Temperature-induced changes in lamellar crystals of monodisperse nylon 6 and nylon 6 6 oligoamides

The structural changes induced as a function of temperature in lamellar crystals of monodisperse oligoamides of nylon 6 and nylon 6 6 have been studied using differential scanning calorimetry, X-ray diffraction and electron diffraction. Three different types of solid-state transformations, differing in their mechanisms, rates and associated transformation temperatures, are evident: unfolding, intersheet shear and, for nylon 6 6 molecules only, a Brill transformation. Unfolding transformations in the chain-folded crystals can occur within 40 min in a temperature range similar to 40 degrees C below the melting temperature, T-m, and can also be induced at lower temperatures over longer time periods. Changes in intersheet shear, from an alternating to a progressive scheme, can be induced at relatively low temperatures, e.g. similar to 70 degrees C below T-m for the once-folded 9-amide nylon 6 oligomer lamellar crystals, if long time periods (similar to days) are employed. Upon heating, all the nylon 6 6 oligomers exhibited Brill transformations prior to melting and they occur rapidly (<second) between 20 and 70 degrees C below T-m. The nylon 6 oligoamides do not undergo a Brill transformation, and all but the 3-amide nylon 6 oligomer, showed behavior analogous to the nylon 6 polymer. The 3-amide nylon 6 oligomer has a room temperature crystal structure that differs markedly from the nylon 6 polymer and exhibits a pattern of behavior closer to that of nylon 6 6. On the basis of these results, we offer structural interpretations for the nature of the intersheet shear, unfolding, and Brill transformation.