Segment-Scale, Force-Level Theory of Mesoscopic Dynamic Localization and Entropic Elasticity in Entangled Chain Polymer Liquids

Abstract: We develop a segment-scale, force-based theory for the breakdown of the unentangled Rouse model and subsequent emergence of isotropic mesoscopic localization and entropic elasticity in chain polymer liquids in the absence of ergodicity-restoring anisotropic reptation motion. The theory is formulated in terms of a conformational N-dynamic-order-parameter Generalized Langevin Equation approach. It is implemented using a field-theoretic Gaussian thread model of polymer structure and closed in a universal manner at the level of the chain dynamic second moment matrix. The physical idea is that the isotropic Rouse model fails due to the dynamical emergence of time-persistent intermolecular contacts determined by the combined influence of local chain uncrossability, long range polymer connectivity and a self-consistent treatment of chain motion and the dynamic forces that hinder it. For long chain melts, the mesoscopic localization length (tube diameter) and emergent elasticity predictions are in near quantitative agreement with experiment. Moreover, the onset chain length scales with the semi-dilute crossover concentration with a realistic numerical prefactor. Distinctive predictions are made for various off-diagonal correlation functions that quantify the full spatial structure of the dynamically localized polymer conformation. As the local uncrossability constraint and/or intrachain bonding spring are softened, the tube diameter is predicted to swell until it reaches the chain radius-of-gyration at which point entanglement localization vanishes in a discontinuous manner. A full dynamic phase diagram for the destruction of mesoscopic localization is constructed, which is qualitatively consistent with simulations and the classical concept of an entanglement degree of polymerization.