Simulated Force Spectroscopy of Polyethylene Crystals

Single crystals of polyethylene can be grown from dilute solvent solutions. The resulting crystals are a few microns across, but are only 10-20 nm thick. The crystals consist of long chains of polymer folded back on themselves. Treatment in concentrated nitric acid can break covalent bonds on the surface leaving free ends, which can be 'picked up' using an AFM probe. Chains can then be pulled out of the bulk crystal, the unfolding process should give a distinct signature in the force curve.

To understand the dynamics of the unfolding process, atomistic molecular dynamics simulations are performed. By holding the end atom of a folded chain in place and slowly moving it away from the surface, a simulated force curve can be produced. These force curves have a periodic nature, with one event for every 2 folds in the chain.

AFM image of a polyethylene crystal.

Force curves for chains with 7 folds. 30 and 40 carbon atoms per fold.

It is also possible to simulate deposition of chains onto the crystal surface. This is achieved by partially pulling a chain from the crystal. Once the chain is part way out of the crystal, it can be lowered back onto the surface. In this model an infinite plane, attached by a hookean spring to a fixed point, represents the AFM cantilever. This pivot point can then be moved towards the surface, as well as oscillated either parallel to or normal to the crystal.

By changing variables such as the rate of deposition, the temperature and the solvent quality it is hoped that the polymer chain will adopt different configurations on the surface.

Single chain being pulled from the bulk crystal.