Intramolecular effects on the thermodynamics of polymers

It has long been assumed that intramolecular screening effects strongly affect the thermodynamics of polymer systems. Thus, models such as the liquid state PRISM theory explicitly separate the intramolecular contributions from intermolecular interactions in enumerating the thermodynamics of polymer systems. Here we show that the normalized difference between the cohesive energy density, η, and the internal pressure, Π≅∂U/∂V]_T, Δ≅(Π - η)/Π, which is essentially equal to zero for small molecules interacting purely through dispersive forces, allows for a direct measure of the intramolecular screening effect for polymer systems. These results are substantiated by analytical results derived from the lattice fluid model equation of state. We have also considered the behavior of several pure polyolefins from molecular dynamics simulations and find that Δ assumes a nearly universal value of 0.25±0.02. While our results stress the importance of intramolecular screening effects in determining the thermodynamics of polymer systems, they also suggest that the assumption that η≈π, which is commonly employed in the application of regular solution theory to polymers, may be grossly in error.