Approach to the description of dynamical polymer coil overlap onset as perceived from NMR

Abstract

A simple model is proposed to describe the polymer coil overlap process as perceived from NMR; it is based on a two‐step motional narrowing mechanism. Firstly, high relaxation frequency motions of monomer units only induce a partial motional narrowing effect leaving a nonzero average tensorial spin coupling ?_D≠0; then motional narrowing is achieved by slow chain diffusion through entanglements :?_D serves as a low NMR reference frequency. The tube renewal process as identified by De Gennes with an equivalent Rouse chain motion is used to predict chain length and temperature dependences of the concentration polymer coil overlap c_ν observed by NMR. It is proposed to define c_ν as the concentration where the motional narrowing condition is fulfilled for all relaxation modes of normal coordinates of the chain except for the slowest one. An attempt to evaluate the terminal relaxation time of entangled chains from NMR measurements is described.