Entanglement, rheology

J. F. Berret, J. Appell, G. Porte. Linear rheology of entangled wormlike micelles. Langmuir 9 2851-2854, 1993. 

The rheological behavior of storage XGs was characterized by steady and dynamic shear rheometry [104,266]. Tamarind seed XG [266] showed a marked dependence of zero-shear viscosity on concentration in the semi-dilute region, which was similar to that of other stiff neutral polysaccharides, and ascribed to hyper-entanglements. In a later paper [292], the flow properties of XGs from different plant species, namely, suspension-cultured tobacco cells, apple pomace, and tamarind seed, were compared. The three XGs differed in composition and structural features (as mentioned in the former section) and... 

As a consequence of the irregular and rough structure of the xylan particles, entanglements between particles are promoted and this fact may explain the poor flow properties of this polymer (Kumar et al., 2002 Nunthanid et al., 2004). Additionally, rheological parameters of xylan powder have also been studied, such as bulk and tapped densities, Hausner ratio, Carr s index, and angle of repose values, and they are summarized in Table 1. 

McLeish, T.C.B. et al. Dynamics of entangled H-polymers Theory, rheology, and neutron scattering. Macromolecules, 32, 6734, 1999. 

Taking into account the relevance of the range of semi-dilute solutions (in which intermolecular interactions and entanglements are of increasing importance) for industrial applications, a more detailed picture of the interrelationships between the solution structure and the rheological properties of these solutions was needed. The nature of entanglements at concentrations above the critical value c leads to the viscoelastic properties observable in shear flow experiments. The viscous part of the flow behaviour of a polymer in solution is usually represented by the zero-shear viscosity, rj0, which depends on the con-... 

The self-similar spectrum is not valid at short times, X < X0, where the details of chemical structure become important (glass transition, entanglements, etc.). The cross-over to the glass transition at short times is typical for all polymeric materials, for both liquids and solids. The critical gel is no exception in that respect. X0 could be used as a characteristic time in the CW spectrum since it somehow characterizes the molecular building block of the critical gel however, it has no direct relation to the LST. At times shorter than X0, the LST has no immediate effect on the rheology. Indirect effects might be seen as a shift in the glass transition, for instance, but these will not be studied here. 

Unique architecturally driven properties that may be expected from hyperbranched polymers will be largely derived from their (a) amplified number of terminal functional groups, (b) new rheological properties based on less chain entanglement and (c) new architectural arrangements that may modulate crystallinity, flow characteristics and glass transition properties in designed systems. 

Contents Chain Configuration in Amorphous Polymer Systems. Material Properties of Viscoelastic Liquids. Molecular Models in Polymer Rheology. Experimental Results on Linear Viscoelastic Behavior. Molecular Entan-lement Theories of Linear iscoelastic Behavior. Entanglement in Cross-linked Systems. Non-linear Viscoelastic-Properties. 

As we conjectured in the introduction, the fundamental role of topology in this approach to entangled polymer dynamics would indicate that changes to the topology of the molecules themselves would radically affect the dynamic response of the melts. In fact rheological data on monodisperse star-branched polymers, in which a number of anionically-polymerised arms are coupled by a multifunctional core molecule, pre-dated the first application of tube theory in the presence of branching [22]. Just the addition of one branch point per molecule has a remarkable effect, as may be seen by comparing the dissipative moduli of comparable linear and star polymer melts in Fig. 5. 

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