Effects of Emulsion Droplet Size on the Structure of Electrospun

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Effects of emulsion droplet size on the structure of electrospun ultrafine biocomposite fibers with cellulose nanocrystals Yingjie Li, Frank K. Ko, and Wadood Y Hamad Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/bm400540v • Publication Date (Web): 21 Jun 2013 Downloaded from http://pubs.acs.org on September 24, 2013

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Effects of emulsion droplet size on the structure of electrospun ultrafine biocomposite fibers with cellulose nanocrystals Yingjie Li1, Frank K. Ko*1 and Wadood Y. Hamad*2 1. Materials Engineering Department, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4 2. FP Innovations, 3800 Wesbrook Mall, Vancouver, BC, Canada V6S 2L9 ABSTRACT

Electrospinning of cellulose nanocrystals (CNC)/poly (lactic) acid (PLA) emulsions has been demonstrated to be an effective dispersion and alignment method to control assembly of CNC into continuous composite ultrafine fibers. CNC-PLA nanocomposite random-fiber mats and aligned-fiber yarns were prepared by emulsion electrospinning. A dispersed phase of CNC aqueous suspension and an immiscible continuous phase of PLA solution comprised the CNC-PLA water-in-oil (W/O) emulsion system. Under a set of specific conditions, the as-spun composite ultrafine fibers assumed core-shell or hollow structures. In these structures, CNCs were aligned along the core, in the core-shell case, or on the wall of the hollow cylinder, in the hollow fiber case. CNCs act as nucleating 1

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agents influencing PLA crystallinity, and improve the strength and stiffness of electrospun composite fibers. The effects of emulsion droplet size on fiber structural formation and CNC distribution within the electrospun fibers have been carefully examined.

KEYWORDS: Cellulose nanocrystals, emulsion electrospinning, ultrafine fibers, poly (lactic) acid

INTRODUCTION

Advances in nanotechnology and increasing awareness around the importance of sustainability and renewable materials have stimulated renewed interest in cellulosic materials. Cellulose is one of the most abundant renewable natural polymers. Nanocrystalline celluloses (NCCs) or cellulose nanocrystals (CNCs), are extracted from lignocellulose materials, e.g. wood pulp, using an acid hydrolysis process.1 It has density of 1.59 g/cm3 and tensile strength approaching 10 GPa.2 As a reinforcement material, CNCs have potential to improve the strength, modulus and toughness of polymer composites.3,4 Super-strong nanocomposite fibers produced by electrospinning remain an elusive challenge owing mainly to the presence of molecular-scale defects in spite of reaching nano-scale fiber diameters. To explore the potential of preparing high-performance nanofibers, CNC could be a suitable choice for reinforcement assuming effective dispersion and well-controlled alignment of CNC within the fiber structure. The dispersion of CNCs in aqueous systems is known to be stable and uniform. The presence of sulfate groups (negative charge) on the surface of CNCs extracted using sulfuric acid hydrolysis facilitates electrostatic stabilization of nanocrystals in aqueous me2


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dia. A direct mixing method can be used to disperse CNCs into water-soluble polymer aqueous solutions. The aqueous suspensions of CNC have been used directly as solvents for polymers such as poly (ethylene oxide)5,6 and poly (vinyl alcohol)7–10 for subsequent electrospinning. Another approach has been to co-axially electrospin aqueous dispersions of CNC particles with a polymer solution (e.g., cellulose/NMMO solution) to directly produce CNC-reinforced electrospun fibers.11 To extend the polymer system from water-soluble to hydrophobic polymers, freezedried CNCs have been re-dispersed into organic solvents (e.g., dimethylformamide, ethanol, tetrahydrofuran/dimethylformamide, 2-propanol/water, dimethylacetamide) to dissolve the target polymer (e.g. poly (lactic acid)12, poly (acrylic acid)13, poly (methyl methacrylate)14, ethylene vinyl alcohol15, cellulose acetate16), and prepare for electrospinning. However, there are obstacles for the use of CNC in systems that are not waterbased and non-polar, because of CNC’s high polarity and tendency for the formation of strong hydrogen bonding. Therefore, solvent exchange14, use of surfactant17, and surface modification18 are alternative methods to improve CNC dispersion in hydrophobic polymers. In addition to the dispersion problem, the alignment of CNCs within the matrix is another issue to be addressed. Orientation of CNCs has been done by various methods, such as shear force19,20, magnetic field21,22, electric field23 or a combination both24. Research has been carried out to exploit the functional characteristics and properties of CNC-filled nanocomposite to create new and specific applications. The application of the electrospun CNC-PLA system in a greenhouse trial confirmed the potential applications of CNCs in agricultural applications, where increasing cellulose nanocrystal content in 3


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fibers speeded up the fiber degradation rate and the Columbia Blue release rate.25 Initial studies have established the potential of CNC as a drug delivery excipient for use alone or in conjunction with other formulations to deliver drugs.26 CNC nanocomposites have also been investigated as tissue engineering scaffolds, and it was found that the dispersed CNC phase created a rigid percolating network within the host matrix.27 Nanofiltration composite membranes have been prepared, containing a thin layer of hydrophilic CNC coating and asymmetric electrospun PAN nanofibrous. The composite membranes showed good mechanical properties and efficient filtrating capacity for nanoparticles and oil/water separation.28 In the lithium ion battery field, CNCs have been used to reinforce poly (ethylene oxide) or poly (vinylidene fluoride-co-hexafluoropropene) making them more suitable candidates for polymer separators in lithium ion batteries.29–33 In the supercapacitor arena, the pyrolysis of CNCs yielded a carbonaceous material possessing a graphitic structure and has been used as electrode.34 In the packaging area, investigations on the barrier properties of CNC-improved materials have mainly focused on water vapor transmission and oxygen permeability.35 CNC also shows potential for optical sensors and optical filters.36,37 Here we explore in-depth water-in-oil (W/O) emulsion system consisting of two immiscible phases, which is designed to uniformly disperse aqueous CNC suspensions into hydrophobic PLA solutions. The paper will carefully assess requirements for creating optimal emulsions by studying the effect of emulsion droplet size on electrospinnability and fiber structure. EXPERIMENTAL SECTION

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Materials. PLA was purchased from NatureWorks® (Grade: 4042D, M n =66,000 g/mol). CNC aqueous suspensions (3.25 wt. %) were prepared by hydrolysis of bleached softwood kraft pulp using 64 wt % H2SO4, at 45 ˚C for 25 min, as described by Hamad and Hu1. Fluorescent isothiocyanate (FITC) labeled CNCs were prepared according to the method of Dong and Roman38. Span 80 was purchased from Sigma Aldrich. Chloroform and toluene were purchased from Fisher Scientific. Preparation and characterization of CNC-PLA emulsions. CNC-PLA water in oil (W/O) emulsions and the FITC-CNC-PLA W/O emulsions were prepared as follows. These emulsions consisted of aqueous phases and an oil phases. The aqueous phase is CNC aqueous suspensions, as the dispersed phase. The oil phase is the 8 wt. % solution of PLA in chloroform and toluene (7:3), as the continuous phase. Span 80 was added to stabilize the emulsion, as surfactants. The weight ratio (PLA: solvents: span 80: CNC suspensions) is 6.8:78.4:4.3:10.5. The reason for selecting this ratio was explained in the thesis written by the first author39. Figure 1 shows the schematic of CNC-PLA emulsion preparation process. The twophase system and the surfactant were shaken by hand, vortexed at 3000 rpm for 2 minutes to mix up, and subsequently sonicated in a sonicator with different sonication power and time. Effects of different sonication power and time on CNC-PLA water-in-oil emulsions droplet sizes were studied. According to the literature40 , the emulsion droplet size is gov0.6 -0.2 ρ2 , where E  dE / dt is the rate of input power erned by the input power, D~E-0.4 γ12

per unit volume, γ12 is the interfacial tension of the dispersed phase 1 against the continu5


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ous phase 2, and ρ 2 is the density of the continuous phase. If the input power is unchanged, emulsion droplets sizes barely changed at different sonication time. Conversely, the emulsion droplet sizes get smaller if input power becomes larger. Three sets of emulsion were prepared with different sonication power and certain time. Emulsion 1 was sonicated in a bath sonicator (Branson Bransonic® 3510 5.5L MT UltraSonic Bath Cleaner) for 1 hour with output power of 20 KW/m3. Emulsion 2 was sonicated in a cell sonicator (Misonix Sonicator® 3000 Ultrasonic Cell Disruptors) for 5 minutes at 225 kW/m3. Emulsion 3 was sonicated in a cell sonicator for 5 minutes at input power of 320 kW/m3. Three sets of emulsions were examined under an optical microscope (Nikon Eclipses LV 100) to determine the sizes of the emulsion droplets, which were measured using ImageJ software. Electrospinning. For the electrospinning of the random fiber mat, the experiment was conducted using a Drum Electrospinning Unit (Kato Tech Inc, Japan). Electrospinning of PLA solutions, CNC-PLA W/O emulsions or the FITC-CNC-PLA W/O emulsions were placed in a horizontally positioned plastic syringe (15 ml) with a flat-end metal needle. The syringe was operated by a syringe pump to feed solutions at a rate of 0.22 ml/h and the needle was subjected to a high voltage DC power of 23 kV relative to a vertically grounded drum rotating at a constant speed of 8 m/min (equivalent to 32 rpm). The drum was positioned 15 cm from the needle tip, which was covered by a sheet of aluminum foil to collect fibers. The fibrous nonwoven mats were dried under vacuum and peeled off for subsequent analysis. The spinning atmosphere was air at 30 – 40 % relative humidity. Electrospinning of aligned fibers was conducted using a disk electrospinning unit, Nanon (MECC CO., LTD). The syringe was operated by a syringe pump to feed solu6


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tions at a rate of 0.70 ml/h and the needle was subjected to a high voltage DC power of 25 kV relative to a vertically grounded disk rotating at a constant speed of 2000 rpm. The disk was positioned 15 cm from the needle orifice. The spinning atmosphere was air at 30 – 40 % relative humidity. Characterization of electrospun fibers. The morphology of electrospun fibers was observed using a Hitachi S-3000N scanning electron microscope (SEM), Hitachi S-4700 field emission scanning electron microscopy (FE-SEM) and Hitachi H-7600 transmission electron microscope (TEM). The distribution of FITC-CNC in electrospun fibers was examined with Olympus BX61W1 FluoView confocal laser-scanning microscope (CLSM). A Fourier-transform infrared spectrometer (FT-IR) (Perkin-Elmer 16 PC FT-IR spectrometer) was used to analyze the chemical structure of CNC/PLA fibers. The spectrum was measured with an average of 16 scans and a resolution of 4 cm-1 over the range of 400 to 4000 cm−1. DSC was performed using a TA Instruments Q1000 DSC. Approximately 10 mg of sample was loaded into sealed pans and allowed to proceed through a heat-cool-reheat cycle in nitrogen atmosphere at a rate of 10 °C/min from -70 °C to 200 °C. Mechanical properties of the fibers were evaluated using a KES-G1 micro-tensile tester (Kato Tech). Ultrafine fibers were tested in both the random fiber mat and aligned fiber yarn configurations. Five specimens were tested for each sample. A 30 mm gauge length was used for the random fiber mat and aligned fiber assemblies. For the random ultrafine fiber mat, the sample had dimensions of 5 mm x 40 mm. The aligned ultrafine fiber bundles were slightly twisted to gain a certain degree of integrity within the yarn. Approximately one twist per centimeter was applied. The weight of each sample was recorded to calculate areal density which was used in the tensile properties calculations. 7


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RESULTS AND DISCUSSIONS CNC characterization. The morphology and dimensions of CNCs were studied by TEM. Figure 2 shows TEM images of CNCs. The diameter is around 5~20 nm, and the length is about 100~500 nm. Fiber morphology. Emulsion 1 was electrospun into smooth random fiber mats and smooth aligned fibers. Figure 3 a shows the SEM image depicting the electrospun random CNC-PLA nanocomposite fibers with average diameter ca. 590 nm. Figure 3 b illustrates the SEM image of the electrospun aligned CNC-PLA nanocomposite fibers, which shows the good alignment of fibers. The average diameter is ca. 1000 nm. CNC detection. To track the presence and distribution of CNC within the fibers, fluorescent-labeled CNC (FITC-CNC) was used in this work. Figure 4 show images of the as-spun FITC-CNC-PLA fibers from emulsion 1. Results indicate that CNC particles are uniformly and continually incorporated into the fibers. To gain further insight into the location and alignment of CNC particles within the composite fibers, TEM and FE-SEM observations of as-spun CNC-PLA nanocomposite fibers from emulsion 1 were carried out. It was found that for thin fibers (diameter below 400~500 nm), the structure of fibers was core-shell and CNC particles were continuously aligned in the core (Figure 6 d). The core diameters were in the range of 5 nm ~ 250 nm. Moreover, for thick fibers (diameter larger than 500 nm), the fiber structure was observed to be a hollow cylinder and CNC particles were evident within the wall of these hollow cylinders (Figure 6 e and f). The inner diameter of the hollow cylinder varied from tens of nanometers to a few hundred nanometers.

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Fiber structure formation mechanism.

The mechanism for structural formation

(core-shell and hollow cylindrical structures) could be a combination of emulsion droplet coalescence in the core of fibers and phase separation during the electrospinning process as illustrated in Figure 5. Emulsion droplet coalescence during the electrospinning process may involve emulsion droplets moving inward, being stretched and merging into the fiber direction.41–45 The inward movement of emulsion droplets might originate from rapid elongation and quick evaporation of solvents.46 In the organic phase, the mixed solvents of chloroform and toluene evaporated faster than water and the viscosity of the polymer phase rapidly increased. The difference in evaporation speed between the dispersed phase and continuous phase induces droplets to move into the center and stabilize. Meanwhile, the emulsion droplets get stretched and deformed parallel to the electric field, which may result from the presence of a leaky dielectric.46 It is well established that dispersion of a conductive aqueous phase in an insulating fluid results in the formation of a leaky dielectric, promoting droplet deformation and elongation into an elliptical shape under an electric field. Afterward, the elongated emulsion droplets emerge into columns, which arise through a dielectrophoresis process under an electric field47,48. Dielectrophoresis is a phenomenon in which a force is exerted on a dielectric particle subjected to a non-uniform electric field.49 This force does not require the particle being charged. In our study, in response to an electric field, the aqueous phase will coalesce into column structures within the oil phase instead of bead-in-string structures before the onset of bending instability during electrospinning. It is therefore understandable that phase separation of aqueous and organic

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phases could subsequently occur. In this case, the aqueous phase is in the core and polymer phase in the shell. If the diameters of the aqueous column containing CNC in the core are small (Figure 5), less water exists in the core, and its evaporation is quite thorough, thereby resulting in fibers with small diameters. The movement of aqueous droplets and the tendency to orient parallel to the electric field helps CNC particles settle into the center of fibers. The thorough evaporation of solvent allows CNCs to be aligned in the core of fibers, thus promoting the formation of CNC core-PLA shell structures (Figure 5). If the diameters of the aqueous columns in the core are large (Figure 5), a larger amount of water is present in the core. However, the shell solidifies first followed by the core. Once the shell solidifies, the average diameter of the fibers does not change. Therefore, the fiber diameter remains relatively large. On the other hand, fast evaporation of the shell solvent and the existence of a slowly solidifying core lead to formation of a hollow cylindrical structure. If the aqueous suspension penetrates the shell, an immediate precipitation of the shell layers of the hollow cylinder embedded with CNC takes place. After water evaporation, CNC particles are able to orientate within the fiber on the wall of the hollow cylinder (Figure 5). We suppose that the different sizes of aqueous phase columns in this phase-separated system could be a consequence of the polydispersed size of emulsion droplets. Polydispersed emulsion 1 consisted of droplets in the range of 1~30 μm in diameter. The average diameter of emulsion droplets was 5.87 μm with a standard deviation of 4.05 μm. As discussed above, emulsion 1 produced core-shell fibers (Figure 6, d and e) and hollow fibers (Figure 6, f). Emulsion 2 produced emulsion droplets with diameters of 2.69±1.18 μm, 10


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and generated fibers with diameters ~ 480 nm. All small fibers have core-shell structure and the diameter of the fiber core is in the range 50 - 300 nm (Figure 6, g). Droplets sizes of emulsion 3 were even smaller than 1 μm, while some aggregates were found within fibers (Figure 6, h). These results are summarized in Table 1and Figure 6. After summarizing and analyzing the 20 TEM images for each condition, it has been verified at lower power (emulsion 1), 80% fibers were core-shell and 20% fibers are hollow; at medium power (emulsion 2), 100% fibers were core-shell; at high power (emulsion 3), all fibers (100%) had CNCs aggregates. It implies that the location and alignment of CNC can be tailored via the control of the droplet sizes. Physico-chemical properties. Figure 7 shows the FT-IR results of electrospun CNCPLA fibers from emulsion 1. The band at around 1050 cm–1 is the most intense in the cellulose spectrum.50 A broad peak in the 3650–3000 cm−1 range is characteristic of cellulose O–H stretching.50 Bands at 1759, 1185, 1130 and 1089 cm−1 represent the backbone ester group for PLA.51 DSC results. DSC was carried out for freeze-dried CNC flakes, the electrospun PLA fibers and CNC-PLA nanocomposite fibers from emulsion 1. The data are presented in Table 2 and Figure 8. Distinct cold crystallization peaks were observed in the nanocomposite fibers. The crystallization temperature, corresponding to the peak of the cold crystallization exotherm decreased with the presence of CNCs. This indicated that CNCs acted as a nucleating agent for PLA. The melting peak of the CNC-PLA nanocomposite fiber appears much sharper than that of PLA fiber, which also confirms the nucleating effect of CNCs. Xiang et. al also reported that the incorporation of CNCs tends to lower the

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cold crystallization temperatures, and demonstrated that CNCs induce PLA crystallization.12 The CNC nucleating phenomena also have been reported by Gray et.al 52. Tensile test. Figure 9 shows typical stress-strain curves of electrospun 8 % PLA random fiber mats (a), CNC-PLA random fiber mats from emulsion 1 (b) and yarns of PLACNC aligned fibers from emulsion 1 (c). The addition of CNC increases Young's modulus and maximum tensile strength but decreases the strain at maximum tensile strength (Table 3). At 5 wt. % CNC, the strength of random CNC-PLA nanocomposite fibers (b) increased by more than 12% and its modulus increased by 163% whereas strain decreased by 54%, compared to the random electrospun PLA fibers (a). The results have been compared between e-spun fibers with/without CNC to verify the CNC reinforcing effect. The strength of the aligned CNC-PLA aligned fibers (c) increased by 70%, its modulus by 147%, and its strain decreased by 82% compared to the random electrospun PLA fibers (b). Random and aligned CNC-PLA fibers have been compared and have been found out the additional alignment can help to improve the mechanical properties. Strain decreased as a consequence of the reinforcing effect. CONCLUSIONS Emulsion electrospinning provides an approach to uniformly disperse hydrophilic CNC particles into hydrophobic polymers to prepare ultrafine fibers. CNC-PLA water-in-oil emulsions were electrospun into random fiber mats and aligned fiber yarns. CNC acts as nucleating agent inducing PLA crystallization, where the addition of CNC provides effective reinforcing effects to the PLA polymer matrix, as confirmed by DSC and tensile testing results. Our experimental findings offer a clear understanding of the mechanism responsible for electrospun fiber formation, and a correlative relationship between emul12


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sion droplet size and fiber structure is proposed. When the emulsion droplet size has an average diameter of 6 μm, the fibre has a combination of core-shell and hollow structures. If the emulsion droplet size is lowered to an average diameter of 3 μm, all the fibres have core-shell structures. Reducing the emulsion droplet size further (< 1 μm) will cause the CNC particles to come out of the aqueous phase and aggregate. CORRESPONDING AUTHOR INFORMATION E-mails: [email protected] and [email protected] ACKNOWLEDGEMENTS The authors acknowledge FP Innovations for financial support under the university partnership program. REFERENCES (1)

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Figure 1. Schematic of emulsion preparation and electrospinning of CNC-PLA emulsions.

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Figure 2.TEM image of CNC.

Figure 3. (a) SEM image of electrospun (b) SEM image of CNC-PLA electrospun CNC-PLA nanocomposite fibers (collector CNC-PLA nanocomposite fibers (collector speed = 32 rpm) depicting random distribu- speed =2000 rpm) depicting aligned distrition.

bution.

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Figure 4. Confocal laser scanning microscopy (CLSM) image of FITC-CNC-PLA fibers. (Dark appearance indicates photo-bleaching of FITC molecules upon exposure to excitation light when using the CLSM.)

Figure 5. The proposed mechanism for fiber structural formation. 20


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Table 1. Effect of the emulsion droplet size on CNC fiber structure.

Emulsion 1 Sonicator input power 20 KW/m3

Emulsion 2

225 KW/m3 320 KW/m3

Average droplets size

About 6 μm

Fiber structure

Hollow and core-shell Core-shell

Emulsion 1 a

d

e

Emulsion 3

About 3 μm

Less than 1 μm CNC aggregates

Emulsion 2

Emulsion 3

b

c

g

h

f

Figure 6. Correlations between emulsion droplet size (a ~ 6 μm, b ~ 3 μm and c

Effects of Emulsion Droplet Size on the Structure of Electrospun

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