(6bx) Design of Functional Polymeric Materials: From Ion Transport to Bio-Inspired Assembly

Lithium polymer batteries offer a number of advantages to standard lithium ion batteries, including an all-solid state structure, increased safety, and the potential to be combined with lithium metal anodes for increased energy density over lithium intercalation anodes. However, the low-temperature (< 80 °C) ionic conductivity of polymer electrolytes has remained a major limitation, and progress in understanding strategies for systematic improvement in ionic conductivity has been dominated a single polymer for elucidating structure-property relationships in ionic conducting polymeric solids, i.e., poly(ethylene oxide) (PEO).

While earning my Ph.D. in Chemical Engineering at UC Santa Barbara under the guidance of Profs. Glenn Fredrickson, Edward Kramer, and Craig Hawker, I focused on the development of poly(allyl glycidyl ether) (PAGE) as an alternative platform for lithium battery polymer electrolytes. PAGE was found to have peak conductivities at [O]/[Li] = 16, with σ > 3×10^-5 S/cm at 25 °C and σ > 5×10^-4 S/cm at 80 °C. Below 60 °C, PAGE has a conductivity that is 10–100 times higher than that of PEO at equivalent salt concentrations with this disparity in conductivities between PAGE and PEO increasing with decreasing temperature. In addition, the synthetic versatility of allyl glycidyl ether as a building block was demonstrated by the preparation and evaluation of various AGE-EO macromolecular architectures that show superior performance to both PAGE and PEO by utilizing copolymerization and thiolene coupling chemistry. In order to develop design rules for polymer electrolytes, a library of structurally similar polyethers, poly(glycidyl ether)s (PGEs) were synthesized by anionic polymerization with controlled molecular weight and polydispersity index. Polymer electrolytes were prepared with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at a molar ratio of [O]/[Li] = 16. Significantly, no correlation between glass transition temperature or room temperature viscosity and ionic conductivity over the range 20–80 °C could be established. Similar to conventional small molecule organic electrolytes, the ionic conductivity increases with the dielectric constant (ε) of the parent polymers over the range ε = 4.8–6.0, indicating a strong dependence of ion conductivity on the ion dissociation equilibrium in the polymer matrix. This hypothesis was further supported by pulse field gradient (PFG) NMR studies of the lithium and fluorine diffusion behavior. Our results suggest that synthetic efforts to create highly conductive polymer electrolytes should focus on increasing the dielectric constant of the parent polymeric material.

In my current research as a postdoctoral associate with Prof. Uli Wiesner and Prof. Lara Estroff at Cornell University, I am developing block copolymer based peptide brushes to investigate the self-assembly of these polymers and their ability to locally control crystallization of small molecules. Proteins, and their smaller peptide components, offer a diversity of self-assembly pathways: not only of the amino acid-based polymer itself, but in biomineralization, a protein matrix directs crystal phase and orientation of inorganic materials. The rich sequence diversity of proteins, however, has not been demonstrated for synthetic three-dimensional (3-D), continuous frameworks. Block copolymers offer long range periodic order of 0-D to 3-D structures through self-assembly and have been utilized to template high-surface area inorganic materials. Nevertheless, control has primarily been spatial while direction of crystal type and crystallographic orientation has remained challenging. By combining the long-range periodic structure direction of block copolymers with the chemical and sequence specificity of peptides, new highly-controlled composite materials might be created. Here I will discuss the synthesis of peptide and peptoid functionalized block copolymers. By altering the amino acid content and order, we investigate the role of sequence control on polymer self-assembly and on directed and templated inorganic materials growth. Ultimately, the development of composite three-dimensional structures with controlled single-crystal facets and large surface area could be expanded beyond biological and mechanical applications to new energy materials with high selectivity towards catalytically active surfaces and to pharmaceutical design.