Mechanical Properties of Electrospun Nanofibers

Leon Marcel Bellan

Electrospinning is a simple and quick technique for producing fibers with nanoscale diameters from a wide range of materials. In this process, a strong electric field causes a viscous solution to form a Taylor cone, from which a thin fluid jet is formed. This fluid jet may harden by a variety of processes and become a continuous fiber. There are various techniques for collecting oriented fibers, including patterning a collector electrode to cause the fibers to jump between specific positions on the substrate [1] and rotating the substrate to allow deposition of fibers in a spiral [2] or linear [3] pattern. We used the scanned electrospinning technique [4, 5] to deposit oriented polymer and glass fibers over trenches for mechanical characterization.

We have deposited polyethylene oxide (PEO) nanofibers over microfabricated trenches and used an atomic force microscope (AFM) to measure their Youngs moduli. The AFM probes were blunted by tapping them against a Si surface for at least 20 minutes so that they didnt pierce the fibers. The fibers were depressed at the center of the trench and force-displacement data was recorded. The Youngs moduli were extracted from the cubic term of a third order polynomial fit to the data. The resulting values were significantly larger than what has been reported for bulk PEO, suggesting molecular orientation. We also deposited fibers composed of polysiloxane (a type of spin-on glass) mixed with polyvinylpyrrolidone (PVP). We measured the Youngs moduli of these fibers both before and after baking them (at which point they become silica glass). The Youngs moduli of these fibers increased by a factor of 10 after baking and agreed with values calculated from the mechanical resonance frequencies of similar fibers from a previous study [6].

We have also demonstrated the ability to electrospin polydicyclopentadiene (PDCPD) fibers. This electrospinning process differs from common electrospinning processes in that the jet hardens not by solvent evaporation but by in-flight polymerization. Initially, dicyclopentadiene monomer was mixed with Grubbs generation II catalyst and began to polymerize. As the polymerization progressed, the solution became more viscous and it was possible to electrospin fibers from it. Several PDCPD fibers were collected over trenches for future mechanical characterization.

We have electrospun PEO and glass fibers over microfabricated trenches and measured their Youngs moduli with an AFM. The resulting values for the PEO fibers were significantly larger than the typical bulk value, which supports previous claims of molecular orientation in electrospun nanofibers. The Youngs moduli of the glass fibers agreed with values calculated from previously measured mechanical resonance frequencies of electrospun glass fibers. We have also demonstrated the ability to electrospin PDCPD fibers from a solution that polymerizes in flight.

Figure 1: Suspended PDCPD fiber.

Figure 2: AFM probe dulled by tapping against Si.

Figure 3: AFM probe cut with FIB.

References:

1. "Electrospinning of Polymeric and Ceramic Nanofibers as Uniaxially Aligned Arrays", Li, D., Yuliang, W., Younan, X., Nano Letters, 3, (8), 1167 (2003).

2. "Polymeric Nanowire Architecture", Kameoka, J., Czaplewski, D., Liu, H. Q., Craighead, H. G., Journal of Materials Chemistry, 14, (10), 1503-1505 (2004).

3. "Electrostatic Field-Assisted Alignment of Electrospun Nanofibres", Theron, A., Zussman, E., Yarin, A. L., Nanotechnology, (3), 384, (2001).

4. "Fabrication of Oriented Polymeric Nanofibers on Planar Surfaces by Electrospinning", Kameoka, J., Craighead, H. G., Applied Physics Letters, 83, (2), 371-373, (2003).

5. "A Scanning Tip Electrospinning Source for Deposition of Oriented Nanofibres", Kameoka, J., Orth, R., Yang, Y. N., Czaplewski, D., Mathers, R., Coates, G. W., Craighead, H. G., Nanotechnology, 14, (10), 1124-1129 (2003).

6. "Fabrication of Suspended Silica Glass Nanofibers from Polymeric Materials Using a Scanned Electrospinning Source", Kameoka, J., Verbridge, S. S., Liu, H. Q., Czaplewski, D. A., Craighead, H. G., Nano Letters, 4, (11), 2105-2108 (2004).