Low-Frequency Amide Modes in Different Hydrogen-Bonded Forms of Nylon-6 Studied by Inelastic Neutron Scattering and Density-Functional Calculations

Abstract

Inelastic neutron-scattering spectra of oriented nylon-6 were measured using the filter−analyzer and time-of-flight techniques. The samples studied were semicrystalline fibers with the crystalline fraction either in the α or the γ form, and selectively deuterated polymer films. Strong features observed in the inelastic spectra correspond to vibrations of methylene groups, however several of the typical amide modes are also identified. The frequencies of observed amide V, VI and VII modes depend on the type of crystal structure. The lowest amide VII mode is located at 27 meV in the α phase, and at 32 meV in the γ phase. Broad boson peaks are observed in both types of fibers, and attributed to their amorphous fractions. The boson peak of the γ fiber is found at a slightly higher frequency, which we explain by a higher cohesive energy density due to stronger hydrogen bonds or their higher density in the γ amorphous phase. Density functional calculations, using the B3LYP functional, were employed to study the vibrational modes of smaller model molecules, including H-bonded couples. These calculations show that hydrogen bonding significantly modifies not just the frequencies of out-of-plane amide vibrations, but the character of mode polarization vectors as well.