Tailoring Water-Resistant and Breathable Performance of

Sep 23, 2016 - Robust Fluorine-Free Superhydrophobic Amino-Silicone Oil/SiO2 Modification of Electrospun Polyacrylonitrile Membranes for Waterproof-Br...
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Tailoring Water-Resistant and Breathable Performance of Polyacrylonitrile Nanofibrous Membranes Modified by Polydimethylsiloxane Junlu Sheng,† Min Zhang,† Yue Xu,† Jianyong Yu,*,†,§ and Bin Ding*,†,§ †

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China § Nanofibers Research Center, Modern Textile Institute, Donghua University, Shanghai 200051, China S Supporting Information *

ABSTRACT: The demand of water-resistant and breathable materials applied to a separation medium and protective garments is steadily increasing. Typical approaches to obtain these functional materials are based on hydrophobic agents and porous substrates with small fiber diameter, tiny pore, and high porosity. However, a fluorinated hydrophobic finishing agent usually employed in providing effective waterproofness is limited with respect to their environmental persistence and toxic potential. Herein, with the aim to keep a balance between the water-resistance and breathability as well as mechanical properties, we fabricate a novel fluoride-free functional membrane by electrospun polyacrylonitrile (PAN) nanofibers modified with polydimethylsiloxane (PDMS). As determined by morphological, DSC, and FT-IR analyses, the curing reaction of PDMS macromolecules formed an abundance of hydrophobic adhesive structures, which improved the waterproof performance dramatically and imparted relative good breathability at the same time. By systematically tuning the curing temperature as well as the concentration of PDMS, the modified PAN membranes with 4 wt % PDMS possessed good water-resistance (80.9 kPa), modest vapor permeability (12.5 kg m−2 d−1), and air permeability (9.9 mm s−1). Compared with pristine PAN membranes, the modified membranes were endowed with enhanced tensile stress of 15.7 MPa. The good comprehensive performance of the as-prepared membranes suggested their potential applications in protective clothing, membrane distillation, self-cleaning materials, and other medical products. Furthermore, the proposed relationship between porous structure and waterproof/breathable property as one considerable principle is applicable to designing functional membranes with different levels of protective and comfortable performance. KEYWORDS: polyacrylonitrile, electrospinning, poly(dimethylsiloxane), surface modification, waterproof/breathable permeability (6.3 kg m−2 d−1) and modest waterproofness (110 kPa). However, PTFE membranes was expensive and easy to lose water resistance when washed a few times, which restricted its practical applications.5 So it is still imminent for searching a universal approach to obtain cost-effective, durable hydrophobic waterproof/breathable membranes with both good water resistance and vapor permeability. Electrospinning, as one versatile technique to produce porous nanofiber membranes, has been of considerable interest within low weight, small pore size, high porosity, and interconnected tortuous channels.6−8 Apart from this, the other unique advantages, such as wide range of raw materials, low-power consumption, convenience to create hydrophobic surface, and controllable porous structure impart an electrospinning strategy to effectively fabricate functional nanofiber

1. INTRODUCTION Waterproof/breathable functional membranes have been developed for the use in fabrics to provide the wearer with a greater level of comfort under many extreme climatic conditions, while still supplying protection of human body from environmental factors, such as rain, wind, and snow.1,2 This permeability to water vapor but resistance to water droplets are not only applied in textile industry but can also be employed in medical applications, membrane distillation, moisture regulation of buildings, and in humidification for the conservation of art and paper.3 There are two major functional membranes classified into hydrophilic thermoplastic polyurethane (TPU) membranes and expanded polytetrafluoroethylene (PTFE) membranes. Hydrophilic TPU films exhibited relative high waterproofness (140 kPa) because of its nonporous structure, while the water vapor transmittance rate (WVTR) was low (2 kg m−2 d−1).4 As for PTFE membranes, due to a large number of very tiny hydrophobic porous channels, the membranes exhibited high water vapor © 2016 American Chemical Society

Received: July 29, 2016 Accepted: September 23, 2016 Published: September 23, 2016 27218

DOI: 10.1021/acsami.6b09392 ACS Appl. Mater. Interfaces 2016, 8, 27218−27226

Research Article

ACS Applied Materials & Interfaces membranes.9−11 Lim et al.12 prepared a Janus polyacrylonitrile (PAN) fabric via electrospining and then thermal treatment, which repelled water on the one side and absorbed water on the other side. Their work indicated electrospining can be used as a new route for the design and development of smart membranes. Park et al.13 first reported electrospun waterproof/ breathable functional membranes, which possessed good moisture permeability (9 kg m−2 d−1) but poor water resistance (3.65 kPa). Kim et al.14 introduced waterborne polyurethane as coating resin to electrospun nanofibrous with improved waterproofness of 84.4 kPa due to the reduced pore size. However, the water vapor permeability decreased to 4.1 kg m−2 d−1 on the contrary. Recently, we synthesized waterborne fluorinated polyurethane and utilized it to modify electrospun nanofibers. Although the resulted membranes also had an increased water resistance (83.4 kPa) and modest breathability (9.2 kg m−2 d−1),15 the synthesis procedure of this fluorinated finishing agent was complicated, and the increase of waterproofness was compromised because of the breathability. As determined by current investigations, a stable hydrophobic surface/interface combined with a small pore size are confirmed to be essential for providing good water repellency. Conventionally, fluorinated or silicon compounds are used for hydrophobic modification due to their low surface free energy. However, fluorinated compounds have been banned in some applications because of their widespread occurrence in the environment as well as their environmental persistence, bioaccumulation, and toxic potential.16 Nowadays, nonfluorine silicone-containing finishing agents like polydimethylsiloxane (PDMS) have been investigated to fabricate the hydrophobic surface/interface.17,18 Xue et al.19 fabricated superhydrophobic poly(ethylene terephthalate) textile surfaces by treating the textiles with alkali followed by dip-coating with PDMS. The coated textiles presented self-cleaning property to acid/alkaline etching. Furthermore, the flexible siloxane (−SiO−) backbone of PDMS make it a potential candidate for fabricating gas separation membranes due to its high intrinsic permeability. Halim’s research group used PDMS to modify PAN microporous membranes to selectively separate carbon dioxide (CO2) from nitrogen (N2).20,21 PDMS worked as a protective precoating layer to prevent the penetration of diluted polymer solution into the porous structure. Both its hydrophobic and protective properties indicated PDMS could be a useful nonfluorine modification agent in electrospun waterproof/ breathable field. In this work, we introduce a fluoride-free and facile strategy to fabricate waterproof/breathable functional membranes utilizing electrospinning and surface modification. PAN nanofibrous membranes (NFM) were chosen as the backbone of the functional membrane due to their desirable properties like thermal stability, good resilient and controllable porous structure. However, there are a few disadvantages, such as moderate hydrophilicity, low moisture absorption, and lack of surface conductivity, which greatly limit its further applications in wearable cloths. The modification of PDMS on PAN nanofibers will alter its surface properties and expand its usages in the areas of waterproof/breathable membranes. PDMS was introduced as a nonfluorinated hydrophobic agent, whose relative low surface tension (23 mN m−1) in combination with microscale roughness inherent in PAN electrospun membranes can effectively improve the waterproof perfomance.22,23 The fabrication of PDMS-modified PAN (PAN@PDMS) NFM is illustrated in Figure 1. PDMS modification not only avoids the

Figure 1. Schematic illustration of the preparation procedure of composite PAN@PDMS NFM.

persistent micropollutants of the perfluorinated compound but also provides adhesive structure and optimized porous characteristics. In order to acquire the functional membranes with both better water-resistance and breathability, this work focused on how different types of porous structure can influence the performance of PDMS-modified membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. PAN powders (Mw = 90 000) was bought from Kaneka Co., Ltd., Japan. PDMS prepolymer and cross-linker (Sylgard 184) were manufactured by Dow Corning Co., America. N,Ndimethylacetamide (DMAc) and n-hexane were supplied by Shanghai Lindi Chemical Reagent Co., Ltd., China. Methylene blue and allochroic silicagel particles were obtained through Aladdin Chemical Reagent Co., China. All reagents were employed without any processing. 2.2. Electrospinning. PAN/DMAc transparent solution with a concentration of 9 wt % was prepared by stirring for 10 h. PAN NFM were fabricated under spinning environment (25 ± 2 °C and 45 ± 2%) through a DXES-3 electrospinning machine (SOF Nanotechnology Co., Ltd., China). The electrospinning process was operated with a feed rate of 1 mL h−1, an applied voltage of 30 kV, and a distance between spinneret and collector of 24 cm. 2.3. PDMS Modification. PDMS prepolymer was blended with cross-linker at constant 10:1 ratio for stirring 20 min and then degassed to remove bubbles. PAN NFM were modified with PDMS/nhexane solutions and then dried in a vacuum oven with various curing temperatures: 80, 100, and 120 °C. PDMS/n-hexane solutions were prepared containing a series concentrations: 2, 4, 6, and 8 wt %. The prepared modified nanofibrous membranes were defined as PAN@ PDMS-x, where x was the various concentrations of PDMS. 2.4. Characterization and Measurements. Thermal properties of pristine and modified membranes were investigated through differential scanning calorimetry (DSC) analysis by a DSC 4000 (PerkinElmer, America). The Fourier transform-infrared spectroscopy (FT-IR) method was employed to characterize the curing reaction of PDMS using a Nicolet 8700 FT-IR spectrometer (America). Surface morphological observations were tested with a scanning electron microscope (SEM, TESCAN VEGA 3, TESCAN Ltd., China). The pore size of the membranes was characterized by capillary flow porometer (CFP-1100AI, PMI Inc., America). Porosity of the samples was tested on the basis of the previously described method.24 The advancing and receding contact angles of the water droplet were tested by a Contact Angle Analyzer (Kino SL200B, America), and the volume 27219

DOI: 10.1021/acsami.6b09392 ACS Appl. Mater. Interfaces 2016, 8, 27218−27226

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ACS Applied Materials & Interfaces of the tested water droplet was 5 μL. Mechanical performance was measured using a current testing equipment with a load-cell of 200 N (XQ-1C, Shanghai New Fiber Instrument Co., Ltd., China). Abrasion properties were also measured referring to ASTM D3884 using previously described strategy.14 At least five measurements of the mechanical tests were carried out and averaged for all samples. The water resistance of the membranes was estimated by water absorption and hydrostatic pressure measurements. Water absorption percentage of pristine and PDMS modified PAN membranes were determined on the basis of formula below:25

W (%) =

Wt − Wi × 100% Wi

(1)

where Wt and Wi are the weights of the samples after and before immersed into the distilled water, respectively. Hydrostatic pressure was measured on the basis of the AATCC 127 test criterion with a water pressure increasing rate of 6 kPa min−1. Breathable performance of the membranes was determined by testing WVTR and air permeability. WVTR testing was performed according to ASTM E96 inverse cup standard with the temperature of 38 °C, relative humidity of 50%, and a wind velocity of 1 m s−1. WVTR values were determined on the basis of the equation below:26

WVTR =

W2 − W1 × 24 S

Figure 2. SEM images of pure PAN NFM (a) and PAN@PDMS-2 NFM prepared with various curing temperatures: (b) 80, (c) 100, and (d) 120 °C.

3.1.2. DSC Analysis and Mechanical Property. In addition to the surface morphology, the curing reaction was also verified by DSC analysis. As displayed in Figure 3a, the DSC

(2)

where the unit of WVTR is kg m−2 d−1, W2 − W1 is the mass variation of distilled water during testing, and S is the area of the samples. Based on the ASTM D 737 criterion, the air permeable property was tested under a differential pressure of 100 Pa. For each sample, water absorption percentage, hydrostatic pressure, WVTR, and air permeability were tested at least three times.

3. RESULTS AND DISCUSSION 3.1. Effect of Curing Temperature. 3.1.1. Morphology and Structure. Waterproof/breathable functional membranes are usually composed of porous substrate and hydrophobic surface. Feng et al. demonstrated that the surface hydrophobicity was dependent on fiber diameter, interfiber distance, porosity, and surface chemistry of the membranes.27 So the morphology and structure of the PDMS-modified membranes were investigated. During the heating process, PDMS with low surface free energy is cross-linked by curing reaction between the vinyl group in the prepolymer and the silicon hydride group in the cross-linker.28 In order to find the optimized curing temperature, the morphology of pristine and modified membranes with 2 wt % PDMS was characterized by SEM, as shown in Figure 2a−d. Pristine PAN nanofibers (average fiber diameter was 213 nm) were smooth and stacked randomly (Figure 2a). After being cured, the membranes were covered with adhesive network between the nanofibers. PDMS can be cross-linked at room temperature, but appropriate heating can accelerate the reaction process. So fixing the heating time at 30 min and the concentration of PDMS at 2 wt %, the crosslinking degree of PDMS showed a gradual increase when elevating the temperature from 80 to 120 °C, as demonstrated from the SEM images. When the curing temperature was 80 °C, there were only small areas of adhesive structure indicating the curing was insufficient. When the curing temperature was continually increased, the area of the adhesive structure became bigger, and the fiber diameter increased slightly from 269 to 285 μm. When PAN@PDMS-2 NFM were cured at 120 °C, there was almost no single fiber in the fibrous structure. Every two or three nanofibers interfused closely together to form adhesive network, and the pores in the membranes were filled with PDMS modification agent.

Figure 3. (a) DSC thermograms and (b) tensile property of pristine PAN and PAN@PDMS-2 NFM cured at various temperatures.

thermogram of pure PAN revealed one obvious exothermic peaks centered at about 286.4 °C, which was corresponding to the peroxidation of PAN.29 However, after modification, PAN@PDMS-2 NFM under curing temperatures of 80, 100, and 120 °C exhibited gradually reduced peroxidation peak at 284.7, 283.9, and 282.0 °C, respectively. This variation trend indicated that greater and greater amounts of PDMS covered on the pristine membranes with elevating the curing temperature, which corresponded to the structural transformation in Figure 2. In addition, the glass transition temperature (Tg) of pure PAN NFM was about 100 °C and was supported by DSC analysis of the previous literature.30 Nevertheless, for the cured PAN@PDMS-2 NFM, Tg increased from 104 to 118 °C with elevating curing temperature from 80 to 120 °C (Figure S3). This change was ascribed to the cross-linked PDMS, which could inhibit the chain mobility of PAN and lead to a higher Tg. 27220

DOI: 10.1021/acsami.6b09392 ACS Appl. Mater. Interfaces 2016, 8, 27218−27226

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ACS Applied Materials & Interfaces Curing temperature not only affected the thermal property but also influenced the mechanical performance, as revealed in Figure 3b and Supporting Information Figure S4. Under heating, the molecules of the prepolymer and the cross-linker would become active to diffusion and form strong chemical bonds between them. So the tensile stress of the modified membranes was higher than the pure PAN NFM. The pristine PAN membranes exhibited the strength of 6.7 MPa and breaking elongation of 60.9%. After being cured at 80 °C, the strength had almost the same stress of 7.2 MPa, which was the result of insufficient cross-linking between the prepolymer and cross-linker. When curing temperature was continuously elevated, the strength increased obviously to 11.7 MPa and possessed larger breaking elongation of 75.9%, indicating the cross-linking reaction of PDMS was sufficient at 100 °C. However, when heated at higher temperature (120 °C), excessive PDMS with poor mechanical property caused the strength slightly decline to 9.6 MPa.31 Based on the above considerations, curing temperature of 100 °C was enough for cross-linking curing to acquire optimized PDMS modification. 3.1.3. FT-IR Analysis. Taking morphological, thermal, and mechanical performance into account, the curing temperature of 100 °C was chosen for further investigation. The crosslinking reaction cured at 100 °C of PAN@PDMS-2 NFM was confirmed using FTIR spectra. As shown in Figure 4, the

Figure 5. (a)−(c) Top view, (d)−(f) side view, and (g)−(i) magnified side view SEM images of PAN@PDMS NFM. These images from left to right corresponded to those membranes with increasing PDMS concentrations: (a) PAN@PDMS-4, (b) PAN@PDMS-6, and (c) PAN@PDMS-8.

which was consistent with the effect of curing temperature on the morphology presented in Figure 2. Moreover, modifications by PDMS can increase the average diameter of the membranes slightly, as shown in Supporting Information Figure S1. From the cross-sectional images in Figure 5 and Supporting Information Figure S2, we can see the modified membranes were closely packed and the nanofibers were bonded with each other. In addition, the thickness of the modified membranes decreased from 35 to 13 μm (Supporting Information Table S1), suggesting PDMS completely filled the membranes rather than only covered the surface of the membranes. The SEM images from middle and reverse view of PAN@PDMS-4 NFM (Figure S5 in the Supporting Information) also confirmed this conclusion. Nevertheless, when the concentration of PDMS was 8 wt %, the nanofibrous structures were damaged and formed compact solid membranes with almost nonporous morphology, as shown in the high-resolution SEM image of this sample (Figure 5i). 3.2.2. Mechanical Performance. Taking the practical applications into consideration, an excellent mechanical performance of the membranes is necessary to satisfy stretch, deformation, and tear during the following laminating process and end use. Therefore, the relationship between mechanical property and concentration of PDMS was also studied. Figure 6a presented the tensile stress and breaking elongation of the relevant electrospun membranes with different concentrations of PDMS. Pure PAN NFM produced a tensile strength of 6.7 MPa and breaking elongation of 60.9%. As for electrospun nanofibers, it is relevant to underscore their major weakening property because the polymer molecule are not aligned and interacting fully. The stress of the membranes modified with 2 wt % PDMS increased to 11.7 MPa. When the quantity of PDMS reached 4 wt %, the stress further increased to 15.7 MPa, which was about 2 times that of the pure PAN NFM, whereas the elongation also increased to 80.0%. This improvement of mechanical performance indicated PDMS modification imparted electrospun membranes with adhesive structure to enlarge the cohesive force of the adjacent nanofibers. However, when the concentration of PDMS was continuously increased to 8 wt %, the tensile property became worse with a reduced tensile stress of 12.3 MPa and elongation

Figure 4. FT-IR spectra of the pristine PAN and PAN@PDMS NFM cured at 100 °C.

pristine PAN NFM exhibited typical CN stretching vibration band of cyan group at 2247 cm−1. The peak at around 1250 cm−1 was ascribed to a stretching vibration of C−O in methyl acrylate remaining in PAN and PAN@PDMS NFM. However, after PDMS modification, the intensity of PAN@PDMS NFM at this peak was larger than that of pristine PAN membranes, which was due to the silicon-methyl bond of PDMS at 1257 cm−1. The intensity of the CN bond in the spectrum of PAN@PDMS NFM weakened because of large amounts of PDMS covering on the membranes. The typical peak of silicon dimethyl bond was at 790 cm−1. Moreover, the characteristic silica oxygen stretching vibration peak at 1081 cm−1 appeared in PAN@PDMS NFM sample, confirming the polymerization of the prepolymer and cross-linker.32 3.2. Effect of PDMS Concentration. 3.2.1. Morphology and Structure. Apart from the curing temperature, PDMS concentration was considered to be a primary factor in affecting the degree of adhesive during the modifying process. To find the most effective PDMS content for the waterproof/breathable properties, the concentrations of the PDMS were regulated. Top view SEM images (Figure 5a−c) displayed that the adhesive structure among the fibrous structure increased significantly along with increasing PDMS concentration, 27221

DOI: 10.1021/acsami.6b09392 ACS Appl. Mater. Interfaces 2016, 8, 27218−27226

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modification can enhance the mechanical performance of electrospun nanofibrous membranes. 3.2.3. Water Resistance Analysis. Cross-linked PDMS elastomers with a water contact angle of 111° for a smooth film are commonly used for fabricating hydrophobic surfaces.33,34 In this work, the surface wettability of the fibrous membranes relied on chemical composition and surface morphology, and the tests were performed by a dynamic method based on the Cassie model, as shown in Figure 7a. In order to investigate the wetting behavior of the PAN@PDMS NFM, the advancing contact angle (θadv) and contact angle hysteresis values were measured, which are exhibited in Figure 7b and Table S1. Figure 7b indicated that pure PAN NFM exhibited hydrophilicity with the θadv of 32°. Introduction of 4 wt % PDMS increased θadv of the modified membranes to 141°; when further increasing PDMS contents, θadv of the membranes decreased from 141° to 127°. The gradual decrease in θadv might be caused by the decreasing porosity (increasing solid area fraction) of the membranes.35,36 Relatively, the contact angle hysteresis changed slightly for the modified membranes with a different concentration of PDMS (Table S1). Low contact angle hysteresis with a value around 7° makes the water droplet roll easily when it comes in contact with the PAN@ PDMS-4 NFM surface. In order to further observe the dynamic wettability of the surfaces, the spreading and repellency of the water droplet were recorded using a high-speed camera system.37 As exhibited in Supporting Information Figure S6, the water droplet spread fast when it touched the pure PAN NFM surface. Simultaneously, the droplet revealed almost no deformation when departing the PAN@PDMS-4 NFM surface, thus confirming the water resistance of the modified membranes. Besides wettability, the porous structure of the relevant membranes was also influenced by the concentration of PDMS. Figure 7b and Supporting Information Figure S7 displayed the porous characteristics with distribution, max-

Figure 6. (a) Tensile performance of PAN@PDMS NFM with various PDMS concentrations. (b) Abrasion testing and (c) photographs of abrasion resistance of pristine PAN and PAN@PDMS-4 NFM.

of 65.0%. This variation tendency suggested an excessive PDMS amount formed a solid-like film leading to decreasing elasticity, which coincided with the change of the mechanical performance as the function of curing temperature. Therefore, the concentration of 4 and 6 wt % modified membranes exhibited relative good tensile stress and breaking elongation. In addition, we chose 4 wt % PDMS as a representative to investigate the abrasion property of the membranes. From the pictures in Figure 6b,c, the abrasive resistance was better for PAN@ PDMS-4 NFM than that of pure PAN membranes after sliding friction in 20 cycles, which confirmed PDMS

Figure 7. Analysis of water resistant performance of PAN@PDMS NFM. (a) Illustration of the water drop on PAN@PDMS-4 NFM. (b) θadv and dmax of relevant PAN@PDMS NFM obtained from various PDMS concentrations. (c) Water adsorption rate and hydrostatic pressure of PAN@ PDMS NFM with different PDMS concentrations. (d) The linear relationship between hydrostatic pressure and cos θadv/dmax of the relevant PAN@ PDMS NFM. 27222

DOI: 10.1021/acsami.6b09392 ACS Appl. Mater. Interfaces 2016, 8, 27218−27226

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Figure 8. Breathable property of water vapor and air of relevant PAN@PDMS NFM with different PDMS concentrations. (a) Porosity of PAN@ PDMS NFM. (b) WVTR and air permeability of PAN@PDMS NFM. (c) The linear relationship between WVTR and porosity. (d) The linear relationship between air permeability and porosity.

imum, average, and smallest pore size of the relevant membranes. Nanofibers of pure PAN NFM stacked together randomly to form a three-dimensional fluffy nonwoven structure with a maximum pore size of 1.51 μm. As expected, PDMS modification endowed pristine membranes with adhesive structure and decreased pore size. The maximum pore size (dmax) for PAN@PDMS-2, PAN@PDMS-4, PAN@ PDMS-6, and PAN@PDMS-8 were 1.15, 1.14, 1.08, and 1.09 μm, respectively. The maximum pore size of PAN@PDMS-8 was almost the same as that of PAN@PDMS-6, indicating that 6 wt % concentration of PDMS was almost enough to fill with the pristine PAN NFM. Due to the transformation of water wettability and pore size of the modified membranes, the water resistant property was investigated by water adsorption and hydrostatic pressure tests, as presented in Figure 7c. Hydrostatic pressure was the pressure at the liquid−vapor interfaces of membranes, extruding water through the pores and preventing the wetting of the pores.38 Water resistance of pure PAN NFM was poor because of the largest water adsorption rate of 83% and the lowest hydrostatic pressure of near 0 kPa. This phenomenon mainly depended on the hydrophilicity of the pristine membranes. On one hand, after the modification of 2 wt % PDMS, the water adsorption rate decreased obviously to 1.1%. When PDMS concentration was further increased, there was no obvious change for the water adsorption rate, and the rate remained in the range of 0.6%, indicating the modified membranes could not adsorb water at all. On the other hand, there was an obvious increase from 61.8 to 80.9 kPa of hydrostatic pressure following elevation of PDMS contents from 2 to 4 wt %. However, when PDMS concentration was continuously increased from 6 to 8 wt %, the hydrostatic pressure decreased from 72.9 to 61.5 kPa on the contrary. From the linear relationship law between hydrostatic pressure and cos θadv/dmax in Figure 7d, we disclosed that the hydrostatic pressure depended on the surface wettability and pore size of the nanofibrous membranes and can be calculated employing the formula below:

P = 4Bγ

cos θadv dmax

(3)

where γ was the surface tension of water drop (72 mN m−1). This formula was close to the Laplace equation described in our previous work.24 The important difference in this work was introducing a geometric factor (B) as a correction coefficient for irregular tortuous pores. Based on the surface tension of water and the slope of the line, the coefficient B was about 0.38 of PAN@PDMS modified membranes. 3.2.4. Breathability of Water Vapor and Air. Waterproof and breathable functional membranes designed for coats should possess three main characteristics: materials should be water repellent, water vapor permeable, and wind resistant.39−41 Apart from water repellency, permeability to water vapor should permit evaporating sweat from the skin’s surface to ensure the wearer feel comfortable; the windproof property prevents cold wind from entering the space between the skin and garments during extreme climate conditions. Sweat vapor breathability and windproofness were usually characterized in terms of WVTR and air transmittance, respectively. The main aim of this study was to find a relationship between breathability and porous structure of the nanofibrous membranes, whose structure was usually represented by its pore size, porosity, and interconnected channels. With an increase in the concentration of PDMS, the structural transformation from fluffy to adhesive resulted in the porosity gradually decreased from 81.0 to 19.7%, as displayed in Figure 8a. The variation of porosity also confirmed that more adhesive structure formed with increasing PDMS concentration (Figure 2 and Figure 5). Figure 8b displayed that WVTR and air permeability had the same decline tendency as that of porosity. Because of the fluffy structure, pristine PAN NFM exhibited a high WVTR of 13.7 kg m−2 d−1 and air permeability of 23.6 mm s−1. When the concentration of PDMS reached 8 wt %, the WVTR decreased to 10.9 kg m−2 d−1 and air permeability declined to 1.6 mm s−1 obviously. The average pore size within the range of 0.65 to 0.96 μm after PDMS modification was far bigger than the size 27223

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hour, the water droplets still stood on the membranes rather than spread or disappeared, indicating the water resistance of the modified membranes. At the same time, the color of allochroic silicagel particles changed from blue to pink, which suggested there was a large amount of water vapor transferring through the membranes. This typical experiment suggested PDMS-modified PAN membranes could be potentially useful for various applications in textile industry, medical products, membrane distillation, moisture regulation of buildings, and in humidification for the conservation of art and paper.

of water vapor and air molecules. On account of this phenomena and according to Darcy’s law, porosity played an important role in WVTR and air permeability.42−44 Therefore, the relationship between porosity and breathability (WVTR and air permeability) was investigated, as shown in Figure 8c,d. WVTR and air permeability demonstrated a linear relationship with porosity. In other words, every single porosity provided a consistent amount of water vapor and air permeability. 3.3. Mechanism Simulation and Demonstration of Waterproof/Breathable Performance. Based on the investigation of the relationship between the performance and the structures, the mechanism of the waterproof/breathable performance of PAN@PDMS membranes corresponded to the pore size effect.45−47 In the case of the PDMS-modified microporous membrane, the maximum pore size (1−2 μm) is between the diameter of a water vapor molecule (0.0004 μm) and the finest water droplets (>100 μm) of atmospheric precipitation. Thus, porosity representing the number of pores had an important effect on the level of the water vapor transmitting property.48,49 PAN@PDMS hydrophobic microporous membranes containing very fine interconnected channels could be waterproof and, at the same time, breathable. A simulation of the mechanism of waterproofness and water vapor permeability for PAN@PDMS hydrophobic membranes was exhibited in Figure 9a.

4. CONCLUSIONS In conclusion, hydrophobic composite PAN@PDMS membranes with relatively good waterproofness and breathability have been prepared by a simple, efficient, and fluoride-free strategy. PDMS as fluoride-free modification agent endowed PAN electrospun membranes with hydrophobic adhesive structure and appropriate pore size due to its low surface energy and curing reaction on the membrane surface. The water-resistance, breathability, and mechanical properties of the nanofibrous membranes were comprehensively tailored by optimizing curing temperature and concentration of PDMS. Compared to the pristine PAN NFM, the modified PAN@ PDMS-4 membranes possess relatively high strength (15.7 MPa), large breaking elongation (80.0%), good hydrostatic pressure (80.9 kPa), modest WVTR (12.5 kg m−2 d−1), and air permeability (9.9 mm s−1). In addition, the suggested relationship between porous structure and waterproof/breathable performance could be one directive principle for designing desired functional membranes with extensive applications in real environments, even under tough conditions. Successful fluoride-free modification by PDMS suggests that this versatile approach has the potential to benefit other substrates like woven and knitted fabrics in waterproof/breathable applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09392. Average fiber diameters of PAN@PDMS NFM; SEM images of pristine PAN and PAN@PDMS-2 NFM from side view; Tg of PAN@PDMS-2 NFM fabricated with various curing temperature; tensile stress and breaking elongation of PAN@PDMS-2 NFM fabricated with various curing temperature; SEM images of PAN@ PDMS-4 NFM from different perspectives; thickness and hysteresis angle of the modified PAN NFM with various concentrations of PDMS; photographs of dynamic measurements of water spreading on the surface of pure PAN NFM and water repellency on the surface of the PAN@PDMS NFM; pore distribution, average, and smallest pore size of PAN@PDMS NFM (PDF)

Figure 9. (a) Mechanism simulation and (b) typical tests demonstrating the waterproof/breathable performance of PAN@ PDMS NFM.

Comprehensively considering the waterproofness, breathability, and mechanical performance, 4 wt % of PDMS concentration was chosen for demonstrating the waterproof and breathable performance, which was carried out using distilled water and allochroic silicagel particles. Herein, water droplets stained with methylene blue dye were used to represent the waterproofness, and silicagel particles were utilized as humidity indicators to confirm the water vapor transforming through the membranes, as presented in Figure 9b. This experiment was carried out at room temperature and low humidity of 20%. The PAN@PDMS-4 NFM was covered on a beaker filled with water. After heating at 100 °C for half an

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86 21 62378202. *E-mail: [email protected] Notes

The authors declare no competing financial interest. 27224

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ACKNOWLEDGMENTS The authors would like to acknowledge the National Natural Science Foundation of China (Nos. 51473030 and 51322304), the Fundamental Research Funds for the Central Universities, and the “DHU Distinguished Young Professor Program”.

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