Semi-crystalline polymers cold-drawing

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. 

The break-up of crystallites and the reformation of the lamellar fragments into microfibrUs is the basis of a theory for the cold-drawing of isotropic semi-crystalline polymers due to Peterlin. " (See also Hosemann et and Robertson .) Both Peterlin and Hosemann assert that the main mechanism is the break-up of each crystallite into approximately twenty smaller units which lie like pearls on a string with their chain axes parallel to the IDD. Many aspects of these theories would seem to be relevant to the deformation of oriented polymers of modest draw ratios. 

Fig. 6.19 Illustration of microstructure evolution upon cold-drawing of semi-crystalline polymers...

I wish to draw attention to some experimental results which seem promising as guidelines for a theory of cold-drawing. A comprehensive discussion of experiments on semi-crystalline polymers is given by Zapas, in another paper of this series. 

Load-deformation curves for polyethylene and polystyrene are shown in Fig. 5.2. Polyethylene necks and cold draws in the manner described above (5.N.1). As it is a semi-crystalline polymer, yielding involves considerable disruption of th.e crystal structure. Slip occurs... 

Fig. 5.48 Schematic illustration of the change in spherulite morphology during the cold-drawing of a semi-crystalline polymer (after Magill).

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