Preparation of Nanofiber Bundles via Electrospinning Immiscible Polymer Blend for Oil/Water Separation and Air Filtration
通过静电纺丝不混相聚合物混合物制备纳米纤维束，用于油/水分离和空气过滤

Abstract 抽象

Nanofiber bundles with specific areas bring a new opportunity for selective adsorption and oil/water or air separation. In this work, nanofiber bundles were prepared by the electrospinning of immiscible polystyrene (PS)/N-trifluoroacetylated polyamide 6 (PA6-TFAA) blends via the introduction of carbon nanotubes (CNTs) or a copolymer of styrene and 3-isopropenyl-α, α’-dimethylbenzene isocyanate (TMI), which was denoted as PS-co-TMI. Herein, CNT was used to increase the conductivity of the precursor for enhancing the stretch of PS droplets under the same electric field, and PS-co-TMI was used as a reactive compatibilizer to improve the compatibility of a PS/PA6-TFAA blend system for promoting the deformation. Those obtained nanofiber bundle membranes showed an increase in tensile strength and high hydrophobicity with a water contact angle of about 145.0 ± 0.5°. Owing to the special structure, the membranes also possessed a high oil adsorption capacity of 31.0 to 61.3 g/g for different oils. Moreover, it exhibits a high potential for gravity-driven oil/water separation. For example, those membranes had above 99% separation efficiency for silicon oil/water and paraffin wax/water. Furthermore, the air filtration efficiency of nanofiber bundle membranes could reach above 96%, which might be two to six times higher than the filtration efficiency of neat PS membranes.
具有特定面积的纳米纤维束为选择性吸附和油/水或空气分离带来了新的机会。在本工作中，通过引入碳纳米管 （CNT） 或苯乙烯和 3-异丙烯基-α、α'-二甲基苯异氰酸酯 （TMI） 的共聚物，通过静电纺丝制备了不混溶的聚苯乙烯 （PS）/N-三氟乙酰化聚酰胺 6 （PA6-TFAA） 混合物，其表示为 PS-co-TMI。本文使用 CNT 增加前驱体的导电性，以增强 PS 液滴在相同电场下的拉伸，并使用 PS-co-TMI 作为反应增容剂来提高 PS/PA6-TFAA 共混体系的相容性，以促进变形。获得的纳米纤维束膜显示出拉伸强度增加和高疏水性，水接触角约为 145.0 ± 0.5°。由于特殊的结构，膜对不同的油也具有 31.0 至 61.3 g/g 的高油吸附能力。此外，它在重力驱动的油/水分离方面表现出很高的潜力。例如，这些膜对硅油/水和石蜡/水的分离效率超过 99%。此外，纳米纤维束膜的空气过滤效率可达 96% 以上，可能比纯 PS 膜的过滤效率高 2 到 6 倍。

1. Introduction 1. 引言

Nanofiber membranes have high specific surface areas, highly interconnected pore structures, nano scale pore sizes, the potential to incorporate active chemistry on a nanoscale, and low initial solidification [1]. However, the fibers in the nonwoven mats are randomly arranged, which results in low mechanical strength and other disadvantage. In order to improve the desirable properties of nanofibers and expand the application of nanofiber membranes, more and more research tend to the fabrication and application of aligned nanofiber bundles or yarns. The nanofiber bundles and yarns are of significant value in a wide range of applications, including tissue engineering [2,3], drug release [4,5], sensors [6,7,8], reinforced composites [9,10] and filtration [3,11].
纳米纤维膜具有高比表面积、高度互连的孔结构、纳米级孔径、在纳米尺度上掺入活性化学的潜力以及低初始凝固 [1]。然而，无纺布毡中的纤维是随机排列的，这导致了机械强度低和其他缺点。为了提高纳米纤维的理想性能并扩大纳米纤维膜的应用，越来越多的研究倾向于制备和应用排列的纳米纤维束或纱线。纳米纤维束和纱线在广泛的应用中具有重要价值，包括组织工程[2,3]、药物释放[4,5]、传感器[6,7,8]、增强复合材料[9,10]和过滤[3,11]。

Electrospinning is one of most simple and versatile ways to fabricate nonwoven membranes with fibers varying from micro to nano scales. With different devices and methods, nanofiber bundles could be achieved by electrospinning [12,13,14], including the auxiliary electrode collecting method [15,16,17], self-bundling electrospinning method [4,18], water bath-collecting method [19,20], etc. For example, Wang and his co-workers [13] produced self-assembling nanofiber yarn bundles via a swirling collector. When the electrospinning started, the nanofibers broke off from the jet and whipped from the collector, as the collector kept swirling. The nanofiber bundles formed between the fiber tips and the ground plate by whipping around the anchor point and contact adjacent bundles. Guan et al. [21] prepared PA66 fiber bundles with two opposite electrode pins as the collector. After post-treatment with 0.05 wt% multiwall carbon nanotubes (MWCNTs), the electric conductivity and tensile strength rapidly increased to 0.2 S/cm and 103 MPa. Wang et al. [22] fabricated polyacrylonitrile (PAN) fiber yarns with the self-bundling electrospinning method. Compared with normal nonwoven PAN membranes, the tensile strength, tensile module and elongation at break of the fiber yarn membrane increased more than 320%, 2670% and 260%, respectively.
静电纺丝是制造纤维从微米到纳米不等的无纺布膜的最简单、最通用的方法之一。通过不同的设备和方法，可以通过静电纺丝[12,13,14]实现纳米纤维束，包括辅助电极收集法[15,16,17]、自捆绑静电纺丝法[4,18]、水浴收集法[19,20]等。例如，Wang 和他的同事 [13] 通过旋转收集器生产了自组装的纳米纤维纱线束。当静电纺丝开始时，纳米纤维从射流中脱落并从捕收机中抽出，因为捕收剂不断旋转。纳米纤维束通过围绕锚点摆动并在接地板之间形成并接触相邻束。Guan 等 [21] 制备了 PA66 纤维束，其中两个相反的电极针作为集电极。用 0.05 wt% 的多壁碳纳米管 （MWCNT） 后处理后，电导率和拉伸强度迅速提高到 0.2 S/cm 和 103 MPa。Wang等[22]用自捆绑静电纺丝法制备了聚丙烯腈（PAN）纤维纱线。与普通非织造PAN膜相比，纤维纱膜的拉伸强度、拉伸模量和断裂伸长率分别提高了320%、2670%和260%以上。

Recently, functional fibers with complex structures, such as islands-in-the-sea [23], cocontinuous [24] and core–sheath [25] structures could be gained by the electrospinning of polymer blends through controlling the phase separation. For example, Tang et al. [26] have prepared PS/PA6 bead-free fibers by electrospinning of the PS/PA6-TFAA blend, which was ascribed to the good electrospinnability of PA6-TFAA at low concentration. Furthermore, the core–shell superfine fibers could be prepared by introducing with an interfacial compatibilizer, which could promote the dispersion of minor components during microphase separation. Wang et al. [27] electrospun liquid crystal/polymer core–sheath fibers formed by phase separation. They could change the width of the polymer sheath, and the diameter of the liquid crystal core increases with controlling the feed rates. In addition, they found that the viscosity gradient resulted in an inward movement of the lower viscosity component, which lead to the core/sheath structure. However, there has been little effort made to prepare nanofiber bundles toward the control of phase separation in electrospinning.
近年来，通过控制相分离，聚合物共混物的静电纺丝可以获得具有复杂结构的功能性纤维，如海中岛[23]、共连续[24]和芯鞘[25]结构。例如，Tang等[26]通过静电纺丝制备了PS/PA6-TFAA共混物的无珠纤维，这归因于PA6-TFAA在低浓度下具有良好的静电纺丝性能。此外，可以通过引入界面相容剂来制备核壳超细纤维，这可以促进微相分离过程中微量组分的分散。Wang等[27]通过相分离形成的静电纺液晶/聚合物芯鞘纤维。它们可以改变聚合物护套的宽度，并且液晶芯的直径会随着进给速率的增加而增加。此外，他们还发现粘度梯度导致较低粘度成分向内移动，从而导致核心/护套结构。然而，很少有人努力制备纳米纤维束以控制静电纺丝中的相分离。

The membrane technique, especially electrospun membranes, has been widely used in the separation of oil–water mixtures with property of high oil removal efficiency and stable effluent quality, which have been applied in food processing, pharmaceutical desalination and fuel cell industries [1,28]. A high hydrophobicity and specific surface could improve the selective adsorption and separation efficiency in water-in-oil mixtures. Nonwoven fabric also is the main product for filtration in air pollution with low cost, light weight and high filtration efficiency [29,30]. The size and structure of fibers and specific surface of membranes are the key to the filtration efficiency [31,32,33].
膜技术，尤其是静电纺丝膜，已广泛应用于油水混合物的分离，具有除油效率高、出水水质稳定等特点，已应用于食品加工、制药脱盐和燃料电池行业[1,28]。高疏水性和比表面积可以提高油包水混合物中的选择性吸附和分离效率。无纺布也是空气污染过滤的主要产品，具有成本低、重量轻、过滤效率高等特点[29,30]。纤维的大小和结构以及膜的比表面积是过滤效率的关键[31,32,33]。

In this study, we used immiscible polystyrene (PS)/N-trifluoroacetylated polyamide 6 (PA6-TFAA) blending with different additives as a model system to fabricate nanofiber bundles after post-treatment with formic acid etching. With a proper amount of conductive particles or compatibilizers as additives, the force of PS droplets in fluid jets had been enhanced, and droplets could be largely stretched, which resulted in nanofiber bundles. Due to the more aligned fiber structure of nanofiber bundles, the higher tensile strength and tensile module were observed. Moreover, the hydrophobicity and selective adsorption for oil had been improved, resulting from the rougher surface morphology and higher specific area. These nanofiber bundle membranes showed significant application in water-in-oil separation and air filtration.
在本研究中，我们使用不混溶的聚苯乙烯 （PS）/N-三氟乙酰化聚酰胺 6 （PA6-TFAA） 与不同添加剂共混作为模型系统，在甲酸刻蚀后制备纳米纤维束。以适量的导电颗粒或相容剂为添加剂，PS 液滴在流体射流中的力得到增强，液滴可以在很大程度上被拉伸，从而产生纳米纤维束。由于纳米纤维束的纤维结构更加对齐，观察到更高的拉伸强度和拉伸模量。此外，由于表面形态更粗糙和比面积更高，油的疏水性和选择性吸附得到了改善。这些纳米纤维束膜在油包水分离和空气过滤方面显示出重要的应用。

2. Materials and Methods 2. 材料和方法

2.1. Material 2.1. 材料

Polystyrene (PS, Mw, 228,800 g/mol) and polyamide (PA6, Mw, 49,400 g/mol) were purchased from Yangzi-BASF Styrenics Company (Hong Kong, China) and UBE Nylon Ltd. (Taphong, Thailand), respectively. Styrene (St), benzoyl peroxide (BPO), toluene, methanol, dichloromethane (CH₂Cl₂), chloroform (CHCl₃), Karl Fischer reagent (without pyridine), silicon oil and paraffin wax were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Sunflower oil was purchased from Luhua Group Co., Ltd. (Shanghai China) 3-Isopropenyl-α,α’-dimethylbenzene isocyanate (TMI) and trifluoroacetic anhydride (TFAA) were purchased from Aladdin Co., Ltd. (Fukuoka, Japan) Carbon nanotubes (CNT) were purchased from Chengdu Organic Chemicals Co. Ltd. (Chengdu, China) Methylene blue and oil red O were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) PA6 was used after dried at 80 °C in vacuum, and all other chemicals were used as received.
聚苯乙烯（PS，Mw，228,800 g/mol）和聚酰胺（PA6，Mw，49,400 g/mol）分别购自扬子巴斯夫苯乙烯公司（中国香港）和宇部锦纶有限公司（泰国塔蓬）。苯乙烯 （St）、过氧化苯甲酰 （BPO）、甲苯、甲醇、二氯甲烷 （CH₂Cl₂）、氯仿 （CHCl₃）、卡尔费休试剂（不含吡啶）、硅油和石蜡购自国药集团试剂有限公司 （中国 上海） 葵花籽油购自鲁华集团有限公司 （中国 上海） 3-异丙烯基-α，α'-二甲基苯异氰酸酯 （TMI） 和三氟乙酸酐 （TFAA） 购自阿拉丁公司， Ltd. （日本福冈） 碳纳米管 （CNT） 购自成都有机化学品有限公司 （中国成都） 亚甲蓝和油红 O 购自上海麦克林生化有限公司 （中国上海） PA6 在 80°C 真空下干燥后使用，所有其他化学品均按收到时使用。

2.2. Preparation of PS-Co-TMI
2.2. PS-Co-TMI 的制备

A random copolymer of St and TMI, denoted as PS-co-TMI, was synthesized by the free radical copolymerization of St and TMI in toluene with BPO as a free radical initiator. The products were precipitated twice in methanol and then filtered and dried under the vacuum. In this work, the Mn of PS-co-TMI was 37,600 g/mol. Details of the synthesis of PS-co-TMI can be found in other papers [34,35,36].
St 和 TMI 的无规共聚物，表示为 PS-co-TMI，是通过 St 和 TMI 在甲苯中自由基共聚而成的，BPO 为自由基引发剂。将产品在甲醇中沉淀两次，然后在真空下过滤、干燥。在本工作中，PS-co-TMI 的 Mn 为 37,600 g/mol。PS-co-TMI 合成的详细信息可在其他论文中找到 [34\u201235\u201236]。

2.3. N-Trifluoroacetylation of PA6
2.3. PA6 的 N-三氟乙酰化

A given amount of PA6, CH₂Cl₂ and TFAA were added to a flask. The molar ratio between TFAA and the amide group of PA6 was about 1.5:1. The volume ratio of CH₂Cl₂ and TFAA was 2:1. After reacting for 12 h at 25 °C, the PA6-TFAA was gained by a rotary evaporator.
将一定量的 PA6 、 CH₂、 Cl₂ 和 TFAA 加入培养瓶中。TFAA 与 PA6 酰胺基团的摩尔比约为 1.5：1。CH₂、 Cl₂ 和 TFAA 的体积比为 2：1。在 25 °C 下反应 12 小时后，通过旋转蒸发器获得 PA6-TFAA。

2.4. Fabrication of Fiber Membranes
2.4. 纤维膜的制造

The precursor of PS/PA6-TFAA was prepared by dissolving PS and PA6-TFAA in CHCl₃ with a concentration of 0.2 g/mL, respectively, and then blended with a volume ratio of 1:1. Five different solutions were prepared varying CNT or PS-co-TMI with 0, 2 and 4 wt%. The electrospinning solution was placed in a 5 mL syringe attached to a stainless needle with an internal diameter of 0.6 mm. The needle was connected to the positive voltage of 12 KV, and the rotary collector was connected to negative voltage of -3 KV, which was applied by a high-voltage power supply (DW-P303-1ACD8, Tianjin Dongwen High-voltages Source Co., Tianjin, China). The distance between the needle tip and collector was maintained at 15 cm, and the flow rate was 3 mL/h, which was controlled by a syringe pump (KDS-200, KD Scientific Inc., Holliston, MA, USA). The spun membranes were dried at 80 °C in vacuum for 6 h to remove residual solvents along with the deacetylation of PA6-TFAA. Then, PS/PA6-TFAA fibers turned out to be PS/PA6 fibers. After that, the membranes were etched by formic acid three times and each time took 24 h, which were marked as e-PS/PA6, e-PS/PA6/2PS-co-TMI, and e-PS/PA6/2CNT with 2 wt% of additives, and e-PS/PA6/4PS-co-TMI, e-PS/PA6/4CNT with 4 wt% of additives.
将 PS 和 PA6-TFAA 分别溶解在浓度为 0.2 g/mL 的_{CHCl 3} 中，然后以 1：1 的体积比混合制备 PS/PA6-TFAA 的前体。制备了 5 种不同的溶液，CNT 或 PS-co-TMI 含量分别为 0、2 和 4 wt%。将静电纺丝溶液放入 5 mL 注射器中，该注射器连接到内径为 0.6 mm 的不锈钢针头上。针接 12 KV 的正电压，旋转集热器接 -3 KV 的负电压，由高压电源（DW-P303-1ACD8，天津东文高压源有限公司，天津，中国）。针尖和收集器之间的距离保持在 15 cm，流速为 3 mL/h，由注射泵（KDS-200，KD Scientific Inc.，Holliston，MA，USA）控制。将纺丝膜在 80 °C 真空中干燥 6 小时，以去除残留溶剂以及 PA6-TFAA 的脱乙酰化。然后，PS/PA6-TFAA 纤维被证明是 PS/PA6 纤维。之后，用甲酸刻蚀膜 3 次，每次用 24 h，分别标记为添加剂含量为 2 wt% 的 e-PS/PA6、e-PS/PA6/2PS-co-TMI 和 e-PS/PA6/2CNT，添加剂含量为 4 wt% 的 e-PS/PA6/4PS-co-TMI、e-PS/PA6/4CNT。

2.5. Oil Selective Adsorption Measurement
2.5. 油选择性吸附测量

The oil adsorption tests were carried on at 25 °C. The spun membranes were placed in a Petri dish containing 30 mL water and 1 g oil. The oil was dyed red by oil red O. After 1 hour’s adsorption, the samples were drained for 2 min and then weighed. The adsorption capacity of membranes was evaluated in three kinds of oil: silicon oil, sunflower oil and paraffin wax. The oil adsorption capacity of membranes was determined from the following equation:
油吸附试验在 25 °C 下进行。 将纺膜置于装有 30 mL 水和 1 g 油的培养皿中。油被油红 O 染成红色。吸附 1 小时后，将样品沥干 2 min，然后称重。评价膜在 3 种油中吸附能力：硅油、葵花籽油和石蜡。膜的油吸附能力由以下公式确定：

Q=𝑚𝑠−𝑚0𝑚0$$\mathrm{Q}=\frac{m_s-m_0}{m_0}$$

(1)

where Q was the oil adsorption capacity (g/g), m_s was the total mass of wet adsorbent (g), and m₀ was the weight of initial adsorbent (g).
式中 Q 为油吸附量 （g/g），m_s 为湿吸附剂总质量 （g），m₀ 为初始吸附剂重量 （g）。

2.6. Oil/Water Separation Performance
2.6. 油/水分离性能

The spun membrane was fixed in between two both-sides-opening tubes with an inner diameter of 3.3 cm. Polyester film was put under the filter as a supporting membrane. The oil/water mixtures were placed in a glass beaker with a mass ratio of 1:100, which was followed by vigorously stirring for 1 h. The mixtures were poured into the upper tube, and the separation carried on due to gravity. Three types of oil/water mixtures, such as silicon/water, sunflower oil/water and paraffin wax/water were analyzed. The water was dyed blue by methylene blue in the silicon/water and paraffin wax/water mixtures.
将纺丝膜固定在两根内径为 3.3 cm 的两侧开口管之间。聚酯薄膜作为支撑膜放在过滤器下。将油/水混合物置于质量比为 1：100 的玻璃烧杯中，然后剧烈搅拌 1 h。将混合物倒入上管中，由于重力而进行分离。分析了三种类型的油/水混合物，例如硅/水、葵花籽油/水和石蜡/水。水在硅/水和石蜡/水混合物中被亚甲蓝染成蓝色。

The separation efficiency of the spun membranes was calculated by the formula:
纺丝膜的分离效率由以下公式计算：

A=𝑀0−𝑀1𝑀0×100%$$\mathrm{A}=\frac{M_0-M_1}{M_0}\times 100\%$$

(2)

where A was separation efficiency (%), M₁ was water content levels after separation determined by Karl Fischer titration (g/g), and M₀ was water content levels before separation by Karl Fischer titration (g/g).
其中 A 为分离效率 （%），M₁ 为卡尔费休滴定法分离后的水分含量 （g/g），M₀ 为卡尔费休滴定法分离前的水分含量水平 （g/g）。

2.7. Air Filtration Performance
2.7. 空气过滤性能

The spun membrane was fixed in between two one-side-opening tubes with an inner diameter of 3.2 cm. Polyester film was put under the filter as a supporting membrane. One of the tubes was connected with the smoke generator and the other one was connected with the smoke detector. The smoke generator could offer smoke with a concentration of 15–25 mg/m³ and wide size distribution from <100 nm to 2 μm. The smoke flow rate used in the efficiency test was 5 L/min.
将纺丝膜固定在两根内径为 3.2 cm 的单侧开口管之间。聚酯薄膜作为支撑膜放在过滤器下。其中一根管子与烟雾发生器相连，另一根管子与烟雾探测器相连。烟雾发生器可提供浓度为 15–25 mg/m³ 的烟雾，以及从 <100 nm 到 2 μm 的广泛尺寸分布。效率测试中使用的烟雾流速为 5 L/min。

The air filtration efficiency of the spun membranes was calculated by the formula:
纺丝膜的空气过滤效率由以下公式计算：

𝜂=𝐶0−𝐶1𝐶0×100%$$\eta =\frac{C_0-C_1}{C_0}\times 100\%$$

(3)

where η was the separation efficiency (%), C₁ was the smoke concentration after separation (mg/m³), and M₀ was the smoke concentration before separation (mg/m³).
其中 η 为分离效率 （%），C₁ 为分离后烟雾浓度 （mg/m³），M₀ 为分离前烟雾浓度 （mg/m³）。

2.8. Characterization 2.8. 特征描述

The AC resistance of the precursors was tested by an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Company, Shanghai, China) with the three-probe method, using a graphite electrode/calomel electrode/graphite electrode. The sinusoidal AC signal was set as 0.5 mV, the frequency range was set as 10⁻²~10⁶ Hz, and the testing temperature was set as 25 °C. Conductivity was calculated by the formula:
使用石墨电极/甘汞电极/石墨电极，通过电化学工作站（CHI660E，上海晨华仪器公司，中国上海）采用三探针法测试前驱体的交流电阻。正弦交流信号设置为 0.5 mV，频率范围设置为 10⁻²~10⁶ Hz，测试温度设置为 25 °C。 电导率由以下公式计算：

σ=𝑑𝑅×𝑆$$\mathsf{\sigma }=\frac{d}{R\times S}$$

(4)

where σ was the conductivity of electrospinning solution (S/cm), d was the distance of the graphite electrode (cm), R was the self-resistance of the precursor (Ω), and S was the area of the graphite electrode below the liquid level (cm²).
其中 σ 是静电纺丝溶液的电导率 （S/cm），d 是石墨电极的距离 （cm），R 是前驱体的自电阻 （Ω），S 是石墨电极低于液面的面积 （cm²）。

The morphologies of electrospun fiber membranes were observed by using scanning electron microscopy (SEM, ZEISS ULTRA 55, Carl Zeiss AG, Oberkochen, Germany) with an accelerating voltage of 5 KV. Before the test, the samples were dried for 2 h in a vacuum oven at 80 °C and then gold sputtered for 4 min. The mechanical properties of membranes were tested using an electronic universal testing machine (UTM2102, Shenzhen Suns Technology Stock Co., Ltd., Shenzhen, China) at a deformation rate of 10 mm/min. The length and width of the samples were 20 and 5 mm. The water contact angle (WCA) with a water volume of 3 μL was measured by a contact angle measuring device (OCA 20, DataPhysics Instruments, Filderstadt, Germany). The water content levels were determined by Karl Fisher titration. The smoke concentration was detected by ELPI (Dekati Ltd., Kangasala, Finland). The pressure drop could be calculated from the height difference marked in the tube U.
使用加速电压为 5 KV 的扫描电子显微镜 （SEM， ZEISS ULTRA 55， Carl Zeiss AG， Oberkochen， Germany） 观察静电纺丝纤维膜的形态。测试前，将样品在 80 °C 的真空烘箱中干燥 2 小时，然后溅金 4 分钟。使用电子万能试验机（UTM2102，深圳市太阳科技股份有限公司，中国深圳）以 10 mm/min 的变形速率测试膜的机械性能。样品的长度和宽度分别为 20 和 5 mm。使用接触角测量设备（OCA 20，DataPhysics Instruments，Filderstadt，Germany）测量水体积为 3 μL 的水接触角 （WCA）。水分含量水平通过 Karl Fisher 滴定法测定。烟雾浓度由 ELPI（Dekati Ltd.，Kangasala，芬兰）检测。压降可以根据管 U 中标记的高度差计算得出。

3. Results 3. 结果

3.1. Morphologies of Electrospun Fibers
3.1. 静电纺丝纤维的形态

To understand the structure of PS/PA6 fibers, the SEM images of as-spun fibers and fibers etched by formic acid are displayed in Figure 1. Smooth PS/PA6 fibers with average diameters of 2.0 μm were obtained, as shown in Figure 1a. After being etched by formic acid, the PS/PA6 fibers became PS fibers and some grooves turned out on the surface, as shown in Figure 1b. However, nanofibers with a diameter of about 200 nm only displayed on the core of e-PS/PA6 fibers, which was observed on the cross-section of the etched fibers in Figure 1c.
为了了解 PS/PA6 纤维的结构，图 1 显示了纺丝纤维和甲酸蚀刻纤维的 SEM 图像。获得了平均直径为 2.0 μm 的光滑 PS/PA6 纤维，如图 1a 所示。PS/PA6 纤维经甲酸蚀刻后变成 PS 纤维，表面出现一些凹槽，如图 1b 所示。然而，直径约为 200 nm 的纳米纤维仅显示在 e-PS/PA6 纤维的纤芯上，这在图 1c 中蚀刻纤维的横截面上观察到。

Figure 1. SEM images of PS/PA6 fibers without (a) and with (b,c) etched by formic acid.
图 1. 无 （a） 和有 （b，c） 的 PS/PA6 纤维经甲酸蚀刻的 SEM 图像。

In the emulsion-like immiscible precursors, PA6-TFAA formed the continuous phase, while PS formed the discontinuous phase [37,38,39], resulting from the different viscosity of PA6-TFAA and PS. The viscosities of PS and PA6-TFAA were 141 and 20 mPa·s. The polymer with lower viscosity preferred to form the continuous phase, which meant PA6-TFAA formed the continuous phase. Furthermore, PS and PA6-TFAA performed phase separation during the electrospinning process due to the incompatibility [23,40]. While in electrospinning, the electric force applying on PA6-TFAA was larger than that on PS, and the velocity gradient formed perpendicular on the interface of PS and PA6-TFAA domains due to different electrospinnability. In the core of fluid jets, high velocity led to PA6-TFAA offering a larger stretch for PS droplets, and the PS droplets stretched, head-to-tail coalesced, and formed the nanofibers. Nevertheless, on the shell of fluid jets, PS droplets did not gain enough stretch before coalescence and formed the solid shells, as the velocity on the shell was lower than that on the core.
在乳液状不混溶前驱体中，PA6-TFAA形成连续相，而PS形成不连续相[37,38,39]，这是由于PA6-TFAA和PS的粘度不同造成的。PS 和 PA6-TFAA 的黏度分别为 141 和 20 mPa·s。粘度较低的聚合物倾向于形成连续相，这意味着 PA6-TFAA 形成连续相。此外，由于不相容性，PS 和 PA6-TFAA 在静电纺丝过程中进行了相分离 [23\u201240]。而在静电纺丝中，施加在 PA6-TFAA 上的电力大于 PS，并且由于静电纺丝性能不同，在 PS 和 PA6-TFAA 畴的界面上形成垂直的速度梯度。在流体射流的核心中，高速导致 PA6-TFAA 为 PS 液滴提供更大的拉伸，并且 PS 液滴被拉伸，从头到尾聚结，形成纳米纤维。然而，在流体射流的壳层上，PS 液滴在聚结前没有获得足够的拉伸并形成固体壳层，因为壳层上的速度低于核心上的速度。

In order to fabricate the perfect nanofiber bundles, two kinds of additives were blended with PS/PA6-TFAA for enhancing the stretch of PS droplets, respectively, which were CNT and PS-co-TMI. Figure 2 showed the conductivity of PS/PA6-TFAA solution with an increment of CNT. With higher conductivity, the fluid jets could gain a larger stretch under the same electric field. In the fluid jets, the random PS droplets were stretched, turned from sphere to ellipsoid and became close head-to-tail. Next, PA6-TFAA between the head and tail of ellipsoid-like PS droplets was squeezed out, and the PS droplets coalesced as rod-like droplets. With coalescence going, PS nanofibers were formed before the solvent disappeared. The surface morphology of PS/PA6 fibers almost remained smooth after introducing CNT, as shown in Figure 3a,d. After being etched by formic acid, the PS nanofiber bundles appeared and were oriented along the fiber on the surface, as shown in Figure 3b,c,e,f. The diameter of nanofibers of bundles was 50–150 nm. As the conductivity of PS/PA6-TFAA with 2 wt% CNT was too low to offer enough stretch to PS droplets in fluid jets, rod-like PS domains were observed (Figure 3b,c).
为了制备完美的纳米纤维束，将两种添加剂与 PS/PA6-TFAA 共混以增强 PS 液滴的拉伸，分别是 CNT 和 PS-co-TMI。图 2 显示了 CNT 增加的 PS/PA6-TFAA 溶液的电导率。具有较高的电导率，流体射流在相同的电场下可以获得更大的拉伸。在流体射流中，随机的 PS 液滴被拉伸，从球体转向椭球体，并从头到尾变得接近。接下来，挤出椭球状 PS 液滴头部和尾部之间的 PA6-TFAA，PS 液滴聚结为棒状液滴。随着聚结的进行，在溶剂消失之前形成了 PS 纳米纤维。引入 CNT 后，PS/PA6 纤维的表面形貌几乎保持光滑，如图 3a、d 所示。经甲酸刻蚀后，出现 PS 纳米纤维束，并沿表面的纤维定向，如图 3b、c、e、f 所示。束的纳米纤维直径为 50-150 nm。由于具有 2 wt% CNT 的 PS/PA6-TFAA 的电导率太低，无法为流体射流中的 PS 液滴提供足够的拉伸，因此观察到棒状 PS 结构域（图 3b，c）。

Figure 2. Conductivity of PS/PA6-TFAA electrospinning solution with different content of CNT.
图 2. 不同 CNT 含量的 PS/PA6-TFAA 静电纺丝溶液的电导率。

Figure 3. SEM images of electrospun PS/PA6/CNT without (a,d) and with (b,c,e,f) etching by formic acid. The contents of CNT were 2 wt% (a–c) and 4 wt% (d–f).
图 3. 无 （a，d） 和有 （b，c，e，f） 甲酸蚀刻的静电纺丝 PS/PA6/CNT 的 SEM 图像。CNT 的含量为 2 wt% （a-c） 和 4 wt% （d-f）。

On the other hand, the isocyanate groups of PS-co-TMI could react with the terminal group of PA6-TFAA to form a graft copolymer of PS-g-PA6-TFAA, which acted as a compatibilizer in the PS/PA6-TFAA system and enhanced the interfacial interaction. From Figure 4a, when the amount of PS-co-TMI was 2 wt%, parts of the surface appeared porous, because some PS migrated to the surface, and a high evaporated solution was used in the precursor [41,42]. After being etched by formic acid, the porous surface remained, the smooth surface transformed to coherent nanofibers, and the cores were filled with nanofibers, which were imperfect nanofiber bundles (Figure 4b,c). When the content of PS-co-TMI increased to 4 wt%, the interfacial interaction between PS and PA6-TFAA had improved. As PA6-TFAA had higher spinnability, more electric force was applied on the PA6-TFAA domain under the same electric field. Therefore, more drag force was transferred from the PA6-TFAA domain to PS droplets when the amount of compatibilizer increased. The PS droplets gained more stretch and transformed from sphere to ellipsoid, rod and nanofibers finally.
另一方面，PS-co-TMI 的异氰酸酯基团可以与 PA6-TFAA 的末端基团反应，形成 PS-g-PA6-TFAA 的接枝共聚物，在 PS/PA6-TFAA 体系中起增容剂的作用，增强界面相互作用。从图4a中可以看出，当PS-co-TMI的量为2 wt%时，部分表面看起来是多孔的，因为一些PS迁移到了表面，并且在前驱体中使用了高蒸发溶液[41,42]。被甲酸蚀刻后，多孔表面仍然存在，光滑的表面转变为连贯的纳米纤维，芯中充满了纳米纤维，这些纳米纤维是不完美的纳米纤维束（图 4b，c）。当 PS-co-TMI 含量增加到 4 wt% 时，PS 和 PA6-TFAA 之间的界面相互作用得到改善。由于 PA6-TFAA 具有更高的可旋转性，因此在相同电场下对 PA6-TFAA 结构域施加了更大的电力。因此，当相容剂的量增加时，更多的阻力从 PA6-TFAA 结构域传递到 PS 液滴。PS 液滴获得了更多的拉伸，并最终从球体转变为椭球体、棒状和纳米纤维。

Figure 4. SEM images of electrospun PS/PA6/PS-co-TMI without (a,d) and with (b,c,e,f) etching by formic acid. The contents of PS-co-TMI were 2 wt% (a–c) and 4 wt% (d–f).
图 4. 无 （a，d） 和有 （b，c，e，f） 甲酸蚀刻的静电纺丝 PS/PA6/PS-co-TMI 的 SEM 图像。PS-co-TMI 的含量为 2 wt% （a-c） 和 4 wt% （d-f）。

Figure 5 displays the tensile stress vs. strain curves for PS and e-PS/PA6 without and with additives. For all samples, the tensile stress–strain curves showed a non-linear elastic behavior until the highest tensile stress. That is because the fibers slipped and oriented under stress loading. When the stress reached its maximum, a drastic reduction in tensile stress happened in e-PS/PA6 with additives. That means the orientation of bundles had completed in the stress loading direction, and the bundle membranes broke at the point of the highest stress. However, the tensile stress decreased slowly after the highest stress for neat PS and e-PS/PA6 fiber membranes, resulting from incomplete orientation of the random fibers. Under the same electric field, PS prefers to form the random fibers because of unstable fluid jets action due to lower electrospinnability. By becoming introduced with 4 wt% CNT, the conductivity of electrospun solution increased almost twice (Figure 2), which enhanced the electrospinnability of PS and formed orientated fibers. On the other hand, the electrospinnability of PA6-TFAA is high enough, resulting in forming more orientated fiber membranes. By adding 4 wt% PS-co-TMI, the compatibilizer could improve the interfacial interaction between PS and the PA6-TFAA domain, which could efficiently enhance the stability of fluid jets and formed orientated fiber membranes.
图 5 显示了无添加剂和有添加剂的 PS 和 e-PS/PA6 的拉伸应力与应变曲线。对于所有样品，拉伸应力-应变曲线都显示出非线性弹性行为，直到达到最高拉伸应力。这是因为纤维在应力载荷下滑动和定向。当应力达到最大值时，添加添加剂的 e-PS/PA6 的拉伸应力急剧降低。这意味着束的取向已在应力加载方向上完成，并且束膜在最高应力点断裂。然而，由于随机纤维的取向不完全，纯 PS 和 e-PS/PA6 纤维膜的拉伸应力在最高应力后缓慢下降。在相同的电场下，PS 倾向于形成无规则纤维，因为较低的静电自触性导致流体射流作用不稳定。通过引入 4 wt% CNT，静电纺丝溶液的电导率几乎增加了两倍（图 2），这增强了 PS 的静电纺丝性能并形成了定向纤维。另一方面，PA6-TFAA 的静电纺丝性足够高，从而形成更定向的纤维膜。通过添加 4 wt% PS-co-TMI，增容剂可以改善 PS 与 PA6-TFAA 结构域之间的界面相互作用，从而有效增强流体射流和形成定向纤维膜的稳定性。

Figure 5. Tensile stress–strain curves of electrospun fibers: PS, e-PS/PA6, e-PS/PA6/4CNT and e-PS/PA6/4PS-co-TMI. The content of CNT or PS-co-TMI was 4 wt%. The fiber membranes electrospun from blended polymer solution were etched by formic acid before being tested.
图 5. 静电纺丝纤维的拉伸应力-应变曲线：PS、e-PS/PA6、e-PS/PA6/4CNT 和 e-PS/PA6/4PS-co-TMI。CNT 或 PS-co-TMI 的含量为 4 wt%。从共混聚合物溶液静电纺丝的纤维膜在测试前用甲酸蚀刻。

The tensile strength and tensile module of different fiber membranes are summarized in Figure 6. The tensile strength and tensile module of PS membranes are lower than 0.10 MPa and 4.9 MPa, respectively. After adding PA6-TFAA into the precursor, the orientation of fibers in the membranes had greatly improved, and the structure changed from bead-on-string to “imperfect” nanofiber bundles, by which the tensile strength increased to 0.89 MPa and the tensile module increased to 20.3 MPa. When blended with 4 wt% PS-co-TMI or CNT into PS/PA6-TFAA, the orientation of fibers had further improved, and perfect nanofiber bundles were formed, which made the tensile strength and tensile module 53–58 times and 13–17.5 times higher than that of neat PS fiber membranes.
图 6 总结了不同纤维膜的拉伸强度和拉伸模量。PS膜的拉伸强度和拉伸模量分别低于0.10 MPa和4.9 MPa。在前驱体中加入 PA6-TFAA 后，膜中纤维的取向大大改善，结构由串珠变为“不完美”纳米纤维束，拉伸强度提高到 0.89 MPa，拉伸模量提高到 20.3 MPa。当与 4 wt% PS-co-TMI 或 CNT 共混到 PS/PA6-TFAA 中时，纤维的取向进一步改善，形成了完美的纳米纤维束，使拉伸强度和拉伸模量分别比纯 PS 纤维膜高 53-58 倍和 13-17.5 倍。

Figure 6. Variation in tensile strength and tensile module of electrospun fibers: PS, e-PS/PA6, e-PS/PA6/4CNT and e-PS/PA6/4PS-co-TMI. The content of CNT or PS-co-TMI was 4 wt%. The fiber membranes electrospun from blended polymer solution were etched by formic acid before tested.
图 6. 静电纺丝纤维的拉伸强度和拉伸模量的变化：PS、e-PS/PA6、e-PS/PA6/4CNT 和 e-PS/PA6/4PS-co-TMI。CNT 或 PS-co-TMI 的含量为 4 wt%。从共混聚合物溶液静电纺丝的纤维膜在测试前用甲酸蚀刻。

3.2. Air Filtration Performance of Electrospun Fiber Membranes
3.2. 静电纺丝纤维膜的空气过滤性能

The filtration performance of membranes was investigated by the smoke of moxibustion tobacco. As shown in Figure 7a, the filtration efficiency of PS and e-PS/PA6 increased from 14 to 54% and 25 to 72%, respectively. However, the filtration efficiency of these membranes was much lower than that of e-PS/PA6/4PS-co-TMI and e-PS/PA6/4CNT, which varied from 88 to 96% and 74 to 89% with the increment of basis weight. Meanwhile, the pressure drop of PS membranes increased from 20 to 60 Pa, which were lowest in these four kinds of membranes (Figure 7b). That means the packing density of PS membranes was low, and this fluffy structure facilitated air flow penetrating through the membranes, resulting in low filtration efficiency. The pressure drop of e-PS/PA6 and e-PS/PA6/4CNT was almost the same, which was between 65 and 130 Pa. The higher filtration efficiency of e-PS/PA6/4CNT could be ascribed to the nanofiber bundle structure. The pressure drop of e-PS/PA6/4PS-co-TMI greatly rose from 150 to 320 Pa with a basis weight of 2.6 to 3.4 g/m²; then, it slightly increased from 320 to 430 Pa with a basis weight of 3.8 to 7.7 g/m², which means the packing density of e-PS/PA6/4PS-co-TMI was the highest in these four kinds of membranes. That helps to further improve the filtration efficiency.
通过艾灸烟的烟雾研究膜的过滤性能。如图 7a 所示，PS 和 e-PS/PA6 的过滤效率分别从 14% 提高到 54% 和 25% 提高到 72%。然而，这些膜的过滤效率远低于 e-PS/PA6/4PS-co-TMI 和 e-PS/PA6/4CNT，随着基重的增加，过滤效率从 88% 到 96% 和 74% 到 89% 不等。同时，PS 膜的压降从 20 Pa 增加到 60 Pa，在这四种膜中最低（图 7b）。这意味着 PS 膜的填充密度较低，这种蓬松的结构有助于气流渗透膜，导致过滤效率低。e-PS/PA6 和 e-PS/PA6/4CNT 的压降几乎相同，都在 65 到 130 Pa 之间。e-PS/PA6/4CNT 较高的过滤效率可归因于纳米纤维束结构。e-PS/PA6/4PS-co-TMI 的压降从 150 Pa 大幅上升到 320 Pa，基重为 2.6 至 3.4 g/m²;然后，它从 320 Pa 略微增加到 430 Pa，基重为 3.8 到 7.7 g/m²，这意味着 e-PS/PA6/4PS-co-TMI 的堆积密度在这四种膜中最高。这有助于进一步提高过滤效率。

Figure 7. (a) Filtration performance of membranes with various basis weight. (b) Pressure drops of membranes with various basis weight. The content of CNT or PS-co-TMI was 4 wt%. The fibers electrospun from blended polymer solution were etched by formic acid before being tested.
图 7. （a） 不同定量的膜的过滤性能。（b） 不同基重的膜的压降。CNT 或 PS-co-TMI 的含量为 4 wt%。从混合聚合物溶液中静电纺丝的纤维在测试前被甲酸蚀刻。

In order to figure out the air filtration behavior, four kinds of membranes with a basis weight of about 5.4 g/m² were chosen for the next discussion, which were denoted as XXX-5, for example PS-5 membrane. The details of membranes are shown in Table 1. The packing density (α) [43] and the quality factor (QF) [44] are described by the following equation:
为了弄清楚空气过滤行为，选择了四种基重约为 5.4 g/m² 的膜进行下一次讨论，它们被表示为 XXX-5，例如 PS-5 膜。膜的详细信息如表 1 所示。堆积密度 （α） [43] 和品质因数 （QF） [44] 由以下公式描述：

α=𝑊𝜌×𝑍$$\mathsf{\alpha }=\frac{W}{\rho \times Z}$$

(5)

QF=−ln(1−𝜂)Δ𝑃$$\mathrm{QF}=-\frac{\ln \left(1-\eta \right)}{\Delta P}$$

(6)

where W is the basis weight of membranes, ρ is the density of the polymer material, Z is the thickness of membranes, η is the air filtration efficiency, and ΔP is the pressure drop.
其中 W 是膜的基重，ρ 是聚合物材料的密度，Z 是膜的厚度，η 是空气过滤效率，ΔP 是压降。

Table 1. Filtration performance of membranes.
表 1. 膜的过滤性能。

The smoke of moxibustion tobacco has a wide size distribution from <100 nm to 2 μm. As Figure 8a shows, the majority of particles was between 0.8 and 2 μm, whose concentration exceeded 3 mg/m³. The thickness of the PS-5 membrane was the largest (47.6 μm, as Table 1 shown), which could hardly stop smoke particles with diameter <300 nm penetrating, and the highest filtration efficiency was just about 30%, which might be attributed to the low packing density [45]. The gap of filtration efficiency between e-PS/PA6/4CNT-5 and e-PS/PA6-5 had broadened when the diameter of smoke particles was lower than 2 μm. That means the surface structure of nanofiber bundles could capture more particles, especially small particles with almost the same packing density and pressure drop (as Table 1 shows). The filtration efficiency of e-PS/PA6/4PS-co-TMI-5 was above 90% for diameter >480 nm and about 85% for diameter <320 nm, as it possessed the highest packing density and special structure of nanofiber bundles. However, the QF of e-PS/PA6/4PS-co-TMI-5 was the lowest because of the highest pressure drop, as Table 1 shows. Due to the strong balance of filtration efficiency and pressure drop, e-PS/PA6/4CNT-5 seemed to be the best filtration membranes with QF of 1.71 × 10⁻⁴ Pa⁻¹.
艾灸烟的烟雾具有从 <100 nm 到 2 μm 的广泛粒径分布。如图 8a 所示，大多数颗粒在 0.8 到 2 μm 之间，其浓度超过 3 mg/m³。PS-5 膜的厚度最大（47.6 μm，如表 1 所示），几乎无法阻止直径为 <300 nm 的烟雾颗粒穿透，最高过滤效率仅为 30% 左右，这可能是由于低堆积密度 [45]。当烟雾颗粒直径小于 2 μm 时，e-PS/PA6/4CNT-5 和 e-PS/PA6-5 的过滤效率差距扩大。这意味着纳米纤维束的表面结构可以捕获更多的颗粒，尤其是具有几乎相同堆积密度和压降的小颗粒（如表 1 所示）。e-PS/PA6/4PS-co-TMI-5 对直径 >480 nm 的过滤效率在 90% 以上，对直径 <320 nm 的过滤效率约为 85%，因为它具有最高的堆积密度和纳米纤维束的特殊结构。然而，e-PS/PA6/4PS-co-TMI-5 的 QF 最低，因为压降最高，如表 1 所示。由于过滤效率和压降的强烈平衡，e-PS/PA6/4CNT-5 似乎是最好的过滤膜，QF 为 1.71 × 10⁻⁴ Pa⁻¹。

Figure 8. (a) Concentration of the smoke particles with different diameter size before filtration. (b) Filtration efficiency to different diameter size of smoke particles. The content of CNT or PS-co-TMI was 4 wt%.
图 8. （a） 过滤前不同直径的烟雾颗粒的浓度。（b） 对不同直径大小的烟雾颗粒的过滤效率。CNT 或 PS-co-TMI 的含量为 4 wt%。

Figure 9 shows SEM images of these four kinds of membranes after filtration with 1, 5, 20, and 30 min. After 1 min filtration, the smoke particles were hardly found in PS-5 and e-PS/PA6-5 membranes. Comparing Figure 9c with Figure 9d, most of e-PS/PA6/4PS-co-TMI-5′s bundles had scattered, which increased the specific surface area and improved the packing density of the membranes. This is the reason more particles were noticed in e-PS/PA6/4PS-co-TMI-5 membranes. In addition to spheroidal particles, wizened particles were observed on the nanofiber bundles (inset picture of Figure 9d,g,h), which indicated liquid particles could be partly adsorbed into the bundles and made the bundles remain at a small size. Moreover, the wizened particles could act as branches on the bundles to increase the surface area, which further improved the filtration efficiency. When the filtration took 5 min, the particles captured by fibers become more, as shown in Figure 9e–h. The wizened particles could be observed frequently on the nanofiber bundles and the bundles were partly covered by the smoke dust, as shown in Figure 9g, h. Furthermore, the bundles stuck together by dust and “dust membranes” formed at the intersection of fibers (Figure 9h). At 20 min, the particles on the fibers become larger in membranes of e-PS-5 and e- PS/PA6-5 (Figure 9i,j). The “dust membranes” started to show at the intersection of bundles in e-PS/PA6/4CNT-5 (Figure 9k) and covered almost the whole membranes of e-PS/PA6/4PS-co-TMI-5 (Figure 9l). Finally, the fibers were covered by the spheroidal particles and the size became obviously larger in PS-5 membranes (Figure 9m) because the fiber could not adsorb liquid inside. The smooth surface with a few spheroidal particles and the dust membranes were observed in e-PS/PA6-5 membranes. The membranes with higher filtration efficiency were blocked by dust, as shown in Figure 9o,p.
图 9 显示了这四种膜在 1 、 5 、 20 和 30 分钟过滤后的 SEM 图像。过滤 1 min 后，PS-5 和 e-PS/PA6-5 膜中几乎未发现烟雾颗粒。将图 9c 与图 9d 进行比较，大部分 e-PS/PA6/4PS-co-TMI-5 束已散开，这增加了比表面积并提高了膜的堆积密度。这就是在 e-PS/PA6/4PS-co-TMI-5 膜中发现更多颗粒的原因。除了球状颗粒外，在纳米纤维束上还观察到枯萎的颗粒（图 9d、g、h 的插图），这表明液体颗粒可以部分吸附到束中，并使束保持小尺寸。此外，枯萎的颗粒可以在束上充当分支以增加表面积，从而进一步提高了过滤效率。当过滤需要 5 分钟时，纤维捕获的颗粒变得更多，如图 9e-h 所示。在纳米纤维束上可以经常观察到枯萎的颗粒，并且纳米纤维束部分被烟尘覆盖，如图 9g、h 所示。此外，纤维束被灰尘和在纤维交叉处形成的“尘膜”粘在一起（图 9h）。20 分钟时，纤维上的颗粒在 e-PS-5 和 e-PS/PA6-5 膜中变大（图 9i，j）。“尘膜”开始出现在 e-PS/PA6/4CNT-5 中的束状物交叉处（图 9k），几乎覆盖了 e-PS/PA6/4PS-co-TMI-5 的整个膜（图 9l）。 最后，纤维被球状颗粒覆盖，PS-5 膜的尺寸明显变大（图 9m），因为纤维无法吸附内部的液体。在 e-PS/PA6-5 膜中观察到表面光滑，带有少量球状颗粒和尘膜。过滤效率较高的膜被灰尘堵塞，如图 9o，p 所示。

Figure 9. EM images of PS-5 (a,e,i,m), e-PS/PA6-5 (b,f,j,n), e-PS/PA6/CNT-5 (c,g,k,o) and e-PS/PA6/PS-co-TMI-5 (d,h,l,p) membranes after filtration. The filtration time was 1 min (a–d), 5 min (e–h), 20 min (i–l) and 30 min (m–p). The content of CNT or PS-co-TMI was 4 wt%. The scale bar was 5 μm.
图 9. 过滤后 PS-5 （a，e，，m）、e-PS/PA6-5 （b，f，，n）、e-PS/PA6/CNT-5 （c，g，k，o） 和 e-PS/PA6/PS-co-TMI-5 （d，h，，p） 膜的 EM 图像。过滤时间为 1 分钟 （a-d）、5 分钟 （e-h）、20 分钟 （-） 和 30 分钟 （m-p）。CNT 或 PS-co-TMI 的含量为 4 wt%。比例尺为 5 μm。

According to the above results, we supposed electrospun fibers captured the dust when continuous flow brought the dust and hit the fiber membranes. Some dust attached to the fibers. With more smoke feeding, the dust particles were able to move along the fibers [31]. Then, the particles aggregated or captured the new one in air flow to form larger particles. As the filtration continued, the particles became large enough to contact each other and aggregated to form dust membranes at the intersection of two fibers. As time went by, the triangular dust membranes formed, and finally, the whole filters were blocked by dust. Because the particles-captured-ability was too weak, e-PS-5 could build the dust membranes (Figure 9m) within 30 min filtration. In addition, e-PS/PA6-5 membranes were able to build the triangular dust membranes (Figure 9o). Due to the powerful particles-captured-ability, the nanofibers bundle membranes were blocked at 30 min for e-PS/PA6/4CNT-5 and 20 min for e-PS/PA6/4PS-co-TMI-5.
根据上述结果，我们假设静电纺丝纤维在连续流动带来灰尘并撞击纤维膜时捕获了灰尘。一些灰尘附着在纤维上。随着烟雾的进料增加，尘埃颗粒能够沿着纤维移动 [31]。然后，颗粒在气流中聚集或捕获新的颗粒，形成更大的颗粒。随着过滤的继续，颗粒变得足够大，可以相互接触，并在两根纤维的交汇处聚集形成尘膜。随着时间的推移，三角形的尘膜形成，最后，整个过滤器都被灰尘堵塞了。由于颗粒捕获能力太弱，e-PS-5 可以在 30 min 过滤内构建尘膜（图 9m）。此外，e-PS/PA6-5 膜能够构建三角形尘膜（图 9o）。由于纳米纤维束膜具有强大的颗粒捕获能力，e-PS/PA6/4CNT-5 和 e-PS/PA6/4PS-co-TMI-5 分别在 30 min 和 20 min 时被封闭。

3.3. Oil/Water Separation Performance of Electrospun Fiber Membranes
3.3. 静电纺丝纤维膜的油/水分离性能

Wettability depends on the chemical composition and surface morphology of the membranes. All the samples shown in Figure 10 were PS membranes, including as-spun neat PS fibers and etched blended electrospun fibers. So, the surface morphology made a difference in the water contact angle (WCA) in this work. The porous, bead-on-string and fluffy structure of as-spun neat PS fibers led to the high WCA of 133.4 ± 1.3°. After adding PA6-TFAA, the WCA of the etched fibers just had a small increase to 139.3 ± 1.3°, resulting from the almost smooth and bead-free fiber structure. With the addition of CNT or PS-co-TMI, the WCA of nanofiber bundle membranes increased to 145.0 ± 0.5°. Although the surface structure of nanofiber bundles was rough in this work, the bead-free and dense surface structure could not make the more abundant hierarchical structure for membranes, which could not further improve the hydrophobicity of membranes.
润湿性取决于膜的化学成分和表面形态。图 10 所示的所有样品都是 PS 膜，包括纺差纯 PS 纤维和蚀刻混纺静电纺丝纤维。因此，在这项工作中，表面形态对水接触角 （WCA） 产生了影响。纺制的纯 PS 纤维的多孔、串珠和蓬松结构导致 133.4 ± 1.3° 的高 WCA。加入 PA6-TFAA 后，刻蚀纤维的 WCA 仅小幅增加至 139.3°± 1.3°，这是由于纤维结构几乎光滑且无珠。随着 CNT 或 PS-co-TMI 的加入，纳米纤维束膜的 WCA 增加到 145.0 ± 0.5°。虽然在这项工作中纳米纤维束的表面结构粗糙，但无珠和致密的表面结构无法为膜提供更丰富的分层结构，从而无法进一步提高膜的疏水性。

Figure 10. Water contact angle of electrospun fibers: PS, e-PS/PA6, e-PS/PA6/4CNT and e-PS/PA6/4PS-co-TMI. The content of CNT or PS-co-TMI was 4 wt%. The fiber membranes electrospun from blended polymer solution were etched by formic acid before being tested.
图 10. 静电纺丝纤维的水接触角：PS、e-PS/PA6、e-PS/PA6/4CNT 和 e-PS/PA6/4PS-co-TMI。CNT 或 PS-co-TMI 的含量为 4 wt%。从共混聚合物溶液静电纺丝的纤维膜在测试前用甲酸蚀刻。

Figure 11 displays the oil selective adsorption capacities of PS fiber membranes with different morphologies. The neat PS fiber membranes showed the minimum adsorption from 10.1 to 17.1 g/g in different oil (as Figure 11a shown). Although neat PS fiber membranes had a fluffy structure which indicated large external pores, the membranes easily shrunk, and the oil desorbed due to the poor tensile module. The nanofiber bundle membranes exhibited excellent oil adsorption from 31.0 to 61.3 g/g. That can be explained by the rough surface and internal penetrated gap of nanofiber bundles, which led to a bigger specific area that was more easily saturated by oil. In addition, the capillary action was enhanced by the internal gap of the bundles; thus, the oil could be quickly adsorbed into the inner void. As Figure 11b shows, the dyed paraffin wax was adsorbed by the e-PS/PA6/4PS-co-TMI membrane in a few seconds. On the other hand, although there was a large void in the core of etched PS/PA6 without an additive, some of them could not be filled with highly viscous oil, as there were a few penetrated grooves on the solid surface, as shown in Figure 1b,c. That is the reason the oil adsorption of e-PS/PA6 without any additive was almost half of that of the etched PS/PA6 with CNT or PS-co-TMI.
图 11 显示了不同形态的 PS 纤维膜的油选择性吸附能力。纯 PS 纤维膜在不同油中显示出 10.1 至 17.1 g/g 的最小吸附（如图 11a 所示）。虽然整洁的 PS 纤维膜具有蓬松的结构，表明外部孔隙较大，但由于拉伸模块较差，膜很容易收缩，并且油会脱附。纳米纤维束膜在 31.0 至 61.3 g/g 范围内表现出优异的吸油性能。这可以用纳米纤维束的粗糙表面和内部穿透间隙来解释，这导致更大的比面积更容易被油饱和。此外，光束的内部间隙增强了毛细管作用;因此，油可以迅速吸附到内部空隙中。如图 11b 所示，染色的石蜡在几秒钟内被 e-PS/PA6/4PS-co-TMI 膜吸附。另一方面，虽然在没有添加剂的情况下蚀刻 PS/PA6 的核心存在很大的空隙，但其中一些无法填充高粘度油，因为固体表面有一些穿孔，如图 1b、c 所示。这就是为什么没有任何添加剂的 e-PS/PA6 的油吸附率几乎是用 CNT 或 PS-co-TMI 蚀刻的 PS/PA6 的一半。

Figure 11. (a) The oil selective adsorption of electrospun fibers: PS, e-PS/PA6, e-PS/PA6/4CNT and e-PS/PA6/4PS-co-TMI. (b) Optical images of paraffin wax (dyed red with oil red O) selective adsorption by e-PS/PA6/4PS-co-TMI with different time. The content of CNT or PS-co-TMI was 4 wt%. The fiber membranes electrospun from blended polymer solution were etched by formic acid before being tested.
图 11. （a） 静电纺丝纤维的油选择性吸附：PS、e-PS/PA6、e-PS/PA6/4CNT 和 e-PS/PA6/4PS-co-TMI。（b） 不同时间 e-PS/PA6/4PS-co-TMI 选择性吸附石蜡（用油红 O 染红）的光学图像。CNT 或 PS-co-TMI 的含量为 4 wt%。从共混聚合物溶液静电纺丝的纤维膜在测试前用甲酸蚀刻。

The water-in-oil emulsion separation efficiency is shown in Figure 12. The neat PS fiber membranes have a minimum separation efficiency of 91.8%, 90.0% and 92.2% in silicon oil/water, sunflower oil/water and paraffin wax/water separation, respectively. The separation efficiency of e-PS/PA6 fiber membranes increase a little bit. Due to the highest hydrophobicity and oil adsorption capacity, the separation efficiency of nanofiber bundle membranes increased to above 99.0% in silicon oil/water and paraffin wax/water separation and 97.7% in sunflower oil/water. In this water-in-oil emulsion separation, when oil/water mixture was poured into the upper tube, the oil was immediately adsorbed by the nanofiber bundle membranes and formed an oil film on the surface of the filters, while water was repelled by hydrophobic membranes or the oil film. As a result of gravity force, oil ran through the membrane and collected. In this case, the pictures of the dyed water removed by e-PS/PA6/4PS-co-TMI membranes are shown in Figure 13. After filtration, the oils were clear.
油包水乳化液分离效率如图 12 所示。纯 PS 纤维膜在硅油/水、葵花籽油/水和石蜡/水中的最低分离效率分别为 91.8%、90.0% 和 92.2%。e-PS/PA6 纤维膜的分离效率略有提高。由于具有最高的疏水性和油吸附能力，纳米纤维束膜在硅油/水和石蜡/水分离中的分离效率提高到 99.0% 以上，在葵花籽油/水中提高到 97.7% 以上。在这种油包水乳液分离中，当油/水混合物倒入上管时，油立即被纳米纤维束膜吸附，并在过滤器表面形成油膜，而水则被疏水膜或油膜排斥。由于重力，油流过膜并收集起来。在这种情况下，e-PS/PA6/4PS-co-TMI 膜去除的染色水的图片如图 13 所示。过滤后，油是透明的。

Figure 12. The oil/water separation efficiency of electrospun fibers: PS, e-PS/PA6, e-PS/PA6/4CNT and e-PS/PA6/4PS-co-TMI. The content of CNT or PS-co-TMI was 4 wt%. The fiber membranes electrospun from blended polymer solution were etched by formic acid before being tested.
图 12. 静电纺丝纤维的油/水分离效率：PS、e-PS/PA6、e-PS/PA6/4CNT 和 e-PS/PA6/4PS-co-TMI。CNT 或 PS-co-TMI 的含量为 4 wt%。从共混聚合物溶液静电纺丝的纤维膜在测试前用甲酸蚀刻。

Figure 13. Optical images of water-in-oil emulsion before (a–c) and after (d–e) filtration with nanofiber bundle membranes (e-PS/PA6/4PS-co-TMI). Oil/water mixtures were silicon oil/water (a,d), sunflower oil/water (b,e) and paraffin wax/water (c,f). Water was dyed blue with methylene blue in silicon oil/water and paraffin wax/water.
图 13. 使用纳米纤维束膜 （e-PS/PA6/4PS-co-TMI） 过滤前 （a-c） 和 （d-e） 油包水乳液的光学图像。油/水混合物为硅油/水 （a，d）、葵花籽油/水 （b，e） 和石蜡/水 （c，f）。在硅油/水和石蜡/水中用亚甲蓝将水染成蓝色。

4. Conclusions 4. 结论

The nanofiber bundles were prepared by electrospinning PS/PA6-TFAA blended with CNT or PS-co-TMI and then etched by formic acid. CNT could increase the conductivity of the precursor in order to improve the drag force of the electric field for PS droplets in jets. Meanwhile, PS-co-TMI used as a reactive compatibilizer could enhance the interfacial interaction of PS and PA6-TFAA, resulting in drag force efficiently transferred from the PA6-TFAA domain to PS droplets. Due to the greater orientation of fibers, the nanofiber bundle membranes had a high tensile strength and tensile module of 1.7 MPa and 63.0–84.7 MPa. As a result of the greater roughness on the surface of the nanofiber bundle membrane, WCA reached 145.0 ± 0.5°, approaching superhydrophobicity. The oil selective adsorption was from 31.0 to 61.3 g/g, resulting from a high internal void and specific area. In addition, the membranes showed a high water-in-oil emulsion separation of above 99% in silicon oil/water and paraffin wax/water. Moreover, the structure of nanofiber bundles also helps to improve air filtration efficiency. The highest air filtration efficiency of the nanofiber bundle membranes could reach above 96%.
通过静电纺丝 PS/PA6-TFAA 与 CNT 或 PS-co-TMI 共混，然后用甲酸刻蚀制备纳米纤维束。CNT 可以提高前驱体的电导率，以提高射流中 PS 液滴的电场阻力。同时，用作反应增容剂的 PS-co-TMI 可以增强 PS 和 PA6-TFAA 的界面相互作用，从而使阻力从 PA6-TFAA 结构域有效地传递到 PS 液滴。由于纤维的取向更大，纳米纤维束膜具有 1.7 MPa 和 63.0–84.7 MPa 的高拉伸强度和拉伸模量。由于纳米纤维束膜表面的粗糙度较大，WCA 在 0.5° ±达到 145.0°，接近超疏水性。油选择性吸附为 31.0 至 61.3 g/g，这是由于较高的内部空隙和比面积造成的。此外，膜在硅油/水和石蜡/水中表现出超过 99% 的高油包水乳液分离率。此外，纳米纤维束的结构还有助于提高空气过滤效率。纳米纤维束膜的最高空气过滤效率可达 96% 以上。