Poly networks stress relaxation

The equilibrium 20 is important not only in the synthesis of linear polysiloxanes but also in their applications. The effects of water vapor on inducing chain cleavage at high temperature are not only reduced molecular weights but also a dramatic increase in the rates of chemically induced stress relaxation at 250 °C in cross-linked poly(dimethylsiloxane) networks under load (70). Slow hydrolytic bond cleavage in cross-linked networks is seen even in studies of stress relaxation in air at room temperature, and appreciable rates of stress relaxation in the loaded networks are measured at temperatures as low as 70 (7i). The stress relaxation is greatly accelerated... 

Another example of network formation is found in PEO (poly(ethylene oxide))-silica systems [58, 59]. At relatively small-particle concentrations, the elastic modulus increases at low frequencies, suggesting that stress relaxation of these hybrids is effectively arrested by the presence of silica nanoparticles. This is indicative of a transition from liquidlike to solidlike behavior. At high frequencies, the effect of particles is weak, indicating that the influence of particles on stress relaxation dynamics is much stronger than their influence on the plateau modulus. 

L. H. Sperling, H. F. George, V. Huelck, and D. A. Thomas, Viscoelastic Behavior of Interpenetrating Polymer Networks Poly(ethyl acrylate)-Poly(methyl methacrylate), J. Appl. Polym. Sci. 14, 2815 (1970). Creep behavior of sequential IPNs. Stress relaxation. Master curves. 

Figure 10.7 (8) illustrates the stress relaxation of a poly(dimethyl siloxane) network, silicone rubber, in the presence of dry nitrogen. The reduced stress, o(t)/(T(0),is plotted,so that under the initial conditions its value is always unity. Since the theory of rubber elasticity holds (Chapter 9), what is really measured is the fractional decrease in effective network chain segments. The bond interchange reaction of equation (10.2) provides the chemical basis of the process. While the rate of the relaxation increases with temperature, the lines remain straight, suggesting that equation (10.2) can be treated as the sole reaction of importance. 

Two samples of such correlations are presented in Fig. 83 for PVC and poly(vinyl butyral) (PVB). One or a few bends in the fracture stress fracture stress with temperature. More complex CR spectrum in the temperature region of P-relaxation and, simultaneously, three bends in the Op vs T plot may be seen for PVC. In this case, a pair of the neighboring creep rate peaks at 200 and 220 K, and the onset of a new peak form 250 K are observed instead of a broad P-relaxation mechanical loss peak with maximum at ca. 240 K. The changes in the slope of the 01 vs T plot occur just at the characteristic temperatures in the CR spectrum of PVC. Similar correlations were also found for PMMA [11], PS [13], and epoxy networks [328]. 

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