Crystalline plastic relaxation

With crystalline plastics, the main effect of the crystallinity is to broaden the distribution of the relaxation times and extend the relaxation stress too much longer periods. This pattern holds true at both the higher and low extremes of crystallinity (Chapter 6). With some plastics, their degree of crystallinity can change during the course of a stress-relaxation test. This behavior tends to make the Boltzmann superposition principle difficult to apply. 

Theoretically, plasticization is reversible. In fact, it induces irrtemal rearrangemerrts and may also facilitate the relaxation of internal stresses. These phenomena are often found when the material has chains of low molecirlar weight or when it has a low degree of crystallinity. Plasticization is characterized by a change in the mechanical properties of the material, resulting in a decrease of the glass transition temperature (T ). 

Left effects of physical aging processes on oriented and semi-crystalline plastics (top relaxation processes, bottom post-crystallization)... 

The increased macromolecule mobility facilitates relaxation of internal stresses in the plastic so that strength can temporarily increase the macromolecules may also become more easily oriented and in semi-crystalline plastics the processes associated with secondary crystallization can proceed more easily [86]. 

The thorough and persistent work on precursor decay (the dependence of Hugoniot elastic limit on propagation distance) of Duvall s Washington State University group was successful in demonstrating that precursor attenuation was due to both stress relaxation and hydrodynamic attenuation. Typical data on crystalline LiF is shown in Fig. 2.7. Observed plastic strain... 

While a plastic usually exhibits not one but many relaxation times, each relaxation time is affected by the temperature in exactly the same manner as another. That is the whole relaxation spectrum shifts in unison along the logarithmic no longer applicable in these materials, because the crystalline morphology changes with the temperature. 

Crystallinity—about.i to 15% (213,232). The creep of plasticized poly(vinyl chloride) polymers as a function of temperature, concentration, and kind of plasticizer has been studied by many workers, including Aiken et ai. (232), Neilscn ct ai. (234), and Sabia and Eirich (243). These last workers also studied stress relaxation (244). In the case of crystalline polymers, plasticizers and Copolymerization reduce the melting point and the degree of Crystallinity. These factors tend to increase the creep and stress relaxation, especially at temperatures approaching the melting point. 

Existence of a high degree of orientational freedom is the most characteristic feature of the plastic crystalline state. We can visualize three types of rotational motions in crystals free rotation, rotational diffusion and jump reorientation. Free rotation is possible when interactions are weak, and this situation would not be applicable to plastic crystals. In classical rotational diffusion (proposed by Debye to explain dielectric relaxation in liquids), orientational motion of molecules is expected to follow a diffusion equation described by an Einstein-type relation. This type of diffusion is not known to be applicable to plastic crystals. What would be more appropriate to consider in the case of plastic crystals is collision-interrupted molecular rotation. 

The effect of additives, others than plasticizers, on the dynamic mechanical properties of styrene polymers have attracted little attention from researchers. Flame retardants such as l,2-bis(tetrabromophthalimide)ethane, crystalline decabromodiphenyl oxide (DBDPO) and antimony trioxide (Sb203) do not affect the a relaxation of aPS [38,39]. 

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