Glass transition regions stress effects

These new variables are necessary to take into account viscoelastic effects linked to molecular motions. These effects are non-negligible in the glassy domain between boundaries a and (3 in the map of Fig. 11.2, and they are very important in the glass transition region (around boundary a). Here, we need relationships that express the effects of s, d (the stress rate may be used instead of the strain rate), and T on the previously defined elastic properties. Also numerical boundary values of elastic properties are required, characterizing unrelaxed and relaxed states (see Chapter 10). 

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

In the same studies, Moehlenpah et al (1970, 1971) obtained master curves for the stress relaxation of their epoxy systems, at least into the glass-to-rubber transition region (Figure 12.4), and demonstrated similar behavior of both the stress relaxation modulus and the tensile modulus as a function of strain rate. As with the strain rate studies mentioned, no effect of filler type on the WLF shift factor was observed. All solid fillers increased the modulus of the system, the fibers being more effective than the spheres. The bubbles, as expected (Nielsen, 1967a), decreased the modulus. 

Good adhesion does not a priori lead to better transfer of the impact energy from the hard phase to the pliant phase. In fact, the pliant phase can only absorb significant amounts of energy in the region of the glass-transition. Furthermore, the energy absorption is so small that it has little effect on the stress ratios within the test sample. Equilibration of stress must therefore proceed from another mechanism. 

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