Morphological and chemical modifications and plume ejection following UV laser ablation of doped polymers: Dependence on polymer molecular weight

Abstract

This work investigates the effect of polymer molecular weight ( M W) on the morphological and chemical modifications and on the plume ejection of doped polymethyl methacrylate (PMMA), and polystyrene (PS) films following irradiation at 248 nm. Micro-bubbles develop in the irradiated films of the low absorbing PMMA-based substrates. The extent and kinetics of the observed morphological changes are respectively larger and last longer in the low M W polymer, as evidenced by optical microscopy and real-time monitoring of transmission of a CW laser. The changes observed in the Raman spectra upon irradiation indicate that degradation occurs to a higher extent in larger M W polymers. Laser induced fluorescence (LIF) probing of the plume reveals the presence of NapH and PhenH products from, respectively, NapI- and PhenI-doped films and a slower ejection process in the plume of low M W polymer. For highly absorbing PS, a less dramatic dependence on M W is observed. Results are discussed in the framework of the bulk photothermal model, according to which ejection requires a critical number of bonds to be broken.