FROM ENTROPY DRIVEN MOTION TO REPTATION — LARGE SCALE DYNAMICS IN POLYMER MELTS

In these lecture notes we discuss chain motion of long linear homopolymers in the melt. We first present pertinent results on the mechanical dynamic properties as well as on chain diffusion. We then turn to the microscopic motions which are behind the macroscopic phenomena and introduce neutron spin echo spectroscopy as a unique tool to investigate such motions. Polymers are systems with a large number of internal degrees of freedom. Therefore, unlike in common liquids the conformational entropy is an important driving force for chain motion giving rise e.g. to rubber elasticity. We present NSE investigations on such motions which are commonly described in the so called Rouse model. A comparative study employing NSE and molecular dynamics simulations explores the limits of this model description. At larger length scales and correspondingly longer times the topological hindrance for motion imposed by the surrounding chains bears on the freedom of chain motion which is gradually restricted to motion along the chain profile. Thus, the chains move like snakes or reptiles, a mechanism called reptation. Again we present NSE data on the space time evaluation of chain motion. They display (i) the existence of defined length scale beyond which topological constraints become important and (ii) show that beyond this length scale the reptation mechanism prevails.