Patent Publication Number: US-2016236232-A1

Title: Electrospinning with reduced current or using fluid of reduced conductivity

Description:
BENEFIT CLAIMS TO RELATED APPLICATIONS 
     This application is a continuation of U.S. non-provisional application Ser. No. 13/787,724 entitled “Electrospinning with reduced current or using fluid of reduced conductivity” filed Mar. 6, 2013 in the names of Ashley S. Scott, Andrew L. Washington, Jr., and John A. Robertson, which is a divisional of U.S. non-provisional application Ser. No. 12/728,070 entitled “Fluid formulations for electric-field-driven spinning of fibers” filed Mar. 19, 2010 in the names of Ashley S. Scott, Andrew L. Washington, Jr., and John A. Robertson, which in turn claims benefit of (i) U.S. provisional App. No. 61/161,498 entitled “Electrospinning Cationic Polymers and Method” filed Mar. 19, 2009 in the names of Ashley S. Scott, John A. Robertson, and Andrew L. Washington, Jr., and (ii) U.S. provisional App. No. 61/256,873 “Electrospinning with reduced current or using fluid of reduced conductivity” filed Oct. 30, 2009 in the names of Ashley S. Scott, John A. Robertson, and Andrew L. Washington, Jr. Each of said non-provisional and provisional applications is hereby incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND 
     The field of the present invention relates to electrospinning of polymer nanofibers or electrospraying of small droplets. In particular, electrospinning with relatively reduced fluid conductivity or with relatively reduced current is disclosed herein. 
     The subject matter disclosed herein may be related to subject matter disclosed in co-owned U.S. non-provisional App. No. 11/634,012 entitled “”Electrospraying/electrospinning array utilizing a replacement array of individual tip flow restriction” filed Dec. 5, 2006 in the names of John A. Robertson and Ashley Steve Scott (Pub. No. US 2008/0131615 published Jun. 5, 2008) and U.S. provisional App. No. 61/161,498 entitled “Electrospinning cationic polymers and method” filed Mar. 19, 2009 in the name of Ashley S. Scott. Both of said applications are incorporated by reference as if fully set forth herein. 
     “Electrospinning” and “electrospraying” refer to the production of, respectively, so-called “nanofibers” or “nanodroplets”, which may be “spun” as fibers or “sprayed” as droplets by applying high electrostatic fields to one or more fluid-filled spraying or spinning tips (i.e., nozzles or spinnerets). The high electrostatic field produces a Taylor cone at each tip opening. The sprayed droplets or spun fibers are typically collected on a target substrate. A high voltage supply provides an electrostatic potential difference (and hence the electrostatic field) between the spinning tip (usually at high voltage) and the target substrate (usually grounded). A number of reviews of electrospinning have been published, including (i) Huang et al, “A review on polymer nanofibers by electrospinning and their applications in nanocomposites,”  Composites Science and Technology , Vol. 63, pp. 2223-2253 (2003), (ii) Li et al, “Electrospinning of nanofibers: reinventing the wheel?”,  Advanced Materials , Vol. 16, pp. 1151-1170 (2004), (iii) Subbiath et al, “Electrospinning of nanofibers,”  Journal of Applied Polymer Science , Vol. 96, pp. 557-569 (2005), and (iv) Bailey,  Electrostatic Spraying of Liquids  (John Wiley &amp; Sons, New York, 1988). Details of conventional electrospinning materials and methods can be found in the preceding references and various other works cited therein, and need not be repeated here. 
     Conventional fluids for electrospinning (melts, solutions, colloids, suspensions, or mixtures, including many listed in the preceding references) typically have significant fluid conductivity (e.g., ionic conductivity in a polar solvent, or a conducting polymer). In addition, conventional methods of electrospinning typically include a syringe pump or other driver/controller of the flow of fluid to the spinning tip, and a conduction path between the high voltage supply and the fluid to be spun. Such arrangements are shown, for example, in U.S. Pat. Pub. No. 2005/0224998 (hereafter, the &#39;998 publication), which is incorporated by reference as if fully set forth herein. In FIG. 1 of the &#39;998 publication is shown an electrospinning arrangement in which high voltage is applied directly to a spinning tip, thereby establishing a conduction path between the high voltage supply and the fluid being spun. In FIGS. 2, 5, 6A, and 6B of the &#39;998 publication are shown various electrospinning arrangements in which an electrode is placed within a chamber containing the fluid to be spun, thereby establishing a conduction path between the high voltage supply and the fluid. The chamber communicates with a plurality of spinning tips. In any of those arrangements, significant current (typically greater than 1 μA per spinning tip) flows along with the spun polymer material. Conventional electrospinning fluids are deposited on metal target substrates so that current carried by the spun material can flow out of the substrate, thereby avoiding charge buildup on the target substrate. Electrospinning onto nonconductive or insulating substrates has proved problematic due to charge buildup on the insulating substrate that eventually suppresses the electrospinning process. 
     SUMMARY 
     A method comprises: dissolving an aromatic side chain polymer in a terpene, terpenoid, or aromatic solvent; dissolving an inorganic salt in a polar organic solvent; mixing the salt solution and the polymer solution; and using the predominantly terpene, terpenoid, or aromatic solvent phase of the mixture as an electrospinning fluid. The fluid is electrospun from one or more spinning tips onto a target substrate. The inorganic salt and the polar organic solvent are chosen so as not to cause substantial precipitation of the polymer upon mixing with the polymer solution. The terpene, terpenoid, or aromatic solvent can comprises D-limonene, the aromatic side chain polymer can comprise polystyrene, the polar organic solvent can comprise dimethyl formamide, and the inorganic salt can comprise LiCl, AgNO 3 , CuCl 2 , or FeCl 3 . The method can further comprise electrospinning the fluid with the fluid and spinning tips electrically isolated from a voltage source that drives the electrospinning. 
     Objects and advantages pertaining to electrospinning or electrospraying may become apparent upon referring to the exemplary embodiments illustrated in the drawings and disclosed in the following written description or appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate schematically an exemplary electrospinning head. 
         FIGS. 2A and 2B  illustrate schematically another exemplary electrospinning head. 
     
    
    
     The embodiments shown in the Figures are exemplary, and should not be construed as limiting the scope of the present disclosure or appended claims. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Electrospinning or electrospraying of polymer-containing nanofibers or small droplets, respectively, can be employed to produce a variety of useful materials. However, scaling up an electrospinning process beyond the laboratory or prototype level has proven problematic. To achieve production-type quantities, multiple electrospinning tips are typically employed in an arrayed arrangement. However, the conductive fluids used and the significant current (typically greater than 1 μA per tip) carried by fibers emerging from each tip lead to impractically large overall current and to undesirable electrostatic interactions among the electrospinning tips and fibers; these limit the number and density of electrospinning tips that can be successfully employed. 
     Electrospinning fluids are disclosed herein that exhibit substantially reduced conductivity relative to conventional electrospinning fluids (while maintaining suitability for electrospinning), at least partly mitigating the undesirable electrostatic interactions described above. One group of such electrospinning fluids comprises mixtures of (i) a solution of polystyrene in D-limonene and (ii) an inorganic salt dissolved in dimethyl formamide. Polystyrene (PS) is a non-polar, non-conductive polymer; D-limonene (DL) is a relatively high-boiling, low vapor pressure, non-polar solvent that occurs naturally in citrus rinds. D-limonene is attractive as a “green,” or environmentally friendly, organic solvent, and is readily available in large quantities as a byproduct of citrus processing. Conventional electrospinning has been attempted using a solution of PS in DL, but it has been observed that the resulting fibers are relatively large (about 700 nm) and of poor quality (Shin et al, “Nanofibers from recycle waste expanded polystyrene using natural solvent,”  Polymer Bulletin , Vol. 55 pp. 209-215 (2005)). 
     Treatment of the PS/DL solution with a solution of inorganic salt in dimethyl formamide (DMF) markedly improves the quality of nanofibers produced by electrospinning a PS/DL fluid. In one example of preparation of the electrospinning fluids, a PS/DL solution is prepared that is between about 10% and about 50% PS by weight, typically between about 20% and about 40% PS by weight, preferably between about 25% and about 35% PS by weight. A solution of about 30% PS by weight in DL can be employed. The measured conductivity of the 30% PS/DL solution is about 0.0 μS/cm (in contrast to a conductivity of about 150 μS/cm for pure DL) and the viscosity is about 3125 cps. As noted above, the PS/DL solution does not produce nanofibers of satisfactory size or quality when used as the electrospinning fluid. 
     To the PS/DL solution is treated with a solution of inorganic salt in DMF. Examples of salts that can be employed are LiCl, CuCl 2 , AgNO 3 , and FeCl 3 ; other suitable inorganic salts can be employed. Salt concentrations in DMF can be between about 0.01% and about 10% salt by weight, typically between about 0.02% and about 5% salt by weight, preferably between about 0.05% and about 1% salt by weight. The PS/DL solution and the salt/DMF solution are mixed in a selected proportion. At higher salt/DMF proportions, the resulting mixture phase-separates over a period of several hours to several days (into separate fluid layers, sometimes also with a solid precipitate). The salt/DMF-treated PS/DL (the phase-separated PS/DL layer, if phase separation occurs) is used as the electrospinning fluid. The table below summarizes some observed results using PS/DL treated with salt/DMF in various proportions. The conductivity of the salt/DMF-treated PS/DL is about 13 μS/cm in each example. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                 wgt % PS 
                   
                 % salt 
                 % salt/DMF 
                   
                 typical fiber 
               
               
                 in DL 
                 salt 
                 in DMF 
                 in PS/DL 
                 viscosity 
                 diameter 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 30% 
                 LiCl 
                 5% wgt 
                 10% 
                 vol 
                 1640 
                 cps 
                 no data 
               
               
                 30% 
                 LiCl 
                 5% wgt 
                 20% 
                 vol 
                 1200 
                 cps 
                 no data 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 30% 
                 LiCl 
                 5% wgt 
                 30% 
                 vol 
                 990 
                 cps 
                 552.5 
                 nm 
               
               
                 30% 
                 AgNO 3   
                 5% wgt 
                 30% 
                 vol 
                 795 
                 cps 
                 516.7 
                 nm 
               
               
                 30% 
                 LiCl 
                 5% wgt 
                 30% 
                 wgt 
                 325 
                 cps 
                 289 
                 nm 
               
               
                 30% 
                 CuCl 2   
                 5% wgt 
                 30% 
                 wgt 
                 540 
                 cps 
                 224 
                 nm 
               
               
                   
               
            
           
         
       
     
     Another exemplary salt/DMF-treated PS/DL comprises a solution of 30% by weight of PS (mw 192k, atactic) dissolved in DL and combined with a solution of CuCl 2  dissolved in DMF, with the amounts of CuCl 2  and DMF chosen to yield a 3:1 mole ratio of CuCl 2  to PS and a 1:1 mole ratio of DMF to DL. The resulting mixture does not phase separate and is used as an electrospinning fluid in exemplary embodiments described hereinbelow, wherein nanofibers between about 250 nm and about 300 nm are consistently produced. More generally, the amounts of salt and polar organic solvent can be chosen to result in (i) a mole ratio between about 0.5:1 and about 20:1 of the salt to the polymer in the mixture and (ii) a mole ratio between about 1:1 and about 1:4 of the polar organic solvent to the terpene, terpenoid, or aromatic solvent in the mixture. 
     The electrospinning of the fluids described above exhibit electrospinning characteristics that differ substantially from those of conventional electrospinning fluids in several ways. Nanofibers can be spun from the salt/DMF-treated PS/DL onto insulating substrates (e.g., Mylar®, Typar®, paper, and so forth) as well as conducting substrates. The flow rate during spinning of the salt/DMF-treated PS/DL electrospinning fluid is substantially larger than that of conventional electrospinning fluids (20-500 μL/min/nozzle while producing nanofibers of less than 500 nm diameter, versus 1-2 μL/min/nozzle for conventional fluids). The current carried by the spun nanofibers is substantially reduced for the salt-DMF-treated PS/DL (less than about 0.3 μA/nozzle versus greater than about 1 μA/nozzle for conventional fluids). The nanofibers produced by electrospinning the salt/DMF-treated PS/DL typically spread over a smaller area when spun than nanofibers spun from conventional electrospinning fluids (e.g., a spot about 0.5 inch in diameter versus about 2 inches in diameter when spun from a nozzle about 7 inches from the target substrate). Instead of requiring a syringe pump or similar mechanism to drive fluid flow (as is the case when using conventional electrospinning fluids), fluid head pressure behind the nozzle can be sufficient to sustain electrospinning of the salt/DMF-treated PS/DL fluid (in one example only about 1 inch of fluid pressure was sufficient). In contrast to nanofibers produced by conventional electrospinning fluids, which can vary widely (for example, from less than 200 nm to greater than 1 μm) in their diameter based on operating conditions such as voltage or flow rate, electrospinning the salt/DMF-treated PS/DL fluid typically produces nanofibers in about a 250-300 nm range over a wider range of operating conditions. In one example, electrospinning with a fluid head pressure of about 1 psi and an applied voltage of 80 kV results in a flow rate of about 59 μL/min/nozzle and fibers of about 278 nm average diameter. In another example, the same pressure and flow rate with an applied voltage of 40 kV yields fibers of about 282 nm average diameter. In yet another example, applying a fluid head pressure of about 10 psi and applying about 80 kV results in a flow rate of about 135 μL/min/nozzle and fibers of about 235 nm average diameter. 
     Alternative solvents can be employed for dissolving the polystyrene (or other polymer); examples of candidate solvents include but are not limited to: limonene derivatives (e.g., carveol or carvone); other terpene-based solvents or terpenoid derivatives (e.g., α-pinene, β-pinene, 2-pinanol, camphene, α-myrcene, cis-α-ocimene, linalool, nerol, geraniol, citronellol, Y-terpinene, α-phellandrene, p-cymene, terpinolene (1,4(8)-menthadiene), isolimonene (2,8-menthadiene), ψ-limonene (1(7),8-menthadiene), or 1(7),4(8)-menthadiene); aromatic solvents (e.g., benzene or toluene); tetrahydrofuran or other ethers; or other similar solvents or mixtures thereof. Alternative non-polar, non-conductive polymers can be employed; examples of candidate polymers include but are not limited to: styrene butadienes, other aromatic side chain polymers, polymethylmethacrylate (PMMA) or other acrylate polymers, polyvinylchloride (PVC), or copolymers or derivatives thereof. Alternative salts can be employed; examples include but are not limited to LiCl, AgNO 3 , CuCl 2 , or FeCl 3 . In addition to treating the polymer solution so that it spins, silver salts can also impart desirable antimicrobial properties onto the deposited electrospun nanofibers. Alternative solvents can be employed for dissolving the salt to treat the non-polar polymer solution that preferably do not reduce the solubility of the polymer in its solvent; examples include but are not limited to: DMF, N-methyl-2-pyrrolidone (NMP), or tetrahydrofuran (THF). 
     The electrospun nanofibers listed in the table above were formed using a conventional electrospinning arrangement, in which a conduction path is established between the electrospinning fluid and the high voltage supply (either through ground or through the supply&#39;s voltage output). However, the salt/DMF-treated PS/DL electrospinning fluids can also undergo electrospinning without any electrical conduction path to the electrospinning fluid (i.e., if the electrospinning fluid and the nozzles are electrically isolated, which means both from the voltage supply and from ground). The high voltage applied to drive the electrospinning process is applied to a plate, screen, or mesh  102  that is electrically insulated from the electrospinning fluid  10  and provided with passages or perforations  103  for the electrospinning nozzles  104  (alternatively, with a single opening that accommodates all of the nozzles  104 ). The nozzles  104  are also electrically insulated from the plate, grid, or mesh  102  and are arranged to convey the electrospinning fluid  10  through the passages  103 , whether flush with ( FIG. 1A ) or extending through ( FIG. 1B ) the plate, grid, or mesh  102 . The applied electrostatic field drives the electrospinning process, but with substantially less current flow per nozzle and substantially less charge deposited onto the target substrate, relative to the conventional arrangement with a conductive path. In the isolated-nozzle arrangement, the nozzles  104  can be formed from an suitable insulating material (e.g., Teflon®, polyethylene, ceramic, glass, and so on). 
     Conventional fluids have also been observed to undergo electrospinning without a conduction path between the fluid and the high voltage supply. It has been observed qualitatively that such isolated-nozzle spinning with conventional fluids requires up to four times the applied voltage to initiate electrospinning (e.g., 60 kV versus 15 kV), produces a substantial, readily visible and audible corona discharge near the nozzle (versus only a audible corona discharge), and deposits substantially more surface charge onto an insulating target substrate (loud, visible spark when discharged versus no audible or visible discharge) compared to isolated-nozzle electrospinning with salt/DMF-treated PS/DL. 
     An exemplary electrospinning head  200  is shown in  FIGS. 2A and 2B . The head  200  comprises (i) a metal plate  202 , (ii) an array of  110  metal tubes  204  (stainless steel in this example) about 1 inch long arranged in a 3 inch by 3 inch square grid pattern on ¼ inch centers with a nozzle end extending about ½ inch from one surface of the plate, and (iii) a bundle of  110  corresponding capillary tubes  206  (Teflon® of other suitable dielectric insulating material), each about 8 inches long and about 450 μm in inner diameter. The capillary tubes  206  are connected to a fluid reservoir (not shown) at one end and are each inserted into the back end (opposite the nozzle end) of a corresponding one of the metal tubes  204 . The fluid reservoir can be controlled to apply a desired level of head pressure on the fluid in the reservoir. A high voltage supply (not shown) applies voltage to the metal plate  202  (and hence all the metal tubes  204 ) to cause electrospinning of the fluid in the reservoir through the capillary tubes  206 . 
     The capillary tubes  206  can be recessed about ½ inch (or other suitable distance) into the metal tubes  204  (as in  FIG. 2A ) so that the electrospinning head  200  acts as a conventional spinning head, with a conduction path between the electrospinning solution and the voltage supply (through metal plate  202  and metal tubes  204 , which are in contact with the spun fibers  20  after they leave the recessed ends of the capillary tubes  206 ). Alternatively, the capillary tubes  206  can be extended out from the nozzle ends of the metal tubes  204  by about ½ inch (or other suitable distance; as in  FIG. 2B ) to act as an electrically isolated nozzle, eliminating any conduction path between the electrospinning solution and the high voltage supply. 
     The arrangements of  FIGS. 2A and 2B  enables comparison of electrospinning behavior and spun material between the differing operating conditions. The capillary tubes  206  act as fluid flow restrictors according to the Hagen-Poiseuille equation and function in a manner analogous to that of the flow restrictors disclosed in US 2008/0131615 (incorporated above). For a given fluid composition and flow rate, the voltage can be adjusted to minimize the amount of fluid that drips or flows (not as fibers) from the spinning head, presumably by optimizing the balance of electrostatic and hydrodynamic forces acting on the fluid jet. Conversely, for a given fluid composition and voltage, the flow rate can be chosen to minimize the amount of fluid that drips or flows (not as fibers) from the spinning head. That optimized flow rate is typically larger when the fluid is spun from electrically isolated capillary tubes  206  than when it is spun from the metal tubes  204  due to the differing balance of those forces. 
     Using head  200 , a salt/DMF-treated PS/DL fluid (the non-phase-separating composition described above) was electrospun onto an insulating Mylar® target substrate, onto a non-insulating scrim substrate, onto an aluminum foil target substrate (in each of those three cases with the substrate resting on a conductive ground plate, screen, or mesh), and onto an electrically isolated aluminum foil target substrate. In all cases a head pressure of about 0.5 psi was applied to the fluid in the reservoir and the flow rate was about 5.7 μL/min/nozzle. The electrospinning fluid was in contact with the high voltage supply through plate  202  and metal tubes  204 . As shown in the table below, when electrospinning a salt/DMF-treated PS/DL fluid, varying the voltage or the nature of the target substrate has remarkably little effect on the average size of the resulting nanofibers. Similar fiber sizes (about 250-300 nm) were obtained when spinning the salt/DMF-treated PS/DL (non-phase separated composition) from electrically isolated nozzles. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                 avg fiber 
               
               
                   
                 substrate 
                 voltage 
                 diameter 
               
               
                   
                   
               
             
            
               
                   
                 scrim on ground plate 
                 40 kV 
                 264.8 nm 
               
               
                   
                 scrim on ground plate 
                 60 kV 
                 239.8 nm 
               
               
                   
                 scrim on ground plate 
                 74 kV 
                 237.0 nm 
               
               
                   
                 scrim on ground plate 
                 90 kV 
                 254.1 nm 
               
               
                   
                 Mylar ® on ground plate 
                 75 kV 
                 288.1 nm 
               
               
                   
                 Mylar ® on ground plate 
                 76 kV 
                 252.4 nm 
               
               
                   
                 Mylar ® on ground plate 
                 80 kV 
                 254.6 nm 
               
               
                   
                 Mylar ® on ground plate 
                 90 kV 
                 247.0 nm 
               
               
                   
                 aluminum foil (grounded) 
                 71 kV 
                 251.7 nm 
               
               
                   
                 aluminum foil (isolated) 
                 71 kV 
                 254.3 nm 
               
               
                   
                   
               
            
           
         
       
     
     It is intended that equivalents of the disclosed exemplary embodiments and methods shall fall within the scope of the present disclosure or appended claims. It is intended that the disclosed exemplary embodiments and methods, and equivalents thereof, may be modified while remaining within the scope of the present disclosure or appended claims. 
     For purposes of the present disclosure and appended claims, the conjunction “or” is to be construed inclusively (e.g., “a dog or a cat” would be interpreted as “a dog, or a cat, or both”; e.g., “a dog, a cat, or a mouse” would be interpreted as “a dog, or a cat, or a mouse, or any two, or all three”), unless: (i) it is explicitly stated otherwise, e.g., by use of “either . . . or”, “only one of . . . ”, or similar language; or (ii) two or more of the listed alternatives are mutually exclusive within the particular context, in which case “or” would encompass only those combinations involving non-mutually-exclusive alternatives. For purposes of the present disclosure or appended claims, the words “comprising,” “including,” “having,” and variants thereof shall be construed as open ended terminology, with the same meaning as if the phrase “at least” were appended after each instance thereof. 
     In the appended claims, if the provisions of 35 USC §112 ¶6 are desired to be invoked in an apparatus claim, then the word “means” will appear in that apparatus claim. If those provisions are desired to be invoked in a method claim, the words “a step for” will appear in that method claim. Conversely, if the words “means” or “a step for” do not appear in a claim, then the provisions of 35 USC §112 ¶6 are not intended to be invoked for that claim.