New interpretation of progression bands observed in infrared spectra of nylon‐m/n

Abstract

A series of progression bands observed in the infrared spectra of nylon-m/n and their model compounds have been interpreted in a new manner on the basis of simply coupled oscillator models of zigzag alkyl chains. Nylon-m/n possesses the methylene sequences of (CH 2 ) m and (CH 2 ) nϪ2 , and so the effective models of m and n Ϫ 2 coupled oscillators, respectively, had previously been assumed for the methylene rocking-twisting mode, for example. However, the spectral patterns of progression bands predicted by this previously proposed model have been found to be inconsistent with those observed for many kinds of nylon samples with various m and n values. It is rather reasonable to assume that the effective numbers of oscillators should be m Ϫ 2 and n Ϫ 4 for the methylene rocking, twisting, and wagging modes of the (CH 2 ) m and (CH 2 ) nϪ2 sequences, respectively. In other words, the infrared progression bands observed for methylene local modes of nylon-m/n may be interpreted reasonably with the data of n-alkane molecules with the chemical formulae CH 3 (CH 2 ) mϪ2 CH 3 and CH 3 (CH 2 ) nϪ4 CH 3 . For the C™C stretching modes, the equivalent n-alkanes are CH 3 (CH 2 ) mϪ1 CH 3 and CH 3 (CH 2 ) nϪ3 CH 3 , respectively. In the simply coupled oscillator model, the vibrational mode of one methylene group is represented by an oscillator, for example. Our new concept is to isolate the terminal oscillator adjacent to the amide group from the other oscillators in the inner parts of the methylene zigzag sequence. This corresponds to a physical situation in which the methylene group adjacent to the amide group shows a different vibrational behavior of larger amplitude than those of the inner methylene sequence, as supported by broad-line NMR data and molecular dynamics calculations reported in the literature. Another possibility is a difference in the electron structure of the methylene unit adjacent to the amide group from that of the inner methylene sequence, resulting in a difference in the force constant and giving a vibrational decoupling between these two types of methylene units.

Conformational disorder in the Brill transition of uniaxially-oriented nylon 10/10 sample investigated through the temperature-dependent measurement of X-ray fiber diagram

“…In the latter case, the molecular chains experience a large-amplitude thermal motion around the chain axis and some of the chains rotate almost freely at enough high temperature. In the Brill transition of nylons, on the other hand, the amide groups of the neighboring chains are connected to each other by the intermolecular hydrogen bonds even in the high-temperature phase above the Brill transition temperature [1][2][3][4][5]. Therefore, different from the case of PE, the molecular chains of nylons cannot rotate perfectly freely around the chain axes because of such a partial constraint at the amide group positions.…”

“…Sample: nylon 10/10 -[-NH(CH 2 ) 10 NHCO(CH 2 ) 8 CO-]-was used here since we have accumulated many experimental data as well as the molecular dynamics calculation results for this sample [1][2][3][4][5]. The pellet, which was supplied kindly by the Shanghai Cellulose Works, Shanghai, China, was evacuated overnight above 100 8C to purge the absorbed water.…”

“…In a series of papers [1][2][3][4][5], we have investigated the structural changes occurring in the so-called Brill transition of aliphatic nylons on the basis of X-ray diffraction, infrared spectra and molecular dynamics calculation. Quantitative analysis of a series of infrared progression bands intrinsic of methylene zigzag sequences clarified in a concrete manner that the chain conformation is disordered remarkably through the trans-gauche exchange around the methyleneamide and metylene-methylene skeletal bonds.…”

“…Therefore, it might be more suitable to define the Brill transition phenomenon, not in terms of the behavior of the X-ray equatorial reflections, rather in terms of the conformational disordering with the intermolecular hydrogen bonds kept unbroken even above the transition temperature region. So far, many people including us have investigated the structural changes in the Brill transition mainly on the basis of the X-ray diffraction [2][3][4][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] and infrared spectral data [1][2][3][4]29,30] and also by the molecular dynamics (MD) calculation [5,31]. In most of the papers, however, the X-ray reflections used in the discussion are almost limited to the main 100 and 010/110 equatorial reflections only.…”

Reversible changes induced by temperature in the spherulitic birefringence of nylon 6 9

“…FTIR is a powerful technique to obtain information about hydrogen bond interactions and the arrangement of methylene sequences in aliphatic polyamides [45,46]. Thus, the wavenumber of the Amide A band (NH stretching mode) is directly related to the strength of intermolecular hydrogen bonds.…”