Polymeric Distribution


Concepts Shown:

Different polymeric molecular structures and their effect on deformation, and, the four stages that linear amorphous polymers undergo as temperature increases


A soft rubber tubing, a plastic spoon (polystyrene), a lighter and liquid nitrogen.


Pass around the soft tubing and the rigid plastic spoon. Now, immerse the tubing in liquid nitrogen and heat the spoon using the lighter. Show the students how one polymer can be turned rigid by cooling and how the other can be turned rubbery by heating. Also show them from the second diagram, how crosslinked, elastomeric and crystalline polymeric materials demonstrate different viscoelastic moduli from linear amorphous polymers at various temperatures. CAUTION: The nitrogen may spew out of the other end of the tubing. Goggles for the instructor and for the student in the front row must be provided.


The right end of the abscissa in the first graph has higher temperatures. At the left end and below the glass transition temperature, Tg, where only elastic deformation can occur, the material is comparatively rigid. The polyethylene spoon is an example of a linear amorphous polymer below its Tg (Tg of polystyrene is 90 degrees C). In the range of Tg, the material is leathery. It can be deformed and even folded, but does not spring back to its original shape. In the rubbery plateau, polymers deform readily but quickly regain their previous shape if the stress is removed. The rubber tubing is such an example. At still higher temperatures, the polymer deforms extensively by viscous flow. All linear amorphous polymers go through the four distinct stages mentioned above, if they are heated or cooled. For instance, when the soft rubbery tubing is immersed in nitrogen, it undergoes its glass transition and becomes rigid. On the other hand, the rigid polystyrene spoon can be heated above its Tg, into its rubbery stage. [eq]. Figure (2) compares the deformation behavior for the different structural variants. A 100 percent crystalline polymeric material (curve 2) does not have a Tg. Therefore, it softens more gradually as the temperature increases, until the melting temperature is approached, at which point fluid flow becomes significant. The behavior of crosslinked polymers is represented in figure (2) by curve (3). A vulcanized rubber, for example, is harder than a nonvulcanized one. Curve (3) is raised more and more as a larger fraction of the possible cross-links are connected. Thermoset qualities of crosslinked polymers arise from the fact that the three-dimensional amorphous structure carries well beyond an imaginable melting temperature. Once the glass temperature is exceeded, elastomeric molecules can be rotated and unkinked to produce considerable strain. If the stress is removed the molecules snap back to their kinked conformations. This rekinking increases with greater thermal agitation at higher temperatures. Thus, curve (4) increases slightly to the right across the rubbery plateau. The elastomer finally reaches the temperatures at which it becomes a true liquid, and flow proceeds rapidly. [eq].


Rahul Pinto

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