We describe the application of the semi-grand canonical Monte Carlo (SGMC) method to analyze and interpret experimental data for nonequilibrium polymer melts and glasses. Experiments that provide information about atomic-level ordering, e.g., birefringence, are amenable to this approach. Closure of the inverse problem of determining the structural detail from limited data is achieved by selecting the lowest-free-energy ensemble of configurations that reproduces the experimental data. The free energy is calculated using the thermodynamic potential of the appropriate semi-grand canonical (SGC) ensemble [NPTΔμ( I )], as defined by the experimental data. To illustrate the method, we examine uniaxially oriented polyethylene melts of average chain length up to C400. The simulation results are analyzed for features not explicitly measured by birefringence, such as the density, torsion angle distribution, molecular scale orientation, and free energy, to understand more fully the underlying features of these nonequilibrium states. The stress-optical rule for polyethylene is evaluated in this way.

No abstract

No abstract

Multi‐scale modeling requires the selection and preservation of information crucial to understanding the behavior of a system at appropriate length and time scales. For a full description of processed polymers, such a model must successfully link rheological properties with atomic‐level structure. We propose a method for the calculation of an important rheological state descriptor, the polymeric configuration tensor 〈QQ〉, from atomistic simulations of oligomers. The method requires no adjustable parameters and can describe anisotropic polymer conformations at conditions of significant deformation. We establish the validity of the atomistic‐to‐macromolecular scaling by comparing the consistency of predictions of 〈QQ〉 among different polyethylene oligomer systems. We use this method with the previously reported semi‐grand canonical Monte Carlo method to deduce macromolecular and atomic‐level structural information interchangeably for systems with flow‐induced orientation.magnified image

SYNOPSISPhase diagrams including tie lines for three ternary solvent-polymer-polymer systems consisting of tetrahydrofuran-polystyrene-polytetrahydrofuran at 30°C have been obtained using size exclusion chromatography. The effect of molecular weight on polymer-polymer compatibility has been investigated. A liquid-liquid equilibria ( LLE ) database has been used to compare UNIQUAC and a modified Flory-Huggins model with respect to their ability to correlate LLE data. Both models contain six adjustable parameters. As previously observed in the literature, the estimation of the UNIQUAC parameters is troublesome due to the model's complexity and the intercorrelation of the parameters. On the basis of better performance and ease of use, the Flory-Huggins model is recommended. The Flory-Huggins model is capable of accurately describing LLE of systems which exhibit quite complex tie line behavior.