Crystallization And Melting Simulations Of Oligomeric Alpha 1 Isotactic Polypropylene

The crystallization and melting of the alpha 1 form of isotactic polypropylene (iPP) was studied with molecular dynamics (MD) simulations, using a fully flexible force field representing the C and I-I atoms atomistically and the CH3 groups as united atoms. Initially, a model crystal lattice of iPP of infinite molar mass was generated from experimental diffraction data, adding the pendant and geminal hydrogens. Next, crystal configurations of finite molar mass were created from this initial model lattice, and it was verified that the force field could preserve the crystal structure, keeping intact the monoclinic symmetry of the crystal. The generated configurations were heated under isobaric conditions, until a first-order transition into the melt state took place. The density change upon melting and the enthalpy of melting were estimated as differences between well-equilibrated ensembles of crystal and melt configurations. In order to calculate the equilibrium melting temperature T-m, composite (sandwich) configurations consisting of both melt and crystal subdomains in contact with each other were generated and subjected to a series of isothermal-isobaric simulations at a variety of temperatures. T-m was determined as that temperature where none of the phases in the sandwich grew at the expense of the other. As proximity to the glass temperature T-g made the dynamics of crystal growth very sluggish, a constraining potential inducing helical conformations along the chains was introduced to accelerate crystallization. Using a novel Gibbs-Duhem integration scheme that utilizes data from both sandwich and single-phase solid and liquid simulations, results for equilibrium solid-liquid coexistence were extrapolated to zero constraining potential. An accurate estimate of T-m was thereby obtained.