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

Coarse-grained molecular dynamics simulations are performed to investigate the origins of the surface features observed in films deposited by the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. The simulations of MAPLE are performed for polymer concentrations up to 6 wt.% and a broad range of laser fluences. The polymer molecules are found to be ejected only 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. The effect of dynamic molecular redistribution in the ejected matrix-polymer droplets, leading to the generation of transient "molecular balloons" in which polymer-rich surface layers enclose the volatile matrix material, has been identified as the mechanism responsible for the formation of characteristic wrinkled polymer structures observed experimentally in films deposited by MAPLE.

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