Extensive molecular-dynamics simulations were performed to study the effect of chain conformational rigidity, controlled by bending and torsion potentials, on self-diffusion in polymer melts. The polymer model employs a novel torsion potential that avoids computational singularities without the need to impose rigid constraints on the bending angles. Two power laws were traditionally used to characterize the dependence of the self-diffusion coefficient on polymer length: D ∝ N-ν with ν = 1 for N < Ne (Rouse regime) and with ν = 2 for N > Ne (reptation regime), Ne being the entanglement length. The simulations, for constant temperature and density, up to N = 250 revealed that, as the chain rigidity increased, the exponent ν gradually increased towards ν = 2.0 for N < Ne and ν = 2.2 for N > Ne. The value of Ne was slightly increased from 70 for flexible chains, up to the point where the crossover became undefined. This behavior was confirmed also by an analysis of the bead mean-square displacement. Subsequent investigations of the Rouse modes, dynamical structure factor, and chain trajectories indicated that the pre-reptation regime, for short stiff chains, was a modified Rouse regime rather than reptation.

Effect of Bending and Torsion Rigidity on Self-Diffusion in Polymer Melts: a Molecular-Dynamics Study. Bulacu, M., Van Der Giessen, E.: Journal of Chemical Physics, 2005, 123[11], 114901