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 CC stretching and CO stretching modes were observed with both excitations. Downshifting and broadening were detected for the CC peak with both excitations and also for the CO peak with the 488 nm excitation due to formation of condensed aromatic rings. Oppositely, upshifting was found with 785 nm excitation for the CO peak and especially for its broad shoulder newly developed at the high wavenumber side. The latter feature was assigned to CO groups attached to polymer chains without conjugation and the bands behaviour was interpreted by breaking of the CC bonds of the polymer, leading to the formation of a crosslinked, disordered and stressed structure with still intact CO groups, due to the increased nuclear damage at the end of the ion track.

► He plasma-based ion implanted PET was studied by Raman using 488 and 785 nm excitations. ► Downshift occurred for CC peak at both wavelengths and for CO peak at 488 nm. ► Upshift and broadening occurred for CO peak at 785 nm excitation. ► New peak appeared at 785 nm excitation, assignable to CO on distorted polymer chain. ► CC and CO peak intensities and CO peak width depended on ion acceleration voltage.