Elucidating the Thermal, Chemical, and Mechanical Mechanisms of Ultraviolet Ablation in Poly(methyl methacrylate) via Molecular Dynamics Simulations

Abstract

[Figure: see text]. Laser ablation harnesses photon energy to remove material from a surface. Although applications such as laser-assisted in situ keratomileusis (LASIK) surgery, lithography, and nanoscale device fabrication take advantage of this process, a better understanding the underlying mechanism of ablation in polymeric materials remains much sought after. Molecular simulation is a particularly attractive technique to study the basic aspects of ablation because it allows control over specific process parameters and enables observation of microscopic mechanistic details. This Account describes a hybrid molecular dynamics-Monte Carlo technique to simulate laser ablation in poly(methyl methacrylate) (PMMA). It also discusses the impact of thermal and chemical excitation on the ensuing ejection processes. We used molecular dynamics simulation to study the molecular interactions in a coarse-grained PMMA substrate following photon absorption. To ascertain the role of chemistry in initiating ablation, we embedded a Monte Carlo protocol within the simulation framework. These calculations permit chemical reactions to occur probabilistically during the molecular dynamics calculation using predetermined reaction pathways and Arrhenius rates. With this hybrid scheme, we can examine thermal and chemical pathways of decomposition separately. In the simulations, we observed distinct mechanisms of ablation for each type of photoexcitation pathway. Ablation via thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case in which an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. A detailed analysis of the processes shows that a critical energy for ablation can describe this complex series of events. The simulations show a decrease in the critical energy with a greater amount of photochemistry. Additionally, the simulations demonstrate the effects of the energy deposition rate on the ejection mechanism. When the energy is deposited rapidly, not allowing for mechanical relaxation of the sample, the formation of a pressure wave and subsequent tensile wave dominates the ejection process. This study provides insight into the influence of thermal, chemical, and mechanical processes in PMMA and facilitates greater understanding of the complex nature of polymer ablation. These simulations complement experiments that have used chemical design to harness the photochemical properties of materials to enhance laser ablation. We successfully fit the results of the simulations to established analytical models of both photothermal and photochemical ablation and demonstrate their relevance. Although the simulations are for PMMA, the mechanistic concepts are applicable to a large range of systems and provide a conceptual foundation for interpretation of experimental data.

Surface morphological modification of crosslinked hydrophilic co-polymers by nanosecond pulsed laser irradiation

“…For laser irradiation at 532 nm, either photochemical or photothermal mechanisms would explain the formation of both simple craters for dr MT and dr/sw MD, and craters with distinct morphologies around for sw MT. The corresponding low attenuation coefficients at 532 nm (Table 1) support the major involvement of a laser ablation mechanism, which massively removes polymeric material at relatively large penetration depths [3].…”

“…The results can be interpreted in terms of the currently accepted mechanisms for ns laser-induced polymer surface modification [1][2][3]5], with a consideration of the relative optical penetration depths l a (≡˛− 1 ) for the various samples at the two LI 's. For laser irradiation at 532 nm, either photochemical or photothermal mechanisms would explain the formation of both simple craters for dr MT and dr/sw MD, and craters with distinct morphologies around for sw MT.…”

“…Laser ablation, i.e., the massive removal of polymeric material is the most prevalent result from ns laser-matter interactions at fluences well above the threshold for surface modification. Precise laser processing usually requires the use of ultraviolet (UV) laser wavelengths in order to avoid thermal effects degrading the quality of the modified surface morphologies, as arising from the use of visible (Vis) and infrared (IR) wavelengths [1,3]. Since 2005, a distinct phenomenon, so-called "laser foaming", has been reported to occur in self-standing films of polymers and biopolymers obtained by casting procedures [4,5].…”

Micro additive manufacturing using ultra short laser pulses

“…However, following Prasad et al, the interplay of chemical, thermal and also mechanical processes is still under investigation. For this purpose computational investigations, which means mainly molecular dynamics (MD) simulations, combined with other techniques such as Monte Carlo (MC) algorithms and mesoscopic approaches are employed [47,50,51]. In their investigation, Prasad et al proved by using a combined MD-MC approach that chemical, thermal and mechanical interactions partially couple during laser processing.…”

Photothermally Directed Assembly of Block Copolymers

“…Photothermal annealing using localized laser heating offers an attractive route toward this goal. Laser illumination has been used for a variety of applications, including treatment of semiconductor and inorganic materials, [77][78][79] laser ablation of polymers, [80][81][82][83][84][85] controlled dewetting, [86][87][88] ordering of photoresponsive materials, [89,90] and the annealing of soft materials such as chemically amplified photoresists. [91][92][93][94] More recently, laser illumination has been applied to block copolymer materials [95] in conjunction with solvent vapor [96] and thermal [97][98][99][100][101] annealing methods.…”