Rex P. Hjelm/Photo Courtesy New Mexico Consortium
Los Alamos National Laboratory and New Mexico Consortium Scientist Rex P. Hjelm gave an invited presentation Friday for the Graduate Seminar Series of the Department of Materials and Chemical Engineering, New Mexico State University, entitled, “Soft Matter Materials Physics by Neutron Scattering Techniques and Applications to Device Engineering”.
Hjelm spoke about scattering from light, electrons, x-rays and neutrons which have led to fundamental discoveries in soft matter physics and processing behavior. Among these techniques, neutron scatter is an important complementary probe of soft matter materials, structure and dynamics. Properties that distinguish neutron methods from others, include salient differences of scatter from the nuclei of light elements and their isotopes, wavelengths and energies that match well with the length and energy scales of these materials, a magnetic moment for studies of magnetic structure, and penetrability into bulk samples and robust sample environments. Of major importance for soft materials research is contrast labeling by the substitution of deuterium for hydrogen.
Hjelm presented how these aspects of neutron techniques provide important insight into soft materials physics and its applications to device engineering, using studies of ionomer polymers in fuel cell membrane electrode assemblies (MEAs) as an example. Commercial MEA ionomers are amphiphilic, by virtue of a hydrophobic Teflon-like main chain with hydrophilic sulphonate- terminated fluoroether pendant chains.
The hypothesis is that ionomer amphiphilic balance can be used as a tool, by considering the thermodynamics and kinetics of the interactions of ionomers with solvent and catalyst interfaces, to understand and control of MEA morphology and performance. In his research, Hjelm used small-angle neutron scattering (SANS) and neutron reflectometry (NR) to test of this notion. With SANS he probed the process-induced evolution of ionomer morphology and interactions using solvents with different ionomer Flory-Huggins interaction parameters.
The results of Hjelm’s research showed that the degree of solvent interaction and kinetics promoted chain entanglements leading to improved MEA durability. Solvent penetration and polymer self-assembly and interactions were also affected by amphiphilic balance. Comparison with transport properties leads to the hypothesis that charge effects and particle aspect ratios are associated with formation of MEA transport channels. During the course of the voltage cycle, Pt catalyst goes though a hydrophobic (Pt metal) to hydrophilic (Pt-oxide) transition. Hjelm used NR to show that this result in the movement of water to and from the interface. This effect is greater with amphiphilic balance and may explain the corresponding difference in hysteresis observed during cyclic voltammetry.