Hello Everyone,
I will be working this summer on optimizing a needleless electrospinning apparatus to increase production rate, then after consistent results and ideal morphology, I will embed LDH nanosorbents in a mat to use as a membrane for water purification.
The basis for electrospinning has been around for about 400 years roughly, William Gilbert was one of the first to observe electrostatic attraction but it was not until 1900 when John Francis Cooley filed the first patent for an electrospinning device that this attraction was harnessed to make materials. Electrospinning uses electric forces to draw charged threads of polymer solutions to create fiber mats with the diameter of the fibers being in the order of some hundred nanometers. The set up for a conventional electrospinning apparatus is simple. A high voltage source, syringe pump, a solution of the desired polymer and a collector plate are all that is needed.
The picture to the left is the conventional set up that I started out on when I began at ASU. When running the set up the high voltage lead is clipped onto the tip of the blunt needle. Placed about 10-15 cm away was the collector plate that has aluminum foil with a ground lead attached to it. It took some time before I could get a full mat to be made with fibers. One of the biggest issues that I ran into was that instead of electrospinning, electrospraying was occurring. Electrospraying occurs when a droplet of a polymer solution is highly charged and it is not able to coagulate it is then overcome by the Coulomb repulsion. Instead of having fibers on the collector plate a powder of the polymer is coated on the collector.
Electrospray on the left and Electrospinning on the right
After researching more on what causes electrospraying to occur I found that to have fibers that stay linked all the way to the collector the molecular cohesion of the polymer must be relatively high. The polymer solution I was using was a 30% by weight PVP with a molecular weight of 29,000. After reading this we decided to switch to a higher molecular weight of PVP (1,300,000 Mw). We started with a 5% (PVP) by weight solution using water as the solvent. After some tweaking, I was finally able to get fibers on the collector plate that formed a mat. After repeating this twice and getting consist results it was time to set up the needless apparatus. The main benefit to a needless set up is that the production rate is increased as well as not having to deal with clogging (tip of the needle). In the conventional apparatus, we could get flow rates anywhere from .3 ml/hr all the way down to .02 ml/hr. Such slow flow rates make it a lengthy process to be able to have a thick mat. The needleless electrospinning setup uses the same basic principles in the conventional setup a conductive wire is placed vertically with a circular collector placed around, there is a tube connected to the syringe on the pump that delivers the solution at a constant rate down the wire. When a high voltage is applied there is a buildup of charge in the solution and when it hits a peak the solution is charged and electrostatic repulsion counteracts the surface tension of the droplets on the wire and a stream of liquid erupts from the wire. As the stream of liquid flies towards the collector, it dries and elongates. Below is a picture of the needless set up that I will be optimizing this summer.
Using the high molecular weight PVP two mats were successfully electrospun. The first mat that was spun used a 10% by weight PVP, the mat that was made after 3 hours was very thin. A small sample was taken to do SEM, from this it was found that the fibers were the correct size with respect to the diameter but there were curlicues in the mat. After seeing the curlicues, the next mat that was spun the polymer concentration was bumped up to 15%, this seemed to do the trick. After three hours of spinning a thick mat was produced and the morphology was almost spot on. There are a few bubbles in the fibers that are currently being troubleshot.
Below are some SEM pictures for both the 15 and 10% PVP solutions.
15 % PVP 2000x
10 % PVP solution 250x
15 % PVP solution 100x
10 % PVP 1000x
-Tanner Rosenthal