Effect of molecular weight on the morphological modifications induced by UV laser ablation of doped polymers

This work investigates the effect of polymer molecular weight (M(W)) on the surface morphology of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in air at 248 nm. In agreement with previous studies, irradiation of PMMA at 248 nm results in surface swelling and bubble formation within the irradiated bulk. Most importantly, the size of bubbles varies sensitively for the different M(W) values, with larger bubbles being formed for the low M(W) systems. Nevertheless, the maximum swelling attains higher values for the high M(W) values (when compared at the corresponding ablation threshold of the systems). Real-time monitoring of transmission of a probing beam shows that morphological changes last longer in the low M(W) polymer. Melting, consistent with a thermal mechanism, occurs, and enough evidence is gathered to provide direct support for the bulk photothermal model, according to which ejection requires that a critical number of bonds is broken. In particular, the observed different morphological effects can be ascribed to the interplay of two factors, namely, of the much more efficient decomposition of the low M(W) polymer to gaseous products and of the dependence of the mechanical polymer properties on M(W). For PS at 308 nm, the changes parallel the ones for PMMA at 248 nm. In contrast, at the strongly absorbed 248 nm, the morphological processes in PS show a less dramatic dependence on M(W). In all, these results are of direct importance for the optimization of laser processing schemes and applications (e.g., tissue processing, laser deposition, laser restoration, etc.).