Dependence of ultraviolet nanosecond laser polymer ablation on polymer molecular weight: Poly(methyl methacrylate) at 248 nm

We rely on a methodology demonstrated previously for assessing the temperature evolution and polymer viscosity changes in the 248 nm irradiation of poly(methyl methacrylate)s (PMMAs) with molecular weights (M-w) ranging from 2.5 to 996 kDa. Briefly, this methodology [G. Bounos , J. Appl. Phys. 98, 084317 (2005)] relies on monitoring the formation of aryl products in the irradiation of polymer doped with iodonaphthalene or iodophenanthrene. The results demonstrate that higher temperatures are attained with increasing M-w. The surface temperatures at the corresponding ablation thresholds are estimated to be similar to 850-900 K for M-w >= 120 kDa vs similar to 600 K for 2.5 kDa PMMA. In addition, for all M-w's, melting is demonstrated (viscosity values of approximate to 10(1) Pa s), but it lasts longer for high M-w PMMAs. We ascribe these differences to the fact that low M-w PMMAs dissociate efficiently to desorbing monomers/oligomers, thereby resulting in more efficient energy removal. Even so, the ablation threshold of the high M-w PMMAs is attained at higher fluences. This can be ascribed to the higher fraction of bonds that is required to be broken, and also plausibly to the higher gaseous product pressures that must be attained, for material ejection to be effected. The results are overall well accounted by the bulk photothermal model. (c) 2006 American Institute of Physics.