(94f) Anisotropy in Layered Polymer-Clay Nanocomposites

The construction of organic polymer and inorganic clay composites has long been studied as mixtures, aqueous suspensions, and intercalation structures. These composites have been shown to act as electrolytes, reinforced materials, and even flame retardants. Specifically, we are interested in designing self-assembled polymer-clay composites with the layer-by-layer (LBL) technique to understand the connection between structure and mateials properties in organic-inorganic LBL assemblies. The influence of structure in a polymer-clay composite can be probed with impedance spectroscopy to track the motion of ions through the composite. An exfoliated polymer-clay composite, where the clay platelets are evenly dispersed, would show isotropic transport of ions. Alternately, a highly ordered composite would show preferential transport relative to nanoplatelet orientation. The study of transport in LBL composites is key to understanding how these materials might be used in a Li-ion battery as a single ion conductor or a fuel cell as a blocking layer for fuel crossover.

Here, we fabricate trilayers of linear poly(ethylene imine) (LPEI), Laponite clay, and poly(ethylene oxide) (PEO) to produced a highly ordered composite. First, positively charged LPEI is adsorbed upon a substrate, then negatively charged Laponite, and lastly, neutral PEO. These three adsorption steps are repeated ad infinitum with the layer-by-layer (LBL) technique to give films with nanometer-scale control (~ 5 nm per trilayer deposition). Laponite clay is of special interest because it is a single ion conductor, effectively eliminating concentration polarization in electrolyte membranes. PEO and LPEI are known polymer electrolytes that help bind the clay platelets together.

Through AFM, SEM, and XRD, we observe a structure where clay nanoplatelets self-assemble in highly oriented sheets parallel to the substrate surface, like bricks with polymer “mortar”. This well-defined structure yields anisotropic ion transport cross-plane and in-plane to the nanoplatelet sheets. Ion conduction is hindered in the cross-plane as ions have trouble navigating a tortuous path around nanoplatelets, and ion conduction is promoted in-plane because of “channels” created by polymer intercalation generated in the LBL technique.

Future work entails investigating the permeation properties of these oriented nano-composites. Like cross-plane ion conduction, gas permeation or fuel crossover is expected to be blunted as the layered clay film blocks gas diffusion.