Influence of Working Temperature on The Formation of Electrospun Polymer Nanofibers
© The Author(s). 2017
Received: 14 September 2016
Accepted: 24 December 2016
Published: 19 January 2017
Temperature is an important parameter during electrospinning, and virtually, all solution electrospinning processes are conducted at ambient temperature. Nanofiber diameters presumably decrease with the elevation of working fluid temperature. The present study investigated the influence of temperature variations on the formation of polymeric nanofibers during single-fluid electrospinning. The surface tension and viscosity of the fluid decreased with increasing working temperature, which led to the formation of high-quality nanofibers. However, the increase in temperature accelerated the evaporation of the solvent and thus terminated the drawing processes prematurely. A balance can be found between the positive and negative influences of temperature elevation. With polyacrylonitrile (PAN, with N,N-dimethylacetamide as the solvent) and polyvinylpyrrolidone (PVP, with ethanol as the solvent) as the polymeric models, relationships between the working temperature (T, K) and nanofiber diameter (D, nm) were established, with D = 12598.6 − 72.9T + 0.11T 2 (R = 0.9988) for PAN fibers and D = 107003.4 − 682.4T + 1.1T 2 (R = 0.9997) for PVP nanofibers. Given the fact that numerous polymers are sensitive to temperature and numerous functional ingredients exhibit temperature-dependent solubility, the present work serves as a valuable reference for creating novel functional nanoproducts by using the elevated temperature electrospinning process.
Electrostatic energy has gradually increased its share from other types of energies (such as mechanical energy, acoustic energy, and thermal energy) in creating nanoproducts through a top-down manner. Popular examples of such nanoproducts are electrospun nanofibers and electrosprayed nanoparticles [1–3]. In these electrohydrodynamic methods, electrostatic energy performs a dominant function in generation; however, other energies, such as thermal, radiant, and mechanical energies, can be combined into the process for an effective production [4–6].
Currently, the development of electrospinning focuses on two directions. One is the large-scale production of electrospun nanofibers for commercial products through edge, multiple-needle, needle-less, and slit electrospinning [7–9]. However, few studies tackled cost reduction and experimental optimization. A recent publication has demonstrated that the reasonable utilization of spinneret material (polypropylene) can save electric energy and improve the aligned effect of electrospun nanofibers . This paper demonstrates a concept that proposes a substantial development space for optimizing experimental or production conditions to create high-quality nanofibers in an economical manner. Among these conditions, such as applied voltage, fluid flow rate, fiber-collecting distance, environmental humidity, and temperature, working fluid temperature has received the least attention in terms of influence on the formation of electrospun polymer nanofibers from solution electrospinning, although reports regarding melt electrospinning can be found in publications .
Another development direction for electrospinning is to generate novel types of nanostructures and nanofibers. These nanostructures include the popular core–sheath fibers, tri-layer nanofibers, Janus nanofibers, and the complicated nanostructures from a combination of core–sheath and Janus [12–15]. However, only slightly over 100 polymers can be electrospun into nanofibers under ambient temperature, thus considerably limiting the potential applications of multiple-fluid electrospinning in creating novel nanostructures and functional nanoproducts. A reasonable selection of temperature of the working fluids can be a useful tool for nanofabrication through electrospinning processes. First, some semicrystalline polymers (such as polyethylene and polypropylene) are dissolved in solvents only at an elevated temperature. Elevated temperature electrospinning creates new types of polymer nanofibers . Second, elevated temperature logically decreases the viscosity of polymer solutions but exerts minimal influence on physical entanglements, which particularly benefit the formation of electrospun nanofibers from polymer species with ultra-high molecular weights . Third, numerous functional nanofibers are created by adding a guest active ingredient into a host filament-forming polymer matrix, whereas numerous ingredients, such as numerous poorly water-soluble drugs, exhibit temperature-dependent solubility . Thus, elevated temperature electrospinning has the potential to expand the capability of electrospinning to generate new functional nanofibers and nanostructures.
Although temperature is a key parameter in electrospinning, the related reports are extremely limited, which is possibly correlated with the simple implementation of electrospinning under ambient conditions. The key in running elevated temperature electrospinning lies in heating and maintaining the working fluid at a constant temperature different from ambient conditions. Steven et al.  utilized a ceramic infrared emitter to manipulate solution temperature up to 110 °C during electrospinning. They declared that infrared flux on the polyethylene solution from the emitter can be precisely controlled by both the variable output controller and the distance between the emitter and the glass syringe. Wang et al.  reported a jacket-type heat exchanger that was exploited to control the temperature of solutions containing polyacrylonitrile (PAN) in dimethylformamide up to 88.7 °C. A circulation of heated silicone oil by a pumping system connected to an oil bath was utilized to adjust the working temperature. Considering the applications of an electric heating film and a temperature regulator, Yu et al. developed an auxiliary heating system to maintain a constant temperature of working fluids for preparing medicated nanofibers [17, 18] and drug-loaded composite microparticles . Desai and Kit  conducted elevated temperature electrospinning to prepare beadless composite nanofibers consisting of chitosan and polyacrylamide. The working temperature of 70 °C was maintained through the circulation of hot air around the syringe and needle. Kin et al.  prepared cellulose nanofibers from its solutions in a mixture of N-methylmorpholine oxide and water through elevated temperature electrospinning but provided no detailed information on the heating unit. De Vrieze et al.  investigated the effects of temperature and humidity on electrospun cellulose acetate nanofibers. The working temperature was adjusted using a polymethylmethacrylate chamber to house the electrospinning system, and the whole setup was placed in a temperature-controlled room, with variations only from 273 to 303 K. These approaches (including infrared radiation, direct heating, indirect heating through flowing air/oil/water, or even storage in a constant temperature room) can be considered when implementing elevated temperature electrospinning over a range of working temperature.
The above-mentioned publications successfully demonstrated the usefulness about the combined utilization of thermal energy and electrostatic energy in creating polymeric nanoproducts with an elevated temperature of the working solutions. However, none of these works systematically investigated the influence of temperature on fiber formation. In the present work, polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) were utilized as the model filament-forming polymers. A synthetic and semicrystalline organic polymer resin with the linear formula (C3H3N)n, PAN is a versatile polymer used to produce a large variety of products, including ultrafiltration membranes, hollow fibers for reverse osmosis, and fibers for textiles. This polymer is used as the chemical precursor in 90% of high-quality carbon fiber production and is also extensively used in electrospinning; PAN nanofibers are good precursors for preparing carbon nanotubes (CNTs) [23, 24]. PVP is a water-soluble polymer made from the monomer N-vinylpyrrolidone. This polymer is soluble in water and other polar solvents. PVP is used as binders in pharmaceutical tablets and as additives in batteries, ceramics, fiberglass, inks, and inkjet paper; this polymer is also used in the production of membranes, such as dialysis and water purification filters, as well as in the solubility enhancement of poorly water-soluble drugs [25, 26]. PAN and PVP are preferred not only because of their extremely broad applications in a wide variety of fields but also because of their special electrospinnability. PAN possesses spinnability in the aprotic solvent N,N-dimethylacetamide (DMAc), which displays a high boiling point of 166 °C. PVP features good spinnability in the typical protic solvent ethanol, with a boiling point of 78.4 °C. The two working solutions can represent almost all types of polymer solutions exploited for electrospinning in creating the corresponding nanofibers.
PAN powders (MW = 80,000) were obtained from Shangyu Baisheng Chemical Technology Co., Ltd. (Shaoxing, China). PVP K90 (MW = 360,000) was purchased from BASF Shanghai Co., Ltd. (Shanghai, China). N,N-Dimethylacetamide (DMAc) and anhydrous ethanol were provided by Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals used were of analytical grade.
Working Fluids and Electrospinning
Two electrospinnable solutions were prepared to implement elevated temperature electrospinning. One solution contains PAN in DMAc with a concentration of 15% (w/v). To ensure a homogeneous working fluid, the PAN solution was agitated over 12 h at 80 °C and then was cooled to the ambient temperature. The other solution was PVP K90 in anhydrous ethanol with a concentration of 9% (w/v) and was prepared under ambient conditions.
Spinning parameters for preparing PAN and PVP nanofibers
20 ± 1
40 ± 1
60 ± 2
80 ± 2
20 ± 1
30 ± 1
40 ± 1
50 ± 1
60 ± 2
The morphology of electrospun fibers was assessed through field-emission scanning electron microscopy (FESEM; Quanta FEG450, FEI Corporation, Hillsboro, OR, USA). Prior to examination, samples were sputter-coated with platinum to prevent charging during FESEM imaging. ImageJ software (National Institute of Heath, Bethesda, MD, USA) was utilized to measure the fiber diameter from SEM micrographs. For each sample, nanofiber size was measured at over 100 points.
The surface tensions and viscosities of the PVP solution were measured as a function of working temperature. The former was carried out with a BZY-1 Surface Tension Tensiometer (Shanghai Hengping Instrument and Meter Factory, Shanghai, China). The latter was conducted using a NDJ-279 rotary viscometer (Machinery and Electronic Factory of Tongji University, Shanghai, China). An HZBZ-08 Automatic saturated vapor pressure measuring instrument (Shanghai Xu-Ji Electric Co., Ltd., Shanghai, China) was exploited to measure the saturated vapor of anhydrous ethanol at different temperatures (20.0–78.0 °C) by using a static method .
Results and Discussion
Implementation of Elevated Temperature Electrospinning
In the present work, the inner structure of the auxiliary apparatus is shown in Fig. 1b. The ambient temperature was maintained at 20 °C under room air conditioners. Other temperature levels exceeding this value were achieved by the heating film. Before applying high voltages to commence electrospinning, the working fluids were first equilibrated for half an hour at the pre-determined temperature. The auxiliary temperature-controlled accessory possessed good temperature-regulated accuracy with a fluctuation of ±2 °C.
Influence of Temperature on The Formation of PAN Nanofibers
The data were fit according to different traditional equations, including linear equation (y = ax + b), power function equation (y = ax b), and one-variable quadratic equation (y = ax 2 + bx + c). Among these equations, the quadratic equation D = 12598.6 − 72.9T + 0.11T 2 showed the best fitting result, with a correlation coefficient of 0.9988 (Fig. 4). In the above equation, D represents the average diameter of nanofibers (nm) and T represents the working temperature (K). According to this equation, an inflection point can be found at T = 342 K, i.e., 69 °C. Under this working temperature, the thinnest PAN nanofiber with a diameter of 250 nm was achieved (as indicated by the red star in Fig. 4).
Wang et al.  produced PAN fibers with a diameter of 65 nm–85 nm from a 6% PAN solution (no information about molecular weight) at a working temperature of 88.7 °C. A scaling law of d = 3.0η 0.74 (d is the fiber diameter and η is the viscosity at the working temperature) was deduced. Thus, they concluded that high temperature induces the production of ultrathin fibers. According to their equation, higher working temperature indicates smaller η of the working fluid and, thus, smaller diameter of resultant nanofibers. The fundamental explanation that supports this result is that an increase in temperature would decrease the viscosity of polymer solutions but exert minimal influence on physical entanglements, which act to prevent capillary breakup for the formation of linear electrospun nanofibers from polymer solutions. Evidently, our results do not agree with this declaration.
The faster solvent evaporation under high temperatures than under ambient conditions should create different rigid PAN fiber precursors (which could not be further drawn under the electrical fields) with less content of the residual DMAc. Logically, the later evaporation of the solvent from the native polymeric nanofibers assembled on the collector would deform the final nanofibers, often generating a rough surface owing to barometric pressure . Thus, the final PAN fibers electrospun at high temperatures exhibited smaller surface indentations and a relatively smoother surface morphology compared with those electrospun at relatively low working temperatures (Fig. 5).
Influence of Temperature on The Formation of PVP Nanofibers
To investigate further the influence of temperature and the related solvent evaporation on the formation of nanofibers, a different solution system consisting of PVP dissolved in anhydrous ethanol (boiling point of 78.4 °C, a protic and volatile solvent, different with DMAc, which is an aprotic solvent with a high boiling point of 166 °C) was explored on the elevated temperature processes. The preparations of PVP nanofibers F5, F6, F7, and F8 at ambient temperature 30, 40, and 50 °C could be implemented continuously and robustly. However, electrospinning at a high temperature of 60 °C for producing PVP nanofiber F9 was fragile and unsteadily. The generation of ethanol vapor from the PVP solution induced the separation of the working fluids into two separate phases in the glass syringe. In the PVP solution, the separated ethanol vapor caused intermittent stoppage of the spinning process and sometimes spurted a pool of liquids.
where p s is the saturated pressure, T s is the temperature, d is the solvent molecular diameter, and k is the Boltzmann constant.
As evidently seen from the equation, the elevation of temperature will result in a thicker Knudsen layer. However, the increase in temperature will simultaneously increase the saturated pressure and consequently result in a thinner Knudsen layer. As the temperature was gradually increased, p s increased faster until the boiling point (Fig. 9b). During temperature increase, the value of T s/p s (which directly reflects the thickness of the Knudsen layer) became smaller and smaller (Fig. 9c). The Knudsen layer thickness presents a virtually linear relationship to temperature, i.e., y = 4.1375 − 0.0455x (R = −0.9898), where y is T s/p s and x is the temperature. At 60 °C, the thickness of the Knudsen layer was only 38% of that at 20 °C. Thus, higher working temperature indicates smaller T s/p s value and thinner Knudsen layer, corresponding to the faster evaporation of the solvent and solidification of the PVP fluid jets. The premature solidification of the fluid jet under an excessively high temperature will exert a negative influence to stop the drawing under the electric field, thereby producing fibers with large diameters.
With PAN solutions in DMAc and PVP K90 solutions in ethanol as the model working fluids, elevated temperature electrospinning processes were successfully carried out to synthesize nanofibers under a series of working temperatures by using a homemade electrospinning system. Regardless of protic solvent (ethanol) or aprotic solvent (DMAc) and their volatilization property, elevated working temperature generated both positive and negative influences on the formation of polymer nanofibers. Temperature elevation decreased surface tension and viscosity, which consequently resulted in facile electrospinning and downsized resultant products. However, temperature elevation also directly influenced the electrical drawing during electrospinning by accelerating the evaporation of the solvent from fluid jets. Excessively high working temperatures led to the premature termination of the electrical stretching of polymer fluid jets, thus exerting a negative influence on the produced fibers with large diameters. The PAN and PVP nanofibers produced under reasonable conditions exhibited fine linear morphology without any observed beads-on-a-string or spindles-on-a-string phenomenon. The fitting equations for PAN and PVP nanofibers are D = 12598.6 − 72.9T + 0.114T 2 (R = 0.9988) and D = 107003.4 − 682.4T + 1.1T 2 (R = 0.9997), respectively. Given the fact that numerous polymers are sensitive to temperature and numerous functional ingredients exhibit temperature-dependent solubility, the present work serves as a valuable reference for creating novel functional nanoproducts through elevated temperature electrospinning. Manipulating the working temperature can also be combined into the coaxial, side-by-side, and tri-axial electrospinning processes to extend the applications of these techniques in creating novel functional nanomaterials.
This work was supported by the National Natural Sciences Foundation of China (No. 51373101), the College Student Innovation Project of USST (No. XJ2016234) and the Key Project of the Shanghai Municipal Education Commission (No.13ZZ113).
G-ZY and J-HY conceived the idea of the project. G-ZY, H-PL, and JW carried out the experiments. D-GY and G-ZY drafted the manuscript. D-GY supervised the project. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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