Radical Environments for Fast Charge Transport in the Stable Radical Polymer, Ptma
Authors: Bobela David C. , Hughes Barbara Katherine , Braunecker Wade A. , Kemper Travis , Larsen Ross E. , Gennett Thomas 
Published: February 2020

Non-conjugated, open-shell polymers with stable radical pendant groups have emerged as a unique class of electroactive materials for use in chemical or electronic devices traditionally dominated by conjugated conducting polymers or inorganic semiconductors. Their stability and reversible redox properties allow the neutral polymers to convert to cation or anion states analogous to doping a semiconductor p or n-type. The charge transport kinetics in these materials can be surprisingly on par with transition metal ion devices thus allowing the exciting opportunity to implement these materials in electronic devices, specifically for energy storage.
The stable radical polymer poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate) (PTMA) is one such redox active polymer that has recently been studied as potential candidate for cathodes in all organic battery devices and charge transport mitigating layers in super-capacitor / battery hybrid devices1. It is clear from the literature that the device properties are intimately connected to the polymer properties, most importantly, the pendant radical. The stable pendant radical is highly localized and therefore the electronic transport through the polymer is apparently governed by electron-hopping mechanisms. However, it is unclear whether the radical environment is optimal for the intended purpose, in this case, charge transport.

For this work, we have used well-known electron paramagnetic resonance (EPR) methods to study the PTMA-TEMPO radical materials to determine structural information about the radical environment. Using synthetic techniques to control the radical mole fraction, X, on the methyl methacrylate backbone, we investigated the dependence of polymer conformation on radical concentration and attempt to link favorable radical environments to conditions of favorable charge transport. We call these oligomers PTMA-X and will present data for various values of X from 100% (PTMA), to 10%.
The EPR data indicate that the majority of radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. This distance is estimated to be about 5.6 Å and is consistent with recent theoretical work2. These small distances promote a strong coupling between radicals which therefore promote conditions of fast charge transport. We see evidence for subtle changes in the radical environment depending on characteristics of the solvent, but overall the radical-radical distances are unaltered. Perhaps most interestingly, the radical-radical distances do not change until there is less than about 60% radical mole fraction. For this regime, the polymer can accommodate larger radical-radical spacing by changing conformation. Small angle neutron scattering (SANS) experiments of these polymers in solution indeed confirm these conformation changes. Since the radical network appears unchanged from 60% to 100%, we expect only small differences in the transport properties such as conductivity to result as a function of X. Hence, these results indicate that PTMA-60 may be as good as PTMA with respect to charge transport, and may therefore serve as a better choice for charge transport layers in devices where gravimetric capacity is highly valued. In this talk, we will present these data, along with SANS and electrical transport measurements, and then discuss their implications on charge transport for these and other related stable radical polymers.
1. Oyaizu, K.; Nishide, H. Radical Polymers for Organic Electronic Devices: A Radical Departure from Conjugated Polymers? Advanced Materials
2009,
21 (22), 2339-2344.
2. Kemper, T. W.; Larsen, R. E.; Gennett, T. Relationship between Molecular Structure and Electron Transfer in a Polymeric Nitroxyl-Radical Energy Storage Material. The Journal of Physical Chemistry C
2014,
118 (31), 17213-17220.