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Preparation of Electrospun Polyurethane/Hydrophobie Silica Gel Nanofibrous Membranes for Waterproof and Breathable Application.

INTRODUCTION

Electrospinning microporous membranes with high waterproofness and good breathability have attracted tremendous attention, owning to the characteristics that can restrict the permeation of liquids and toxic gas, but allow the water vapor to escape [1]. In general, they have a lot of potential applications such as protective clothing, membrane distillation, filter, and tissue engineering [2-5]. According to the previous studies, methods for fabrication of the membranes with excellent hydrophobicity were mainly done by decorating the surface with low-surface energy materials and controlling the surface structure of the membranes. For example, Jin et al. [6] prepared breathable and superhydrophobic polyurethane (PU) electrospun webs with silica nanoparticles. Li et al. [7] fabricated fluorinated PU nanofibrous membranes with carbon nanotubes via electrospinning, in order to enhance the waterproofness and mechanical properties of membranes. Wang et al. [8] reported the tortuously structured poly(vinyl chloride)/PU fibrous membranes for high efficiency fine particulate filtration. They found the porous structure of nanofibrous membranes could be regulated by tuning the diameter and packing density.

Electrospinning, as a versatile and cost-effective technique, can be used to produce continuous polymer fibers with diameters ranging from nanometer to submicron scale, which exhibits the high porosity and large specific surface area [9, 10], These nanofibers can be expected to have potential applications in several fields such as filtration, oil-water separation, and antifouling [11-13], Recently, some researchers have investigated the combinations of the polymer and inorganic compounds to improve the surface properties of membranes, such as fluorinated polystyrene)/single-walled carbon nanotubes, PU/Ti[O.sub.2]-fly ash, and bio-based polyurethane (BPU)/m-silica [14-16]. To the best of our knowledge, only few reports were about the fabrication of electrospun composite nanofibers modified by hydrophobic silica gel (HSG). The main goal of this study was to develop PU/HSG composite fibrous membranes with high tensile strength, desirable thermal properties, and excellent waterproof-breathable performance.

PU possesses a wide range of desirable properties such as high hydrophobicity, adjustable pore structure, and reinforced mechanical properties, which have already been used in manufacturing industry [17-20], However, the waterproofness of pure PU nanofibrous membranes remains insufficient to allow them to compete with commercial membranes in practical applications [21], Therefore, it is necessary to make the surface modification of PU nanofibrous membranes. Due to the simplicity and low-cost of the manufacturing process, the incorporation of HSG and the thermal treatment provide a typical strategy for preparation of high-performance composite membranes.

In this study, the HSG was prepared via modifying silica sol-gel with the dimethyldiethoxysilane (DDS) to achieve a hydrophobic surface. After the introduction of HSG, the surface of PU fibers was enriched with silicon-containing segments and showed hydrophobic property, which increased considerably the mechanical, thermal, and waterproof-breathable properties of the membranes. In addition, the thermal post-treatment at the optimized temperature endowed the PU/HSG nanofibrous membranes with higher waterproofness and breathability. The morphology, porosity, pore size, water resistance, and water vapor permeability of PU/HSG nanofibrous membranes have been systematically investigated by adjusting the HSG contents and temperatures of heat treatment. The novel nanofibrous composite membranes have great potential to be used as the functional materials for fabricating separation media, filter, outdoor sportswear, and chemical protective clothing.

EXPERIMENTAL

Materials

PU ([M.sub.w]= 150,000 g/mol) was obtained from BASF Co., Ltd., China. N,N-dimcthylformamide (DMF), ethanol, hydrogen chloride (HCl), and ammonia were supplied by Hangzhou Gaojing Fine Chemical Co., Ltd., China. Tetraethoxysilane (TEOS, Si[O.sub.2] content [greater than or equal to] 28%) and DDS were purchased from Shanghai Aladdin Chemical Co., China. All chemicals were of analytical grade and were used as received without further purification.

Synthesis of HSG

First, a certain amount of TEOS was dissolved in 5 mL of ethanol solution (2 mol/L), followed by addition of 1 mL HCl solution (2 mol/L) drop by drop. The mixture was stirred for 5 h in a 50[degrees]C water bath and then 0.5 mL N[H.sub.3][H.sub.2]O (2 mol/L) was slowly added. The homogenous silica sol-gel solution was obtained by vigorously stirring for another 5 h at room temperature. Then 0.5 mL DDS was dropped into the solution and the reaction was allowed to proceed at 70[degrees]C for 24 h, which removed most of the hydrophilic hydroxyl groups of the silica gel surface. The resultant HSG was washed twice in sequence with toluene and absolute ethanol. Finally, the obtained HSG was stored at 25[degrees]C and 50% RH.

Preparation of PU/HSG Composite Fibrous Membranes

The obtained HSG were scattered in the 9 mL DMF with ultrasonic treatment for 5 h at ambient temperature. Then, PU was dissolved in the above solution, followed by continuous mechanical mixing for 48 h to obtain the homogeneous PU/ HSG composite solutions. The mixture of colloid and PU was further homogenized via ultrasonic treatment for 2 h prior to the electrospinning process. The concentration of PU/HSG composite solutions was 18 wt%, and the amount of HSG was controlled to 1,3, and 5 wt% with respect to the polymer PU.

The PU/HSG solutions were electrospun at a feed rate of 0.6 mL/h by using the spinning equipment (KDS100, KD Scientific Inc., USA), as shown in Fig. 1a. A high voltage of 15 kV was applied to the needle tip and the distance between the spinneret and rotating collector (rotating rate of 300 rpm/min) was 15 cm. The relevant temperature and humidity of environmental parameters were 25[degrees]C [+ or -] 2[degrees]C and 50% [+ or -] 2% RH. After electrospinning, the membranes were dried under vacuum for 2 h to remove the residual solvent, and the thickness of obtained membranes was 30 [+ or -] 5 [micro]m. Then, the electrospun membranes were treated with the temperatures of 60, 80, 100, and 120[degrees]C for 5 h. The hydrophobic modification of PU/HSG membranes is shown in Fig. 1b and the corresponding membranes were denoted as PU/HSG-x, where x (wt) is the concentration of HSG.

Characterization for Fibrous Membranes

The viscosity and the conductivity of the solutions were examined by viscometer (NDJ-9S, Shanghai Pingxuan Scientific Instruments Co., Ltd., China) and the conductivity meter (DDS-307A, Yidian Scientific Instruments Co., Ltd., China), respectively. FTIR spectra were performed by the Nicolet Fourier Transform spectrophotometer (AVATAR 5700, US) with an attenuated total reflection (ATR) accessory. The chemical composition of the modified silica gel was investigated by using the X-ray photoelectron spectroscopy (XPS, K-Ahpha, USA), with the X-ray source of Al K[alpha] at 1,486.6 eV and 300 W. Scanning electron microscopy (FE-SEM, Ultra55, Germany) equipped with an energy dispersive X-ray spectroscopy (EDS) detector was used to observe the fibrous morphology and determine the elemental distribution of nanofibers. The mechanical properties were measured by applying Multifunctional Mechanical Tester (KES-G1, KATO TECH, Japan) at a speed of 0.3 mm/s. Rectangular specimens were cut into 30 mm x 5 mm and the tests were repeated five times to ensure the reproducibility. Thermal characterizations were investigated by using a thermogravimetric analyzer (PerkinElmer Pyris 1 series) under a nitrogen atmosphere at a heating rate of 10[degrees]C/min. The BET surface areas of samples were measured using [N.sub.2] adsorption-desorption isotherms with a surface area analyzer (ASAP 2020 HD88 Research-grade ultra-high performance fully automatic gas adsorption system, USA). The pore size distribution was characterized by bubble point method using a capillary flow porometer (CFP-1500AE, Porous Materials, Inc. USA). All electrospun mats of 30 mm x 30 mm (thickness d = 30 [+ or -] 5 pm) were used in this study. Porosity of the fibrous membranes was measured by the following equation

porosity = (1- [[rho].sub.1]/[rho]) X100% (1)

where [rho] is the density of the raw PU materials and [[rho].sub.1] is the density of the obtained fibrous membranes.

Measurements of Waterproof-Breathable Performances

Static water contact angle (WCA) was measured by using a video contact angle measuring instrument (JY-82B, Chengde DingSheng Test Machine Equipment Co., Ltd., China). A droplet of deionized water (3.0 [micro]L) was dropped on the surface of the film at room temperature. The air permeability was measured according to ASTM standard test method by using an air permeability tester (YG461E, Wenzhou Fangyuan Instruments Co., Ltd., China), with a pressure drop of 100 Pa. The hydrostatic pressure of the membranes was examined according to AATCC 127 standard test methods by using a Hydro Tester (FX 3000-IV, TEXTEST AG Zurich/Switzerland), with a water pressure increasing rate of 6 kPa/min. The water vapor transmission (WVT) rate was evaluated according to the GB/T 12704.22009 standard by using a fabric moisture permeability testing instrument (YG(B)216-X, Wenzhou DaRong Textile Instrument Co., Ltd., China), under the conditions of 38[degrees]C and 90% RH. The WVT rate was calculated with the following equation:

WVT = [[DELTA]m - [DELTA]m']/[A x t] (2)

where WVT rate is in g/[m.sup.2]/day, Am is the quality of each sample after the test, [DELTA]m' is the quality of each sample before test, A is the permeation area ([m.sup.2]), and t is the time of testing (1 h). Each type of samples was tested for three times and the results were expressed as mean [+ or -] SD.

RESULTS AND DISCUSSION

Structural Characterizations

Hydrophobic modification of the silica gel was performed by removing hydrophilic hydroxyl groups and immobilizing the dimethyldiethoxy groups at the end of the gel network, as shown in the Fig. 1c. The groups generated at the end of the silica gel increased the compatibility and dispersibility of HSG in the organic solvent, which was beneficial to fabricate the uniform electrospun solution, and the interaction between PU and HSG could be enhanced. The elemental composition of the HSG was characterized by the XPS analysis. As displayed in Fig. 2a, the detected atoms molar ratio of C, O, and Si on the surface of HSG confirmed that the chemical reaction occurred between the hydroxy group and an ethoxy group. In the XPS C's core-level spectrum of HSG (Fig. 2b), curve-fitted peaks of C--H, C--Si, and C--O bonds were observed at 284.5, 285.1, and 286 eV, respectively. Figure 2c presents the Ols spectrum consisting of of three fitted curves with binding energies of 532.3 (C=0), 532.9 (C-0), and 533.6 eV (Si-O). Meanwhile, Si2p curve could be resolved into three component peaks of 102.5 (O-Si-O), 103.5 (C-Si), and 104.2 eV (Si[(O).sub.4]) [22, 23], as depicted in Fig. 2d.

Also the FTIR spectra of HSG have been carried out in Fig. 3a in order to further investigate the DDS groups in modified silica gel. Typical peaks of the alkyl groups were observed at ~2960 and 2840 [cm.sup.-1], which were attributed to the C--H symmetric and asymmetric stretching vibration. This result demonstrated the successful modification of the silica gel, which was consistent with the XPS analysis. In addition, the FTIR (ATR) spectra of PU/HSG (0, 1, 3, and 5 wt%) composite mats are presented in Fig. 3b. The typical absorption feature for carbamate group was found at 3330 [cm.sup.-1]. Peaks at 1724 and 1220 [cm.sup.-1] were assigned to C=0 and C--O stretching vibration [24, 25]. The absorption at 1096 [cm.sup.-1] was attributed to the Si--O--Si vibrational peak and the O--Si--O gave the absorption peak at 774 [cm.sup.-1] [26], Four spectra exhibited little difference except the absorptive intensity at 1724 and 774 [cm.sup.-1]. The intensity of the peak at 774 [cm.sup.-1] was obviously increased with increasing the HSG content from 1 to 5 wt%. Enhanced peaks of N--H and C--O groups could be a response to the polar functionalities or the amidation reactions between free amine and hydroxyl, which indicated the physical or chemical interaction existing between PU and HSG [27, 28].

Furthermore, the surface chemical composition of PU/HSG composites was investigated by using the EDS, as shown in Fig. 4. The spectrum of the HSG exhibited the following atomic composition (Fig. 4a): 25.41% C, 36.74% O, and 27.73% Si. In comparison to the original PU fibers (Fig. 4b), PU/HSG composite membranes had the obvious Si contents, as shown in Fig. 4c, which indicated that the HSG was combined into the PU fibrous membranes during the electrospinning process. As seen in Fig. 4d, corresponding elemental mapping of PU/HSG-3% composite membranes provided the information on distribution of the C, N, O, and Si atoms on the individual fiber. Si element mapping corresponded to the distribution of HSG on the fiber and these results were coherent with the XPS and FTIR experiment results.

Surface Morphology and Microstructure

SEM images of PU/HSG nanofibrous membranes obtained with various HSG contents and the corresponding diameter distribution histograms are presented in Fig. 5. As shown in Fig. 5a, pure PU nanofibers showed relatively smooth surfaces and homogeneous characteristics in the fiber diameter. As the HSG loading in the hybrid increased from 1 to 5 wt% shown in Fig. 5b-d, there were more protrusions formed on the fibers surface and the nanofiber diameter became fine accompanied with an irregular distribution. Compared with the pure PU nanofibrous membranes, the PU/HSG composite membranes provided a high ratio of surface area to volume (Fig. 6d). It could be speculated that the increase in specific surface area is closely related to the decoration of HSG. Hence, the as-prepared nanofibrous membranes with nonuniform structure and low-surface energy were expected to exhibit excellent hydrophobic properties [29].

As shown in Fig. 5, the average fiber diameter of PU/HSG fibrous membranes formed with 0, 1, 3, and 5 wt% HSG contents were 639, 411, 331, and 434 nm, respectively. It indicated that the fiber diameter would be obviously decreased with increasing the HSG contents. The solution parameters have been investigated in previous studies, which greatly influence the morphology of fibrous membranes [30, 31]. As the viscosity and conductivity of solutions have a contrary effect on electrospun fiber diameter. Therefore, the increase or decrease in fiber diameter was decided by the competition between solution viscosity and conductivity. As can be seen in Table 1, the viscosity and the conductivity of the solutions were both increased with increasing the concentrations of HSG. When the HSG content reached to 3 wt%, the optimized fiber diameter (331 nm) could be achieved.

Apart from that, the porous structure of PU/HSG nanofibrous membranes would be affected by the addition of HSG. Figure 6a indicated that the pore sizes of all the samples were in the range of 0.21-1.10 [micro]m, and the smallest pore size (0.48 [micro]m) could be obtained with the HSG content of 3 wt%. The [d.sub.max] (the maximum pore size) and porosity are also important factors in determining the waterproof and breathable performance of the membranes [32]. As depicted in Fig. 6b and c, the [d.sub.max] (from 1.4 to 0.85 [micro]m) and the mean pore size (from 0.85 to 0.53 [micro]m) of the relevant membranes were dramatically reduced due to the decrease in fiber diameter. However, the porosity of composite membranes was almost unchanged at gradually increasing the HSG content, which can facilitate the membranes to keep high breathability.

Mechanical and Thermal Properties

Figure 7a shows the typical tensile stress-strain curves of the PU/HSG nanofibrous membranes. The tensile strength, Young's modulus, and elongation at break are listed in Table 2. In comparison with the pure PU, the electrospun PU/HSG composite membranes exhibited an increase in the tensile strength and Young's modulus with increasing the amount of HSG. When the concentration of HSG was 5 wt%, the breaking strength of PU/HSG membranes increased from 5.1 to 6.9 MPa, which resulted from the increased adhesive structure between the fibers and HSG. According to the FTIR spectra, well physical or chemical interaction between PU and HSG ensures the efficient load transfer from polymer matrix to the PU/HSG fibers, which lead to high mechanical stabilities of composite membranes [33, 34], In addition, the better mechanical properties also confirmed the existence of hydrogen bonding between the polymer and colloid web or the amidation reactions between free amine and hydroxyl [35].

To evaluate the effect of HSG on the thermal stability, TGA curves of electrospun PU/HSG nanofibrous membranes containing different amounts of HSG are shown in Fig. 7b. In this figure, one can see the single step thermal degradation of pure PU nanofibers was started from about 295[degrees]C and completed at about 550[degrees]C. The curves of PU/HSG fibrous membranes could be roughly divided into two main stages according to the weight loss. The initial decomposition occurred below 310[degrees]C, which was attributed to the irreversible breakage of the urethane linkages. And the second stage from 425[degrees]C to 450[degrees]C results from the decomposition of HSG side chain [36, 37], It indicated that the thermal stabilities of the PU/HSG nanofibers were gradually improved with the increasing amount of HSG [38], The reason is that the HSG could provide more physical barrier, which was conducive to limit the transfer of thermal energy.

Waterproof and Breathable Performances

As shown in Fig. 8a, the WCA of PU/HSG composite membranes with 0, 1, 2, 3, and 5 wt% HSG were 109.2[degrees], 119.5[degrees], 126.3[degrees], 133.6[degrees], and 130.2[degrees], respectively. The WCA of the membranes increased as the amount of HSG increased, which can be ascribed to the effect of the surface roughness. When the HSG contents increased from 3% to 5%, the WCA were gradually decreased due to the increase in fiber diameter and the pore size [39]. With the increasing of HSG concentration, surface energy could effectively decrease and increase the surface geometry of the composite membranes, and thus leading to the high water resistance. As shown in Fig. 8b, the hydrostatic pressure of PU/ HSG fibrous membranes was regularly increased from 1.51 to 4.26 kPa with the addition of HSG. As displayed in Fig. 8c and d, with the concentration of HSG increasing to 5 wt%, the WVT rate and air permeability were, respectively, decreased to 7.5 kg/[m.sup.2]/day and 8.7 L/[m.sup.2]/s remarkably. This would be attributed to that membrane with the decreased porosity possessed less interconnected passageways for WVT [40].

Thermal Treatment

The waterproofness and breathability of PU/HSG nanofibrous membranes were further investigated by heat treatment with different temperatures. Figure 9 presents that the morphology and diameter of PU/HSG nanofibrous membranes were critically affected by tuning the temperatures. When the temperature elevated to 100 or 120[degrees]C, which was between the glass transition ([T.sub.g]) and melting, the bonding between the nanofibers came into forming and gradually increased as shown in Fig. 9d and e. It was clearly observed that the fiber diameter of PU/HSG-3% composite membranes gradually decreased from 331 to 325 nm with increasing the temperatures of heat treatment. It can be assigned to that the adhesion structure among adjacent fibers could make the molecular chains stretch and rearrange under the effect of thermal treatment [41].

As shown in Fig. 10b, when the temperatures increased to 120[degrees]C, the WCA of PU/HSG-3% nanofibrous membranes was 142[degrees] exhibiting the hydrophobic surface of the electrospun mats. As can be seen in Fig. 10c, the hydrostatic pressure increased obviously from 4.3 to 5.45 kPa on increasing the temperatures, which was due to the decrease of [d.sub.max] (from 0.87 to 0.64 [micro]m) [42]. As depicted in Fig. 10a, the higher porosity (58%) could be obtained at the temperature of 120[degrees]C, which provided more interconnected passageways for vapor transmission. As a result, the WVT rate (from 7.02 to 8.01 kg/[m.sup.2]/day) and air permeability (from 9.01 to 9.20 L/[m.sup.2]/s) slightly enhanced because of the increased porosity, as displayed in Fig. 10d. These investigations indicated that the WCA, WVT rate, and air permeability could be regulated by adjusting the HSG content and temperatures of heat treatment, which provides a facile and efficient method for producing the fibrous membranes with various waterproofness and breathability.

CONCLUSIONS

In this article, the transformation of silica gel from hydrophilicity to hydrophobicity was realized through chemical treatment with DDS. The obtained PU/HSG composite nanofibrous membranes with hydrophobic surface and optimized porous structure possessed the excellent waterproof, breathable, and mechanical performance. Porous structures (pore size and porosity) of the fibrous membranes were systematically optimized by tuning the concentration of HSG as well as the heating temperature. Overall, PU/HSG-3% composite fibrous membranes with the heating temperature of 120[degrees]C exhibited good waterproofness of 5.45 kPa, large air permeability of 9.20 L/[m.sup.2]/s, and high mechanical properties with tensile strength of 9.8 MPa. Moreover, the asprepared fibrous membranes presented good thermal stability with the high temperature of thermal degradation, which would be greatly useful in some fields such as protective clothing, water purification, and tissue engineering.

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Xianyuan Gu (ID), (1,2) Ni Li (ID), (1,2,3) Jin Cao, (1) Jie Xiong (1,2)

(1) College of Materials and Textile, Zhejiang Sci-Tech University, Hangzhou, China

(2) Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Zhejiang Sci-Tech University, Hangzhou, China

(3) Department of Materials Engineering, University of British Columbia, Vancouver, Canada

Correspondence to: J. Xiong; e-mail: jxiong@zstu.edu.cn Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 11272289; contract grant sponsor: Zhejiang Provincial Natural Science Foundation of China; contract grant number: LY16E030007

DOI 10.1002/pen.24726

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Schematic illustration of (a) electrospinning, (b) preparation of the PU/HSG composite membranes, and (c) terminal modification of silica gel. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. (a) XPS wide-scan spectra for HSG. High resolution XPS spectra of (b) C Is, (c) O Is, and (d) Si 2p for HSG. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. (a) FTIR spectra of the HSG and (b) FTIR (ATR) spectra of PU/HSG with various weight ratios (0, 1, 3, and 5 wt%). fColor figure can be viewed at wiIeyonlinelibrary.com]

Caption: FIG. 4. EDS analyses of (a) HSG, (b) Pure PU, and (c) PU/HSG-3. (d) Elemental mapping images of the selected area on PU/HSG-3% nanofibers. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. SEM images and fiber diameter distribution of PU/HSG fibrous membranes with various HSG contents: (a) 0, (b) 1, (c) 3, and (d) 5 wt%.

Caption: FIG. 6. (a) Pore size distribution, (b) [d.sub.max] and porosity, (c) mean pore size, and (d) BET surface area of PU/HSG fibrous membranes containing different contents of HSG. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. Stress-strain curves and TGA curves of the nanofibrous mats: (a) Pure PU, (b) PU/HSG-1%, (c) PU/HSG3%, (d) PU/HSG-5%. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. (a) WCA, (b) hydrostatic pressure, (c) WVT rate, and (d) air permeability of the composite membranes fabricated from various contents of HSG. [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 9. SEM images of PU/HSG-3% membranes with different temperatures of thermal treatment: (a) untreated, (b) 60, (c) 80, (d) 100, and (e) 120[degrees]C.

Caption: FIG. 10. (a) [d.sub.max] and porosity, (b) WCA, (c) hydrostatic pressure, and (d) WVT rate and air permeability of PU/HSG-3% composite membranes with various temperatures. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Properties of PU, PU/HSG solutions, and mean
fiber diameter of the membranes with different
concentrations of HSG.

HSG content   Conductivity    Viscosity
(wt%)         ([micro]S/cm)    (MPa s)

0                 2.31          2350
1%                2.89          2812
3%                3.16          3215
5%                3.25          3642

HSG content   Mean fibrous    Standard deviation of
(wt%)         diameter (nm)    mean fibre diameter

0                  639                139.3
1%                 411                182.1
3%                 331                70.5
5%                 434                 174

TABLE 2. Mechanical properties of PU/HSG-x
composite nanofibrous membranes.

           Tensile strength     Elongation at          Young's
Sample          (MPa)             break (%)         modulus (MPa)

PU/HSG-0   5.1 [+ or -] 0.2    119 [+ or -] 5.6    1.5 [+ or -] 0.1
PU/HSG-1   5.7 [+ or -] 0.1   114.7 [+ or -] 3.8   1.8 [+ or -] 0.2
PU/HSG-3   6.3 [+ or -] 0.3   110.9 [+ or -] 6.8   2.1 [+ or -] 0.2
PU/HSG-5   6.9 [+ or -] 0.2   103.3 [+ or -] 2.6   2.3 [+ or -] 0.1
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Author:Gu, Xianyuan; Li, Ni; Cao, Jin; Xiong, Jie
Publication:Polymer Engineering and Science
Article Type:Report
Date:Aug 1, 2018
Words:4992
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