Travis W. Kemper

Close Packing of Nitroxide Radicals in Stable Organic Radical Polymeric Materials

The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that 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. Below 60% coverage, the polymer rearranges to accommodate larger radical-radical spacing. These findings are consistent with theoretical calculations and help explain some experimentally determined electron-transport properties.

Quenching of the Perylene Fluorophore by Stable Nitroxide Radical-Containing Macromolecules

Stable nitroxide radical bearing organic polymer materials are attracting much attention for their application as next generation energy storage materials. A greater understanding of the inherent charge transfer mechanisms in such systems will ultimately be paramount to further advancements in the understanding of both intrafilm and interfacial ion- and electron-transfer reactions. This work is focused on advancing the fundamental understanding of these dynamic charge transfer properties by exploiting the fact that these species are efficient fluorescence quenchers. We systematically incorporated fluorescent perylene dyes into solutions containing the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radical and controlled their interaction by binding the TEMPO moiety into macromolecules with varying morphologies (e.g., chain length, density of radical pendant groups). In the case of the model compound, 4-oxo-TEMPO, quenching of the perylene excited state was found to be dominated by a dynamic (collisional) process, with a contribution from an apparent static process that is described by an ∼2 nm quenching sphere of action. When we incorporated the TEMPO unit into a macromolecule, the quenching behavior was altered significantly. The results can be described by using two models: (A) a collisional quenching process that becomes less efficient, presumably due to a reduction in the diffusion constant of the quenching entity, with a quenching sphere of action similar to 4-oxo-TEMPO or (B) a collisional quenching process that becomes more efficient as the radius of interaction grows larger with increasing oligomer length. This is the first study that definitively illustrates that fluorophore quenching by a polymer system cannot be explained using merely a classical Stern-Volmer approach but rather necessitates a more complex model.

Primary Radiation Defect Production in Polyethylene and Cellulose

Irradiation effects in polyethylene and cellulose were examined using molecular dynamics simulations. The governing reactions in both materials were chain scissioning and generation of small hydrocarbon and peroxy radicals. Recombination of chain fragments and cross-linking between polymer chains were found to occur less frequently. Crystalline cellulose was found to be more resistant to radiation damage than crystalline polyethylene. Statistics on radical formation are presented and the dynamics of the formation of radiation damage discussed.

Modeling reaction pathways of low energy particle deposition on polymer surfaces via first principle calculations.

The chemical processes that lead to polystyrene surface modification via low energy deposition of C(2)H(+), C(2)F(+), CH(2), CH(2)(+), and H(+) radicals and ions are examined using first principles calculations. Specifically, the reaction mechanisms responsible for products identified in classical molecular dynamics with reactive empirical bond-order potentials are examined using density functional theory. In addition, these calculations consider how the presence of charges on the incident particles changes the result for the CH(2) system through the comparison of barriers, transition states, and final products for CH(2) and CH(2)(+). The structures of the reaction species and energy barriers are determined using the B3LYP hybrid functional. Finally, CCSD/6-31G(d,p) single point energy calculations are carried out to obtain optimized energy barriers. The results indicate that the large variety of reactions occurring on the polystyrene surface are a consequence of complex interactions between the substrate and the deposited particles, which can easily be identified and characterized using advanced computational methodologies, such as first principle calculations.

Mechanisms for hyperthermal polyatomic hydrocarbon modification of PMMA surfaces from molecular dynamics simulations

Classical molecular dynamics simulations are performed to determine the mechanisms by which hyperthermal hydrocarbon polyatomics, which are present in low-energy plasmas, chemically modify polymer surfaces. In particular, C2H, CH3, and C3H5 are deposited on an amorphous poly (methyl methacrylate) (PMMA) substrate with kinetic energies of 4, 10, 25, and 50 eV and compared to the deposition of H at the same energies. The short-range forces on the atoms are determined using the second generation reactive empirical many-body potential, while the long-range forces are determined using a Lennard-Jones potential. The simulations predict that at all these incident energies, the chemical modification of the PMMA is limited to within a nanometer of the surface. Atoms, fragments, and incident polyatomics are further predicted to chemically attach to specific sites on the PMMA monomers at low energies and to attach to a wider range of sites at higher energies. However, no appreciable cross-linking between polymer cha...