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Research Article pubs.acs.org/journal/ascecg
Electrospinning Biopolymers from Ionic Liquids Requires Control of Different Solution Properties than Volatile Organic Solvents Oleksandra Zavgorodnya,† Julia L. Shamshina,‡ Jonathan R. Bonner,§ and Robin D. Rogers*,†,‡,∥ †
Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States 525 Solutions, Inc., 720 Second Street, Tuscaloosa, Alabama 35401, United States § CFM Group, LLC, 2135 University Blvd., Tuscaloosa, Alabama 35401, United States ∥ Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada ‡
S Supporting Information *
ABSTRACT: We report the correlation between key solution properties and spinability of chitin from the ionic liquid (IL) 1ethyl-3-methylimidazolium acetate ([C2mim][OAc]) and the similarities and differences to electrospinning solutions of nonionic polymers in volatile organic compounds (VOCs). We found that when electrospinning is conducted from ILs, conductivity and surface tension are not the key parameters regulating spinability, while solution viscosity and polymer concentration are. Contrarily, for electrospinning of polymers from VOCs, solution conductivity and viscosity have been reported to be among some of the most important factors controlling fiber formation. For chitin electrospun from [C2mim][OAc], we found both a critical chitin concentration required for continuous fiber formation (>0.20 wt %) and a required viscosity for the spinning solution (between ca. 450−1500 cP). The high viscosities of the biopolymer−IL solutions made it possible to electrospin solutions with low, less than 1 wt %, polymer concentration and produce thin fibers without the need to adjust the electrospinning parameters. These results suggest new prospects for the control of fiber architecture in nonwoven mats, which is crucial for materials performance. KEYWORDS: Chitin, Biomass, Surface tension, Conductivity, 1-Ethyl-3-methylimidazolium acetate
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INTRODUCTION
form is not employed due to their poor solubility in conventional VOCs.2−9 Since the discovery that certain ionic liquids (ILs) can directly dissolve biopolymers such as cellulose, electrospinning of raw biopolymers has seen much attention.10−24 Very recently, we designed a relatively large-scale (0.3 L) setup suitable for electrospinning of biopolymers from the IL 1-ethyl3-methylimidazolium acetate ([C2mim][OAc]) that demonstrated that appropriate voltage, feed rate, and working distances could be achieved for large-scale fiber production, with satisfactory throughput and continuous production.25 Nonetheless, electrospinability and fiber formation are strongly dependent on solution properties, as known from extensive studies of electrospinning from VOCs.26−34 For example, in electrospinning from VOCs, the proper chain entanglement, required for fiber stretching, is regulated by viscosity, polymer concentration, and polymer molecular weight (MW), while the surface tension and conductivity of the solution direct the formation of either fibers or
Electrospinning of synthetic polymers from volatile organic solvents (VOCs) is an industrially important process to produce fibers with submicron diameters.1 Depending on the conditions, electrospinning allows tailoring the properties of the fibers to obtain desired morphologies. However, the rising cost of finite petroleum reserves coupled with a great amount of synthetic plastics polluting both oceans and land have increased the demand for manufacture of products from abundant Natural resources (e.g., agricultural feedstock). Nature’s renewable polymers can act as direct substitutes for synthetic polymers and have already attracted significant societal attention in many industries in this regard. In order for biopolymeric products to overtake plastics, we need to make them readily available for use and open doors to innovation through development of fundamentally different processing technologies, starting from the production of raw materials to their processing into variety of shapes and forms needed for specific applications. While VOCs are still suitable for the electrospinning of a few biopolymers including silk, polylactic acid, and soluble biopolymer derivatives such as cellulose acetate, the electrospinning of most biopolymers in their native © 2017 American Chemical Society
Received: March 21, 2017 Revised: April 24, 2017 Published: April 27, 2017 5512
DOI: 10.1021/acssuschemeng.7b00863 ACS Sustainable Chem. Eng. 2017, 5, 5512−5519
Research Article
ACS Sustainable Chemistry & Engineering droplets.35−37 The electrical conductivity of the solution can be tuned by addition of conductive additives to electrospinning solutions such as inorganic salts (e.g., KH2PO4, NaCl) or organic salts (ILs, e.g., hexamethylimidazolium chloride, 1ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium phosphate) which improves both spinability and fiber formation of a polymer from VOCs.38−43 It has also been shown that incorporation of ILs into a VOC−polymer solution prior to an electrospinning process results in hybrid “ionogel” materials, where the ILs stays entrapped within the electrospun biopolymeric matrix.44−47 There are not many studies addressing the effect of these parameters on electrospinning from ILs,48 which are drastically different from VOCs. Indeed, ILs are composed of ions (attracted by the presence of Coulombic interactions) and possess physicochemical characteristics which can be quite different from conventional VOCs.49−51 Many ILs are nonvolatile, more viscous than VOCs, highly conductive (conductivity values are in a far greater range than those for VOCs), and capable of dissolution of biopolymers not soluble in VOCs. Only a few, rather unrelated, studies have been reported, where changes in solution parameters were shown to be important for electrospinning.11,14 Han’s group reported that both viscosity and surface tension of a cellulose solution in 1-allyl-3methylimidazolium chloride ([Amim]Cl) was excessive for electrospinning and required solution doping with dimethyl sulfoxide (DMSO) that improved solution flow and regulated the resulting diameter of the electrospun fibers.11 Similarly, the effect of surface tension of ILs on the fiber morphology was demonstrated by Freire et al. on electrospinning of cellulose solutions from [C2mim][OAc] doped with long chain surface active 1-decyl-3-methylimidazolium chloride ([C10mim]Cl) IL that was used to reduce the surface tension of an IL mixture.14 In the study we report here, we investigated the effect of key solution parameters needed for electrospinning of biopolymers from ILs and addressed the differences brought about by utilization of ILs as electrospinning solvent and conductive media instead of VOCs. ILs containing basic acetate [OAc]− or chloride [Cl]− anions and alkylimidazolium cations were identified as the most effective at dissolving biopolymers.52−54 Acetate-based imidazolium ILs such as 1-ethyl3-methylimidazolium acetate [C2mim][OAc] are recognized as excellent media for the nonderivatizing dissolution and modification of chitin,55 also having low vapor pressure, high thermal stability, and a relatively low viscosity at room temperature. In our previous papers, as a model system for electrospinning, we investigated the spinability of IL-extracted chitin solutions in [C2mim][OAc].25,48 In this study, as a continuation of our previous work, we correlated the solution properties with electrospinability and architecture of the formed fibers. This work establishes a basis for electrospinning of biopolymers from ILs, exemplified with different types of chitin by providing an understanding of the relationship between solution properties and electrospinnability, crucial for developing electrospun materials with predictable fiber size distribution.
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Chitinous Biomass. P-Chitin: Thermally Preprocessed Shrimp Shells. Shells were received from the Gulf Coast Agricultural and Seafood Cooperative in Bayou La Batre, AL. The shrimp shells were processed with a screw press and dried at a special facility by heating them in fluidized bed dryer to 190 °C. The final moisture content after the drying was less than 5 wt %. The thermally dried material was crushed to particles with a 0.635 cm diameter or below by a hummer mill and shipped to The University of Alabama. The received shrimp shells were ground using an electric lab mill (Model M20 S3, Ika, Wilmington, NC) and sieved through a set of four (1000, 500, 250, and 125 μm) brass sieves with wire mesh (Ika Labortechnik, Wilmington, NC). The particles sized
