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Formation of Bead-Free and Core-Shell Superfine Electrospinning Fibers Under the Assistance of Another Polymer and an Interfacial Compatibilizer.

INTRODUCTION

Electrospinning has become a versatile technique for producing continuous fibers with micrometer and nanometer diameters from a variety of polymer solutions or melts [1-3]. This technology has attracted growing interest due to the potential applications in filtration [4-6], sensor [7, 8], drug delivery [9, 10], and tissue engineer [11, 12]. The successful application of electrospinning fibers mainly depends on their morphologies. The desired morphology is often bead-free superfine fiber. For example, bead-free superfine fibers showed more advantages as water-poof breathable fabrics because their water vapor transport rate and resistance to water penetration were much higher than those of bead-on-string fibers [13]. Moreover, their tensile strength and elongation at break were also better than those of bead-on-string fibers.

However, bead-on-string fibers are often main products, and even in some case, fibers are not produced at all during electrospinning. This may result from two main factors: solution characteristics and processing parameters. The solution characteristics include viscosity [14-17], conductivity [18, 19], and surface tension [14]. For instance, using N,N-dimethylformamide (DMF) or tetrahydrofuran (THF) as an electrospinning solvent, polystyrene (PS) could not form bead-free fibers when solution concentration is below 20%. Increasing the solution concentration to 30%, uniform bead-free fibers with an average diameter of 1.5-2.5 [micro]m were obtained with DMF as a solvent, whereas ribbon-like fibers were produced with THF one due to its higher evaporation rate and lower surface tension [14]. Other researchers tried to control processing parameters, including applied voltage [20-22], distance between the spinneret and the collector [20, 22], temperature [23], and humidity [24], to produce bead-free superfine fibers. When the concentration of polyaniline (PAN) was maintained at 4.5%, the morphology transformation from bead-on-string to bead-free structure was achieved by enhancing the electrospinning temperature from 50[degrees]C to 60[degrees]C [23].

With the development of electrospinning technology, electrospinning of polymer blend has attracted extensive attention in recent years. Some polymers recognized for their poor spinability in the past can be successfully electrospun into ultrafine fibers by using another polymer as an additive. Cellulose acetate (CA) alone could not produce bead-free fibers by electrospinning approach [25]. However, an introduction of 20% polyurethane (PU) changed the morphology of electrospinning fibers into bead-free structure with an average diameter of 0.36 [micro]m. Moreover, electrospinning of polymer blend could combine the advantages of polymer components to improve properties of the final fiber mats. For instance, after adding 30% polycarbonate (PC), Young's modulus of as-electrospun blend fibers increased to 55 MPa which is almost four times higher than that of pure PU fibers [26].

Because most polymer pairs are mutually immiscible, they tend to separate into two phase regimes and then coalesce with a rapid solvent evaporation in the process of electrospinning. The interfacial interaction should be an important parameter for electrospinning of polymer blend. As is well known, compatibilizers such as block or graft copolymers are often used to reduce the interfacial tension and enhance the interfacial adhesion in the melt processing of polymer blend. However, the role of compatibilizer in electrospinning of polymer blend is still unclear. In this work, using polyamide 6 (PA6) as minor component polymer and a copolymer (PS-co-TMI) of styrene (St) and 3-isopropenyl-[alpha], [alpha]-dimethylbenzene isocyanate (TMI) as a reactive compatibilizer, their influences on the electrospinning behavior and fibrous structure of PS solution were systematically investigated.

EXPERIMENTAL

Materials

PS ([M.sub.w] = 228,800 g/mol) and PA6 ([M.sub.w] = 49,400 g/mol) with the content of terminal amine of 60.3 [micro]mol/g were purchased from Yangzi-BASF Styrenics Company and UBE Nylon Ltd., respectively. Chloroform (CH[Cl.sub.3]), dichloromethane (C[H.sub.2][Cl.sub.2]), THF, methanol, formic acid, and benzoyl peroxide (BPO) were purchased from Sinopharm Chemical Reagent Co., Ltd. Trifluoroacetic anhydride (TFAA), TMI, and 9-(methylamino-methyl)anthracene (MAMA) were purchased from Aladdin Co., Ltd.

Synthesis of PS-co-TMI Containing Anthracene Moieties

PS-co-TMI was synthesized by free radical copolymerization of St and TMI in toluene using BPO as a free radical initiator. Details on the polymerization principle and preparation procedures can be found elsewhere [27]. In this work, the TMI content and [M.sub.n] of PS-co-TMI were 7.7 wt% and 37,600 g/mol, respectively.

To determine very small amounts of in situ forming compatibilizers, MAMA-containing anthracene moieties was introduced into PS-co-TMI chain to get fluorescent-labeled compatibilizer [28-30]. The preparation procedure was as follows: PS-co-TMI (5 g) was first dried at 80[degrees]C for 8 h in a vacuum oven and then reacted with MAMA (0.1 g) in THF (25 mL) at 60[degrees]C for 8 h. The fluorescent-labeled PS-co-TMI was obtained by precipitation in cold methanol and drying at 80[degrees]C for 12 h in a vacuum oven. The MAMA content in the fluorescent-labeled PS-co-TMI was 1.9 wt%, and the TMI content dropped to 5.8 wt%.

N-Trifluoroacetylation of PA6

PA6 and PS cannot directly form a homogenous solution because PA6 is insoluble in a classic organic solvent such as THF and CH[Cl.sub.3] at room temperature. Nevertheless, our previous study showed that PA6 became soluble in such solvents after they were reacted with TFAA leading to N-trifluoroacetylated derivatives (PA6-TFAA) [27, 29]. The procedure of N-trifluoroacetylation on PA6 was as follows: 1 g of PA6 was added into a two-neck flask followed by purging with nitrogen. Five milliliters of CH2C12 and 2 mL TFAA were successively introduced into the flask. After 12 h of reaction at room temperature, PA6-TFAA was obtained by rotary evaporation at 30[degrees]C to remove C[H.sub.2][Cl.sub.2] and excess TFAA.

Electrospinning of Polymer Solution

Before electrospinning, three kinds of solutions using CHC13 as a solvent, PS solution, PS/PA6 solution, and PS/PA6/PS-co-TMI solution were prepared. For the latter two solutions, PS concentration was 0.10 g/mL. For a typical electrospinning process, the polymer solution was first placed in a 5 mL syringe (WZ-50c6, Smiths Medical Instrument Co., Ltd., Zhejiang) attached to a stainless needle with an inner diameter of 0.6 mm. This needle tip was then connected to the positive electrode of a variable high-voltage power supply of type DW-P303-1ACD8 (Tianjin Dongwen High-voltages Source Co.). The ground electrode was connected to the metallic collector wrapped with aluminum foil. The distance between needle tip and collector was maintained at 15 cm, and then, the polymer solution with a flow rate of 3 mL/h was electrospun at a voltage of 15 kV. Finally, the obtained fibers on the metallic collector were dried under vacuum at 90[degrees] C for 12 h to remove any residual solvents along with the deacetylation of PA6-TFAA.

Characterization

A rotational rheometer (RS6000, Thermo Scientific HAAKE MARS) was used to characterize rheological behaviors of polymer solutions. The test was performed using a dynamic mode in a frequency range from 100 to 0.1 Hz at 25[degrees]C.

The reaction extent of PS-co-TMI and PA6 after electrospinning was measured by Gel Permeation Chromatography (GPC) (Waters 1525/2414, Waters, USA) equipped with a refractometer and a UV detector [31]. Before measurement, the reacted PS-co-TMI should be separated from the electrospun fibers. For that purpose, those fibers were immersed in THF for 48 h at room temperature to remove PS and unreacted PS-co-TMI. To completely remove PS and unreacted PS-co-TMI, the above process was repeated three times to get the insoluble materials, a mixture of PA6, and the formed copolymer of PS-co-TMI and PA6. The above insoluble materials were dissolved in THF to form a solution of 6 mg/mL for GPC measurement with a UV detector at 367 nm. It is easy to obtain the amount of reacted PS-co-TMI because the MAMA has very strong UV-absorption at 367 nm, whereas the PA6 and PS do not have any absorption at this wavelength [32].

The morphologies of as-electrospun products were observed by a scanning electron microscopy (SEM) of type ZEISS ULTRA55. Before SEM observation, samples were dried for 2 h in a vacuum oven at 90[degrees]C and then gold sputtered for 3 min. They were analyzed by SEM at a voltage of 5 kV. An energy dispersive spectrometer (EDS) (Inca Energy Coater, Oxford, UK) was used to analyze the element content of their surface. Scion Image software was used to determine the sizes of fibers and beads.

The tensile properties of as-electrospun mats were measured by a universal test machine (UTM2102, Shenzhen Suns Technology Stock Co. Ltd.). Samples were cut into slices of 20 mm in length and 5 mm in width. All tensile tests were carried out with a crosshead speed of 10 mm/min at room temperature.

RESULTS AND DISCUSSION

Electrospinning Behavior of PS Solution

The electrospinning of PS/CH[Cl.sub.3] solution was performed at 25[degrees]C-30[degrees]C and 60%-75% RH. The resulting morphologies are shown in Fig. 1. When the concentration of PS solution was lower than 0.10 g/mL, the as-electrospun products completely or mostly consisted of collapsed beads (Fig. la and b). This may occur because the viscosity of such a dilute polymer solution is too low to construct sufficient entanglement of polymer chains so that the initiating jet was transformed into many droplets and formed the bead structure. Upon increasing PS concentration to 0.15 and 0.20 g/mL, the viscosity increased to 74 and 141 mPa s as shown in Table 1. The resulting morphology turned into bead-on-string structure (Fig. lc and d). As PS concentration further increased to above 0.25 g/mL, the viscosity dramatically increased and the resulting morphology was evolved into bead-free fibers. Obviously, the morphology of electrospinning fiber indeed depends on the viscosity of polymer solution. In other words, bead-free fibers can only be produced when polymer concentration is above a certain level. However, it must be noted that the diameter of bead-free fibers greatly increased to above 10 [micro]m when the polymer concentration is above the critical concentration, that is, the bead-free superfine fibers cannot be achieved by increasing polymer concentration.

Electrospinning Behavior of Immiscible Polymer Blend

PA6 possesses a good electrospinnability to produce bead-free superfine fibers even as a dilute solution [33-35]. Can PA6 as an additive of PS solution promote the formation of bead-free superfine fibers? To answer this question, the electrospinning of PS solutions (0.10 g/mL) with different PA6 contents was carried out. Before electrospinning, PA6 was subjected to N-trifluoroacetylation to become soluble in CH[Cl.sub.3]. SEM images of those obtained electrospun fibers are shown in Fig. 2. As expected, the introduction of 20% PA6 based on the mass of PS can change the morphology of electrospun fibers from bead structure to bead-on-string structure. Further increasing the dosage of PA6 to 25% and 33%, the number of beads in bead-on-string structure was decreasing and their sizes were getting increasingly smaller. When the dosage of PA6 reached 50%, completely bead-free fibers were obtained.

As mentioned above, one reason for morphology change of those electrospun fibers after the addition of PA6 could be the increase of solution viscosity. From Table 2, it can be found that the introduction of PA6 hardly changed the solution viscosities. Even when the dosage of PA6 reached 50% that can completely form bead-free fibers, the viscosity of electrospun solution was slightly increased from 37 to 59 mPa s, which was much lower than the critical viscosity of PS solution being able to produce bead-free electrospun fibers. Obviously, the increase of viscosity is not the main reason for the formation of bead-free fibers by introducing PA6.

To figure out the formation mechanism of bead-free fibers, the as-electrospun PS/PA6 mats were etched by THF or formic acid to reveal the distribution and morphology of PA6 or PS domain. Before being etched, the fibrous membranes were first dried in a vacuum oven at 80[degrees]C for 12 h to convert PA6-TFAA into PA6. In this way, the PA6 phase could not be etched any more by THF. This can be confirmed by two following evidences: one is that after the as-electrospun PS/PA6 (80/20 by mass) mats are etched by THF, the residual mass fraction of 17.0 wt% is close to the initial mass percentage of PA6 phase; the other one is that the contents of carbon and oxygen elements on the residual surface measured by EDS are about 82.1 wt% and 17.9 wt% close to those of PA6 (carbon: 83.9 wt%, oxygen: 16.1 wt%). The morphologies of the residual PA6 phase after THF etching are shown in Fig. 3. It is obvious that, irrespective of the dosage of PA6, the beads almost completely disappear and the residual PA6 phase shows a fibrous structure. Moreover, the diameter of those fibers dramatically decreases to nanometer scale.

However, after electrospun PS/PA6 (80/20 by mass) mats were etched by formic acid, the mass reduces by 17 wt% which means that the PA6 phase is completely removed. However, their surface morphologies hardly change, which contain a large number of pores induced by rapid volatilization of solvents cooling the jet surface to condense water vapor into water droplet [36]. They show similar phenomena for electrospun PS/PA6-TFAA fibrous mats with 25 wt%, 33 wt%, and 50 wt% PA6. All of the above results may imply that the electrospun fibers present a core-shell structure with PS as the shell and PA6 as the core, which can be confirmed from the cross-section morphology of electrospun fibers etched by formic acid as shown in Fig. 4.

Based on the above results, the formation mechanism of bead-free PS fiber under the assistance of PA6 can be finally described as follows: in the presence of high voltage electric field, electrospinning solution was ejected from the needles following by rapid solvent evaporation. As a result, a microphase separation took place due to the immiscibility of PA6-TFAA and PS. In view of a lower solubility in CHCI3 and a smaller component, PA6-TFAA should be dispersed in the PS/CHCI3 matrix. Under the persistent electric drawing force, each dispersed PA6-TFAA microphase is elongated to form fiber structure. Simultaneously, PS is separated out from the solution and covers the surface of PA6-TFAA fiber, and then forms the final shell-core fiber.

Electrospinning Behavior of Reactive Compatibilized Polymer Blend

As is well known, PS-co-TMI can be used as a reactive compatibilizer for the PS/PA6 melt mixing because its isocyanate group is able to react with the terminal amine group of PA6, leading to the formation of a PS-g-PA6 graft copolymer. Based on the analysis of GPC with a UV detector at 367 nm, it can be noticed that, in the process of mixing and electrospinning of PS/PA6/PS-co-TMI solution, about 50% of PS-co-TMI participated in the reaction with PA6. The morphologies and tensile properties of electrospinning PS/PA6 mats with PS-co-TMI as a reactive compatibilizer are shown in Figs. 5 and 6, respectively. It can be found that, after adding 2% or 4% PS-co-TMI, the number of shuttle-like beads greatly decreased in contrast to that in the absence of PS-co-TMI (Fig. 2a), and the tensile strength also increased. For example, the introduction of 4% PS-co-TMI can increase tensile strength of PS/PA6 mats from 0.06 to 0.08 MPa. When the content of PS-co-TMI increased to above 6%, bead-free superfine fibers were obtained, and the tensile strength at 8% PS-co-TMI reached 0.12 MPa. Obviously, it can be concluded that the introduction of interfacial compatibilizer into polymer blend solution can promote the formation of superfine electrospinning fiber. From Table 3, it can be deduced that the introduction of PS-co-TMI increased slightly the solution viscosities, which may imply that the formation of bead-free fibers by the introduction of PA6-TFAA is not due to the viscosity factor.

To further ascertain the electrospinning mechanism of the immiscible polymer blend solution in the present of interfacial compatibilizer, SEM micrographs of as-electrospun mats etched by formic acid or THF are shown in Fig. 7. After being etched by formic acid, the morphologies and diameters of as-electrospun fibers hardly changed. Moreover, from EDS measurement, it can be found that there do not exist any oxygen element on the surface of the fibers, which may imply that the fibrous outer layer is PS. However, after being etched by THF, the number of beads in the as-electrospun mat dramatically decreased and even disappeared. The oxide element showed up on the surface of fibers and beads, which further confirms the inner core of as-electrospun fiber, is PA6. Therefore, the addition of PS-co-TMI does not change the core-shell structure of electrospinning fibers.

From Fig. 7, it should be also noted that after removing PS shell, the PA6 fibers as the core become more and more homogeneous with the increase of PS-co-TMI content. Obviously, the introduction of compatibilizer has a big effect on the core of as-electrospun fiber. In the electrospinning process of polymer blend solution, the minor component as the dispersed phase undergoes microphase separation from the matrix and the stretching deformation of strong electrical field. This recalls a similar mixing-drawing extrusion process of immiscible polymer blend in which the minor component first dispersed itself in the matrix under screw shearing action and then deformed the dispersed phase into microfiber by drawing extrudates along the extrusion direction. Our previous results [37] had showed that an introduction of interfacial compatibilizer could improve the dispersion uniformity and promote the deformation of dispersed phase into microfiber in the mixing-drawing extrusion process. With this idea, the introduction of interfacial compatibilizer leading to the formation superfine fibers can be ascribed to the mechanism shown in Fig. 8. PS-co-TMI first reacts with PA6 to form a copolymer PS-g-PA6 which can act as the interfacial compatibilizer of PS and PA6. After ejection from the needle, solvent evaporation and then microphase separation leads to dispersed PA6 spheres. Simultaneously, the copolymer PS-g-PA6 migrates into the interface to reduce the dispersed phase size and enhance the dispersion uniformity. Subsequently, the persistent electric drawing force drives the spherical PA6 to deform into the fibrillar structure. In the meantime, the PA6 chain of copolymer PS-g-PA6 still inserts itself into PA6 domains, whereas PS chains stay in PS phase. This means that the drawing force was transferred more efficiently from PA6 to PS domains through the copolymer PSg-PA6 dragging at the interface. As a result, the PS fluid is easier to uniformly cover the surface of PA6 fibers and form bead-free superfine fibers.

CONCLUSIONS

In this work, the electrospinning behavior of PS solution was studied under the assistance of polyamide 6 (PA6) as another polymer component and a copolymer (PS-co-TMI) of St and TMI as a reactive compatibilizer. The bead-free fiber cannot be obtained when PS/CH[Cl.sub.3] solution is below 0.25 g/mL. However, the diameter of obtained fiber exceeds 10 pm. After 50% PA6 based on the PS mass was introduced into the PS/CHCI3 solution, bead-free fibers with PA6 as the core and PS as the shell are obtained at a 0.10 g/mL of PS solution and their diameter greatly decrease. The reason for morphology change of those electrospun fibers after the addition of PA6 is ascribed to its good electrospinnability promoting the formation of PS fibers. Furthermore, PS-co-TMI that can react with PA6 to produce PS-g-PA6 as an interfacial compatibilizer is introduced to the PS/PA6 electrospinning solution. It can be found that after adding 2% or 4% of PS-co-TMI, the number of shuttle-like beads greatly decreases. When the content of PS-co-TMI increased to 6% and 8%, bead-free superfine fibers were obtained. Obviously, an interfacial compatibilizer results in the formation of superfine fibers. On the one hand, the interfacial compatibilizer can promote the dispersion of minor component during microphase separation. On the other hand, the copolymer PS-g-PA6 dragging at the interface is easier to promote the deformation of PA6 into microfiber core and drive PS to uniformly cover the surface of PA6 fibers and form bead-free superfine fibers.

ACKNOWLEDGMENTS

The authors thank the National Key Research and Development Program of China (2016YFC1100801), the Fundamental Research Funds for the Central Universities (2017FZA4024), and the State Key Laboratory of Chemical Engineering (SKLChE-13D) for their financial support.

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Yin Tang, (1) Lian-Fang Feng, (1,2) Xue-Ping Gu, (1,2) Cai-Liang Zhang [iD] (1,2)

(1) State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310017, People's Republic of China

(2) Institute of Zhejiang University--Quzhou, 78 Jiuhua Boulevard North, Quzhou, 32400, People's Republic of China

Correspondence to: C.-L. Zhang; e-mail: zhangcailiang@zju.edu.cn

Contract grant sponsor: the Fundamental Research Funds for the Central Universities; contract grant number: 2017FZA4024. contract grant sponsor: the National Key Research and Development Program of China; contact grant number: 2016YFC1100801. contract grant sponsor: the State Key Laboratory of Chemical Engineering; contract grant number: SKL-ChE-13D.

DOI 10.1002/pen.25130

Caption: FIG. 1. SEM images of as-electrospun fibers from PS solutions at different concentrations: 0.05 g/mL (a), 0.10 g/mL (b), 0.15 g/mL (c), 0.20 g/mL (d), 0.25 g/mL (e), and 0.35 g/mL (f).

Caption: FIG. 2. SEM images of the as-electrospun fibers from PS/PA6 solutions with different contents of PA6: 20% (a, b), 25% (c, d), 33% (e, f), and 50% (g, h). The PS concentration in CHC13 was fixed at 0.10 g/mL.

Caption: FIG. 3. SEM images of etched fibers of PS/PA6-TFAA with different contents of PA6-TFAA by formic acid (a, b, e, f, i, j, m, n) and by THF (c, d, g, h, k, 1, o, p). The concentrations of PS: 0.10 g/mL, the content of PA6-TFAA: 20% (a-d), 25% (e-h), 33% (i-1), and 50% (m-p).

Caption: FIG. 4. SEM image of cross-section of electrospun PS/PA6-TFAA fiber etched by formic acid. The concentration of PS was 0.10 g/mL, and the content of PA6-TFAA was 50%.

Caption: FIG. 5. SEM images of the as-electrospun fibers from PS/PA6 solution with different contents of PS-co-TMI. The concentration of PS: 0.10 g/mL, the concentration of PA6: 0.02 g/mL, and the content of PS-co-TMI was 2% (a), 4% (b), 6% (c), and 8% (d) based on the mass of PS.

Caption: FIG. 6. Tensile curves of the as-electrospun fibers from PS/PA6 solution with and without PS-co-TMI. The concentration of PS: 0.10 g/mL, the concentration of PA6: 0.02 g/mL, and the content of PS-co-TMI was 4% and 8% based on the mass of PS.

Caption: FIG. 7. SEM image of electrospun PS/PA6-TFAA fibers with different contents of PS-co-TMI after being etched by the formic acid (a, c, e and g) and THF (b, d, f, and h). The concentration of PS and PA6-TFAA were 0.10 g/mL and 0.02 g/mL, and the content of PS-co-TMI is 2% (a, b), 4% (c, d), 6% (e, f), and 8% (h, g) based on the mass of PS.

Caption: FIG. 8. Schematic mechanism of electrospinning on the compatibilized PS/PA6 blend solution. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Viscosities of PS/CH[Cl.sub.3] solutions at 100
[s.sup.-1] of shear rate.

Concentration (g/mL)   0.05   0.10   0.15   0.20   0.25   0.35
Viscosity (mPa s)      24     37     74     141    267    4,058

TABLE 2. Viscosities of PS/PA6 solutions ([C.sub.PS] = 0.10 g/mL)
at 100 [s.sup.-1] of shear rate.

Content of PA6 (%)   0    20   25   33   50
Viscosity (mPa s)    37   31   36   41   59

TABLE 3. Viscosities of PS/PA6-TFAA/PS-co-TMI solutions
([C.sub.PS] = 0.10 g/mL, [C.sub.PA6-TFAA] = 0.02 g/mL) at
100 [s.sup.-1] of shear rate.

Content of PS-co-TMI (%)   0    2    4    6    8
Viscosity (mPa s)          31   32   32   34   36
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Author:Tang, Yin; Feng, Lian-Fang; Gu, Xue-Ping; Zhang, Cai-Liang
Publication:Polymer Engineering and Science
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Date:Jul 1, 2019
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