STRUCTURAL STUDIES ON NYLONS WITH MORE THAN ONE DIRECTION OF HYDROGEN BONDS. Jordi Puiggalí. Departament d'Enginyeria Quimica, ETS d'Enginyers Industrials, Universitat Politècnica de Catalunya, Diagonal 647, Barcelona 08028, Spain

Although nylons have been known for well over 60 years, there still remains considerable confusion in the literature concerning the nature and stability of their crystalline structures. However all the stable structures reported at low temperature have a single hydrogen bond direction as a common characteristic. More confusing is the nature of the pseudohexagonal structures attained when nylons are heated above the "Brill transition temperature" and also the metastable pseudohexagonal structures characteristic of quenched samples. Whereas earlier models postulated random amide rotational jumps of 60deg. to explain the high temperature hexagonal phases (Brill, Prakt. Chem. 161, 49, 1942), the later experimental evidences suggest transitions involving methylene segment librations (Wendolowski et al, Science 247, 431 1990).

In recent years, we have carried out a systematic effort aimed at investigating polyamides in which an isolated methylene group is placed between two amide groups. Although a pseudohexagonal packing is characteristic, the experimental data point to structures with either three (Belle et al, Polymer 35, 1231, 1994), two (Aceituno et al, Macromolecules in press) or one (Franco et al, Macromolecules 27, 4284, 1994) hydrogen bond direction(s) which are different from the conventional [[gamma]] form of nylons. Crystallographic and quantum mechanical studies on model compounds support these new structures and also reveal that some dicarbonylic units as glutaryl residues (Navarro et al, J. Am. Chem. Soc. 117, 7307, 1995), tend to be in a folded conformation, where the two C-O directions are rotated. With these results on hand we study the nylons 65 and 55 as representatives of glutaric acid derivatives. Experimental data from uniaxially oriented fibers and lamellar crystals show a monoclinic or a pseudohexagonal unit cell, depending on the diamine unit. However the results suggest similar structures where each molecule is linked to its four neighbors by a network of hydrogen bonds made by amide groups in two different orientations. Particular conformations for the glutaryl residues play a decisive role in establishing such a unique structures. Temperature-induced structural changes have also been studied, showing that the monoclinic packing of nylon 65 reverts to a pseudohexagonal one at 190deg. C, Modifications are believed to occur without changes in the hydrogen bond system.