Superoleophilic and Flexible Thermoplastic Polymer Nanofiber

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Superoleophilic and Flexible Thermoplastic Polymer Nanofiber Aerogels for Removel of Oils and Organic Solvents Jianwei Lu, Dandan Xu, Junkan Wei, Shan Yan, and Ru Xiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07004 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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ACS Applied Materials & Interfaces

Superoleophilic and Flexible Thermoplastic Polymer Nanofiber Aerogels for Removal of Oils and Organic Solvents Jianwei Lu, Dandan Xu, Junkan Wei, Shan Yan. Ru Xiao* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 2016, P. R. China Correspondence to: Ru Xiao (E-mail: [email protected])

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ABSTRACT Chemical crosslinked poly (vinyl alcohol-co-ethylene) (EVOH) nanofiber aerogels (NFAs) were fabricated employing an economical and facile freeze-drying process. The manufactured chemical crosslinking nanofiber aerogel was successfully confirmed by scanning electron microscopy (SEM), attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR), and X-ray diffraction (XRD). The resulting aerogels showed high porosity (>99%), superior elasticity, elastic durability， high hydrophobicity and superoleophilicity without any other hydrophobic modification. The crosslinked EVOH NFAs exhibited excellent absorption capacity (ranging from 45-102 times their own weight) when exposed to various oils and organic solvents, which was observed to be higher than for most reported sorbents in the literature. Consequently, it is envisaged that the crosslinked EVOH NFA would play an important role in many fields of pollution removal.

KEYWORDS:

crosslinked;

EVOH

NFA;

elasticity;

superoleophilicity; pollution removal


hydrophobicity;

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Introduction With the rapid development of economic globalization and industrialization, the emission of pollutants (mainly petroleum products and toxic organic solvents) pose a clear threat to our living environment and ecosphere,1-3 so their elimination/reduction is accordingly urgent. Several current approaches have been developed to tackle the problems, such as sorbents,4-5 skimmers,6 and burning. However, most of the methods exhibit poor efficiency or have expensive costs.7 Therefore, it is essential to further design efficient, low cost materials with high absorption for pollutants.

In this regard, aerogels are attractive for their high porosity, high continuity, low density , large specific surface area and high absorption capacity, 8which makes them one of the best candidates for the removal of oil/organic pollutants and the separation of oil/water emulsion.6, 9-10 However, traditional inorganic particle aerogels generally suffer from low strength and high fragility with poor mechanical stability,11 thus limiting their practical applications in many fields.12 Consequently, aerogels with excellent mechanical properties are highly sought after. Previous attempts have proved that the introduction of continuous organic fibrous structures would effectively reinforce the mechanical and resultant properties of the aerogels.13-14 Therefore, organic fibers ( cellulose nanofibers,15-16 PAN nanofibers,17 etc) are proposed to represent an optimal material to obtain aerogels with promising performance for the absorption of pollutants.

Nanofibers are at the forefront of fibrous materials, given their high aspect ratio, superfine diameter, high strength and high absorption,18-19 which allows for their assembly into three dimensional (3D) nanofiber aerogels (NFAs). Several methods have been developed for the preparation of nanofibers, including electrospinning,20-23 melt blowing,24-25 polymerization,26-27

phase separation28-29 and melt blending

extrusion30-31 Thermoplastic polymer nanofibers–poly(vinyl alcohol-co-ethylene) (EVOH) nanofibers, prepared by melt extrusion were selected as the building blocks,



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in light of their abrasion resistance, biodegradability, biocompatibility, good processability, low cost and nontoxicity to humans. 32-33 To the best of our knowledge, the thermoplastic polymer NFAs have not been previously reported in the literature. The EVOH NFAs were produced from the precursor EVOH nanofibers suspension using a gelation-free and facile freeze-drying process. However, the challenge of creating the thermoplastic polymer aerogels from EVOH nanofibers lies in the inability to form strong binding interactions between individuals. This is ascribed to the inability of EVOH nanofibers suspension to form a stable sol-gel system, which is often found in the preparation of cellulose aerogels34-35 and boehmite nanofiber aerogels.36 Instead, the EVOH nanofibers are interlinked solely by physical forces (hydrogen bonding and electrostatic forces37) in the prepared aerogel, and the physical binding can be easily disrupted by any external force,38 therefore limiting the use as absorbents.

Motivated by this and in order to improve their mechanical, physical and absorptive capacity, the chemical crosslinking method would be utilized for the aerogels herein reported. A frequently used crosslinking agent glutaraldehyde (GA) was chosen to bound the nanofibers tightly together. The condensation reaction would occur between the hydroxyl groups in EVOH nanofibers and the aldehyde groups in GA, upon heat (ca. 75°C).39-40 Simultaneously, as the polymerization process occurs, the hydrophilic hydroxyls were replaced by the hydrophobic keto groups, leading to the hydrophobic obtained aerogel. The hydrophilic EVOH NFAs exhibited hydrophobic and superoleophilic only by crosslinking without any other hydrophobic modification (silanization,41-42 TiO2 coating,43-44 phase separation45 and pyrolysis .46)

In this work, we first explored the preparation of the chemically crosslinked thermoplastic polymer NFAs –EVOH NFAs with low density, high porosity (> 99%), excellent mechanical properties, hydrophobic (water contact angel > 145°) and superoleophilicity by a versatile and simple freeze-drying method. The aerogels exhibit superabsorbent properties (ranging from 45 to 102 times their own weight) for


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oil/organic solvent and rapid shape recovery ability. Consequently, these aerogels are anticipated to have a great potential for pollutants removal and environmental protection.

Experimental section Materials Poly(ethylene-co-polyvinyl alcohol) (EVOH, Model: ET3803) with the density of 1.17 g cm-3 and 38 mol % ethylene was supplied by Nippon Gohsei, Japan. Cellulose acetate butyrate ester (CAB, Model: 381-20, butyryl content 37 wt %, acetyl content 13.5 wt %, and hydroxyl content 1.8 wt %) was purchased from the Eastman Chemical Company (The United States). Acetone with the density of 0.7845 g cm-3 was supplied by Beijing YanShan Petrochemical Corporation (China). Tert-butanol (t-BuOH, >98%), Acetic acid (99.5%) and Glutaraldehyde aqueous solution (GA, 25 wt %) were obtained from Sinopharm Chemical Reagent Co., Ltd.

Preparation of EVOH nanofibers The EVOH nanofibers used in this experiment were prepared according to a previously published procedure.47 Briefly, The EVOH resin and CAB powders were dried for 24 h at 80 °C in a vacuum drying oven, and then EVOH resin was mixed with CAB powders as a sacrificial matrix in a blend ratio of CAB/EVOH=80/20. The mixtures were fed into the corotating twin-screw extruder (D=16mm, L/D=40; EUROLAB16, Thermo-Haake Co.) with a screw speed of 50 rpm and a melting zone temperature ranging from 200 to 225 °C. The extrudate was water-cooled to ambient temperature. Finally, EVOH nanofibers were obtained after removing CAB from the extrudate of CAB/EVOH via a Soxhlet extractor in acetone.

Preparation of EVOH nanofiber suspensions In a typical experiment for the preparation of EVOH NFAs with a density of 12.3 mg cm-3. 1g of EVOH nanofibers were dispersed in 100 ml of an acetic acid aqueous



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solution (1 vol %) and t-BuOH mixture (weight ratio of 4:1). Uniform EVOH nanofiber suspensions were obtained by homogenizing the mixture for 30 min at 12000 rpm with an IKA T25 homogenizer, and then a weighted amount of crosslinker GA (25 wt%) was added to the EVOH nanofiber suspension with mechanical stirring for 10 min to obtain the native EVOH nanofiber suspensions.

Fabrication of crosslinked EVOH NFAs The obtained suspensions were poured into the desired plastic container. The EVOH nanofiber suspensions were then frozen at -65°C in a cryogenic refrigerator. The frozen samples were frozen-dried in a lyophilizer (VirTis Freezemobile 25EL Sentry 2.0, USA) for 48 h to obtain the uncrosslinked EVOH NFAs. The resulting aerogels were crosslinked in a vacuum oven at 75°C for 3 h, giving rise to crosslinked EVOH NFAs finally.

Characterization The densities were calculated by measuring the mass and volume of the EVOH NFAs. The morphology of EVOH NFAs was characterized using a scanning electron microscopy (SEM, S-3000N, Hitachi Ltd., Japan). The functional groups of EVOH NFAs were characterized using an attenuated total reflection-Fourier transform infrared spectrometer (ATR-FTIR, Nicolet 6700, Nicolet Inc., USA) in the range of 4000-400 cm-1 with a resolution of 4 cm-1. The X-ray diffraction (XRD) patterns of EVOH NFAs were characterized using a X-ray diffractometer (XRD, D/max-2550PC, Rigaku Co., Japan). The samples were scanned in 2θ angles from 5° to 60° with a scanning rate of 15°/min at ambient temperature. Water contact angle (WCA) and oil contact angle (OCA) measurement with water/oil volume 3 µL was performed using a contact angle goniometer (CA, OCA40 Micro, Dataphysics Ltd., Germany) to evaluate wettability of EVOH NFAs. The compression measurements were characterized using Instron testing system (Model 5969, Instron Co., USA) fitted with a 100N load. Cylindrical EVOH NFAs with a diameter of ~30 mm and a height of ~28 mm were used. The stress-strain curves with strain of 40, 60 and 80% were


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measured at a strain rate of 30 mm/min. The cyclic compression test was measured at a strain rate of 300 mm/min. Density of EVOH NFAs The density of EVOH NFAs was calculated by the following formula (1):

ρ = mV

(1)

Where ρ is the density of the EVOH NFAs, m is the mass of the EVOH NFAs, V is the volume of the EVOH NFAs.

Porosity of EVOH NFAs The porosity of EVOH NFAs was calculated by the following formula (2):

η (%) =

V0 − m / ρ ×100% V0

(2)

Where η is the porosity of the EVOH NFAs, V0 is the volume of the EVOH NFAs,

m is the mass of the EVOH NFAs, ρ is the density of EVOH nanofibers. Absorbency of oil/organic solvent First, the dried crosslinked EVOH NFA (about 40 mg) was immersed into pure oil and pure organic solvent at ambient temperature. Once the crosslinked EVOH NFA was completely filled with the oil/organic solvent, then taken out and removed excess oil/ organic solvent on the surface of the aerogel with a filter paper. Afterwards, the saturated aerogel was weighted quickly to avoid evaporation of the absorbed oil/organic solvent. The weight of the crosslinked EVOH NFA before and after absorption was recorded. The absorbency of aerogel was calculated by the following formula (3): Q (%) =

W − W0 × 100% W0

(3)

Where Q is the liquid absorbency of the EVOH NFAs, W0 and W are the weight of the aerogel before and after liquid absorption, respectively.

Results and discussion ACS Paragon Plus Environment


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Morphology and structure of the NFAs Figure 1 describes the fabrication process of EVOH NFAs, which have been successfully designed by a versatile, facile, and gelation-free fibrous freeze-shaping method. The EVOH nanofibers were prepared by melt blending extrusion with the diameter ranging from 150 to 500nm, and assembled into an intricate interlaced network in the resulting EVOH NFAs through freeze-drying method. We furthermore employed the GA crosslinker to convert the physical weak bonds to strong covalent bonds between EVOH nanofibers by means of an in-situ crosslinking method. The introduction of the GA results in the decrease of the distance between EVOH nanofibers, the volume of samples tended to generate slight shrinkage, and the density of aerogels which was observed to increase from 10.8 to 12.3 mg cm-3.

Herein, the frozen samples were dried under vacuum condition, resulting in the pressure reduction around the samples, thus allowing the solvent among the EVOH nanofibers to sublimate from the solid phase to gas phase.48 The solvent in gas phase was removed using a vacuum pump, and the porous structure was formed as illustrated in Figure 2. Whilst a minimal force would bind together the EVOH nanofibers upon sublimation, complete removal of the solvent would result in stronger inter-fiber interactions, hence preventing the porous structure from falling apart. Figure 2 shows the cross-sections of EVOH NFAs before and after crosslinking treatment. From Figure 2a and b clearly illustrates that the pores size (15-30µm) in the analyzed samples were much larger than for most previously reported aerogels.49-50 In this experiment, the cryogenic refrigerator was used to slowly freeze the EVOH nanofiber suspension, leading the larger and more resolved crystals of ice. The latter could potentially lead to big cracks and poor integrity of the generated samples.51-52 In order to effectively decrease the volume expansion of the solvent in the samples during freezing process, t-BuOH which has a low surface tension (23.6×10-3N·m-1 at ambient temperature) was chosen as the suspension carrier.53 The optimum content of t-BuOH was 20 % (w/v), the variation of pore size inside the formed aerogels was negligible.54


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As shown in Figure 2a and b the aerogel exhibits an open porous network structure of uniform fibers, with the pores highly interconnected. From Figure 2a and c, it is observed that the nanofibers were stacked and twined together in uncrosslinked EVOH NFA, which is associated to the weak intermolecular interaction between EVOH nanofibers (such as hydrogen bond, Van Edward force, etc.). Such weak force can be disrupted by posttreatment methodologies, thus limiting the application of the aerogels. It is therefore necessary to increase these intermolecular in the aerogels. To this end, GA was utilized as the crosslinking agent to form stable carbonyl crosslinking bonds among the EVOH nanofibers. Figure 2b and d show the cross-section of the crosslinked EVOH NFAs. Unlike the uncrosslinked samples, the crosslinked EVOH NFAs exhibited obvious 3D network porous structure with strong bonding among the nanofibers (Figure 2d). It was also observed from Figure 2b that the EVOH nanofibers were slightly deformed, which can be ascribed to the in-situ crosslinking between GA and EVOH nanofibers, highlighting the structural rigidness. In short, the presence of the GA crosslinker would decrease the distance between EVOH nanofibers, causing these to be closely packing with a resulting increased density of the aerogel.

Upon heat treatment in a vacuum oven at 75 °C, the polymerization between the hydroxyl and the aldehyde groups would occur to form keto groups, forming stable network structure and thus generating stiffer aerogels, as shown in Figure 3a. ATR-FTIR analyses were carried to corroborate the interaction between the GA interacted with the hydroxyl groups of EVOH, as depicted in Figure 3b. The uncrosslinked EVOH NFAs were observed to exhibit a broad absorption band at 3347cm-1 associated to the vibration stretching mode of the –OH groups. This is proposed to be related to intermolecular hydrogen-bonding interactions among the hydroxyl groups. Upon heat treatment, the intensity of the –OH band was observed to decrease distinctly, whilst the vibrational band associated to the C=O stretching showed at ca 1713cm-1 for crosslinked EVOH NFAs, resulting from addition of the



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crosslinking agent. The C=O stretching vibration band at 1713 cm-1 was furthermore associated to ketone functional group, hence being able to differentiate from the aldehyde in the GA and conforming the successful polymerization.

From Figure 3c, it is clear that the XRD pattern of uncrosslinked EVOH NFAs shows significant difference from the XRD pattern of crosslinked EVOH NFAs. The crystallinity (40.57%) of uncrosslinked EVOH NFAs is much higher than that (30.51%) of crosslinked EVOH NFAs. This is proposed to be associated to the presence of the crosslinker GA. The occurrence of in-situ crosslinking would compromise the integrity and regularity of polymer molecular chains, and form network structure, which would result in restricted flexibility of molecular chains. Therefore, the crystallinity of aerogel would decrease significantly. The XRD patterns provide further experimental evidence of the success of the in- situ crosslinking.

Surface wettability of the NFAs Contact angle (CA) represents a crucial indicator of the surface wettability of aerogels. In this regard, the porous rough structure in EVOH NFAs could reduce the water-solid contact area, hence improving the surface wettability of aerogels.55

As shown in Figure 4a, a water droplet of 3µl dipped on the surface of the aerogel prepared without crosslinking agent, it was absorbed immediately and the water contact angle (WCA) decreased from 130.2° to 0° in 30s, with comparable hydrophilicity. This can be attributed to the hydrophilic hydroxyl groups in the EVOH NFAs surface, which could form hydrogen-bond between EVOH nanofibers and water. Upon crosslinking, the WCA of EVOH NFAs was observed to increase significantly. It was determined that (Figure 4b) the WCA of crosslinked EVOH NFAs without any hydrophobic treatment could still maintain at 144.4° after 3 min. The latter was associated to a number factors: (1) upon heat treatment, many hydroxyl groups in the EVOH nanofibers reacted with the aldehyde groups to form keto groups, surrounded by long hydrophobic alkyl chains. These keto groups on the surface of


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crosslinked NFAs would enhance their hydrophobicity; (2) most of hydrophilic hydroxyl groups in the EVOH nanofibers were reacted, which would reduce the hydrophilicity of the resulting aerogels. The uncrosslinked EVOH NFAs also exhibit oleophilicity (Figure 4c), indicating the amphipathy of uncrosslinked EVOH NFAs, thus limiting the efficiency of uncrosslinked EVOH NFAs for the removal of pollutants from water. The superoleophilicity of crosslinked EVOH NFAs was confirmed (Figure 4d). The samples absorbed an oil droplet in less than 1 s and the oil contact angle (OCA) of the crosslinked EVOH NFAs was reduced to 0° accordingly. Therefore, the crosslinked EVOH NFAs exhibits high hydrophobicity and superoleophilicity, which represents a key aspect for oil/organic solvent absorption and water purification.

The crosslinked EVOH NFA could be immersed in water only by a pair of tweezers (Figure 4f), with the aerogel returning the surface of the water on removal of the pair of tweezers, with negligible water absorption. In turn, a superior absorption ability of the crosslinked EVOH NFA was observed when immersed in oil/organic solvent. This implies that the high hydrophilic and superoleophilic crosslinked EVOH NFAs could remove the oil and organic solvent in the water.

Mechanical properties of the NFAs The crosslinked EVOH NFA with a density of 12.3 mg/cm3 without any addition of rigid fillers exhibited robust mechanical properties, allowing to sustain repeated large deformation without collapse, which is beneficial to practical application in many fields.38

From Figure 5a (Movie S1 in the Supporting Information) , it is illustrated that the crosslinked EVOH NFA could bear a 60% compressive strain and exhibit complete shape recovery on release of the loading. In turn, the uncrosslinked EVOH NFA was not elastic and was permanently deformed after loading (Figure 5b and Movie S2 in the Supporting Information). This was attributed to the fact that nanofibers



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interactions via chemical covalent bonds in the crosslinked EVOH NFA, hence able to sustain external force compression, whereas the weaker physical forces in the uncrosslinked EVOH NFA would collapse upon compression. The compressive stress-strain curves of the crosslinked EVOH NFAs at different maximum strain of 40, 60 and 80% were shown in Figure 5c, which furthermore illustrate excellent elasticity of the crosslinked EVOH NFAs. These curves showed a highly nonlinear and closed hysteresis, which are typical of viscoelastic, energy-dissipative, and highly deformable materials.56 From the curves, two distinct stages were displayed, that is, linear elastic behavior region at low strain, corresponding to the pores were gradually deformed and the elastic bending of the EVOH nanofibers; the stress increased steeply at high strain, corresponding to densification of pores and the EVOH nanofibers impinged upon each other.17 The stress was 24 KPa at 80% strain, higher than most pure polymeric aerogel,8, 38 which is clearly attributed to stiffer pore walls upon crosslinking with GA.

The particularly flexible aerogel (vide supra) also exhibited outstanding compression cycling performance. The stress-strain curves of the crosslinked EVOH NFA at maximum strain of 60% during repeated loading/unloading cycles are shown in Figure 5b. The obvious hysteresis loops in the curves and a plastic deformation (8.6% at 250th, 13.6% at 500th) were observed after repeated cycles of compression, which were ascribed to energy-dissipation of the aerogel during the cyclic compression process. Whilst a plastic deformation of 20–30% at strain of 60% is typical for polymeric materials, and many other fibrous aerogels exhibit plastic deformation of larger than 20% at this strains.55

Oil/organic solvent absorbency of the NFAs Due to its porous structure, hydrophobicity and superoleophilicity, excellent mechanical properties, the crosslinked EVOH NFA is an ideal candidate material for removing oils and other organic pollutants from water. As shown in Figure 6a (Movie S3 in the Supporting Information), a small piece of the crosslinked EVOH NFA with


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density of 12.3 mg/cm3 was placed on the surface of the diesel-water mixtures. The diesel (dyed with Sudan red III) was absorbed completely after 60 s. Similarly, when the crosslinked EVOH NFA was exposed to organic solvent and water mixtures, the organic solvent was also absorbed immediately by the aerogel (Figure 6b and Movie S4 in the Supporting Information). In addition, the crosslinked EVOH NFAs have stiff pore walls and small pores, which are proposed to be able to hold the absorbed oil/organic solvent and when removed from the mixtures.9 The oil/organic solvent (except chloroform because of its high density) filled aerogels would float on the surface of the water without any oil or organic solvent release.

To further investigate the oil/organic absorption performance of crosslinked EVOH NFAs, various types of oils (gasoline, diesel oil, soybean oil, etc.) and organic solvents (phenoxin, chloroform, toluene) were tested (Figure 6c). These liquids are common pollutants in our daily life as well as a result of laboratory and industry work. The absorption of aerogels was evaluated by weight gain (wt %), which was defined as the weight of the absorbed substance per unit weight of the dried crosslinked EVOH NFA. As shown in Figure 6c, the crosslinked pure EVOH NFAs without any hydrophobic treatment exhibited superior sorption performance for oil/organic solvent. This may be associated to the following factors: (1) the oil/organic solvent could be effectively and quickly absorbed by these hydrophobic and oleophilic functional groups on the surface of aerogels; (2) the low density and high porosity of the aerogels make them exhibit a great amount of absorption. In general, the sorption capacities for oils and organic solvents ranged from 45 to 102 times the weight of aerogel, which can be associated to its hydrophobicity and superoleophilicity, as well as the porous structure in the crosslinked EVOH NFAs that can significantly enhance their absorbency.57 The outstanding oil/organic solvent absorbency of crosslinked EVOH NFA was observed to be much higher than that of previously reported sorbents, such as activated carbon (

Superoleophilic and Flexible Thermoplastic Polymer Nanofiber

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