Dresden 2017 – wissenschaftliches Programm
CPP 29.6: Vortrag
Dienstag, 21. März 2017, 15:15–15:30, ZEU 250
Loss of entanglement or shear-induced ordering in polymer melts — •Ulrich Scheler — Leibniz-Institut für Polymerforschung Dresden e.V.
The transverse NMR relaxation T2 is most suited to investigate polymer chain dynamics.In polymers commonly at least two components are observed, which are attributed to the entangled and free polymer chain segments respectively. In the present study different polysiloxane melts with a narrow molecular-weight distribution have been used. Under shear in a Couette cell one commonly assumes shear-induced ordering, which would result in restricted degrees of freedom and hence a shortened T2. However in most cases a prolonged T 2 has been observed under shear, which can be explained by either a loss of entanglements or an increased mesh size between entanglements based on a local crowding of entanglements. A shortened T2 as signature of shear-induced ordering is observed only for short-chain polymers, which exhibit a single exponential decay, indicative of the absence of entanglements. That implies that the loss of entanglements is the process dominating the local polymer dynamics, while shear-induced ordering happens as well for entangled polymers.