Polymers yield point

Molecular weight also influences the static fracture response as well as the mechanical behavior of UHMWPE at large strains [7]. For example, beyond the polymer yield point, the hardening or cold drawing portion behavior in uniaxial tension is sensitive to the molecular weight. Figure 2.2 illustrates the tme-stress strain curve in uniaxial tension (room temperature, 30 mm/min) for two grades of UHMWPE, in comparison with HDPE. 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Phthahc resins are usually processed to an acid number of 25—35, yielding a polymer with an average of 1800—2000. The solution viscosity of the polymer is usually followed to ascertain the polymer end point. The resin is cooled to 150°C and hydroquinone stabilizer (150 ppm) is added to prevent premature gelation during the subsequent blending process with styrene at 80°C. The final polymer solution is cooled to 25°C before a final quaUty check and dmmming out for shipment. 

The other models can be appHed to non-Newtonian materials where time-dependent effects are absent. This situation encompasses many technically important materials from polymer solutions to latices, pigment slurries, and polymer melts. At high shear rates most of these materials tend to a Newtonian viscosity limit. At low shear rates they tend either to a yield point or to a low shear Newtonian limiting viscosity. At intermediate shear rates, the power law or the Casson model is a useful approximation. 

KCl-polymer (potassium chloride-polymer) muds can be classified as low solids-polymer muds or as inhibitive muds, due to their application to drilling in water-sensitive, sloughing shales. The use of polymers and the concentration of potassium chloride provide inhibition of shales and clays for maximum hole stability. The inverted flow properties (high yield point, low plastic viscosity) achieved with polymers and prehydrated bentonite provide good hole cleaning with minimum hole erosion. 

Below T, most amorphous polymers show a more or less linear stress-strain curve with no yield point (Fig. 18.8)... 

Thermoplastic polymers subjected to a continuous stress above the yield point experience the phenomenon of cold-drawing. At the yield point, the polymer forms a neck at a particular zone of the specimen. As the polymer is elongated further, so this neck region grows, as illustrated in Figure 7.7. 

In many applications we consider the yield point of a polymer to be its point of failure. We base this definition on whether the material is still fit for its end use once it has yielded. Thus, if the teeth on a polymer gear wheel yield, they will change shape and may become useless. Alternatively, packaging film may yield, but still maintain its barrier properties and be fit for continued use. 

Creep is the long-term deformation of a polymer under a sustained or intermittent load at stresses below the yield point. We see creep at work when we place furniture on carpet. If we... 

Voids within a sample are a major cause of internal haze. We see the effect of voiding when we stretch polymers, such as high density polyethylene and isotactic polypropylene, that have distinct yield points and clearly defined necks (as discussed earlier in this chapter). The... 

How do we determine the tensile modulus, tensile yield point, elongation at break and tensile strength of a polymer What characteristics of the polymer define these properties ... 

Even in cases where the rigid polymer forms the continuous phase, the elastic modulus is less than that of the pure matrix material. Thus two-phase systems have a greater creep compliance than does the pure rigid phase. Many of these materials craze badly near their yield points. When crazing occurs, the creep rate becomes much greater, and stress relaxes rapidly if the deformation is held constant. 

The continuous chain model includes a description of the yielding phenomenon that occurs in the tensile curve of polymer fibres between a strain of 0.005 and 0.025 [ 1 ]. Up to the yield point the fibre extension is practically elastic. For larger strains, the extension is composed of an elastic, viscoelastic and plastic contribution. The yield of the tensile curve is explained by a simple yield mechanism based on Schmid s law for shear deformation of the domains. This law states that, for an anisotropic material, plastic deformation starts at a critical value of the resolved shear stress, ry =/g, along a slip plane. It has been... 

Polymerization was carried out in NaOH aqueous solution. Table 4 summarizes the reaction conditions and polymer yields. The first step of the polymerization was hydrolysis and oligomerization of the silylethers at low temperatures from 90°C to 150 °C. In order to increase the polymer yield and the softening point of the polymers, the second step was carried out at higher temperatures under reduced pressure, removing alcohol and water, 1H-NMR, 13CNMR and IR spectroscopy of each polymer show that these polymers have the polysiloxane structure substituted with a phenol group. 

For many polymers and metals the yield point is not clearly defined. In these cases the offset yield strength is used. To obtain this value, a line parallel to the linear portion of the curve is drawn such that... 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

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