Nonlinear Rouse-chain relaxations in obstacle media

The entropic interplay of a nonlinearly deformed chain in obstacle media provides elementary knowledge into single-chain dynamics and nonlinear viscoelasticity of polymer liquids flowing in porous or entangled media. This work scrutinized, using Brownian dynamics simulation, the relaxation dynamics of nonlinearly deformed Rouse chains suspended in 2D array of point obstacles. The simulation disclosed a highly confined, anisotropic path of chain retraction that forces a hierarchy of Rouse retraction propagating from the outer segments toward the central ones. Such a constrained Rouse retraction was generally more retarded than free Rouse retraction in dilute system. The classical Doi theory on a 1D Rouse chain captures a similar feature, yet it generally underpredicts the degree of chain retraction as observed in the simulation and seemingly predicts an incorrect dependence on the strain magnitude. For sufficiently long chains, the simulation revealed, in addition, a long-persisting tail of chain stretch well beyond the Rouse regime. This long-time anomaly has further been noted to be closely correlated with another previously unnoticed, dominantly diffusive coil-size relaxation taking place on time scales between the Rouse time and the reptation time. The last feature has been attributed to a general coupling between terminal chain retraction and early-stage chain reptation for relatively long chains. In light of the present findings for obstacle media, we discuss recent experimental trends noted for entangled polymer solutions.