Assessment of the attained temperatures and of melting in the nanosecond irradiation of doped poly(methylmethylacrylate) at 308, 248, and 193 nm via the examination of dopant reactivity

The thermal and structural changes effected to poly(methylmethylacrylate) (PMMA) upon irradiation at 308, 248, and 193 nm are assessed via the examination of the formation yields of the products formed by the photolysis of iodoaromatics (iodonaphthalene and iodophenanthrene-ArI-) dopants. Specifically, the main aryl product, the hydrogen-substituted derivative ArH, is formed via a thermally activated process (hydrogen-atom abstraction); thus, its formation efficiency reflects the temperature evolution in the substrate following UV irradiation. In the case of iodonaphthalene dopant, biaryl species (1,1-binaphthalene and perylene) are also formed via diffusion-limited reaction of the aryl radicals; thus, their yield reflects the extent of polymer melting. To this end, laser-induced fluorescence is employed for the quantification of the aryl products formed in the substrate as a function of the irradiation fluence. At all wavelengths, the ArH amount scales linearly with F-laser at low fluences, but at higher fluences, it increases sharply reaching a plateau near the ablation threshold. Only quantitative differences concerning the fluence onset of the ArH increase and the amount of product remaining in the substrate are observed. Simulations accounting for the temporal and spatial evolutions of the temperature reproduce well the observed F-laser dependences. The quantitative differences in the extent of ArH formation are well accounted by the extent of the heat diffusion to the sublayers. Thus, contrary to many previous suggestions, a thermal process is demonstrated to be dominant at the three wavelengths. Concerning the biaryl species, their yield decreases from 308 to 193 nm. The simulation of their formation yield provides semiquantitative information about the polymer viscosity changes (melting) upon irradiation at the three wavelengths. Besides the mechanistic implications, the study also provides insight into the factors affecting the extent of chemical modifications in laser processing of polymers and organic substrates in general. In particular, the reduced extent of chemical modifications upon ablation at strongly absorbed wavelengths is indicated to be crucial for the success of these procedures. (c) 2005 American Institute of Physics.