Free radicals – compounds with an unpaired electron that are created under normal cellular metabolism as well as via chemical reactions or electromagnetic irradiation – can cause different changes in polymers. Many of these changes are due to recombination reactions and can cause increased yellowing or alterations to a polymer’s bending strength.
While much is known about free radicals and their general relationship to health, additional research has examined the role of radicals in polymers, thereby providing researchers with a more granular look at their behavior. The free radicals in polymers, particularly those found on the surface of polymers’ film, may be generated in part through a process of electron or ion bombardment. Electron spin resonance (ESR) measurements may provide information on the structural and chemical properties of these radicals, as the ESR signal can study the production and recombination of the compounds in plasma polymers.
A recent study by a group from Germany used an ESR analysis of plasma-polymerized thin films and assessed radical generation, lifetimes, activation energies, and temperature behaviors. To begin their experiment, the study researchers took high purity silicon wafers to be used as substrates for fluorocarbon plasma polymer films. The spin number, respective spin density, and radical relaxation kinetics of the coatings were all calculated. Additionally, the researchers coated the silicon wafer substrates as well as the glass pipettes with fluorocarbon polymer. The measurements were obtained with a bench-top ESR spectrometer.
Recombined radicals showed a deviation from a linear growth in the number of radicals, and researchers discovered that a strongly broadened ESR signal of the radicals that were in the fluorocarbon plasma polymers. A persistent increase in the number of radicals were observed in a linear line during the radical generation process.
No substantial change was observed in radical decay in relation to the change in the various temperatures. Nearly no radical decay was observed at 173 K. According to radical decay measurements, radicals in plasma-polymerized C4F8 coatings had lower activation energy than radicals in plasma-polymerized C3F6 films.
The study also found that bimolecular reaction, a second-order recombination process, could model the radical number decay. The researchers also calculated the radical density of fluorocarbon films with variation in thickness of approximately 1.5×1020 spins.cm-3. They added that the width of the ESR line was contingent on the polymer film’s fluorine content as well as the ESR spin density. Ultimately, the increasing fluorine content as well as the spin density correlated with an increased line width.
Haupt M, Barz J, Oehr C. Creation and Recombination of Free Radicals in Fluorocarbon Plasma Polymers: An Electron Spin Resonance Study. Plasma Processes and Polymers. 2008;5:33–43.