Twist solitons in ordered phases of n-alkanes

Abstract

Twist solitons are important topological defects in the crystalline and rotator phases of normal alkanes and polyethylene (PE). They facilitate molecular motion along the stem axis in crystalline lamellae, as well as molecular rotation in rotator phases RI and RII. Here, we determine the conformation, formation energy, mobility, and fluctuation free energy of twist solitons in n-alkanes and PE by a combination of techniques, including all-atom simulations and analytical/numerical calculations. We find the soliton formation energy to be about 15 kT for the crystal, 4 kT for RI, and only about kT in the RII phase. We model stochastic transport of solitons across a PE crystalline lamella, and compute a jump rate for stems at 350 K of 30 000 s⁻¹, comparable to NMR results; in the RI phase of C₂₃ n-alkanes, we predict a jump rate of 1.5 × 10¹⁰s⁻¹, comparable to molecular dynamics (MD) observations. We calculate corrections due to twist fluctuations, which reduce the soliton free energy by of order kT. Thus the RII phase should be regarded as a phase in which molecules twist freely with essentially no barrier.