Laser-induced thermal degradation and ablation of polymers: bulk model

Ablation of organic polymers is described on the basis of photothermal bond breaking within the bulk material. Here, we assume a first order chemical reaction, which can be described by an Arrhenius law, Ablation starts when the density of broken bonds at the surface reaches a certain critical value . In order to understand the ablation behavior near the threshold fluence, phi(th), non-stationary regimes are considered. The present treatment revea ls several qualitative differences with respect to models which treat ablat ion as a surface process: (i) Ablation starts sharply with a front velocity that has its maximum value just after the onset. (ii) The transition to qu asi-stationary ablation is much faster. (iii) Near threshold, the ablated d epth has a square-root dependence on laser fluence, phi - phi(th). (iv) Wit h phi approximate to phi(th), ablation starts well after the laser pulse. ( v) The depletion of species is responsible for the Arrhenius tail with flue nces phi less than or equal to phi(th). (C) 1999 Elsevier Science B.V. All rights reserved.