Diffusing Diffusivity in Dynamics of Unentangled Polymer Melts

The exact results for Rouse chain dynamics in the crowded environment modeled by diffusing diffusivity (N. Ahamad and P. Debnath, Phys. A 2020,549, 124335) in explaining polymer melt dynamics is explored. Theory for p=0 mode accounts for anomalous and non-Gaussian diffusion in the center of mass (c.m.) and monomer dynamics of united atom molecular dynamics simulation trajectories of unentangled polyethylene melts, with excellent agreement. Accurate estimates of scaling relations for c.m. and monomer mean square displacement with time interval t are obtained. The corresponding c.m. and monomer diffusion coefficients as a function of t confirm diffusing diffusivity in polymer melts. It is shown shown that the dependent parameters of the modified Rouse model expressed in terms of all modes are reducible to an identical set of per mode parameter values on application to real data. Analysis of velocity autocorrelation functions indicates that the theory is tested on simulation trajectories of polymer melts with friction. The theory estimates for additional power-law exponents in the exponential of displacement distributions agree with the data. The results show fairly convincingly that the Rouse model with diffusing diffusivity may find scope as an alternate theoretical formalism for polymeric liquids.