(251i) Molecular Dynamics Simulation of Twist Solitons in Isotactic Polypropylene Crystals

Twist solitons are topological defects that allow chains to move in crystalline polyethylene (PE), and stabilize rotator phases in n-alkanes. Rotator phases are partially ordered phases in which nearly all-trans chain "stems’’ are packed on a regular lattice but rotationally disordered. Solitons are localized twists in a chain; when solitons propagate across a crystalline lamella, the chain rotates and translates along its axis. Less is known about twist solitons in isotactic polypropylene (iPP), and rotator phases in iPP are speculative. To determine if twist solitons stabilize rotator phases in iPP as they do in alkanes, we have used all-atom molecular dynamics to simulate twist solitons in iPP, to determine their shape, mobility, and formation energy. In our simulations, iPP twist solitons are represented by an extra 2π/3 twist “trapped” on a periodically connected iPP molecule in a crystalline system. The soliton formation energy is a balance between intramolecular twist stiffness arising from the dihedral potential, and an intermolecular orientational potential resulting from interactions with neighboring molecules. We measure the soliton shape by monitoring the "twist angle’’ defined by the dihedral angle between successive pendant methyls along the average chain direction. In the alpha phase, solitons are well defined and narrow, with a width of less than six monomers at the iPP melting temperature. This implies a high formation energy for solitons in the alpha phase. We anticipate broader solitons with lower formation energies in the rotator phase, because the more disordered arrangement and lower density of chains should result in a weaker intermolecular potential. Dynamically, solitons in the alpha phase diffuse along the chain axis. We obtain the soliton diffusion coefficient from the mean-square displacement versus time. Qualitatively, solitons in iPP diffuse much more slowly than in alkanes, consistent with a much higher friction coefficient of a more localized defect.