Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation

Abstract

The physical mechanisms and molecular-level picture of laser-induced material ejection from frozen solutions of polymer molecules in a volatile matrix are investigated in a series of coarse-grained molecular dynamics simulations. The simulations are performed for polymer concentrations up to 6wt% and laser fluences covering the range from the regime where molecular ejection is limited to matrix evaporation from the surface up to more than twice the threshold fluence for the onset of the collective molecular ejection or ablation. The results of the simulations are related to experimental observations obtained in matrix-assisted pulsed laser evaporation (MAPLE) thin film depositions and are used to address unresolved research questions that are of direct relevance to MAPLE performance. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected in the ablation regime and are always incorporated into polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. The entanglement of the polymer molecules facilitates the formation of intricate elongated viscous droplets that can be related to the complex morphologies observed in polymer films deposited by MAPLE. Analysis of the state of the irradiated target reveals a substantial increase of the polymer concentration and complex surface morphology generated in the new surface region by the ablation process. The ramifications of the computational predictions for interpretation of experimental data and the directions for future experimental exploration are discussed based on the physical picture of molecular ejection and transport in MAPLE emerging from the simulations.