Session G42: Dynamics of Ionizable Polymers

Morphology and Ion Dynamics in Neat and Hydrated Sulfonated Polymers

An understanding of morphology and dynamics in single-ion conducting (SIC) polymers is needed to design these polymers for use as electrolytes. In melt SICs, the ions tend to self-assemble into nanoscale ionic aggregates, and the morphology of these aggregates affects the ion dynamics. I will describe atomistic molecular dynamics (MD) simulations of a precise SIC with a polyethylene backbone and sulfonated phenyl groups placed every 5th carbon along the backbone (p5PhSA). The ionic aggregates form percolating clusters, and the scattering profiles calculated from simulations are in good agreement with x-ray scattering. We characterize the shape of the percolated aggregate and provide evidence that the ions move by "shuttling" along the ionic aggregate. When hydrated, the acid form of the polymer has excellent proton conductivity. Increasing water content leads to an increase in the characteristic spacing between hydrophilic domains. This swelling can be obscured in X-ray scattering due to a loss of scattering contrast between the hydrophobic and hydrophilic domains. The MD simulations reveal that the systems are still nano-phase separated, even when X-ray scattering appears to indicate otherwise. Hydronium ion diffusivities calculated from the MD simulations follow the experimentally-measured trends in conductivity and are consistent with changes in the nanoscale morphology with changing water content. I will discuss comparisons with other SICs and implications for the future design of ion-conducting polymers.   

Viscoelastic Response of Ionizable Polymers Melts and Dense Suspensions: Role of Ionic Assemblies

Driven by their technological significance, numerous  efforts have probed the structure and dynamics of ionizable polymers, all pointing to the fact that their structure and dynamics are driven by the association of the ionic groups, which act as physical crosslinkers. These polymers form heterogenous media that consist of ionic and non-ionic regions that differ in their characteristics, where the interrelation between the response of the different domains determine their overall reaction to external perturbation. Here the viscoelastic response of polystyrene sulfonate melts and dense suspensions studied by non-equilibrium molecular dynamics simulations will be discussed. The polymer sulfonation levels were varied from the ionomer regime where the ionic clusters are prominently isolated to the polyelectrolyte regime where ionic networks are often formed.  The systems were exposed to both shear and elongational flows and the measured viscosity was correlated with chain conformation and the ionic cluster characteristics. With increasing shear rates, the ionic assemblies break up in both regimes and align with the flow direction, reducing the shear viscosity, though the chains are only slightly stretched. In extensional flow, the ionic clusters are dynamic, rapidly breaking and reforming.  For low suflonation fractions, the chains are stretched while at higher sulfonation fractions there is a very broad distribution of end-to-end distances of the chains. Molecular understanding that underlines the response of these model ionic polymers will be discussed.   

Ion Conduction, Morphology, and Network Formation in a Double-Helix Ionic Polymer Composite Electrolyte

Collective intermolecular interactions can give rise to surprising material properties.  I will describe a distinct class of polymeric ion conductors that we term molecular ionic composites (MICs).  MICs are an integrated combination of ionic fluids with the sulfonated Kevlar®-like polymer “PBDT.” PBDT forms a double helix that provides a rigidity persistence length of ~ 1 micrometer along the rod axis (20X that of DNA), and thus represents a new 1D material building block.  MICs appear to consist of a collective electrostatic network that enables their transport and mechanical properties.  MICs simultaneously possess high mechanical stiffness (E’ up to ~ 1 GPa) and yet liquid-like motions of ions inside (conductivity up to 8 mS/cm), and stability to 300°C.  MICs show promise for enabling, e.g., high density and safe Li and Na batteries, as well as a host of other electrochemical and molecular separations devices.

To characterize these materials, we employ a range of NMR methods that include spectroscopy and diffusometry, and we combine these with X-ray scattering, molecular dynamics simulations, electrochemistry, and microscopy.  I will discuss the phenomena that give rise to MIC properties, and will provide insights into the interplay between multi-scale structure and ion transport in these nanoconfined systems.   

Insights into the transport mechanisms of water through polymer membranes from quasielastic and inelastic neutron scattering

Polymer membranes that facilitate or inhibit ion transport are critical for a range of technologies, spanning fuel cells, flow batteries, ion exchange columns, and filtration for clean water.  A common thread that spans these applications is that their function is coupled to the transport of water through the membrane.  There is a significant need to understand the nature by which the water and other penetrants move through the membrane.   In this presentation, I will focus on the general case of water diffusion through three types of materials with very different network structures: tightly cross-linked epoxy resins that are used in structural composites and exhibit very little swelling, loosely cross-linked polyamide membranes that are used as the active layer in a reverse osmosis desalination membranes that contain polar groups and exhibit moderate swelling upon hydration, and ion containing block copolymer alkaline fuel cell membranes that contain transient ionic cross-links and swell significantly with water.  A combination of techniques will be used to characterize the swollen structure and dynamics of the membrane materials relative to their transport behavior. Positron annihilation lifetime spectroscopy will be used to quantify the intermolecular packing efficiency in the material, small angle neutron scattering will be used to quantify the microstructure of the water-rich domains in the hydrated materials, and a combinate of infrared and nuclear magnetic resonance spectroscopy will be used to quantify interactions between water and the membrane.  Quasielastic and inelastic neutron scattering will be used to quantify both the polymer dynamics of the membrane and the dynamics of the water molecules diffusing through the membranes.  The insights from these measurements will be used to discuss the different mechanisms that are used to describe water transport in these materials ranging from solution-diffusion to pore-flow transport.   

Charged Polymers with Various Ionic Additives: Phase Behavior, Ion Clustering, and Ion Transport Properties

Theoretical and experimental analyses of the thermodynamic properties of charged polymers have provided insights into how to control nanostructures via electrostatic interactions and improve the ionic conductivity without compromising mechanical strength, which is crucial for practical applications. In this talk, I would like to discuss methods to control the self-assembly and ion diffusion behavior of charged block copolymers by varying the type of tethered ionic moieties, local concentration of embedded ions with controlled electrostatic interactions, and nanoscale morphology. I discuss with particular emphasis on the structure–transport relationship of charged block copolymers using various ionic additives to control the phase behavior electrostatically, as well as the ion-transport properties. My group's prolonged surge and synthetic advances are pushing the frontiers of charged block copolymers to have virtually homogenous ionic domains with suppressed ion agglomeration via the nanoconfinement of closely bound ionic moieties, resulting in efficient ion conduction. I also would like to introduce the latest results of our groups on acid-tethered polymers by focusing on the design and synthesis of bifunctional polymers. Through the introduction of two types of functional moieties to precise positions of polymer backbones, ion distribution at distances of several angstroms, ion aggregation at several nanometers, and microphase separation at a few tens of nanometers could be modulated. Computational and experimental analyses have provided insights into how to improve the ionic conductivity across multiscale self-assembled structures of bifunctional polymer electrolytes without compromising mechanical strength.