Preparation of bamboo-hat-shaped deposition of a poly(ethylene terephthalate) fiber web by melt-electrospinning

Abstract

Melt-electrospinning is an efficient and environmentally friendly technique for producing nano/micro fibers. In the present work, the electric field distribution was analyzed by the use of ANSYS software; four poly(ethylene terephthalate) (PET) melt-electrospun fiber films were prepared within different times; bamboo-hat-shaped deposition on the plating collector was discovered and an enlarged longitudinal sectional view was exhibited for the appearance changes of four films after a series of exact external dimension measurements of fiber films. Due to bulging distribution of the electric field and accumulated charges in the film, fibers with a diameter range from 3 to 7 um were produced in different spinning times and the deposition of PET fibers was bulging out. The surface morphology and structure of PET fibers produced in four consecutive 15-minute periods were investigated by scanning electron microscopy; it was found that fibers become thinner with the expansion of the deposition circle. In addition, porous and self-bonded structures in fiber films appeared late in the spinning process. The differential scanning calorimetry measurement reveals that PET fibers become crystallized and the degree of crystallinity increases with the increase of whipping radius; the melting temperature improves slightly and, by contrast, the cooling crystal temperature of various fiber films decrease late in the spinning process. The technique of melt-electrospinning to fabricate bamboo-hat-shaped fiber films is quite conducive for further researches to achieve three-dimensional structural material in a particular shape.

Keywords

melt-electrospinning electric field simulation poly(ethylene terephthalate) fibers bamboo-hat-shaped deposition morphology and thermal properties

Electrospinning is a process for manufacturing nano/micro fibers in an external electric field that has attracted much interest in recent decades. In general, electrospinning can be performed using either polymer melts or polymer solutions; therefore, solution-electrospinning and melt-electrospinning technique are named, respectively. Due to lack of volatile solvents, melt-electrospinning has more advantages over solution-electrospinning, such as environmental friendliness, low costs and for some applications where solvent toxicity and accumulation are limited.^1–3

Since there is a higher viscosity of polymer melt in comparison with solution,⁴ nowadays the main research work of melt-electrospinning is focused on apparatus to heat polymers,^5–8 provide high voltage^9,10 or modify needles;¹¹ and on parameter optimizations to realize the preparation of sub-micro and even nanofibers and on some applications, such as drug delivery^12,13and protective materials.¹⁴ In addition, there is a new research focus on melt-electrospinning. As melt-electrospinning leads typically to micron diameter fibers, the path of polymer jets that are electrically charged in melt-electrospinning is predictable,¹⁵ with some researchers studying fiber deposition from polymer melts based on this direct writing.^16–18 Meanwhile, the 3D structure or shape of fibers^19–21 or fiber assemblies^22,23 can meet the application needs with their good properties. So, how jets move and deposit with a unique structure by melt-electrospinning is becoming one of the hottest research topics.

Poly(ethylene terephthalate) (PET) is one of the most important semi-crystalline polyesters, with low costs and outstanding performance;^24–26 furthermore, the fiber products are one of the most important applications of PET. Researchers have demonstrated that the PET fiber is a kind of material of great use for the reinforcement. Even PET fibers are considered to be especially adapted to fabricate artificial ligaments for their plasticity, flexibility and high mechanical resistance to rupturing.²⁷ However, there are a few choices about solvents to dissolve PET for solution-electrospinning and, also, those solvents are dangerous.^28,29 Therefore, the melt-electrospinning technique is quite an alternative way for producing PET ultrafine fibers.

In our previous work on melt-electrospinning, the device has be designed by ourselves, and the process parameters of PET melt-electrospinning have been optimized to produce PET ultrafine fibers from 0.9 to 8 micrometers. In addition, there is an apparently bulging core structure in the center of the circular PET fiber film, but the area close to the edge is quite thin, and from center to edge, its thickness gradually decreases, which is very similar to the shape of a bamboo hat. In this study, the electrical field distribution in our melt-electrospinning configuration was simulated by ANSYS software and bamboo-hat-shaped deposition of PET fibers by melt-electrospinning was described. The external dimensions of PET fiber films by melt-electrospinning were investigated and re-exhibited with a longitudinal sectional view after the measurements of productivity, of film radius and of the film thickness in different locations by coating a thickness gauge. Moreover, the surface morphology and structure of PET fiber films were analyzed using a scanning electron microscope (SEM); the thermal properties were studied by using a differential scanning calorimeter (DSC). The results suggest that there is a bulging distribution of electrical field and, during the melt-electrospinning process, the polymer jet deposits with expanded layers in the shape of a bamboo-hat; the PET fibers become thinner and there is more porosity and self-bonding in fiber films late in the spinning process; PET fibers show higher melting temperature than raw PET chips and get much crystallization late in the spinning process. This bamboo-hat-shaped ultrafine fiber film is expected to be used as a three-dimensional (3D) structure material, such as cell scaffolds for tissue engineering. The environmentally friendly and efficient technique of melt-electrospinning for fabricating 3D structure fiber films is quite conducive to achieving materials in other concrete shapes.

Experimental details

Materials and setups

The intrinsic viscosity of the PET chips used in the present study was 0.66 dL/g, purchased from Jiangsu (China) and vacuum dried at 140℃ for 24 h before spinning. The experimental setup (Figure 1) included a high-voltage supply (DW-P203-10AC, China) connected to the metal nozzle, a custom-made melt reservoir made of copper, an air compressor (Tp751, China), two adjustable heating devices for the polymer melt and environmental temperature, respectively, and a square plate collector(from aluminum, side length 15 cm). The polymer was molten in a metal hopper surrounded by a circular heating coil separated by insulation material; the flow rate was controlled by a valve connecting the air compressor to the top entry of the reservoir. The hot-coil and thermocouple temperatures were adjustable up to 320℃ and 100℃, respectively. With the use of a spotlight, good observation and capture of jet motion were better achieved.

Simulation of electric field

The ANSYS software (ANSYS Inc., USA) was employed and a two-dimensional (2D) axisymmetric finite element model was used to simulate the electric field on account of the symmetric structure of our setup (Figure 1). The finite element model of the spinning system mainly contained a metal nozzle, aluminum collector and atmosphere, as shown in Figure 2. Herein the origin o represents the center of the plating aluminum collector; the x-axis was parallel to the collector and the y-axis was perpendicular to the collector, respectively. The vertical distance between the nozzle and the collector was 50 mm; the length of the collector was 150 mm. High voltage, 30 kV, was applied to the nozzle at the excitations, and the aluminum foil collector and the boundaries at an infinite distance were set as zero potential. After meshing and solving by the software, the electric field strength and its distribution were established.

Melt-electrospinning

According to the previous work, to determine a consecutive spinning process, the desirable process parameters consisted of the voltage (30 kV), the distance between the nozzle and collector (5 cm), the flow rate (4 kPa), the melt temperature (255℃) and environment temperature (40℃). In this experiment, to investigate the change of fiber deposition during the melt-electrospinning process, under the above process parameters and relative humidity (58%), four kinds of PET fiber films were produced within four defined times, set at 15, 30, 45 and 60 minutes. Subsequently, the melt-electrospun fiber films were removed along with the aluminum foil from the plating collector.

Observation of jet motion and fiber deposition

To record polymer jet motion during the melt-electrospinning process and appearance changes of the fiber deposition after a defined time, two high-speed cameras (KEYENCE vm-6000, Japan) at a rate of 60 frames per second (fps) were used to capture the jet path and dimensions of fiber films from two different directions.

Characterization

External dimensions of the PET melt-electrospun fibers films

The four PET melt-electrospun fibers films were photographed with a camera and then measured from the photos with AutoCAD software; thus, the changes of the fiber film radius were obtained. The weights of films in different depositing times were weighted by a high-precision electronic balance (JK-2104N, China). The locations for thickness measurement were marked with dots at 5 mm intervals along the radical direction from the center to the edge of each fiber film and then the thickness of different dots distributed in fiber films were acquired with a coating thickness gauge (Fischer MP0R, USA). Eventually the shape mode of fiber deposition was built by the AutoCAD software.

SEM observation

The PET melt-electrospun fibers were observed with a SEM (Hitachi SU-1510, Japan). Each sample was sputter coated with gold for analysis. Five SEM images taken from different sample locations in each film were used to measure the fiber diameter. The mean diameter of the fiber was calculated using 100 measurements that were randomly selected. The AutoCAD software was used to determine the fiber diameters.

DSC investigation

The thermal properties of raw PET chips and PET films melt-electrospun in different times were investigated with the DSC (Q200, China). Approximately 10 mg of each sample was used for the DSC study to avoid the possible thermal lag, the flow rate of N2 was set at 25 ml/min, and the DSC curves were recorded from 10 to 280℃ with a scanning rate of 10℃/min. The experiments were repeated three times on each sample; hence, the average value was presented. The enthalpy of crystallization and the enthalpy of melting were calculated based upon the areas under the solid–liquid phase change peaks of PET using the thermal analysis software affiliated with the equipment.

Results and discussion

Electric field simulation

The vectors and voltage distribution of the electric field of melt-electrospinning were calculated by the ANSYS software, as is shown in Figure 3. The arrows show the direction of the electric field, and their length is proportionate to the strength at that position; the color map indicates the voltage distribution. It is easily seen that there is an axisymmetric and unevenly electric field created in this electrospinning configuration. Figure 4(a) shows the electric potential and electric field strength along the y-axis; the maximum electric field strength (3012.3 V/mm) is concentrated on the nozzle. In the area where y < 35 mm, the potential and electric field strength both increase linearly, while in the area where 35 < y < 50 mm, the potential and electric field strength both increase sharply to the maximum. Figure 4(b) indicates the electric field strength along the x-axis at four different levels above the collector. We can find that even at the same level, the values of electric field strength are quite different. As for these four levels, they all have a highest value appearing in the center where x = 0 and then they all decrease along the x-axis. Also there is a turning point in the area about 35 < |x| < 40 mm, in the center area (|x| < 35 mm), that is, the higher above the collector, the higher the electric field strength; in the edge area, the strength comparison results of these four levels are reversed. So there is a bulge in the electric field strength distribution at the same level in this melt-electrospinning configuration.

The external dimensions and shape of fiber films in different depositing periods

In this work, fiber films were deposited and collected on plating aluminum foils. The representative external dimensions of fiber film deposited within different time are shown in Figure 5(a)–(d). Obviously, there is a trait in common in these four images: the conical bulge in the center of the circular fiber films. Moreover, as the depositing time goes on, the larger the deposition becomes and the larger the bulge that forms.

To investigate the special shape of these fiber films, a series of measurements of external dimensions was carried out, including radius, weight and thickness of the PET fiber films. The average radii of these four fiber films are 28.1, 37.0, 43.0 and 52.2 mm, respectively. Figure 5(e) reveals the changes in weight of each deposition within different times and, in addition, the differences in weight are also marked in red. In Figure 5(e), the weight of the fiber film or productivity of melt-electrospinning increases non-linearly; within the forth 15-minute period from the 45th minute to the 60th minute, there are many more fibers depositing on the collector; this is inconsistent with the expanding rate in the radius of each 15 minutes. In consideration of circular deposition of all films with high symmetry in this technique of melt-electrospinning, the thickness of film could be obtained along the direction of the radius to simplify the experiment. Points being measured were marked at 5 mm intervals in a line from the center to the edge of each film, as shown in Figure 5(f)–(g), and then the average thickness of points located in the circle of the same radius at fiber films were examined by coating a thickness gauge 10 times; herein, the thickness of all points in different radii of each fiber film were acquired, as shown in Figure 6. Finally, the longitudinal sectional view of fiber film with superimposed layers as the time went on was drawn from the actual ratio of x and y by AutoCAD software, as shown in Figure 7.

PET fiber film with this bulging structure in melt-electrospinning can be attributed to the uneven electric field distribution and accumulated charges in the fiber film. In the case of melt-electrospinning, external high voltage is applied to the nozzle and then a polymer melt droplet will be charged so that the surface tension of the melt droplet surrenders to the electrical forces and meanwhile the jet is initiated.³⁰ Until the jet gets earthed, electric charges are stored among the jet. At the beginning of the deposition, the falling jet touches the conductive aluminum foil directly and the charges can be wiped out easily. However, with the increase of jet deposition, the late deposited fibers are separated gradually from the foil by former deposited fibers. In consideration of the non-conductive polymer of PET, it is reasonable that the electric charges are accumulated among fiber film little by little; herein, the potential of the film becomes larger. Meanwhile, because of the bulging distribution of the electric field in this spinning configuration, the falling jet then moves by the repulsive force towards the edge of the deposited fiber film where the potential is relatively low, resulting in the larger potential difference and electric field strength between nozzle and edge. Then deposition becomes expanded bit by bit. Finally, with the expansion of the fiber deposition, the charges spread over the edge film and the electric field strength in the center of film become larger again than the edge, so fibers are willing to deposit in the center. As a result, the thickness of the center area of the film is larger than the edge film and there is a bulging PET fiber film in the shape of bamboo-hat forms.

Surface morphology and structure of four kinds of fiber films in different depositing times

We randomly take fibers deposited near the edge of circular film as samples to evaluate the morphologies of four kinds of fibers films generated in varies time periods using a SEM. The representative morphologies of melt-electrospun PET fibers deposited in different time periods are shown in Figure 8. The fibers are cylindrical with smooth surfaces and distribute randomly in each melt-electrospun PET fiber film; the fiber diameters range from 3 to 7 µm in four films, but there are quite uniform diameters in each film. Moreover, there are more porous and self-bonded structures in fiber film in late deposition, as shown in Figure 8(d).

As the depositing time goes on melt-electrospun fibers tend to be thinner, but there appears to be a fluctuation when the depositing time is set at 45 minutes and the thinnest fibers are manufactured. This means that the polymer melt becomes the largest drawn in that time period. Such a result might be attributed to the change of electric field, stretch distance and the rate of polymer fed on the nozzle. As the consecutive fibers deposit on the plate gradually, the circle of fiber film becomes larger and much thicker due to the accumulated electric charges as the above context mentioned and with that the polymer jet is forced to be more inclined to keeping away from the center; thus, the jet experiences more whipping (Figure 9 illustrates the rising radius of the polymer jet whipping motion at different time points, where r means the whipping radius from center to depositing position) and subsequently fibers are thinner. However, high-speed drawing induces the increase of the feeding rate of polymer melt to nozzle.³¹ According to the law of mass conservation and in condition of no polymer decomposition, the more polymer is fed, the more fibers will deposit. In Figure 5, it is easy to find that the weight of the 60-minute deposition is much larger than that of the former 45-minute deposition, that is to say, much more polymer melt is fed to the nozzle; in addition, the electric field strength of the outer position where it is too far away from center appears relatively weak (shown in Figure 4). So it is another circumstance whereby the diameter of fiber deposits in 60 minutes become larger slightly after two continual declines.

The porous structure in the late depositing process is also owing to the low charge-transfer efficiency. An increasing number of charges accumulating among thinner fibers are unable to be transferred away to air, so repulsive force would expand the space between deposited fibers, herein making it more porous in fiber films. The reason for self-bonded behavior between fibers is that the heat cannot be dissipated out in a timely manner in the deposition process in the case of the high-speed spinning with a large number of polymer melt and high-speed depositions. On the fast-spinning fibers deposit in a very short time, then the following fibers cover the former fibers immediately, in which the former fibers do not cool perfectly in this short time, resulting in bonding-structure. However, there may be potential use of this fiber film with a porous and self-bonded structure.

Thermal properties of raw PET chips and four kinds of PET fibers deposited in different times

Figure 10 shows the thermal behavior of the four kinds of PET melt-electrospun fibers generated in different times, and they are compared with raw PET chips; the degree of crystallinity $χ c$ (%) and temperatures of major peaks from DSC thermograms of various PET fibers are summarized in Table 1. Only one peak can be found in the DSC curve of raw PET chips, while two peaks are easily found in each curve of the electrospun fiber film and these four curves appear similar to each other (Figure 10). After drying in a vacuum oven at 140℃ for 24 h, there is no glass transition temperature or cooling crystallinity exhibiting in the raw PET chips curve, since the crystal structure becomes perfect gradually during the drying process,³² and the melt point is at about 254℃ with melting enthalpy of 43.66 J/g; therefore, the crystal degree of PET chips is quite high (Table 1). From the DSC curves of four PET fibers, it can be observed that the peak of cooling crystal temperature decreases with the expanded whipping radius, while the melting point improves. The cooling crystal enthalpies of four PET fibers are 27.12, 32.43, 30.79 and 31.19 J/g, respectively, and their melt enthalpies are 48.89, 55.19, 54.45 and 56.24 J/g, respectively. Then the degree of crystallinity of raw PET and its product could be calculated based on the following equation:

χ c = \frac{Δ H f - Δ H c}{Δ H f 100 %} \times 100 %

here

Δ H f

is the heat of fusion,

Δ H c

is the heat of cold crystallization and

Δ H f 100 %

is the heat of fusion for 100% crystalline PET 140 J/g.³¹ According to Table 1, the crystallinity

χ c

of various depositions in different times in the spinning process shows an upward trend, except PET chips, which are dealt with by long-time pretreatment in a vacuum oven. The reason is that different times in the spinning process means different whipping radii of the jet with the expansion of circle deposition, so in the late spinning process, the whipping degree is very large; therefore, the fibers become long-time and exhibit full drawing, easily leading to obtaining better orientation. Perfect drawing increases the regularity of molecular chain sequence in PET. Consequently, the crystallization induced by orientation happens and the fiber films get larger crystallinity in the late melt-electrospinning process. To some extent, improving the melt point in these four fiber films is consistent with a high degree of crystallinity making the high melt point. In contrast, the cooling crystal peak decreases and this means that the fibers crystallize easily.

Table 1.

Summary of differential scanning calorimeter curves for different poly(ethylene terephthalate) (PET) fibers.

Chips/fibers	T_c (℃)	T_m (℃)	$χ c$ (%)
Raw PET chips	N/A	254.03	31.19
First 15-minute deposition	128.6	254.03	15.55
Second 15-minute deposition	124.3	254.08	16.26
Third 15-minute deposition	123.1	254.28	17.62
Forth 15-minute deposition	122.7	254.45	17.89

T_c: cooling crystal temperature; T_m: melting temperature.

Conclusion

Electric field distribution in melt-electrospinning was analyzed by the ANSYS software and four PET melt-electrospun fiber films were prepared within different times, 15, 30, 45 and 60 minutes, respectively. The external dimensions of these four fiber films were measured, including radius and weight of film and thickness of sampling points along the radial direction. Further, an enhanced longitudinal sectional view was obtained and the overlaid depositions in every 15-minute time period were marked in a different color. The bamboo-hat-shaped deposition of PET fibers on the plate in melt-electrospinning was demonstrated with the analysis of a combination of electric field and accumulated electric charges. The surface morphology was investigated using a SEM. SEM images reveal that the fibers are cylindrical with a smooth surface and are quite uniform in diameter and deposit randomly; fibers become thinner near 3 µm as the spinning time goes on. More porous and self-bonded structures in the late spinning process are also illustrated in the SEM images. The DSC measurement indicates that PET fibers become crystallized in spinning and the degree of crystallization increases gradually. Meanwhile, the melting point of fibers also improves with the increase of the whipping radius; however, the cooling crystal temperature decreases slightly during the melt-electrospinning process. This special structural ultrafine fiber film is promising to be used as 3D cell scaffold in tissue engineering; the technique of melt-electrospinning to fabricate bamboo-hat-shaped fiber films is quite conducive for further research to achieve 3D structural material in a certain shape.

Footnotes

Funding

This work was supported by the Nation High Technology Research and Development Project of China (863; No.2012AA030313) and in part by the Natural Science Foundation of Jiangsu Province (No. SBE201201094).

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