Polymorphism
Some polymers can exhibit more than one crystalline form, depending on the method used for the preparation of the crystal.
This behavior is exhibited by nylon 6,6.
One of the structures is depicted in PC Fig. 7.24, reproduced from Bunn, C. W.; Garner, E. V. "The Crystal Structure of Two Polyamides" Proc. Roy. Soc. (London) 1947, 189A, 39-68.
Perhaps a more famous example, because of its important implications for the conformations of proteins, is the behavior of polyalanine.
Poly(L-alanine) can exist in either of two very different crystalline forms.
- A right-handed alpha helix in which the chain is relatively compact (the translation per residue along the helix axis is only 0.15 nm) with intrachain hydrogen bonds.
- A beta sheet in which the chain is more highly extended (the translation per residue along the chain axis is 0.345 nm, which is more than twice as large as the result in the alpha helix), with interchain hydrogen bonds.
Infrared linear dichroism is an example of a spectroscopic technique that can distinguish between these two crystalline forms.
Some other poly(amino acids), such as polyglycine, also exhibit two very different crystal structures, depending on the method of sample preparation.
Return to the index
September 13, 2001
Wayne L. Mattice: wlm@polymer.uakron.edu