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

Abstract

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, φ 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, φ− φ th. (iv) With φ≈ φ th, ablation starts well after the laser pulse. (v) The depletion of species is responsible for the Arrhenius tail with fluences φ≤ φ th.