Flow-induced degradation

Flow-induced degradation is intimately related to the nonequilibrium conformation of polymer coils and any attempt to interpret the process beyond the phenomenological stage would be incomplete without a sound understanding of chain dynamics. To make the paper self-contained and to provide a theoretical basis for the discussion, we have included some fundamental models of polymer dynamics in the next section which may also serve as a guideline for future work in the field of polymer degradation in flow. For the first-time reader, however, this section is not absolutely necessary. Further, any reader familiar with molecular rheology or interested only in experimental results can skip this section, only to go back whenever a reference is needed. 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

In flow-induced degradation, K is strongly dependent on the chain length and on the fluid strain-rate (e). According to the rate theory of molecular fracture (Eqs. 70 and 73), the scission rate constant K can be described by the following equation [155]... 

The molecular weight of the polymer can influence the rate of flow-induced degradation in at least two respects first, molecular mobility (characterized by... 

Flow-induced degradation in solution is a complex function of polymer concentration which can alter the rate of chain scission in several respects. It can modify ... 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

The flow-induced destruction of networks of both near-spherical and rodlike particulates suspended in polymer matrices can be reversed with time in quiescent conditions. It is of limited interest, since it may take hours, which will cause thermal degradation. Larson (90)... 

The compression-molded part, by definition, does not have flow-induced orientation. Comparison of compression-molded part properties with those of an injection-molded part can show the effect of melt temperature on properties. In the compression-molded article without flow-induced orientation, the impact strength remains constant until a certain melt temperature is surpassed and then decreases. This thermal degradation effect can be attributed to the polybutadiene component, which acts as an initiation site for oxidative degradation of the matrices. 

All the previous systems were polydisperse in nature, so that the coil-stretch transition was broad (because the relaxation time depends upon molecular weight). Also, only a part of the molecular weight distribution (the longest molecules) becomes stretched by the flow. It therefore proved impossible to determine the dependence of the coil-stretch or interaction behavior upon molecular weight. For these reasons, we examine the behavior of a model system, monodisperse atactic polystyrene (a-PS) in the 0 solvent decahydronaphthalene. We will explore separately, both by assessment of molecular strain and macrorheology, the onset of interaction behavior, particularly as a function of molecular weight, and the associated flow-induced degradation. 

Mechanical or flow-induced degradation of polymers may occur under extreme shear or extrusion flow conditions giving rise to unfolding of macromolecules in extensional flow with subsequent chain scission. 

Viscoelastic flow behaviour of dilute polymer solutions in porous media is described as a method for characterization of polymer-solvent-temperature systems. Porous media flow tests provide information on the solution state of polymer solutions and the molecular weight of the polymers used. Furthermore, flow-induced and thermally induced degradation effects - frequently observed in polymer solutions - can be characterized by the measurement of viscoelastic effects in flow through porous media. Decrease of molecular weight and changes of the conformation of macromolecules in solution are important parameters in these processes. 

Characterization of the shear- and elongational flow-induced degradation of macromolecules... 

The question is frequently discussed as to what flow-induced mechanisms cause a break of the chain of macromolecules in solution and the decrease of the molar mass or decrease of viscosity this engenders. The viscoelastic flow behaviour of dilute polymer solutions reacts very sensitively to changes in molecular weight degradation effects of all kinds. This applies both to mechanically and thermally induced influences and also to aging effects [31 which are frequently observed in polymer solutions. 

Odelle, J.A. Keller, A. Miles, M.J. A method for studying flow-induced degradation verification of chain halving. Polym. Comm. 24 (1983) 7-10... 

Hyperhomocysteinemia, in coronary arteries increased TNF-a expression, which enhanced oxidative stress ultimately impairing flow-induced dilation that can be reversed by superoxide dismutase (SOD) (147). Also, in EC Hey inhibits dimethylarginine dimethylamino-hydrolase (DDAH) enzyme activity by direct interaction (enzyme that degrades asymetric dimethylarginine [ADMA]), causing ADMA to accumulate and inhibit NO synthesis, which might explain how Hey impairs NO dependent vasodilation (148). 

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