## Abstract

Detailed interpretations of the X-ray diffraction patterns of fibres and sheets of 66 and 6.10 polyamides (polyhexamethylene adipamide and sebacamide respectively) are proposed. The crystal structures of the two substances are completely analogous. Fibres of these two polyamides usually contain two different crystalline forms, $\alpha $ and $\beta $, which are different packings of geometrically similar molecules; most fibres consist chiefly of the $\alpha $ form. In the case of the 66 polymer, fibres have been obtained in which there is no detectable proportion of the $\beta $ form. Unit cell dimensions and the indices of reflexions for the $\alpha $ form were determined by trial, using normal fibre photographs, and were checked by using doubly oriented sheets set at different angles to the X-ray beam. The unit cell of the $\alpha $ form is triclinic, with a = 4$\cdot $9 A, b = 5$\cdot $4 A, c (fibre axis) = 17$\cdot $2 A, $\alpha $ = 48${\textstyle\frac{1}{2}}$ degrees, $\beta $ = 77 degrees, $\gamma $ = 63${\textstyle\frac{1}{2}}$ degrees for the 66 polymer; a = 4$\cdot $95 A, b = 5$\cdot $4 A, c (fibre axis) = 22$\cdot $4 A, $\alpha $ = 49 degrees, $\beta $ = 76${\textstyle\frac{1}{2}}$ degrees, $\gamma $ = 63${\textstyle\frac{1}{2}}$ degrees for the 6.10 polymer. One chain molecule passes through the cell in both cases. Atomic coordinates in $\alpha $ crystals were determined by interpretation of the relative intensities of the reflexions. The chains are planar or very nearly so; the oxygen atoms appear to lie a little off the plane of the chain. The molecules are linked by hydrogen bonds between C$\chembond{2,0} $O and NH groups, to form sheets. A simple packing of these sheets of molecules gives the $\alpha $ arrangement. An alternative packing of the sheets gives a two-molecule triclinic cell, and this is the structure proposed for the $\beta $ form. Streaks on the layer lines (but not on the equator) of fibre photographs are attributed to an arrangement of molecular sheets in which $\alpha $ and $\beta $ groupings occur at random. The optical properties of fibres and doubly oriented sheets are consistent with the structure determined by the X-ray method. Unoriented specimens are spherulitic; optical evidence on the structure of the spherulites is discussed. Some striking similarities between the crystallography of the polyamides and that of the fibrous proteins are discussed.