Beáta Kavcová, Vinicius T. Santana, David Nečas, Lucie Janů, Martina Janůšová, Lenka Zajíčková
Free radicals in amine plasma polymers deposited under mild ion bombardment and their kinetics
Surface and Coatings Technology 539 (2026) 133937
Long-lived free radicals in bioactive surfaces provide a highly reactive platform for direct covalent binding of biomolecules without the need for additional chemical crosslinkers. Here, we report a high concentration of long-lived (≥100 days) free radicals (~10¹⁸ spin/cm³, measured by electron paramagnetic resonance -- EPR) in amine plasma polymer (PP) thin films prepared from cyclopropylamine (CPA), which increases with applied discharge power. After deposition, the spin density decreases steeply over the first 5 days, then levels off to about half its initial amount. A radical-kinetics model accounting only for diffusion and surface recombination successfully described the time evolution and confirmed the presence of two radical types with different diffusion coefficients. Therefore, the EPR spectra were refitted considering two peaks, resulting in the separation into two species with vastly different decay rates. One, with g-factor of 2.004, indicates a nitrogen- or oxygen-centered radical, stable in time with a lower diffusion coefficient. The other, with g-factors below 2.002, corresponds to a single-bonded carbon-centered radical, which decreases with aging time. The high radical density is not induced by the effect of energetic ions, but follows from the unique structure of the CPA monomer. Since we discovered that the film deposition proceeds at nearly the same rate during the off-time of the pulsed plasma mode as during the on-time and that the radical density scales with the on-time power rather than average power, we concluded that CPA plasma polymerization is mainly driven by biradical reactions based on Yasuda's growth mechanism for plasma polymerization.
DOI: 10.1016/j.surfcoat.2026.133937
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