The Quantum Chemistry Research on the Polyethylene Surface Modification of He Atmospheric Pressure Plasma Jet

Abstract In order to investigate the plasma modification mechanism on the polymer surface, quantum chemistry simulations based on the first-principle theory were carried out to explore the possible microscopic reactions. The helium atmospheric plasma jet (APPJ) 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 plasma plume. The surface characterization illustrates that the oxygen-containing functional groups including C=O, C-O, O-H and C=C were introduced and shown different radial distribution pattern after treatment. On account of the electronic effect theory, the n-heptane was chosen as a simplified model of the LDPE fragment. Measured bond energies were used to calibrated the correlation data and validated calculation results. The bond energies indicates the possibility of direct bond breaking caused by high-energy particles in plasma. By estimating the Gibbs free energy changes of each step of designed routes, it was found that reactive species can activate chemical reactions. Most of the radical involved steps had 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. With a high spin multiplicity and electronic attraction from oxygen atoms, C-C bonds can be broken down to insert oxygen atoms to from ethers and peroxy bonds. Hydroxyl and atomic H radicals are able to destroy fragile peroxy bonds, thereby producing terminated carbon chains for further oxidation. These reaction patterns can explain plasma modification mechanisms on polymer surface.