Regensburg 2010 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 23: Polymer Dynamics

CPP 23.5: Talk

Wednesday, March 24, 2010, 11:15–11:30, H48

Multiple quantum NMR observation of reptation and constraint release in polymer melts — •Fabian Vaca Chavez and Kay Saalwaechter — Institute of Physics, Martin-Luther-University Halle-Wittenberg, Halle (Saale), Germany

In his seminal paper, de Gennes [1] propose a qualitative explanation of the polymer melts dynamics, far above the entanglement molecular weight Me, in terms of a reptative, snake-like motion of the chain through a mesh of fixed topological constraints (entanglements) set by the other chains. Here we show results from 1H multiple quantum (MQ) NMR on a benchtop spectrometer [2], probing the validity of the tube model of polymer dynamics, which combines the reptation concept with the Rouse theory for unentangled chains. This fixed-tube model is insuficient for the quantitative description of actual mechanical data, and ongoing discussions focus on including dynamics of the tube itself, caused by contour-length fluctuations (CLF), arising from chain-end motions of the test chain, or constraint release (CR), arising from matrix chain motions. We also observe characteristic deviations from the tube model predictions up to high molecular weights, and show that CR processes are responsible for modified chain modes faster than actual reptation. Our results extend previous observations by neutron spin-echo spectroscopy (NSE), whose limited dynamic range poses limitations to the study of well-entangled systems.

[1] P. G. de Gennes, J. Chem. Phys., 55, 572 (1971). [2] K. Saalwaechter, Progr. NMR Spectrosc., 51, 1 (2007).