| A comprehensive investigation of the thermodynamic properties of lattice polymers with structured monomers is performed through Monte Carlo type computations. The model of lattice polymers with structured monomers, also known as the extended lattice model, has been introduced as the underlying model of the lattice cluster theory (LCT), a thermodynamic theory for polymer mixtures in the liquid state. In the extended lattice model, the chemical structure of the macromolecules is mapped onto a cubic lattice. The resolution of the model is limited to closely bound groups of atoms, such as CHn groups, which occupy the sites of the lattice. The computations serve as a test of the mathematical accuracy of the theory and provide additional information that is not captured by the LCT. Thermodynamic quantities like the internal energy, the chemical potential, and the specific heat are computed for melts and binary blends of lattice polymers with structures similar to those of polyethylene, polypropylene, polyethylethylene, and polyisobutylene. The influence of monomer structure on the vapor-liquid coexistence curves of melts and the liquid-liquid coexistence curves of binary blends is also investigated. The predictions of the LCT for the thermodynamic properties of linear chains (polyethylene structure) are shown to be in better agreement with the computed values than the predictions of older theories. Although the accuracy of the LCT is expected to deteriorate as the complexity of the monomers' structure is increased, a reliable theory has to describe at least qualitatively the monomer structure effect. We find qualitative disagreements between the predicted influence of monomer structure on important thermodynamic properties and the effect of monomer structure observed in the computations. The qualitative disagreements arise for properties of both melts and binary blends and appear to be caused primarily by inaccurate counting of intermolecular contacts by the LCT. |
