Behavior Semi-Crystalline Polymers

What about semi-crystalline polymers Their viscoelastic behavior is much more complex, because of the superposition of the behavior of the crystalline and amorphous domains. This superposition is not necessarily linear, and coupling of the responses can occur, particularly as the degree of crystallinity of a sample is increased and the amorphous domains become constrained by the crystalline domains. Because of this, the behavior of semi-crystalline polymers is much less uniform than their amorphous counterparts, displaying individual idiosyncrasies that often have to be described separately. 

FIGURE 13-85 Schematic representation of the (10 second) modulus versus temperature behavior obtained from a sample of high density polyethylene. 

We have now examined some important aspects of the viscoelastic behavior of polymeric materials and discussed the relaxation processes responsible for these properties. 

There are two important points that you should take with you to the next section. First, time and temperature are inextricably 

FIGURE 13-86 Tan 5 data for a low density (blue) and linear (red) polyethylenes [redrawn from the data of H. A. Flocke, Kolloid-Z. Z. Polym., 180, 188 (1962)]. 

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Figure 1.26 summarizes the property behavior of amorphous, crystalline, and semicrystalline materials using schematic diagrams of material properties plotted as functions of temperature. Again, pressures affect the transition temperatures as schematically depicted in Fig. 1.27 for a semi-crystalline polymer. 

By considering the influence of the interphase upon annealing, it is possible to shed a little light on the welding behavior of semi-crystalline polymers, which has received much less attention than that of amorphous polymers. Because of the ill-defined morphology of the interphase the welding characteristics of semi-crystalline polymers are quite different from amorphous polymers and are far from well understood. 

Let s start by looking at a simple polymer, polyethylene, that has a lot going on in its stress/strain plots (Figure 13-38). Flexible, semi-crystalline polymers such as this (where the T of the amorphous domains is below room temperature) usually display a considerable amount of yielding or cold-drawing, as long as they are not stretched too quickly. For small deformations, Hookean elastic-type behavior (more or less) is observed, but beyond what is called the yield point irreversible deformation occurs. 

Final properties of an injection-molded product, made of semi-crystalline polymers, strongly depend on the flow-induced microstructure (crystallinity, morphology, orientation, etc.). Particularly, the enhancement in polymer crystallization rate crucially changes the soUdiiication behavior, and the oriented microstructure leads to a local anisotropy in thermal and mechanical properties. 

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