Polymers with charges on their backbones, along with neutralizing small molecule counterions, are termed polyelectrolytes or ionomers. The chief distinguishing factors between these different terms is the state of aggregation of the charges: in the case of polyelectrolytes, a significant fraction of the counterions are dissociated from the chain and can move freely through the system. In contrast, in the case of ionomers, nearly all counterions are strongly condensed onto the chain, and additionally these neutral charge pairs may form neutral aggregates of many ion pairs. While the attractiveness of these materials comes from the fact that they selectively transport cations alone, this is mitigated by the fact that the presence of isolated aggregates can make ion conduction a very slow transport process. The goal is to understand the factors which control the ion pairing to form isolated dipoles, and the further factors which drive these dipoles to self-assemble into aggregates. The dielectric constant of a relatively nonpolar polymer will be changed by adding a high dielectric constant solvent and vary temperature over a wide range. Aggregation will be studied directly through STEM and both SAXS and SANS. Less direct techniques will also be used, such as mechanical rheology and DSC, where self-assembly of ion pairs into aggregates is associated with a signature that is akin to a glass transition and rheology strongly depends on whether the counterions are free, paired or clustered. Recently developed dielectric spectroscopy methods will determine free ion content and mobility in ionomers, directly assessing the extent of ion pair formation. These studies will be complimented by computer simulations, employing Monte Carlo and Molecular Dynamics methods.
Non-Technical Summary The intellectual merit of this research will be an improved understanding of ion-pairing and ion-pair clustering, culminating in the development of a new model that fully details the transition from polyelectrolyte to ionomer. Such ion-containing polymers are of considerable interest since they have been proposed for use in actuators, fuel cell membrane electrode assemblies and for the cation conduction medium for advanced batteries. Hence, this research should facilitate polymer design for actuators, fuel cells and batteries. Materials development in the energy field is expected to play a very important role in the future of the United States economy and way of life. Graduate students trained in this 'energy materials' arena will be in enormous demand in both US industry and academia. PSU and Columbia have superb undergraduates and research motivates them to attend graduate school (14/23 of our undergraduate researchers have gone on to graduate school in science and engineering over the past 10 years) with many current undergraduates interested in 'energy materials'
Long chain polymers with covalently bonded ions have small unattached counterions that maintain a net zero charge for these materials. In a polar environment (such as water) some of the counterions dissociate from the polymer chain and a free to conduct; such ion-containing polymers are referred to as polyelectrolytes (poly for polymer and electrolyte because they are analogous to other electrolytes such as common table salt in water, whose ions also dissociate and conduct). In a non-polar environment (such as the neat molten state or in a low-polarity solvent) the counterions all pair with the bonded ions on the polymer chain and then the same polymer is called an ionomer. A tiny fraction of the ions in the ionomer state still can conduct but the ionic conductivity is vastly smaller. For various applications, such as ionic actuators (artificial muscles) that bend when an electric field is applied, advanced battery membranes that allow counterions to transport between electrodes and supercapacitors that store large quantities of electrical energy by polarizing counterions, we need ionomers that have a larger fraction of conducting ions. In other words, for many applications we want ionomers to behave more like polyelectrolytes. Our research aims to understand what is needed to get ionomers to conduct counterions and ultimately should lead to ionomer membranes with higher ionic conductivity. We have learned in our research that to conduct Li+ counterions for a lithium battery, the ionomers need three important design criteria. (1) The ionomers must have anions covalently bonded to the polymer that only bind Li+ weakly, such as tetraphenyl borate which has less than half the binding energy of more conventional anions. (2) The ionomers need to be more polar than conventional polymers, accomplished by incorporating polar side groups such as carbonate in the polymer. (3) The ionomers need to have neutral parts that have strong specific solvation of the Li+ counterion. For example poly(ethylene oxide) has ether oxygen atoms in the chain that surround and solvate Li+ counterions. Consequently, Colby’s research group is currently constructing ionomers with carbonate side groups and tetraphenyl borate anions that are both attached to the polymer chain using spacers having multiple ether oxygens for Li+ solvation.
