Solid organic compounds consisting of ordinary small molecules tend to be crystalline, that is, the molecules pack themselves in regular three-dimensional arrays. Polymers are different; they can be amorphous (totally lacking positional order on the molecular scale) or semicrystalline (containing both crystalline and amorphous regions in the same sample). The semicrystalline case can be pictured according to the "fringed micelle" model (an admittedly poor choice of name). Here is a two-dimensional representation:
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"Fringed micelle" model |
| In polymers, the degree of crystallinity ranges: |
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Semicrystalline polymers have true melting temperatures (Tm) at which the ordered regions break up and become disordered. In contrast, the amorphous regions soften over a relatively wide temperature range (always lower than Tm) known as the glass transition (Tg). Fully amorphous polymers do not exhibit Tm, of course, but all polymers exhibit Tg. Above these temperatures, polymers are liquids. For more information, read ahead in chapers 11 and 12 of Cowie.
Examples of amorphous polymers: conventional polystyrene and poly(methyl methacrylate).
Examples of crystalline polymers: polyethylene, PET polyester.
A few special polymers show intermediate phases. Such materials are liquid crystals, and are the basis for a large active field of research in both industry and academia. Discussion of liquid crystalline polymers is beyond the scope of this course, and the interested student is refered to chapter 16 of Cowie, and to the following monographs:
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