Kevin G. Honnell

Site–site correlations in short chain fluids

Intramolecular and intermolecular site–site correlations in short chain fluids are obtained via Monte Carlo simulation for volume fractions ranging between 0.05 and 0.35. The chains are modeled as pearl necklaces of freely jointed hard spheres; chains composed of 4 and 8 beads are studied. The intramolecular distribution between a pair of beads separated by a fixed number of segments along the chain is found to be remarkably independent of the position of the pair along the chain. At low densities the intermolecular site–site pair distribution function at contact is found to be much less than one due to the ‘‘correlation hole’’ effect. The contact value increases as the density is increased, and decreases as the chain length is increased. We use the intramolecular correlations measured to obtain polymer reference interaction site model predictions for the intermolecular site–site distribution function. We find that the theory accurately reproduces the local structure of the fluid, but significantly overestimates the contact value of the distribution function, especially at low densities. A comparison of freely jointed chain results with simulations of chains with fixed bond angles and torsional rotations treated in the rotational isomeric state approximation shows that the correlation hole is more pronounced in freely jointed chains. We test a superposition approximation used to evaluate the three body term in the pressure equation for chain molecules. We find that the three‐body term is sizeable, and that the superposition approximation significantly underestimates the three‐body contribution.

Local structure of polyethylene melts

Polymer‐RISM (Reference‐interaction‐site‐model) theory is used to examine the local structure of a dense polyethylene melt near the freezing point. Predictions for the static structure factor are found to be in near quantitative agreement with new x‐ray diffraction data obtained at 430 K and 1 atm.

Reference interaction site model theory of polymeric liquids: Self‐consistent formulation and nonideality effects in dense solutions and melts

The reference interaction site model (RISM) integral‐equation approach to polymeric liquids is generalized to allow a self‐consistent determination of single‐chain and intermolecular pair correlations. Nonlinear medium‐induced effects on intrachain statistics are described at the level of self‐consistent pair interactions. Tractable schemes to implement the self‐consistency aspect are formulated for semiflexible and rotational isomeric state chain models, and applied numerically to concentrated solutions and melts of semiflexible polymers. Theoretical results are in good agreement with off‐lattice molecular dynamics simulations, and a rich dependence of the renormalized persistence length on temperature, aspect ratio, density, and degree of polymerization is found. The general formalism for polymer alloys is sketched and the potentially important role of local density and concentration fluctuations as nonuniversal mechanisms for inducing conformational perturbations is emphasized. A detailed analysis is m...

Diffraction by macromolecular fluids

X-Ray diffraction data on n-alkane fluids ranging from n-butane to polyethylene are analysed to yield the relative contributions from intra- and inter-molecular scattering. The intramolecular contributions are calculated from an exact rotational-isomeric-state model at short length scales, and approximated by a semiflexible chain model on global scales. With increasing chain length, the intermolecular pair distribution functions show a general decrease in short-range structure along with the predicted self-screening, or de Gennes ‘correlation hole’, whereby sites on different chains are partially excluded from close approach to sites on a given chain.