| Abstract |
Laser ablation of polymers constitutes the basis for a wide range of applications ranging of restoration of artworks to medical applications. There are several reasons for the difficulties in establishing the mechanism(s), but the most important one appears to be the limitations in the systematic examination of the influence of material parameters. To overcome this problem, we turn to the examination of polymer molecular weight (Mw) on laser induced processes and in ablation. In the comparison of polymers of different Mws, the chemical and optical properties are nearly identical, and the systems differ only in the number of bonds. Thus, the relevance of the various mechanisms that have been suggested for UV ablation of polymers can be directly tested. The study concentrates on Poly(methylmethacrylate) (PMMA) and on Polystyrene (PS) which examined as model systems of varnishes in the painted artworks at three excimer wavelengths (193 nm, 248 nm and 308 nm). At weakly absorbed wavelengths, the ablation thresholds increase with increasing Mw, whereas at strongly absorbed wavelengths, the ablation thresholds are nearly the same. Different trends in the morphological changes are observed. For a more detailed assessment of the mechanism, a methodology for assessing the temperature evolution and polymer viscosity changes was developed by relying on monitoring the kinetics of products formation by photolysis of photosensitive compounds (probes) dispersed within the examined polymers. For both PMMA and PS, at all wavelengths, high temperatures well above the ceiling decomposition temperatures are attained. At weakly absorbed wavelengths higher temperatures are attained with increasing Mw whereas at strongly absorbed wavelengths, the temperatures are about the same independently of polymer Mw. In addition, for all Mws, melting is demonstrated. These results unequivocally demonstrate that UV laser ablation is a thermal process; yet, significant deviations between the experimental temperatures and theoretically predicted ones on the basis of the bulk photothermal model (relying on conventional decomposition of polymer to monomers/oligomers) are noted. To eludicdate the reason for this discrepancy, the nature of the ejected material has been characterized by SEM and AFM examination of the material deposited on a surface (under vacuum). For low Mw, decomposition to monomers and oligomers is extensive; whereas for high Mw the process is uncompleted. In parallel, the photoscattering experiments show that at weakly absorbed wavelengths, the translational distributions of the ejecta from low Mw are very broad and slow, whereas the ones from the high Mw systems are very sharp and peak at high velocities, indicating that ejection from the high Mw systems is much more impulsive. At strongly absorbed wavelengths the distributions are high, nearly the same for both high and low Mw. The indications by the examination of translational distributions have been confirmed by piezoelectric measurements of the pressure developed in the substrates. We argue that the observed features are similar to the characteristics of explosive boiling observed in the fast superheating of simple compounds (liquids). A model for the explosive boiling of polymers is developed and shown that it can consistently account for all observations, as well as for most inconsistencies noted in previous studies on laser ablation of polymers. Besides the mechanistic implications, the results are also of direct relevance to the optimization of laser processing schemes, since in a number of applications (e.g. pulsed laser deposition of polymeric films, laser restoration of artworks, medical applications) the molecular weight may vary a lot from case to case. Most surprisingly the influence of Mw has not taken into account on the optimization of laser processing schemes. Especially in the restoration of artworks, even on the same artwork, the upper layers appear a higher degree of polymerization (because of UV exposure and humidity) In the last part of thesis, we present a preliminary study on the chemical modifications effected to PMMA doped with iodonaphthalene or iodophenanthrene upon femtosecond UV (500 fs, λ=248 nm) irradiation. The most important observation is that product formation is quite limited and selective upon irradiation at high fluences close to the ablation thresholds. A plausible explanation based on formation at weak plasma within the substrate is advanced. At any rate, the results indicate that in the fs processing of biopolymers, besides the well known excellent etching morphology, an additional factor for the success relates to the high selectivity of the induced chemical modifications. This thesis constitutes the basis for 9 publications in international high quality peer reviewed scientific journals (e.g. Journal of Physical Chemistry B, Journal of Applied Physics), one chapter in a scientific book related to ablation of molecular substrates and numerous presentations in national and international scientific conferences. Four more articles are in the processes of preparation and will be submitted to scientific journal shortly.
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Examination of the mechanisms of UV Laser Ablation of polymers
