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 reveals several qualitative differences with respect to models which treat ablation 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 quasi-stationary ablation is much faster. (iii) Near threshold, the ablated depth has a square-root dependence on laser fluence, phi - phi(th). (iv) With phi approximate to phi(th), ablation starts well after the laser pulse. (v) The depletion of species is responsible for the Arrhenius tail with fluences phi less than or equal to phi(th). (C) 1999 Elsevier Science B.V. All rights reserved.