Decoupling of ion- and photon-activation mechanisms in polymer surfaces exposed to low-temperature plasmas

The modification of polypropylene (PP) by an argon plasma is emulated in a particle beam experiment. An ion beam deflector, used to steer argon ions from an electron-cyclotron-resonance (ECR) plasma source towards the sample, suppresses the UV and VUV photons generated in the plasma volume. The modification of PP surface by 200 and 500 eV ions is monitored by in situ Fourier transform infrared spectroscopy (FTIR). One observes a transition from an initial region of fast etching to a steady state without chemical modification and lower etching rate. This behavior is attributed to the progressive graphitization at the surface due to ion bombardment. An anti-synergism arises by adding UV photons because of cross-linking of the polymer at the subsurface region, which renders the etch rate much smaller compared to the etch rate by ion only impact.