Melting Point and Solid–Liquid Coexistence Properties of α1 Isotactic Polypropylene as Functions of Its Molar Mass: A Molecular Dynamics Study

The melting point ( T m ) of the α1 form of isotactic polypropylene (iPP) as a function of its molar mass was studied with molecular dynamics (MD) simulations. The 11mer, 32mer, and 47mer systems were simulated, with the latter displaying a T m representative of the long-chain polymer. Adopting a methodology developed in previous work, composite (sandwich) configurations consisting of both melt and crystal subdomains in contact with each other were generated, and T m was determined as that temperature where none of the phases in the sandwich grew at the expense of the other. To deal with the sluggish dynamics of solidification and melting, a constraining potential that drives chains toward helical conformations was added to the Hamiltonian, solid–liquid equilibration was achieved at high temperature in the presence of this potential, and T m was ultimately obtained through gradual removal of the constraining potential in single-phase solid and liquid simulations by Gibbs–Duhem integration. The enthalpy difference Δ fus H and entropy difference Δ fus S between single-phase solid and liquid were obtained in the absence of the constraining potential. During the melting process the up–down configuration of chains, the structure factor, the order parameter describing chain orientation relative to the c crystallographic axis, and motions characteristic of the rotator phase were studied along with the density and enthalpy.