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crystalline polymer
A polymer showing crystallinity.One or two-dimensional order leads to mesophase structure.The range of order may be as small as about in one (or more) crystallographic direction(s) and is usually below in at least one direction.
Crystalline and Amorphous Polymers
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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:
"Fringed micelle" model
In polymers,the degree of crystallinity ranges:
from 0% (amorphous glasses)
to >90% (highly crystalline)
and is affected by:
structure
temperature
molecular weight
stereochemistry
processing conditions
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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.
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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:
Liquid-crystalline Polymers,R.A.Weiss and C.K.Ober,eds.; ACS Symposium Series No.435,1990.
Side-chain Liquid Crystalline Polymers,C.B.McArdle,ed.; Blackie,1989.
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