Title:
ATOMISTIC SIMULATIONS OF ATACTIC POLYPROPYLENE: INITIAL GUESS OPTIMIZATION AND LONG-RANGE CORRECTION EVALUATION
ATOMISTIC SIMULATIONS OF ATACTIC POLYPROPYLENE: INITIAL GUESS OPTIMIZATION AND LONG-RANGE CORRECTION EVALUATION
Author(s)
Trevino Garrido, Nohemi Dxandi
Advisor(s)
Ludovice, Peter J.
Meredith, J. Carson
Grover, Martha A.
Meredith, J. Carson
Grover, Martha A.
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Abstract
It was shown by this research, that after about 75% of initial guess chain polymerization, chains would began to collapse into compact conformations due to the self-avoiding nature of the chains. This resulted in structural inaccuracies in the initial guess that would not be ameliorated by energy minimization or molecular dynamics. Thus, the effect of density on the structural and energetic components of initial guess generation was investigated in this research. Findings showed significant asymmetry in chains generated with the present established initial guess method. Additionally, cohesive energy density, diffusivity, and entanglement of atomistic models of atactic polypropylene are strongly sensitive to density changes and were improved at densities that were even higher than the experimental density. However, there were only slight changes to structural properties such as mean square radius of gyration, the characteristic ratio, and chain symmetry as density was modified. Additional initial guess methods were developed such as integrating minimization into the polymerization process, filtration of initial guesses based on their structural properties, multi-step minimization, an expanding periodic boundary method, and optimization of forcefield parameters for initial guess generation.
Additionally, a simple, rapid, and empirical method was developed for calculating energy and pressure long-range corrections. This method involved iteratively replicating the periodic lattice of the polymer system and matching the cutoff to the new size of the system. Thus, the potential energy is dependent on the number of cells in the lattice and the potential energy eventually converged to provide the long-range corrections. The cohesive energy also converged since the isolated energy of the chains is a constant value and could thus also be rapidly calculated.
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Date Issued
2021-10-01
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Text
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Thesis