Multiscale computational modeling of high-pressure phase stability, structure, and thermophysical properties of compressible polyolefin solutions

by Moeed Shahamat

Institution: McGill University
Department: Department of Chemical Engineering
Degree: PhD
Year: 2015
Keywords: Health And Environmental Sciences - Environmental Sciences
Record ID: 2057918
Full text PDF: http://digitool.library.mcgill.ca/thesisfile130499.pdf


The knowledge of high-pressure phase behavior and phase equilibria of polyethylene (PE) in hydrocarbon solutions is an integral part of the process design and manufacturing of PE via solution polymerization. This thesis focuses on the study of fundamental polymer thermodynamics and key mechanisms that govern phase stability in polyolefin solutions via combined thermodynamics-molecular modeling algorithms.Force field-molecular dynamics simulations are utilized to bridge the gap between experimentally observed macro-scale phase separation phenomena and molecular-level details of fundamental studies of macromolecular thermodynamics in polymer-solvent systems. In this context, the main contributions of the present thesis work focus on molecular thermodynamic characterization of the pressure-induced phase separation (PIPS) mechanism and lower critical solution temperature (LCST) fluid phase behavior of PE solution; high-pressure thermodynamic and structural properties of binary and ternary solutions of PE + hexane and PE + hexane + ethylene, respectively; improvement of the computational efficiency and accuracy of the isobaric-isothermal and canonical ensemble simulations; overcoming the practical challenges involved in the implementation of equation of state theories.A fully-atomistic molecular mechanics force field combined with molecular dynamics is implemented to compute solubility parameter, liquid phase density, structure, and internal pressure of HDPE and hexane over a broad range of pressures. Based upon the knowledge of pressure and temperature dependence of solubility parameters the binary interaction parameter is computed to shed light on phase stability predictions in PIPS mechanism and LCST phase behavior. A molecular-level explanation for the change in cohesive properties and structure of PE and hexane upon raising the external pressure is provided. Additionally, a relation is established between cohesive energy density and internal pressure for the solvent and polymer as a function of pressure. A comparison is reported between electrostatic algorithms of switch function and the particle mesh Ewald method, and also the effect of grid spacing on the computational accuracy of electrostatic energy contribution is revealed.This thesis also implements the state of the art molecular modeling methods and equation of state modeling to report on the pressure dependence of binary PE solution density for various polymer compositions, required to solve the phase equilibria and kinetics of compressible polymer solutions. The effect of the cut-off radius of intermolecular potentials on the non-bonded forces and densities of the polymer-solvent mixture with the objective of improving the computational efficiency of molecular dynamics simulations is investigated and an optimized cut-off distance is suggested for high-pressure molecular mechanics modeling of compressible polyolefin solutions. An atomistic-level analysis of the impact of pressure on the structure of PE-solvent mixture is also provided.The isobaric-isothermal…