Two-wavelength Raman study of poly(ethylene terephthalate) surfaces modified by helium plasma-based ion implantation

The surface of poly(ethylene terephthalate) (PET) was modified by helium plasma-based ion implantation (He PBII). The untreated and surface modified samples were characterised with optical absorption spectroscopy and two-wavelength micro-Raman spectroscopy excited with 488 nm and 785 nm light sources, allowing to examine the chemical bonding configuration of the surface layers on different depths and by selective enhancement of vibrations of different structural units. Upon treatment, simultaneously with the development of the broad D and G bands, a gradual decrease of the peaks corresponding to the Cdouble bond; length as m-dashC stretching and Cdouble bond; length as m-dashO stretching modes were observed with both excitations. Downshifting and broadening were detected for the Cdouble bond; length as m-dashC peak with both excitations and also for the Cdouble bond; length as m-dashO peak with the 488 nm excitation due to formation of condensed aromatic rings. Oppositely, upshifting was found with 785 nm excitation for the Cdouble bond; length as m-dashO peak and especially for its broad shoulder newly developed at the high wavenumber side. The latter feature was assigned to Cdouble bond; length as m-dashO groups attached to polymer chains without conjugation and the bands behaviour was interpreted by breaking of the Cdouble bond; length as m-dashC bonds of the polymer, leading to the formation of a crosslinked, disordered and stressed structure with still intact Cdouble bond; length as m-dashO groups, due to the increased nuclear damage at the end of the ion track.