2.3 s, while the on U-side WCA changed from 91° to 0° within 2.5 s. The similar trend of WCAs on both sides can be attributed to the different water transport process. When dropped on the E-side, water can penetrate through the electrosprayed layer followed by transporting into the unelectrosprayed side, while on the U-side, it spread directly into the unelectrosprayed cotton fabric which was hydrophilic. Still frames from fabrics electrosprayed with PSF, EC, PA6, and PAN showed the same OWT 7



property and the WCA changes underwent a similar trend (as shown in Figure S2). The transportation time for 10 µl water needed to penetrate through the fabrics from the E-side to the U-side was different among the five electrosprayed OWT fabrics, as shown in Table S1. For PET, EC, and PA6 electrosprayed OWT fabrics, water can penetrate through the fabrics within 3 s. While for PSF and PAN electrosprayed OWT fabrics, it took over 4 s for water to penetrate through the fabrics, which could be ascribed to the different wettability and initial water contact angle. Among the five polymers, PSF was the most hydrophobic, and the cooperation of the hydrophobicity and roughness resulted in longer time for water to penetrate through the fabric. PAN was hydrophilicity polymer, although the roughness gradient structure-generated-SEGC could drive water transporting from E-side to U-side, the hydrophilicity of PAN slowed the transportation speed, resulting in the longer transportation time. OWT property was observed in the five electrosprayd fabrics; even for fabrics electrosprayed with hydrophilic PA6 with -NH- in the main chain and PAN with -CN as the side group, the electrosprayed fabrics still possessed OWT property. This unique feature could be mainly attributed to the SEGC arising from the roughness gradient across the fabric thickness. The water remining of the electrosprayed fabric after dropping water on both sides of the fabric was also tested. As shown in Figure 3a, when water was on PET electrosprayed side, it can transport from the E-side to the U-side spontaneously. After the process, there was no water residual when the E-side was covered with dry paper. However, as shown in Figure 3b, when dropped on the U-side, it only spread and penetrate the cotton fabrics and can’t transport through the E-side, which made paper became blue when the dry paper was covered on it. Similar phenomena can be observed in PSF and EC electrosprayed fabrics (see Figure S3b-c). However, water residual existed in both sides of fabrics electrosprayed by PA6 and PAN, as shown in Figure S3d-e. It can be attributed to the hydrophilicity of PA6 and PAN, which made the electrosprayed layer with the water retention ability and water can’t totally penetrate through the fabric. Besides, the critical breakthrough pressure for water to penetrate from one side to the other side of the fabric was also measured by a purpose-built device. As shown in Figure S4, when water was 8


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fed from the E-side, the pressure was negative because water can transport through the fabric from the E-side spontaneously; while when water was fed from the U-side, the pressure was positive because extra energy was needed for water to penetrate through the electrosprayed layer. For PSF, EC, and PET electrosprayed fabrics, water needed higher pressure to transport through the fabrics. While for PAN and PA6 electrosprayed fabric, lower pressure was needed because of the hydrophilicity of the polymers, which possessed a certain water retention ability and made water need lower pressure to pass through.

3.3 Chemical analysis Figure 4a shows the FTIR spectra of cotton fabrics before and after electrosprayed with PET. The peak at 3338 cm-1-3273 cm-1 was contributed by the vibration of hydroxyl group.18 The intensity of this mode in the spectrum of the E-side became lower than the control fabric because of the electrosprayed PET layer. The peak at 2895 cm-1 came from the symmetrical stretching vibration of -CH2 from the cotton fabric.19 Several new vibrational modes occurred in the cotton fabric electrospray coated with PET. The vibration peak at 1715 cm-1 was from the -C=O bond from the chemical structure of PET. The sharp peak at 1408 cm-1 was ascribed to the angular deformation of C-O-H from the carboxylic groups.20 The peaks at 1240 cm-1 and 1093 cm-1 originated from the vibration of -C-O group. The peak at 1016 cm-1 indicated an aromatic substitution pattern.21 The peak at 725 cm-1 was related to the bending of -C-H .22 The FTIR spectrum of the U-side was similar to that of the control fabric. The results indicated that the rough electrosprayed layer existed only on the E-side. Figure 4b shows the XPS spectrum of the control fabric and the E-side of PET electrosprayed cotton fabric. After electrosprayed with PET, the content of the C element on the E-side increased from 64.64% to 69.31%, while the content of the O element decreased from 31.36% to 28.68%. Chemical analysis results of the cotton fabrics electrosprayed with PSF, EC, PA6, and PAN are shown in Figure S5-S8. The strong peaks of PSF shown in Figure S5a were at 1585 cm-1 and 1486 cm-1 [the vibration of C=C from aromatic rings],23 1240 cm-1 [the vibration of 9



Ф-O-Ф],24 and 834 cm-1 [the vibration of -CH].24 Except for the peaks of O1s and C1s, the peak of S1s from -S=O group was observed in XPS survey spectrum (see Figure S5b). The characteristic peaks of EC coated sample (Figure S6a) were almost the same as that of the control fabric except for the peak at 2976 cm-1, which represented the ethoxyl group of EC.25 The FTIR spectrum of the cotton fabric electrosprayed with PA6 was shown in Figure S7a. Compared with the spectrum of the control fabric, three new peaks of PA6 at 2862 cm-1, 1639 cm-1, and 1542 cm-1 can be seen on the E-side, which came from the symmetric C-H stretching vibrations in the methyl groups, the stretching of C=O and the bending of N-H from amide, respectively.26, 27 As shown in Figure S8a, three new peaks at 2242 cm-1, 1731 cm-1, and 1666 cm-1 were presented in the FTIR spectrum of the E-side of PAN coated cotton fabric, which came from the C≡N and the C=O stretching of methyl methacrylate, and the partially conjugated C=N structure, respectively.28, 29 The content of elements on the cotton fabric also changed before and after being electrosprayed. Among the five electrosprayed fabrics, the characteristic peaks of the polymers and the changes of element contents were only observed in the E-side, which confirmed that the rough electrosprayed layer only existed on the E-side and a roughness gradient structure-generated-SEGC was formed along the fabric thickness.

3.4 Roughness analysis The electrosprayed fabric is schematically illustrated in Figure 5a. A rough layer was formed on fabric’s single side. Besides, electrosprayed layer generated a thickness gradient along the fiber of the fabric. In order to distinguish the layer deposited by electrospraying and to present the roughness gradient vividly, Rhodamine B was mixed with the polymer solution as a fluorescence indicator. The confocal images electrosprayed and control fabrics are shown in Figure 5b. The light green area in the image of the electrosprayed fabric was the electrosprayed layer on the surface of the fabric, which was rough and consist of micro-to-nano particles connected by nanofiber (Figure 1a), while the unelectrosprayed area was dark because no fluorescence indicator existed. Small 10


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thickness gradient of the electrosprayed layer can be seen from the confocal image, which was coincident with the schematic in Figure 5a. The confocal images of the OWT fabrics electrosprayed with PSF, EC, PA6, and PAN, and the coating thickness of the five OWT fabrics are shown in Figure S9. Similar changes were observed in the confocal images (Figure 5b and Figure S9a-d). The thickness of the EC electrosprayed layer was 22.10 µm which was the largest among the electrosprayed layers due to its large particle sizes (as shown in Figure S1b). The thickness of the PAN electrosprayed layer was the smallest, which was 17.43µm. And the thicknesses of the PET, PSF, and PA6 layers were 18.46 µm, 18.56 µm, and 21.70 µm, respectively. The average thickness of the five electrosprayed layers was 19.67±2.12 µm. Further, the morphologies of the control fabric and the PET electrosprayed layer are shown in Figure 5c-d. The control fabric was smooth, whereas the electrosprayed layer was rough and consisted of micro-to-nano particles connected by ultrafine nanofibers. The morphology of the cast film (inset of Figure 5c) showed smooth surface, similar to that of the control fabric. Because of the roughness difference between the smooth unelectrosprayed fiber and the electrosprayed layer, a roughness gradient structure-generated-SEGC was formed across the electrosprayed fabric thickness. The morphologies of the PSF, EC, PA6, and PAN cast films and the electrosprayed layers are shown in Figure S10-S13. All samples possessed a roughness gradient across the fabric thickness. Similar smooth morphology of the cast film was observed for the other polymers as PET except for PA6. The morphology of the PA6 cast film was rough because the high volatility and the low boiling point of FA (100.8 ℃). FA could evaporate very rapidly, leaving limited time for the cast film to smoothen the surface before solidification. Because of the hydrophilicity of the control fabric and the OWT property of the electrosprayed fabric, it was difficult to characterize the WCA changes with the electrospraying of polymers. To explore the effect of roughness on the WCA changes of control fabric and electrosprayed fabric, the cast film and electrosprayed layer of the polymer were prepared on the aluminum foil. The comparison between the WCAs of the cast film and the electrosprayed layer on the aluminum 11



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substrate could demonstrate clearly the effect of the roughness of the electrosprayed layer on the WCA, which to some extent reflected the surface energy difference between the U-side and the E-side. Additionally, the roughness of the cast film and the electrosprayed layer were characterized, which could be considered as an estimation of the roughness difference between the E-side and the U-side. Contour arithmetic mean deviation (Ra) was used to present the surface roughness. Ra, characterized by the linear roughness in this paper, was calculated by the following equation:30

= |( )|

(1)

where y(x) is the value of the roughness and l is the test length. The linear roughness values of the cast film and the electrosprayed layer are shown in Figure 5e. The linear roughness of the PET cast film was 0.338 µm, while that of the electrosprayed layer was 0.610 µm. The insets show the outline of the linear roughness. As shown in the insets in Figure 5e, the y(x) curve of the electrosprayed layer changed a lot, while the y(x) curve of the cast film showed little change. The obvious contrast of the linear roughness in Figure 5e could be explained by the surface structural/morphology difference between the cast film and the electrosprayed layer, as shown in Figure 5c-5d. Accordingly, a roughness gradient structure was formed from the side with the electrosprayed layer and the unelectrosprayed side of the fabric, which resulted in SEGC across the fabric thickness. The SEGC created by this roughness gradient structure could be evaluated by the different WCAs when dropping water on the cast film and the electrosprayed layer (shown in Figure 5f). When water (5 µl) was dropped on the cast film, WCA varied from 85° to 77.5 ° with a D-value of 7.5 ° within 5 min; when 5 µl water was dropped on the electrosprayed layer, WCA varied from 122.5° to 118° with a D-value of 4.5 ° within 5 min. The PET electrosprayed layer had considerable higher WCA in comparison with the smooth cast PET film, demonstrating that the rough surface would increase the hydrophobicity of the E-side of the electrosprayed fabric and create surface energy gradient across the fabric. To further verify that it was the roughness gradient derived SEGC that mainly contributed to the OWT ability, both hydrophilic and hydrophobic polymers were used for electrospraying on the one 12


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side of the fabric. The linear roughness and WCA changes with time of the cast films and electrosprayed layers of PSF, EC, PA6, and PAN were studied (shown in Figure S10-S13). Table 2 listed the values of the linear roughness and WCA changes with time for the cast film and electrosprayed polymer layer on aluminum foil. For all polymers tested, the linear roughness of the cast film was lower than that of the electrosprayed layer, which was consistent with the SEM images with the smooth cast film and the rough electrosprayed layer shown in Figure S10-S13. The roughness difference between the cast film and the electrosprayed layer indicated that the roughness gradient structure-generated-SEGC formed across the electrosprayed fabric thickness. For the cast film, the linear roughness of PA6 was larger than that of the other polymers because of the rough surface of the PA6 film (Figure S 12a); for the electrosprayed layer, the EC electrosprayed layer showed the largest linear roughness due to the large size of micro-to-nano particles (Figure S1). Among the five polymers, PAN and PA6 showed the largest ∆WCA for both cast film and the electrosprayed layer, which could be ascribed to the hydrophilic/polar groups of -NH- and -CN on the molecular chains. When water droplet contacted with the hydrophilic groups, it could spread on the surface, resulting in smaller WCA. ∆WCA of PET, PSF, and EC showed little changes because of no hydrophilic groups in these polymer chains. It was interesting that for PAN and PA6, WCA

0 min

of the cast film was larger than that of the electrosprayed layer, whereas for

PET, PSF, and EC, the WCA 0 min of the cast film was smaller than that of the electrosprayed layer. The result could be attributed to the different chemical structures. The hydrophilic group -CN of PAN and -NH- of PA6 were easy to contact with water, and the electrosprayed layer possessed larger specific surface area than the cast film, which would make water droplet spread more easily on the electrosprayed surface, resulting in lower WCA 0 min compared with the WCA 0 min of the cast film. Using the electrospraying technique, a rough surface consisted of micro-to-nano particles or micro-to-nano particles connected by ultrafine nanofiber was obtained on the surface of fabric. The roughness gradient across the electrosprayed fabric thickness was formed, which contributed to the 13



WCA gradient across the electrosprayed fabric thickness. Besides, the WCA gradient along the fabric thickness created a surface energy gradient according to Wu’s methods and Young’s equation.31, 32 Therefore, the roughness gradient structure-generated-SEGC did form along the fabric thickness by the electrospraying process. When water was on the U-side, it could penetrate quickly into the fabrics due to the hydrophilicity of the unelectrosprayed cotton fabric and stopped by the electorsprayed layer. While when it was on the E-side, it can obtain the capillary force from the unelectrosprayed fabric to overcome the blocked force generated by the roughness gradient structure. When water transported to the interface of the rough layer and the smooth fabric, the driving force from the roughness gradient structure-generated-SEGC across the electrosprayed fabric thickness and the capillary force from the fabric helped water to transport through and spread into the hydrophilic fabric matrix.3, 4, 33-35 In this way, the electrosprayed fabric possessed the OWT ability. The difference of chemical composition and roughness on the E-side and the U-side of the electrosprayed fabric would both generate surface energy gradient across the fabric thickness. While the observed OWT property of the electrosprayed fabrics might be attributed to synergistic effects of chemical composition and roughness difference on the E-side and the U-side, the roughness gradient structure-generated SEGC would be considered as the key factor for the OWT property. The fabrics electrosprayed with five polymers with different chemical structures all showed similar OWT property, suggesting that the chemical composition played limited role in determining the OWT property. Additionally, the surface energy gradient from the chemical composition difference was expected to be relatively weak because of lack of low surface energy substances in these fabrics with electrosprayed polymer layer. Therefore, the surface chemistry difference would only play minor role in deciding the OWT property of the fabrics. For example, both EC and cellulose of cotton fabric had very similar chemical structures, which would result in the same surface chemistry on E-side and U-side (i.e. no surface chemistry/chemical composition

14


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gradient), yet the EC electrosprayed fabric showed the OWT property. These results demonstrated that the SEGC in these fabrics was largely generated from the roughness gradient structure.

3.5 Air permeability test Besides the OWT property, the air permeability of the electrosprayed fabrics was evaluated. An air permeability tester was used for the permeability test. Figure 6 shows the air permeability of the control fabric and the electrosprayed fabrics. The air permeability of the control fabric was 38.5 cm3 cm-2 s-1. And the air permeability of the electrosprayed fabrics (from 27.7 cm3 cm-2 s-1 to 38.0 cm3 cm-2 s-1) was slightly lower than that of the control fabric. Among the five electrosprayed fabrics, PAN electrosprayed fabric possessed the largest air permeability (38.0 cm3 cm-2 s-1), while the air permeability of EC electrosprayed fabric was the smallest (27.7 cm3 cm-2 s-1), which can be attributed to the particle size difference of the electrosprayed layer. As shown in Figure S1b and Figure S1d, the size of the electrosprayed EC particles was the largest and the EC particles filled some gap between single fibers, which made air a little harder to pass through. While the PAN particles were the smallest and only existed on the surface of the single fiber, which made air easier to pass through. The air permeability of PET, PSF and PA6 electrosprayed fabrics was 33.6, 31.8, and 30.3 cm3 cm-2 s-1, respectively. The air permeability test indicated that although with a slightly decrease, the electrospraying treatment didn’t affect the air permeability of the fabrics. Because of the advanced OWT ability and excellent air permeability, the prepared fabrics could be applied in various fields (e.g., sportswear).

4. Conclusion OWT fabrics has been prepared by constructing a rough layer consisted of micro-to-nano structure on one side of the fabric by electrospraying. We have demonstrated that the roughness gradient structure-generated-SEGC across the thickness of the electrosprayed fabric played an important part in the OWT property. Five polymers with hydrophobic or hydrophilic main chains or 15



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side groups were used as the electrospray precursors. With similar morphology of the electrosprayed layer (e.g. micro-to-nano particles or micro-to-nano particles connected by ultrafine nanofibers), all the electrosprayed fabrics showed an advanced OWT ability, even for those being electrosprayed with a layer of hydrophilic polymers. Additionally, the electrosprayed rough layer showed no effects on air permeability of the fabrics. These foundational studies indicated that polymers without low surface energy components/elements such as fluorine or silicon, and polymers with hydrophilic groups could be used to prepare OWT fabric, as long as a roughness gradient structure-generated-SEGC across the fabric thickness was designed. The method would broaden the materials/structures for designing of novel OWT fabrics with high performance for various applications.

Supporting Information SEM images cotton fabrics electrosprayed with polymers (Figure S1), Still frames to show dropping water on the electrosprayed fabrics (Figure S2), water remaining of the fabric (Figure S3), breakthrough pressure of the fabric (Figure S4), chemical analysis of fabrics (Figure S5-S8), the electrosprayed layer thickness of the OWT fabrics electrosprayed by the five polymers (Figure S9), FESEM images and WCA changes of the cast film and electrosprayed layer (Figure S10-S13), and transportation time for 10µl water to transport from E-side to U-side (Table S1).

Acknowledgements This work was supported by the Science and Technology Plans of Tianjin (15PTSYJC00230, 15PTSYJC00250

and

14TXGCCX00014),

the National Natural Science Foundation of China (51573136), and the Natural Science Foundation of Tianjin (grant numbers 15JCYBJC17800).

References 16


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Figure 1. (a) Schematic of the electrospraying treatment and the FESEM images of the cotton fabric before and after electrosprayed with a layer of PET; (b) illustration of water transport process when dropping water on the U-side or the E-side of the fabric electrosprayed with polymer.

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Figure 2. Still frames to show dropping water (15 µl) on (a) the control fabric, (c) the PET electrosprayed E-side and U-side; contact angle changes when water was dropped on (b) the control fabric and (d) the PET electrosprayed cotton fabric.

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Figure 3. Water remaining of PET electrosprayed fabric when dropped water on (a) E-side and (b) U-side.

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Figure 4. Chemical analysis of the cotton fabric before and after electrosprayed with PET: (a) FTIR spectra; (b) XPS survey spectra.

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Figure 5. (a) schematic of the electrosprayed fabric; (b) confocal images of the control fabric and the electrosprayed fabric; FESEM images of (c) the control fabric (the inset:PET cast film) and (d) the electrosprayed layer; (e) linear roughness of the PET electrosprayed layer and the PET cast film (Insets: the outlines of the linear roughness); (f) WCA changes of the PET electrosprayed layer and the PET cast film.

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Figure 6. Air permeability of (a) control fabric; and fabrics electrosprayed by (b) PET, (c) PSF, (d) EC, (e) PA6, (f) PAN.

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Table 1. Electrospraying parameters of the different solutions. Electrospraying solution

Spinning distance [cm]

Rotation speed [rpm]

Flow rate [ml/h]

Applied voltage [kV]

PET/HFIP

15

60

2.0

20

PSF/DMSO

15

60

1.0

20

EC/DMF

15

60

1.0

20

PAN/DMF

15

60

2.0

20

PA6/FA

15

60

0.008

28

27


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Table 2. Linear roughness and WCA changes of the polymers. Cast film Polymer

Electrosprayed layer

Linear roughness [µm]

WCA 0 min [°]

WCA 5 min [°]

∆WCA [°]

Linear roughness [µm]

WCA 0 min [°]

WCA 5 min [°]

∆WCA [°]

PET

0.338

85

77.5

7.5

0.610

122.5

118.5

4

PSF

0.275

92

89.5

2.5

0.631

123

121.5

1.5

EC

0.360

76.5

71.5

5

1.023

129.5

128

1.5

PA6

0.450

87.5

52.5

35

0.646

69

38.5

30.5

PAN

0.375

82.5

51.5

31

0.555

61.5

26

35.5

28


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TOC

29


One-Way Water Transport Fabrics Based on Roughness Gradient

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