Molecular modeling of thermodynamic properties, microstructure, and phase behavior of polymer systems
Chapman, Walter G.
Doctor of Philosophy
Fluids are defined as complex due to their size, shape, polydispersity, or specific intermolecular and intramolecular interactions. Success in modeling complex fluids and their mixtures is contingent upon the ability of the molecular model to describe specific interactions, and capture the size and shape effects governing the phase behavior of the systems. Molecular models based on Wertheim's Thermodynamic Perturbation Theory of first order incorporate detailed information regarding the architecture of the molecules and their microscopic interactions, thus representing fluids and their mixtures with a high degree of realism. In particular, the Statistical Associating Fluid Theory (SAFT), as well as its later versions (e.g., Perturbed Chain-SAFT), have emerged as powerful tools for modeling complex fluid systems. The developments presented in this dissertation can be broken down into two components that separately focus on the bulk and interfacial aspects of the considered systems. The bulk part focuses on modeling phase behavior of polyethylene solutions. Based on experimental studies of the phase behavior of Linear Low Density Polyethylene, a simple and effective modeling concept for branched polyolefins is proposed in the framework of PC-SAFT. The model, extensively validated by comparisons to experimental data, is used to study the influence of a short-chain branching distribution on the phase behavior of polyolefins. The PC-SAFT equation of state was the underlying thermodynamic model used in the studies of branched polyolefins. A shortcoming of PC-SAFT identified during the work---systematically erroneous predictions of the phase behavior of polymer solutions at high polymer concentrations---motivated the development of a new equation of state for chain fluids. The SAFT-Dimer model describes the phase behavior of long chain fluids and polymers with high accuracy. On the interface side, a recently developed density functional theory (DFT), based on Wertheim's Thermodynamic Perturbation Theory of first order, is applied in conjunction with SAFT to predict the interfacial properties of hydrocarbons and polymers. The self-consistency of the bulk and interfacial models is of critical importance to several applications in which interfacial properties of the considered systems need to be predicted based on the readily available bulk properties.