ABSTRACT:The molecular and crystal structures of aliphatic polyurethane series -[O-(CH2)m-OCONH-(CH2)6-NHCO-]p-with m=2-6, have been studied by X0 ray diffraction methods. Two types of molecular structures have been found. The one (m=3, 5 and type II of m=2) is similiar to the model proposed for r-phase of polyamides, where the hydrogen bonding requirements between adjacent chains are satisfied by tilting the urethane group planes approximately 30° with respect to the fiber axis causing a shortening of the fiber identity periods. The other (m=4, 6 and type I of m=2) is similar to the a-phase of polyamides, where the molecular chains have planar zigzag conformation. These structures are well confirmed by the comparison of the observed and calculated intensities of X-ray diffraction spots. 4, 6-polyurethane (Perlon U) has an X-ray diffraction pattern similiar to that of the unsubstituted polyamides. The chain repeat distance (fiber period) was found to be 19.1 A by Zahn and Kohler\ which is in accordance with that predicted for a fully extended planar zigzag chain. The similarity in the X-ray data between polyamides and polyurethanes suggests that the latter has a conformation of side-by-side zigzag chains with hydrogen bondings between neighbouring chains. This view was pointed out by Borchert2, who proposed the two-molecule unit cell, a=4.95 A, b(fiber axis)=l9.2 A, c=S.69 A, a=90°, ~= 60.2°, and r=170-176°. Zahn and Winter 3 reported the four-molecule unit cell, a=9.55 A,