Defense Date

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Mechanical and Nuclear Engineering

First Advisor

Dr. Hani El-Kaderi

Second Advisor

Dr. John Hackett

Third Advisor

Dr. Brian Hinderliter

Fourth Advisor

Dr. Karla Mossi

Fifth Advisor

Dr. James McLeskey

Abstract

Fluoropolymers are employed in countless end-user applications across several industries. One such fluoropolymer is polytetrafluoroethylene. This research is concerned with studying and understanding the thermal behavior of polytetrafluoroethylene. Such understanding is critical to predict its behavior in diverse service environments as the polymer ages and for allowing bottom up design of improved polymers for specific applications.

While a plethora of experiments have investigated the thermal properties of polytetrafluoroethylene, examining these properties using molecular dynamics simulations remains in its infancy. In particular, the current body of molecular dynamics research on polytetrafluoroethylene has primarily focused on studying polytetrafluoroethylene phases, its physical nature, and its helical conformational structure. The present study is the first molecular dynamics simulations research to study polytetrafluoroethylene behavior near the glassy transition temperature. Specifically, the current research utilizes molecular dynamics simulations to achieve the following objectives: (a) model and predict polytetrafluoroethylene glassy transition temperature at different molecular weights, (b) examine the impact of glassy transition temperature on the volume-temperature and thermal properties, (c) study the influence of molecular weight on polytetrafluoroethylene melt and glassy state, and (d) determine the governing forces at the molecular level that control polytetrafluoroethylene glassy transition temperature. Achieving the aforementioned objectives requires performing four major tasks. Motivated by the scarcity of polytetrafluoroethylene force fields research, the first task aims to generate and test polytetrafluoroethylene force fields. The parameters were produced based on the Optimized Potentials for Liquid Simulations All Atom model. The intramolecular parameters were generated using the automated frequency matching method while the torsional terms were fitted using the nonlinear least squares algorithm. The intermolecular partial atomic charges were obtained using Northwest Computational Chemistry software and fitted using the restrained electrostatic potential at (MP2/6-31G*) level of theory. The final set of parameter was tested by calculating polytetrafluoroethylene density using molecular dynamics simulations.

The second task involves building polytetrafluoroethylene amorphous structure using molecular dynamics at periodic boundary conditions for polytetrafluoroethylene cell at different molecular weights. We use the amorphous structure in the molecular dynamics simulations in consistence with research evidence which reveals that polymer properties such as the specific volume will differ as the polymer passes the glassy transition when it is in the amorphous phase structure whereas no variation occurs when the polymer passes the glassy transition while it is in the crystalline structure. The third task includes testing polytetrafluoroethylene melt phase properties: density, specific heat, boiling point, and enthalpy of vaporization. In the fourth and final task, we performed molecular dynamics simulations using NAnoscale Molecular Dynamics program. This task involves the polymer relaxation process to predict polytetrafluoroethylene mechanical behavior around the glassy transition temperature. Properties that are affected by this transition such as density, heat capacity, volumetric thermal expansion, the specific volume, and the bulk modulus were examined and the simulated results were in good agreement with experimental findings.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

5-8-2015

Available for download on Wednesday, May 06, 2020

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