Experiments and modeling studies have identified thermal, chemical, and mechanical processes as likely sources of laser ablation in polymeric materials. In our earlier study, molecular dynamics simulations with an embedded Monte Carlo based reaction scheme have been used to investigate the role of each of these processes separately, in a poly(methyl methacrylate) substrate irradiated by ultraviolet lasers. In the present study, using the same substrate, we allow for interactions among these processes, to better model the real ablation process, and investigate how it affects the system evolution during ablation. In the purely thermal case, chemical reactions are allowed to occur via the thermo-mechanical bond break and radical formation leading to subsequent gas formation. Similarly, in purely photochemical cases, additional bond breaks and reactions occur due to high temperatures and stresses. In all cases, it is observed that the ablation process is extended over longer time scales (laser pulse width) resulting in higher yields. The thermo-mechanical bond breaks and ensuing reactions ensure that the substrate remains hotter for a long time, causing more fragmentation and ejection of the substrate. The mechanism of ejection for all thermal and chemical pathways is found to be both the thermo-mechanical in nature, driven by critical fraction of broken bonds, as well as chemical in nature, governed by near complete disintegration of the polymer matrix into monomers, small polymer fragments, and gas molecules. The formation of volatile gases, just underneath the surface assists in the ejection of the fragmented substrate. In all cases, secondary damage due to thermo-mechanical bond break process stimulated by high temperature and stresses, and by chemistry of the polymeric substrate, is found to contribute significantly more than the direct laser photochemical or photothermal damage, to the substrate and plume evolution as well as total ablation yield.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films