Molecular motion in polyamides

Abstract It is well known that aliphatic polyamides with even numbers of methylene groups between amide groups melt at higher temperatures than those with odd numbers of methylene groups. Crystallographic studies in the past have sought to relate the packing of the amide groups to the variations in melting point, but these studies have been made almost exclusively at room temperature. In the present study, series of polyamide fibers were examined by x‐ray diffraction over a wide range of temperatures. In the even‐numbered polyamides, the triclinic structure occurring at room temperature shifts to a more symmetrical structure in which the chains assume hexagonal packing. This latter packing occurs in the odd‐numbered polymers at room temperature. The polar groups in the latter occur in layers which are normal to the chain axes. The layers of polar groups in the even‐numbered polyamides are tilted with respect to the chain axes at room temperature, and remain tilted at high temperatures even though the lateral chain packing becomes the same as the lower‐melting (odd‐numbered) members of the series. The spacing between polar groups along the chain decreases with increasing temperature, but the decrease is relatively less in the even‐numbered than in the odd‐numbered polymers, and is less in the more polar than in the more paraffinic compounds. These changes are ascribed to torsional and rotational motions of chain segments in the crystallites. Proton magnetic resonance studies demonstrate the onset of motion of the hydrocarbon segments and compare qualitatively the constraints to chain motion among isomeric members of the series. Some implications regarding the melting process are discussed.