Diffusivity of wormlike particles in isotropic melts and the influence of local nematization

Systems of wormlike particles are studied by molecular dynamics computer simulations in the isotropic phase. The particles are modeled as a chain of nine soft repulsive, partially overlapping, spherical sites. These particles are characterized by a varying degree of internal flexibility: from very stiff and rodlike to very deformable and stringlike. Their self-diffusion coefficients have been evaluated at many thermodynamic conditions. Maintaining the temperature constant and sufficiently low, from the stiff rod case and letting the worms become more flexible, the self-diffusion coefficients first steeply increase, then reach a plateau. This behavior is rationalized in terms of local nematic domains. The presence of the latter significantly affects the equation of state of the wormlike particles as well, with the density of the stiffest rods being, respectively, the highest at lower temperatures and the lowest at higher temperatures. This fact might have significant implications for the theory of liquid-crystalline phase behavior in systems of flexible particles. If the density of all systems is fixed at the equilibrium value of the stiffest case, which is a way to single out the effects of flexibility, self-diffusion coefficients may exhibit a moderately nonmonotonic behavior: subsequently to the above-mentioned steep increase, they may reveal a shallow maximum for intermediate degrees of internal flexibility degrees, then gently decrease. This behavior is tentatively associated with the different effects that internal flexibility has on the average length and width of a wormlike particle.