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Abstract:
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Vacuum-ultraviolet (VUV) irradiation (kexc: 172 ± 12 nm) of polystyrene films in the presence of oxygen
produced not only oxidatively functionalized surfaces, but generated also morphological changes.
Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization,
enhanced aggregation or printing, processes leading to morphological changes open new possibilities of
microstructurization. Series of experiments made under different experimental conditions brought
evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene
pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen,
ozone) produced in the gas phase between the VUV-radiation source and the substrate. However,
oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight
and eventually to mineralization of the organic substrate is initiated by electronic excitation of the
polymer leading to C–C-bond homolysis and to a complex oxidation manifold after trapping of the
C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation
could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after
washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily
functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the
polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the
fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of
the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred
radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects,
optical microscopy, friction-mode AFM) and disproportionation (fluorescence). |