Chain-folded lamellar crystals of aliphatic polyamides. Investigation of nylons 4 8, 4 10, 4 12, 6 10, 6 12, 6 18 and 8 12

Chain-folded single crystals of the seven even-even nylons: 4 8, 4 10, 4 12, 6 10, 6 12, 6 18 and 8 12 have been grown from solution and their morphologies and structures studied using transmission electron microscopy; both imaging and diffraction. Sedimented mats were examined using X-ray diffraction. The solution grown single crystals are lath-shaped lamellae. Diffraction from these crystals, at room temperature, reveals that three crystalline forms are commonly present. The crystals are composed of chain-folded, hydrogen-bonded sheets; the linear hydrogen bonds within the sheets generate a progressive shear of the chains. The sheets are found to stack in two different ways; some of the sheets stack with progressive shear, to form 'alpha-phase' crystals; other sheets stack with alternate up and down stagger, to form 'beta-phase' crystals. Both the alpha- and beta-crystals give two strong diffraction signals at spacings of 0.44 and 0.37 nm; these signals represent a projected inter-chain distance within a hydrogen-bonded sheet (actual value 0.48 nm) and the inter-sheet spacing, respectively. Some crystals also show an additional diffraction signal at 0.42 nm; this signal is characteristic of the pseudo-hexagonal phase, a phase usually only found at high temperatures. The melting points of solution grown crystals of this even-even nylon series decrease with decreasing linear density of hydrogen bonds. On heating, the strong diffraction signals in both alpha- and beta-phases move together and meet, as is the case for other even-even nylons. The lowest temperature at which the two signals first have the same spacing is termed the Brill temperature. For all the nylons of the present study the Brill temperature is coincident with the melting temperature, and the two strong signals meet at the spacing (0.42 nm) of the pseudo-hexagonal phase. The behaviour of these nylons is compared and contrasted with that of nylon 6 6, where only the a-phase is found at room temperature and, on heating, the Brill temperature is found to occur in the range 95-35 degrees C below the melting point at 265 degrees C. (C) 1997 Elsevier Science Ltd.