Degradation shear

Molecular Weight. Measurement of intrinsic viscosity in water is the most commonly used method to determine the molecular weight of poly(ethylene oxide) resins. However, there are several problems associated with these measurements (86,87). The dissolved polymer is susceptible to oxidative and shear degradation, which is accelerated by filtration or dialysis. If the solution is purified by centrifiigation, precipitation of the highest molecular weight polymers can occur and the presence of residual catalyst by-products, which remain as dispersed, insoluble soHds, further compHcates purification. 

Aqueous Solution Viscosity. A special solution preparation method is used for one type of measurement of aqueous solution viscosity (96). The appropriate amount of poly(ethylene oxide) resin is dispersed in 125 mL of anhydrous isopropyl alcohol by vigorous stirring. Because the resin is insoluble in anhydrous isopropyl alcohol, a slurry forms and the alcohol wets the resin particles. An appropriate amount of water is added and stirring is slowed to about 100 rpm to avoid shear degradation of the polymer. In Table 4, the nominal resin concentration reported is based on the amount of water present and ignores the isopropyl alcohol. 

Processing Aids. Stearic acid [57-11-4] or other fatty acids and/or metal soaps of fatty acids are added to reduce shear degradation and mill sticking during mixing. Sorbitan monostearate (ICI s Span 60) is one of the best processing aids to reduce mill sticking. 

There are different reasons to discard a column a column can be damaged by irreversible adsorption of reactive polymer samples. Small amounts of styrene oligomers are known to permanently elute from styrene-divinylbenzene materials with tetrahydrofuran as the eluent, which means a continuous shear degradation of the separation material and consequently a decrease of the packing quality this observation is very important if fractions are collected and used for further analyses, e.g., for the determination of infrared (IR) spectra. One can presume that similar effects are present with other organic materials too. 

Shear degradation is another parameter that is very difficult to quantify and it is not fully understood. Several studies have been made on this subject (16-19) and the topic has been reviewed (20). We have found in our own work (21) that NBS 1476, a low-density polyethylene, is very susceptible to shear degradation in the GPC column and that the amount of degradation that will occur is very dependent on both flow rate and the particle size of the packing. 

Am columns. However, for typical polyethylenes produced using Ziegler-Natta catalysts, which do not have a high molecular weight tail, shear degradation was not a concern. 

Compounding and palletizing of the latex blended material is sometimes done by melt mixing. In such cases precautions must be taken to avoid thermal and shear degradation of the blended polymer. 

In a detailed study on shear degradation of DNA, Adam and Zimm found a complex dependence of kc on solution viscosity. Considering that the macromolecules can rupture only after tumbling had brought them into the right configuration, these authors proposed to include solution viscosity into the pre-exponential factor A (l/r)s) [84],... 

Fig. 38. Action of entanglements in simple shear degradation. The tension on the qth link from the center is given by fq = f0(l — 4q2/n2), where f0 qsy f(L) is the tension on center link (f(L) = function of chain length) and n = number of links per chain.

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

In the semi-dilute regime, the rate of shear degradation was found to decrease with the polymer concentration [132, 170]. By extrapolation to the dilute regime, it is frequently argued that chain scission should be nonexistent in the absence of entanglements under laminar conditions. No definite proof for this statement has been reported yet and the problem of isolated polymer chain degradation in simple shear flow remains open to further investigation. 

Neglecting the thermal energy contribution, the rate for shear degradation was obtained as ... 

Pseudozan is an exopolysacchaiide produced by a Pseudomonas species. It has high viscosities at low concentrations in formation brines, forms stable solutions over a wide pH range, and is relatively stable at temperatures up to 65° C. The polymer is not shear degradable and has pseudoplastic behavior. The polymer has been proposed for enhanced oil-recovery processes for mobility control [1075]. 

Table 5. Results from shear degradation measurements in two different rheometers (capillary, Couette-type) compared with model predictions of narrowly distributed polystyrene samples in toluene... 

For viscous fluids, the velocity may be constrained by the allowable pressure drop or shear degradation of the fluid (e.g. large molecules breaking down into smaller molecules from high shear rates). Typical values are given in Table 13.6. 

Zr(lV) has been used to crosslink HEC (64). Crosslinked CMHEC tends to show more shear degradation on passing through pumps and small orifices in downhole tools than crosslinked HPG fluids... 

Shear stability undergoes irreversible shear degradation reversible shear thinning. 

Compared to partially hydrolyzed polyacrylamide, xanthan gum is more expensive, more susceptible to bacterial degradation, and less stable at elevated temperatures (1). However, xanthan gum is more soluble in saline waters, particularly those containing divalent metal ions generally adsorbs less on rock surfaces and is substantially more resistant to shear degradation (1,34). The extensional viscosity of the semi-rigid xanthan molecule is less that that of the flexible polyacrylamide (263). 

Proper well completion, particularly perforation design, can reduce polyacrylamide shear degradation during entry into the formation (313). 

While performance of resource recovery processes have been significantly improved in the past two decades, further efforts must be directed to 1. minimize shear degradation, 2. control adsorption, 3 control process rheology, and 4 provide appropriate chemical stability. These are the properties which fundamental and applied research must target during the next decade. 

McCormick, C.L., et al. "Development of Laboratory Screening Tests to Predict Polymer Performance in Enhanced Oil Recovery (I). Shear Degradation, Viscosity, and Electrolyte Studies," prepared for DOE under contract No. EF-77-S-05-5603, 1977. 

Chang, H.D., Darby, R. "Effect of Shear Degradation on the Rheological Properties of Dilute Drag Reducing Polymer Solutions," J. Rheol.. 1983, 27(1), 77-88. 

In the second section, improved column technology is emphasized. The effects of operational variables on the performance of the chromatographic system are considered. Some of the operational variable concerns are shear degradation of high molecular weight polymers, the use of mixed solvent systems, and the optimization of resolution for analysis of oligomers and small molecules. 

A reliable procedure for determination of molecular parameters number, weight and z-averages of the molecular weight (Mj, i = n, w and z respectively) for polyethylenes, PE, by means of Size Exclusion Chromatography, SEC, has been developed. The Waters Sci. Ltd. GPC/LC Model 150C was used at 135 C with trichlorobenzene, TCB, as a solvent. The standard samples as well as commercial stabilized and not stabilized PE-resins were evaluated. The effects of sampling, method of solution preparation, addition of antioxidant(s), thermal and shear degradation were studied. The adopted procedure allows reproducible determination of and M , with a random error of 4% and M2, with 9%, within 2 to 72 hrs from the initial moment of preparation of solutions. 

Shear Degradation of Very High Molecular Weight Polymers in Gel Permeation Chromatography... 

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