Direct observation of the transition from free to constrained single segment motion in entangled polymer melts

Incoherent neutron-spin-echo spectroscopy (NSE) has been employed to directly determine the time-dependent mean-squared segment displacement 〈 x ( t ) 2 〉 of a polymer chain in the melt covering the transition from free to constraint Rouse relaxation along the virtual tube of the reptation model. The predicted transition of the time dependence of 〈 x ( t ) 2 〉 from 〈 x 2 〉∝ t 1/2 to ∝ t 1/4 is clearly corroborated by the incoherent NSE results.