Environmentally Friendly and Breathable Fluorinated Polyurethane

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Environmental friendly and breathable fluorinated polyurethane fibrous membranes exhibiting robust waterproof performance Jing Zhao, Yang Li, Junlu Sheng, Xianfeng Wang, Lifang Liu, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08885 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017

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ACS Applied Materials & Interfaces

Environmental Friendly and Breathable Fluorinated Polyurethane Fibrous Membranes Exhibiting Robust Waterproof Performance Jing Zhao,†,ǁ Yang Li,‡,ǁ Junlu Sheng,† Xianfeng Wang,*,† Lifang Liu,† Jianyong Yu,†,§ and Bin Ding*,†,§ †

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua

University, Shanghai 201620, China ‡

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

Materials Science and Engineering, Donghua University, Shanghai 201620, China §

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

* Corresponding author at: College of Textiles, Donghua University, Shanghai 201620, China. Fax: +86 21 62378202. E-mail: [email protected]; [email protected].

ACS Paragon Plus Environment


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ABSTRACT Waterproof and breathable membranes which provide high level of protection and comfort are promising core materials to meet the pressing demand for future upscale protective clothing. However, creating such materials characterized with environmental protection, high performance and ease of fabrication has proven to be a great challenge. Herein, we report a novel strategy to synthesize fluorinated polyurethane (C6FPU) containing short perfluorohexyl (-C6F13) chains, and introduced it as hydrophobic agent into polyurethane (PU) solution for one-step electrospinning. A plausible mechanism about the dynamic behaviour of fluorinated chains with the increasing C6FPU concentration was proposed. Benefiting from the utilization of magnesium chloride (MgCl2), the maximum pore size of fibrous membranes were dramatically decreased. Consequently, the prepared PU/C6FPU/MgCl2 fibrous membranes exhibited excellent hydrostatic pressure of 104 kPa, modest water vapour transmission (WVT) rate of 11.5 kg m-2 d-1, and desirable tensile strength of 12.4 MPa. The facile fabrication of PU/C6FPU/MgCl2 waterproof and breathable membranes not only match well with the tendency of environmental protection but also fully meet the requirements for high performance under extremely harsh environments.

KEYWORDS: electrospinning, environmental friendly, waterproof and breathable, short perfluorohexyl chains, magnesium chloride


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1. INTRODUCTION Waterproof and breathable membranes which can not only prevent water droplets permeation but also allow water vapor transmission have been extensively employed in a variety of applications ranging from protective garments, aerospace, building materials to aquaculture industry.1-3 These membranes can be divided into two categories, one is hydrophilic nonporous membranes and the other is hydrophobic microporous membranes. Thermoplastic polyurethane (TPU) hydrophilic membranes possess good hydrostatic pressure benefiting from the nonporous structure, but provide the wearer with a low level of comfort due to the poor moisture vapor permeability. Typical hydrophobic microporous membranes such as polytetrafluoroethylene (PTFE) stretching membranes exhibit good water resistance and moisture vapor permeability properties, but they are difficult to be decomposed and recovered, at the same time they suffer from the complicated fabrication process. Thus, it is an imminent task to search a simple method to fabricate waterproof and breathable membranes with environmental protection and high performance simultaneously.4 Currently, electrospinning as the most effective method to fabricate continuous fibers with diameters as low as a few nanometers, can be applied to obtain not only natural and synthetic polymers, but also ceramics and metals.5-7 As one of the advanced fibrous materials, electrospun membranes combine small pore size, high porosity, good mechanical strength, as well as fine flexibility, which have been considered as good candidate for developing waterproof and breathable materials.8,9 In 2007, polyurethane (PU) was chosen to fabricate waterproof and breathable laminates for the first time, and the fabric displayed good water vapor transmission (WVT) rate over 9 kg m-2 d-1, but the hydrostatic pressure was as low as 3.7 kPa.10 Later, Ge et al. developed a series of PU fibrous membranes with different diameters by utilization of electrospinning, and the prepared membranes exhibited improved



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waterproofness (5.7 kPa), but declined moisture vapor permeability (7.9 kg m-2 d-1).11 The reasons for low water resistance of obtained membranes were the poor hydrophobic property and large pore size. On one hand, to improve the hydrophobicity of fibrous membranes, they synthesized fluorinated polyurethane with long perfluorooctyl chain (-C8F17), and introduced it into PU electrospinning solution, imparting to the electrospun membranes amphiphobicity with water contact angle (WCA) of 156o and oil contact angle (OCA) of 145o.12 As a consequence, the resultant membranes showed an elevated water resistance (39.3 kPa), and good moisture vapor permeability (9.2 kg m-2 d-1). On the other hand, to decrease the pore size of electrospun fibrous membranes, Li et al. exploited carbon nanotubes (CNTs) to prepare waterproof and breathable membranes using the same hydrophobic agent.13 Thanks to the addition of CNTs, the conductivity of electrospinning solutions sharply increased, which boosted the stretching effect of jet stream, resulting in the formation of thin fibers and small pore size. Ultimately, the hydrophobic membranes showed improved waterproofness (108 kPa) and good water vapor permeability (9.2 kg m-2 d-1), which can be perceived as good alternatives for various applications. However, there are more and more evidences indicating that fluorinated hydrophobic agents with long perfluoroalkyl chain (-CnF2n+1, n ≥ 8) such as perfluorooctanoic acid (PFOA) and perfluorooctane sulphonate (PFOS) can resist degradation, accumulate in organisms, and possess the long biological half lives, which are harmful to both human beings and the environment.14-18 Therefore, the applications of organic fluorinated compounds with long perfluoroalkyl chain have already been strictly limited by many countries including Canada, America, and China. Fortunately, hydrophobic agents with short perfluoroalkyl chain (-CnF2n+1, n ≤ 6) don’t present notable bioaccumulation and persistence. On the contrary, they will be decomposed by the organism in a short period of time, and their breakdown products are innoxious. Currently, extensive research focus on developing environmental friendly fluorinated materials as substitutes for the traditional fluorine-containing polymers associated with


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PFOA and PFOS.19-21 In our previous study, we synthesized waterborne fluorinated polyurethane (WFPU) with short perfluorohexyl (-C6F13) chains, and utilized it as hydrophobic agent to modify polyacrylonitrile (PAN) nanofibrous membranes by post-treatment.22 The as-prepared fibrous membranes were endowed with good waterproofness (83.4 kPa), and high moisture vapor permeability (12.5 kg m-2 d-1). However, the disadvantages associated with the post-processing membranes are complicated treatment steps and inhomogeneity of the hydrophobic coating, fluorinated components by one-step electrospinning may provide a good solution for the abovementioned problems.

Scheme 1. (a) Chemical structure of the synthesized C6FPU. (b) Schematic illustration of fabrication process and performance demonstration of PU/C6FPU/MgCl2 waterproof and breathable membrane. In this work, we synthesized fluorinated polyurethane (C6FPU) containing short perfluorohexyl (-C6F13) chains, and fabricated environmental friendly waterproof and breathable membranes with robust performance by utilization of one-step electrospinning (Scheme 1). The wettability as well as porous structure of fibrous membranes were finely regulated by adjusting C6FPU and magnesium chloride (MgCl2) concentration, respectively. In particular, surface chemical compositions of fibrous



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membranes were investigated with the help of X-ray photoelectron spectroscopy (XPS), and a probable assumption about the dynamic behavior of perfluoroalkyl chains with the increasing C6FPU concentration was proposed. Moreover, the effects of MgCl2 concentration on the porous structure, water resistance, moisture vapor permeability, and mechanical property were comprehensively studied. As a result, the fibrous membranes with good hydrophobicity, high porosity and small pore size were fabricated, which presented excellent comprehensive properties.

2. EXPERIMENTAL SECTION 2.1. Materials. PU (Desmopan 9370AU, density=1.1 g cm-3) was obtained from Covestro CO., Ltd., Germany. Polytetrahydrofuran (PTMEG, Mn=1000), 4,4'-Methylenebisphenylisocyanate (MDI), triethylene glycol (TEG), N,N-dimethylformamide (DMF), dimethylacetamide (DMAc) and MgCl2 were supplied by Aladdin Chemical Reagent Co., Ltd., China. 2-Perfluorohexyl ethyl alchol (TEOH-6) was supplied by Fuxin Hengtong Fluorine Chemicals Co. Ltd. 2.2. Synthesis of C6FPU. In order to improve hydrophobic property, C6FPU with double terminal perfluorohexyl chains was synthesized through a stepwise polymerization reaction (as displayed in Figure S1, Supporting Information).23-26 In the preparation stage, PTMEG was dehydrated under vacuum at 110 °C for 5 h, in addition, 12.5 g purified MDI and 17 g DMF were mixed together at room temperature. In the first reaction step, 7.5 g PTMEG was added into a four-mouth flask under a dry nitrogen environment at 60 °C for 0.5 h and with a stirring speed of 150 rpm. Then the mixture of MDI and DMF was added dropwise (2 seconds per drop) into the flask, and the reaction was maintained at 60 °C for 0.5 h. In the second reaction step, chain extender TEG of 2 g was added dropwise into the above mixture, the temperature was kept at 65 °C for 1 h, which was the crucial stage in the whole synthesis process. In the end, 12.74 g TEOH-6 and 5 g DMF was added as end capping reagent into the reaction solution and stirred at 75 °C for 2 h. The obtained polymer was precipitated and washed to get


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the final C6FPU using the same method, as described in our previous work.13 The Fourier transform infrared (FT-IR) spectrum of C6FPU was presented in Figure S2, 1H and 19F nuclear magnetic resonance (NMR) spectra of C6FPU were presented in Figure S3. 2.3. Preparation of Electrospinning Solutions. Briefly, for preparation of PU/C6FPU solutions, PU and C6FPU were dissolved in DMAc with stirring for 10 h. The concentrations of C6FPU were 0, 1, 2, and 3 wt%, respectively. For preparation of PU/C6FPU/MgCl2 solutions, PU and C6FPU were added to the MgCl2/DMAc mixture and stirred for 10 h. The concentration of C6FPU was 2 wt%, and the concentrations of MgCl2 were 0.004, 0.008, 0.012, and 0.016 wt%, respectively. The concentration of PU was kept 12 wt% in all the solutions. 2.4. Fabrication of the Fibrous Membranes. The electrospinning process was carried out using the DXES-3 spinning equipment (Shanghai Oriental Flying Nanotechnology Co., Ltd., China). Polymer solutions were loaded into five plastic syringes, with the constant feed rate of 4 mL per hour. The continuous jet stream was generated under a high voltage of 50 kV, and the relevant fibers were deposited onto the paper-covered receiver at the tip-to-collector distance of 24 cm. The temperature and relative humidity of the electrospinning chamber were kept at 25 ± 2 °C and 80 ± 5%, respectively. The corresponding fibrous membranes were put in the vacuum oven for 2 h under 80 °C to remove the residual solvent, and for all fibrous membranes the thickness was 20 ± 2 µm. The resultant PU/C6FPU fibrous membranes containing various C6FPU concentrations were denoted as PU, PU/C6FPU -1, PU/C6FPU-2 and PU/C6FPU-3. The resultant PU/C6FPU-2/MgCl2 fibrous membranes containing different MgCl2 concentrations of 0.004, 0.008, 0.012, and 0.016 wt% were denoted as PU/C6FPU-2/MgCl2-1, PU/C6FPU-2/MgCl2-2, PU/C6FPU-2/MgCl2-3, and PU/C6FPU-2/MgCl2-4, respectively. 2.5. Characterization. The morpholoy of electrospun membranes were studied by scanning electron



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microscope (SEM, TESCAN VEGA 3, TESCAN Ltd., Czech Republic). Thickness of the membranes were obtained via CHY-C2 thickness gauge (Labthink Instruments Co., Ltd., China). The surface element composition of the fibrous membrane was analyzed using XPS (Perkin Elmer PHI 5000CESCA system) with Mg Kα (1253.6 eV) as the radiation source. And the X-ray gun was conducted with the applied voltage of 14 kV under a power of 250 W, and the takeoff angle was 90°. WCA of the fibrous membrane was characterized by a Kino SL200B Contact Angle Analyzer with the water volume of 3 µL. Porous structure was analyzed by CFP-1100AI capillary flow porometer purchased from Porous Materials Inc., USA. Porosity was calculated using the following formula:27 ( ) Porosity = ×100%

where represents the density of the raw material, signifies the density of fibrous membrane. 2.6. Measurements. The WVT rate which represented the breathable performance was measured according to ASTM E96 inverse cup standard under the certain temperature (38 °C), relative humidity (50%) and wind velocity (1 m s−1), and was tested by YG601H water vapor transmission tester (Ningbo Textile Instruments Co., Ltd., China). WVT rate values were calculated as following:28 WVT rate =

×24

where the unit of WVT rate is kg m−2 d−1, signifies the mass of test cup before test, is the mass of test cup after test, is the test area. According to the ASTM D 737 criterion, the air permeable properties were tested under a differential pressure of 100 Pa. The hydrostatic pressure which represented waterproof property was tested by YG812C hydrostatic pressure tester (Nantong Hongda Experiment Instruments Co., Ltd., China), based on AATCC 127 test criterion, at a 6 kPa min-1 pressure increasing rate. The results of WVT rate, air permeability, and hydrostatic pressure were determined by three samples. Mechanical performance was determined by a XQ-1C tensile tester (Shanghai New Fiber


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Instrument Co., Ltd., China). For each fibrous membrane, strip sample was stretched under 10 mm min-1 crosshead rate, and the sample width was 3 mm. The result was calculated by fifteen samples.

3. RESULTS AND DISCUSSION

Figure 1. SEM images of (a) PU, (b) PU/C6FPU-1, (c) PU/C6FPU-2, and (d) PU/C6FPU-3 fibrous membranes. 3.1 PU/C6FPU Fibrous Membranes. 3.1.1. Morphology. We designed fluorine-containing waterproof and breathable fibrous membranes based on three criteria: (1) the wettability of fibrous membranes should be hydrophobic to avoid the surface being wet when encountering water droplets, (2) the fibers should accumulate layer by layer to form a porous structure characterized by interconnected channels, high porosity as well as small pore size, and (3) the fibrous membranes must possess good mechanical properties which could be subject to external force to ensure normal use.29,30 The first requirement was achieved by the introduction of synthesized C6FPU, and the last two requirements were realized by electrospinning combing with regulation of electrospinning parameters. To obtain hydrophobic surface with small amount of addition contents, we adjust the concentration of C6FPU from 0 to 3 wt%, and the corresponding morphological changes were studied (Figure 1). The representative SEM images of PU/C6FPU fibrous membranes revealed as 3D porous structure, which consisted of randomly



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distributed fibers providing enormous amounts of interconnected channels for media to transmit. Obvious adhesion structures among contiguous fibers without C6FPU addition could be noted from Figure 1a, which could be attributed to the profuse residue of DMAc. After the introduction of C6FPU, adhesion structures among the fibers decreased gradually with the increasing of C6FPU concentration (Figure 1b-d), which could be a consequence of accelerated solidification of jet stream deriving from the decreased surface tension of electrospinning solutions, similar results have also been reported in previous studies.12,31 In addition, the average fiber diameter varied within the range of 470 to 520 nm as C6FPU concentration increased, which could be perceived as no essentially changed, since the viscosities and conductivities of PU/C6FPU solutions didn’t vary too much (Table S1, Supporting information).


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Figure 2. (a) XPS spectra and (b) WCA of PU/C6FPU fibrous membranes with different C6FPU concentrations.

Figure 3. Representation of the perfluoroalkyl chains behavior with the gradually increased C6FPU concentration before and after electrospinning process. 3.1.2. Surface Wettability. The wettability is a very significant property for waterproof and breathable membranes, which is determined by both chemical element composition and surface geometrical microstructure.32 XPS and WCA measurements were employed to analyze the surface property of PU/C6FPU membranes (Figure 2). Generally speaking, fluorinated segments prefer to migrate to the fiber surface, which contributed to the low surface energy.33-35 To provide insight into the influence of C6FPU concentration on surface chemistry composition of PU/C6FPU membranes, XPS characterization was carried out to measure the surface elemental composition with a depth of ~10 nm (Figure 2a). It is worth noting that all the PU/C6FPU membranes contain characteristic fluorine (685-695 eV for F1s) peak as expected, indicating that the fluorine atom distributed on the surface of electrospun fibers.5 The XPS data for the three PU/C6FPU membranes are presented in Table S2



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(Supporting information), fluorine/carbon (F/C) ratio increased as C6FPU concentration increasing from 1 to 2 wt %. However, further increased C6FPU concentration to 3 wt %, the F/C raio notably decreased from 0.75 to 0.69, which could probably be related to the aggregation of perfluoroalkyl chains. The proposed mechanism for perfluoroalkyl chains dynamic behavior with the gradually increased C6FPU concentration before and after electrospinning process is presented in Figure 3. In the beginning, the concentration of C6FPU is low, the number of perfluoroalkyl chains increased with the gradual addition of C6FPU. However, just like surfactants who possess the critical micelle concentration (CMC), as the C6FPU concentration reaches a certain critical value, there comes to a point where the perfluoroalkyl chains start to aggregate, which can be called critical aggregation concentration (CAC).36 Once above CAC, further addition of C6FPU only leads to increase in the number of aggregates in solution. Under such circumstances, fluorinated segments are encased in hydrogen segments, similar results have also been reported in previous study.37 Besides, fibers formed from the solutions at a fairly rapid speed in electrospinning process, and the whole process may be too fast for the fluorinated segments changing from wrapped to become unwrapped and move to the surface. Therefore, the F/C ratio decreased when C6FPU concentration was 3 wt%. WCA of PU/C6FPU fibrous membranes are exhibited in Figure 2b. PU membranes without C6FPU addition exhibited an initial angle of 105°, which was related to the intrinsic property of PU and rough surface of electrospun fibers. As C6FPU concentration increased to 1 wt%, the contact angle increased to 135°, which was ascribed to the sharply increased F/C ratio (Table S2). In contrast, the contact angle didn’t change obviously with the C6FPU concentration increased to 3 wt %, it is because WCA depends on two factors: one is chemistry composition, and the other is surface roughness, especially the latter plays a more important role in determining the WCA value when the F/C ratios have no evident difference.38-40


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Figure 4. (a) Pore size distribution, (b) dmax and porosity of PU/C6FPU fibrous membranes with different C6FPU concentrations. 3.1.3. Porous Structure. Except for wettability, the effects of C6FPU concentration on the pore size of PU/C6FPU membranes were also investigated. Since fibrous membranes belong to porous media, under a given external pressure, liquid water would give priority to the pore with larger diameter, thus the maximum pore size (dmax) was determined by bubble point test method. For fibrous membranes with same surface wettability but different pore size, dmax greatly influences the hydrostatic pressure, and the smaller dmax contributes to higher hydrostatic pressure.41,42 The porous structure including pore size distribution, dmax together with porosity of the relevant membranes were displayed in Figure 4. As presented in Figure 4a, pore size of PU/C6FPU membranes varied within the range of 0.39 to 7.06 µm,



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and there was an obvious increased tendency for the average pore size with the increasing of C6FPU concentration. dmax and porosity of PU/C6FPU membranes are shown in Figure 4b, as the C6FPU concentration increased, dmax of the fibrous membranes increased from 3.50 to 7.16 µm, and porosity also increased from 40.7% to 48.8%. These results can be ascribed to the decreased bonding points and the fluffy accumulated fibers with the increasing content of C6FPU.

Figure 5. (a) Hydrostatic pressure and WVT rate, (b) stress-strain curves of PU/C6FPU fibrous membranes with various C6FPU concentrations. 3.1.4. Waterproof, Breathable and Mechanical Properties. As we expected, the improved hydrophobic property and smaller dmax lead to the elevated hydrostatic pressure. The waterproofness of PU/C6FPU membranes with 0, 1, 2, and 3 wt% C6FPU were 4.8, 25.9, 28.6, and 20.3 kPa, respectively,


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which was the result of the combined effect of surface wettability and dmax. Meanwhile, as the concentration of C6FPU increased, the moisture vapor permeability increased from 12.2 to 13.0 kg m-2 d-1, as shown in Figure 5a. It could result from increscent porosity induced by the decreased adhesion structure, which brought about more interconnected channels for water vapor to transmit, thus leading to better moisture vapor permeability. Mechanical strength is a very critical property that can’t be ignored in practical application. In general, the fracture process under external force was bound up with the geometric arrangement and adhesion structures among the fibers. As can be seen from the SEM images (Figure 1), bonding structures could be the main factor that affected mechanical strength. Figure 5b displayed the stress-strain curves of PU/C6FPU fibrous membranes with various C6FPU concentration, which could be clarified by the two-step fracture mechanism.43 It can be described as follows: As a small initial external force is applied, the non-oriented fibers among the bonding points tend to align with the stretching direction, leading to the appearance of first nonlinear elastic behavior. With the force continuously increase, the curve displays a linear elasticity which is related to the intrinsic property of PU/C6FPU membranes. In the end, the disrupture of PU/C6FPU membranes derives from the break of aligned fibers among the bonding points. Consequently, PU fibrous membrane without C6FPU addition exhibited a tensile strength of 6 MPa, breaking elongation of 371.1%. And it took on a tendency of decrease with the increasing C6FPU concentration due to the declined bonding points. Considering the waterproofness, moisture vapor permeability, as well as mechanical strength, ultimately, we choose C6FPU concentration with 2 wt% for further study.



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Figure

6.

SEM

images

of

(a)

PU/C6FPU-2/MgCl2-1,

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(b)

PU/C6FPU-2/MgCl2-2,

(c)

PU/C6FPU-2/MgCl2-3, and (d) PU/C6FPU-2/MgCl2-4 fibrous membranes. 3.2 PU/C6FPU/MgCl2 Fibrous Membranes. 3.2.1. Morphology. The optimization of porous structure with small pore size and high porosity for fibrous membranes was realized by utilization of divalent metal salt MgCl2. Figure 6 showed the SEM morphologies of PU/C6FPU-2/MgCl2 fibrous membranes with various MgCl2 contents. PU/C6FPU-2 fibrous membranes without MgCl2 addition exhibited an average fiber diameter of 470 nm (as described above). The fibers stacked more densely and fiber uniformity became much better benefiting from the utilization of MgCl2 which improved the conductivity of electrospinning solutions (Figure 6a-d). When the MgCl2 concentration was 0.004 wt%, the membranes revealed a decreased average fiber diameter of 370 nm, which resulted from the enhanced stretching effect to the charged jet stream.44 As the MgCl2 contents increased to 0.012 wt%, the average fiber diameter reached the minimum value of 295 nm. However, further increased the MgCl2 concentration to 0.016 wt%, the fiber diameter increased to 380 nm, which could probably be due to the excess solution extruded from the tip resulted from the sharply increased conductivity (Table S3, Supporting information).


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Figure 7. (a) Pore size distribution, (b) dmax and porosity of PU/C6FPU-2/MgCl2 fibrous membranes with various MgCl2 concentrations. 3.2.2. Porous Structure. In the meantime, porous structures of PU/C6FPU-2/MgCl2 fibrous membranes with different concentrations of MgCl2 were investigated (Figure 7). The pore distribution displayed a gradually narrower tendency (Figure 7a), which could be interpreted by the thinner fibers and the improved uniformity with increasing MgCl2 concentration from 0.004 to 0.012 wt%, and the PU/C6FPU-2/MgCl2-3 fibrous membranes presented the narrowest pore size distribution. However, with the MgCl2 concentration increased to 0.016 wt%, the pore size distribution broadened, which could be ascribed to the increased fiber diameter of PU/C6FPU-2/MgCl2-4 fibrous membranes, similar results have also occurred with respect to dmax. PU/C6FPU-2/MgCl2-3 fibrous membranes showed the



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minimum dmax of 0.94 µm. On account of fibers tightly accumulated to form closely packed structures, the porosity of the PU/C6FPU-2/MgCl2 fibrous membranes declined compared with fibrous membranes without MgCl2 addition. The porosity decreased slowly from 30.1% to 25.7% with MgCl2 concentration increasing from 0.004 to 0.012 wt%, and then increased to 29.2% when the MgCl2 concentration was 0.016 wt%, which was consistent with the change tendency of fiber diameter. In addition, it should be noted that the WCA of PU/C6FPU-2/MgCl2 fibrous membranes didn’t change significantly as the MgCl2 concentration gradually increased (Figure S4, Supporting information).

Figure 8. (a) Hydrostatic pressure and WVT rate, (b) tensile strength and elongation of fibrous membranes with various MgCl2 concentrations. (c) Waterproof and breathable performance demonstration,

(d)

lyophobic

property,

and

(e)

large-scale

PU/C6FPU-2/MgCl2-3 fibrous membrane.


product’s

photograph

of

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3.2.3. Properties and Demonstration. To provide insight into the effects of surface wettability and porous structure on the comprehensive performances, the water resistance of the PU/C6FPU-2/MgCl2 fibrous membranes were investigated by testing hydrostatic pressure, as exhibited in Figure 8a. There was an obvious increase from 61.7 to 104.7 kPa of hydrostatic pressure following the increase of MgCl2 concentration from 0.004 to 0.012 wt%, which could be ascribed to the decreased dmax. However, when the MgCl2 concentration continuously increased to 0.016 wt%, the hydrostatic pressure decreased to 75.3 kPa on the contrary, which was caused by the enlarged pore size. Apart from water pressure resistance, vapor permeability was also vital performance for waterproof and breathable membranes. Figure 8a displayed that WVT rate had the same change tendency as that of porosity, PU/C6FPU-2/MgCl2 fibrous membranes exhibited WVT rate ranging from 11.5 to 11.7 kg m-2 d-1 indicating that PU/C6FPU-2/MgCl2 fibrous membranes possess good moisture vapor permeability. Moreover, the PU/C6FPU-2/MgCl2 fibrous membranes displayed good windproof property, and the air permeability was 4.70 mm s-1 with MgCl2 concentration increased to 0.012 wt% (Figure S5, Supporting information). Importantly, the PU/C6FPU-2/MgCl2-3 fibrous membranes exhibit relatively good comprehensive performance compared with the commercialized polyurethane films and coatings, and the properties are very close to the electrospun PU membranes with long perfluorakyl chain (Figure S6 and Table S4, Supporting information). Taking into consideration actual use process, mechanical properties of PU/C6FPU-2/MgCl2 waterproof and breathable membranes are investigated (Figure 8b). The tensile strength of PU/C6FPU-2/MgCl2 fibrous membranes containing MgCl2 concentrations of 0.004, 0.008, 0.012, 0.016 wt% were 9.7, 10.1, 12.4, and 10.7 MPa, respectively, and the elongation were 255.9%, 242.2%, 234.6%, and 245.2%, respectively. The increase in tensile strength resulted from the improved degree of orientation due to the increased conductivity, and the decrease in elongation could be attributed to the



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increased dimensional stability. Young’s modulus is a measure of the ease with which the fibrous membranes undergo deformation, for PU/C6FPU-2/MgCl2 fibrous membranes, it increased gradually from 5.8 to 9.1 MPa with the MgCl2 concentration increasing from 0.004 to 0.016 wt% (Figure S6, Supporting information), which could be ascribed to the efficient stretching effect at higher conductivity of solutions. While energy to break represents the toughness and durability of the fibrous membranes, when MgCl2 concentration increased from 0.004 to 0.012 wt%, it increased from 12.4 to 16.2 mJ m-3, and further increase MgCl2 concentration to 0.016 wt%, it declined to 15.5 mJ m-3.45 Therefore, PU/C6FPU-2/MgCl2-3 fibrous membranes exhibited the best mechanical property with high tensile stress of 12.4 MPa, large elongation of 234.6%, Young’s modulus of 9.1 MPa and energy to break of 16.2 mJ m-3. Moreover, the TGA curve displayed that PU/C6FPU-2/MgCl2-3 fibrous membranes also have good thermal stability, in which two different slopes of weight reduction can be observed. The first one is related to the breakage of urethane bonds, and the second one is related to the thermal decomposition of polyol.46-48 (Figure S8, Supporting information). Particularly, waterproof and breathable performance demonstration of the prepared membranes have been carried out, as exhibited in Figure 8c. The beaker filled with boiling water was covered by PU/C6FPU-2/MgCl2-3 fibrous membrane, and it was notable that water droplets were suspended on the hydrophobic surface, but a great amount of water vapor could pass through the membrane freely.49 As we all know, fluorine-containing hydrophobic agent can impart to the surface both hydrophobicity and oleophobicity, which is far superior to the fluorine-free hydrophobic agent.50,51 As a demonstration, the surface of fibrous membranes were tested with respect to hydrophobic and antifouling properties. Herein, colored water, oil, coffee and milk were employed to contaminate the surface (Figure 8d). When water droplets were dripped dropwise on the membrane, they could smoothly roll down the surface without leaving any trace. With respect to antifouling properties, a dribble of liquid including sunflower oil,


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coffee and milk slid off from the membrane without wetting the surface. More importantly, PU/C6FPU-2/MgCl2-3 fibrous membrane can be scaled up easily with the existing multijets electrospinning equipment, and a large-scale (65×60 cm) fibrous membrane was obtained (Figure 8e). By utilization of larger electrospinning device, it is extremely likely to achieve industrial production for a large quantity. As a result, PU/C6FPU-2/MgCl2-3 fibrous membranes with high performance could be perceived as a promising candidate for waterproof and breathable application.

4. CONCLUSIONS In conclusion, we have presented a facile method to fabricate environmental friendly waterproof and breathable membranes with robust performance. By utilization of a novel polymerization strategy, we successfully synthesized C6FPU hydrophobic agent with short perfluorohexyl chains. Through analysis of surface chemistry composition with the help of XPS, the plausible mechanism that perfluoroalkyl chains increased first then aggregated with the gradually increased C6FPU concentration was proposed. Benefiting from the introduction of MgCl2, uniform fibers and dramatically decreased dmax were achieved. In the end, the resultant PU/C6FPU-2/MgCl2-3 fibrous membranes exhibited excellent hydrostatic pressure (104 kPa), high WVT rate (11.5 kg m-2 d-1), as well as good tensile strength (12.4 MPa), suggesting their potential use in a variety of applications such as protective garments, medical equipment, sophisticated electronics, and building materials.

ASSOCIATED CONTENT Supporting Information Stepwise polymerization route of C6FPU (Figure S1). FT-IR spectrum of C6FPU (Figure S2). 1

19

F and

H NMR of C6FPU (Figure S3). Viscosity, surface tension and conductivity of PU/C6FPU solutions

(Table S1). XPS data of PU/C6FPU membranes with different concentration of C6FPU (Table S2). Viscosity, surface tension and conductivity of PU/C6FPU/MgCl2 solutions (Table S3). WCA of



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PU/C6FPU-2/MgCl2 membranes with different MgCl2 concentrations (Figure S4). Air permeability of PU/C6FPU-2/MgCl2 membranes with various MgCl2 concentrations (Figure S5). Comparison of waterproof and breathable performance for electrospun membranes and commercialized polyurethane materials. (Figure S6). Comparison of waterproof and breathable performance for PU membranes with different perfluorakyl chain (Table S4). Young’s modulus and energy to break of PU/C6FPU-2/MgCl2 fibrous

membranes

with

various

MgCl2

concentrations

(Figure

S7).

TGA

curve

of

PU/C6FPU-2/MgCl2-3 fibrous membrane (Figure S8).

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

Zhao and Yang Li contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51473030, 51503028 and 51673037), the Shanghai Rising-Star Program (No. 16QA1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2016019), the National Key R&D Program of China (No. 2016YFB0303200) and the Fundamental Research Funds for the Central Universities.

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TOC Graphic 95x38mm (600 x 600 DPI)


Environmentally Friendly and Breathable Fluorinated Polyurethane

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