Low-Frequency Vibrational Modes of Nylon 6 Studied by Using Infrared and Raman Spectroscopies and Density Functional Theory Calculations

Far-infrared (FIR) and low-frequency Raman bands in the 90-400 cm(-1) region of crystalline nylon 6 in alpha form were assigned based on comparisons of experimental spectra and quantum mechanical calculations. A fragment methodology was applied in the calculations for explicit consideration of interchain interactions and crystal symmetry. The main features in both Raman and FIR spectra were reproduced well, which enabled the band assignments based on density functional theory and the significant improvement of the conventional assignments for which there had been a big dispute. Temperature dependence of the experimental FIR spectra has revealed that both bands at 222 and 111 cm(-1) are characteristic of the alpha-form structure. Their intensities linearly decreased with increasing temperature with marked two transition points, which correspond to glass and Brill transitions. Both bands can be indicators of the lattice length of alpha-form nylon 6. On the basis of the calculations, the FIR and Raman bands at similar to 100 cm(-1) were successfully assigned to methylene torsion and transverse motion of amide groups in which NH and O atoms move out of the amide plane. Decomposition of the calculated spectra revealed that the intensities at, similar to 100 cm(-1) in both spectra mainly originate from the amide groups and only secondarily from the methylene groups. Moreover, the FIR intensities at similar to 100 cm(-1) were nearly perfectly governed by the amide groups, which could be a reason why this FIR band is particularly sensitive to hydrogen bonds among the low-frequency bands. The FIR band at 222 cm(-1) was assigned to methylene torsion and transverse motion of NH groups. Both FIR bands at 222 and 111 cm(-1) contain perpendicular motions of methylene and amide groups. This will be a reason for their sensitivity to interchain interactions in alpha-form nylon 6. Contrarily, the FIR band at 294 cm(-1) is in parallel polarization to the chain direction and assigned to a deformation of C-CH2-CH2 and bending motion of C = O in the amide plane. This is the reason why this band is not sensitive to the structural transitions of nylon 6. Our previous works revealed that in regions of 125 and 70 cm(-1,) there are specific vibrational peaks of crystalline polyesters primarily arising from out-of-plane motion of ester groups. We can find a similarity in 125 and 70 cm(-1) regions between crystalline polyesters and nylon 6 that both polymers show specific out-of-plane vibrational peaks around 100 cm(-1), which are sensitive to the lattice length among polymer chains.