The quantum chemistry research on the polyethylene surface modification of He atmospheric pressure plasma jet

Abstract To investigate the plasma modification mechanism on the polymer surface, quantum chemistry simulations based on the first-principle theory were conducted to explore the possible microscopic reactions. A helium atmospheric plasma jet was used to treat the low-density polyethylene (LDPE) surface. Experimental spectroscopy methods identified reactive species, including O 2 , OH, atomic hydrogen, and oxygen, on the surface spreading area of the plasma plume. The surface characterization illustrated that the oxygen-containing functional groups, including C=O, C–O, O–H, and C=C, were introduced. Notably, these groups displayed different radial distribution patterns after treatment. On account of the electronic effect theory, n -heptane was chosen as a simplified model of the LDPE fragment. Measured bond energies were used to calibrate the correlation data and validate the calculation findings. The bond energies indicated the possibility of direct bond breaking caused by high-energy particles in the plasma. By estimating the Gibbs free energy changes at each step of the designed routes, it was found that reactive species can activate chemical reactions. Most of the radical involved steps exhibited a spontaneous tendency or a low energy barrier. The hydroxyl and atomic O radicals stripped the hydrogen atoms from carbons to trigger and stimulate the oxidation process, grafting C=O and C–OH groups on the carbon chain. Due to the high spin multiplicity and electronic attraction from oxygen atoms, C–C bonds can be broken down and insert oxygen atoms to form ethers and peroxy bonds. Hydroxyl and atomic H radicals can destroy fragile peroxy bonds, thereby producing terminated carbon chains for further oxidation. These reaction patterns can explain plasma modification mechanisms on polymer surfaces.