Crystallization and Melting Behavior (Tm)

In its solid form, a polymer can exhibit different morphologies, depending on the structure of the polymer chain as well as the processing conditions. The polymer may exist in a random unordered structure termed amorphous. An example of an amorphous polymer is polystyrene. If the structure of the polymer backbone is a regular, ordered structure, then the polymer can tightly pack into an ordered crystalline structure, although the material will generally be only semicrystalline. Examples are polyethylene and polypropylene. The exact makeup and architecture of the polymer backbone will determine whether the polymer is capable of crystallizing. This microstructure can be controlled by different synthetic methods. As mentioned above, the Ziegler-Natta catalysts are capable of controlling the microstructure to produce stereospecific polymers. The types of microstructure that can be obtained for a vinyl polymer are shown in Fig. 1.6. The isotactic and syndiotactic structures are capable of crystallizing because of their highly regular backbone, while the atactic form would produce an amorphous material. The amount of crystallinity actually present in the polymer depends on a number of factors, including the rate of cooling, crystallization kinetics, and the crystallization temperature. Thus, the extent of crystallization can vary greatly for a given polymer and can be controlled through processing conditions.

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