Ionic Liquid Platform for Spinning Composite Chitin–Poly(lactic acid

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An Ionic Liquid Platform for Spinning Composite Chitin-Poly(lactic acid) Fibers Julia L. Shamshina, Oleksandra Zavgorodnya, Paula Berton, Pratap K. Chhotaray, Hemant Choudhary, and Robin D. Rogers ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01554 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Sustainable Chemistry & Engineering

An Ionic Liquid Platform for Spinning Composite Chitin-Poly(lactic acid) Fibers Julia L. Shamshina,1 Oleksandra Zavgorodnya,2 Paula Berton,3 Pratap K. Chhotaray,2 Hemant Choudhary,2 and Robin D. Rogers2,4* 1

Mari Signum, Mid-Atlantic, 3204 Tower Oaks Boulevard, Rockville, MD 20852, USA

2

Department of Chemistry, The University of Alabama, 250 Hackberry Ln, Tuscaloosa, AL 35487, USA

3

Chemical and Petroleum Engineering Department, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada

4

525 Solutions, Inc., 720 2nd Street, Tuscaloosa, AL 35403, USA

E-mail of corresponding author: [email protected]

Abstract In the design of stronger chitin fibers reinforced with poly(lactic acid) (PLA), an ionic liquid (IL)-based approach was developed in which both polymers were co-dissolved in an 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]) and wet-jet spun into composite fibers. Chitin, directly extracted from shrimp shell had a solubility in the IL of 2.75 wt%, while PLA of MW 700,000 g/mol, had a solubility of 49 wt%. Keeping the IL saturated in chitin, homogeneous solutions of chitin and PLA could be obtained up to 27 wt% (relative to the IL) PLA. Spinning dopes were prepared by maintaining the chitin concentration relative to the IL at 1.75 wt% and adding PLA in chitin to PLA weight ratios of 1:0.1 through 1:1 (PLA concentrations of 0.175 wt% to 1.75 wt% relative to the IL). Homogeneous chitin/PLA fibers could be spun when the chitin to PLA ratio was between 1:0.1 and 1:0.3. The tensile strength and plasticity of the fibers depended on the chitin to PLA ratio with the highest plasticity (8.8 vs. 3.0% for pure chitin fibers), strength (112 vs. 71 MPa), and stiffness (5.9 vs. 4.2 GPa) observed for fibers with a chitin to PLA ratio of 1:0.3. Studies of the fracturing surface of the fibers indicated that fracturing occurred through an initial disruption of the interactions between polymer chains, followed by complete fiber breakage. The work not only demonstrates that homogeneous composite fibers can be spun using a biopolymer and PLA additive, but also suggests a versatile

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platform for preparation of multiple biopolymer-PLA materials using solution processing methods.

Keywords: Chitin; Co-dissolution; 1-Ethyl-3-methylimidazolium acetate; Fiber extrusion; Ionic liquid; Poly(lactic acid); Tensile strength; Young’s modulus.


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Introduction Biopolymers, as substitutes for synthetic plastics, have gained significant attention during the last decade due to their biodegradability, low cost, and minimal environmental impact. 1 Among biopolymers, chitin (Figure 1), the second most abundant polymer after cellulose, offers the advantage of easy functionalization through its acetamide groups. In particular, our group has developed high-performance chitinous sorbents in the form of dry-wet spun and electrospun fibers2-4 which, for example, when functionalized with amidoxime, can be used for extracting of uranium from seawater.5,6 However, when these sorbents were evaluated for applications like metal recovery from seawater, the dry-wet spun fibers displayed strengths insufficient to meet performance criteria, with tensile strength significantly weaker than analogous synthetically tailored materials.5 One strategy to impart strength to biopolymeric materials is through the incorporation of special solid fillers and reinforcements that allow the introduction of control over intermolecular hydrogen bonding interactions between two polymers.1 For our work, poly(lactic acid) (PLA), a synthetic polymer derived from renewable natural monomers, offers good mechanical properties depending on its molecular weight (MW) and degree of crystallinity.7,8 However, PLA cannot act as the sorbent itself because it lacks functional groups suitable for further functionalization (Figure 1), it is brittle, and it possesses low hydrophilicity, and hence poor water-barrier properties.9,10 Although advertised as degradable, PLA is highly resistant to UV-light irradiation. Its complete degradation to carbon dioxide and water requires high humidity and/or elevated temperature. 11 Under regular environmental conditions, PLA is known to be highly stable,11 which makes it ideal for environmentally-sound sorbents that would degrade if lost in seawater. A

B

Figure 1. Structures of chitin (A) and poly(L-lactic acid) (PLA) (B).



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Due to the properties described above, PLA is better used in the form of composites or blends, and hence combining easy-to-functionalize chitin with the strong and stable PLA is a logical combination. Nonetheless, mixing these polymers represents a big challenge. PLA is traditionally melt-processed, the methods for making PLA composites include those that improve the dispersion and interfacial adhesion in respective mixtures. Since chitin, like other biopolymers, cannot be melted, to prepare chitin–PLA composites, nanocrystalline or microfibrilated chitin is added in the form of nanowhiskers and physically mixed with the melted PLA matrix.10,12-14 After such physical mixing, techniques that have been used to prepare the composite materials are restricted to casting and evaporating, non-aqueous solvent dispersion, polymer grafting, freeze-drying, and hot pressing.15 A single example was found in literature of Chitin-PLA nanogels prepared by using PLA dissolved in chloroform mixed with a CaCl2methanolic chitin solution followed by sonication and solidification in an excess of methanol as anti-solvent.16 Using the above preparation techniques often results in agglomeration and non-uniform distribution of the biopolymer due to its poor solubility, compromising the mechanical properties of the composites compared to neat PLA.10,12 To obtain more uniform biopolymer dispersions in the PLA matrix in melt processes, biopolymers are often first stabilized by plasticizers or surfactants, or prepared as solvent dispersions (e.g., in water or alcohols) which are mixed with melted PLA and then extruded,17-20 Another strategy includes dissolving the PLA in an organic solvent and adding solid biopolymers (e.g., cellulose microfibrils) to form a suspension, followed with roller-mixing to obtain a more uniform dispersion.21-23 This last method slightly improved the mechanical properties of PLA due to more uniform polymer distribution, however, neither process results in formation of a ‘uniform’ material because of the formation of miscible binary mixtures on a molecular level. Even for ‘soluble’ chitin or cellulose derivatives, such as chitosan or cellulose acetate, there is no method to produce uniform solutions. By far, ‘mixed-solvent methods’ are used to prepare most blends, such as PLA/DMSO24 or PLA/chloroform25 mixed with chitosan/acetic acid. Other efforts to produce PLA materials reinforced with biopolymers report extrusion of plasticized chitosan-PLA blends, 26 often followed by molding/shaping, 27 or simple coatings 28 (e.g., chitosan-coated PLA).


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Ionic liquids (ILs, known as salts that are liquid below 100 °C29) can be designed as solvents for biopolymers,30,31 or be used as a medium in which a synthetic polymer may be dissolved to enable further chemical transformations.32-39 Dissolved alone or with other polymers, IL-polymer solutions can be used to prepare spun fibers, films, hydrogels, beads, and electrospun mats providing a route to a host of new materials.2,4,5,40-53 For example, we recently demonstrated that specialty composite fibers could be obtained from chitin-alginic acid solutions in IL through a dry-jet wet spinning process, resulting in homogeneously blended fibers.48 Remarkably, despite the huge increase in interest in PLA-based materials (e.g., in 3D printing), there are only two studies where chitin54 or cellulose55-PLA blends were made using an IL, both via a ‘mixed solvent’ method, where the IL was used as solvent for the biopolymer, while PLA was dissolved in either chloroform55 or dichloromethane.56 Still, we have not found studies where PLA and chitin or cellulose were simultaneously dissolved and processed in an IL. Instead, ILs have been explored for PLA degradation (using [C2mim][OAc] at 170 °C),56 as PLA plasticizers,57-59 or as PLA additives to provide bacteriostatic/antibacterial properties.60 We also note that in virtually every instance, biopolymers have been added to PLA to improve the performance of the PLA, while our goal was to use PLA as an additive to improve the performance of our biopolymeric materials. Here, we report our efforts to improve the mechanical strength of chitin fibers by direct co-dissolution of chitin and PLA in the IL [C2mim][OAc] to prepare homogeneously blended spinning dopes. We hypothesized that by codissolving the polymers in the same solvent we could achieve uniform blending and tune the properties of the blended materials by changing the chitin to PLA ratio. We explored whether this route would provide a platform for controlling mechanical robustness of spun Chitin-PLA fibers needed for sorbent applications, by tuning the mechanical properties and plasticity of the materials through variations in fiber composition.

Results and Discussion We started our study to impart controllable strength in chitin fibers to be used as sorbents, by first determining under what conditions we could obtain homogeneous chitin/PLA blends. Blended fiber properties are usually influenced by the constituent polymer components, their relative amounts, and mixing conditions. For chitin, we selected chitin extracted from thermally pre-treated, pressed, and ground shrimp shell biomass. The chitin was isolated by microwave5 ACS Paragon Plus Environment


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assisted dissolution using [C2mim][OAc], followed by coagulation in water, washing, and ovendrying, as we have previously described.48 Purity of the dry IL-extracted chitin was measured using Fourier-Transform Infra-Red spectroscopy (FT-IR), powder X-ray diffraction (PXRD), and solid-state CP-MAS

13

C Nuclear Magnetic Resonance spectroscopy (CP-MAS

13

C NMR).

The diffraction patterns of chitin showed peaks at 9.4, 11.9, 19.3°, typical for chitin.61 FT-IR spectra also showed peaks representative for chitin.62-65 These included C-O and C=O (amide I) stretches at 1660 cm-1 and 1630 cm-1, N-H (amide II) at 1558 cm-1, C-O (CH deformation of the β-glycosidic bond) at 895 cm-1, N-H stretching (asymmetric) at 3265 cm-1, N-H stretching (symmetric) at 3100 cm-1, N-H deformation at 1560 cm-1, O-H stretching at 3480 cm-1, C-H in CH3 at 2965 cm-1, C-H in CH2 at 2927 cm-1, C-H in CH3 (asymmetric) at 2883 cm-1, C-O-C ring (asymmetric) at 1157 cm-1, and C-C stretch at 566 cm-1. The chitin was not found to be contaminated with the IL, as the main peaks associated with the IL include carbonyl C=O of the acetate anion at 1656 cm-1, in-plane vibrations of the imidazolium ring around 1554 cm-1, C–H stretching contributions at 3094 cm-1 (consisting of two strongly overlapping bands), and C-H stretching at 3206 cm-1.66 Spectra of purified chitin, as well as comparison of spectra between pure chitin and chitin contaminated with the IL are shown in the ESI. Finally, solid-state multiCP

13

C NMR was used as previously shown67 to assess chitin purity and demonstrated no

deacetylation, as well as the absence of the proteins. PLA with the highest molecular weight (MW) available on the market (700,000 g/mol or 6.5 dl/g), was purchased and used as received. Before preparing blends of the polymers, we first determined the solubility for each polymer in [C2mim][OAc]. The solubility of chitin was determined by adding 10 mg of chitin to a loosely capped vial containing 5 mL of the IL, equipped with a stirring bar and placed into oil heating bath, at 100 °C for 15 h. The stirring while heating was continued until all the chitin was dissolved, and then an additional 10 mg chitin was added to the solution. This process was repeated until the chitin concentration in the IL solution reached its saturation limit, i.e., additional stirring and heating did not result in disappearance of undissolved chitin. The solubility, which depends on the polymer MW and the biomass source, was 2.75 wt%, in agreement with previous reports.48 When the PLA solubility in [C2mim][OAc] was evaluated in the same fashion, the polymer was found to be ~18 times more soluble than chitin (up to 49 wt%). The co-solubility of chitin and PLA in [C2mim][OAc] was then determined. We first loaded 6 ACS Paragon Plus Environment

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2.75 wt% chitin and 49 wt% PLA (both relative to the mass of IL), with heating and stirring as described above, but a large amount of PLA (identified by FT-IR) remained undissolved even after 24 h. To maintain our primary polymer concentration (chitin), the amount of PLA was decreased to 44, 40, 35, 30, 29, 28, and 27 wt%, while the chitin concentration was kept constant. A homogeneous solution of chitin and PLA was obtained at 2.75 wt% chitin and 27 wt% PLA (both relative to the mass of the IL), corresponding to 0.141 g of chitin and 1.384 g of PLA (i.e., Chitin:PLA = 1:9.8 w/w), dissolved in 5 g of the IL. This suggested that in its supporting role as an additive, we were not limited by PLA co-solubility. Attempts to pull fibers were based on the room temperature dry-jet wet spinning set-up (Figure S1, ESI) we have previously reported.46 In this apparatus, the spinning dope is pulled through the coagulation bath, wrapped around set of godet rollers. The voltage settings for the godets were 0.10 V and 1.8 V. The spinning was conducted at a rate slow enough to allow IL removal and formation of a monofilament. When fibers form they are collected on a spool rotated manually at the same speed and thoroughly washed by placement of the spool into DI water which is exchanged periodically with fresh DI water (see Experimental). After complete washing, the fibers are hung and air-dried. Since we had previously determined that chitin fibers prepared from 1.75 wt% chitin solutions had optimal mechanical properties,48 we prepared control 100% chitin fibers for this study at this spinning dope concentration. Neat PLA control spinning dopes were prepared through thermal dissolution of PLA from 1.75 through as high as 49 wt% in [C2mim][OAc] by heating in an oil bath for 15 h. Attempts to use any of the PLA solutions were unsuccessful independently of PLA concentration (up to the solubility limit of 49 wt%). All PLA solutions dripped into the coagulating bath forming flakes as the IL was washed out. Chitin-PLA composite spinning dopes were prepared by simultaneous thermal dissolution of chitin and PLA in the IL, keeping the chitin concentration at 1.75 wt% (i.e., 178 mg chitin in 10 g IL). The dissolution was conducted at 100 oC over 15 h. The chitin to PLA weight ratios chosen for study were 1:0.1 (abbreviated here as Chitin-PLA-1), 1:0.25 (Chitin-PLA-2), 1:0.3 (Chitin-PLA-3), 1:0.5 (Chitin-PLA-4), 1:0.75 (Chitin-PLA-5), and 1:1 (Chitin-PLA-6) (Table 1) corresponding to solution concentrations of PLA to IL of from 0.17 to 1.75 wt% (relative to the IL). The viscosity of these solutions significantly increased with PLA concentration.



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Table 1. Spun Chitin and blended Chitin-PLA fibers.

Sample Name Chitin-PLA-6 Chitin-PLA-5 Chitin-PLA-4 Chitin-PLA-3 Chitin-PLA-2 Chitin-PLA-1 Chitin

Concentration relative to [C2mim][OAc] Chitin, PLA, wt% wt% a 1.750 1.313 0.875 1.750 0.578 0.438 0.175 1.750 0

Chitin:PLA Ratio

a

1:1 1:0.75 1:0.5 1:0.3 1:0.25 1:0.1 1:0

Chitin concentration in IL was fixed to be 1.75 wt% of the IL.48 VISCOLab 3000 limit (10,000 cP).

b

Viscosity (cP, 25 °C)

Fiber Diameter, µm

10695 ± 10.2% b 10985 ± 6.9% b 11123 ± 6.8% b 6026 ± 6.7% 5686 ± 6.9% 4842 ± 0.8% 2912 ± 0.8%

125 ± 27 172 ± 33 116 ± 42 106 ± 4 90 ± 2 122 ± 8 111 ± 3

Note that these values are just above the

All composites were spun under the same experimental conditions noted above for the study of the controls. All of the prepared composite solutions were spun into fibers, however ChitinPLA-4, Chitin-PLA-5, and Chitin-PLA-6 exhibited excessive solutions viscosity preventing their uniform extrusion (see optical images of these fibers, below). The wet 100% chitin fibers were non-transparent with a yellowish tint, while all PLA-containing fibers were white. Once airdried, the chitin fibers darkened to a light brown color, as did the Chitin-PLA fibers with essentially no visual differences between the chitin and composite fibers once dried. This change in color was also observed for air-dried chitin films,41 and is indicative of densification during drying. The average dry fiber diameters were determined with an optical microscope by measuring five different fibers inspected for uniformity. The fibers with high PLA content, Chitin-PLA-4, Chitin-PLA-5, and Chitin-PLA-6, appeared to have non-uniform fiber diameters (Figure 2, Table 1), which might be due to the higher overall concentration of polymers which often results in non-uniform diameters,48 or interference of PLA with the hydrogen bond chitin network. In addition, these fibers were very weak and brittle after drying, either breaking during the airdrying process or to touch when dry. As a result, we excluded Chitin-PLA-4, Chitin-PLA-5, and Chitin-PLA-6 fibers from further characterization.


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Figure 2. Optical microscopy images of wet fibers (400x objective): Chitin-PLA-4 (A) and Chitin-PLA-5 (B) and of dry fibers (100x objective): 100% Chitin (C), Chitin-PLA-1 (D), Chitin-PLA-2 (E), Chitin-PLA-3 (F), Chitin-PLA-4 (G), Chitin-PLA-5 (H), and Chitin-PLA-6 (I). The morphology of the fibers was investigated with scanning electron microscopy (SEM) which revealed surfaces with visible striations indicating sufficient stretching of the spun fibers (Figure 3). The presence of striations on the fiber surface is consistent with differences in coagulation rate between the surface and bulk phase, reported previously for chitin fibers.40



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Figure 3. SEM images (magnification 100x and 1500x (insets)) of Chitin (A), Chitin-PLA-1 (B), Chitin-PLA-2 (C), and Chitin-PLA-3 (D). Spectroscopic Characterization The structure and composition of the spun fibers was studied using Fourier Transform Infra-Red (FT-IR) spectroscopy. The FT-IR spectrum of the commercial PLA pellets (Figure 4) showed the two characteristic bands at 1753 cm-1 and 869 cm-1 corresponding to C=O and C-COO stretching in PLA, respectively. 68 For neat chitin fibers, characteristic groups included the acetamide group (-NH-C(O)-CH3) which exhibits a carbonyl C=O stretch (amide-I) split into two components (due to hydrogen bonding between amide moieties), at 1646 and 1613 cm-1, and an amide-II band at 1539 cm-1 (Figure 4, right).62


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Figure 4. FT-IR spectra of chitin and Chitin-PLA composite fibers. Chitin (blue), Chitin-PLA-1 (dark red), Chitin-PLA-2 (dark green), Chitin-PLA-3 (purple), PLA (black) and chitin–PLA physical mixture (grey) made of the same composition as Chitin-PLA-3). Right: Expanded view of the 1200–2000 cm-1 regions. In the blended fibers, the characteristic peaks of both PLA and chitin were observed but were slightly shifted as compared to the neat polymers (Figure 4, right). The characteristic peak of PLA at 1753 cm-1 shifted to lower frequency ~1733 cm-1, while the split chitin amide-I bands at 1646 and 1613 cm-1, which have different intensities, became of nearly equal intensity in the blended fibers. Contrarily, both the location and intensity of the PLA’s C=O peak at 1753 cm-1 do not change in the spectrum of a physical mixture of these two polymers (Chitin to PLA = 1:0.3), confirming hydrogen-bonding between the C=O groups of PLA and the amide (-C(O)NH) groups of chitin in the blended fibers. The intensity of the PLA-related peaks increases with increasing PLA content and were observed to be significantly smaller as compared to those of chitin. No peaks related to [C2mim][OAc] were observed in the FT-IR spectra of the fibers, confirming removal of the IL during washing.

Thermal Analysis Thermogravimetric Analysis (TGA) The thermal stability of the fibers was studied using TGA. We report here the temperature, at 11 ACS Paragon Plus Environment


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which 5% of the sample had decomposed during heating (T5%onset) and the temperature at which the rate of change in the mass of a sample was the fastest (Tmax) determined from the 1st derivative of the TGA curves.69 Thermal degradation of the neat chitin fibers occurred in a single step with T5% of 248 °C and Tmax of 356 °C (Figure 5). The PLA sample exhibited a two-step decomposition, with a very subtle transition between the steps. The first step with T5% of 245 °C (Tmax of 300 °C) resulted in a weight loss of about 8.8 % (Table 2), indicating the presence of some remaining tin catalytic impurities, 70 - 72 left after polymerization. This step was followed by a second step, the decomposition of PLA, with T5% of 320 °C (Tmax of 377 °C). Table 2. Thermal analyses of Chitin, PLA, and Chitin-PLA fibers. TGA Sample

PLA pellet Chitin-PLA-3 Chitin-PLA-2 Chitin-PLA-1 Chitin fiber

T5% °C Step 1, Step 2, °C °C 245 320 287 289 294 248 -

Tmax °C Step Step 2, 1, °C °C 300 377 364 363 360 356 -

DSC 1st cycle Tg °C 66 -

Tm1 °C 174 160 182 -

2nd cycle Tm2 °C 180 204 182 190 -

Tg °C 53 -

Tm °C 167 -

- No transition was detected.

Figure 5. Thermogravimetric analysis curves of PLA and Chitin and Chitin-PLA composite fibers. Chitin (blue), PLA (black), Chitin-PLA-1 (dark red), Chitin-PLA-2 (dark green), and Chitin-PLA-3 (purple). Right: Expanded view of the 200-350 °C regions. 12 ACS Paragon Plus Environment

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Even though neat PLA exhibited a two-step mass loss, all Chitin-PLA blended fibers decomposed in a single step, with a decomposition temperature higher than neat chitin, but lower than neat PLA, confirming uniform blending. The value of T5% depended on the amount of PLA, and decreased with increasing PLA content.

Differential Scanning Calorimetry (DSC) The phase morphology of the blends was determined using DSC. Generally, blends with two main constituents being completely mixed show no apparent phase separation as opposed to composites, in which mixing is seldom complete and thus phase separation is apparent.73 The thermal transitions in the Chitin-PLA fibers were evaluated by DSC in the temperature range from 0 to 220 °C, at a heating/cooling rate of 10 °C/min, and the melting temperatures (Tm, the transition peak maxima) and glass transitions (Tg) were recorded. In the 1st heating cycle for the PLA pellets, two endothermic peaks were observed at 66 and 178 °C corresponding to the Tg and Tm, respectively (Figure 6 and Table 3), in agreement with reported values.74 All blended fibers demonstrated melting split into two events well-spaced in temperature, Tm1 and Tm2, one peak being much smaller than the second. The first melting event (Tm1) took place at 182 °C (Chitin-PLA-1), 160 °C (Chitin-PLA-2), and 174 °C (Chitin-PLA-3). The second melting event (Tm2) was close to that for the PLA pellets, namely 190 °C (Chitin-PLA-1), 182 °C (Chitin-PLA-2), and 204 °C (Chitin-PLA-3). It is worth noting that Tm2 temperature shifted to higher values for all blends when compared to the melting point of neat PLA (178 °C), and not down as would be expected from melting point depression. In addition, these two melt peaks are very different in size, with Tm1 being much smaller than Tm2. Such thermal behavior has been reported to be a function of crystallization temperature, crystallization time, and heating rate depending primarily on the size and perfection of initial crystals, 75 or to the formation of a second type of crystal/crystallite size population with different surface free energy. As semicrystalline PLAs can be found in multiple forms, Tm in a range of 130–230 °C (depending on structure) have been reported. When compared with each other, minor melting peak broadening was observed for the Chitin-PLA-1 and Chitin-PLA-2 fibers comparing to Chitin-PLA-3 (Figure 6A). We also observed suppression of Tg when compared with neat PLA. In the 1st cooling cycle, no crystallization events were observed for either PLA or the blends. 13 ACS Paragon Plus Environment


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Figure 6. DSC (Exo ↑) thermograms of the 1st heating (A), 1st cooling (B), and 2nd heating cycles (C) curves of Chitin (blue), PLA pellets (black), Chitin-PLA-1 (dark red), Chitin-PLA-2 (dark green), and Chitin-PLA-3 (purple). 14 ACS Paragon Plus Environment

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The 2nd heating cycle showed a glass transition for PLA at 53 °C, followed by cold crystallization at 128 °C, and then melting at 167 °C (Figure 6 B, C), as previously reported.70,76,77 For pure chitin fibers, only a weak endothermic peak at ~80-130 °C related to the relaxation of the acetamide groups in the polymer’s backbone was detected.78 The Chitin-PLA blends did not show any thermal transitions in the 2nd heating cycle. Such suppressed PLA crystallization in composites has been previously reported for PLA-poly(ethylene oxide) (PEO) binary mixtures.79

Mechanical Properties The tensile properties of the spun fibers were tested using a MTS Q-Test 25 equipped with a pneumatic grip designed to hold fibers. Four fibers of each type with uniform cross-section were tested using a load cell with capacity of 22.4 N and cross-head speed of 1.27 mm/min. As expected, the neat chitin fibers demonstrated the lowest tensile strength (~70 MPa, Figure 7, Table 3) although the value was slightly lower when compared to published data for chitin fibers drawn from the same IL, but under slightly different conditions and a different chitin source (160-218 MPa).48 For neat PLA (MW 530,000, slightly lower than used by us, but the highest reported in the literature), solution-spun fibers were reported to exhibit tensile strengths of 2801000 MPa, 16-18% elongation at break and Young’s modulus of 7-9 GPa.80 Blended fibers normally exhibit properties which depend significantly on the mechanical properties of the components and their amounts, as well as on the blend microstructure and interfacial interactions.81 In our composite fibers, the fibers with the lowest concentration of PLA (Chitin-PLA-1) demonstrated no statistically significant improvement in strength (83±5) MPa, Figure 7, Table 3) compared to neat chitin fibers (71±2 MPa), although we recognize the higher overall amount of polymers present. Higher concentrations of PLA did result in stronger fibers as predicted, with tensile strengths of ~110 MPa observed for both Chitin-PLA-2 and Chitin-PLA-3 (Table 3). The latter two composites represent an ~1.5-times increase in tensile strength over the neat chitin fibers. Quick One Way Analysis of Variance (ANOVA) statistics tests were conducted using Sigma Plot software (Version 11.0). Before conducting the ANOVA test, the software verified the assumptions of normality of distribution and homogeneity of the variance test. The p-value obtained for ANOVA was more than the significance level for all groups, suggesting the data being normally distributed; the tests of homogeneity of variance showed 15 ACS Paragon Plus Environment


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homoscedasticity of the data. To determine which groups have significant differences, we computed Tukey’s post hoc test, to isolate the groups that differ from the others using a multiple comparison procedure through pairwise multiple comparison. The results revealed that tensile strength values for chitin, and Chitin-PLA-1 were statistically the same, but were statistically different from the values for Chitin-PLA-2 and Chitin-PLA-3. The Chitin-PLA-2 and ChitinPLA-3 results were statistically the same. The statistics are provided in the ESI. The Young's modulus significantly increased from 4.2(2) GPa for pure chitin fibers, to ~6 GPa for all blends. However, the Young’s modulus was essentially independent of PLA concentration. This suggests that some shortcomings in chitin fiber mechanical properties, such as brittleness might be overcome by even minor additions of PLA. We have found no literature examples of drawing fibers of PLA from an IL. It was reported, however, that fibers produced from PLA exhibited tensile strength of 260 – 1000 MPa, depending on MW and fiber pulling method. Low MW (23,000 and 58,000 g/mol) PLA possesses strength of only 58 MPa,79 although these fibers were prepared by injection molding. Higher values of tensile strength (202-267 MPa)8 were obtained for higher MW PLA (380,000 g/mol), for melt-extruded fibers. Finally, the highest fiber strength was observed for the highest MW PLA with MW 350,000 and 530,000 spun from toluene solvent.79,80 Comparison of chitin fiber strength with literature values for fibers of chitin drawn from [C2mim][OAc] solution under slightly different conditions and from a different chitin source,40 showed that our chitin fibers had lower tensile strength than previously reported 160 - 218 MPa, perhaps because of the different chitin source being used, and different biomass batch.


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Figure 7. Stress-strain curves of Chitin and Chitin-PLA composite fibers: Chitin (blue), ChitinPLA-1 (dark red), Chitin-PLA-2 (dark green), and Chitin-PLA-3 (purple). Table 3. Tensile measurements and literature comparisons. Sample

Strength, MPa

Yield Strength,1 MPa

Young’s modulus, GPa

Strain, %

Chitin Chitin-PLA-1 Chitin-PLA-2 Chitin-PLA-3

71±2 83±5 110±12 112±7

50±4 43±6 55±5 59±4

4.2±0.2 6.4±0.3 5.8±0.3 5.9±0.3

3.0±0.2 5.2±1.8 8.1±0.5 8.8±0.4

Literature Values PLA (MW 530,000) fibers drawn from toluene80 PLA (MW 350,000) fibers drawn from toluene79 Melt extruded PLA (MW 380,000) fibers8 PLA (MW 23,000) materials by injection molding80 PLA (MW 58,000) materials by injection molding80

280-1000

-

7-9

16-18

260-800

-

6-10

12-26

202-267

-

3-5

3.3-5.5

59

-

3.55

1.5

58

68

3.75

5.5

Fracture Morphology



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To better understand the effect of blending on the fiber properties, we also examined the fracturing surface of the fibers. As seen from SEM cross-sections (Figure 8), the fibers were homogeneously blended with the presence of non-uniform edges in all fibers along the crosssection. The observed edges indicate that the fibers start failing at different stresses before the failure was propagated across the full cross-section. The fracture induced flow was located at the surface and bulk volume of the fibers. Considering that the studied fibers showed similar crosssection diameters, their fracturing underwent a similar mechanism, i.e., an initial disruption of the interactions between polymer chains, followed by complete fiber breakage.

Figure 8. SEM images (700x) of fiber cross-sections after the tensile strength tests: (A) Chitin; (B) Chitin-PLA-1; (C) Chitin-PLA-2; (D) Chitin-PLA-3.

Conclusions The solubility of both chitin extracted from shrimp shells using [C2mim][OAc] and high molecular weight PLA (MW 700,000 g/mol) in this same IL allows the co-dissolution of both polymers in a wet-jet spinning process to spin stronger chitin fibers. Despite the fact that solutions of only PLA at any concentration up to its solubility limit of 49 wt% were not able to form fibers, homogeneous composite fibers could be spun when the chitin concentration was 1.75 wt% and the chitin:PLA ratio was equal or below 1:0.3. The ability to co-dissolve biopolymers with PLA suggests a versatile platform for preparation of multiple biopolymer-PLA materials using solution processing methods. 18 ACS Paragon Plus Environment

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Blends of chitin and PLA in weight ratios of 1:0.25 and 1:0.3 exhibited an ~1.5x increase in tensile strength compared to neat chitin, 110±12 and 112±7 MPa, respectively. The plasticity of the fibers increased from 3% for chitin fibers to 5% even for blends with the lowest concentration of PLA. For fibers with larger PLA load, plasticity increased to 8.1% and 8.8% for blends of chitin to PLA with weight ratios of 1:0.25 and 1:0.3 respectively. Young's modulus increased from 4.2±2 GPa, to ~6 GPa for all blends, independent of PLA concentration. The observed improvements in fiber strength at weight ratios of chitin/PLA of 1:0.2 and 1:0.3 is possibly due to hydrogen-bonding interactions between the C=O of PLA and the amide (-C(O)NH-) of chitin in the blended fibers, as suggested by spectroscopic examination. In addition, the fracture pattern of the fibers suggested that fracturing occurred through an initial disruption of the interactions between polymer chains, followed by complete fiber breakage. While this proof-of-concept study illustrates a new technique to produce chitin-PLA composites with improved mechanical properties, there is much work to be done. Further optimization should be addressed during the commercialization step, including optimization of the equipment, to find greater gains in composite fiber strength to compete with the array of strong plastic fibers on the market today. Mass-manufacturing of chitin fibers would require a large-scale supply of chitin and commercial scale fiber pulling machines, optimization of the process parameters, and of course solvent recycling. At least for the first one, chitin supply at large scale, a start-up company Mari Signum Mid-Atlantic (Mari Signum)82 has recently been formed as a chitin and chitin materials production company. It is currently scaling-up the technology of chitin production from crustacean waste and we hope that economies of scale for the chitin polymer will drive continued innovation in new chitin products. New chitin-based products will need the same time and attention that plastics have received for so long in order to compete in an open market, regardless of whether the chitin-based fibers are more environmentally-benign or not.

Materials and Methods Materials The ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc], purity >95%) was purchased from IoLiTec, Inc. (Tuscaloosa, AL). Poly(L-Lactic acid) (PLA) with molecular weight of ~700,000 (6.5 dl/g), was purchased from Polysciences, Inc. (Warrington, PA). 19 ACS Paragon Plus Environment


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Deionized (DI) water, obtained from a commercial deionizer (Culligan, Northbrook, IL) with specific resistivity of 16.82 MΩ•cm at 25 °C, was used for all experiments. The shrimp shells were received from the Gulf Coast Agricultural and Seafood Cooperative (‘COOP’) in Bayou La Batre, AL, where the shrimp shells were dried at a dedicated facility. This was done by first pressing the biomass with a screw press (to get rid of the majority of the water), followed by temperature treatment at up to 190 °C in a fluidized bed dryer, until the biomass was less than 5 wt% wet. The dried shrimp shells were pulverized to particles

Ionic Liquid Platform for Spinning Composite Chitin–Poly(lactic acid

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