Ejection of matrix-polymer clusters in matrix-assisted laser evaporation: Coarse-grained molecular dynamics simulations

Abstract

Molecular-level dynamic simulations are performed to investigate the mechanisms of molecular ejection and transport in laser ablation of frozen polymer solutions, as related to the matrix-assisted laser evaporation (MAPLE) technique for polymer film deposition. Coarsegrained description of molecular matrix and polymer molecules is used in the model, allowing for large-scale simulations of the ejection of multiple polymer molecules from MAPLE targets with different polymer concentrations, from 1 to 6 wt.%. The ejection of polymer molecules is observed only above the threshold for the collective material ejection (ablation). Ablation is driven by the phase explosion of the overheated matrix material, which proceeds through the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets and gas-phase matrix molecules. The polymer molecules resist the decomposition of the transient foamy liquid structure and stabilize the matrix droplets. In all simulations the polymer molecules are ejected as parts of large matrixpolymer droplets/clusters that are likely to retain a large fraction of matrix material at the time of the deposition on a substrate. The ejection and transport of large matrix-polymer droplets is related to high-resolution scanning electron microscopy (SEM) images of polymer films deposited in MAPLE, where morphologies of the films are found to be indicative of active processes of matrix vaporization and escape from the deposited matrix-polymer droplets.