Infrared studies of chain folding in polymers. X. polycaprolactam

The 1210-, 1233-, and 1288—cm^?1 infrared bands in the α form of nylon 6 are shown to result from a unique conformation in a tightly folded chain with adjacent reentry. The assignment of the 1210- and 1288-cm^?1 bands to motions of a gauche nitrogen-methylene group in the fold and the 1233-cm^?1 band to motions of a gauche carboxyl-methylene group in the fold is supported by the infrared studies of the α form of nylon 6, the γ form of nylon 6, poly-D(-)β-methyl- ? -caproamide (PBCA), and the high- and low-temperature forms of the cyclic dimer of nylon 6. The γ- form of nylon 6 and PBCA do not exhibit a unique conformation in the fold. The 1198-cm^?1 band can be assigned to the trans conformation of the amide group which exists in the normal planar zigzag conformation in the α-form crystal. A mechanism for the α to γ iodine-induced transition in nylon 6 is proposed showing the role of the fold in maintaining a minimum of molecular disordering during the transformation.