WO2018098464A1 - Systems and methods of production and use of thermoplastic and thermoplastic composite nanofibers - Google Patents

Abstract

Systems and methods discussed herein are directed towards forming a plurality of melt-spun polymer-based fibers without using a solution or solvent to dissolve the polymer-based material prior to melt-spinning. The fibers may be fabricated using an extruder setup or a coated source wire or wires to directly form the fibers, which may be collected on a stationary or rotating target in an ordered or random array, and which may be removed for further use or processing.

Description

SYSTEMS AND METHODS OF PRODUCTION AND USE OF

THERMOPLASTIC AND THERMOPLASTIC COMPOSITE NANOFIBERS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application Serial No. 62/427,102 filed November 28, 2016, and entitled "Systems and Methods of Production and Use of Thermoplastic Nanofibers," which is hereby incorporated herein by reference in its entirety. This application also claims benefit of U.S. provisional patent application Serial No. 62/582,786 filed November 7, 2017, and entitled "Systems and Methods of Production and Use of Thermoplastic and Thermoplastic Composite Nanofibers," which is also hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This research was sponsored by the National Science Foundation under grant

NSF CMMI-1538048.

BACKGROUND

[0003] Polymer fibers may be employed for a variety of industrial and medical applications, the range of applications may be based in some cases on the type and dimensions of fibers produced.

BRIEF SUMMARY OF THE DISCLOSURE

[0004] In an embodiment, a method for fabricating solvent-free polymer fibers, comprising: heating a source wire by applying a first voltage across a coil disposed about the source wire; melting, in response to heating the source wire, a material disposed on the source wire as a layer; and generating an electrostatic field between the source wire and a target by applying a second voltage between the source wire and the target; and drawing the melted material from the source wire and toward the target in response to the electrostatic field to form a fiber. In an embodiment, the material comprises a polymer that may be a thermoplastic polymer, and may further comprise a one-dimensional or a 2-dimensional material distributed in the polymer. In an embodiment, the material comprises about 1 wt% to about 10 wt% of the one- dimensional or the two-dimensional material. In an embodiment, the method further comprises collecting the fiber on the target and/or rotating the target while collecting the fiber. An average diameter of the fiber fabricated via the method is less than 20 microns, and in some examples less than 10 microns, and the source wire is vertically oriented or oriented at an acute angle relative to horizontal.

[0005] In an embodiment, an alternate method for fabricating solvent-free polymer fibers, comprising: (a) heating a source wire; (b) melting a layer of a material disposed on the source wire in response to (a); (c) generating an electrostatic field between the source wire and a target which may be rotating; (d) drawing the melted material away from the source wire and towards the target to form a plurality of fibers; and (e) collecting the plurality of fibers on the target. In an embodiment, (a) comprises: applying a first voltage across a coil disposed about the source wire, the first voltage is between 0.10 V and 5.0 V, and (c) comprises applying a second voltage between the source wire and the rotating target to generate the electrostatic field, and the second voltage is between 10 and 40 kV. In an embodiment, the material comprises a theremoplastic, and in some embodiments the material comprises a plurality of 1 D or 2D materials, wherein the plurality of 1 D or 2D materials are aligned in the plurality of fibers collected in (e) to comprise at least one of a target thermal, mechanical, or magnetic property, and an average diameter of the plurality of fibers is less than 10 microns.

[0006] In an embodiment, a method for fabricating solvent-free polymer fibers, comprising: disposing a plurality of polymer pellets in an extruder coupled to a melt- spinning apparatus; melt-spinning the plurality of polymer pellets to form a plurality of melt-spun fibers in an air temperature from about 20°C to about 400°C, wherein the plurality of melt spun fibers do not comprise a solvent. In an embodiment, the method further comprises reducing, prior to melt-spinning, an average size of the plurality of pellets, wherein an average size of the plurality of pellets after the reducing is at least 1/10 of an initial average size, wherein the plurality of pellets comprise polylactic acid (PLA), wherein each pellet of the plurality of pellets disposed in the grinder comprises a maximum diameter of 5 mm, and wherein the plurality of pellets disposed in the grinder comprises a mass of less than 55 mg. In an embodiment, the method further comprises forming the plurality of melt spun fibers each comprising a fiber diameter, wherein a ratio of the average fiber diameter to a diameter of the plurality of pellets is from 1 : 10 to 1 :1000 and weaving the plurality of melt-spun fibers into a fabric.

[0007] Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a detailed description of the exemplary embodiments disclosed herein, reference will now be made to the accompanying drawings in which:

[0009] FIG. 1 is a schematic view of an embodiment of a system for fabricating polymer fibers in accordance with principles described herein;

[0010] FIG. 2 is a microscopy image of polymer fibers manufactured according to certain embodiments described herein;

[0011] FIG. 3 is a graphical illustration of the fiber diameter as a function of the length of the collected fiber on a spool according to certain embodiments described herein;

[0012] FIG. 4 is a graphical illustration of the fiber diameter as a function of the extrusion flow rate according to certain embodiments described herein;

[0013] FIG. 5 is a photograph of an embodiment of a system for fabricating polymer fibers in accordance with principles described herein;

[0014] FIG. 6 is graphical illustration of the fiber diameter as a function of the amount of time the fiber was electrospun according to certain embodiments described herein;

[0015] FIG. 7 is graphical illustration of the diameter of the melt-electrospun fibers as function of pellet size according to certain embodiments described herein; [0016] FIG. 8 is a histogram of the melt electrospun fiber diameters for three pellet sizes (the largest, the smallest and the pellet with the median size) according to certain embodiments described herein;

[0017] FIG. 9 is a is graphical illustration of a polymer jet radius profile the dimensionless final fiber radius as compared to the dimensionless spinline coordinate at various processing temperatures;

[0018] FIG. 10 is a graphical illustration of the dimensionless final fiber radius as a function of location of heating according to certain embodiments described herein;

[0019] FIG. 1 1 is a graphical illustration of the dimensionless final polymer fiber radius obtained on the collector as a function of location of heating according to certain embodiments described herein;

[0020] FIG. 12 is a graphical illustration of the dimensionless final fiber radius as a function of air temperature (T_air) according to certain embodiments described herein;

[0021] FIGS. 13A and 13B are schematic view of embodiments of melt-electrospinning systems in accordance with the principles described herein;

[0022] FIG. 14 is a schematic view of an embodiment of a system for fabricating polymer fibers in accordance with principles described herein;

[0023] FIG. 15 is a graphical and visual comparison of the fiber diameters of fibers manufactured with the system of FIG. 14 and conventional systems of edge-based and syringe-based melt electrospinning; and

[0024] FIG. 16 is a graphical illustration of the thickness of a layer of material on a source wire as compared to a time of electrospinning the fibers without the fibers breaking during manufacture of fibers with the system of FIG. 14.

DETAILED DESCRIPTION OF THE DISCLOSED EXEMPLARY EMBODIMENTS

[0025] The following discussion is directed to various exemplary embodiments. However, one of ordinary skill in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

[0026] The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

[0027] In the following discussion and in the claims, the terms "including" and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to... ." Also, the term "couple" or "couples" is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection of the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms "axial" and "axially" generally mean along or parallel to a particular axis (e.g., central axis of a body or a port), while the terms "radial" and "radially" generally mean perpendicular to a particular axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. As used herein, the terms "approximately," "about," "substantially," and the like mean within 10% (i.e., plus or minus 10%) of the recited value. Thus, for example, a recited angle of "about 80 degrees" refers to an angle ranging from 72 degrees to 88 degrees.

[0028] Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.1 1 , 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, R|, and an upper limit, R_u, is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R|+k*(R_u-R|), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, 50 percent, 51 percent, 52 percent, 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed.

[0029] Disclosed herein are system and methods for the scalable production of polymer fibers via melt electrospinning. That is, systems and methods disclosed herein are related to particle-based melt-electrospinning to generate thin polymeric fibers. For example, embodiments of methods disclosed herein can be employed to manufacture nonwoven, continuous and straight nanofibers in environmentally sustainable and scalable fashion via major innovations in melt-electrospinning. In some embodiments, the size of the Taylor cone is reduced by limiting the rate of supply of the melted polymer by utilizing polymer microfibers or particles as the precursor for melt-electrospun nanofibers.

[0030] As compared to conventional solution electrospinning techniques, which have been previously employed to generate fibers, embodiments of systems and methods disclosed herein do not use solvent(s), and thus, offer the potential for lower cost and environmentally safer manufacture of polymer fibers (e.g., eliminate the need for ventilation and disposal of the solvents). It should also be appreciated that the produced melt-spun fibers are solvent-free, and thus, can be used in biomedical applications where solvent free fibers are required without the need for further processing and related costs that may be necessary to ensure biocompatibility of conventional polymer fibers that may include solvents and residues. Solvents may be conventionally employed to dissolve/melt polymers to form fibers. In addition, embodiments of methods and systems disclosed herein use particles (e.g., powders comprising particles having a maximum size under about 1 .00 mm) as the raw material for the fiber production, which leads to a reduction of the resulting diameter of fibers, which enhances the potential uses of the fibers as filter material for liquid and gaseous application, as wound healing material, and other applications where an increased surface area (resulting from the use of smaller fibers) may be desired. That is, increasing the range of diameters produced by using embodiments of systems and methods disclosed herein enables the production of polymer fibers with smaller, more uniform and consistent diameters, which increases the available surface area of a matrix or other group (plurality) of separate or interwoven fibers, thereby facilitating interaction between the polymer fiber and other types of matter through the surface of the fiber. In some embodiments, nanofiber "mats" of woven polymer fibers have a relatively high porosity and surface-to- volume ratio, which makes the mats suitable for energy-efficient and pollutant-specific water purification, or wound dressing materials with wound care materials embedded into the mat. In an embodiment, the wound healing materials may be added directly to the fibers, while in other embodiments, the wound healing materials may be added or enhanced subsequent to formation of the fibers/and or the mat. [0031] As described herein, an exemplary polymer suitable for melt-electrospinning, based on melt-process-ability, thermal degradation temperature and melting point was identified. Microfibers were fabricated via melt-extrusion, and, in some embodiments, the microfibers or segments thereof (e.g., particles) are used as the precursor of melt electrospun nanofibers. It is however to be noted that the source of the particles may not necessarily be fibers, and particles can also be obtained by grinding larger polymer pieces. Also disclosed herein is a validation of a correlation between the geometrical constraints on the polymer source and the diameter of melt electrospun fibers.

[0032] Referring now to FIG. 1 , an embodiment of a system 100 to fabricate polymer fibers is shown. In this embodiment, system 100 includes a collector 102, a twin screw extruder 104, a heater 120, a pump 106, a die 1 14, a voltage generating source 108, a heat source 1 12, and a target or collector 1 16. Die 1 14, voltage generating source 108, heat source 1 12 are disposed in a temperature controlled environment 1 18, the heat source 1 12 generates a melt-spinning temperature used in conjunction with the electrostatic forces generated by the voltage generating source 108 so that the spun fibers will remain pliable during manufacture and collection on the target 1 16. That is, if the fibers are attempted to be formed in an environment with a temperature that is below a predetermined temperature specific to the materials employed, the fibers may break and interrupt the process because the environment is too cold. In various examples, the target 1 16 is a surface that may be stationary, rotating, or combinations thereof and is employed to collect fibers formed. The target 1 16 may be a flat or curved surface, or may be configured as a cylinder, elliptical cylinder, or other geometry configured to continuously receive/collect fibers without breaking the fibers. Thus, one of the advantages presented by the systems and methods discussed herein is that there can be continuous formation of fibers without breakage until a polymer fed into the system 100 is used up or substantially used up, such that the remaining weight percentage of materials disposed in the system 100 when breakage occurs in a manner that interrupts the spinning, less than about 5 weight % of the starting material remains un-spun. In other examples, less than about 1 wt. % of the starting material remains un-spun.

[0033] A bulk polymer(s) is fed in powdered form into the collector 102, and may be further ground into finer particle sizes via a grinder in extruder 104. In general, collector 102 may take various forms and be configured with an inlet where the bulk polymer(s) (e.g., powered bulk polymer(s)) is fed and an outlet that supplies the bulk polymer(s) to extruder 104. In this embodiment, collector 102 has a diameter of 7.0 cm and can be rotated by a 12V DC motor to a speed up to 400 RPM. In one example, collector 102 with a 7.0 cm diameter is rotated at about 80 RPM (about a 29 cm/s tip velocity). In an embodiment, 120 (e.g., a hot plate) is coupled to or disposed in proximity to collector 102 to heat bulk the polymer or to the extruder 104, depending upon the embodiment, in order to melt the materials.

[0034] The extruder 104 receives the bulk polymer(s) from collector 102 and pushes the bulk polymer materials through a nozzle (not shown) at an outlet 104A of the extruder 104 that is disposed in the environment 1 18. For purposes of clarity and further explanation, a coordinate axes system is shown in FIG. 1 . The coordinate axes system includes a first axis 122, a second axis 124, and a third axis 126. Axes 122, 124, 126 are orthogonal. First axis 122 extends parallel to the direction at which the extruded polymer(s) exit extruder 104. A jet created by the fibers formed from the melt at the outlet 104A (e.g., from the nozzle of the extruder) may have a polymer melt radius profile represented by both angles 128A, 128B measured upward and downward from first axis 122, where each angle 128A, 128B may be from about 0 degrees to about 30 degrees relative to the first axis 122. While angles 128A, 128B are illustrated as being measured upward and downward in the general direction of the second axis 124, in other embodiments, the splay (polymer melt radius profile) may extend in other directions around the first axis 122.

[0035] In embodiments described herein, the bulk polymer(s) comprises a thermoplastic polymer (e.g., polylactic acid, polycaprolactone, etc.) or combinations of thermoplastic polymers. As discussed herein, a thermoplastic polymer is one that exhibits plastic properties upon heating and can be heated and cooled repeatedly without losing its plastic properties. A plurality of 1 D and/or 2D material(s) such as graphene, graphite, carbon nanotubes (single or multi-walled), nanowires, or the like may optionally be included with the bulk polymer. Such 1 D and 2D materials may be capable of self-assembly and exhibit desired thermal, mechanical, electrical, magnetic properties, or combinations thereof depending upon the application intended for the fibers. In some examples, the 1 D and/or 2D materials may be aligned with respect to a spinning direction of the fibers, e.g., aligned in the same direction (parallel to) the spun fibers, or perpendicular to the direction of the spun fibers, or within an angular range of the spinning since the fibers are pulled through the electrostatic field. In one embodiment, the plurality of fibers may be formed by a melted material pulled via the electrostatic field at an angle substantially parallel to a normal plane extending from the tip of an extruder or other device, such as a wire which acts as a melt source. In another embodiment, the melt may be pulled through the field at an angle that is 0 degrees to +/-30 degrees from the normal plane or a defined axis. As used herein, the term "1 D" and "one-dimensional" may be used to describe materials such as nanofibers and nanorods that have relatively high aspect-ratios (ratio of length-to- width) of 1000 or more with at least one dimension being from 1 nm to 100 nm, and the terms "2D" and "two-dimensional" may be used to describe materials that have a thickness that is 100 times or more thinner than the other two dimensions of the material. The self-assembly of these materials may be driven by van der Waals forces and cause the 1 D or 2D materials to orient in a predetermined configuration. In contrast to 1 D and 2D materials, zero-dimension (0D) materials may comprise particles that are isolated from each other by space or in suspension, such as nanodispersions or nanoclusters (clusters of nano-sized particles that are in contact with each other).

[0036] Pump 106 is coupled to extruder 104 and introduces the polymer into temperature-controlled environment 1 18. The voltage generating source 108 within the temperature-controlled environment 1 18 propels the polymer from the extruder 104 forward via the electrostatic force created, the melted polymer may be referred to as "jets" as it leaps/is pulled from the extruder 104 and/or the die 1 14. In this embodiment, the voltage generating source 108 creates an electrostatic potential from about 5kV to about 30kV to propel the polymer towards the target 1 16. Heat source 1 12 coupled to or disposed within environment 1 18 maintains a predetermined temperature or temperature range in the environment 1 18. In general, the environment 1 18 may be maintained at a temperature that is within, for example, 5%, 10%, 25%, or another percentage of the melting point of the polymer disposed in collector 102.

[0037] This melt-spinning in FIG. 1 may occur when materials such as polymer powders and/or pellets are disclosed in the collector 102 which feeds the materials to the extruder 104, one or both of the collector 102 or the extruder 104 may be heated via the heater 120. The pump 106 promotes movement of the melting polymer material (e.g., melting polymer powders and/or pellets) into the electrostatic field generated by the voltage generating source 108. The melting polymer material may optionally be further extruded through the die 1 14 that may have one or more perforations via which the fiber diameters are tuned (controlled). In some embodiments, no die 1 14 is used and the fibers formed are collected on the target 1 16 from the extruder 104 without passing through a die 1 14 or other intermediary structure. The fibers generated in the electrostatic field in the environment 1 18 are collected on the target 1 16. This process may be continued until fibers break at the tip of the twin screw extruder 104. In one example, the polymer was extruded through a die 1 14 with a diameter of 0.5mm. In this experiment, the extruder 104 output flow was a function of the residual polymer inside the extruder 104 as well as the extruder torque. Thus, for a given torque, the diameter of the extruded fibers gradually and monotonically reduced with time (and the reduction of the residual polymer inside the extruder).

[0038] Example 1

[0039] Selection of the bulk polymer for scalable production of fibers

[0040] Polylactic acid (PLA) polymer was used to produce polymer fiber using the system of FIG. 1 . In one example, the PLA polymer was a biosourced commercial product (Ingeo 6202D, NatureWorks), which can form fibers in conventional microfiber systems with as little as 0.5 g/9000 m (denier per filament or dpf). A cylindrical fiber at this dpf would produce an 8 micron diameter filament suitable for further processing. With a 30 g/10 min melt flow index (MFI) at 210° C, the material has flow characteristic suitable to accept nano-constituents, such as carbon nanotubes (CNTs), employed to reheat the microfiber for electrospinning into nanofiber. In an embodiment, the melt spinning temperatures (e.g., the temperature of the environment 1 18 in FIG. 1 ) may be from about 220° C to about 240° C with high fiber draw velocities that support a scaled-up (increased production) nanofiber process.

[0041] Fabrication of Microfibers as the precursor to Melt-electrospun fiber

[0042] Polylactic Acid (PLA) microfibers were used as the precursor (base material) to form melt-electrospun fibers via melt-extrusion by using a twin-screw extruder as discussed in FIG. 1 . The experimental details and results are described hereinbelow. [0043] PLA pellets (from Nature Works), also referred to herein as "particles," were used to fabricate microfibers via particle based melt-electrospinning. The particles ranged in starting size (maximum diameter) from about 100 μιη to about 1 .0 mm, which may be reduced during processing but before melt-spinning.

[0044] The diameter of the extruded fibers were in the range of 50-150 μιη. Measurements of fiber diameter were averaged over every one meter length of the fiber, with four diameter measurements taken at each meter to get a reliable average diameter. Measurements were made on the optical microscopy software directly.

[0045] Results and Discussion

[0046] FIGS. 2- 4 are results of experiments using the system 100 in FIG. 1 . FIG. 2 is a microscopy image of one of the polymer fibers manufactured according to certain embodiments of the present disclosure. The optical image of extruded fibers is shown at 50X magnification. FIG. 3 shows the fiber diameter as a function of the length of the collected fiber on the spool. As expected, the diameter of the fiber decreases with extrusion length. That is because the fiber extrusion will lower the residual mass inside the extruder. Hence, the pressure difference generated by the extruder will decrease, which will lower the output flow rate and the diameter of the extruded fibers. Each extrusion experiment began with about 4 g of PLA, and continued until the residual mass was about 2 g, after which the pressure difference was not sufficient to allow for a stable extrusion of PLA.

[0047] FIG. 4 is a graph illustrating the fiber diameter as a function of the extrusion flow rate according to certain embodiments of the present disclosure. As shown in FIG. 4, a residual mass of 3.0-3.4 g in the extruder allowed controlled fabrication of PLA microfibers with diameters adjustable from about 40 μιη to about 120 μιη. In other embodiments, fibers with diameters smaller than 40 μιη may also be fabricated, for example, by employing higher extrusion temperatures to make the flow more drawable (e.g., able to be extruded with ease and with minimized or zero unintentional breakage) or faster drawing velocities. While the diameter of the fibers is a function, at least in part, of the residual mass, and thus, varies during the extrusion process, the variation of the fiber diameter is very gradual such that within each continuous 1 m of the extruded fiber, the diameter change is within the -5% of the average diameter of that segment. Thus, for scale tests, embodiments described herein offer the potential to be used to fabricate sufficiently long fibers as the precursors for melt-electrospun nanofibers.

[0048] Example 2

[0049] Particle-based electrospinning: geometrical confinement of Taylor Cone

[0050] The hypothesis that the geometrical confinements on the precursor of melt- electrospun fibers can lower the diameter of the fibers was tested. For this purpose, PLA particles (pellets) with characteristic length scales of ~1 mm to 6 mm were used as the precursor. The pellets were subsequently melt-electrospun and their diameters were measured via optical microscopy. The details of the experiment and the results are explained below.

[0051] The melt electrospinning setup contained a high negative polarity power supply (N030HP1 , Acopian). As shown in FIG. 5, a PLA pellet was placed on the corner of a steel plate, and a stationary collector of aluminum foil was placed 7 cm away from the polymer tip. The steel plate was heated on a hot plate (PC-620, Corning) to 400° C, though the polymer reached considerably lower temperatures (~220-240°C) as measured with an IR thermometer gun. The temperature of the hot plate was chosen so that the polymer temperature stayed between the melting temperature and the degradation temperature. The negative power supply used to energize the polymer melt and initiate the electrospinning could be adjusted. That is because to initiate electrospinning, the polymer melt has to be at -5-30 kV with respect to the target, while the polymer melt has to be at the same electric potential as the hot plate to avoid undesired discharging. Thus, the hot plate was grounded while the electric potential of the target was reduced to -20-30 kV with the aid of the negative power supply,

[oooi] To investigate the dependence of the diameter of electrospun fibers on the size of the pellets, two approaches were carried out which employed two types of targets using the system of FIG. 1 . First, the electrospun fibers spun from a single pellet on a moving belt target were collected. During the electrospinning process, as the originally disposed pellet is moved to exit at the electrospinning jet, the pellet size was reduced, and the diameter of the electrospun fibers corresponding to the pellet size after this reduction was measured to observe any correspondence between the pellet size and fiber diameter. In this experiment, electrospinning was performed for 10 minutes. Readings of fiber diameters were reported corresponding to approximately each 90 seconds of the experiment. In the second experiment, different pellet sizes from 52 mm (average PLA pellet size) to 1 .3 mm (1/40 of average PLA pellet size) were used. Pellet volume was calculated by measuring the characteristic length scales of the pellets via optical microscope and assuming spherical shapes. To initiate the electrospinning, the pellet was directly placed on the steel plate and allowed to melt for 5 minutes. After that point, the high voltage power supply was turned on at -27kV. The polymer and steel plate were grounded. Electrospinning was performed for 15 seconds after initial jet formation. A video camera was used to monitor the start of the electrospinning jet. The starting pellet size (diameter) was from about 100μιη to about 2 mm, and the ratio of a pellet size (diameter) to the diameter of the electrospun fibers were from 100:1 to 1000:1 .

[0052] The diameter of the electrospun fibers were determined using optical microscopy (SZX16, Olympus). ImageJ was used to analyze the images. For each case, 200 diameter readings were taken. The average value of the diameters, their standard deviation, and histogram of the diameter sizes were recorded.

[0053] Results and Discussion

[0054] FIG. 6 is graph illustrating fiber diameter as a function of time the fiber was electrospun. A "continuous" electrospinning was performed with a moving belt target as described above. As shown in FIG. 6, the diameter of the electrospun fibers reduced with time because the formation of the electrospun fibers is accompanied by a reduction of the pellet size, according to conservation of mass principle. The reduced pellet size imposes stronger geometrical constraints on a Taylor cone, thus, the size of the cone gets reduced with time. The reduction in the size of the Taylor cone leads to a reduction in the fiber size, as demonstrated in FIG. 6, which confirms a correlation between the Taylor cone size and fiber diameter. The methods disclosed herein may be referred to as "particle-based electrospinning." A multi-scale physics based model of melt-electrospinning was developed based on solving a system of equations of conservation of mass, momentum and energy in jet which is subjected to radiation heating and electrostatic forces. The correlations between the size of the Taylor cone, the volumetric heat given to the electrospinning jet, the temperature of the electrospinning environment are discussed herein. These factors and material properties may control the convective heat loss, and the diameter of the melt- electrospun fibers, which may be targeted in order to obtain fibers of a particular dimension (or range of dimensions) in order to achieve a target range of material properties.

[0055] FIG. 7 illustrates the diameter of the melt-electrospun fibers as function of pellet size. To obtain this data, various pellet sizes were melted on a stationary target. The pellet size is presented in log-scale. As shown in FIG. 7, reducing the initial particle size (volume) by 40 times (from -52 g to 1 .3 g) reduced the fiber diameter by a factor of 2.

[0056] FIG. 8 is a histogram of the melt electrospun fiber diameters for three pellet sizes (the largest, the smallest and the pellet with the median size) as a function of the starting mass (m_s). As shown in FIG. 8, the fiber diameter is a function of the initial particle size and not directly its mass or volume. Therefore, it may be more instructive to compare the fiber diameters as a function of the characteristic length scales of the particles. Thus, here the "equivalent diameter" of the pellets is defined as the diameter of a sphere with the same volume (and mass) of the pellet. For example, the largest pellet size with an equivalent diameter of about 4.3 mm resulted in melt electrospun fibers with a diameter distribution that peaked at a fiber diameter of about 6.7 μιη. Moreover, by employing the smallest pellet which has an equivalent diameter of about 1 .35 mm, the diameter distribution of the melt electrospun fibers shifted to lower diameters, resulting in a diameter distribution peak at -3.7 μιη.

[0057] Both electrospinning experiments with moving belt and stationary target with varying pellet size suggested that a reduction in the pellet size can impose geometrical constraints on the Taylor cone, which will in turn lead to smaller electrospun fibers. With the smallest starting volume tested, the diameter of melt electrospun fibers averaged about 3.6 urn. This is significantly thinner than when the starting volume is a full pellet, which gave an average diameter of about 6.7 urn.

[0058] Example 3

[0059] Multi-Physics Model of Melt Electrospinning

[0060] A modeling framework has been established which can describe the evolution of the polymer melt radius in the process of melt-electrospinning as a function of various physical parameters involved in the process. Governing equations of the model are presented below:

[0061] Mass: nR²v = V. [0062] Momentum:

pw'

p- + £ + £ + =- ^{+ (t} - >>¾>¾' ⁺

R

[0063] Charge: R²KE_t + 2nRvo = I.

[0064] Electric field:

E(z)

[0065] Tangential projection of electric field: E_t(z) =

1+ (R')²

[0066] Energy:

[0067] In the above set of equations, 'R' is the radius of the jet, V is the local velocity of the jet, 'V' is the applied volumetric flowrate, 'p' is the density, 'g' is the acceleration due to gravity, 'F_T' is the viscoelastic tensile force in the jet (computed from constitutive Giesekus equation fitted from rheological data), 'γ' is surface tension, 'σ' is the charge density, 'ε' is the local permittivity, '£₀'is vacuum permittivity, Έ' is the local electric field, 'E_t' is the tangential projection of the electric field, 'K' is the electrical conductivity, T is the electric current, V is the voltage applied between the nozzle and the collector, 'Ro' is the initial radius of the polymer jet, 'd' is the nozzle-collector separation distance, 'C_p' is the polymeric heat capacity, T is temperature, 'h' is the convective heat transfer coefficient, 'Τ_∞' is the external air temperature and 'Q' is the volumetric heat source term. Primes indicate derivatives w.r.t 'z'. The model was been formulated as an initial value problem of five coupled first order ordinary differential equations. Using a set of five initial conditions, it was solved numerically using ode15s in MATLAB.

[0068] Results and Discussion

[0069] FIGS. 9-12 are associated with experimental results of a system discussed in the modeling of melt electrospinning above. FIG. 9 is a plot of a polymer jet radius profile against the dimensionless final fiber radius as compared to the dimensionless spinline coordinate at various processing temperatures. In general, dimensionless numbers are those in which a first length is divided by a second measurement of the same type, e.g., a final fiber radius divided by an initial fiber radius would give you a dimensionless final polymer fiber radius. The polymer jet radius profile is a measurement of an angle of splay of the fibers are they are formed, e.g., if an axis extends from the extruder where the melt exits, some fibers will be pulled parallel to this axis via the electrostatic force, and some will be pulled within an angle of this axis, for example, from 0 degrees to about 30 degrees from the axis. FIG. 9 shows the polymer jet radius profile for scenarios including (1 ) when no heat is provided (Q=0), (2) when heat is turned on in between 20 and 40 (as indicated on the y-axis which is the dimensionless spinline coordinate (Q=2000)). FIG. 9 reflects manufacturing conditions comprising a constant air temperature for the melt spinning at about 20°C.

[0070] FIG. 10 is a plot of dimensionless final fiber radius as a function of location of heating. FIG. 10 shows these results at a low Q (Q = 100) corresponding to different values of air temperature (T_air). For the case when Q = 0, no downstream heat source was present. The polymer fiber radius did not thin after about a distance of 20 dimensionless units away from the nozzle of the extruder. When the heat source is turned on at that point (Q = 2000) and a considerable degree of thinning of the polymer fiber was observed.

[0071] FIG. 1 1 is a graph of the dimensionless final polymer fiber radius obtained on the collector as a function of location of heating. FIG. 1 1 shows the variation of the dimensionless final polymer fiber radius obtained on the collector as a function of location of heating for a low value of Q (Q = 100) and at different values of air temperature. FIG. 1 1 shows results for a high value of Q (Q = 2000) at different values of air temperature (Tar).

[0072] FIG. 12 is a graph of dimensionless final fiber radius as a function of air temperature (T_ai_r). When the heat source is turned on between 20 < dimensionless spinline coordinate < 40, the variation of the final polymer fiber radius with the air temperature for varying values of Q (100 & 2000) was observed.

[0073] In particular, FIG. 12 illustrates air temperatures corresponding to a low value of Q (Q = 100) and a high value of Q (Q = 2000) when heat is turned on between 20 < dimensionless spinline coordinate < 40.

[0074] For a low value of Q (Q = 100), the minimum final radius of the polymer fiber was achieved when operating at the highest allowable air temperature and if the heating is started from the beginning of the melt spinning process/run. For a high value of Q (Q = 2000), as seen from Figure 1 1 and Figure 12 (Q = 2000 in which heat is turned on between 20 < dimensionless spinline coordinate < 40), a maxima in the dimensionless final fiber radius at an intermediate value of air temperature between 20°C and 400°C. It is believed this occurs because, if too much thinning occurs before the volumetric heating is turned on, the volumetric heating makes little difference. In other words, in sufficiently thin fibers, the volumetric heat is dissipated through convection.

[0075] Referring now to FIGS. 13A and 13B, schematic views of alternative embodiments of systems for producing polymer fibers via melt-electrospinning are shown. In FIG. 13A, the polymer melt is fed to the electrostatic field through the holes of a grate; and in FIG. 13B, the polymer melt is fed to the electrostatic field through the holes of a perforated sheet. The grate or a perforated sheet, as shown in FIGS. 13A and 13B, respectively, may be employed to reduce an average size of the polymer fiber such that, after the electrospinning, the fiber is at least 1 /10 of the size of a diameter of a hole of the grate or perforated sheet. For example, if the in-plane dimensions of the melt-electrospinning jet produces a larger diameter fiber than desired, the polymer melt is supplied through the holes of a grate or perforated sheet (as in FIGS. 13A and 13B) with a hole in-plane diameter in the range of 50 microns to 4 millimeters. In an alternate embodiment, an initial dimension of the polymer jet may be defined in part by size of the holes of the grate or the perforated sheet.

[0076] Example 4

[0077] Referring now to FIG. 14, an embodiment of a system 200 for producing electrospun polymer fibers is shown. In this embodiment, system 200 includes an elongate linear or straight source wire 202 coated with a coating or layer 204 of a polymer material 205 used to form the polymer fibers. While a single source wire 202 is represented in FIG. 14, in alternate embodiments, a plurality of source wires 202 of the same or varying dimensions, properties, and coating thicknesses and materials may be used and arranged in various manners and angles. The box 212 in FIG. 14 represents a temperature-controlled environment 212, this environment 212 may be at an ambient air temperature (e.g., 20-25.5 °C) up to a temperature determined by a melting temperature of the material 205 that may be, for example, from 200 °C to 400 °C or more.

[0078] Source wire 202 has a central or longitudinal axis 203, a length L₂₀₂ measured axially relative to axis 203 between a first or lower end 202A and a second or upper end 202B, and a width or diameter W₁₀₂ measured radially relative to axis 203. In FIG. 14, source wire 202 is vertically oriented such that an angle a between axis 203 and horizontal is 90°. However, in other embodiments, angle a is an acute angle between 0° and 60°. As will be described in more detail below, the vertical or generally upright orientation of source wire 202 results in the flow of melted polymer material 205 towards the first end 202A of source wire 202 under the force of gravity and the electrostatic forces applied to the polymer during the spinning process. The length L-202 and width W₂₀₂ of wire 202 can vary. In this embodiment, the width W₂₀₂ (e.g., diameter) of the source wire 202 is between about 50.0 microns and about 2.0 mm.

[0079] In general, wire 202 can be made of any suitable metal or metal alloy such as nickel, (Ni), copper (Cu), aluminum (Al), titanium (Ti), gold (Au), silver (Ag), platinum (Pt), iron (Fe), or combinations thereof. In this embodiment, source wire 202 is made of nickel (Ni).

[0080] The layer 204 of material 205 is mounted directly on source wire 202 and has a thickness T₂₀₄ measured radially relative to axis 203 from the radially inner surface of layer 204 contacting the outer surface of the source wire 202 and the radially outer surface of layer 204 distal source wire 202. In this embodiment, thickness T₂₀₄ is between about 5 microns and 300 microns. Layer 204 is preferably uniformly and evenly distributed about source wire 202 such that the composition and thickness of layer 204 is uniform along the circumference and length of source wire 202.

[0081] Material 205 forming layer 204 comprises one or more polymers, and more specifically, comprises one or more thermoplastic polymers (e.g., polylactic acid, polycaprolactone, etc.). In addition, material 205 may optionally include a plurality of 1 D and/or 2D material(s) such as graphene, graphite, carbon nanotubes (single or multi-walled), nanowires, or the like. Such 1 D and 2D materials may be capable of self-assembly and exhibit desired thermal, mechanical, electrical, magnetic properties, or combinations thereof depending upon the application intended for the fibers. In embodiments including 1 D and/or 2D materials, material 205, and hence layer 204, comprises about 1 .0 wt% to 10.0 wt % of the 1 D and/or 2D materials with the balance of material 205 comprising the polymer(s). In other examples, this wt. % of 1 D and/or 2D materials may be greater than 10 wt. %, for example, up to 50 wt. % with the balance of material 205 comprising the polymer(s). There are no solvents or additives used in the material 205 that forms the layer 204, in contrast to conventional methods of producing polymer fibers such as solution-based electrospinning. That is, in one embodiment, the starting materials that form material 205 and layer 204 consist of one or more thermoplastic polymers, and in another embodiment, consist of one or more thermoplastic polymers and one or more 1 D or 2D materials but no additional solvents or additives are added to the material 205 used to coat the source wire 202 to form the layer 204. In some examples, one or more applications of the material 205 may be employed to form the layer 204 via the formation of sub-layers which may be of equal or varying thicknesses and/or compositions.

[0082] Referring still to FIG. 14, a heating wire or coil 208 is wrapped around the source wire 202. A first voltage V1 is applied to heating coil 208, which generates thermal energy via Joule heating. The thermal energy produced by coil 208 must be sufficient to induce melting of material 205 of layer 204 (e.g., the thermal energy must be sufficient to raise the temperature of the material 205 to its melting point). In this embodiment, first voltage V1 applied to coil 208 is less than 5.0 V, and in some applications, may be less than 1 .0 V. In general, the first voltage V1 applied to coil 208 can be varied to control the thermal energy applied to material 205, and hence, the rate at which material 205 melts. As previously described, source wire 202 is vertically oriented or oriented at an acute angle relative to horizontal such that the melted material 205 flows generally downward to the lower end of source wire 202 via gravity.

[0083] A power supply 206 generates a second voltage V2 between the source wire 202 and a target 210 to initiate the melt electrospinning process. In particular, the second voltage V2 applied between source wire 202 and target 210 generates an electrostatic field therebetween that draws melted polymer fibers from the lower end or tip of source wire 202 onto target 210 since, upon melting, the material 204 is pulled gravitationally and then pulled into the electrostatic field towards the target 210. In this embodiment, the second voltage V2 is between about 10.0 kV and 20.0 kV, in other embodiments, V2 may be less than 30.0 kV. In this embodiment, target 210 rotates during the melt electrospinning to collect the produced fiber. However, in other embodiments, the target (e.g., target 210) may be stationary or there may be a secondary target to which the fibers are transferred. The polymer fibers produced in this manner with system 200 and collected on target 210 can have diameters or widths of about 0.25 microns to 5.0 microns, or, in some embodiments, may be from about 5.0 microns to about 10.0 microns. The diameters or widths of the resultant polymer fibers collected on target 210 can be tuned by manipulating one or more parameters of the process including, without limitation, the composition of material 205, the first voltage V1 used to heat the source wire 202 (and associated thermal energy generated), the second voltage V2 (and the associated electrostatic force), or combinations thereof. In general, the melt electrospinning using source wire 202 can be executed for any suitable period of time. In some embodiments, the melt electrospinning using source wire 202 is performed for a period of at least 1.0 minute, from 1.0 to 5.0 minutes, or from 5.0 to 10.0 minutes, or more than 10.0 minutes.m as examples. Thus, one of the advantages presented by the systems and methods discussed herein is that there can be continuous formation of polymer fibers without breakage until the coating 204 is completely or substantially used up, such that the remaining weight percentage of the coating 204 on the source wire 202 when breakage occurs in a manner that interrupts the spinning, less than about 5 weight % of the starting material remains un-spun. In other examples, less than about 1 wt. % of the starting material remains un-spun.

[0084] In one example, material 204 was PCL and the first voltage V1 was sufficient to general thermal energy capable of heating the material 204 to at least about 90°C, which is above the melting point of PCL. In addition, the second voltage V2 was about 14-16 kV between source wire 202 and target 210 to initiate the melt electrospinning process. Such second voltage V2 is near the minimum voltage to initiate the process, and was sufficient to lead to continuous spinning of the fibers. As discussed herein "continuous" spinning is spinning of the fibers that is started and stopped intentionally, or that stops when the coating material is substantially all melted from the source wire, as opposed to processes which terminate when the fibers break. The average fiber diameters produced in this example was 6.0 ± 1.0 microns.

[0085] The polymer fibers collected by target 210 may be removed and employed as- is, or the fibers may be removed and then further processed such as by adding coatings to the fibers, treating the fibers in solution, thermal treating the fibers, etc. In some embodiments, further processing of the fibers occurs while the fibers are on the target 210.

[0086] In various embodiments, the target 210 may collect an ordered array of fibers or a random array of fibers. As discussed herein, an "ordered" array is one where the plurality of fibers are arranged in a particular direction, directions, or within a directional (angular) tolerance, for example, to produce a desired property or range of properties (thermal, electrical, magnetic, mechanical, or combinations thereof). An ordered array may further comprise a predetermined spacing among and between individual fibers or bundles of fibers, where a bundle of fibers is two or more fibers arranged in the same or substantially the same direction. In other examples, an ordered array is an intentional deposition of each fiber or fiber bundle that may be based on a spacing between bundles/fibers, a direction of deposition (e.g., of the fiber/bundle and/or an orientation of 1 D/2D materials of the melt), a desired thickness of a final array structure, a desired shape of the final array structure, or other dimensions or properties of a final array structure collected on the target. A random array may comprise a plurality of overlapping fibers in various directions that may also produce a desired property or range of properties (thermal, electrical, magnetic, mechanical, or combinations thereof). While the source wire 202 is illustrated as being substantially perpendicular to the target 210 in FIG. 14, in alternate embodiments it may be at an angle relative to the target 210.

[0087] The geometry of source wire 202 concentrates the electrostatic field around one end (e.g., based on gravity, whichever end of the wire is closer to the ground), as a result of which the initial diameter of the electrospinning jet is significantly reduced. In some examples, the source wire has a consistent diameter, and in other examples, the source wire comprises a diameter that tapers towards (e.g., gets smaller closer to) an end of the source wire where the melting polymer is pulled/pushed via the electrostatic forces towards the target. Thus, this example geometrically confines the jet, without adding solid walls or boundaries, but rather by manipulating the local electrostatic fields.

[0088] Referring now to FIG. 15, a comparison between the diameters of polymer fiber produced with system 200 previously described and conventional methods of edge- based and syringe-based melt electrospinning is shown. The results shown in FIG. 15 were obtained using a system such as that in FIG. 14, the experiment was performed in ambient air with a coating thickness of 70-100 microns, a spinning duration from 30 seconds to 1 minute, and the source wire angle was about 50 degrees with respect to normal (e.g., with respect to an axis at 90 degrees from a normal plane). The voltage applied to form the electrostatic field was from 15-20 kV. As shown in FIG. 15, with a wire diameter from about 200 microns to about 1000 microns, the plurality of fibers produced with system 200 were less than 10 microns in diameter. This is in contrast with the conventional methods that produced fibers with an average diameter greater than 15 microns or greater than 20 microns.

[0089] Referring now to FIG. 16, a graph illustrating the thickness of a layer of polymer material on a source wire (e.g., thickness of layer 204 of material 205 on source wire 202) as a function of time of electrospinning the fibers without the fibers breaking using system 200 previously described is shown. The experiment At the start of electrospinning, the layer of polymer material had a thickness of about 160-180 microns. As the polymer starts to flow, it initially accumulates near the tip where it is pulled into the electrostatic field, so the thickness initially goes up (from 160 microns to 180 microns). The region in FIG. 16 labeled "nearly stable melt electrospinning" is a region in which the melt electrospinning is producing a plurality of fibers within a predetermined tolerance without breakage. This period is shown from 8 s to 14 s but may extend beyond that range, for example, until the coating material is substantially all melted off of the source wire. While FIGS. 15 and 16 illustrate results from a PCL coating, other coating materials of other polymers and/or 1 D and/or 2D materials may be employed.

[0090] While exemplary embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the compositions, systems, apparatus, and processes described herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order and with any suitable combination of materials and processing conditions.

Claims

1 . A method for fabricating solvent-free polymer fibers, comprising:

heating a source wire by applying a first voltage across a coil disposed about the source wire;

melting, in response to heating the source wire, a material disposed on the source wire as a layer; and

generating an electrostatic field between the source wire and a target by applying a second voltage between the source wire and the target; and

drawing the melted material from the source wire and toward the target in response to the electrostatic field to form a fiber.

2. The method of claim 1 , wherein the material comprises a polymer.

3. The method of claim 2, wherein the material is a thermoplastic polymer.

4. The method of claim 2, wherein the material further comprises a one- dimensional or a 2-dimensional material distributed in the polymer.

5. The method of claim 4, wherein the material comprises about 1 wt% to about 10 wt% of the one-dimensional or the two-dimensional material.

6. The method of claim 1 , further comprising collecting the fiber on the target.

7. The method of claim 6, further comprising rotating the target while collecting the fiber.

8. The method of claim 1 , wherein an average diameter of the fiber is less than 10 microns.

9. The method of claim 1 , wherein the source wire is vertically oriented or oriented at an acute angle relative to a horizontal axis.

10. A method for fabricating solvent-free polymer fibers, comprising:

(a) heating a source wire;

(b) melting a layer of a material disposed on the source wire in response to

(a);

(c) generating an electrostatic field between the source wire and a rotating target;

(d) drawing the melted material away from the source wire and towards the rotating target to form a plurality of fibers; and

(e) collecting the plurality of fibers on the rotating target.

1 1 . The method of claim 10, wherein (a) comprises:

applying a first voltage across a coil disposed about the source wire.

12. The method of claim 1 1 , wherein the first voltage is between 0.10 V and 5.0 V.

13. The method of claim 12, wherein (c) comprises applying a second voltage between the source wire and the rotating target to generate the electrostatic field.

14. The method of claim 13, wherein the second voltage is between 10 and 40 kV.

the second voltage

15. The method of claim 10, wherein the material comprises a theremoplastic.

16. The method of claim 1 1 , wherein the material comprises a plurality of 1 D or 2D materials, wherein the plurality of 1 D or 2D materials are aligned in the plurality of fibers collected in (e) to comprise at least one of a target thermal, mechanical, or magnetic property.

17. The method of claim 10, wherein an average diameter of the plurality of fibers is less than 20 microns.

18. A method for fabricating solvent-free polymer fibers, comprising:

disposing a plurality of polymer pellets in an extruder coupled to a melt- spinning apparatus;

melt-spinning the plurality of polymer pellets to form a plurality of melt-spun fibers in an air temperature from about 20°C to about 400°C, wherein the plurality of melt spun fibers do not comprise a solvent.

19. The method of claim 18, further comprising: reducing, prior to melt-spinning, an average size of the plurality of pellets, wherein an average size of the plurality of pellets after the reducing is at least 1/10 of an initial average size.

20. The method of claim 18, wherein the plurality of pellets comprise polylactic acid (PLA).

21 . The method of claim 18, wherein each pellet of the plurality of pellets disposed in the grinder comprises a maximum diameter of 5 mm.

22. The method of claim 18, wherein the plurality of pellets disposed in the grinder comprises a mass of less than 55 mg.

23. The method of claim 18, further comprising forming the plurality of melt spun fibers each comprising a fiber diameter, wherein a ratio of the average fiber diameter to a diameter of the plurality of pellets is from 1 : 10 to 1 : 1000.

24. The method of claim 18, further comprising weaving the plurality of melt-spun fibers into a fabric.