Human Skin-Like, Robust Waterproof, and Highly Breathable Fibrous

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Surfaces, Interfaces, and Applications

Human Skin-Like, Robust Waterproof, and Highly Breathable Fibrous Membranes with Short Perfluorobutyl Chain for Eco-Friendly Protective Textiles Jing Zhao, Xianfeng Wang, Lifang Liu, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10408 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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ACS Applied Materials & Interfaces

Human Skin-Like, Robust Waterproof, and Highly Breathable Fibrous Membranes with Short Perfluorobutyl Chain for Eco-Friendly Protective Textiles Jing Zhao,†,§ Xianfeng Wang,*,†,§ Lifang Liu,*,†,§ Jianyong Yu,†,§ and Bin Ding†,§ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Textiles,

Donghua University, Shanghai 201620, China §

Innovation Center for Textile Science and Technology, Donghua University, Shanghai 200051, China

*E-mail: [email protected]; [email protected].

KEYWORDS: human skin-like, eco-friendly, waterproof and breathable, short perfluorobutyl chain, fibrous membranes

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

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ABSTRACT Flexible smart membranes with superior waterproofness and extreme breathability are highly desirable for wearable uses. However, present waterproof and breathable materials suffer from limited performance efficiency, alarming environmental risks, and complicated fabrication procedures. We report on eco-friendly fibrous membranes with human skin-like robust waterproof and highly breathable capabilities that can be prepared via a facile electrospinning strategy. A novel polyurethane elastomer (C4FPU) possessing double terminal short perfluorobutyl (-C4F9) chain is synthesized for the first time, and incorporated into polyurethane (PU) fibers matrix endowing the membrane with mighty and durable hydrophobicity. Additionally, the employment of AgNO3 greatly decreased the maximum pore size (dmax), contributing to the dramatically enhanced waterproofness. The resulting PU/C4FPU/AgNO3 fibrous membranes exhibit comprehensive properties of exceptional hydrostatic pressure (102.8 kPa), excellent water vapor transmission rate (12.9 kg m-2 d-1), high mechanical property (9.8 MPa), and significant antibacterial efficacy against E. coli and S. aureus. The successful synthesis of these intriguing membranes may provide a promising candidate for the new generation of key building blocks of the upscale protective garments.


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1. INTRODUCTION With the same function as human skin, waterproof and breathable membranes which can block the penetration of water droplets, whereas permit moisture vapor to transport, have been widely used in weather protective clothing, medical instrument, membrane distillation, and construction materials.1-3 The mainstream products of waterproof and breathable membranes mainly include two types: hydrophilic membranes and microporous membranes.4,5 With respect to hydrophilic nonporous membranes, they usually show excellent waterproofness but at a cost of decreased water vapor transmission (WVT) rate since they breathe via an absorption-diffusion-desorption process. In terms of hydrophobic microporous membranes, their pore size are smaller than the finest rain droplet but larger than the water vapor molecule, thus they can provide good comprehensive properties with equilibrium of waterproofness and breathability. It is obvious that water vapor transport through the pores is much faster than diffusion through a solid, therefore, the microporous membranes should display superior water vapor permeability and provide the wearer with high level of comfort.6 However, as far as we know, the representative polytetrafluoroethylene (PTFE) microporous membranes decompose slowly in the environment and have difficulty in recovering.7 Therefore, it is very imperative for us to fabricate human skin-like robust waterproof and highly breathable membranes with environmental friendliness through a facile and scalable approach. Until now, a variety of strategies including biaxial stretching,8 template-based methods,9 wet coagulation, and melt blown have been employed to produce microporous waterproof and breathable membranes. However, these methods are limited by raw materials, complicated in fabrication process, high cost, and most importantly difficult in regulating waterproofness and breathability simultaneously. Electrospinning,10-15 as a convenient and effective route to fabricate micro/nanofibers, has been developed rapidly in recent years. The electrospun fibers which are characterized by small pore size,



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high porosity, controllable porous structure, and ease of surface modification, have been regarded as a useful approach for preparing waterproof and breathable membranes. Until now, a wide variety of waterproof and breathable fibrous membranes have been studied, such as polyurethane (PU),16 polyacrylonitrile (PAN),17 polyvinylidene fluoride,18 polypropylene,19 and so on. Among these investigations, PU has attracted wide attention due to its excellent elasticity, wear-comfort, and easy care performance. However, for electrospun PU membranes, the intrinsic poor hydrophobicity as well as the large maximum pore size (dmax) brought about low hydrostatic pressure of 3.7 kPa (WVT rate was 9 kg m-2 d-1).20 With the purpose of improving hydrophobic property of PU fibers, Ge et al.21 introduced a synthesized polyurethane (FPU) containing perfluoroalkane segment (-C8F17) into pristine PU membranes, which greatly enhanced the water resistant property (39.3 kPa), along with medium moisture permeability (9.2 kg m-2 d-1). Subsequently, Zhang et al.22 decreased dmax of the fibrous membranes by introducing LiCl into PU/FPU solution, the resultant membranes displayed good waterproofness (82.1 kPa) and breathability (10.9 kg m-2 d-1). However, fluorinated polymers containing long perfluoroalkyl group (Rfn, n ≥ 8) produce perfluoroalkylsulfonate or perfluoroalkyl acid, such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) which are persistent in the environment, accumulate in human and animal tissues, and have long biological half-lives.23-25 Keeping these alarming hazards in view, many countries have taken certain regulatory measures which recommend the use of fluorochemicals with short perfluorinated carbon atoms (Rfn, n≤6) due to their less toxicity and lower bioaccumulative potentials.26,27 Thus, developing environmentally friendly fluorinated polymers to supersede traditional fluorides is the focus of material scientists. It is believed that compounds with shorter fluorocarbon chain would be significantly less toxic and may be expected to have a higher security.28 Presently, the commercially available and most advanced water repellents are waterborne fluorinated chemicals


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containing four perfluorinated atoms (Rfn, n=4), which can impart the fabrics or membranes with hydrophobicity through finishing process. However, the shortcomings related to the products are the inhomogeneity and poor durability of the hydrophobic coatings as well as the complex treatment process.29 Therefore, the elaboration of waterproof and breathable membranes with solvent-based fluorinated chemicals (Rfn, n=4) incorporated in fibers is of great significance but very challenging.

Figure 1. Chemical structure of synthesized C4FPU and the schematic representation of the preparation of human skin-like, eco-friendly waterproof and breathable membrane. Here, we present a robust methodology for fabricating robust waterproof, highly breathable and eco-friendly waterproof breathable membranes by incorporating synthesized novel polyurethane (C4FPU) elastomer with double terminal perfluorobutyl (-C4F9) chains and AgNO3 in fibers matrix via a facile electrospinning method (Figure 1). The fluorine atomic percentage on the surface and in the bulk of the fibrous membrane were determined by X-ray photoelectron spectroscopy (XPS) and element analysis, indicating that fluorinated segment migrated to the surface of the fiber. In addition, the influence of AgNO3 concentrations on porosity, dmax, pore size distribution as well as wettability of the fibrous membranes were systematically investigated. The good hydrophobicity, small dmax, high porosity and release of Ag ions endowed the fibrous membranes with robust waterproofness, excellent breathability, good tensile strength along with significant antibacterial property, which revealing good candidates in protective textiles.



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2. EXPERIMENTAL SECTION 2.1.

Materials.

PU

(Desmopan

9370AU)

was

bought

from

Covestro

Co.,

Ltd.

Poly(oxytetrmethylene)glycol (PTMG, Mn=1000), triethylene glycol (TEG), silver nitrate (AgNO3), 4,4'-diphenylmethane diisocyanate (MDI), dimethyl formamide (DMF), and N,N-dimethylacetamide (DMAc) were purchased from Aladdin Chemical Reagent Co., Ltd., China. 2-(perfluorobutyl) ethyl alchol (TEOH-4) was provided by Wuhan Savoil Chemicals Co. Ltd. Sodium chloride was supplied by Shanghai Lingfeng Chemical Reagents Co., Ltd., China. Yeast exact powder was bought from Oxoid Ltd., United Kingdom. Agar powder, peptone and sodium hydroxide were bought from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis of C4FPU. C4FPU with -C4F9 chain situated on the double terminal of the polymer chain was designed due to the facile movement of perfluoroalkane segment to the surface of the fibers during electrospinning, so as to achieve good hydrophobicity. It was synthesized via a stepwise polymerization reaction. 15 g anhydrous PTMEG was added into the four necked flask, and the stirring speed of the agitator arm was 200 rpm, at the temperature of 60 °C for 0.5 h. 25 g purified MDI together with 20 g DMF were weighed out and blended to form a transparent uniform solution, and then added dropwise into the container under nitrogen environment for 0.5 h. After reacted for 1 h, 3.5 g TEG as the chain extender was added into the aforesaid mixture, and the reaction was performed at 65 °C for 2 h. Finally, 18.5 g TEOH-4 and 10 g DMF was added into the flask, the polymerization was continued for another 2 h. The resulting product was purified using the previous method.30 The polymerization reaction process was illustrated in Figure S1, the Fourier transform infrared spectrum was displayed in Figure S2, nuclear magnetic resonance spectra of 1H, 19F and the chemical structure of synthetic C4FPU were presented in Figure S3. 2.3. Preparation of Fibrous Membranes. A series of PU/C4FPU solution containing different C4FPU


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concentrations (0, 1, 2, and 3 wt %) and 14 wt % PU was made by adding C4FPU and PU into DMAc solvent with stirring for 5 h. The prepared electrospun membranes were denominated as PU and PU/C4FPU-x (x stood for the C4FPU concentration). PU/C4FPU-2/AgNO3 solution containing various AgNO3 concentrations (0.005, 0.01, 0.015 and 0.02 wt %) was prepared by adding C4FPU and PU into AgNO3/DMAc solution with violent stirring for 5 h, and the concentrations for PU and C4FPU were kept

14

and

2

wt

%,

respectively.

The

obtained

membranes

were

denominated

as

PU/C4FPU-2/AgNO3-y (y was 1, 2, 3, and 4 with the increasing of AgNO3 concentration). The solution was fed into a syringe with a capillary tip (10 ml). The anode of the high voltage power supply was connected to the syringe needle tip, and the cathode was connected to the ground drum. The electrospinning experiment was conducted using a DXES-N spinning equipment (SOF Nanotechnology Co., China), the impressed voltage was 45 kV, with constant infusion rate of 2 ml h-1, and the receive distance was 22 cm. The ambient temperature was 25 ± 2 °C and relative humidity was 75 ± 5 %. For the preparation of PU/C4FPU-2 flat film, the solution which containing 14 wt % PU and 2 wt % C4FPU was poured on a glass plate and swept to be flat, and then dried in the vacuum oven at 80 °C for 2 h. 2.4. Characterization of Electrospun Membranes. SEM images of prepared samples were taken by the TM 3000 SEM (hitachi Ltd., Japan HQ) with an acceleration voltage of 20 kV. The aperture size of the prepared membranes was tested by CFP-1100AI capillary flow porometer (Porous Materials Inc. USA). Porosities for the obtained membranes were computed using following formula: Porosity =

ρ1 − ρ 2 × 100% ρ1

where ρ1 stands for the density of the polymer, and ρ 2 refers to the density of electrospun membranes. Analysis of the chemical element composition of the surface for the fibrous membranes were performed



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by XPS. In particular, XPS was tested with an ESCALAB 250Xi XPS system (Thermo Fisher Scientific Ltd., UK), using a monochromatic Al Kα source. A VarioELLIII element analyzer was used to determine the element compositions of the whole membrane, according to JY/T 017-1996 test method. Water contact angle (WCA) tests were performed by Kino SL200B contact angle goniometer using liquid droplets of 3 µL in volume.

2.5. Measurements of the Fibrous Membranes. Waterproofness of the fibrous membranes were measured through the hydrostatic pressure tester (YG812C, Nantong Hongda Experiment Instruments Co., Ltd.), with an increased pressure rate of 6 kPa min-1, using a black woven fabric as the substrate. The result is based on measuring the first three drops that have penetrated through the membrane under the continuously increased applied pressure. Breathability of the membranes was tested by YG601H moisture permeability equipment (Ningbo Textile Instruments Co., Ltd.), using same method as before.31 The WVT rate were obtained with the following formula:

WVT rate =

m − m0 × 24 A

where m is the mass before test, m 0 is the mass after test, A is the test area, and the unit of the value is kg m-2 d-1. Tensile property measurements was performed by XQ-1C tensile instrument (Shanghai New Fiber Instrument Co., Ltd.). The membranes were tailored into small pieces with a certain width, the clamping head distance was 10 mm, and the final result was calculated by 15 small pieces. The antibacterial activity of fibrous membranes was tested against the Gram-negative E. coli (strain 25923) as well as against Gram-positive S. aureus (strain 25922) bacteria. Both strains were obtained from the American Type Culture Collection. The disc diffusion method was carried out in Luris-Bertani medium solid agar petri plate, the PU and PU/C4FPU-2/AgNO3-3 membranes were cut into a square shape with 0.5 cm diameter, autoclaved for 15 min at 120 °C to sterilize, and were placed on the agar plate of E.

coli and S. aureus, which were then incubated at 37 °C for 24 h and inhibition zones were carefully


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observed.

3. RESULTS AND DISCUSSION

Figure 2. SEM images of PU/C4FPU fibrous membranes containing various C4FPU concentrations of (a) 0, (b) 1, (c) 2, and (d) 3 wt %, respectively. (e) dmax and mean pore size, and (f) porosity of PU/C4FPU membranes at various C4FPU concentrations.

3.1. PU/C4FPU Fibrous Membranes. 3.1.1. Morphology and porous structure of PU/C4FPU fibrous membranes. A systematic design of desired human skin-like, high-performance and eco-friendly waterproof and breathable materials requires a combination of three significant characteristics: (i) durable excellent hydrophobic surface and small pore size, which can prevent water droplet penetrating into the macroporous membranes under a relatively high liquid pressure,32 (ii) high porosity, which provide more interconnected passageways for water vapor to transmit,33 and (iii) high strength fibers with closely interlaced nonwoven geometry, which can ensure enough resistance under external forces to satisfy the practical application. In this work, the construction of eco-friendly, durable hydrophobic



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surface was realized by the employment of synthesized C4FPU, which was introduced into electrospinning solution directly as hydrophobic agent at a small amount. The small pore size and high porosity were coordinately attained by regulating the content of AgNO3. The good mechanical property was obtained by choosing PU as the polymer matrix together with optimized fiber morphology. In the first phase of our investigation, a series of C4FPU concentrations (1, 2, and 3 wt %) were tailored, and the effects of C4FPU concentrations on morphologies, porous structure, wettability as well as waterproof and breathable performance were fully studied. As presented in Figure 2a, pristine electrospun PU membrane exhibited a 3D nonwoven geometry with average fiber diameter of 641 nm, and these fibers mostly stuck together due to the excess residual solvent. After the introduction of C4FPU, the morphologies of fibrous membranes were significantly changed, as presented in Figure 2b-d. The adhesive structure gradually decreased with the increment of C4FPU concentration, which could be attributed to the reduced surface tension (Table S1, Supporting Information). The average diameter of the relevant fibrous membranes were 693, 718, and 748 nm, respectively (Figure S4, Supporting Information), which showed slightly increase with the increment of C4FPU concentration, that was because the viscosity of the electrospinning solution didn’t alter too much. Due to the gradually increased fiber diameter and decreased bonding points which helped to form larger space, dmax regularly increased from 2.76 to 4.34 µm, and mean pore size simultaneously enlarged from 1.18 to 1.72 µm, as exhibited in Figure 2e. Additionally, with the increment of C4FPU concentration from 1 to 3 wt %, the porosity of the electrospun membranes improved from 30.7% to 41.4%, as shown in Figure 2f, which could be ascribed to the reduced adhesions sparing more space among fibers.

3.1.2. Surface chemistry and wettability of PU/C4FPU fibrous membranes. Wetting property of a solid surface is governed by both its surface energy and geometric microstructure.34-36 The surface chemical composition of electrospun PU/C4FPU membranes at different C4FPU concentrations were


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confirmed by XPS, as exhibited in Figure 3a. For all the three membranes, the bands located at 689.4±0.2, 284.6±0.3, 532.6±0.1, and 400.4±0.1 eV were associated with the characteristic peaks of F1s, C1s, O1s and N1s, respectively. With the increment of C4FPU concentration from 1 to 2 wt %, the fluorine atomic percentage among C, N, O, and F atoms increased from 23.3% to 25.2%, and the F/C atomic ratio also increased from 0.39 to 0.44 (Table S2, Supporting Information), indicating that more fluorine atoms were distributed on the surface of electrospun fiber. However, further increased C4FPU concentration to 3 wt %, the fluorine atomic percentage decreased to 24.6% and the F/C atomic ratio simultaneously reduced to 0.42. It was credited to the critical aggregation concentration (CAC) of C4FPU, beyond which only added the number of aggregates of fluorinated chains as observed in previous reported works.37 To provide insight into the functionalization feature of C4FPU in surface modification, the fluorine atomic percentage on the surface and in the bulk (denoted as FMs and FMb, respectively) of PU/C4FPU-2 fibrous membrane were compared by combination of XPS and elemental analysis. On one hand, XPS provided the fluorine atomic percentage of the fiber surface, and FMs was the above mentioned 25.2%. On the other hand, through the results of elemental analysis and numerical calculation (Table S3, Supporting Information), the FMb was 1.1%. The fluorine enrichment factor (E) is defined as the ratio of fluorine atomic percentage on the surface to the bulk of the fibrous membrane38, i.e. E=FMs/FMb. For PU/C4FPU-2 fibrous membrane, the surface enrichment factor was about 23. The result has confirmed that the fluorinated moieties had migrated toward the air/membrane interface, leading to the fluorine enrichment on the fibers surface, which was in good agreement with the previous studies.39,40 In order to clarify and explain this phenomenon thoroughly, PU/C4FPU-2 flat film with nearly non-porous structure and extraordinarily low porosity (~1%) was prepared via dry-casting method. It is noteworthy that fluorine content on the surface of flat film (FAs, 27.9%) was a little higher



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than FMs, which could be because the space in the porous structure of fibrous membrane occupied some test area. As a consequence, the preferential surface segregation of perfluorobutyl chain toward the air-fiber interface occurred in the electrospinning and solidification process. Actually, it is the great difference in the surface energy between fluorinated chains and non-fluorinated chains that contributed to the driving force for fluorinated chains segregating to the air-fiber interface,41 as illustrated in Figure 3c.

Figure 3. (a) XPS spectra of PU/C4FPU fibrous membranes. (b) Fluorine atomic percentage of the surface and bulk of PU/C4FPU-2 fibrous membrane, and the fluorine atomic percentage of the surface of PU/C4FPU-2 flat film. (c) Schematic representation of surface enrichment of fluorinated segment on the membrane during the fabrication process. (d) WCA of electrospun PU/C4FPU membranes containing various C4FPU contents. Subsequently, the wettability of electrospun PU/C4FPU membranes were evaluated by testing the


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WCA. As demonstrated in Figure 3d, pristine PU fibrous membranes showed limited hydrophobicity with a WCA of 110°, which depended on the inherent wettability of PU polymer and the hierarchical roughness of fibrous membranes. Benefiting from the introduction of perfluorobutyl chain, PU/C4FPU fibrous membranes with 1, 2, and 3 wt % C4FPU displayed enhanced hydrophobicity with WCA of 120.4°, 121.4°, and 122.5°, respectively, which was resulted from the lowered surface energy and the larger roughness brought about by the decreased adhesion structure.

Figure 4. Hydrostatic pressure and WVT rate of PU/C4FPU fibrous membranes at different C4FPU concentrations.

3.1.3. Waterproof and breathable performance of PU/C4FPU fibrous membranes. Waterproofness is one of the critical properties for waterproof and breathable membranes, and it mainly depends on the intrinsic hydrophobicity of materials and dmax of the fibrous membranes. At a given hydraulic pressure, water droplets would be prone to come out from the pores with larger diameter, i.e., pores with smaller diameter need higher pressure to break through.42,43 As shown in Figure 4, although with a small dmax, PU membranes without C4FPU addition displayed a low waterproofness with hydrostatic pressure of 8.5 kPa, which was attributed to the poor hydrophobicity of PU materials. When the C4FPU concentration was 1 wt %, the waterproofness increased to 22.2 kPa, which was mainly due to the greatly improved hydrophobicity of PU/C4FPU-1 fibrous membranes. With the same reason, PU/C4FPU-2 fibrous



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membranes obtained from 2 wt % C4FPU exhibited the best water resistant property with hydrostatic pressure of 25.1 kPa. However, further increment of C4FPU content to 3 wt % resulted in a decreased waterproofness of 21.8 kPa, that was because the wettability of fibrous membranes were almost the same, dmax had become the main factor that affected hydrostatic pressure. On the other hand, due to electrospun fibers assemble into an interconnected porous structure which are quite applicable for water vapor to transport, fibrous membranes generally present definite breathability required for protective clothing. Thus, WVT rate of electrospun PU/C4FPU membranes at various C4FPU concentrations were measured to evaluate breathable performance. As demonstrated in Figure 4, breathability of pristine PU membrane was 10.8 kg m-2 d-1, with C4FPU concentration increased to 3 wt %, it gradually increased to 12.0 kg m-2 d-1, which could be ascribed to elevated porosity resulting in more connected pathways for moisture vapor to diffusion. Mechanical properties of fibrous membranes are of great importance for practical applications. Electrospun PU membranes presented good tensile strength of 8.7 MPa as well as large elongation of 376.2%, which would be related to the abundant adhesion structure. With the increment of C4FPU concentration, the tensile strength gradually decreased to 5.2 MPa, and the elongation also reduced to 321.3% (Figure S5, Supporting Information). Since fiber diameter and fiber orientation had no much difference, the decreased bonding points would be response for the result predominantly. Considering the overall properties of the PU/C4FPU fibrous membranes, we choose PU/C4FPU-2 membranes for further investigation.


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Figure 5. SEM images of PU/C4FPU-2/AgNO3 membranes containing various AgNO3 concentrations of (a) 0.005, (b) 0.010, (c) 0.015, and (d) 0.020 wt %, respectively. (e) Average diameter, (f) pore size distribution, and (g) dmax and porosity of PU/C4FPU-2/AgNO3 membranes containing different AgNO3 concentrations.

3.2. PU/C4FPU-2/AgNO3 Fibrous Membranes. 3.2.1. Morphology and porous structure of PU/C4FPU-2/AgNO3 fibrous membranes. It is apparent that thinner electrospun fibers lead to smaller pore size and dmax, as well as contribute to higher hydrostatic pressure. In order to improve water resistant property of PU/C4FPU-2 fibrous membranes, AgNO3 was introduced into electrospinning solution. The SEM images for PU/C4FPU-2/AgNO3 membranes containing different AgNO3 concentrations revealed a typical 3D nonwoven structure, as elucidated in Figure 5a-d. The appearance of more fibers could be ascribed to the addition of small amount of AgNO3 in the electrospinning solution, which greatly enhanced the conductivity (Table S4, Supporting Information). The increscent electric force and elevated whipping of spinning fluid jet directly induced to form fibers with small diameter as well as rapid solvent evaporation, thus the bonding points also decreased. As shown in Figure 5e, the average diameter decreased from 460 to 350 nm with the increment of AgNO3



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concentration from 0.005 to 0.02 wt %. As can be seen from Figure 5f, regular pore size distribution could be observed after the introduction of AgNO3. The pore size of PU/C4FPU-2/AgNO3-1 fibrous membrane ranged from 0.35 to 1.35 µm, and their distribution concentrated at 0.83 µm. With the increment of AgNO3 concentration, the pore size became smaller and more uniform, which mainly distributed at 0.71, 0.68 and 0.59 µm, respectively. Furthermore, due to the thinner fiber diameter and closely packed arrangement, dmax of electrospun PU/C4FPU-2/AgNO3 membranes gradually declined from 1.35 to 1.00 µm, and at the same time porosity also reduced directly from 40.5% to 30.2%, as displayed in Figure 5g. In addition, benefiting from the utilization of AgNO3 which resulted in the formation of more fibers without excess adhesions, the surface roughness of PU/C4FPU-2/AgNO3 fibrous membranes increased. Comparing with PU/C4FPU-2 fibrous membranes which had a WCA of 121.4°, PU/C4FPU-2/AgNO3 fibrous membranes presented an elevated WCA of ~137° (Figure S5, Supporting Information).

3.2.2. Comprehensive properties of PU/C4FPU-2/AgNO3 fibrous membranes. Just as we have expected, the waterproofness of PU/C4FPU-2/AgNO3 fibrous membranes greatly increased benefiting from the utilization of AgNO3. As presented in Figure 6a, the waterproof properties of PU/C4FPU-2/AgNO3 fibrous membranes were 61.6, 90.3, and 102.8 kPa, which improved dramatically with the decrement of dmax. However, waterproofness of PU/C4FPU-2/AgNO3-4 membrane decreased to 82.2 kPa, which could be ascribed to the formation of visible holes on the membrane resulted from the violent instable whipping of solution jet. On the other hand, with the increment of AgNO3 concentration from 0.005 to 0.02 wt %, the WVT rate decreased from 13.4 to 12.7 kg m-2 d-1, which was resulted from the decreased porosity hindering the diffusion of water vapor. Subsequently, we compared water resistant property and water vapor permeability of PU/C4FPU-2/AgNO3-3 fibrous membranes with other waterproof breathable materials. As can be seen


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from Figure 6b, commercial PU films presented a low waterproofness of 40-43 kPa, and modest breathability of 7.8-8.2 kg m-2 d-1. In contrast, PU coatings exhibited better water resistant property with waterproofness of 62-70 kPa, but decreased vapor permeability with WVT rate of 2.4-3 kg m-2 d-1. Benefiting from the unique structure characteristics including thin fiber, small pore diameter as well as high porosity, waterproof and breathable fibrous membranes demonstrated good overall properties. Electrospun PAN fibrous membranes with post treatment presented waterproofness of 74-80 kPa, and breathability of 11.4-12.5 kg m-2 d-1.31,44 PU/FPU membranes with long fluorocarbon chain showed waterproofness of 86 kPa, and breathability of 11.9 kg m-2 d-1.45 Electrospun PU membranes containing short perfluorohexyl chain exhibited good waterproof breathable properties (104 kPa, 11.5 kg m-2 d-1).46 Fortunately, PU/C4FPU-2/AgNO3-3 eco-friendly waterproof and breathable membranes in this work possessed superior overall performance with waterproofness of 102.8 kPa, and breathability of 12.9 kg m-2 d-1. Figure 6c demonstrated the typical stress-strain curves of the four electrospun membranes at different AgNO3 concentration. The samples demonstrated non-linear elastic behavior at the first region with a small external force, and then displayed a typical linear increase until break, which could be interpreted as a two-step break mechanism. When small external force was applied to the membrane, the non-aligned fibers were compelled to align along the stress direction, giving rise to the first nonlinear elastic behavior. With external load increased, the curve exhibited a linear elastic behavior deriving from the intrinsic property of PU fibers. As a result, the aligned individual fibers break resulted in the destruction of the whole membrane. Owing to the intense friction among adjacent thin fibers, the electrospun PU/C4FPU-2/AgNO3 membranes showed similar robust strength of 8.8, 9.2, 9.8, and 9.7 MPa, respectively, which were superior to the electrospun PU/C4FPU-2 membranes. Thus, in terms of waterproof and breathable performance as well as mechanical strength, PU/C4FPU-2/AgNO3-3 fibrous



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membrane exhibited the best comprehensive properties. Antibacterial activity is a desired property of solid surfaces. The antibacterial property of PU/C4FPU-2/AgNO3-3 fibrous membrane was assessed via the disk diffusion technique,47 with pristine PU membranes for comparison purpose. Figure 6d showed that PU/C4FPU-2/AgNO3-3 fibrous membrane displayed apparent antibacterial activity. Two S. aureus colonies and a small E. coli colony grown in the surrounding area of PU/C4FPU-2/AgNO3-3 fibrous membrane, and the inhibition zone against S. aureus was larger than that of E. coli. On the contrary, the pronounced growths of S. aureus and E. coli colonies were observed in the surrounding area of pristine PU membrane, indicating that the PU fibrous membrane exhibited poor antibacterial effect. As far as we know, the broad-spectrum antibacterial property could be ascribed to the release of Ag ions from PU/C4FPU-2/AgNO3-3 fibrous membrane.

Figure 6. (a) Hydrostatic pressure and WVT rate, (b) comparison of hydrostatic pressure and WVT rate


Page 19 of 26 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

among


representative

waterproof

and

breathable

materials.

(c)

Stress-strain

curves

of

PU/C4FPU-2/AgNO3 electrospun membranes at different AgNO3 contents. (d) Antibacterial activity of PU/C4FPU-2/AgNO3-3 fibrous membranes against S. aureus and E. coli.

4. CONCLUSIONS In summary, here we reported a facile and scalable approach for the fabrication of human skin-like, high-performance and eco-friendly waterproof and breathable membranes. A novel C4FPU with double terminal short perfluorobutyl chain has been successfully synthesized for the first time and incorporated into electrospun PU membranes directly, endowing the fibrous membranes with good hydrophobicity. By comparing fluorine atomic percentage on the surface and in the bulk of the fibrous membrane, it has been confirmed that perfluorobutyl chain segregated to the air-fiber interface. Benefiting from the utilization of AgNO3, the waterproofness was significantly improved because of the dramatically decreased dmax, at the same time the fibrous membranes were also endowed with significant antibacterial property. Consequently, the PU/C4FPU-2/AgNO3-3 fibrous membrane exhibited robust water resistant property of 102.8 kPa, excellent moisture vapor permeability of 12.9 kg m-2 d-1 along with high mechanical property of 9.8 MPa. It is believed that this work is of great significance for the design and large-scale production of human skin-like robust waterproof and highly breathable membranes with eco-friendliness via a benign route, which implies good candidates in versatile applications ranging from protective clothing to medical fields.

ASSOCIATED CONTENT Supporting Information Schematic illustration of the synthesis of C4FPU (Figure S1). 1H,

19

F NMR spectra, and chemical

structure of synthesized C4FPU (Figure S2). FT-IR spectrum of synthesized C4FPU (Figure S3).



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Average fiber diameter of PU/C4FPU membranes (Figure S4). Tensile properties of PU/C4FPU fibrous membranes at different C4FPU concentrations (Figure S5). WCA of PU/C4FPU-2/AgNO3 membranes (Figure S6). Compositions and properties of PU/C4FPU polymer solution (Table S1). XPS results for PU/C4FPU membranes with various content of C4FPU (Table S2). Elemental analysis results of PU/C4FPU-2 fibrous membrane (Table S3). Compositions and properties of PU/C4FPU-2/AgNO3 polymer solution (Table S4).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Fax: +86 21 62378202

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (51473030, 51503028 and 51673037), the Shanghai Rising-Star Program (16QA1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2016019), the National Key R&D Program of China (2016YFB0303200) the Fundamental Research Funds for the Central Universities (2232016A3-03), and the Military Logistics Research Project (AWS14L008).

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TOC


Human Skin-Like, Robust Waterproof, and Highly Breathable Fibrous

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