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Blog
Amorphous orientation
July 30, 2008
While I have been discussing crystal orientation for the last few blog postings, of primary importance to the physical properties of the film, and in particular the mechanical properties, is the amorphous orientation. Indeed, for some oriented polymers such as polystyrene (PS), polyimides (PI), polymethylmethacrylate (PMMA) where there are no crystals, all of the orientation is in the amorphous phase. Molecular orientation is easily visualized and related to residual stress due to orientation using polarized light due to the stress - optical coefficient. An example of polarized visualization of orientation is shown in Figure 1, where the different colors represent different levels of residual stress due to the orientation in the walls and base of the cup, which was oriented by thermoforming.
Recall that during orientation, the amorphous phase, containing the tie molecules, pulls the crystals to realign them. While the crystals resist deformation by increasing the stiffness, it is the amorphous phase which actually caries the load in the polymer. Also as noted in previous postings, the amorphous orientation is the source of thermal shrinkage. Crystalline orientation is discussed more than amorphous orientation because it is relatively easy to measure with X-ray diffraction, while measuring amorphous orientation is more difficult and is usually determined by subtracting the crystalline orientation from the total orientation.
Figure 1: Optical stress patterns in a deep-drawn, thermoformed (oriented) cup. Color variations show variations in residual stress frozen in the oriented molecules.
Posted by Eldridge M. Mount on July 30, 2008 | Comments (0)