Bio-Inspired Polymer Membranes For Resilience of Electrochemical Energy Devices

Electrochemical energy storage and conversion devices such as batteries and fuel cells function by creating an electrical charge difference across two electrodes that drive electron flow through a circuit to power devices. The polymer film (membrane) that separates these electrodes is critical for controlling ion transport and must be mechanically robust. Synthetic membrane performance and durability can be dramatically improved by imbuing the membrane with the ability to sense and adapt to the local electrochemical environment, similar to biological membranes. The overarching goal of this project is to discover the basic science governing self-regulation of mechanical properties and self-healing for ionically-bonded polymer and gel membranes within electric fields.

My research entails setup, running, and analysis of molecular dynamics (MD) simulations of polymers with ionically charged functionalities to determine the influence of an external electric field on the strength of ionic bonds between polymer chains and how this interaction depends on concentration of ionic bonds, any solvent, free ions, polymer backbone rigidity, chemical composition of the polymer. Next step is to determine how ionic bond strength and concentration among linear polymer chains influences polymer mechanical properties (stiffness, strength, toughness, viscoelasticity) and self-healing for different polymer backbone rigidities and chain lengths.

This year has primarily been used to establish simulation and experimental infrastructure. We have established both coarse grained (CG) and fully atomistic (FA) molecular dynamics simulations. The purpose of the CG simulation is to enable rapid exploration of the parameter space. The purpose of the FA simulation is (1) to verify that the CG results are reasonable and (2) to allow direct comparison with our future experimental results. Preliminary deformation simulations have been run with each of these equilibrated models at 100 K. The CG models exhibit an essentially bilinear behavior with a gradual transition between the two linear regions (Figure 1).

In terms of experimental infrastructure, the group has configured an experimental setup to perform uniaxial tensile tests with a large electric field across the specimen. MMD (Mechanics for Material Design) lab purchased a CellScale UStretch device and assembled it, and also designed, built, and revised a parallel plate high voltage setup to apply a field across the specimen, perpendicular to the tensile direction. The ionic polymers were synthesized by radical co-polymerization of 2-Hydroxyethyl acrylate with either an anionic monomer (potassium sulfopropyl-acrylate) or a cationic monomer (ethyl-trimethylammonium chloride acrylate) with 4,4’-Azobis(4-cyanovaleric acid) as the radical initiator in water. The polymers were purified in dialysis bags to remove low molecular weight impurities (monomers and oligomers). The fractions of functional monomers were determined by comparing the integration values for each of the methylene protons in the 1H-NMR spectrum of the polymer.

Future experimental work is to characterize the first set of polymers first in solution and then in the solid state. In solution, we will apply an electric field across a droplet of polymer in solvent and watch the migration of the polymer chains. In order to image ionic crosslinking separating by electric field we will color each polymer with a different acrylic dye (Figure 2). In terms of MD simulations, we will be focusing on the CG model. We will first make some improvements and then use it to explore the importance of different physical parameters – the charge on the ionic beads and the fraction of ionic beads


To check out the analysis codes used in the project, follow this link.