Giannis Bounos

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

We rely on a methodology demonstrated previously for assessing the temperature evolution and polymer viscosity changes in the 248nm irradiation of poly(methyl methacrylate)s (PMMAs) with molecular weights (Mw) ranging from 2.5to996kDa. Briefly, this methodology [G. Bounos et al., 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 Mw. The surface temperatures at the corresponding ablation thresholds are estimated to be ∼850–900K for Mw⩾120kDa vs ∼600K for 2.5kDa PMMA. In addition, for all Mw’s, melting is demonstrated (viscosity values of ≈101Pas), but it lasts longer for high Mw PMMAs. We ascribe these differences to the fact that low Mw PMMAs dissociate efficiently to desorbing monomers/oligomers, thereby resulting in more efficient energy removal. Even so, the ablation threshold of the high Mw PMMAs is attained at h...

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

The thermal and structural changes effected to poly(methylmethylacrylate) (PMMA) upon irradiation at 308, 248, and 193nm 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 Flaser at low fluences, but at higher f...

Femtosecond Laser Cleaning of Painted Artefacts; Is this the Way Forward?

The laser cleaning of painted artefacts relies on the synergy of thermal, photochemical and photomechanical effects, which are involved in laser ablation. A crucial issue, however, for a successful cleaning intervention is the spatial confinement and control of these effects for safeguarding the original surface from potential damage. Extensive studies have shown that in many cases there is an optimum interplay of laser and material parameters, which resulted in successful laser cleaning applications. The laser pulse duration is an important parameter in this context. The scope of this work has been the exploration of any advantages, which may be offered by using ultrafast UV (248 nm) lasers for the cleaning application of sensitive painted artworks. To achieve this goal, comparative study on the ablation rate and threshold of femtoand nanosecond laser pulses of typical varnishes (dammar, mastic, etc.) have been performed. Femtosecond pulses appear to be superior in terms of the spatial resolution and etching resolution and this fact has been demonstrated for both technical samples and original objects. Additionally, possible induced photochemical modifications have been investigated by monitoring the photoproduct laser-induced fluorescence of varnish-systems doped with low concentrations of welldefined photosensitive dopants (e.g. PhenI). It is established that irradiation with fs UV laser pulses results in minimal photochemical modifications. Importantly, the amount of photochemical products is largely independent from the optical properties (i.e. absorptivity) of the varnish. Considering the recent advances in ultrafast laser technology, the use of such lasers appears to provide a viable approach.

Enhanced Water Vapor Transmission through Porous Membranes Based on Melt Blending of Polystyrene Sulfonate with Polyethylene Copolymers and Their CNT Nanocomposites

A novel concept for the use of an immiscible and non-meltable polymer, such as sodium polystyrene sulfonate (PSSNa), in order to prepare polyethylene non-woven breathable membranes is described. Membranes were fabricated by melt compounding of properly functionalized PE (P(E-co-AA)) and PSSNa (P(SSNa-co-GMA)) copolymers in the presence of water soluble polyethylene glycol (PEG). The inability of PSSNa derivatives to be melted was overcome by using PEG, which was easily meltable thus inducing PSSNa processability improvement. PEG was removed after membrane fabrication and therefore also acted as a porogen. Carbon nanotubes, functionalized with PSSNa moieties or alkyl groups, were also incorporated in the membranes with the aim of improving the porous connectivity and increasing the water vapor transmission rate. The morphology of the membranes was investigated through Scanning Electron Microscopy (SEM). Water vapor transmission rate (permeation) (WVTR) measurements for the porous membranes showed increased values in comparison with the neat PE ones. A further increase of WVTR was observed with the addition of CNTs to the polymer membranes.

Novel Aspects of Materials Processing by Ultrafast Lasers: From Electronic to Biological and Cultural Heritage Applications

Materials processing by ultrafast lasers offers several distinct possibilities for micro/nano scale applications. This is due to the unique characteristics of the laser-matter interactions involved, when sub-picosecond pulses are employed. Prospects arising will be discussed in the context of surface and in bulk laser induced modifications. In particular, examples of diverse applications including the development and functionalization of laser engineered surfaces, the laser transfer of biomolecules and the functionalization of 3D structures constructed by three-photon stereolithography will be presented. Furthermore, the removal of molecular substrates by ultrafast laser ablation will be discussed with emphasis placed on assessing the photochemical changes induced in the remaining bulk material. The results indicate that in femtosecond laser processing of organic materials, besides the well acknowledged morphological advantages, a second fundamental factor responsible for its success pertains to the selective chemical effects. This is crucial for the laser cleaning of sensitive painted artworks.

Analysis of plume following ultraviolet laser ablation of doped polymers: Dependence on polymer molecular weight

This work investigates the effect of polymer molecular weight MW on the plume characteristics of poly(methyl methacrylate) (PMMA) and polystyrene (PS) films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI) following irradiation in vacuum at 248nm. Laser-induced fluorescence probing of the plume reveals the presence of ArH products (NapH and PhenH from, respectively, NapI- and PhenI-doped films). While a bimodal translational distribution of these products is observed in all cases, on average, a slower translational distribution is observed in the low MW system. The extent of the observed dependence is reduced as the optical absorption coefficient of the film increases, i.e., in the sequence NapI∕PMMA, PhenI∕PMMA, and PS-doped films. Further confirmation of the bimodal translational distributions is provided by monitoring in situ the temporally resolved attenuation by the plume as it expands in vacuum of a continuous wave helium–neon laser propagating parallel to the substrate. Results are di...

Effect of molecular weight on the physicochemical modifications induced in the UV laser ablation of doped polymers

This work investigates the effect of polymer molecular weight MW on the UV ablation of iodo-naphthalene- and iodo-phenanthrene-doped poly(methyl methacrylate) PMMA, and polystyrene PS films following irradiation at 248 nm. For irradiation at weakly absorbed wavelengths, the ablation threshold increases with increasing MW. However, at strongly absorbed wavelengths, the difference in the ablation thresholds is much smaller, or minimal. In parallel, bubble formation due to accumulation of gas produced by polymer and dopant decomposition differs depending on MW. For highly absorbing PS, the differences of behaviour show a less dramatic dependence on MW. These results are explained within the framework of the bulk photothermal model, according to which ejection requires that a critical number of bonds is broken. In all, they are of direct importance for the optimisation of laser processing schemes and applications and provide the first indication of explosive boiling in UV ablation of polymers.

Fundamental studies of the effect of molecular weight on the UV laser ablation of polymers

In this work we present a systematic study on the effect of MW on UV laser ablation of two polymers, poly(methyl methacrylate) PMMA and polystyrene, PS, doped with the iodo-derivatives of the aromatics naphthalene (Napl) and phenanthrene (Phenl). To this purpose we have examined the processes induced in the films of the doped polymers following ablation and the properties of the ejecta by using a number of different techniques

The potential of femtosecond lasers for the cleaning of painted artefacts

In this paper, the potential of femtosecond pulse lasers for the cleaning application of sensitive painted artworks are discussed. A comparative study on the ablation rate and threshold of femto- and nano-second laser pulses of typical varnishes (dammar, mastic. etc) and pigments (red and white lead, vermilion, ochres etc.) have been performed