Our current research interests are nanocomposites and ion-containing polymers. We design and fabricate new polymer nanocomposites containing carbon nanotubes and now metal nanowires with the aim of improving the mechanical, electrical and thermal properties. With ion-containing polymers our focus is uncovering the nanoscale structure using a variety of state-of-the-art instruments for both static and dynamic experiments and then correlating the structure with the ion conduction as in batteries or fuel cells. Additional research interests include block copolymers, particularly block copolymers that incorporate nanoparticles and charged blocks. Our approach relies both on experimental and simulation results to give the fullest understanding of the systems we study.
(Sponsors: NSF-DMR, ARO-MURI, DOE, Sandia National Laboratory, Kraton Polymers)
Precisely-Spaced Acid Groups Produce Unique Morphologies ( details here and here)
When polymer synthesis is controlled such that functional groups are evenly spaced along a linear polymer, the result is new morphologies. We first reported this in collaboration with Prof. Ken Wagener's group, who used acyclic diene metathesis to produce linear polyethylene with carboxylic acid groups every 9th, 15th, or 21st carbon atom. Our X-ray scattering results found that the acid groups form dimers that drive the formation of acid-rich layers perpendicular to the polymer backbone of the PE crystallites. The spacing between the acid layers correlates with the number of carbon atoms between acid groups. When neutralized with metal cations, such as zinc, these precise copolymers form ionic aggregates whose positions are tightly correlated. At the highest acid content studies, the ionic aggregates self-assemble with cubic ordering. These hierarchical structures might provide additional control over ion transport in polymers or alter their mechanical properties.
Structure in Polymer Single-Ion Conductors (details)
Motivated by the advantages presented by polymer electrolytes for light weight batteries, we are investigating new polymers in an effort to understand what is necessary for the ions to conduct efficiently. Recently, we have investigated a family of materials with well-defined poly(ethylene oxide) (PEO) spacers between sulfonated phthalates that were neutralized with different monovalent metals (M): Li, Na, or Cs. The PEO crystal thickness is defined by the PEO spacer length. The local state of the ionic groups changes from primarily isolated ion pairs to aggregated structures as the cation size decreases from Cs to Li. These findings compare favorably with ab initio calculations performed by our collaborators.
Morphology and Ionic Conductivity in Ionic Liquid Containing Polymers and Polymerized Ionic Liquids (details)
As part of a multidisciplinary research team, we are investigating polymer and ionic liquid mixtures and polymerized ionic liquids for electroactive applications including electromechanical devices and high-performance membranes. Using block copolymers where one block is selectively solvated by an ionic liquid, we were able to correlate changes in ionic conductivity with the block copolymer morphology. Anisotropic morphologies were produced and subsequently characterized by X-ray scattering. When lamellae were oriented in the plane, the in-plane conductivity (purple) was significantly higher than the through-plane conductivity (red), while the conductivity was direction-independent when the morphologies have a continuous conductive path. Finally, significantly higher ionic conductivities can be achieved in a block copolymer/IL solid-state film compared to a homopolymer/IL film at the same IL content (wt%) because the glassy, non-conductive microdomains excludes IL, which produces a higher local IL concentration in the conductive phase.
(Sponsors: NSF-MWN, NSF-MRSEC, Dupont)
Electrical Conductivity in Polymer Nanocomposites with Rod-like Nanoparticles (details here and here)
Our group has looked extensively at polymer nanocomposites (see our review articles here and here). While we have previously studied the effect of carbon nanotube (CNT) orientation on the mechanical properties of polymer nanocomposites (more), recently we have turned our attention to understanding their electrical behavior. Using a two-pronged approach of simulation and experiments, we are investigating the link between structure and electrical behavior in CNT- and metal nanowire-polymer nanocomposites. We have shown that electrical percolation depends on the nanowire aspect ratio (L/D) (more) and degree of alignment (more) in addition to filler volume fraction. We have excellent agreement between our simulations and experimental system and demonstrate that the most widely used analytical model is insufficient for systems with modest aspect ratios.
Resistive Switching in Silver Nanowire-Polystyrene Nanocomposites (details here and here)
Excitingly, some silver nanowire-polystyrene nanocomposites demonstrate resistive switching - a first for a bulk composite. Resistive switching materials demonstrate dramatically different low- and high-field resistivities and are of interest for applications as memory materials.
Polymer Diffusion in Nanocomposites (details here and here and here)
Nanoparticles present a new frontier for understanding polymer dynamics in complex, nanoscale environments. We find that the addition of single- or multi-walled carbon nanotubes produces a minimum in the tracer diffusion coefficient with increasing nanoparticle concentration. When the tracer molecule is large relative to the nanoparticle, the diffusion in the vicinity of the nanoparticle appears to become anisotropic leading to the minimum in diffusion coefficient. Simulations using a trap model that defines a trap size and the extent of slowing perpendicular to the cylindrical trap reproduce both the initial decrease in diffusion attributed to isolated traps and the recovery above the critical filler volume fraction corresponding to trap percolation. Nanoparticles influence polymer diffusion in fascinating ways and will refine our understanding of polymer reptation and might also inform the study of biopolymer diffusion in living systems as well as the processing of polymer nanocomposites.