University of Puerto Rico at Humacao Department of Physics and Electronics

Transcription

1 University of Puerto Rico at Humacao Department of Physics an Electronics Experiment # 6: Electrospinning I Neliza León Brito Prof. Nicholas Pinto FISI February 27, 2007

2 I. Purpose: The purposes of this experiment is to measure, using the scanning electron microscope (SEM), an compare the iameter of human hair an fibers obtaine using electrospinning. II. Introuction: Electrospinning traces its roots to electrostatic spraying, which was first escribe more than 100 years ago. In electrostatic spraying, charge is injecte into a liqui, typically -30 kv, from an electroe. The charge liqui is separate some istance from a secon electroe (target) of opposite polarity to establish a static electric fiel. At a characteristic voltage a suspene roplet forms a Taylor cone an a fine jet of polymer releases from the surface in response to the tensile forces generate by interaction of an applie electric fiel with the electrical charge carrie by the jet. For liquis with a finite conuctivity, charge roplets are isperse from the tip of the Taylor Cone an are elivere to the target. If the liqui consists of a polymer melt or a polymer in solution an the concentration of that polymer is sufficiently high to cause molecular chain entanglement, a fiber, rather than a roplet, is rawn from the tip of the Taylor cone. The jet is then elongate an whippe continuously by electrostatic repulsion until it is eposite on the groune collector. Whipping ue to a bening instability in the electrifie jet an concomitant evaporation of solvent, an, in some cases, reaction of the materials in the jet with the environment allow this jet to be stretche to nanometer-scale iameters. [1-3] A basic electrospinning system consists of a charge polymer solution (or melt) that is fe through a small opening or nozzle (usually a neele or pipette tip). Because of its charge, the solution is rawn towar a groune collecting plate (usually a metal screen, plate, or rotating manrel), typically 30 cm away, as a jet. A picture of the setup for electrospinning is shown on figure 1. [1] For this experiment the polymer to be use will be polyethylene oxie. Polyethylene oxie (PEO) is a polymer compose of repeating subunits of ientical structure, calle monomers, shown below, HO-(CH 2 -CH 2 -O) n -H Poly (ethylene oxie) refers to an oligomer or polymer of ethylene oxie. Historically PEO has tene to refer to longer polymers. PEO is a liqui or low-melting soli, epening on its molecular weights. It is prepare by polymerization of ethylene oxie. 2

3 Figure 1. Setup use for electrospinning Figure 2 an 3 shows a comparison of ifferent objects with their sizes. Figure 1 also shows the kin of instrument that shoul be use to observe the objects in the ifferent scales. 3

4 Electrospinning I Figure 2. Comparison of the sizes of ifferent objects an the instrument that must be use to observe the object 4

5 Electrospinning I Figure 3. Comparison of sizes of natural an man-mae things [4] To observe the etails of the fibers prouce by the electrospinning technique a scanning electron microscope (SEM) must be use. In a typical SEM, electrons are thermionically emitte from a tungsten or lanthanum hexaborie (LaB6) cathoe an are

6 accelerate towars an anoe; alternatively, electrons can be emitte via fiel emission (FE). Tungsten is use because it has the highest melting point an lowest vapour pressure of all metals, thereby allowing it to be heate for electron emission. The electron beam, which typically has an energy ranging from a few hunre ev to 0 kev, is focuse by one or two conenser lenses into a beam with a very fine focal spot size 1 nm to nm. The beam passes through pairs of scanning coils in the objective lens, which eflect the beam in a raster fashion over a rectangular area of the sample surface. Through these scattering events, the primary electron beam effectively spreas an fills a tearrop-shape volume, known as the interaction volume, extening from less than 100 nm to aroun µm into the surface. Interactions in this region lea to the subsequent emission of electrons which are then etecte to prouce an image. X-rays, which are also prouce by the interaction of electrons with the sample, may also be etecte in an SEM equippe for energy-ispersive X- ray spectroscopy or wavelength ispersive X-ray spectroscopy. [] III. Results: The precursor solution use in this experiment was compose of 100 mg of polyethylene oxie issolve in 10 ml of chloroform. The solution was putte in a syringe an a voltage ifference of kv was applie between the syringe an the collector (aluminum foil). A piece of aluminum foil was cut an examine in the SEM. The photos are shown on figure 4 an. Also, a photo from human hair was taken as shown 6 an 7. Figure 4. Low magnification photo of the fiber 6

7 Figure. High magnification photo of the fiber Figure 6. Low magnification photo of a human hair. 7

8 Figure 7. High magnification photo of a human hair. The iameter of the fiber was measure an the average was calculate as shown below. f f = 1 2 ( ) = = µ m 3 4 µ m For the human hair, h h = ( ) = = 7.08µ m µ m IV. Conclusions: In this experiment fibers with sub-micron iameters were obtaine using the electrospinning technique. Their iameter was approximately 70 times less than the human 8

9 hair measure. It coul be conclue that this is a technique to get small fibers easily an using low energy. V. References: [1] [2] [3] [4] [] 9