Molecular dynamics simulations of laser ablation of polymers

Open Access
Author:
Conforti, Patrick F
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 09, 2008
Committee Members:
  • Barbara Jane Garrison, Committee Chair
  • Mark Maroncelli, Committee Member
  • Albert Welford Castleman Jr., Committee Member
  • Vincent Henry Crespi, Committee Member
Keywords:
  • molecular dynamics
  • laser ablation
  • polymethyl methacrylate
  • photochemistry
Abstract:
Molecular dynamics simulation is used to elucidate the mechanisms of ablation in polymers. A novel computational approach is constructed which appropriately models both thermal and chemical processes in poly(methyl methacrylate), or PMMA, following irradiation events. A Monte Carlo scheme uses predetermined reaction chemistry, energetics, and rate kinetics of the polymer material and allows the incorporation of chemical reactions during classical simulations. The kinetics data determines the probability for each type of reaction to perform based on radical survival times and Arrhenius reaction rates. Electronic structure calculations establish the activation and reaction energetics of each of the possible chemical decomposition pathways. The hybrid simulations permit the complex interplay between chemistry and other physical processes to be studied without the need for complex potentials or expensive ab initio dynamics. Using the molecular dynamics-Monte Carlo simulation technique, thermal and chemical excitation channels are separately studied with a coarse-grained PMMA system. The mechanism of ablation for thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case where an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. The photolytic decomposition process is shown to be more efficient in initiating ablation. Stress is important in initiating ejection when a short pulse width is used causing a propagating pressure wave. Additionally, following excitation, molecular dopants can lead to ablation either through localized chemical decomposition of the polymer or the mechanical cleavage of the polymer bonds. The simulations provide insight into the influence of thermal and chemical processes in PMMA and facilitate greater understanding of the complex nature of polymer ablation.