Highly Durable Superhydrophobic Polymer Foams Fabricated by

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Applications of Polymer, Composite, and Coating Materials

Highly Durable Superhydrophobic Polymer Foams Fabricated by Extrusion and Supercritical CO2 Foaming for Selective Oil Absorption Hao-Yang Mi, Xin Jing, Yue-Jun Liu, Lengwan Li, Heng Li, Xiang-Fang Peng, and Huamin Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Materials & Interfaces

Highly Durable Superhydrophobic Polymer Foams Fabricated by Extrusion and Supercritical CO2 Foaming for Selective Oil Absorption Hao-Yang Mi1, 3, a*, Xin Jing1, a, Yuejun Liu1, Lengwan Li4, Heng Li3, Xiang-Fang Peng4, and Huamin Zhou2* 1

School of Packaging and Materials Engineering, Hunan University of Technology, Zhuzhou, 412007, China 2

State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, 430000, China

3

Faculty of Construction and Environment, Hong Kong Polytechnic University, Hong Kong, 518000, China 4

Department of Industrial Equipment and Control Engineering

South China University of Technology, Guangzhou, 510640, China

Corresponding Authors: H.Y. Mi E-mail: [email protected] H. Zhou E-mail: [email protected]

a

the first and second authors contribute equally to this work.

Keywords:

Superhydrophobic;

polymer

foam;

polytetrafluoroethylene

ACS Paragon Plus Environment

durability;

oil

absorption;

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: The severe water contamination caused by oil leakage is calling for low cost and high performace absorbent materials for selective oil removal. In this study, a scalable green method was proposed to produce polypropylene (PP)/polytetrafluoroethylene (PTFE) composite foams via conventional processing techniques including twin screw extrusion and supercritical carbon dioxide foaming. To produce the superhydrophobic foam, microsized and nano-sized PTFE particles were melted blended with PP and subsequently foamed. Ascribed to the nanofibrillation of micro-sized PTFE during processing, the fabricated foam exhibited a special highly porous structure with PTFE nanofibrils and nanoparticles uniformly distributed on the pore surfaces within the PP matrix, which resulted in a remarkable high water contact angle of 156.8° and a low contact angle hysteresis of 1.9°. Different from traditional surface modified superhydrophobic absorbers, the foams prepared are entirely superhydrophobic which means they remain superhydrophobic when being fractured or cut. Moreover, they are highly durable and maintained superhydrophobic when subjected to ultrasonication and mechanical sanding. When used in selective oil absorption, the durable foams exhibited excellent absorption efficiency, and high stability in repetitive and long term use. These advantages make the PP/PTFE foam a promising superabsorbent material for water remediation.


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1. Introduction Oil leaking is becoming a severe environmental problem that significantly pollutes open water and affects the ecosystem.

1-2

Developing low cost and high performance oil absorbent materials is an urgent need for

water remediation.

3-4

Inspired by the lotus leaf effect in nature, superhydrophobic materials are attracting

tremendous attention in various applications including self-cleaning, corrosion resistance, anti-icing, and drag reduction.

5-9

Recently, researchers found that when superhdrophobic modification was applied to porous

materials, the functionalized porous materials can be used for selective oil absorption and oil/water separation, which provides a new path for oil recycling and water remediation. 10-11 So far, commonly investigated superhdrophobic porous materials can be roughly classified into three categories based on the materials and fabrication techniques. Surface modification of pristine porous substances, such as melamine sponge,

12

polyurethane sponge,

13-14

nickel sponge

15

and copper mesh,

11

is a

simple way to render porous material superhydrophobicity. The modification is usually achieved by particulate coating and chemical grafting, which essentially creates a low surface energy rough surface on the pristine porous substances. 16-17 However, the modification process is usually performed in a batch to batch manner in aqueous or organic solutions, during which vast amount of modification solutions are wasted and may induce further water contamination. Moreover, the disassociation of particles is another potential environmental threat when the modified sponges were used in long-term.

12, 18

Another attractive superabsorbent materials are

ultralight carbon based aerogels because of their low densities and high absorption capacities.

19-20

These

aerogels are normally fabricated via pyrolysis of nature materials (i.e. cotton, wood and even bread) and polymer foams,

21-23

lyophilization of hydrogels like graphene24, carbon nanotube

hydrothermal process followed by lyophilization.

26

14

and cellulose

25,

and

However, these fabrication methods have relatively high

cost, and are difficult to be scaled up. In addition, the durability of the aerogels is usually low in long-term. Alternatively, researchers have been trying to use conventional low cost processing methods to produce superhydrophobic porous polymeric materials. The most frequently used method is phase inversion from polymer solutions.

27-28

For example, a superhydrophobic poly(vinylidene fluoride) (PVDF) membrane

prepared by phase inversion could separate oil from oil-water emulsion.


29.

However, phase inversion has


obvious drawbacks, such as the use of organic solvent, difficult to fabricate 3D samples, low hydrophobicity, high density and low absorption capacity. Recently, supercritical gas foaming (SGF) showed potential capability to be used in the fabrication of superhydrophobic polymer foams. SGF uses supercritical gas, like CO2 and N2, as physical blowing agent to create porous structures within polymer matrix when the gas turns from supercritical state to gas state during sudden pressure drop.

30-31

SGF is a promising cost effective and environmental friendly processing method

that is attracting more and more attentions from various applications, such as lightweight automobile products, thermal/acoustic insulation, electro-magnetic interference (EMI) shelding, tissue engineering, packaging, and absorption. 32-34 Attempts have been made to fabricate superhydrophobic foams via extrusion foaming in recent years.

35-36

However, the foams produced often have lower hydrophobicity and absorption capacity when

compared with particulate modified foams and carbon based aerogels. Hence, there is still a great room for the foams fabricated by SGF to improve. Aiming to produce a low cost, high performance and highly durable superhydrophobic foam, we propose a new methodology in this study based on conventional melt extrusion and SCF. Low surface energy polytetrafluoroethylene (PTFE) was combined with polypropylene (PP) via twin screw extrusion and the composite was then foamed using supercritical CO2. We recently found that microsized PTFE particles can be in situ fibrillated into web-like nanofabrils when been processed with polymers via twin screw extrusion due to the high shear stress generated by the rotation of screws. 37-39 Taking this advantage, microsized and nanosized PTFE particles were used to create a special multidimensional hierarchical structure in the PP/mnPTFE foam, that is microsized PP pores decorated with web-like PTFE nanofibers and PTFE nanoparticles. This special structure resulted to a remarkable superhydrophobicity with a water contact angle (WCA) of 156.8° and a low contact angle hysteresis (CAH) of 1.9° on the fractured surface of the foam. Most importantly, the foams are entirely superhydrophobic and highly robust since they remain superhydrophobic when been fractured, cut, sonicated or even sanded. Moreover, the excellent performance of the PP/mnPTFE foam in selective oil absorption was demonstrated as well in this study.


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2. Experimental 2.1. Materials Polypropylene (PP) pellets were purchased from Sinopec. Two types of PTFE powders with particle size in micro scale and nano scale were obtained from Fluon. Chemical reagents used for oil absorption tests, such as hexane, chloroform, octane, toluene, and oil O red, were purchased from Sigma–Aldrich and used as received without further purification. Deionized water was used throughout the experiment. 2.2 Superhydrophobic foam fabrication To fabricate PP/mnPTFE composite, 1kg PP pellets were mixed with 25 g micro PTFE (mPTFE) and 25 g nano PTFE (nPTFE) using a mechanical mixer followed by drying at 75 °C for 2 h. The mixture was then loaded into a twin screw extruder with barrel temperature set to 100°C, 150°C, 170°C, 190°C, 200°C, and 200°C from the hopper to the die. The melt blending was performed at a screw rotation speed of 100 rad/min. The extrudate was cooled in water and granulated using a pelletizer. The PP/mnPTFE composite pellets were sufficiently dried and compressed into 0.5 mm sheets using hot embossing at 190 °C. The obtained PP/mnPTFE sheets were used for the supercritical CO2 foaming process. The PP/mnPTFE sheets were sealed in a mold with rectangular cavity followed by injection of supercritical CO2. The temperature and pressure of the cavity was maintained at 180 °C and 20MPa for 2 h to allow PP/mnPTFE to saturate supercritical CO2. Then the pressure was rapidly released and the mold was cooled using circulating chilled water, in order to prevent pore collapse and achieve micro porous structures. After about 10 min cooling, the superhydrophobic PP/mnPTFE foam was obtained. The bulk foam readily has superhydrophobicity without needing of any additional treatment. The foam was cut or fractured into pieces for subsequent experiments. For comparison, PP/mPTFE foam with only microsized PTFE (50 g mPTFE in 1 kg PP) was also fabricated under the same condition. 2.2. Morphology and wettability characterization



The morphology of fabricated PP and PP/PTFE foams was observed using a digital LEO GEMINI 1530 scanning electron microscope (SEM) (Zeiss, Germany) at a voltage of 3 kV. The surfaces of the foams were cleaned with nitrogen and sputtered with a thin film of gold prior observation. The diameter of pores for the foams was measured using Image Pro-plus software from SEM images. The water contact angle (WCA) measurements were carried out at room temperature with a Dataphysics OCA 15 optical contact angle measuring instrument. A cone tip needle was use for the WCA measurement based on the sessile drop method. A blunt needle was used for the measurement of contact angle hysteresis (CAH). In the measurements, 7 µL DI water was dispensed and resorbed on material surfaces. CAH was calculated from the difference between advancing angle and receding angle. Contact angles of PP/mnPTFE foam to different aqueous solutions with pH values ranging from 1 to 13 were measured as well. 2.3. Absorption capacity and efficiency The absorption capacity of foam was measured by weighing the foam before and after saturation with solvent. The weight gain (g/g) was calculated by,

AC  (m  m0 ) / m0 where m0 and m represent the mass of the foam before and after saturation. The selective oil absorption efficiency was tested by using the foam to absorb specific amounts of oil from water. The absorption efficiency (%) was calculated by,

AE  (m  m0 ) / ml 100 where (m-m0) represents the mass of oil absorbed from the water and ml is the original mass of oil added to the water. 2.4 Reusability and durability The PP/mnPTFE foam was saturated with oils, and then the oils were removed by distillation at 100°C for 15 min. The absorption-desorption was repeated 10 times on the same specimen, in order to investigate the change in absorption capacity in multiple cycles.


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PP/mnPTFE foams were subjected to ultasoinication in ethanol for up to 60 min to evaluate the effect of ultrasonication on wettability. PP/mnPTFE foams were also abrased using sand paper under 1 kg load for up to 20 m to evaluate the effect of surface destruction on their wettability. The WCA of specimens were measured during these tests to evaluate the durability of the PP/mnPTFE foams.

3. Results and Discussion 3.1. Fabrication and morphology Aiming to develop low cost and highly durable superhydrophobic foams that are readily available for oil absorption, we chose conventional polymers (PP and PTFE) as raw materials and environmental friendly melt extrusion and supercrifical CO2 foaming as fabrication methods. According to the basic Cassie-Baxter theory, surface energy and surface microstructures are two key factors for the wettability of a surface.

40

From equation 1, it is clear that improving the intrinsic

contact angle (θ) and area fraction of water in contact with the gas are two ways to achieve higher apparent contact angle (θ*). θ can be enhanced by lowering the surface energy, while fLG is normally increased by introducing complex surface microstructures.41

cos  *  f SL cos   f LG

(1)

where, θ* is the apparent contact angle, θ is the contact angle on the flat surface (intrinsic contact angle), fSL is the fraction of the projected area of the solid in contact with the liquid, and fLG is the fraction of the projected area of the liquid in contact with the gas. In order to reduce the surface energy, PTFE was incorporated into PP matrix by conventional melt blending using twin screw extrusion. As depicted in Fig. 1a, PTFE particles with different sizes were mixed with PP matrix. The mPTFE exhibits irregular shape and an average diameter of 13.7±3.9 µm, while the average size of nPTFE is 241.2±56.9 nm and it presents a sphere or ellipse morphology as shown in Fig. S1. Two types of PTFE particles were introduced into PP matrix because mPTFE has lower cost and has a special ability to be in situ fibrillated into nanofiber webs under strong shear force during twin screw extrusion at high temperature.

37-39, 42

The addition of nPTFE could further reduce



the surface energy and increase the complexity of hierarchical microstructure, which is highly beneficial for enhancing hydrophobicity.43 In situ fibrillated PTFE nanofibrils and PTFE nanoparticles achieved uniform distribution in PP matrix with the help of twin screw extrusion (Fig. 1b). The PP/mnPTFE composite was then saturated with supercritical CO2 which is highly compatible with polymer melt (Fig. 1c). Upon foaming, CO2 transferred from supercritical state to gas state, and led to immediate volume expansion (Fig. 1d). Importantly, many favourable activities took place during the foaming process. First, the voids created in the PP/mnPTFE foam renders the material absorption ability; Second, the PTFE nanofibrills and nanoparticles that are initially embedded in PP matrix are exposed on the surface of pore walls; Third, a multiscale hierarchical structure was created by the micro pores as well as the PTFE nanofibrills and nanoparticles on the foam surface and inside of the foam. Therefore, the fabricated foams are entirely superhydrophobic and can maintain or even improve their hydrophobicity when been cut, fractured or sanded, resulting to ultrahigh stability and durability (Fig. 1e). Due to their multiscale hierarchical porous structure and low surface energy, PP/mnPTFE foams are expected to have superhydrophobicity and superoleophilicity simultaneously and thus can be used for selective oil absorption (Fig. 1f).

Fig. 1. Illustration of the fabrication process of entirely superhydrophobic PP/mnPTFE foam. (a) PP/mnPTFE composites, (b) nanofibrillation of PTFE by twin screw extrusion, (c) saturating PP/mnPTFE with supercritical CO2, (d) foaming process of PP/mnPTFE, (e) PP/mnPTFE foam


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maintains superhydrophobicity when been fractured, and (f) PP/mnPTFE foam can be used for selective oil absorption.

This study was designed to verify the above methodology and to investigate the properties of the developed PP/mnPTFE foams. Recent studies have shown that PTFE can form nanofibrillated weblike structures in polymer matrix, while excessive PTFE dosage would induce unfibrillated PTFE particles embedded in polymer matrix and led to week points in the composite material.

37-39

Uniform

filler distribution is critical for nanocomposite materials to achieve property improvement. It is found in our preliminary experiments that with a loading content of 2.5% for both mPTFE and nPTFE, PTFE nanofibrils and nanoparticles were able to be uniformly distributed in PP matrix without aggregation (Fig. S2). The CO2 foaming process was also optimized, in order to obtain the optimum porous structures. As shown in Fig. S3, it was found that increase foaming pressure and temperature will lead to lager pore size, more open pores, and more exposed PTFE nanofibrils. However, pores would collapse if temperature and pressure were too high. The optimum foaming condition (20 MPa and 180 °C) was used to fabricate the composite foams. The morphology of fabricated PP/mPTFE foam and PP/mnPTFE foam was shown in Fig. 2. It can be seen that uniform micro sized pores were formed in the foaming process. From the high magnification images, it was clearly seen that numerous nanofibrills were presented on the pore walls for both PP/mPTFE foam and PP/mnPTFE foam. This was because web-like PTFE nanofibrils have lower expansion ratio than PP matrix, thus initially embedded PTFE nanofibrils were exposed on the pore surface during the growth of bubbles in PP matrix (Fig. 2a). On the contrary, PTFE nanoparticles were able to freely move along with the PP matrix during expansion, therefore, nanoparticles exposed on pore surfaces were much less than the nanofbrils. Nevertheless, PTFE nanoparticles were presented on the fractured surface which would reduce the surface energy and enhance the surface roughness (Fig. 2b). The average pore size was measured from the SEM images (Fig. S4). The PP/mPTFE foam had a pore size of 11.5±2.9 µm, and the pore size was 9.4±2.3 µm for PP/mnPTFE foam. The smaller pore size of PP/mnPTFE foam was because PTFE nanoparticles acted as nucleation sites in the initial stage of pressure release. The



heterogeneous nucleation of bubbles caused higher pore density and resulted smaller pores.44-45 In addition, it was found that the pores formed were interconnected with each other showing an open porous structure, which is highly favourable for the application of oil absorption.

Fig. 2. SEM images showing the morphology of (a) PP/mPTFE foam and (b) PP/mnPTFE foam.

3.2 Superhydrophobicity and stability The wettability of as prepared foam surfaces and fractured surfaces was evaluated by the water contact angle (WCA) test. The measurement was repeated on the same sample after they were stored in 150 °C oven or in liquid nitrogen for up to 3 h in order to investigate their tolerance to extreme temperatures. As shown in Fig. 3, the WCA of foam surface was lower than that of fractured surface for both PP/mPTFE foam and PP/mnPTFE foam, which was because the fractured surface is usually rougher than the foam surface where pore collapse often happens. The WCAs of PP/mPTFE foam surface and fracture surface were 136.8°, and 142.7° respectively, while the WCAs of PP/mnPTFE foam surface and fracture surface were 151.6°, and 156.8° respectively. The contact angle hysteresis (CAH) of PP/mPTFE foam surface was 42.4°, but it was 4.9° for the PP/mnPTFE foam surface. The high WCA and low CAH of PP/mnPTFE foam were attributed to two reasons. First, the finer pores of


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PP/mnPTFE foam were able to increase the amount of pore walls that can support water bead and thereby increase the number of air pockets under water the droplet. The larger pores of the PP/mPTFE foam, although supposes to have larger air pockets, but insufficient interfacial surface tension usually causes sagging of water bead and finally result to an impregnate Wenzel state. 10, 46 Second, it has been found that increase the complexity of material surface by introducing multidimensional and hierarchical structures are beneficial for improving hydrophobicity.

12, 47-48

The introduction of PTFE

nanoparticles created additional microstructures apart from PP micro pores and PTFE nanofibrils. As illustrated in Fig. 4a, the multiscale hierarchical structure composed of low surface energy PTFE is highly favourable for improving the apparent contact angle according to equation 2 which explains that the apparent contact angle can be increased by adding multiple levels of micro/nano structures on material surface. 46, 49

cos *n  (1  f LG ,n )cos *n 1  f LG ,n

(2)

where, n = 1, 2, 3… is the number of layers of hierarchical structures, fLG is the projected area fraction of the liquid–gas interface of layer n. θ*n and θ*n-1 are the apparent contact angles of the hierarchically structured substrate with n and n-1 layers of nanostructures. The fabricated foams showed excellent stability in extreme temperature conditions as shown in Fig. 3. When submerged in liquid nitrogen, all foams showed negligible reduction in WCA of less than 1°, indicating low temperature treatment has no effect on the microstructure and surface chemistry of the foams. It was found that when stored at 150° for 3 h, the decrease of WCA of all foams is still within 2°. This outstanding thermal resistance was attributed to the thermal barrier effect of PTFE which was reported in our previous work. 39 Furthermore, the PP/mnPTFE foam also resistant to corrosive liquids as shown in Fig. S5. The WCA of PP/mnPTFE foam was over 153° when tested using aqueous solutions with pH ranging from 1 to 13, suggesting their high stability of superhydrophobicity in harsh environment.



Fig. 3. Water contact angle (left) and contact angle stability in extreme temperatures (right) of (a) PP/mPTFE foam surface, (b) PP/mPTFE fracture surface, (c) PP/mnPTFE foam surface, and (d) PP/mnPTFE fracture surface.

Unlike surface engineered foams or particulate modified sponges, which usually lose superhydrophobicity when the functional layer is damaged,

12, 50

the PP/mnPTFE foams developed in

this study are entirely superhydrophobic attributing to their porous structure and uniformly distributed PTFE nanofibrils and nanoparticles. This special structure renders the foam remarkable robustness of superhydrophobicity. As demonstrated in Movie 1, the cut surface of a PP/mnPTFE foam showed very low water adhesion. The WCA of the cut surface was 154.1°, and the CAH was 3.1°. The water droplet can easily move across the cut surface suggesting high water repellence. When the PP/mnPTFE foam was fractured in liquid nitrogen, the superhydrophobicity of fractured surface was even higher as demonstrated in Movie 2. The water droplet showed negligible adhesion on the surface and can freely move across the fluctuant surface with no dragging effect. The WCA and CHA of fractured foam surface were 156.8° and 1.9°, respectively, which is rarely achieved by polymeric


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materials. 17, 51-52 Another possible reason for their outstanding superhydrophobicity is the formation of re-entrant contact in the soil-liquid-gas composite interface as illustrated in Fig. 4a. The angle (θ) between solid-liquid interfacial surface tension (γSL) and liquid-gas interfacial surface tension (γLG) is an acute angle for re-entrant contact circumstance, thus the resultant surface tension γ (equation 3) was greater than the normal contact circumstance. 53

  ( SL   SG ) cos    LG cos(   )

(3)

where, α is the re-entrant angle. The gravitational force of water droplet is balanced by the sum of γ from all soil-liquid-gas composite interfaces. γ normally increases as the sagging of water droplet on the structured surface due to the increase of the composite interface profile, and the surface would be impregnated with water if γ is not sufficient to support the water droplet.

54

On the contrary, γ is higher in the re-entrant contact

circumstance, thus, the balance would be achieved at a higher level, which means the water droplet could maintain a more sphere shape and the apparent contact angle would higher as well. As can be seen from Fig. 3d and Movie 2, the water droplet well maintained sphere shape on the fractured PP/mnPTFE surface which might because of the presence of re-entrant structure on foam surface. The superhydrophobicity of PP/mnPTFE foam is also demonstrated by the formation of an air layer when been immersed in water (Fig. 4b), and the rolling off of water droplets from the surface of PP/mnPTFE foam (Fig. 4d).



Fig. 4. (a) Schematic illustration of contact state of water droplet on the surface of PP/mnPTFE foam, (b) an air layer was formed when the PP/mnPTFE foam was immersed in water, (c) PP/mnPTFE foam can float on water and immerse in hexane, (d) water droplet rolling off PP/mnPTFE foam surface, (e) superoleophilicity and superhydrophobicity demonstration of PP/mnPTFE foam, (f) PP/mnPTFE foam remain superhydrophobic after absorbing oil.

3.3 Selective absorption performance and durability Although PP/mnPTFE foams possess excellent superhydrophobicity, they are still superoleophilic as demonstrated in Fig. 4c, in which PP/mnPTFE foam was floating on top of water, while it was immersed in hexane suggesting the air in the foam has been replaced by hexane. Fig. 4e demonstrated the superoleophilicity and superhydrophobicity of the foam. Fig. 4f showed that the PP/mnPTFE foam remained superhydrophobic after absorbing hexane, which verified its capability to be used for selective oil absorption. The absorption capacity of PP/mnPTFE foam to various oils and organic solvents was investigated. As shown in Fig. 5a, the absorption capacity of PP/mnPTFE foam ranges from 4.6 ~ 9.1 times weight gain depending on the density of different oils and solvents. Although the absorption capacity of the PP/mnPTFE foam was lower than some superabsorbers based on surface modified sponges and ultralight graphene aerogels10,

12, 55,

it outperformed all traditional absorbent

materials (such as activated carbon, Bagasse, and wood fibers) and many polymer based absorbent


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materials (such as polypropylene web and poly (vinylidene fluoride) aerogel) as compared in Table S1. Moreover, the PP/mnPTFE foam possesses unique advantages including low fabrication cost, easy to scale up, entirely superhydrophobicity, and robust performance, which make it a promising selective oil absorption material for water remediation in practical applications.

Fig. 5. (a) absorption capacity of PP/mnPTFE foam towards various oils and organic solvents, (b) absorption capacity stability of PP/mnPTFE foam to hexane and chloroform in ten absorptiondesorption cycles.

Furthermore, it is also important to investigate the stability of absorption capacity in repetitive use. PP/mnPTFE foams were used to absorb hexane and chloroform for 10 cycles. As shown in Fig. 5b, the absorption capacity of PP/mnPTFE foams maintained about the same when absorbing hexane and chloroform, and the absorbed oils can be fully removed by distillation in 10 cycles. To verify the long term stability, the absorption capacity measurement was carried out on a PP/mnPTFE foam stored in air for 5 month. As shown in Fig. S6, the foam maintained about the same capacity in 5 month. As a demonstration, PP/mnPTFE foams were also used to selective absorb hexane atop of water (Fig. 6a) and chloroform under water (Fig. 6b). The process was shown in Movie 3, from which it is seen that PP/mnPTFE foams can quickly absorb both light oil and heavy oil from water and remain hydrophobic after absorption. Noteworthy, it is clearly seen that air bubbles came out from the foam when absorbing chloroform under water, indicating the air in the interior pores were replaced by oil due to the superoleophilicity of the foam and the capillary effect. The selective absorption efficiency of the foams was also investigated by measuring the weight of oil absorbed and the loss of water during



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absorption. As shown in Fig. S7, the absorption efficiency maintained about 100% in 5 cycles of selective absorption of both hexane and chloroform, indicating high selectivity when absorbing oils from water using PP/mnPTFE foam.

Fig. 6. Demonstration of selective oil absorption using PP/mnPTFE foam (a) absorbing hexane atop water and (b) absorbing chloroform under water. Oils were died red using Oil O Read.

Hitherto, a number of superhydrophobic foams with high WCAs have been developed, while the durability is one of the major issues limiting their practical application. When subjected to high frequency

vibration

(i.e.

ultrasonication)

or

mechanical

abrasion,

particulate

modified

superhydrophobic sponges usually face a particle disassociation issue, and ultralight superhydrophobic aerogels often suffer from permanent deterioration, which not only cause the reduction of their hydrophobicity, but also lead to a potential secondary contamination to the water source.

12, 18, 27, 56

To

investigate the durability of PP/mnPTFE foams, they were subjected to ultrasonication in ethanol, and intense abrasion with sand paper. The change of surface morphology and WCA was examined as shown Fig. 7. After ultrasonication for 60 min, the PP/mnPTFE foam surface maintained highly porous structure with the same pore size, and no obvious surface deformation was observed (Fig. 7a and b). As a result, the PP/mnPTFE foam maintained about the same WCA during sonication (Fig. 7g), and still showed a high WCA of 155.6° after 60 min sonication (Fig. 7c). The absorption capacity of specimens after ultrasonication was measured. It was found that the absorption capacity also remained


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about the same regardless of the absorbed oil (Fig. S8). These results indicate that the PTFE nanofibrils and nanoparticles were embedded tightly in PP matrix and were barely affected by sonication. When PP/mnPTFE foams were subjected to sand paper abrasion for 20 m (Fig. 7h), the

Fig. 7. (a and b) Morphology and (c) WCA of PP/mnPTFE foam after ultrasonication in ethnol for 60 min, (d and e) morphology and (f) WCA of PP/mnPTFE foam after abrasion using sand paper for 20m, (g) WCA of PP/mnPTFE foam during ultrasonication test, (h) photograph showing the sand paper abrasion test, and (i) WCA of PP/mnPTFE foam during sand paper abrasion test.

foam structure was seriously damaged as shown in Fig. 7d and e. Pores were torn and collapsed showing an elongated pore morphology because of the back-and-fourth mechanical sanding. Nevertheless, the foam surface maintained a highly porous structure, and the WCA was gradually reduced from 156.8° to 150.2° when been abrased for 20 m. In the absorption test, the foams showed



gradually decreased absorption capacity as well, which might be caused by the damage of foam surface (Fig. S9). Although both WCA and absorption capacity were slightly decrease in the abrasion test, the PP/mnPTFE foam remained superhydrophobic, which is a good indication of their high durability. Moreover, due to the entirely superhydrophobic nature of PP/mnPTFE foam, the foam surface can be easily resorted at any time by removing the damaged surface through cutting or fracturing, which leads to an unlimited theoretical lifespan for the PP/mnPTFE foams. Considering the low fabrication cost and environmental friendliness of the PP/mnPTFE foams, they hold high potential to be used as selective oil absorbers in practical application. Furthermore, this study also provides new insights into the scalable fabrication of superhydrophobic foams, and the method proposed in this study should be widely applicable to a variety polymers.

Supporting Information SEM images of PTFE particles, nanocomposites and foams, pore size distribution of foams, WCA to aqueous solutions with different pH, selective absorption efficiency, and the absorption performance change when subjected to ultrasonication and abrasion are available in supporting information (PDF). Water droplet on cut surface of PP/mnPTFE foam (AVI) Water droplet on fractured surface of PP/mnPTFE foam (AVI) Selective absorbing hexane atop water and chloroform under water (AVI)

Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (51603075; 21604026), the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (P2019-027).

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Highly Durable Superhydrophobic Polymer Foams Fabricated by

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