Self-Consistent Real Space Free Energy Calculations for Polyethylene and Isotactic Polypropylene Crystals and Interfaces

Abstract

Surface free energies for (100) and (010) orthorhombic polyethylene (PE) and α1 isotactic polypropylene (iPP) are calculated with self-consistent real space quasi-harmonic lattice dynamics methods. PE is used as a model system to systematically explore the accuracy of real space calculations versus traditional reciprocal space calculations for the bulk and interfacial properties of crystalline polymers. A linear potential is added to the atomistic Hamiltonian of the crystal to minimize the free energy of crystals with interfaces. (010) iPP is found to have a lower surface free energy than (100) iPP at temperatures relevant to crystallization. PE and iPP surface free energies are found to have opposite temperature dependences, ultimately because the PE slab stem packing becomes increasingly denser than the bulk as temperature increases while the opposite is true of iPP. Real space calculations give good agreement with experimental results and reciprocal space calculations for bulk PE and iPP lattice parameters, as well as calculated surface free energies of PE, with a reasonably sized crystal supercell and simpler calculations than in reciprocal space.