Model for photothermal laser ablation of polymer-like materials

The main feature of polymeric materials is the hierarchy of bonds between molecular groups. There are 'strong' covalent bonds connecting neighbor molecular groups of the same chain, and 'weak' molecular bonds between neighbor groups that belong to different polymer chains. The existing theories of laser ablation of polymers either do not take into consideration this feature (photothermal surface evaporation models) or do not take into account the movement of the interface between condensed and gaseous phases. An important step in this direction has been taken in(1,2) where the so-called bulk or volume model of laser ablation of polymers has been developed. In this model ablation of organic polymers is described on the basis of photothermal bond breaking within the bulk material. Here a first order chemical reaction is assumed, which can be described by Arrhenius law. Ablation starts when the density of broken bonds at the surface reaches a certain critical value. The position of the interface thereafter is fixed with this critical number. It has been shown, in particular, that the movement of the interface between the condensed and gaseous phases during laser ablation is of great importance. In the present paper we develop this model changing the Stefan-like boundary condition at the ablation interface with the physical Frenkel-Wilson condition, which is more appropriate physically and, on the other hand, more convenient for numerical calculations. According to this model, activation energy for elimination of a short enough polymer chain from the surface is proportional to the sum of the energies of weak bonds connecting this chain with the surface. We compare predictions of this model with the previously derived Stefan-like bulk model and with the predictions of surface photothermal model with respect to kinetics and dynamics of single-pulse laser ablation by nanosecond pulse. The parameters used in numerical calculation correspond to the KrF excimer laser ablation of Polyimide.