Effect of molecular weight on the physicochemical modifications induced in the UV laser ablation of doped polymers

This work investigates the effect of polymer molecular weight M_W on the UV ablation of iodo-naphthalene- and iodo-phenanthrene-doped poly(methyl methacrylate) PMMA, and polystyrene PS films following irradiation at 248 nm. For irradiation at weakly absorbed wavelengths, the ablation threshold increases with increasing M_W. However, at strongly absorbed wavelengths, the difference in the ablation thresholds is much smaller, or minimal. In parallel, bubble formation due to accumulation of gas produced by polymer and dopant decomposition differs depending on M_W. For highly absorbing PS, the differences of behaviour show a less dramatic dependence on M_W. These results are explained within the framework of the bulk photothermal model, according to which ejection requires that a critical number of bonds is broken. In all, they are of direct importance for the optimisation of laser processing schemes and applications and provide the first indication of explosive boiling in UV ablation of polymers.