ABSTRACT
Abstract-The irradiation of a polymer by UV light effects photochemical surface modifications, if the photons are sufficiently absorbed by the substrate. A photochemical process can be considered as irradiation of the substrate in some reactive or inert atmosphere. In the presence of a low or nonabsorbing atmosphere and a strongly absorbing substrate, the actual reaction takes place only at the substrate surface, where radical processes are initiated. Four different types of reactions are possible: (I) recombination of radicals, (II) cross-linking of polymer chains, (III) addition of radicals from the reactive atmosphere and (IV) addition of bi-functional molecules resulting in cross-linking. The scope of this work was to study the occurrence of cross-linking of the polymer itself (reaction type II) and deposition of cross-linked thin layers (reaction type IV) following a photochemical surface treatment in the presence of bi-functional molecules. Topography measurements using atomic force microscopy (AFM) show a decrease in the tortuosity of the surface, which is seen as an indication of photo-induced molecular entanglement/cross-linking. Nanoindentation measurements confirmed that surface stiffness within the indentation depth was strongly affected by UV irradiation in the presence of different bi-functional media. Micro-thermomechanical (µTMA) measurements showed that the surfaces of samples irradiated in octadiene and argon had much less thermal expansion and lower softening/melting temperatures than the control sample, which is an indication of decrease in crystallinity because of the occurrence of cross-linking in the near-surface layer. In the case of film treated in the presence of diallylphthalate (DAP), depending on the local structure, either strong decrease of melting temperature or no melting point was found up to 300°C.
