Modeling chain stiffness and attractive interaction for polymeric systems in the NPT ensemble

Theoretically based equations of state (EOS) serve as practical tools for obtaining equilibrium properties of polymers. Most of the theoretically based EOS in current use are pressure-explicit. However, chemical engineering operations are usually carried out at constant temperature and pressure thereby making volume-explicit EOS more practically relevant. Significant fractions of engineering polymers are made up of non-flexible chains and yet the EOS for polymeric systems in current use do not explicitly take account of chain stiffness. In this work, we present a model for polymeric systems which include the effect of chain stiffness and attractive contributions based on a simple volume-explicit hard chains EOS. Chain stiffness is modeled based on non-additive size interactions. We used approximations based on a known exact solution for one-dimensional square well fluid to model the attractive contributions in three-dimensional polymeric system.