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Blog
Crystal orientation and necking
July 28, 2008
As I discussed in an earlier posting, during orientation from the solid state, semicrystalline polymers show a particular reorientation of the crystals as the polymer deforms. When this occurs it is oftentimes accompanied by a distinct change in dimensions of the polymer part in the rejoin of the reorientation. This is seen as a distinct change in cross section of the part and is called “necking."
Figure 1 is a picture I took of a polymer filament obtained from my rheometer during a viscosity test. Therefore it is an air quenched sample, the polymer is LLDPE. I took the fiber and pulled it at room temperature to orient it. As the force was raised the polymer began to stretch orient, with the transition from unstretched to stretched polymer occurring at the neck. The neck propagated towards the unstretched polymer as the overall length of the polymer fiber increased. In the transition region between the fully stretched and the unstretched portions, the crystalline sections are rearranging as the amorphous regions are elongated as drawn in the “Crystal rotation during film orientation” posting.
Figure 1: Magnified picture of a neck formed in a polymer at room temperature as the polymer stretched. The change in diameter marks the location of the change in fiber orientation
Posted by Eldridge M. Mount on July 28, 2008 | Comments (0)