Surface cross-linking of polycarbonate under irradiation at long wavelengths

Depth profiles analyses were performed on a cross section of a photoaged BPAPC substrate by measuring the glass transition temperature and the Young modulus at a microscopic scale. The aim was to define the impact of photochemical modifications induced under UV irradiation upon the physical properties of this polymer. It is well known that ageing of BPAPC begins by a direct phototransformation of the macromolecules according to the photo-Fries rearrangements. These reactions lead to an increase of the free volume and, as a consequence, to a decrease of the temperature associated with the glass transition while in the meantime, the Young modulus of the material is not modified. As ageing proceeds (longer irradiation times), oxidation of the polymer takes place. This oxidative degradation leads to the formation of a cross-linked structure on the exposed surface. This oxidative degradation is associated with an increase of the glass transition temperature and of the Young modulus, whereas in the bulk of the material ageing involves chain scissions characterised by a decreased glass transition temperature.