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Electrospinning of nylon-6,66,1010 terpolymer
EUROPEAN POLYMER JOURNAL
European Polymer Journal 42 (2006) 1696–1704
www.elsevier.com/locate/europolj
Electrospinning of nylon-6,66,1010 terpolymer Yan Li
€ , Zhengming Huang b, Yandong Lu
a,*
c
a
b
School of Materials Science and Engineering, Tongji University, Shanghai 200092, China School of Aeronautics, Astronautics and Mechanics, Tongji University, Shanghai 200092, China c Shanghai Plastic Research Institute, Shanghai 200090, China Received 13 June 2005; received in revised form 7 February 2006; accepted 8 February 2006 Available online 23 March 2006
Abstract Ultrafine nylon fibers were prepared by electrospinning of nylon-6,66,1010 terpolymer solution in 2,2,2-trifluoroethanol (TFE). The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 fibers were investigated by scanning electron microscope (SEM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and tensile test. The effects of electrospun process parameters such as solution concentration, voltage and tipto-collector distance on the morphology and the average size of the electrospun fibers were also studied. The results show that the spinnable concentration of nylon-6,66,1010/TFE solution is in the range of 6–14 wt%, and higher solution concentration favors the formation of uniform fibers without beads. The diameters of the electrospun fibers increase with increasing the solution concentration and decrease slightly with increasing the voltage and needle tip-to-collector distance. But no obvious morphology changes were found with the increase of the voltage and collection distance. DSC and WAXD results suggest that the electrospun nylon-6,66,1010 membranes have lower crystallinity than those of the corresponding casting films. The electrospun nylon-6,66,1010 membrane obtained from the 14 wt% concentration exhibits the largest tensile strength and elongation at break. 2006 Elsevier Ltd. All rights reserved. Keywords: Electrospinning; Electrospun fiber; Nylon-6,66,1010; Membrane; Crystallinity
1. Introduction Polymer fibers with the average diameter ranging from 10 to 50 lm have been wildly used in textiles or composite reinforcement fields, and these fibers are commonly produced by the traditional methods such as melt spinning, dry spinning and wet spinning [1,2]. Recently, a new and useful process, electrospinning, has been developed to produce polymer
*
Corresponding author. Tel./fax: +86 21 65980009. E-mail address: [email protected] (Y. Li).
fibers with submicrometer or nanometer diameters (usually ranging from 100 nm to 5 lm) [3–6]. Due to the small size of the electrospun polymer fibers, the membranes collected from electrospun fibers possess a large surface area per unit mass and a very small pore size [7,8]. These characteristics make the electrospun fibers have many potential applications such as optical materials, sensor materials, nanocomposite materials, tissue scaffolds, wound dressings, drug delivery systems, in filtration and as protective clothing [3,9–11]. Electrospinning technique was first introduced by Formals in 1934 [12]. It involves the use of high
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.02.002
Y. Li et al. / European Polymer Journal 42 (2006) 1696–1704
voltage to charge the polymer solution placed within a syringe. The polymer solution can form a droplet stabilized by its surface tension at the end of the needle tip of the syringe. However, when the applied voltage exceeds a critical value at which the electrostatic force overcomes the surface tension, a stable jet of liquid could be ejected from the droplet. Due to bending instability, the jet is subsequently stretched by many times to form much smaller polymer fibers [13]. The fibers can be collected on a collection screen in the form of nonwoven membranes or on some special collection equipment in the form of aligned yarns or single fiber [14]. More than 30 different types of polymer fibers have been prepared by electrospinning including polyolefine, polyamides, polyester, polycarbonate, poly(vinyl alcohol), polyacrylonitrile, polyethylene oxide (PEO), polystyrene as well as biopolymers like polycaprolactone, protein, DNA or electric conducting polymers like polyaniline, and so on [15]. These polymer fibers have different structures and morphologies. The following parameters are found to affect the electrospinning process and accordingly affect the structure and morphology of the polymer fibers [16]: (i) polymer solution properties such as viscosity, surface tension, conductivity; (ii) processing parameters such as voltage, hydrostatic pressure in the syringe, the distance between the syringe and collection screen; (iii) surrounding conditions, such as temperature, humidity and air velocity. It has been proved that higher solution viscosity (or concentration) and applied voltage will result in a larger fiber. For example, Deitzel et al. have pointed out that the fiber diameter increased with increasing polymer concentration, which was accordant to a power law relationship [2]. The beads are often found on the electrospun fibers and considered as the ‘‘by products’’ of the electrospun fibers. Reneker et al. have found that higher viscosity, higher net charge density and lower surface tension favor the formation of fibers without beads by studying the electrospinning of PEO [10,17]. Therefore, optimal polymer fibers with suitable diameter and without beads can be obtained by controlling the parameters listed above. Polyamides (nylon) are often used to produce fibers with the diameter in the order of 30 lm by conventional melt spinning. However, the fibers with much smaller diameters are preferred for many industrial applications. For example, the nylon fiber-reinforced composites will have good transpar-
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ent property if the fiber diameter is lower than that of light wavelength. So the novel spinning methods with the advantages in producing smaller fiber should be developed to fulfill the industrial requirements, and electrospinning is proved to be a powerful method. Some nylon samples, such as nylon-6, nylon-12 and nylon-4,6, have already been electrospun into ultrafine fibers in the submicroscopic scale [18]. As a pioneering work in the application of electrospun fibers, Bergshoef et al. had prepared transparent epoxy composites reinforced with the electrospun nylon-4,6 fibers with diameters in the range of 30– 200 nm [9]. They found that the Young’s moduli and fracture stress of the fiber-reinforced epoxy composites were 36 and 3 times higher than those of the epoxy samples without the electrospun fibers, respectively. Nylon-6,66,1010 terpolymer is a kind of copolymerization nylon species, which has low melting point, excellent mechanical property, wearing resistance, oil resistivity, heat resistance and can be used for the production of mono- and multifilaments, fibers, films and nets [19]. However, no research on electrospinning of nylon-6,66,1010 terpolymer has been performed. In this study, we reported the electrospinning of nylon-6,66,1010/2,2,2-trifluoroethanol solution for the first time. The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 terpolymer were studied. The effects of solution concentration, applied voltage and needle tip-to-collector distance on the morphology and the average size of the electrospun nylon-6,66,1010 fibers were also investigated. 2. Experimental 2.1. Materials Nylon-6,66,1010 terpolymer (PSGN-150) was received from Shanghai Sailient Chemical Co., Ltd. The melting temperature was in the range of 140–160 C and the copolymerization mass ratio of 6/66/1010 was 10/20/70. 2,2,2-Trifluoroethanol (TFE) was purchased from Sigma–Aldrich Co., which was used directly without further purification. 2.2. Preparation and properties of polymer solution Various polymer solutions with concentrations ranging from 6 to 14 wt% were prepared at room temperature by dissolving the nylon-6,66,1010 in TFE, and stirred by magnetic force to speed the dis-
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solution. These solutions were electrospun at different voltages (12–21 kV) and needle tip-to-collector distance (6–18 cm) from a 5 ml syringe with a 16gauge needle. Solution viscosities were determined by a rotational viscometer (NDJ-79, Electrical Machinery Plant of Tongji University, China) with a rotational speed of 750 rpm at 25 C.
and surface tension. The anode of the high-voltage power supply was connected to a copper wire, which was immersed in the polymer solution. The cathode was connected to the aluminum foil. By applied voltage between the anode and cathode, the droplet was instantly disintegrated into fibers that deposited on the aluminum foil at last. All electrospun samples were dried in a vacuum oven at room temperature for 24 h to remove the residual solvent.
2.3. Morphology 2.7. Tensile properties The morphology of the electrospun nylon6,66,1010 fibers was observed with a scanning electron microscope (HITACHI S-2360) manufactured by Japan Electron Optical Laboratory. The diameters of the electrospun nylon-6,66,1010 fibers were measured directly from the printed SEM micrographs of fibers. 2.4. DSC analysis The thermal properties were measured using a DSC, Pyris 1, Perkin Elmer Co., USA. Two kinds of samples were scanned. One was the membranes electrospun from 8 wt%, 11 wt% or 14 wt% nylon6,66,1010 solutions. The other was the casting films prepared by slowly evaporating the solvent from the corresponding nylon-6,66,1010 solutions. About 5 mg sample was heated from room temperature to 200 C at a heating rate of 20 C/min. 2.5. WAXD Wide angle X-ray diffraction (WAXD) measurements were performed under a Dmax-rC X-ray diffractometer with CuKa radiation (40 kV, 30 mA). The diffraction scans were collected at 2h = 10–40 with the scanning rate of 4/min. 2.6. Electrospinning process The experimental apparatus used for electrospinning was the same as those reported by Frenot [16]. A high-voltage electric field for electrospinning process was produced by a changeable high-voltage power supply (DC-high-voltage generator, Beijing machinery and electricity institute high-voltage technology company). The polymer solution was placed in a 5 ml syringe attached to a 16-gauge needle and then it was fixed above a grounded collector (aluminum foil). The polymer solution formed a droplet at the tip of the syringe due to its weight
The electrospun nylon-6,66,1010 membranes with a planar dimension of width · gauge length = 10 mm · 30 mm used for tensile testing were prepared according to Huang’s method [20]. The tensile properties were tested with an Instron testing machine (Model 4302) with a load cell of 10 N at 23 C. A crosshead speed of 10 mm/min was used for all of the specimens tested. 3. Results and discussion 3.1. Spinnable concentration In our study, continuous nylon-6,66,1010 fibers were successfully electrospun from the nylon6,66,1010/TFE solutions in the concentration range of 6–14 wt%. For the lower concentration of nylon6,66,1010 solution of 6 wt%, no continuous fibers but beads and some fiber segments could be obtained. For the higher concentration of nylon6,66,1010 solution of 14 wt%, the electrospinning process became difficult. On the one hand, the nylon-6,66,1010 sample did not dissolve easily in the TFE to form such concentrated solution. On the other hand, higher viscous fluid balls could be gradually gathered outside the tip of the needle after the solvent volatilization. As a result, no matter how high an electric voltage had been applied, no fibers could be obtained on the collector ultimately. Hence, the spinnable concentration of nylon6,66,1010/TFE solution in electrospinning is 6– 14 wt%. 3.2. Viscosity of polymer solutions Hsiao et al. had reported that the solution concentration or the corresponding viscosity was one of the most effective variables to control the electrospun fiber morphology [21]. We also investigated the effect of the nylon-6,66,1010/TFE solution
Y. Li et al. / European Polymer Journal 42 (2006) 1696–1704 1800
Viscosity, mPa.s
1500 1200 900 600 300 0 6
8
10
12
14
Concentration, wt% Fig. 1. Solution viscosity as a function of nylon-6,66,1010/TFE solution concentration.
properties on the electrospinning process. Fig. 1 shows the relationship between the spinnable concentrations and the corresponding viscosities of the nylon-6,66,1010/TFE solutions. The viscosities of nylon-6,66,1010 solutions increased sharply from 115 mPaÆs to 1646 mPaÆs with the increase of the nylon-6,66,1010 solution concentration from 6 to 14 wt%. 3.3. Influence of polymer solution concentration on fiber morphology The SEM photographs and size distributions of the electrospun nylon-6,66,1010 fibers with different polymer solution concentrations are shown in Fig. 2 (at voltage of 18 kV, needle tip-to-collector distance of 14 cm). The size distribution of every kind of electrospun fibers was statistic from 200 fibers. A lot of beads could be seen on the electrospun fibers prepared from the 6 wt% nylon-6,66,1010 solution, as shown in Fig. 2(a). As the concentration of the nylon-6,66,1010 solution increased, the beads gradually disappeared and the shape of the beads gradually changed from spherical to spindle-like and (Fig. 2(b)). Only very little beads could be found on the fibers electrospun from the 11 wt% nylon6,66,1010/TFE solution (Fig. 2(c)). Continuous and smooth fibers could be formed at 14 wt% when the other processing parameters were constant (Fig. 2(d)). The formation of beaded fibers had been observed widely [7]. They were regarded as ‘‘by products’’ often formed in electrospinning. It was found that the formation of beaded fibers is related to the instability of the jet of polymer solution, the solution viscosity, net charge density carried by the
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electrospinning jet and surface tension of the solution [22]. Higher viscosity favored the formation of fibers without beads [23]. This was proved again by the electrospinning of nylon-6,66,1010. It can also be seen that the nylon-6,66,1010 fiber diameters became larger with the increase of the solution concentration. The 6 wt% nylon-6,66,1010 solution resulted in the smallest fiber diameter, being around 230 nm. The fibers electrospun from 8 wt%, 11 wt% and 14 wt% nylon-6,66,1010 solutions had an average diameter of about 310 nm, 380 nm and 487 nm, respectively. In addition, the fibers electrospun from 6 wt% nylon-6,66,1010 solution had narrower distributions compared with those of the other concentration solutions. In short, the morphology and the average diameter of the electrospun nylon-6,66,1010 fibers can be effectively controlled by polymer concentration. In the spinnable concentration range, the higher the solution concentration, the better the fiber morphology and the bigger the fiber diameter. Although, the size of the electrospun nylon6,66,1010 fibers can be controlled by increasing the concentration of polymer solution, the size distribution of the fibers is really broad. Several authors have also found the broad size distribution of other electrospun polymer fibers [4,7,24]. In my opinion, there are at least three reasons for the broad size distribution: (1) during the electrospinning, the polymer droplet disintegration process induced by a high-voltage power supply may not be uniform, which results in the broad fiber size distribution; (2) the broad fiber size distribution may be associated with the formation of a large number of satellite droplets during the breakup of the solution jet [2]; (3) many parameters can influence the formation of fibers such as solution concentration, surface tension, solvent used, voltage, temperature, and relative humidity. The soft fluctuations of these parameters during the experiments (probably inevitable) may broaden the fiber size distribution. 3.4. Influence of the voltage and the needle tip-tocollector distance on fiber morphology The effects of the voltage and the needle tip-tocollector distance on the morphology of nylon6,66,1010 fibers were also investigated. Continuous and smooth fibers could be formed in the electrospinning voltage ranging from 12 to 21 kV, keeping the polymer concentration at 11 wt% and the needle tip-to-collector distance at 14 cm. However, no
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Fig. 2. SEM photographs and size distribution of electrospun nylon-6,66,1010 fibers as functions of solution concentration at voltage of 18 kV and needle tip-to-collector distance of 14 cm: (a) 6 wt%; (b) 8 wt%; (c) 11 wt%; and (d) 14 wt%.
Y. Li et al. / European Polymer Journal 42 (2006) 1696–1704
obvious difference in the fiber morphology was found in spite that the average fiber diameter decreased slightly with increasing electrospinning voltage. The effect of the needle tip-to-collector distance on the electrospun nylon-6,66,1010 fibers’ morphology is shown in Fig. 3 (at nylon-6,66,1010 solution concentration of 11 wt% and the voltage of 18 kV). Similar to the influence of electrospinning voltage, we found that the fibers morphology hardly changed and the fiber size decreased slightly with the increase of needle tip-to-collector distance. According to the statistic results based on 200 fibers, the fiber’s average diameters were 398 nm at 6 cm, 375 nm at 10 cm, 380 nm at 14 cm and 368 nm at 18 cm, respectively. The slight decrease in fiber size electrospinning with increasing needle tip-to-collector distance due to the longer distance could give more time for the solvent to evaporate and facilitate the charged fluids to split more number of times [25].
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11 wt% nylon-6,66,1010/TFE solution and the membrane electrospun from the corresponding solution is shown in Fig. 4. The casting film and the fiber membrane possessed almost the same spectra, which showed the very obvious characteristic absorption bands of amide groups and methylene segments of polyamides. Their assignments were as follows: 3290 cm 1 (H-bonded N–H stretch vibration), 3080 cm 1 (N–H in-plane bending), 1640 cm 1 (Amide I, C@O stretch), 1540 cm 1 (Amide II, C–N stretch and CO–N–H bend), 940 cm 1 (Amide IV, C–CO stretch), 721 cm 1 ((CH2) > 4, wag), and 690 cm 1 (Amide II, N–H out-of-plane bend). The similar IR spectra of the casting film and the fiber membrane indicated that the molecular structures of the nylon-6,66,1010 sample do not change on the application of high voltage during electrospinning. 3.6. DSC and WAXD
3.5. Infrared spectra A representative infrared spectrum of the casting film prepared by evaporating the solvent from the
From the viewpoint of the commercial applications of the materials, the crystalline properties of the electrospun fibers may be one of the most
Fig. 3. SEM photographs of electrospun nylon-6,66,1010 as a function of the needle tip-to-collector distance: (a) 6 cm, (b) 10 cm, (c) 14 cm, and (d) 18 cm. The nylon-6,66,1010 solution concentration was 11 wt% and the voltage was 18 kV.
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Transmittance
(a)
(b)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber, cm-1 Fig. 4. Infrared spectrum of the nylon-6,66,1010 samples. (a) Casting film prepared by evaporating solvent from the 11 wt% nylon-6,66,1010/TFE solution; and (b) fiber membrane collected from the electrospun 11 wt% nylon-6,66,1010/TFE solution.
important properties. Here, we performed DSC and WAXD experiments on two kinds of samples to evaluate the crystalline properties of nylon-6,66, 1010. One was the electrospun nylon-6,66,1010. membrane, the other was the nylon-6,66,1010 casting film prepared by slowly evaporating the TFE from the corresponding solution. It was difficult for the 6 wt% nylon-6,66,1010 solution to form the fiber membrane due to the existence of the abundant beads. Hence, the 6 wt% nylon-6,66,1010 sample was not studied. The melting enthalpy and melting temperature of the electrospun nylon-6,66,1010 membranes and the corresponding casting films determined by DSC are shown in Table 1. It can be seen that the electrospun nylon-6,66,1010 membranes and the corresponding casting films had almost the same melting temperature (about 145 C), however, the electrospun nylon-6,66,1010 membranes show lower melting enthalpy than those of the corresponding casting films. For example, the melting enthalpy of the membrane electrospun from 8 wt% nylon-6,66,1010 soluTable 1 Melting enthalpy and melting temperature of the electrospun nylon-6,66,1010 membranes and the corresponding casting films Samples
Tm (C)
Hf (J/g)
8 wt% electrospun membrane 11 wt% electrospun membrane 14 wt% electrospun membrane 8 wt% casting film 11 wt% casting film 14 wt% casting film
145.9 145.2 144.9 146.4 146.7 145.3
27.7 29.0 33.5 34.7 37.7 42.9
tion was 27.7 J/g, which was lower than that of the corresponding nylon-6,66,1010 casting film (34.7 J/ g). This indicates that the electrospun nylon6,66,1010 fibers have lower percentage of crystallinity compared with that of the corresponding casting sample. The process of electrospinning, especially the later stages of electrospinning, could be understood as the rapid solidification process of the stretched macromolecular chains under the high elongational rate. The rapid solidification limited the development of crystallinity because the macromolecular chains had no time to form crystalline registration. However, during the preparation of the casting film the macromolecular chains had enough time to adjust their conformations with the evaporation of the solvent slowly and formed the relative high crystalline registration at last. Zong also observed the low crystallinity of the electrospun poly (L-lactic acid) (PLLA) fibers compared with those of the corresponding casting films [21]. The decrease in crystallinity of the electrospun nylon-6,66,1010 fiber membranes compared with that of the corresponding casting films also can be confirmed by WAXD examination. The WAXD results of the 8 wt% and 14 wt% nylon-6,66,1010 samples were taken as the typical examples (Fig. 5). The WAXD patterns of both the nylon6,66,1010 electrospun membranes and the corresponding casting films exhibited two peaks. The two diffraction peaks at about 2h = 20 and 2h = 23 were the (1 0 0) and (0 1 0,1 1 0) of the aphase crystals of typical triclinic form of nylons, respectively [26]. However, the diffraction peaks associated with the electrospun nylon-6,66,1010 membranes were relatively lower and broader compared with the higher and sharp peaks of the corresponding casting films. This probably indicates the easy packing of the crystal in the casting films compared with those of the electrospun membranes. At the same time, it can also be seen that the diffraction peaks of the electrospun nylon-6,66,1010 membranes shift a little toward the low angle direction compared with those of the casting films. According to the Bragg equation [27], the d-spacings of the electrospun membranes were broader than those of the corresponding casting films. This could be explained that the macromolecular chains could not be arranged tightly due to the rapid evaporation of solvent during the electrospinning and formed loose structure. Therefore, WAXD results again proved the lower crystallinity in the electrospun nylon-6,66,1010 membranes.
Y. Li et al. / European Polymer Journal 42 (2006) 1696–1704
Intensity
8wt%
casting film
electrospun membrane
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120% and 155%, respectively. Huang et al. reported that the beads on the fiber surface considerably reduced the cohesive force between the fibers of the electrospun nanofiber membrane and resulted in poorer mechanical properties of the electrospun nanofiber membrane [20]. Hence, the best mechanical properties of the membrane electrospun from the 14 wt% nylon-6,66,1010/TFE solution could be attributed to its good fiber surfaces without beads. 4. Conclusions
10
15
20
25
30
35
40
2θ , degree
Intensity
14wt%
casting film
electrospun membrane
10
15
20
25
30
35
40
2θ , degree Fig. 5. WAXD patterns of the electrospun nylon-6,66,1010 membrane and corresponding casting films.
Acknowledgement This study was financially supported by the Shanghai Nano Special Foundation (0352nm091).
3.7. Mechanical properties It should be noted that the mechanical properties measurements were carried out on the electrospun membranes prepared from the 8–14 wt% nylon6,66,1010/TFE solutions except of the 6 wt% nylon-6,66,1010/TFE solution. The specimen thickness, ultimate strength and elongation at break are shown in Table 2. With the nylon-6,66,1010/TFE solution concentration increasing from 8 wt% to 14 wt%, the ultimate strength and the elongation at break of the electrospun membranes increased Table 2 Tensile properties of the electrospun fiber membranes prepared from the different concentration nylon 6/66/1010/TFE solutions
Specimen thickness (lm) Ultimate strength (MPa) Elongation at break (%)
The nylon-6,66,1010 nanofibers were successfully prepared by electrospinning. The spinnable concentration of the nylon 6/66/1010 solution is in the range of 6–14 wt%. Higher solution concentration favors the formation of uniform ultrafine fibers without beads. The diameter of the electrospun fibers increases with the increase of the solution concentration and decreases slightly with increasing the voltage and the needle tip-to-collector distance. The electrospun nylon-6,66,1010 membranes have lower crystallinity than that of the corresponding casting films. The nylon-6,66,1010 membrane with uniform and no-bead fibers electrospun from the 14 wt% solution exhibits the best tensile strength and elongation at break.
8 wt%
11 wt%
14 wt%
37 4.6 16.7
42 8.0 33.3
39 10.1 42.6
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European Polymer Journal 42 (2006) 1696–1704
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Electrospinning of nylon-6,66,1010 terpolymer Yan Li
€ , Zhengming Huang b, Yandong Lu
a,*
c
a
b
School of Materials Science and Engineering, Tongji University, Shanghai 200092, China School of Aeronautics, Astronautics and Mechanics, Tongji University, Shanghai 200092, China c Shanghai Plastic Research Institute, Shanghai 200090, China Received 13 June 2005; received in revised form 7 February 2006; accepted 8 February 2006 Available online 23 March 2006
Abstract Ultrafine nylon fibers were prepared by electrospinning of nylon-6,66,1010 terpolymer solution in 2,2,2-trifluoroethanol (TFE). The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 fibers were investigated by scanning electron microscope (SEM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and tensile test. The effects of electrospun process parameters such as solution concentration, voltage and tipto-collector distance on the morphology and the average size of the electrospun fibers were also studied. The results show that the spinnable concentration of nylon-6,66,1010/TFE solution is in the range of 6–14 wt%, and higher solution concentration favors the formation of uniform fibers without beads. The diameters of the electrospun fibers increase with increasing the solution concentration and decrease slightly with increasing the voltage and needle tip-to-collector distance. But no obvious morphology changes were found with the increase of the voltage and collection distance. DSC and WAXD results suggest that the electrospun nylon-6,66,1010 membranes have lower crystallinity than those of the corresponding casting films. The electrospun nylon-6,66,1010 membrane obtained from the 14 wt% concentration exhibits the largest tensile strength and elongation at break. 2006 Elsevier Ltd. All rights reserved. Keywords: Electrospinning; Electrospun fiber; Nylon-6,66,1010; Membrane; Crystallinity
1. Introduction Polymer fibers with the average diameter ranging from 10 to 50 lm have been wildly used in textiles or composite reinforcement fields, and these fibers are commonly produced by the traditional methods such as melt spinning, dry spinning and wet spinning [1,2]. Recently, a new and useful process, electrospinning, has been developed to produce polymer
*
Corresponding author. Tel./fax: +86 21 65980009. E-mail address: [email protected] (Y. Li).
fibers with submicrometer or nanometer diameters (usually ranging from 100 nm to 5 lm) [3–6]. Due to the small size of the electrospun polymer fibers, the membranes collected from electrospun fibers possess a large surface area per unit mass and a very small pore size [7,8]. These characteristics make the electrospun fibers have many potential applications such as optical materials, sensor materials, nanocomposite materials, tissue scaffolds, wound dressings, drug delivery systems, in filtration and as protective clothing [3,9–11]. Electrospinning technique was first introduced by Formals in 1934 [12]. It involves the use of high
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.02.002
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voltage to charge the polymer solution placed within a syringe. The polymer solution can form a droplet stabilized by its surface tension at the end of the needle tip of the syringe. However, when the applied voltage exceeds a critical value at which the electrostatic force overcomes the surface tension, a stable jet of liquid could be ejected from the droplet. Due to bending instability, the jet is subsequently stretched by many times to form much smaller polymer fibers [13]. The fibers can be collected on a collection screen in the form of nonwoven membranes or on some special collection equipment in the form of aligned yarns or single fiber [14]. More than 30 different types of polymer fibers have been prepared by electrospinning including polyolefine, polyamides, polyester, polycarbonate, poly(vinyl alcohol), polyacrylonitrile, polyethylene oxide (PEO), polystyrene as well as biopolymers like polycaprolactone, protein, DNA or electric conducting polymers like polyaniline, and so on [15]. These polymer fibers have different structures and morphologies. The following parameters are found to affect the electrospinning process and accordingly affect the structure and morphology of the polymer fibers [16]: (i) polymer solution properties such as viscosity, surface tension, conductivity; (ii) processing parameters such as voltage, hydrostatic pressure in the syringe, the distance between the syringe and collection screen; (iii) surrounding conditions, such as temperature, humidity and air velocity. It has been proved that higher solution viscosity (or concentration) and applied voltage will result in a larger fiber. For example, Deitzel et al. have pointed out that the fiber diameter increased with increasing polymer concentration, which was accordant to a power law relationship [2]. The beads are often found on the electrospun fibers and considered as the ‘‘by products’’ of the electrospun fibers. Reneker et al. have found that higher viscosity, higher net charge density and lower surface tension favor the formation of fibers without beads by studying the electrospinning of PEO [10,17]. Therefore, optimal polymer fibers with suitable diameter and without beads can be obtained by controlling the parameters listed above. Polyamides (nylon) are often used to produce fibers with the diameter in the order of 30 lm by conventional melt spinning. However, the fibers with much smaller diameters are preferred for many industrial applications. For example, the nylon fiber-reinforced composites will have good transpar-
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ent property if the fiber diameter is lower than that of light wavelength. So the novel spinning methods with the advantages in producing smaller fiber should be developed to fulfill the industrial requirements, and electrospinning is proved to be a powerful method. Some nylon samples, such as nylon-6, nylon-12 and nylon-4,6, have already been electrospun into ultrafine fibers in the submicroscopic scale [18]. As a pioneering work in the application of electrospun fibers, Bergshoef et al. had prepared transparent epoxy composites reinforced with the electrospun nylon-4,6 fibers with diameters in the range of 30– 200 nm [9]. They found that the Young’s moduli and fracture stress of the fiber-reinforced epoxy composites were 36 and 3 times higher than those of the epoxy samples without the electrospun fibers, respectively. Nylon-6,66,1010 terpolymer is a kind of copolymerization nylon species, which has low melting point, excellent mechanical property, wearing resistance, oil resistivity, heat resistance and can be used for the production of mono- and multifilaments, fibers, films and nets [19]. However, no research on electrospinning of nylon-6,66,1010 terpolymer has been performed. In this study, we reported the electrospinning of nylon-6,66,1010/2,2,2-trifluoroethanol solution for the first time. The morphology, crystallinity and mechanical properties of the electrospun nylon-6,66,1010 terpolymer were studied. The effects of solution concentration, applied voltage and needle tip-to-collector distance on the morphology and the average size of the electrospun nylon-6,66,1010 fibers were also investigated. 2. Experimental 2.1. Materials Nylon-6,66,1010 terpolymer (PSGN-150) was received from Shanghai Sailient Chemical Co., Ltd. The melting temperature was in the range of 140–160 C and the copolymerization mass ratio of 6/66/1010 was 10/20/70. 2,2,2-Trifluoroethanol (TFE) was purchased from Sigma–Aldrich Co., which was used directly without further purification. 2.2. Preparation and properties of polymer solution Various polymer solutions with concentrations ranging from 6 to 14 wt% were prepared at room temperature by dissolving the nylon-6,66,1010 in TFE, and stirred by magnetic force to speed the dis-
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solution. These solutions were electrospun at different voltages (12–21 kV) and needle tip-to-collector distance (6–18 cm) from a 5 ml syringe with a 16gauge needle. Solution viscosities were determined by a rotational viscometer (NDJ-79, Electrical Machinery Plant of Tongji University, China) with a rotational speed of 750 rpm at 25 C.
and surface tension. The anode of the high-voltage power supply was connected to a copper wire, which was immersed in the polymer solution. The cathode was connected to the aluminum foil. By applied voltage between the anode and cathode, the droplet was instantly disintegrated into fibers that deposited on the aluminum foil at last. All electrospun samples were dried in a vacuum oven at room temperature for 24 h to remove the residual solvent.
2.3. Morphology 2.7. Tensile properties The morphology of the electrospun nylon6,66,1010 fibers was observed with a scanning electron microscope (HITACHI S-2360) manufactured by Japan Electron Optical Laboratory. The diameters of the electrospun nylon-6,66,1010 fibers were measured directly from the printed SEM micrographs of fibers. 2.4. DSC analysis The thermal properties were measured using a DSC, Pyris 1, Perkin Elmer Co., USA. Two kinds of samples were scanned. One was the membranes electrospun from 8 wt%, 11 wt% or 14 wt% nylon6,66,1010 solutions. The other was the casting films prepared by slowly evaporating the solvent from the corresponding nylon-6,66,1010 solutions. About 5 mg sample was heated from room temperature to 200 C at a heating rate of 20 C/min. 2.5. WAXD Wide angle X-ray diffraction (WAXD) measurements were performed under a Dmax-rC X-ray diffractometer with CuKa radiation (40 kV, 30 mA). The diffraction scans were collected at 2h = 10–40 with the scanning rate of 4/min. 2.6. Electrospinning process The experimental apparatus used for electrospinning was the same as those reported by Frenot [16]. A high-voltage electric field for electrospinning process was produced by a changeable high-voltage power supply (DC-high-voltage generator, Beijing machinery and electricity institute high-voltage technology company). The polymer solution was placed in a 5 ml syringe attached to a 16-gauge needle and then it was fixed above a grounded collector (aluminum foil). The polymer solution formed a droplet at the tip of the syringe due to its weight
The electrospun nylon-6,66,1010 membranes with a planar dimension of width · gauge length = 10 mm · 30 mm used for tensile testing were prepared according to Huang’s method [20]. The tensile properties were tested with an Instron testing machine (Model 4302) with a load cell of 10 N at 23 C. A crosshead speed of 10 mm/min was used for all of the specimens tested. 3. Results and discussion 3.1. Spinnable concentration In our study, continuous nylon-6,66,1010 fibers were successfully electrospun from the nylon6,66,1010/TFE solutions in the concentration range of 6–14 wt%. For the lower concentration of nylon6,66,1010 solution of 6 wt%, no continuous fibers but beads and some fiber segments could be obtained. For the higher concentration of nylon6,66,1010 solution of 14 wt%, the electrospinning process became difficult. On the one hand, the nylon-6,66,1010 sample did not dissolve easily in the TFE to form such concentrated solution. On the other hand, higher viscous fluid balls could be gradually gathered outside the tip of the needle after the solvent volatilization. As a result, no matter how high an electric voltage had been applied, no fibers could be obtained on the collector ultimately. Hence, the spinnable concentration of nylon6,66,1010/TFE solution in electrospinning is 6– 14 wt%. 3.2. Viscosity of polymer solutions Hsiao et al. had reported that the solution concentration or the corresponding viscosity was one of the most effective variables to control the electrospun fiber morphology [21]. We also investigated the effect of the nylon-6,66,1010/TFE solution
Y. Li et al. / European Polymer Journal 42 (2006) 1696–1704 1800
Viscosity, mPa.s
1500 1200 900 600 300 0 6
8
10
12
14
Concentration, wt% Fig. 1. Solution viscosity as a function of nylon-6,66,1010/TFE solution concentration.
properties on the electrospinning process. Fig. 1 shows the relationship between the spinnable concentrations and the corresponding viscosities of the nylon-6,66,1010/TFE solutions. The viscosities of nylon-6,66,1010 solutions increased sharply from 115 mPaÆs to 1646 mPaÆs with the increase of the nylon-6,66,1010 solution concentration from 6 to 14 wt%. 3.3. Influence of polymer solution concentration on fiber morphology The SEM photographs and size distributions of the electrospun nylon-6,66,1010 fibers with different polymer solution concentrations are shown in Fig. 2 (at voltage of 18 kV, needle tip-to-collector distance of 14 cm). The size distribution of every kind of electrospun fibers was statistic from 200 fibers. A lot of beads could be seen on the electrospun fibers prepared from the 6 wt% nylon-6,66,1010 solution, as shown in Fig. 2(a). As the concentration of the nylon-6,66,1010 solution increased, the beads gradually disappeared and the shape of the beads gradually changed from spherical to spindle-like and (Fig. 2(b)). Only very little beads could be found on the fibers electrospun from the 11 wt% nylon6,66,1010/TFE solution (Fig. 2(c)). Continuous and smooth fibers could be formed at 14 wt% when the other processing parameters were constant (Fig. 2(d)). The formation of beaded fibers had been observed widely [7]. They were regarded as ‘‘by products’’ often formed in electrospinning. It was found that the formation of beaded fibers is related to the instability of the jet of polymer solution, the solution viscosity, net charge density carried by the
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electrospinning jet and surface tension of the solution [22]. Higher viscosity favored the formation of fibers without beads [23]. This was proved again by the electrospinning of nylon-6,66,1010. It can also be seen that the nylon-6,66,1010 fiber diameters became larger with the increase of the solution concentration. The 6 wt% nylon-6,66,1010 solution resulted in the smallest fiber diameter, being around 230 nm. The fibers electrospun from 8 wt%, 11 wt% and 14 wt% nylon-6,66,1010 solutions had an average diameter of about 310 nm, 380 nm and 487 nm, respectively. In addition, the fibers electrospun from 6 wt% nylon-6,66,1010 solution had narrower distributions compared with those of the other concentration solutions. In short, the morphology and the average diameter of the electrospun nylon-6,66,1010 fibers can be effectively controlled by polymer concentration. In the spinnable concentration range, the higher the solution concentration, the better the fiber morphology and the bigger the fiber diameter. Although, the size of the electrospun nylon6,66,1010 fibers can be controlled by increasing the concentration of polymer solution, the size distribution of the fibers is really broad. Several authors have also found the broad size distribution of other electrospun polymer fibers [4,7,24]. In my opinion, there are at least three reasons for the broad size distribution: (1) during the electrospinning, the polymer droplet disintegration process induced by a high-voltage power supply may not be uniform, which results in the broad fiber size distribution; (2) the broad fiber size distribution may be associated with the formation of a large number of satellite droplets during the breakup of the solution jet [2]; (3) many parameters can influence the formation of fibers such as solution concentration, surface tension, solvent used, voltage, temperature, and relative humidity. The soft fluctuations of these parameters during the experiments (probably inevitable) may broaden the fiber size distribution. 3.4. Influence of the voltage and the needle tip-tocollector distance on fiber morphology The effects of the voltage and the needle tip-tocollector distance on the morphology of nylon6,66,1010 fibers were also investigated. Continuous and smooth fibers could be formed in the electrospinning voltage ranging from 12 to 21 kV, keeping the polymer concentration at 11 wt% and the needle tip-to-collector distance at 14 cm. However, no
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Fig. 2. SEM photographs and size distribution of electrospun nylon-6,66,1010 fibers as functions of solution concentration at voltage of 18 kV and needle tip-to-collector distance of 14 cm: (a) 6 wt%; (b) 8 wt%; (c) 11 wt%; and (d) 14 wt%.
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obvious difference in the fiber morphology was found in spite that the average fiber diameter decreased slightly with increasing electrospinning voltage. The effect of the needle tip-to-collector distance on the electrospun nylon-6,66,1010 fibers’ morphology is shown in Fig. 3 (at nylon-6,66,1010 solution concentration of 11 wt% and the voltage of 18 kV). Similar to the influence of electrospinning voltage, we found that the fibers morphology hardly changed and the fiber size decreased slightly with the increase of needle tip-to-collector distance. According to the statistic results based on 200 fibers, the fiber’s average diameters were 398 nm at 6 cm, 375 nm at 10 cm, 380 nm at 14 cm and 368 nm at 18 cm, respectively. The slight decrease in fiber size electrospinning with increasing needle tip-to-collector distance due to the longer distance could give more time for the solvent to evaporate and facilitate the charged fluids to split more number of times [25].
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11 wt% nylon-6,66,1010/TFE solution and the membrane electrospun from the corresponding solution is shown in Fig. 4. The casting film and the fiber membrane possessed almost the same spectra, which showed the very obvious characteristic absorption bands of amide groups and methylene segments of polyamides. Their assignments were as follows: 3290 cm 1 (H-bonded N–H stretch vibration), 3080 cm 1 (N–H in-plane bending), 1640 cm 1 (Amide I, C@O stretch), 1540 cm 1 (Amide II, C–N stretch and CO–N–H bend), 940 cm 1 (Amide IV, C–CO stretch), 721 cm 1 ((CH2) > 4, wag), and 690 cm 1 (Amide II, N–H out-of-plane bend). The similar IR spectra of the casting film and the fiber membrane indicated that the molecular structures of the nylon-6,66,1010 sample do not change on the application of high voltage during electrospinning. 3.6. DSC and WAXD
3.5. Infrared spectra A representative infrared spectrum of the casting film prepared by evaporating the solvent from the
From the viewpoint of the commercial applications of the materials, the crystalline properties of the electrospun fibers may be one of the most
Fig. 3. SEM photographs of electrospun nylon-6,66,1010 as a function of the needle tip-to-collector distance: (a) 6 cm, (b) 10 cm, (c) 14 cm, and (d) 18 cm. The nylon-6,66,1010 solution concentration was 11 wt% and the voltage was 18 kV.
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Transmittance
(a)
(b)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber, cm-1 Fig. 4. Infrared spectrum of the nylon-6,66,1010 samples. (a) Casting film prepared by evaporating solvent from the 11 wt% nylon-6,66,1010/TFE solution; and (b) fiber membrane collected from the electrospun 11 wt% nylon-6,66,1010/TFE solution.
important properties. Here, we performed DSC and WAXD experiments on two kinds of samples to evaluate the crystalline properties of nylon-6,66, 1010. One was the electrospun nylon-6,66,1010. membrane, the other was the nylon-6,66,1010 casting film prepared by slowly evaporating the TFE from the corresponding solution. It was difficult for the 6 wt% nylon-6,66,1010 solution to form the fiber membrane due to the existence of the abundant beads. Hence, the 6 wt% nylon-6,66,1010 sample was not studied. The melting enthalpy and melting temperature of the electrospun nylon-6,66,1010 membranes and the corresponding casting films determined by DSC are shown in Table 1. It can be seen that the electrospun nylon-6,66,1010 membranes and the corresponding casting films had almost the same melting temperature (about 145 C), however, the electrospun nylon-6,66,1010 membranes show lower melting enthalpy than those of the corresponding casting films. For example, the melting enthalpy of the membrane electrospun from 8 wt% nylon-6,66,1010 soluTable 1 Melting enthalpy and melting temperature of the electrospun nylon-6,66,1010 membranes and the corresponding casting films Samples
Tm (C)
Hf (J/g)
8 wt% electrospun membrane 11 wt% electrospun membrane 14 wt% electrospun membrane 8 wt% casting film 11 wt% casting film 14 wt% casting film
145.9 145.2 144.9 146.4 146.7 145.3
27.7 29.0 33.5 34.7 37.7 42.9
tion was 27.7 J/g, which was lower than that of the corresponding nylon-6,66,1010 casting film (34.7 J/ g). This indicates that the electrospun nylon6,66,1010 fibers have lower percentage of crystallinity compared with that of the corresponding casting sample. The process of electrospinning, especially the later stages of electrospinning, could be understood as the rapid solidification process of the stretched macromolecular chains under the high elongational rate. The rapid solidification limited the development of crystallinity because the macromolecular chains had no time to form crystalline registration. However, during the preparation of the casting film the macromolecular chains had enough time to adjust their conformations with the evaporation of the solvent slowly and formed the relative high crystalline registration at last. Zong also observed the low crystallinity of the electrospun poly (L-lactic acid) (PLLA) fibers compared with those of the corresponding casting films [21]. The decrease in crystallinity of the electrospun nylon-6,66,1010 fiber membranes compared with that of the corresponding casting films also can be confirmed by WAXD examination. The WAXD results of the 8 wt% and 14 wt% nylon-6,66,1010 samples were taken as the typical examples (Fig. 5). The WAXD patterns of both the nylon6,66,1010 electrospun membranes and the corresponding casting films exhibited two peaks. The two diffraction peaks at about 2h = 20 and 2h = 23 were the (1 0 0) and (0 1 0,1 1 0) of the aphase crystals of typical triclinic form of nylons, respectively [26]. However, the diffraction peaks associated with the electrospun nylon-6,66,1010 membranes were relatively lower and broader compared with the higher and sharp peaks of the corresponding casting films. This probably indicates the easy packing of the crystal in the casting films compared with those of the electrospun membranes. At the same time, it can also be seen that the diffraction peaks of the electrospun nylon-6,66,1010 membranes shift a little toward the low angle direction compared with those of the casting films. According to the Bragg equation [27], the d-spacings of the electrospun membranes were broader than those of the corresponding casting films. This could be explained that the macromolecular chains could not be arranged tightly due to the rapid evaporation of solvent during the electrospinning and formed loose structure. Therefore, WAXD results again proved the lower crystallinity in the electrospun nylon-6,66,1010 membranes.
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Intensity
8wt%
casting film
electrospun membrane
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120% and 155%, respectively. Huang et al. reported that the beads on the fiber surface considerably reduced the cohesive force between the fibers of the electrospun nanofiber membrane and resulted in poorer mechanical properties of the electrospun nanofiber membrane [20]. Hence, the best mechanical properties of the membrane electrospun from the 14 wt% nylon-6,66,1010/TFE solution could be attributed to its good fiber surfaces without beads. 4. Conclusions
10
15
20
25
30
35
40
2θ , degree
Intensity
14wt%
casting film
electrospun membrane
10
15
20
25
30
35
40
2θ , degree Fig. 5. WAXD patterns of the electrospun nylon-6,66,1010 membrane and corresponding casting films.
Acknowledgement This study was financially supported by the Shanghai Nano Special Foundation (0352nm091).
3.7. Mechanical properties It should be noted that the mechanical properties measurements were carried out on the electrospun membranes prepared from the 8–14 wt% nylon6,66,1010/TFE solutions except of the 6 wt% nylon-6,66,1010/TFE solution. The specimen thickness, ultimate strength and elongation at break are shown in Table 2. With the nylon-6,66,1010/TFE solution concentration increasing from 8 wt% to 14 wt%, the ultimate strength and the elongation at break of the electrospun membranes increased Table 2 Tensile properties of the electrospun fiber membranes prepared from the different concentration nylon 6/66/1010/TFE solutions
Specimen thickness (lm) Ultimate strength (MPa) Elongation at break (%)
The nylon-6,66,1010 nanofibers were successfully prepared by electrospinning. The spinnable concentration of the nylon 6/66/1010 solution is in the range of 6–14 wt%. Higher solution concentration favors the formation of uniform ultrafine fibers without beads. The diameter of the electrospun fibers increases with the increase of the solution concentration and decreases slightly with increasing the voltage and the needle tip-to-collector distance. The electrospun nylon-6,66,1010 membranes have lower crystallinity than that of the corresponding casting films. The nylon-6,66,1010 membrane with uniform and no-bead fibers electrospun from the 14 wt% solution exhibits the best tensile strength and elongation at break.
8 wt%
11 wt%
14 wt%
37 4.6 16.7
42 8.0 33.3
39 10.1 42.6
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