Temporary networks junctions

Both these models find their basis in network theories. The stress, as a response to flow, is assiimed to find its origin in the existence of a temporary network of junctions that may be destroyed by both time and strain effects. Though the physics of time effects might be complex, it is supposed to be correctly described by a generalized Maxwell model. This enables the recovery of a representative discrete time spectrum which can be easily calculated from experiments in linear viscoelasticity. 

Entanglement (coupling) The localised twisting or looping of polymer chains about each other, leading to the formation of a temporary network stmeture in uncross-linked polymers. The molar mass between the junctions is M. [90Alg]... 

The rubbery plateau can be "stabilised" by cross-linking, the regions of rubbery flow and liquid flow are completely suppressed if enough chemical cross-links are introduced to serve as permanent network junctions in place of the temporary chain entanglements. Crystallisation is a kind of physical cross-linking with (numerically) many junctions. It is understandable that the amorphous state is more or less "stabilised" by crystallisation, so that the transition becomes less pronounced. 

The transient net work model is an adaptation of the network theory of rubber elasticity. In concentrated polymer solutions and polymer melts, the network junctions are temporary and not permanent as in chemically crosslinked rubber, so that existing junctions can be destroyed to form new junctions. It can predict many of the linear viscoelastic phenomena and to predict shear-thinning behavior, the rates of creation and loss of segments can be considered to be functions of shear rate. 

Characterization of Poiymer Networks by Transverse Relaxation. The NMR transverse magnetisation relaxation experiments were extensively used for quantitative analysis of the density of chemical cross-links, temporary and trapped chain entanglements and physical network junctions which are formed in fllled rubbers, semicrystalline and ionic containing elastomers, and for determination of the molecular-scale heterogeneity of polymer networks ((83), and reference therein). 

Even N, the number of chains per unit volume, is not a simple concept because junction points can be either (permanent) chemical cross-links or (temporary) physical entanglements. Not all cross-links will be effective, as seen in Figure 3.8, which shows (a) loose loops , where a chain folds back on itself, and (b) loose ends , where a chain does not contribute to the network [6]. 

Effects of Entanglements. An increase in the functionality of junctions suppresses their fluctuations and therefore fluctuations in chain dimensions are decreased. Also, the entanglements along the chains that come from the neighboring chains may further act as if they were additional (albeit temporary) junctions. Consequently, the modulus of such networks may exceed that of an affine network. 

Network theories temporary junction network with Gaussian chains... 

The change in the linear dependence of jjq on M to the 3.4 power dependence for flexible chain polymers is believed to be due to the formation of an entanglement network or temporary physical junctions between the polymer chains. In the case of rod-like molecules, the hindrance of free rotation of the rod-like molecule by neighboring molecules serves as the entanglements. 

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