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| Home > Publications database > Packing length-dependence of the transition from unentangled to entangled dynamics in polymer melts |
| Conference Presentation (Other) | FZJ-2015-02924 |
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2015
Abstract: It is well-known that the packing model (Fetters and co-workers) shows the relationship between microscopic chain properties (chain stiffness, monomer bulkiness) with macroscopic rheological behavior via the entanglement molecular weight (Me). However, what is less recognized is that the critical molecular weight (Mc) and reptative molecular weight (Mr) which denote transitions between unentangled (Rouse) and entangled (tube) dynamics and reptation-contour length fluctuations (CLF) to pure reptation dynamics respectively, also show dependence on the packing length (p). In this contribution, we highlight two aspects of this dependence:1. Based on zero-shear viscosity data for six different polymers, from our own measurements and from literature, we show that there's a packing length dependence of the two crossovers (Mc, Mr) and that the trend is for the two to merge at a certain critical value of the packing length (p*). Further, analysis of the loss moduli (G'') for several well-entangled polymers with different packing lengths reveals a p-dependence of the high-frequency slope of the terminal peak contrary to the predicted value of -1/4 by tube models with CLF.2. Based on preliminary measurements of the dynamic structure factor by neutron spin echo spectroscopy, we show that the onset for the effect of entanglements on the dynamics (i.e. deviation from Rouse behavior) starts in fact from Me and manifests by suppression of long- wavelength modes. From measurements on a series of polymers with different molecular weights (around Mc) and with different packing lengths, we examine how this deviation from Rouse behavior might proceed differently depending on the packing length of the polymer.Both aspects hint at possible missing ingredients in the current modeling framework (tube model) for entangled polymer melts, particularly on the understated importance of the packing length (p).
Keyword(s): Key Technologies (1st) ; Soft Matter, Macromolecules, Complex fluids, Biophysics (1st) ; Soft Condensed Matter (2nd)
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