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Article Cite This: ACS Omega 2019, 4, 10767−10774
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Fabrication of Guar-Only Electrospun Nanofibers by Exploiting a High- and Low-Molecular Weight Blend Tahira Pirzada,† Barbara V. de Farias,† Hsiao Mei Annie Chu,†,‡ and Saad A. Khan* Department of Chemical & Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, United States
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S Supporting Information *
ABSTRACT: We present a facile approach to electrospin nanofibers of guar galactomannan by blending high- and low-molecular weights (MWs) of guar. We discover that while neither native high MW guar nor hydrolyzed low MW guar is electrospinnable on its own, their combination leads to synergism in producing defect-free nanofibers. Such an approach of fabricating nanofibers from blending multiple MWs of the same polymer may provide an easy route to produce nanofibers of biopolymers which are typically hard to electrospin. Rheological studies reveal that a limiting amount of native guar is needed for electrospinnability, and for those systems that have the proportionate amount of native guar, there is a critical total concentration above which fibers form. Interestingly, a plot of blend viscosity versus guar concentration reveals two power-law regimes with an inflection point, above which fiber formation can be achieved akin to the behavior observed for pure (i.e., nonblend) polymers.
1. INTRODUCTION Biopolymer-based electrospun nanofibers are extensively used in applications that include filtration, drug delivery systems, tissue engineering, and wound dressing.1−8 The natural abundance and biodegradability of these polymers make them ideal candidates for environment-friendly applications where they are replacing their synthetic counterparts. Guar galactomannan (commonly referred as guar) is a natural polysaccharide which is inexpensive, naturally abundant, and biodegradable, approved as a GRAS (generally regarded as safe) substance by Food and Drug Administration (FDA), and is used in many industrial applications because of its unique inherent properties. It is composed of a linear backbone of β1,4-linked mannose (M) units with α-1,6-linked galactose (G) side units randomly distributed along the backbone (Figure 1a). The ratio of mannose to galactose units ranges from 1.6:1 to 1.8:1,9 and the molecular weight (MW) of native guar (abbreviated as NG henceforth) is approximately 2 × 106 Da.
Being a natural polymer, guar can be hydrolyzed by enzymes such as β-mannanase and α-galactosidase, which cleave the polymer backbone and side units, respectively, and also alter its rheological properties and sites of intermolecular interaction (Figure 1b).10−16 Enzymes that hydrolyze guar are secreted specifically by the native microflora present in human colon, thereby rendering guar the exclusive potential of delivering oral drugs targeting the colon and gastrointestinal tract.17,18 This ideology has been explored by researchers seeking to use guar to orally deliver drugs to the colon in the form of hydrogel discs,6,19 microspheres,18 and compacted or coated matrix tablets.20 Its rheological properties are also utilized in paper, cosmetics, and textile industry to provide stability and better adhesion to various products.21 Guar seed endosperm is a source of water soluble gum which is used as a stabilizer, thickener, and emulsifier in various food products such as ice creams, cake mixes, sauces, fruit drinks, and dressings,22 where the rheological properties are important as they give information about stability and flow properties under processing and usage conditions. Moreover, the high affinity of guar gum for water is utilized in the agriculture industry, where it is used to improve water retention and antibacterial properties of the soil.6 Electrospinning offers a simple approach for fiber fabrication from polymer solutions of appropriate viscosity under the influence of an electric field.23 Its ability to produce nonwoven fibrous media in the nanometer range with interconnected pore volume and a high surface area to volume ratio is explored in diverse fields such as filtration devices, smart textiles for
Figure 1. (a) Structure of guar galactomannan (left). (b) Reduction in viscosity of guar galactomannan solution (1% w/w) as a function of time upon incubation with the enzyme β-mannanase (1 × 10−6 units/ mL) (right). © 2019 American Chemical Society
Received: April 1, 2019 Accepted: June 7, 2019 Published: June 20, 2019 10767
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Figure 2. SEM images of electrospun NG and its blend with low-MW hydrolyzed guar, referred to as SF. (a) 1 wt % NG, (b) 47 wt % SF, (c) 0.6 wt % NG/18.8 wt % SF (3:2), (d) 0.5 wt % NG/23.5 wt % SF (1:1), and (e) 0.25 wt % NG/35.25 wt % SF (1:3).
severalfold, but it is quite a challenging task which remains largely unexplained. Woerdeman et al. have demonstrated the effects of low- and high-MW glutenin components in wheat protein on its electrospinnability without the use of a polymer template.38 Recently, Ewaldz and Brettmann have published an interesting review on the development of molecular interactions during electrospinning of polymer blends;39 however, their focus was mainly on polymer−polymer and polymer−small molecules interactions and a brief description of polymer-free electrospinning of cyclodextrins and tannic acid, leaving many questions related to biomolecule electrospinning unanswered. Although there are research studies published on guar-synthetic polymer [mostly PVA or poly(ethylene oxide) (PEO)] composite nanofibers, to the best of our knowledge, there is just one reported attempt to electrospin guar without blending with synthetic polymers.40 Their approach was to electrospin commercial guar gum through a series of purification steps involving filtration membranes to ensure complete removal of insoluble residues which is one of the major reasons behind aggregation during electrospinning. However, we report a comparatively straight forward approach to electrospin a blend of low-MW (∼1.6 × 104 Da) SF with high-MW (∼2 × 106 Da) NG to develop sufficient entanglements in the electrospinning mixture to produce nonbeaded and uniform fibers consisting of guar only. Scanning electron microscopy (SEM) is used to examine the role of blending in nanofiber fabrication, while rheological properties of the blend are studied to understand the relationship between its rheology and electrospinnability.
protective clothing, wound healing, tissue scaffolds, reinforced composites, and biosensors.1,5,7,23−26 However, electrospinning of pure biopolymers is still considered a challenge because biopolymers have markedly high MW (often in the 106 Da range), are naturally highly polydisperse, and in most cases, have very complex molecular structures. In addition, many biopolymers are saturated with hydroxyl (−OH) and amine (−NH2) groups that tend to form strong hydrogen bonds leading to high solution viscosity or gel formation, both of which are detrimental to electrospinning. Hans Tromp et al.8 have tried to electrospin locust bean, tara, guar, konjac, and xanthan gums along with other polysaccharides and have observed the formation of droplets or unstable drops that were unable to reach the collector plate. They attributed the unsuccessful fiber formation of these gums to either a low zeroshear viscosity or, in the case of high zero-shear viscosity, a strong shear thinning behavior. Notably, biopolymers tend to be very rigid molecules in their native states, and their stiffness proves to be problematic when bending and stretching of the solution jet occur during electrospinning.27−30 To overcome some of these limitations, biopolymers are often blended with a synthetic polymer which has more flexible chains and can induce a degree of elasticity in the blended solution so that fibers can stretch without breaking up into droplets.29−33 Some groups have also reported electrospinning of blends of gums such as tragacanth and almond gum with various polymers for the ease of electrospinnability.34−37 Nevertheless, the presence of a synthetic polymer in the fiber mat limits its efficacy in the applications that require the presence of pure biopolymers. We have applied the aforementioned hypothesis to initially electrospin NG gum blended with poly(vinyl alcohol) (PVA), as our experiments indicate that NG solutions are too viscous even at 1% (w/w) and could not form a stable jet. Blending such highly viscous polymers with shorter chain polymers, whether synthetic or natural, can increase their degree of flexibility which might lead to formation of bead-free nanofibers. We observed that PVA (MW ≈ 200 000) blended well with NG to produce homogeneous solutions which electrospun uniform, bead-free fibers. Following the NG/PVA example of blended solutions that overcome electrospinnability limitations, we have blended NG aqueous solution with a solution of lower MW partially hydrolyzed guar called Sunfiber (SF) to produce nanofibers that were comprised solely of guar. Electrospinning a biopolymer without mixing with a synthetic polymer can increase its usefulness and functionality
2. RESULTS AND DISCUSSION 2.1. Electrospinning of Guar Blends. Solutions of NG ranging in concentration from 0.5 to 2 wt % did not produce a stable jet or bead-free nanofibers over a range of electrospinning parameters (voltage, flow rate, and tip-to-collector distance) described in Section 4.4. Figure S1a in Supporting Information displays a representative SEM image of a 1 wt % NG sample that has been electrospun. At low flow rates, the solution formed a drop at the syringe tip which gradually increased in size and dripped. At high flow rates, large drops of the solution sprayed onto the collector. Similarly, at low voltages, the solution dripped, and at high voltages, the solution rapidly splayed into smaller jets and sprayed onto the collector. Although blended with 8 wt % PVA solution in different compositions (Table S1), the resultant solutions 10768
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Figure 3. Shear viscosity of (a) 1 wt % NG, 47 wt % SF and NG/SF blends 1, 2, and 3; (b) corresponding blends B1, B2, and B3 (compositions as demonstrated in Table 1), illustrating the dominant effect of % NG on viscosity rather than total polymer concentration.
Table 1. Comparison of NG/SF Blend Characteristics blend
NG (wt %)
SF (wt %)
total polymer concentration
1 2 3
0.6 0.5 0.25
18.8 23.5 35.25
19.4 24.0 35.5
B1 B2 B3
0.6 0.4 0.14
18.8 19 19.26
SF/NG in fiber
surface tension (dynes/cm)
31 69.7 ± 0.4 47 68.7 ± 1.2 141 68.1 ± 0.6 Keeping Total Polymer Concentration Constant at 19.4% 19.4 31 69.7 ± 0.4 19.4 47 68.1 ± 1.2 19.4 138 67.8 ± 0.7
conductivity (μS/cm)
average fiber diameter (nm)
1496 ± 5 1565 ± 13 1707 ± 5
479 ± 58 336 ± 82 753 ± 68
1496 ± 5 1582 ± 11 1668 ± 4
479 ± 58 350 ± 77 N/A
viscosity than 1 wt % NG guar solution (Figure 3a). However, blending of NG with SF produces a synergistic effect leading to the formation of a stable jet during electrospinning that produces bead-free nanofibers. Before choosing the ratios for blends 1 (3:2), 2 (1:1), and 3 (1:3), we systematically blended solutions of 1 wt % NG and 47 wt % SF in various ratios, 9:1, 1:9, 8:2, 2:8, 7:3, 3:7, and so forth, all the way to a 1:1 ratio. The blends discussed here were the only ones that produced nanofibers by electrospinning, and their corresponding SEM micrographs are displayed in Figure 2. For the remaining blends that were attempted, the results varied between no fiber formation (in the extreme ratios such as 1:9, 9:1) to highly beaded fiber formation (for various other ratios). Table 1 compares the characteristics of the three NG/SF blends that exhibited nanofiber formation (i.e., Figure 2c−e). Blend 1 containing 0.6% NG/18.8% SF (Figure 2c) produced fibers with 479 ± 58 nm diameter, and blend 2 containing 0.5% NG/23.5% SF (Figure 2d) shows similar fiber diameter within error bars. However, further decrease in the NG content (0.25% NG/35.25% SF) for blend 3 (Figure 2e) produces substantially larger diameter fibers (753 ± 68 nm). Several factors need to be considered in analyzing this data including blend composition, total polymer concentration, surface tension, conductivity, and viscosity. Table 1 indicates that an increase in the SF content in the blends leads to an increase in conductivity and slight decrease in the surface tension. As it is well established that enhancement in conductivity and decrease in surface tension of a solution facilitates electrospinnability and formation of bead-free thinner fibers,8,23,27 we can rule out these properties as playing a role in our system. For example, blend 3 has the highest conductivity and lowest surface tension amongst its counterparts (blends 1 and 2) and yet produces the largest fiber diameter. And blend B3 with the largest conductivity and lowest surface tension within its group does not even produce fibers. Table 1 also reveals that the total polymer concentration increases as we go from blend 1 to blend 3. For a pure polymer, an increase in total polymer
formed a stable jet and produced thick mats of nanofibers as shown in Figure S1c−g in the Supporting Information. It is worth mentioning that when mixed together, NG/PVA blends are capable of producing bead-free nanofibers with as low a concentration as 4.8 wt % of PVA (Figure S1f in the Supporting Information), while it has been already established that even if other electrospinning conditions are considered, PVA-only solution is not capable of producing bead-free fibers in the concentration range of 4−5.6 wt %.41 Electrospinnability of a lower concentration of PVA when mixed with guar is indicative of strong interactions between both the polymers which are responsible for a rise in entanglements between the polymer chains, leading to its electrospinnability at lower than reported concentrations. To produce nanofibers whose sole chemical composition was guar, a 47 wt % solution of SF was used to blend with 1 wt % NG solution in deionized water. Please note that on electrospinning, neither of the individual solutions produced bead-free nanofibers even at the saturation concentration (Figure 2a,b). However, when 1 wt % NG was blended with 47 wt % SF at 3:2 (3 part 1 wt % NG and 2 part 47 wt % SF), 1:1 and 1:3 ratios by weight, the resulting blended solutions produced bead-free nanofibers on electrospinning (Figure 2c− e). Although electrospinning of blends of different forms of the same polymer such as poly(methyl methacrylate) (PMMA; isotactic PMMA and syndiotactic PMMA)42 has been reported before, to our knowledge, this phenomenon of promoting electrospinnability in a high-MW biopolymer by blending it with a lower MW version of the same biopolymer has not been previously reported in the literature. NG solutions are viscous even at low concentrations (Figure 3a) because of their high hydrodynamic volume43 and hydrogen bonding, and are therefore hard to electrospin, whereas only beaded nanofibers are obtained from SF due to the fact that the low MW, enzyme-hydrolyzed polymer chains are too short and rigid to form sufficient entanglements necessary for electrospinning. Even at a concentration of ∼47 wt %, SF exhibits a much lower 10769
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find the viscosity of blend B2 to be lower than that of B1. Such a decrease in viscosity, which we can attribute to a lower NG/ SF ratio, could possibly explain the smaller fiber diameter observed in B2 (Table 1, Figure 4). In general, the steady shear viscosity of the samples decreases progressively as we evaluate blends B1, B2, and B3. Even though the total polymer concentration in all three blends is the same (Table 1), the decreasing NG/SF ratio in B1, B2, and B3, respectively, can be attributed to the observed drop in steady-state viscosity. As SF is essentially a low viscosity Newtonian (Figure 3a), NG is the only component that contributes to viscoelasticity. With electrospinnability depending on the viscoelasticity of the solution, the NG content of blend B3 is too low to cause any nanofiber formation. This point is illustrated by comparing blend 3 which produces bead-free fibers with a large diameter (Figure 2e) with blend B3 which exhibits no fiber formation. Both blend 3 and B3 have the same SF/NG ratio, with only ∼0.7% of the polymer composition being from NG. However, the total NG content in the solution phase of B3 is substantially higher (0.25% in B3 vs 0.14% in blend 3), leading us to hypothesize that perhaps there is a lower limit of NG (and concomitantly total polymer) needed for creating nanofibers. Consistent with this notion, both blend 2 (Figure 2d, Table 1) and B2 (Figure 4b, Table 1) produce nanofibers as the NG content in the blends is significantly high to do so. However, both blends have statistically similar fiber diameters (as confirmed by a ttest at a = 0.5) even though the polymer concentrations are different. The viscosity argument does not hold true in this case as well (as it did not for blends 2 and 3), as blend 2 with a higher viscosity would then lead to a higher fiber diameter. We believe the similarity of the fiber diameter could be fortuitous or because both these blends have similar blend MWs (around 57 kDa), a topic discussed in the last part of the study. Nevertheless, this result underscores the complexity of the system and needs more in-depth study by varying MWs of the blends. We intend to pursue this as a follow-up to this initial work by creating hydrolyzed guar of different MWs using enzymes and examining blends of these with NG. 2.2. Specific Viscosity, Blend Composition, and Electrospinnability. To further examine the role of composition/concentration on electrospinnability, two of the NG/SF blends that produced nanofibers, containing 0.5% NG/23.5% SF (blend 1) and 0.6% NG/18.8% SF (blend 2) (Figure 1c,d) were serially diluted starting with the original blend treated as 100% down to 70, 60, 50, 40, 30, 20, and 10%. The zero-shear viscosity of each dilution was measured and plotted as a function of NG concentration in wt % (Figure 5) to examine any correlation between concentration and solution electrospinnability. We find the zero-shear viscosity to exhibit a power-law behavior with two distinct slopes. The viscosity increases more slowly at lower concentration with a much steeper slope following the inflection point. Several features are apparent from the figure. The values for the initial slope are ∼1.9 and ∼1.6 for the blends containing 0.6% NG/18.8% SF and 0.5% NG/23.5% SF, respectively, and the values for the second slope (after the inflection point) are ∼5.1 and ∼4.4, with the inflection point at an NG concentration of ∼0.17 wt %, which corresponds to a total polymer concentration of ∼3.5−4.0%. The concentration dependence is higher than the predicted relationship for semidilute unentangled solutions (∼1.25) and for the semidilute entangled regime than the predicted relationship (∼3.75).44 These higher values of the
concentration results in an increase in viscosity and a concomitant increase in fiber diameter. Such a simple notion of increased total concentration leading to higher viscosity does not hold true with our system (Figure 3a) which is not only a blend but comprised of two disparate MWs. If one examines the viscosity of the three blends in Figure 3a, we find the viscosities of blends 1 and 2 to be similar, suggesting that this could be the reason for these blends to have similar fiber diameters. However, this argument does not hold for blend 3 which exhibits a much lower viscosity but a larger fiber diameter. These results suggest important roles played by blend composition particularly the NG content, as well as total polymer concentration. In order to decouple the effects of the NG content from total polymer concentration on the fiber diameter, three different blends (B1, B2, and B3) were prepared, all with similar SF/NG ratios as of the previous sample sets (column 5, Table 1) but maintaining same total polymer concentration in all the sets, that is, 19.4 wt % (column 4, Table 1). We find blends B1 and B2 to show nanofiber formation; however, the fiber diameter decreases (Table 1, Figure 4a,b) from 479 ± 58
Figure 4. Effect of blend composition on electrospinnability. (a) B10.6 wt % NG/18.8 wt % SF, (b) B20.4 wt % NG/19 wt % SF, and (c) B30.14 wt % NG/19.26 wt % SF.
to 350 ± 77 nm as we go from B1 to B2. In stark contrast, blend B3 (Figure 4c) exhibits complete lack of electrospinnability, as is evident from the absence of fibers altogether. It is worthwhile to mention here that even though blend B3 contains a higher content of SF (19.26%) as compared to the NG content (0.14%), the SF content is still not high enough to produce highly beaded nanofibers as produced by 47 wt % SF solution in Figure 2b. A somewhat consistent picture of the observed phenomenon regarding samples B1−B3 can be garnered by examining the viscosity profiles of these samples, as shown in Figure 3b. We 10770
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Figure 5. Change in zero-shear viscosity with NG concentration. The polymer entanglement concentration is similar in both cases.
exponents are indicative of a rise in polymer association and interaction.45 When compared to a similar plot made for the chitosan−PEO system,46 the slopes are closer1.3 and 6.0 with the critical concentration reported at 2.9 wt %. It should be noted that even though the slopes of the graph are different for both samples, the zero-shear viscosities are effectively the same at the highest concentration as we started with undiluted samples having similar viscosities (Figure 3a). The inflection point in viscosity versus concentration plots has been deemed critical for pure polymers because it separates the region of electrospinnability from the region of nonelectrospinnability.41,45,47,48 To examine if this premise holds true for our blends, the dilutions immediately before and after the inflection point were electrospun and compared to the original undiluted blend. SEM images of the dilutions of 0.6% NG/18.8% SF and 0.5% NG/23.5% blends are shown in Figures 6 and 7, respectively. Figure 6 exhibits transition from “bead only” formation to “beads + fibers” at the inflection
Figure 7. SEM images corresponding to various blend concentrations of 0.5% NG/23.5% SF. From (a−e): 20, 30, 40, 50 and 100% of the original blend composition shown in Figure 4.
Figure 6. SEM images corresponding to various blend concentrations of 0.6% NG/18.8% SF. From (a−g): 20, 30, 40, 50, 60, 70, and 100% of the original blend composition corresponding to Figure 4. 10771
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point and finally to bead-free fibers for the original undiluted blend. Similarly, SEM micrographs in Figure 7 display the transition from “bead-only” formation in 20 and 30% dilutions to the formation of “beads + fibers” at 40 and 50% dilutions, which correlates with the presence of the critical concentration between 30 and 40% as indicated in Figure 5. The concentration which can electrospin bead-free fibers has been reported in the literature for pure polymers to be 2.5− 3 times the critical concentration.41,45,47,48 Interestingly, the same relation also seems to hold for our blends, as we find bead-free fibers formed from a solution at approximately three times the critical concentration. Our results taken together reveal composition and concentration of blends to be an important determinant in electrospinnability. We attempt in Figure 8 and Table S2 to
Further analysis of the blends will be part of our subsequent work to understand the effect of solution composition on its electrospinnability.
3. CONCLUSIONS In this study, we established an approach to electrospin nanofibers of guar galactomannan by blending high MW NG with its hydrolyzed low MW analogue. The ability to obtain nanofibers of blends of the same material without resorting to the use of a second electrospinnable polymer is particularly beneficial in food and pharmaceutical applications, where the presence of a second polymer may be an impediment. The approach described here to fabricate nanofibers from the blends of otherwise nonelectrospinnable biopolymers can be easily applied to other biopolymers that are hard to electrospin, such as pectin and locust bean gum. We found that neither the native high MW guar nor the hydrolyzed low MW guar was electrospinnable at the conditions we used; however, their combination provided a synergistic effect leading to formation of nanofibers, while the composition and concentration of the blend dictated its electrospinnability. Furthermore, the change in viscosity when measured against increasing blend concentration for a specific blend composition underwent a two-stage increase with two different slopes, with the critical concentration at the intersection point separating the region of electrospinnability from the region of nonelectrospinnability. Such a critical concentration has been established before for pure polymers but not for blends, suggesting the universality of the approach in determining regimes for nanofiber formation. 4. MATERIALS AND METHODS 4.1. Materials. NG gum used in this study was purchased from Sigma-Aldrich, USA. The polymer was supplied in the form of a dry, yellow powder and was purified and lyophilized prior to use. The MW of NG was determined to be ∼2 × 106 Da by gel permeation chromatography (GPC). PVA (MW ≈ 200 000) was purchased from Sigma-Aldrich and was used as received. Commercially available enzymatically degraded guar, called Sunfiber R (abbreviated as SF henceforth), with a MW of ∼1.6 × 104 Da (measured using GPC) was obtained from Taiyo International, Inc. and was used as received. Sodium azide was added to all guar solutions to serve as a bactericide and was obtained from Sigma-Aldrich, USA. 4.2. Sample Preparation. NG gum was purified in bulk to remove all the particles that are insoluble in water. According to previously published protocols on guar purification,10,12 solutions of guar were prepared by sprinkling guar powder slowly into a vortex of deionized water created with a Fisher Dynamix mixer to the concentration of 10 g/L, mixed vigorously for 2 h followed by magnetic stirring overnight. The resulting solution was centrifuged at 10 000g for 30 min, and the supernatant was collected and mixed with two volumes of ethanol. Guar precipitates in the ethanol/water mixture, and the precipitate was collected and lyophilized at 100 mTorr for 48 h. Afterward, dried guar was crushed to a fine powder using a mortar and pestle and was stored in a desiccator until used. A 1 wt % solution of the purified guar was prepared in deionized water to which 0.2 mg/mL sodium azide was added as the bactericide. PVA solutions were made by adding the desired amount of PVA pellets to deionized water and left to stir overnight in a 60 °C oil bath. Solution of the lower MW guar, SF, was prepared in the same manner as mentioned above but
Figure 8. Effect of the NG content on blend MW.
examine electrospinnability in terms of blend MW, in which blend MW has been calculated by weighing each component MW with its weight fraction. Figure 8 reveals that at low average blend MW, the only structures formed are beads; as the average blend MW increases, fiber formation becomes more and more pronounced, going from thicker to thinner in diameter with increase in the MW (Figure 8, Table S2). Finally, we observe that for higher MW samples in the order of 102 kDa, the predominant nanostructure obtained is a mixture of beads and nanofibers. Note that Table S2 in the Supporting Information presents more details on the composition of the different blends and their MWs. Figure 8 seems to suggest that there may be a window for obtaining nanofibers in terms of MW and NG composition, that is, the NG content between 0.4 and 0.7 wt % and blend MW between 50 and 100 kDa with the total polymer content in the intermediate range. This NG content-blend MW window may explain why blend B2 (Table 1) has similar fiber diameter to that of the sample with 57.3 kDa MW (blend 2, Table 1; also Table S2). Blend B2 has an NG composition of 0.4 wt % and a blend MW of ∼56 kDa, putting it in the same approximate location in Figure 8 with blend 2 that has a MW of 57.3 kDa. An interesting feature to note from Figure 8 is that we are creating nanofibers of nonelectrospinnable SF by adding small amounts of NG to it. This is reminiscent of the idea of using associative polymers such as guar and hydrophobically modified comb-like polymers to facilitate electrospinning of non- or poorly electrospinnable systems.49 10772
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without any purification. Prior to mixing with NG, SF was dissolved in deionized water at increasing concentrations until the solution was saturated at room temperature at ∼47 wt %. All guar solutions were prepared overnight to allow the guar powder to swell and dissolve slowly and uniformly and were then stored at 4 °C until use to prevent degradation. All rheology and electrospinning experiments were carried out within three weeks of solution preparation to avoid polymer degradation. 4.3. Solution Characterization. Rheological experiments were conducted with a Discovery Hybrid Rheometer (DHR3) (TA Instruments, New Castle, DE) using a 4 cm, 1° cone-andplate geometry. All experiments, unless specified, were performed at room temperature. Because the steady shear response of guar and guar blend solutions is sensitive to shear history, a pre-shear was applied at a strain rate of 5 s−1 for 180 s followed by a rest period of 120 s, as already reported in the literature.49 Zero-shear viscosity measurements were conducted over a range of shear stresses (between 0.1 and 100 Pa), with every measurement taken in triplicates to ensure reproducibility within ±5%. Surface tension of the solutions was measured using the pendent drop method with the help of a First Ten Angstroms Goniometer. An average volume of 5 μL was used to determine surface tension of the respective solutions at 25 °C. Conductivity of the solutions was measured with the help of a Thermo Scientific Orion Star Conductivity Meter at an average temperature of 25 °C. All the measurements were conducted in triplicates. 4.4. Electrospinning. The electrospinning setup consisted of an aluminum collector plate, a precision syringe pump (Harvard Apparatus, Holliston, MA), and a high-voltage power supply (Gamma High Voltage Research, model D-ES30 PN/ M692 with a positive polarity). The syringe pump controlled the flow of polymer solution loaded in a 10 mL syringe to a stainless-steel capillary metal hub needle (22 gauge) which was connected to the positive electrode of the power supply. The collector plate was grounded, and the fibers were collected on aluminum foil wrapped on the collector plate. The electrospinning parameters were adjusted to obtain a stable Taylor cone during the electrospinning process. The flow rate was changed from 0.1 to 1 mL/h, voltage was varied from 5 to 15 kV, and the tip-to-collector distance was varied from 5 to 20 cm. All experiments were conducted at 45 ± 3% relative humidity. While the role of humidity in electrospinning can be interesting,50 we defer this topic for a future study. 4.5. Nanofiber Characterization. Fiber morphology was analyzed by SEM with a FEI XL30 microscope with a field emission gun operated under high vacuum at 5 kV. Because our polymers are nonconductive, the nanofiber mats were sputter-coated with a thin film of gold with a Denton Vacuum Desk IV sputter coater prior to imaging to prevent charging. The average fiber diameter was determined by measuring diameters of 100 individual fibers from multiple SEM images using ImageJ software.
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SEM images of electrospun NG and NG/PVA blends; comparison of NG/PVA blends; and NG/SF blend composition, MW, and electrospinnability (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 919-515-4519. Fax: 919-5153465. ORCID
Tahira Pirzada: 0000-0003-0896-5002 Saad A. Khan: 0000-0002-1530-8249 Present Address ‡
Pfizer Inc., 1211 Sherwood Avenue, Richmond, VA, 232201212, USA. Author Contributions †
Co-first authors.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.M.A.C. was supported by the Nonwovens Institute at North Carolina State University. Microscopy characterization was performed at Duke University’s Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI).
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REFERENCES
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00902. 10773
DOI: 10.1021/acsomega.9b00902 ACS Omega 2019, 4, 10767−10774
ACS Omega
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DOI: 10.1021/acsomega.9b00902 ACS Omega 2019, 4, 10767−10774
