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Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer
Journal Pre-proof Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer Sergey N. Malakhov, Sergey N. Chvalun
PII:
S2352-4928(19)30463-5
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100729
Reference:
MTCOMM 100729
To appear in:
Materials Today Communications
Received Date:
19 July 2019
Revised Date:
26 October 2019
Accepted Date:
28 October 2019
Please cite this article as: Malakhov SN, Chvalun SN, Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100729
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer Sergey N. Malakhov*, Sergey N. Chvalun National Research Center “Kurchatov Institute”, 123182, Moscow, Russia *Corresponding author, e-mail: [email protected]
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Graphical abstract
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Highlights Nonwoven materials from EOC with average fiber diameter 3.9-4.7 µm were made. Materials, produced from heated solution, have “shish-kebab” surface morphology. Produced EOC materials perform superhydrophobic and oleophilic properties.
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Abstract Nonwoven materials with an average fiber diameter of 4.7 µm and 3.9 µm were prepared by electrospinning of a heated solution and melt of the thermoplastic polyolefin elastomer – ethylene-octene copolymer. The materials prepared from the melt have fibers with a smooth surface, while materials produced from a heated solution have “shish-kebab” morphology with highly extended surface. X-ray diffraction analysis data demonstrates increasing of the mesophase content in the fibers prepared from the melt and the orthorhombic crystalline phase – for a heated solution. The produced nonwovens show superhydrophobic and oleophilic properties, high sorption capacity (up to 109 g/g for motor oil) and can be used for various applications, e.g. oil removal from water surface and matrices in tissue engineering. Keywords: Electrospinning; Ethylene-octene copolymer; Nonwoven materials; Oil sorption
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1. Introduction Nowadays electrospinning is one of the most popular methods for producing of nonwoven materials. They are applied in the different fields, e.g. filtration, medicine, electronics, etc. [1, 2]. So far fibers have been obtained from a large variety of polymeric, organic and inorganic materials. At the same time a number of polymers, first of all polyolefins, are presented in publications rather rarely due to their limited solubility, what makes difficult to obtain fibers by the “traditional” solution method. The melt electrospinning (firstly described by Larrondo and Manley in 1981 [3]) is more preferable for such polymers but it is more difficult technologically, so the total number of papers about the melt method is much less than that on the solution one [4]. For a number of applications (from medicine to electronics), the fibers from elastomers are very promising due to their high elongation at break, good elasticity and low residual deformation. However, because of the aforementioned limitations in processing, nonwoven materials are usually produced from a limited number of polymers, e.g. polyurethanes [5-10], polycaprolactone [11] and its copolymers [12-14]. Also there are few papers devoted to the fabrication of fibers from “exotic” elastomeric materials such as poly(1,8-octanediol citrate) [15], poly(decanediol-co-tricarballylate) [16], poly(urea-siloxane) [17] etc. At the same time a wide class of polymers – thermoplastic polyolefin elastomers (with high chemical resistance, which allows to use them in aggressive environments, but makes difficult the processing by solution methods) – has actually remained uncovered by researchers. One of those promising polyolefins is ethylene-octene copolymer (EOC) developed using a metallocene catalyst by the Dow Chemical Co, which exhibits high elasticity (elongation at break is up to 1200%), thermoplastic processability and compatibility with other polyolefins. EOC can be used as linear polymer, after crosslinking or grafting [18, 19], in the form of composites with various fillers [20-24], and also as an impact modifier for other polyolefins [25] or compatibilizing agent for polymer blends [26]. Thus, the present study is aimed at fabrication and investigation of morphology and structure of nonwovens from the thermoplastic polyolefin elastomer – ethylene-octene copolymer by solution and melt electrospinning.
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2. Experimental 2.1. Materials Fibers were spun from ethylene-octene copolymer “Engage” (0.885 g/cm3, 8.2 mol. % of octene, Dow Chemical Company). Sodium stearate was purchased from Sigma-Aldrich; toluene was purchased from Komponent-Reaktiv (Russia). All reagents were used as received, without any further purification. 2.2. Electrospinning An experimental setup based on a single screw extruder Brabender Plasti-Corder PLE-330 with four heating zones (Fig. 1a) was used for melt electrospinning. To prevent premature degradation of the polymer, the temperature of first three zones (screw zones, T1-T3) was set at 200°С. Electrospinning was performed at nozzle temperature (T4) of 310-340°C. The extruder was grounded, and the high voltage (135 kV) was applied to the cylindrical drum (collecting electrode) equipped with an electric motor drive. The diameter and width of the drum were 15 and 25 cm, respectively; the rotation speed was 1.5-2 rpm and the distance between the nozzle
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and the collector was 45 cm. Sodium stearate (3 mass.%) was used as an additive to decrease viscosity and increase conductivity of the melt [27, 28]. For solution electrospinning (Fig. 1b) a 12 wt.% solution of EOC in toluene was used, the polymer was preliminarily dissolved for 12 hours at 105°C. The process was carried out in a oilheated stainless steel cell at 100°C, the high voltage (45 kV) was applied to the nozzle, and the distance between the nozzle and the collector was 30 cm.
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Fig. 1. Experimental setups for producing nonwoven materials by electrospinning of melt (a) and a heated solution of ethylene-octene copolymer (b): 1 – high voltage supply, 2 – feeding device, 3 – polymer, 4 – nozzle, 5 – fiber, 6 – collecting device.
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2.3. Characterization Morphology of nonwoven materials was visualized using scanning electron microscopy (SEM; Phenom ProX, accelerating voltage 5 kV). IR spectra were recorded with a Thermo Scientific Nicolet iS5 spectrometer with an iD5 ATR accessory. X-ray diffraction (XRD) analysis of the samples was performed on a Rigaku SmartLab diffractometer (CuKα-radiation). Contact angles were measured with a KRUSS DSA30E system, the droplet volume was 5 μl.
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2.4. Oil sorption/desorption measurements The maximum sorption capacity of the material was determined using 5W-40 synthetic motor oil (produced by ExxonMobil) as a reference test medium. A 4x4 cm nonwoven sample was weighed, placed in a beaker containing 200 ml of motor oil for 5 minutes, and then removed, held for 60 seconds to drain the unsorbed oil and re-weighed. After that, the material was placed into the sand core funnel and oil was removed from the oil-loaded sample with a vacuum pump for 2 minutes and then the sample was weighed again to determine the mass of residual oil. The desorption and subsequent sorption measurements were repeated for five cycles to evaluate the reusability and recoverability of the fibrous material. 3. Results and discussion SEM reveals that the diameter of fibers produced by electrospinning of a heated solution of EOC in toluene is in the range from 1.5 to 14 μm with an average outer diameter of 4.7 μm (Fig. 2a). The packing density of the produced material was 6.9%. It is important to note that the fibers have a very complex morphology with a high surface area. One can observe the fibers with a “shish-kebab” morphology, and bundle-like structures of fine (less than 1 µm in diameter) fibers. The formation of such structures can be explained by peculiarities of the process of their formation. Since, the solution of EOC in toluene thickens very quickly upon cooling and loses its
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ability to process, electrospinning from solution was carried out at a temperature of 100°C. At the same time, the melting of EOC occurs in the range of 35-90°C: the hexagonal mesophase melts at 35-50°C, and the orthorhombic crystalline phase – at 80-90°C [23]. Accordingly, in this case a “hybrid” process was used – the spinning was carried out from a solution with a temperature higher than the melting point of the bulk copolymer. In general, polymer jet solidifies due to the solvent removal in the case of solution electrospinning or cooling of the fibers for the melt process. Transition of EOC from a heated liquid jet to a solid fiber is very complex process and includes several stages, i.e. evaporation of the solvent, microphase separation and polymer crystallization. The consistent sequences of these processes lead to formation of “shish-kebab” morphology. At first, orthorhombic phase crystallizes and assembles elongated fiber and then the rest of polymer forms the hexagonal mesophase in periphery of “shish-kebab” structure. It should be noted that it is difficult to adjust the average fiber diameter by changing the temperature of the solution. On the one hand, the maximum temperature is limited to the boiling point of toluene, which is about 110°C. On the other hand, our attempts to reduce the temperature of spinning solution led to a dramatic increase in solution viscosity, which makes impossible formation of the fibers.
Fig. 2. SEM images of EOC nonwoven materials, produced by electrospinning of a heated solution (a) and melt (b); inset – optical image of water droplet on melt spun material after 60 seconds from deposition.
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The electrospinning of EOC from the melt begins at a temperature of about 310°C, but a monofilament with a diameter of 80-100 μm is produced on the collecting electrode instead of a nonwoven material. As the temperature of the nozzle increases, the viscosity of the melt decreases and the conductivity increases, resulting in a decrease in the diameter of the fibers. The average fiber diameter was about 28 μm at 320°C and 9.1 μm at 330°C. The electrospinning of EOC from the melt at 340°C allows to produce nonwoven material with fiber diameter in range of 1.9–8.2 μm and an average diameter of 3.9 μm (Fig 2b) and packing density of 6.1%. This is comparable with the results obtained earlier with polyamide-6 [27] and polypropylene [28] under similar conditions. In contrast to the fibers produced from a heated solution, these materials are characterized by a rather smooth surface. It should be noted that the EOC in the fibers retains its elastic properties (elongation at break for films prepared from the obtained nonwoven materials exceeds 1000%).
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Thus, the obtained materials were characterized by similar values of the average fiber diameter and packing density of the fabrics, but significantly different surface morphology of the surface of the fibers. According to X-ray data (Fig 3a), the EOC samples are characterized by the presence of hexagonal mesophase (reflection (100) at 19.7°) and orthorhombic crystalline phase (reflection (110) at 21.1° with a shoulder (200) at 23.3° and reflection (020) at 36.1°) [23, 29] in the pellets and nonwoven materials. The degree of crystallinity was 18% for pellets, 16 and 13% for nonwoven material from solution and melt, respectively. At the same time, the ratio of crystalline phases in fibers in comparison to the initial pellet changes: for the materials obtained by electrospinning of a heated solution, orthorhombic crystalline phase content increases; in the materials, produces by the melt electrospinning, increasing mesophase content is observed. The increase of the mesophase content in nonwovens produced from the melt can be explained by the complexity of the formation of stable crystals in the melt process, in which rapid crystallization of the polymer is accompanied by its high-speed drawing in the electric field. In the IR spectra of the EOC samples (Fig 3b), characteristic bands of polyolefins are observed, i.e. antisymmetric and symmetric C-H stretching in methylene groups (2916 and 2849 cm-1, respectively); C-H deformation in methyl groups (1378 cm-1); C-H deformations in methylene groups: scissoring (1467 cm-1), wagging (1302 cm-1), rocking (719 cm-1) [22]. In addition, a new band appears in the IR spectrum of a material produced from the melt in the region of 1560 cm-1, it corresponds to the antisymmetric stretching of the carboxylate anion of sodium stearate [28]. The absence of absorption bands in the region of 1700–1750 cm-1 (which can be assigned to C=O stretching) indicates that there are no significant thermooxidative degradation of the polymer during melt electrospinning process [30]. Contact angle (CA) measurements revealed that the EOC is a rather hydrophobic polymer: average water CA for the pure EOC film is 112°. Addition of 3% of the sodium stearate (which is an ionic surfactant) leads to a slight increase in the hydrophilicity of the film, average CA decreases to 100°. The produced nonwoven materials (both from solution and melt) possess superhydrophobic and oleophilic properties: water CA is 147-150° (Fig. 2, inset) and oil droplets applied to the material are instantly absorbed, which allows the use of these nonwovens to remove oil spills from water surface. Thus, the addition of sodium stearate does not lead to a noticeable decrease in the contact angles of nonwovens; similar results were previously observed in case of melt electrospinning of polyamide-6 [27] and polypropylene [28].
Fig. 3. XRD patterns (a) and FTIR spectra (b) of EOC pellet (1), nonwoven material produced from a heated solution (2) and nonwoven material, produced from melt (3).
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One of the applications of nonwoven materials is the removal of oil spills from the surface of the water, and the sorption capacity of commercially available materials usually does not exceed 30 g/g [31]. The nonwoven materials (both from solution and melt) obtained in this work are characterized by significantly higher maximum sorption capacity for motor oil, i.e. 95 g/g for fabrics produced by melt electrospinning and 109 g/g for fabrics produced from heated solution. The obtained result is comparable with the sorption capacities for other promising nonwoven materials [32]. In addition, the reusability of the materials was investigated; and the sorption capacitiy of nonwovens at different sorption-desorption cycles is illustrated in Fig. 4.
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Fig. 4. Oil retained in the samples in the cyclic sorption/desorption measurements for materials produced from heated solution (1) and melt (2)
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As can be seen from the presented data, the sorption capacity of the material decreases upon repeated use and levels out at about 45-50 g/g, which may be due to a significant increase in the packing density of the material during oil desorption. It should be noted that during the second and subsequent sorption cycles, the sorption capacity of materials obtained from a heated solution is slightly lower than for materials produced from melt, which can be explained by incomplete removal of oil from the “shish-kebab” fibers. The residual mass of oil in all desorption cycles is in the range of 2-3 g/g for materials produced from the melt and 5-6 g/g for materials obtained from the heated solution, which can also confirm incomplete removal of oil from the highly extended surface of fibers from the heated solution. Based on the presented data, it can be concluded that the sorption capacity of the obtained materials is primarily determined by the packing density of the fabrics (as evidenced by a decrease in capacity by half after several sorption-desorption cycles), and to a much lesser extent depends on the surface morphology of the fibers.
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4. Conclusions Thus, new nonwoven materials were obtained by electrospinning of melt and a heated solution of ethylene-octene copolymer. The average fiber diameter in produced nonwovens was 3.9 and 4.7 μm for melt and solution processes, respectively. It was found that the materials have significantly different fiber morphology: smooth surface for melt electrospinning, and complex “shish-kebab” surface for processing from heated solution. Both materials show superhydrophobic and oleophilic properties. The obtained materials are suitable for removal oil from the water (maximum sorption capacity is more than 100 g/g for motor oil), as matrices for cell cultures and as scaffolds in tissue engineering. Declaration of Competing Interest
None. Acknowledgments The reported study was supported by Russian Science Foundation according to the research project № 18-73-00328. The studies were carried out using equipment of the Resource Centers (“Optics”, “Polymer” and “X-Ray”) of the National Research Center “Kurchatov Institute”.
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[15] L. Zhu, Y. Zhang, Y. Ji, Fabricating poly (1, 8-octanediol citrate) elastomer based fibrous mats via electrospinning for soft tissue engineering scaffold, J. Mater. Sci. Mater. Med. 28 (2017) 93. [16] H.M. Ismail, S. Zamani, M.A. Elrayess, W. Kafienah, H.M. Younes, New threedimensional poly (decanediol-co-tricarballylate) elastomeric fibrous mesh fabricated by photoreactive electrospinning for cardiac tissue engineering applications, Polymers 10 (2018) 455. [17] G. Hochleitner, E. Fürsattel, R. Giesa, J. Groll, H.W. Schmidt, P.D. Dalton, Melt Electrowriting of Thermoplastic Elastomers, Macromol. Rapid Comm. 39 (2018) 1800055. [18] K. Sirisinha, D. Meksawat, Comparison in processability and mechanical and thermal properties of ethylene–octene copolymer crosslinked by different techniques, J. Appl. Polym. Sci. 93 (2004) 1179-1185. [19] C. Jiao, Z. Wang, Z. Gui, Y. Hu, Silane grafting and crosslinking of ethylene–octene copolymer, Eur. Polym. J. 41 (2005) 1204-1211. [20] C.S. Wu, H.T. Liao, Modification of polyethylene–octene elastomer by silica through a sol– gel process, J. Appl. Polym. Sci. 88 (2003) 966-972. [21] H.T. Liao, C.S. Wu, Organic–inorganic polymeric nanocomposites involving novel titanium tetraisopropylate in polyethylene–octene elastomer, J. Polym. Sci. Pol. Phys. 42 (2004) 42724280. [22] G. Latta, Q. Lineberry, R. Ozao, H.Y. Zhao, W.P. Pan, Thermal properties of ethylene octene copolymer (Engage)/dimethyldioctadecyl quaternary ammonium chloride-modified montmorillonite clay nanocomposites, J. Mater. Sci. 43 (2008) 2555-2561. [23] N.P. Bessonova, S.V. Krasheninnikov, A.P. Korobko, E.A. Tomilina, M.Yu. Meshchankina, S.N. Chvalun, Structures and properties of low-crystallinity polyolefins modified with nanodiamonds, Polym. Sci. Ser. A 57 (2015) 819-828. [24] A. Tesarikova, D. Merinska, J. Kalous, P. Svoboda, Influence of clay nanofillers on properties of ethylene‐octene copolymers, Polym. Composites 39 (2018) 4581-4593. [25] A. Arostegui, M. Gaztelumendi, J. Nazabal, Toughened poly (butylene terephthalate) by blending with a metallocenic poly (ethylene–octene) copolymer, Polymer 42 (2001) 9565-9574. [26] Y.L. Wang, X. Hu, H. Li, X. Ji, Z.M. Li, Polyamide-6/poly (lactic acid) blends compatibilized by the maleic anhydride grafted polyethylene-octene elastomer. Polym.-Plast. Technol. 49 (2010) 1241-1246. [27] S.N. Malakhov, S.I. Belousov, M.A. Shcherbina, M.Yu. Meshchankina, S.N. Chvalun, A.D. Shepelev, Effect of Low Molecular Additives on the Electrospinning of Nonwoven Materials from a Polyamide-6 Melt, Polym. Sci. Ser. A 58 (2016) 236-245. [28] S.N. Malakhov, S.I. Belousov, A.S. Orekhov, S.N. Chvalun, Electrospinning of nonwoven fabrics from polypropylene melt with additions of stearates of divalent metals, Fibre Chem. 50 (2018) 27-32. [29] R. Androsch, J. Blackwell, S.N. Chvalun, B. Wunderlich, Wide- and Small-Angle X-ray Analysis of Poly(ethylene-co-octene), Macromolecules 32 (1999) 3735-3740. [30] Sugimoto, M., Shimada, A., Kudoh, H., Tamura, K., & Seguchi, T. (2013). Product analysis for polyethylene degradation by radiation and thermal ageing. Radiation Physics and Chemistry, 82, 69-73. [31] Q.F. Wei, R.R.Mather, A.F. Fotheringham, R.D. Yang, Evaluation of nonwoven polypropylene oil sorbents in marine oil-spill recovery, Mar. Pollut. Bull. 46 (2003) 780-783.
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[32] R. Sarbatly, D. Krishnaiah, Z. Kamin, A review of polymer nanofibres by electrospinning and their application in oil–water separation for cleaning up marine oil spills, Mar. Pollut. Bull. 106 (2016) 8-16.
PII:
S2352-4928(19)30463-5
DOI:
https://doi.org/10.1016/j.mtcomm.2019.100729
Reference:
MTCOMM 100729
To appear in:
Materials Today Communications
Received Date:
19 July 2019
Revised Date:
26 October 2019
Accepted Date:
28 October 2019
Please cite this article as: Malakhov SN, Chvalun SN, Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100729
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Multifunctional nonwoven materials, produced by electrospinning of a heated solution and melt of ethylene-octene copolymer Sergey N. Malakhov*, Sergey N. Chvalun National Research Center “Kurchatov Institute”, 123182, Moscow, Russia *Corresponding author, e-mail: [email protected]
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Graphical abstract
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Highlights Nonwoven materials from EOC with average fiber diameter 3.9-4.7 µm were made. Materials, produced from heated solution, have “shish-kebab” surface morphology. Produced EOC materials perform superhydrophobic and oleophilic properties.
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Abstract Nonwoven materials with an average fiber diameter of 4.7 µm and 3.9 µm were prepared by electrospinning of a heated solution and melt of the thermoplastic polyolefin elastomer – ethylene-octene copolymer. The materials prepared from the melt have fibers with a smooth surface, while materials produced from a heated solution have “shish-kebab” morphology with highly extended surface. X-ray diffraction analysis data demonstrates increasing of the mesophase content in the fibers prepared from the melt and the orthorhombic crystalline phase – for a heated solution. The produced nonwovens show superhydrophobic and oleophilic properties, high sorption capacity (up to 109 g/g for motor oil) and can be used for various applications, e.g. oil removal from water surface and matrices in tissue engineering. Keywords: Electrospinning; Ethylene-octene copolymer; Nonwoven materials; Oil sorption
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1. Introduction Nowadays electrospinning is one of the most popular methods for producing of nonwoven materials. They are applied in the different fields, e.g. filtration, medicine, electronics, etc. [1, 2]. So far fibers have been obtained from a large variety of polymeric, organic and inorganic materials. At the same time a number of polymers, first of all polyolefins, are presented in publications rather rarely due to their limited solubility, what makes difficult to obtain fibers by the “traditional” solution method. The melt electrospinning (firstly described by Larrondo and Manley in 1981 [3]) is more preferable for such polymers but it is more difficult technologically, so the total number of papers about the melt method is much less than that on the solution one [4]. For a number of applications (from medicine to electronics), the fibers from elastomers are very promising due to their high elongation at break, good elasticity and low residual deformation. However, because of the aforementioned limitations in processing, nonwoven materials are usually produced from a limited number of polymers, e.g. polyurethanes [5-10], polycaprolactone [11] and its copolymers [12-14]. Also there are few papers devoted to the fabrication of fibers from “exotic” elastomeric materials such as poly(1,8-octanediol citrate) [15], poly(decanediol-co-tricarballylate) [16], poly(urea-siloxane) [17] etc. At the same time a wide class of polymers – thermoplastic polyolefin elastomers (with high chemical resistance, which allows to use them in aggressive environments, but makes difficult the processing by solution methods) – has actually remained uncovered by researchers. One of those promising polyolefins is ethylene-octene copolymer (EOC) developed using a metallocene catalyst by the Dow Chemical Co, which exhibits high elasticity (elongation at break is up to 1200%), thermoplastic processability and compatibility with other polyolefins. EOC can be used as linear polymer, after crosslinking or grafting [18, 19], in the form of composites with various fillers [20-24], and also as an impact modifier for other polyolefins [25] or compatibilizing agent for polymer blends [26]. Thus, the present study is aimed at fabrication and investigation of morphology and structure of nonwovens from the thermoplastic polyolefin elastomer – ethylene-octene copolymer by solution and melt electrospinning.
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2. Experimental 2.1. Materials Fibers were spun from ethylene-octene copolymer “Engage” (0.885 g/cm3, 8.2 mol. % of octene, Dow Chemical Company). Sodium stearate was purchased from Sigma-Aldrich; toluene was purchased from Komponent-Reaktiv (Russia). All reagents were used as received, without any further purification. 2.2. Electrospinning An experimental setup based on a single screw extruder Brabender Plasti-Corder PLE-330 with four heating zones (Fig. 1a) was used for melt electrospinning. To prevent premature degradation of the polymer, the temperature of first three zones (screw zones, T1-T3) was set at 200°С. Electrospinning was performed at nozzle temperature (T4) of 310-340°C. The extruder was grounded, and the high voltage (135 kV) was applied to the cylindrical drum (collecting electrode) equipped with an electric motor drive. The diameter and width of the drum were 15 and 25 cm, respectively; the rotation speed was 1.5-2 rpm and the distance between the nozzle
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and the collector was 45 cm. Sodium stearate (3 mass.%) was used as an additive to decrease viscosity and increase conductivity of the melt [27, 28]. For solution electrospinning (Fig. 1b) a 12 wt.% solution of EOC in toluene was used, the polymer was preliminarily dissolved for 12 hours at 105°C. The process was carried out in a oilheated stainless steel cell at 100°C, the high voltage (45 kV) was applied to the nozzle, and the distance between the nozzle and the collector was 30 cm.
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Fig. 1. Experimental setups for producing nonwoven materials by electrospinning of melt (a) and a heated solution of ethylene-octene copolymer (b): 1 – high voltage supply, 2 – feeding device, 3 – polymer, 4 – nozzle, 5 – fiber, 6 – collecting device.
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2.3. Characterization Morphology of nonwoven materials was visualized using scanning electron microscopy (SEM; Phenom ProX, accelerating voltage 5 kV). IR spectra were recorded with a Thermo Scientific Nicolet iS5 spectrometer with an iD5 ATR accessory. X-ray diffraction (XRD) analysis of the samples was performed on a Rigaku SmartLab diffractometer (CuKα-radiation). Contact angles were measured with a KRUSS DSA30E system, the droplet volume was 5 μl.
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2.4. Oil sorption/desorption measurements The maximum sorption capacity of the material was determined using 5W-40 synthetic motor oil (produced by ExxonMobil) as a reference test medium. A 4x4 cm nonwoven sample was weighed, placed in a beaker containing 200 ml of motor oil for 5 minutes, and then removed, held for 60 seconds to drain the unsorbed oil and re-weighed. After that, the material was placed into the sand core funnel and oil was removed from the oil-loaded sample with a vacuum pump for 2 minutes and then the sample was weighed again to determine the mass of residual oil. The desorption and subsequent sorption measurements were repeated for five cycles to evaluate the reusability and recoverability of the fibrous material. 3. Results and discussion SEM reveals that the diameter of fibers produced by electrospinning of a heated solution of EOC in toluene is in the range from 1.5 to 14 μm with an average outer diameter of 4.7 μm (Fig. 2a). The packing density of the produced material was 6.9%. It is important to note that the fibers have a very complex morphology with a high surface area. One can observe the fibers with a “shish-kebab” morphology, and bundle-like structures of fine (less than 1 µm in diameter) fibers. The formation of such structures can be explained by peculiarities of the process of their formation. Since, the solution of EOC in toluene thickens very quickly upon cooling and loses its
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ability to process, electrospinning from solution was carried out at a temperature of 100°C. At the same time, the melting of EOC occurs in the range of 35-90°C: the hexagonal mesophase melts at 35-50°C, and the orthorhombic crystalline phase – at 80-90°C [23]. Accordingly, in this case a “hybrid” process was used – the spinning was carried out from a solution with a temperature higher than the melting point of the bulk copolymer. In general, polymer jet solidifies due to the solvent removal in the case of solution electrospinning or cooling of the fibers for the melt process. Transition of EOC from a heated liquid jet to a solid fiber is very complex process and includes several stages, i.e. evaporation of the solvent, microphase separation and polymer crystallization. The consistent sequences of these processes lead to formation of “shish-kebab” morphology. At first, orthorhombic phase crystallizes and assembles elongated fiber and then the rest of polymer forms the hexagonal mesophase in periphery of “shish-kebab” structure. It should be noted that it is difficult to adjust the average fiber diameter by changing the temperature of the solution. On the one hand, the maximum temperature is limited to the boiling point of toluene, which is about 110°C. On the other hand, our attempts to reduce the temperature of spinning solution led to a dramatic increase in solution viscosity, which makes impossible formation of the fibers.
Fig. 2. SEM images of EOC nonwoven materials, produced by electrospinning of a heated solution (a) and melt (b); inset – optical image of water droplet on melt spun material after 60 seconds from deposition.
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The electrospinning of EOC from the melt begins at a temperature of about 310°C, but a monofilament with a diameter of 80-100 μm is produced on the collecting electrode instead of a nonwoven material. As the temperature of the nozzle increases, the viscosity of the melt decreases and the conductivity increases, resulting in a decrease in the diameter of the fibers. The average fiber diameter was about 28 μm at 320°C and 9.1 μm at 330°C. The electrospinning of EOC from the melt at 340°C allows to produce nonwoven material with fiber diameter in range of 1.9–8.2 μm and an average diameter of 3.9 μm (Fig 2b) and packing density of 6.1%. This is comparable with the results obtained earlier with polyamide-6 [27] and polypropylene [28] under similar conditions. In contrast to the fibers produced from a heated solution, these materials are characterized by a rather smooth surface. It should be noted that the EOC in the fibers retains its elastic properties (elongation at break for films prepared from the obtained nonwoven materials exceeds 1000%).
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Thus, the obtained materials were characterized by similar values of the average fiber diameter and packing density of the fabrics, but significantly different surface morphology of the surface of the fibers. According to X-ray data (Fig 3a), the EOC samples are characterized by the presence of hexagonal mesophase (reflection (100) at 19.7°) and orthorhombic crystalline phase (reflection (110) at 21.1° with a shoulder (200) at 23.3° and reflection (020) at 36.1°) [23, 29] in the pellets and nonwoven materials. The degree of crystallinity was 18% for pellets, 16 and 13% for nonwoven material from solution and melt, respectively. At the same time, the ratio of crystalline phases in fibers in comparison to the initial pellet changes: for the materials obtained by electrospinning of a heated solution, orthorhombic crystalline phase content increases; in the materials, produces by the melt electrospinning, increasing mesophase content is observed. The increase of the mesophase content in nonwovens produced from the melt can be explained by the complexity of the formation of stable crystals in the melt process, in which rapid crystallization of the polymer is accompanied by its high-speed drawing in the electric field. In the IR spectra of the EOC samples (Fig 3b), characteristic bands of polyolefins are observed, i.e. antisymmetric and symmetric C-H stretching in methylene groups (2916 and 2849 cm-1, respectively); C-H deformation in methyl groups (1378 cm-1); C-H deformations in methylene groups: scissoring (1467 cm-1), wagging (1302 cm-1), rocking (719 cm-1) [22]. In addition, a new band appears in the IR spectrum of a material produced from the melt in the region of 1560 cm-1, it corresponds to the antisymmetric stretching of the carboxylate anion of sodium stearate [28]. The absence of absorption bands in the region of 1700–1750 cm-1 (which can be assigned to C=O stretching) indicates that there are no significant thermooxidative degradation of the polymer during melt electrospinning process [30]. Contact angle (CA) measurements revealed that the EOC is a rather hydrophobic polymer: average water CA for the pure EOC film is 112°. Addition of 3% of the sodium stearate (which is an ionic surfactant) leads to a slight increase in the hydrophilicity of the film, average CA decreases to 100°. The produced nonwoven materials (both from solution and melt) possess superhydrophobic and oleophilic properties: water CA is 147-150° (Fig. 2, inset) and oil droplets applied to the material are instantly absorbed, which allows the use of these nonwovens to remove oil spills from water surface. Thus, the addition of sodium stearate does not lead to a noticeable decrease in the contact angles of nonwovens; similar results were previously observed in case of melt electrospinning of polyamide-6 [27] and polypropylene [28].
Fig. 3. XRD patterns (a) and FTIR spectra (b) of EOC pellet (1), nonwoven material produced from a heated solution (2) and nonwoven material, produced from melt (3).
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One of the applications of nonwoven materials is the removal of oil spills from the surface of the water, and the sorption capacity of commercially available materials usually does not exceed 30 g/g [31]. The nonwoven materials (both from solution and melt) obtained in this work are characterized by significantly higher maximum sorption capacity for motor oil, i.e. 95 g/g for fabrics produced by melt electrospinning and 109 g/g for fabrics produced from heated solution. The obtained result is comparable with the sorption capacities for other promising nonwoven materials [32]. In addition, the reusability of the materials was investigated; and the sorption capacitiy of nonwovens at different sorption-desorption cycles is illustrated in Fig. 4.
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Fig. 4. Oil retained in the samples in the cyclic sorption/desorption measurements for materials produced from heated solution (1) and melt (2)
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As can be seen from the presented data, the sorption capacity of the material decreases upon repeated use and levels out at about 45-50 g/g, which may be due to a significant increase in the packing density of the material during oil desorption. It should be noted that during the second and subsequent sorption cycles, the sorption capacity of materials obtained from a heated solution is slightly lower than for materials produced from melt, which can be explained by incomplete removal of oil from the “shish-kebab” fibers. The residual mass of oil in all desorption cycles is in the range of 2-3 g/g for materials produced from the melt and 5-6 g/g for materials obtained from the heated solution, which can also confirm incomplete removal of oil from the highly extended surface of fibers from the heated solution. Based on the presented data, it can be concluded that the sorption capacity of the obtained materials is primarily determined by the packing density of the fabrics (as evidenced by a decrease in capacity by half after several sorption-desorption cycles), and to a much lesser extent depends on the surface morphology of the fibers.
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4. Conclusions Thus, new nonwoven materials were obtained by electrospinning of melt and a heated solution of ethylene-octene copolymer. The average fiber diameter in produced nonwovens was 3.9 and 4.7 μm for melt and solution processes, respectively. It was found that the materials have significantly different fiber morphology: smooth surface for melt electrospinning, and complex “shish-kebab” surface for processing from heated solution. Both materials show superhydrophobic and oleophilic properties. The obtained materials are suitable for removal oil from the water (maximum sorption capacity is more than 100 g/g for motor oil), as matrices for cell cultures and as scaffolds in tissue engineering. Declaration of Competing Interest
None. Acknowledgments The reported study was supported by Russian Science Foundation according to the research project № 18-73-00328. The studies were carried out using equipment of the Resource Centers (“Optics”, “Polymer” and “X-Ray”) of the National Research Center “Kurchatov Institute”.
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