Kinetics of Photocrosslinking and Surface Attachment of Thick Polymer Films

We describe the formation process of surface-attached polymer networks generated through photochemically induced C,H insertion crosslinking (CHic). To achieve surface attachment, light must be able to penetrate all the way to the interface between the film and the substrate to trigger the necessary reactions. Although light penetration is typically a nonissue for thin films below 1 μm, it may become difficult for thicker layers. In this contribution, we show that a critical light dose is needed to ensure full percolation. This critical dose can be derived from Lambert–Beer's law and percolation theory. Both experiments and calculations lead to a model, which is able to predict the percolation points of films as a function of light intensity and film thickness, even for very thick films. The only parameters that need to be determined for this model are the linear absorption coefficient of the polymer, its change with increasing irradiation time due to photobleaching, and the percolation point of a thin film.