Publications

We, the Postdoc Professional Development Program (PD2P) leadership team, wrote these postdoc guidelines to be a starting point for communication between new postdocs, their staff mentors, and their managers. These guidelines detail expectations and responsibilities of the three parties, as well as list relevant contacts. The purpose of the Postdoc Program is to bring in talented, creative people who enrich Sandia's environment by performing innovative R&D, as well as by stimulating intellectual curiosity and learning. Postdocs are temporary employees who come to Sandia for career development and advancement reasons. In general, the postdoc term is 1 year, renewable up to five times for a total of six years. However, center practices may vary; check with your manager. At term, a postdoc may apply for a staff position at Sandia or choose to move to university, industry or another lab. It is our vision that those who leave become long-term collaborators and advocates whose relationships with Sandia have a positive effect upon our national constituency.

Ionomers, polymers containing a small fraction of covalently bound ionic groups, have potential as solid, single ion conducting electrolytes in future batteries. However, the ions tend to form aggregates, making counterion diffusion unacceptably slow. A key materials design question is how molecular properties affect ionic aggregation and counterion dynamics. Recent experimental advances have allowed synthesis and extensive characterization of ionomers with a precise, constant spacing of charged groups. Because the molecular architecture is controlled and these materials show increased ionic aggregate ordering versus their randomly spaced analogs, this set of experiments is ideal for direct comparisons with molecular simulations. We perform molecular dynamics simulations of coarse-grained ionomers with either periodically or randomly spaced charged beads. The charged beads are placed either in the polymer backbone (ionenes) or as pendants on the backbone. To understand the range of ionic aggregate morphologies possible in real materials, we vary the spacing of charges along the chain, degree of randomness (from periodic to random block to fully random), and dielectric constant. The well-known "ionomer peak" in the scattering is present in all cases. The peak is significantly more intense for pendant ions with a long periodic spacing of charged beads, which form roughly spherical aggregates. This morphology is in qualitative contrast to the extended aggregates of ionenes that show increased counterion diffusion. Depending on the degree of randomness in spacing of charged beads along the chain, counterion diffusion can increase or decrease versus that of the precisely spaced materials. Possible implications for ionomer electrolyte design will be discussed. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Ionomers--polymers containing a small fraction of covalently bound ionic groups--have potential application as solid electrolytes in batteries. Understanding ion transport in ionomers is essential for such applications. Due to strong electrostatic interactions in these materials, the ions form aggregates, tending to slow counterion diffusion. A key question is how ionomer properties affect ionic aggregation and counterion dynamics on a molecular level. Recent experimental advances have allowed synthesis and extensive characterization of ionomers with a precise, constant spacing of charged groups, making them ideal for controlled measurement and more direct comparison with molecular simulation. We have used coarse-grained molecular dynamics to simulate such ionomers with regularly spaced charged beads. The charged beads are placed either in the polymer backbone or as pendants on the backbone. The polymers, along with the counterions, are simulated at melt densities. The ionic aggregate structure was determined as a function of the dielectric constant, spacing of the charged beads on the polymer, and the sizes of the charged beads and counterions. The pendant ion architecture can yield qualitatively different aggregate structures from those of the linear polymers. For small pendant ions, roughly spherical aggregates have been found above the glass transition temperature. The implications of these aggregates for ion diffusion will be discussed.

The microscopic Polymer Reference Interaction Site Model theory has been applied to spherical and rodlike fillers dissolved in three types of chemically heterogeneous polymer melts: alternating AB copolymer, random AB copolymers, and an equimolar blend of two homopolymers. In each case, one monomer species adsorbs more strongly on the filler mimicking a specific attraction, while all inter-monomer potentials are hard core which precludes macrophase or microphase separation. Qualitative differences in the filler potential-of-mean force are predicted relative to the homopolymer case. The adsorbed bound layer for alternating copolymers exhibits a spatial moduluation or layering effect but is otherwise similar to that of the homopolymer system. Random copolymers and the polymer blend mediate a novel strong, long-range bridging interaction between fillers at moderate to high adsorption strengths. The bridging strength is a non-monotonic function of random copolymer composition, reflecting subtle competing enthalpic and entropic considerations.