Degradation shear-induced

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Low-molecular weight chains do not experience enough shear force to induce scission. Watson et al." demonstrated (by the intrinsic viscosity characterization of masticated NR) that the limiting molecular weight for the shear-induced degradation is in the order of 0.7-1.0 X 10 Frenkel independently speculated that shear-induced cleavage occurs near the midpoint of the polymer chain. 

In order to obtain solutions with the desired flow properties, shear-induced degradation should be avoided. From mechanical degradation experiments it has been shown that chain scission occurs when all coupling points are loose and the discrete chains are subjected to the velocity field. Simple considerations lead to the assumption that this is obtained when y) is equal to T sp(c-[r ]) (Fig. 18). The critical shear rate can then easily be evaluated [22]. 

Below a critical concentration, c, in a thermodynamically good solvent, r 0 can be standardised against the overlap parameter c [r)]. However, for c>c, and in the case of a 0-solvent for parameter c-[r ]>0.7, r 0 is a function of the Bueche parameter, cMw The critical concentration c is found to be Mw and solvent independent, as predicted by Graessley. In the case of semi-dilute polymer solutions the relaxation time and slope in the linear region of the flow are found to be strongly influenced by the nature of polymer-solvent interactions. Taking this into account, it is possible to predict the shear viscosity and the critical shear rate at which shear-induced degradation occurs as a function of Mw c and the solvent power. 

Fig. 34. Flow curve of narrowly distributed polystyrene in toluene (Mw = 23.6 106 g/mol, c = 0.1 g/ ml). Arrow indicates critical shear at which shear-induced degradation occurs...

Examination of Figure 7 shows that stannous octoate strongly catalyzed the already rapid (3.0) ratio polymerization. The highest torque was realized in uncatalyzed Polymer 9 but prompt, substantial melt viscosity reduction, indicative of polymer degradation via shear-induced chain cleavage followed. Catalyzed Polymer 11 showed polymerization die-out at about 4 minutes of reaction time but no polymer degradation tendency is apparent even at these high torque and temperature levels. 

Shear-induced degradation see Degradation, mechano-chemical... 

Observations of hydrodynamic shear-induced degradation of long polymer molecules first appeared in the literature approximately 60 years ago when Frenkel [17]... 

Remember when dealing with proteins that (1) the small physical size of the molecule is below the continuum limit and thus the molecule is not susceptible to shear induced degradation (2) structures that control activity are a result of relatively weak hydrophobic interactions and hydrogen bonds that result in an unstable active structure and finally (3) proteins are surface active due to the amphipathic nature, and thus large surface-to-volume ratios should be avoided. 

Double-stranded RNA molecules, although only discussed in a limited fashion, hold the best traits of DNA and proteins, i.e., siRNA s structure does not lend itself to high surface activity, and thus degradation by absorption to solid- or gas-liquid interfaces is not an issue. Furthermore, siRNA s small size is well below the continuum limit, and therefore, the molecule is not susceptible to shear-induced degradation. 

Two reactions can occur during wear oxidative degradation as a result of frictional heating in the contact zone and mechanochemical degradation initiated by shear-induced rupture of chemical bonds. Present evidence favors the latter process. For example, in the absence of oxygen, the wear of cis-polyisoprene changes to resemble that of c/x-polybutadiene, whereas the wear of poly(ethylene-co-propylene) is unaltered (Gent and Pulford, 1983). These results are in accord with the response of these materials to free radical reactions. 

Lengsfeld CS, Anchoidoquy TJ (2002) Shear-induced degradation of plasmid DNA. J Pharm Sci 91 1581-1589... 

The durability and reliability of a polymer-based product are determined by a number of factors inherent to the material itself (for example, crystallinity, average molecular weight), to its processing (for example, shear-induced degradation, process-induced thermal degradation), and to its service environment (such as, temperature, humidity and the presence of vibrations) [11]-... 

Protein products are especially prone to changes of the tertiary and quaternary structure of the molecules and thereby loss of activity. Degradation is induced by heat and shear stress. Rigorous shaking and administration by peristaltic pumps should be avoided. 

The Gaussian scission probability distribution function, with a preference for mid-chain scission, is frequently encountered in shear-induced mechanochemical degradation. The parabolic distribution, on the contrary, indicates a preference for chain-end degradation. This phenomenon has been reported in the hydrolysis of dextran, as a result of chain branching. 

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