CHAIN-FOLDED LAMELLAR CRYSTALS OF ALIPHATIC POLYAMIDES - INVESTIGATION OF NYLON-4-8, NYLON-4-10, NYLON-4-12, NYLON-6-10, NYLON-6-12, NYLON-6-18 AND NYLON-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 the ir morphologies and structures studied using transmission electron mic roscopy; both imaging and diffraction. Sedimented mats were examined u sing X-ray diffraction. The solution grown single crystals are lath-sh aped lamellae. Diffraction from these crystals, at room temperature, r eveals that three crystalline forms are commonly present. The crystals are composed of chain-folded, hydrogen-bonded sheets; the linear hydr ogen bonds within the sheets generate a progressive shear of the chain s. The sheets are found to stack in two different ways; some of the sh eets stack with progressive shear, to form 'alpha-phase' crystals; oth er sheets stack with alternate up and down stagger, to form 'beta-phas e' crystals. Both the alpha- and beta-crystals give two strong diffrac tion signals at spacings of 0.44 and 0.37 nm; these signals represent a projected inter-chain distance within a hydrogen-bonded sheet (actua l value 0.48 nm) and the inter-sheet spacing, respectively. Some cryst als 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 crys tals of this even-even nylon series decrease with decreasing linear de nsity of hydrogen bonds. On heating, the strong diffraction signals in both alpha- and beta-phases move together and meet, as is the case fo r other even-even nylons. The lowest temperature at which the two sign als first have the same spacing is termed the Brill temperature. For a ll 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 thes e 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 t emperature is found to occur in the range 95-35 degrees C below the me lting point at 265 degrees C. (C) 1997 Elsevier Science Ltd.