Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
In Search of Stronger/Cheaper Chitin Nanofibers through Electrospinning of Chitin−Cellulose Composites Using an Ionic Liquid Platform Julia L. Shamshina,† Oleksandra Zavgorodnya,‡ Hemant Choudhary,‡ Brandon Frye,§ Nathaniel Newbury,§ and Robin D. Rogers*,‡,∥ †
Mari Signum Mid-Atlantic, LLC, 3204 Tower Oaks Boulevard, Rockville, Maryland 20852, United States College of Arts & Sciences, The University of Alabama, Tuscaloosa, Alabama 35487, United States § Anton Paar USA, Inc., 10215 Timber Ridge Drive, Ashland, Virginia 23005, United States ∥ 525 Solutions, Inc., PO Box 2206, Tuscaloosa, Alabama 35403, United States
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S Supporting Information *
ABSTRACT: The ability of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) to solubilize natural biopolymers (e.g., chitin and cellulose) without any chemical modification has been used to develop a onepot process to prepare a spinning dope by extracting chitin from shrimp shell, codissolving microcrystalline cellulose (MCC, DP = 270), and electrospinning nanomats from shrimp shell-extract/MCC solutions. The resulting spinning dopes were prepared with optimal viscosity of 380 to 900 cP, and conductivity and surface tension of ∼2.8 mS/cm and ∼36 dyn/cm, respectively; however, nanofibers could only be prepared when the chitin/MCC ratios in SS-extract/ MCC solution were between 9/1 and 6/4. Compared to nanomats electrospun from shrimp shell-extract solution, 7/3 chitin/MCC composite nanomats demonstrated a 2-fold improvement in hardness and 3-fold improvement in elasticity, although further increase in MCC content resulted in lowering both parameters which nonetheless were higher than the pure chitin nanomats. This one pot process for preparing spinning dopes directly from shrimp shell-extract is a viable method to prepare chitin/MCC composites of improved strength/elasticity at lower costs. KEYWORDS: Biopolymers, Chitin, Cellulose, Composites, Ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), Electrospinning, Nanofibrous mats (nanomats), Strength, Elasticity
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INTRODUCTION Accumulation of nondegradable polymeric waste in both landfills and oceans is a major environmental concern raised in the past decade.1,2 Using biomass feedstock or its waste products for recovering of biopolymers to be used as substitutes to synthetic polymers is a more sustainable approach that has been applied to make different products.3−6 In our own efforts of plastic replacement, we have developed technologies for the preparation of fibers, films, and beads via solution processing of biopolymers dissolved in the ionic liquid (IL) 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]).7−10 Recently, as a continuation of these efforts, we have shown that high molecular weight (MW) regenerated chitin extracted from shellfish waste9 could easily be processed into nanomats through electrospinning from [C2mim][OAc].11−13 We have not only demonstrated the concept of electrospinning of the biopolymer from the IL but also achieved scale up of the process,12 and have shown that fiber formation depends primarily on the solution properties (i.e., conductivity, surface tension, and viscosity or polymer concentration).13 However, © XXXX American Chemical Society
wide application of biopolymers in material development in general, and chitin nanomats in particular, are still limited due to often insufficient mechanical strength as compared to synthetic analogs and the expense of isolating and purifying the chitin from raw crustacean shells. Our previous work has focused on electrospinning of regenerated chitin that we obtained by first extracting the chitin from raw shrimp shells using an IL, and then coagulating it in an antisolvent to remove solids, salts, and proteins.12,13 The extra purification steps of coagulating, isolating, and then redissolving it in fresh IL add cost and thus for practical commodity materials and applications, it would be advantageous to be able to conduct electrospinning directly from the original chitinous extract solution. Electrospinning a solution of chitin crustacean biomass right after the extraction step with simple centrifugation to remove any solids, would significantly reduce the costs; Received: July 9, 2018 Revised: August 24, 2018 Published: September 12, 2018 A
DOI: 10.1021/acssuschemeng.8b03269 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Scheme 1. Electrospinning of Regenerated (purified) Chitin (left) Compared to Electrospinning of Shrimp Shell-Extract (SSextract) Solution (right)
materials, suggests this approach is a viable method to electrospin composite SS-extract/MCC dopes and prepare chitin/MCC composites of improved strength at lower costs.
however, nanofibers electrospun from such shrimp shell-extract solution (abbreviated SS-extract) are anticipated to be weaker than necessary for certain applications due to the presence of impurities, thus requiring reinforcement.14 Indeed, dry-wet jet spun monofilament fibers, pulled from SS-extract solution, exhibited ca. 1.8-times inferior strength than fibers obtained by the same method using regenerated purified and then redissolved, chitin.9 We hypothesized that desirable strength improvement could be achieved using another biopolymer, cellulose, known for its remarkable strength and often used as a reinforcement polymer in composite materials.15−18 For example, microcrystalline cellulose (MCC) with Young’s modulus of 29−36 GPa has been shown to reinforce numerous biopolymeric materials,19−23 and is often used in industrial applications in place of unrefined cellulose directly from plants. Because of its low cost and commercial availability, MCC is often used as a binding filler in, for example, drug delivery systems and as additive to commodity plastics to increase materials rigidity.24,25 In this way, MCCreinforced low density polyethylene (LDPE) demonstrated an ca. 1.6-fold increase in the Young’s modulus when compared to nonmodified LDPE.20 A similar increase in mechanical strength was observed by others for regenerated chitin/MCC films, when composites were prepared through an IL process.26 On the basis of the solubility of both chitin9 and MCC27 in [C2mim][OAc], we settled on the codissolution of both polymers in the same IL. Such attempts of simultaneous processing of chitin and MCC from ILs have been shown to be possible in certain applications, such as making macrofibers (via dry-wet spinning), beads, and films (via biopolymer dissolution and casting or via ion-gel coagulation methods) from 1-ethyl-3methylimidazolium propionate, 1-butyl-3-methylimidazolium chloride, and [C2mim][OAc].28−32 Both chitin and cellulose have each been successfully electrospun from IL solution;33−37 however, we are unaware of any reports where composites of these were electrospun from a single solution of both biopolymers. To test our hypothesis, we electrospun a series of SS-extract/ MCC solutions with different biopolymer ratios from [C2mim][OAc], and employed regenerated chitin and SS-extract without MCC as our benchmarks. Our investigation of the effects of low MW MCC on solution properties, observations on fiber formation, and testing of mechanical properties of the resultant
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RESULTS AND DISCUSSION In order to conduct electrospinning using regenerated (purified) chitin, the chitin needs first to be isolated through multiple steps including microwave-assisted extraction in [C2mim][OAc], centrifugation of undissolved calcium carbonate residue and proteins, coagulation of the supernatant using an antisolvent, multiple washing steps to remove the IL, and drying (Scheme 1). The purified regenerated chitin must then be ground and redissolved in fresh IL prior to electrospinning.12,13 Our first intent with this work was to reduce the time, costs, energy, and solvent usage by eliminating the many isolation and purification steps. We questioned whether simple centrifugation to remove the solids from the SS-extract would obviate the need for the chitin purification steps. The main difficulty is that crustacean shells contain not only chitin but also large amounts of proteins (often covalently bound), mineral salts, and a small amount of lipids. Our earlier work had already suggested that microwaveassisted chitin extraction using [C2mim][OAc] followed by centrifugation would completely remove IL-insoluble calcium carbonate.9 In reality, electrospinning into a water bath is not much different from the coagulation process noted in Scheme 1 and we have previously shown this coagulation step provides regenerated chitin of high purity.9 Preparation of Spinning Dopes. Shrimp Shell-Extract. To prepare SS-extract for the electrospinning trials, shrimp shell biomass was first obtained from raw uncooked shrimps (see Materials and Methods for details). The collected washed, ovendried (80 °C), ground, and sieved (95%) was used as received from IoLiTec, USA (Tuscaloosa, AL). Microcrystalline cellulose (MCC, DP = 270) was obtained from Sigma (Sigma-Aldrich, St, Louis, MO). Deionized (DI) water was obtained from a commercial deionizer (Culligan, Northbrook, IL) with specific resistivity of 16.82 MΩ·cm at 25 °C. Biomass. Frozen shrimp used for chitin extraction were purchased from Skinner’s Seafood, Dauphin Island, AL. The shrimp were thawed, peeled, and visible shrimp meat removed. The backs of the shells were collected and washed 5-times with tap water followed by oven-drying (Precision Econotherm Laboratory Oven, Winchester, VA) at 80 °C for 2 days. The dry shells were ground with a Janke & Kunkel mill (Ika Labortechnik, Wilmington, NC) for 5 min and sieved through 1000, 500, 250, and 125 μm brass sieves with wire mesh (Ika Labortechnik, Wilmington, NC). Shrimp shell particles
