CHAIN-FOLDED LAMELLAR CRYSTALS OF ALIPHATIC POLYAMIDES - COMPARISONS BETWEEN NYLON-4-4, NYLON-6-4, NYLON-8-4, NYLON-10-4, AND NYLON-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 mo rphologies studied using transmission electron microscopy, both imagin g and diffraction. Sedimented mats were examined using X-ray diffracti on. All these nylons have room temperature crystal structures that rel ate to that reported for nylon 6 6, yet there are differences, reflect ing the differences in the amide group distribution. At room temperatu re, all the crystals are composed of chain-folded, hydrogen-bonded she ets; the hydrogen bonds within the sheets form a progressive shear pat tern, and, in addition, the sheets themselves are sheared progressivel y parallel to the sheet plane so that they generate triclinic unit cel ls. 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 stron g and characteristic diffraction signals of the room temperature tricl inic structure, at spacings 0.44 nm (projected interchain/intrasheet d istance) 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 equa l) is in the range 140-190 degrees C. In each case, the triclinic stru cture gradually transforms into a pseudohexagonal structure as the tem perature rises. The melting points of solution-grown crystals of this series of even-even nylons decrease with the linear hydrogen bond dens ity. This series of nylons is unique since in each case the chain fold s must be in the diamine alkane segment.