Chain-folded lamellar crystals of aliphatic polyamides. Comparisons between nylons 4 4, 6 4, 8 4, 10 4, and 12 4

Chain-folded single crystals of the five even-even nylons 4 4, 6 4, 8 4, 10 4, and 12 4 were grown from solution and their structures and morphologies studied using transmission electron microscopy, both imaging and diffraction. Sedimented mats were examined using X-ray diffraction. All these nylons have room temperature crystal structures that relate to that reported for nylon 6 6, yet there are differences, reflecting the differences in the amide group distribution. At room temperature, all the crystals are composed of chain-folded, hydrogen-bonded sheets; the hydrogen bonds within the sheets form a progressive shear pattern, and, in addition, the sheets themselves are sheared progressively parallel to the sheet plane so that they generate triclinic unit cells. The magnitude of this intersheet shear may differ between nylons; it is dependent on the details of the amide decoration pattern on the hydrogen-bonded sheet faces. In all five nylons studied, the two strong and characteristic diffraction signals of the room temperature triclinic structure, at spacings 0.44 nm (projected interchain/intrasheet distance) and 0.37 nm (intersheet distance), move together and merge as they do for single crystals of nylon 6 6. For each of the X 4 nylons, the Brill temperature (lowest temperature where the spacings are equal) is in the range 140-190 degrees C. In each case, the triclinic structure gradually transforms into a pseudohexagonal structure as the temperature rises. The melting points of solution-grown crystals of this series of even-even nylons decrease with the linear hydrogen bond density. This series of nylons is unique since in each case the chain folds must be in the diamine alkane segment.