The semi-crystalline polyethylene is comprised of regions of crystalline structure and regions of amorphous material. Single macromolecules may be part of each of these regions and act as 'tie' molecules that hold the material together (a). As deformation starts, plastic deformation begins in the amorphous regions and the tie molecules are

uncoiled and start to allign (b) (c) (d). The allignment of the macro-molecules generates crystalline regions in the previously amorphous zones. Rotation of the pre-existing crystalline regions also takes place and the final configuration (e) shows the structural change just prior to fracture. This structural change can be seen as an increased transparency of the sample since the crystalline regions scatter visible light less than the amorphous zones.


From: Callister, "Materials Science and Engineering," Wiley (1997)