Nylon 6 can exist in either of two stable crystal structures, the α crystal with hydrogen bonds between antiparallel chains or the γ crystal with hydrogen bonds between parallel chains. Both structures have been observed in the same highly annealed fiber, suggesting that the polymer should not be regarded as a pure compound but a multicomponent mixture. Nylon 6 can also exist in a series of metastable crystal structures which vary continuously in size, perfection, and structural parameters from a pseudohexagonal structure to either of the two stable forms. A single equatorial reflection in the wide-angle x-ray pattern (i.e., pseudohexagonal structure) will not distinguish which of the two stable forms will result upon annealing. Measurement of the 0,14,0 reflection in oriented fibers will distinguish the major trend, but it is still uncertain to what extent annealing conditions can favor one stable form over the other. The variation in the unit cell parameters as a function of annealing has been accurately measured, and the observed phenomena can be qualitatively described by postulating various balances between the basic forces which hold the crystal together (i.e., hydrogen bonding, dipole–dipole interaction, van der Waals attraction, and covalent bonding).
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