Energy dissipation and strength

FIGURE 10.28 Tensile strength of polyurethane elastomers versus T — Tg Tg ranging from -67 to -17°C). (From Smith (1969).) 

FIGURE 10.29 Work-to-break (W/,) versus energy dissipated W ) on stretching almost to the breaking elongation. (From Grosch et al. (1966).) 

Wb and Wd are expressed in joules per cubic meter. Those materials that require the most energy to bring about rupture—the strongest elastomers—are precisely those in which the major part of the energy is dissipated before rupture. 

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In many commercial polymer blends the notched Izod impact strength at room temperature, NIRT, shows high synergism—values 30 to 100 times above the additive value have been reported. These large effects originate in a change of the mechanism of fracture energy dissipation, and/or the fundamental mechanism of fracture [Wefer, 1982). 

Particles dispersed in an aqueous medium invariably carry an electric charge. Thus they are surrounded by an electrical double-layer whose thickness k depends on the ionic strength of the solution. Flow causes a distortion of the local ionic atmosphere from spherical symmetry, but the Maxwell stress generated from the asymmetric electric field tends to restore the equilibrium symmetry of the double-layer. This leads to enhanced energy dissipation and hence an increased viscosity. This phenomenon was first described by Smoluchowski, and is now known as the primary electroviscous effect. For a dispersion of charged hard spheres of radius a at a concentration low enough for double-layers not to overlap (d> 8a ic ), the intrinsic viscosity defined by eqn. (5.2) increases... 

At low speeds and high temperatures, viscous energy dissipation is minimized, and the tear strength approaches a lower limit Go of about 20 J/m (Figure 1.11). This is of the same order of... 

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