Ionomer Dynamics in Confinement: Fundamental Insights from Dielectric Spectroscopy

NON-TECHNICAL SUMMARY:

This project will create knowledge on the dynamic properties and the motions of ions in ion-containing polymeric materials, which will facilitate understanding and optimization of next-generation power sources and electrochemical devices. In particular, the performance of these materials will be explored when their dimensions are restricted to only a few nanometers. The research deals with the way the properties of ion-containing polymers change when confined inside porous membranes, in comparison to the bulk material. For example, there is evidence that some polymer molecules will be absorbed onto the pore surface (strong guest/host interaction) resulting in significant slowing of polymer motions and reduced ion conductivity. These surface effects can be negated when the pores are chemically treated to create weak guest/host interactions, resulting in enhanced ion conductivity. The findings of this work can have a significant impact on the future development of power sources and electrochemical devices, particularly those with nanometer feature sizes.

In terms of human resources, this research project will create learning opportunities for two graduate students. Undergraduate students will also participate meaningfully, through the Penn State Women in Science and Engineering (WISE) Research program, senior thesis projects, and as part of the Penn State NSF-REU program on soft materials. Program participants will be encouraged to engage in outreach activities, particularly those connected with the WISE Institute.

TECHNICAL SUMMARY:

This project targets an important unexplored area: the role of nanoscale confinement on ion transport in conductive ionomers. The mobility of ionic species in confined geometries is an important topic in polymer physics and has future practical relevance in the design and processing of ion-containing polymer nanostructured devices. The motivation for investigating how nanoscale confinement influences ion transport in ionomers arises from the quest to understand how their properties and performance change as their dimensions are restricted to length scales of a few nanometers. To this end, a comprehensive investigation of the molecular dynamics of two conductive ionomer systems confined in cylindrical silica nanopores is proposed. Silica membranes are particularly advantageous for these experiments, as pores with diameters in the range of 4 to 10 nm can be readily achieved. Strong interfacial interactions between the host membrane and guest ionomer will lead to slower dynamics, while spatial restriction at the nanometer length scale can have the reverse effect of enhancing ion transport. Dielectric spectroscopy is an ideal tool to investigate the dynamics of polymers in nanoporous media owing to its ability to probe molecular fluctuations over a wide frequency and temperature range. Aggregation of ion dipoles in ionomers has important consequences for ion conduction, particularly through its influence on polymer segmental dynamics, which in turn is generally coupled with ion transport. The influence of confinement on ion dipole aggregation has not been explored previously, but it is essential to do so in this investigation to provide a complete understanding of the role of confinement on the molecular dynamics.