Direct observation of polymer dynamics: Mobility comparison between central and end section chain segments

In this work the motions of chain segments in either the central 50% of a p olymer or in the two end sections (25% each) were probed. Motions of the ce ntral (or end) section were measured using bilayer welding samples of deute rium-labeled polymers. Symmetric triblock poly(styrene) (HDH) was paired wi th fully deuterated or fully protonated poly(styrene) of nearly equal molec ular weight. Changes in the deuterium profile of a given pair were directly linked to the motions of a given section of the chain. The behavior of the deuterium depth profiles was monitored using specular neutron reflectivity (SNR). Rouse and reptation model predictions for the behavior of chain seg ments in the end and center sections were developed using computer simulati ons and minor chain reptation calculations. These dynamics models are repre sentative of two broad classes of dynamics theories: tubeless (Rouse) and t ubed (reptation). These experiments were thus designed to discriminate betw een tubeless and tubed dynamics. Segments in the central sections showed a distinct lag in crossing the interface, while segments in the end sections crossed the interface continuously. This chain centers' lag behavior is a d istinctive signature of the reptation model and is not predicted by tubeles s models. These experiments offer a simple and direct observation of the hi ghly anisotropic tube motions of entangled polymer melt chains, providing s trong support for the use of reptation to describe dynamics of melt polymer s at the weld interface.