Ion Self-Assembly and Transport in Ionomeric Electrolytes
- Author:
- Lu, Keran
- Published:
- [University Park, Pennsylvania] : Pennsylvania State University, 2016.
- Physical Description:
- 1 electronic document
- Additional Creators:
- Milner, Scott Thomas
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- Open Access.
- Summary:
- Ionomers are ion-containing polymers in which one of the ionic species is covalently linked to the polymer. This group of polymers has diverse applications ranging from commercial packaging to battery electrolytes. The dielectric constant of the polymer distinguishes purely structural ionomeric materials from those that are used to transport ions. In this dissertation, we apply physical modeling to describe self-assembly and ion transport in ionomeric systems. The physical modeling is supported by atomistically-informed, coarse-grained simulations that expand the useful time and length scales accessible through molecular simulations, at the cost of short-time details. The statistical description of ion aggregate populations and ion/charge transport in this dissertation complement the earlier atomistic and bead-spring simulations that provide more localized details of ion aggregate conformations and PEO-driven transport mechanisms.The majority of the dissertation focuses on self-assembly and ion transport the ionomeric electrolyte, sodium-neutralized poly(PEO-co-sulfoisophthalate). Despite the PEO backbone, which is a stronger dielectric than polyethylene or polystyrene, the ions are still aggregated in this material. The packing constraints associated with the PEO backbone drives ion aggregates to be stringlike in morphology. We find that a worm-like micelle equilibrium well describes the size-distribution of ion aggregates in simulations of the ionomer. The ion strings are composed of ions alternating in charge, and can be viewed as a series of dipoles in which the end groups have energetic costs relative to the center because of uncompensated dipoles. On occasion, free ions will coordinate to the sides of stringlike chains, creating excess charge. These charges are more likely to be positive, and result in ion aggregates becoming progressively more positive with size. We find that excess charges are participating in an ion-aggregate mediated transport mechanism through consecutive coordination with ion pairs and higher order clusters. This mechanism is highly efficient and allows positive charged to be relayed through a series of ion aggregate nodes faster than any individual cation. The extent to which charge transport is expedited relative to cation transport is quantified through the recovery of the f factor, a metric of collective motion, at longer timescales.In studies of partially sulfonated PEO ionomers, we found isophthalate groups are highly ordered. The isophthalate groups in the unsulfonated ionomer self-assemble in the absence of any ionic groups. In partially sulfonated PEO ionomers, sulfonated and unsulfonated isophthalate groups also attract each other. Electrostatic and hydrophobic forces drive ions and isophthalate groups, respectively, to self-assemble in partially-neutralized systems. We modeled aggregates of ions and of isophthalate groups (sulfonated and unsulfonated) with a two-component wormlike micelle model. In two- component systems, the species with the more costly endcap is concentrated in larger aggregates, reducing the fraction of their population acting as endcaps. This results in aggregate compositions that are size dependent when the endcap energies of the two species are not equal. Evaluating trends of the endcap energies from both types of aggregates in tandem reveals why ion aggregates become increasingly positive in charge with increasing mass.The wormlike micelle description of ion aggregates breaks down at sufficiently low background dielectric constants. In systems in which the dielectric constant of the backbone PEO is artificially lowered, we find that ion aggregates undergo a transformation from stringlike to sheetlike morphologies. The large, sheetlike aggregates in our low dielectric constant systems further self-assemble into ordered, lamellar structures because of depletion attraction generated by the smaller aggregates. Even though our simulation is in a cubic box of edge length 30 nm, the self-assembled lamellar structures still span more than half the box size, suggesting even larger simulations may be needed. We believe depletion forces will eventually produce spherical structures composed of lamellar sheets, which could reconcile the large, spherical ion structures observed in electron microscopy with the chain-packing costs associated with such large structures, if the ions were randomly packed.In summary, we have found that electrostatic, hydrophobic, and depletion attractions could all potentially play an important role in the self-assembly of ionomeric systems. The extent to which each driving force is important depends on the strength of electrostatic interactions and the chemical structure of the ionomer. These structures interact with ions in the system, which has consequences on the ionic conductivity. The characterization of self-assembly in ionomeric systems is made possible through bottom-up coarse-graining of the corresponding united atom simulations.
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- Dissertation Note:
- Ph.D. Pennsylvania State University 2016.
- Reproduction Note:
- Microfilm (positive). 1 reel ; 35 mm. (University Microfilms 13871877)
- Technical Details:
- The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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