Crystallization of polyethylene and polytetrafluoroethylene by density-functional methods

Molecular density-functional theory is extended to address the crystallization of chemically realistic polymers. The polymer (RISM) reference interaction site model integral-equation approach is employed to calculate the liquid-state structural information required as "input" into our density-functional theory. The single-chain structure is described by the rotational isomeric state model, and the accuracy of both the theoretically calculated single-chain and liquid structures have been verified by direct comparison with Monte Carlo simulation and x-ray scattering, respectively. The driving forces for the crystallization of polymers are found to be completely different from those in monatomic systems and can be understood in terms of an effective "chain-straightening force" (which results from chain packing) combined with a background attractive potential. Remarkably, the predicted melting temperatures for polyethylene and polytetrafluoroethylene at atmospheric pressure are within a few degrees of the experimental values, and the density-temperature phase diagrams are also in good agreement with experiment. Chemically unrealistic, coarse-grained models of polymer structure appear to be inadequate for the crystallization phenomenon, which is found to be quantitatively sensitive to interchain attractive forces and melt compressibility. The aspect ratios in polyethylene and polytetrafluoroethylene melts at the phase transition are predicted to be virtually identical.