Atomic Scale Description of Ionic Behavior in Polymer Nanopores

Abstract

Polyethylene terephthalate (PET) is a carbon-based polymer commonly used in plastic containers. Due to its high melting point, mechanical strength, and selectivity for chemical modification, PET has become an excellent material for use in nanotechnology applications. In particular, PET membranes have been used to build very small pores with nanometer-scale diameters, so called “nanopores”. Several interesting phenomena have been observed in PET nanopores, such as ionic current rectification, reverse rectification due to divalent cations, and nanoprecipitation. However, understanding the physical basis behind such phenomena is still a challenge. We have used molecular dynamics (MD) simulations to study the ionic transport properties of PET nanopores, including the conduction of KCl under different pH conditions and the effect of divalent ions on the ionic conduction and nanoprecipitation. To carry out these simulations, we have developed a protocol to build PET nanopores: First, we constructed a periodic model of bulk PET; then, we created a PET nanopore by removing atoms from a conical region and patching the exposed ends with benzoic groups, the PET surface reproducing the surface charge observed in experiments; finally, the PET nanopores are solvated and simulated under a variety of voltage biases using different ionic species, such as K(+1), Cl(-1), Ca(2+) and HPO4(2-) ions. We applied the protocol and found that it resulted not only in good agreement with experimental data, but also provided an atomic description of the ion dynamics in PET nanopores. Specifically, we observed the enhancement of ionic current due to the surface charge, the permanent binding of Ca(2+) ions to the PET surface, and the dynamics of HPO4(2-) ions inside PET nanopores.