Nanopottery: coiling of electrospun polymer nanofibers

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Fall 2010 entry: Anna Wang

References

Nanopottery: coiling of electrospun polymer nanofibers H-Y Kim, M Lee, K-J Park, S Kim and L. Mahadevan, Nanoletters, 10, 2138, 2010.

Introduction

Whether a system favours stretching modes or deformation is a result of the interplay between internal elastic and viscous forces and external forces such as inertia and gravity. Problems of this nature are relevant over several orders of magnitude – from the small scale problems in soft-matter to geophysics. Kim et al study the spontaneous coiling of polymer nanofilaments and examine the forces involved to determine a simple scaling law.

Experimental setup

Figure 1. Previously suggested setups for depositing nanofibres, jet duration 0.7s. The deposited structures are all chaotic. (a) Using an Al plate spaced closely to a capillary tip, voltage difference 10000kV. (b) Using a glass sheet above a sharp ground tip.
Figure 2. New setup resulting in nanocoils. (a) The setup inclues a capillary placed close to a grounded collector tip. (b) Snapshot of the nanojet coiling into a cylinder, radium 470nm. (c) Snapshots of coiling process.
Figure 3. SEM of coiled structure on top of stainless steel collector tip.

Polymeric filaments can be electrospun onto a substrate by hanging a drop of polymer solution at a capillary needle tip, and using a strong electric field to draw out a nanoscale jet. Figure 1 shows previous attempts to deposit filaments of polymer – in all these cases, the deposition was chaotic due to the bending instability of the jet from its surface charges.

The setup used in this study is shown in Figure 2. The main modification is the use of a strongly focused electric field at the ground. Solutions of poly(ethylene oxide) of varying concentrations (6, 10 and 14 wt% used, MW = 300000) and hence viscosities and permeabilities were ejected using an electric field exceeding 1.2e6 V/m. The polymer emerged as a 1mm diameter drop which then formed a jet and dried out rapidly. The constant diameter of the jet in Figure 2b) is indicative of the jet being effectively dry and a solid when it impinged on the target. The jet had a radius of ~50nm, whirling at ~10000rpm to form a nanocoil with radius ~3µm, height 40µm.

Characterisation

Figure 4. Coil radii plotted according to the scaling law. The different shapes correspond to different concentrations of PEO.

The dominant forces acting on the fibre are the electrostatic force, inertial forces and gravitational forces. The last two were found to be 5 and 4 orders of magnitude smaller than the electrostatic force and hence negligibly small.

Considering the spooling radius R of the system as a balance between the electrostatic torque and the elastic torque, it was found that R ~ (Y/εE)1/3r where Y is the Young’s modulus of the fibre, r is the radius of the fibre, E is the strength of the electric field and ε is the permittivity of free space. The values of all of these variables are readily available except Y.

To determine the Young’s modulus of the fibres, atomic force microscopy was used to probe fibres which were hung over a microtrench on a silicon wafer. Figure 4 shows that the coiling radius does indeed follow the simple scaling law as E, Y and r are varied.

Conclusions

Regular coiling of polymeric fibres was experimentally demonstrated, and the physics behind it was demonstrated via a simple scaling law. Such regularly coiled structures may be of use in nanoscale magnets, bioscaffolds and nanochannels.