Study of hydrogen bond dynamics in Nylon 6 crystals using IR spectroscopy and molecular dynamics focusing on the differences between α and γ crystal forms

Abstract

Proton dynamics of hydrogen bonds (HBs) in the α and γ form of Nylon 6 were investigated by Born–Oppenheimer molecular dynamics (BOMD). Our results show differences in the dynamic effects of interchain HB interactions between the α form and the γ form of Nylon 6. Analysis of the time course of the geometrical parameters of HBs along the BOMD simulations has shown that HBs are dynamically favored in the γ form of Nylon. The quantization of the NH stretching mode enables a detailed discussion of the strengths of HB interactions. Solving the Schrödinger equation for the snapshots of one‐dimensional proton potentials, extracted from the ab initio MD, enables the consideration of anharmonicity, thermodynamics, and approximate quantum effects on proton movement. A larger red shift of the NH stretching band was observed in the γ form compared with the α form. Our study shows that HBs are more stabilized in the γ form than in the α form, which is mainly due to the higher number of HBs. The distribution of HBs along the trajectory clearly reveals the preference of the γ form. The quantization of the NH motion enables the discussion of the differences in the IR spectra between the two forms.

Aliphatic polyamides (nylons): Interplay between hydrogen bonds and crystalline structures, polymorphic transitions and crystallization

“…CO and NH groups are nearly collinear in the γ‐form in contrast with the slight deviation of linearity postulated for the α‐form (i.e., see that amide groups do not lie exactly in the zig‐zag plane of molecular chains in Figure 1). 14 Therefore, it has been suggested that the γ‐form has stronger hydrogen bonds than the α‐form, a feature that is in agreement with FTIR and NMR observations 19 but that is controversial when computation data are considered 20 . In general, thermodynamic stability between α and γ forms is similar (i.e., nylon 6 21 ) and consequently one or the other form can be obtained depending on specific solvent (e.g., exposition to iodine solutions 18 ), thermal or mechanical treatments, processing conditions (e.g., spinning rate and postdrawing treatments 22 ), incorporation of fillers and nucleating agents, 23 mixing with other polymers, 24 and logically on the selected crystallization conditions (e.g., solvent, substrate and temperature for solution, epitaxial and melt crystallizations).…”

Overview and critical survey of polyamide6 structural habits: Misconceptions and controversies

“…However, several authors later stated that the α crystal form has stronger H‐bonds than the γ form as mainly judged from its better thermal stability. [ 41,90a,93,100–102 ] Using respectively ab initio and group contribution modeling Dasgupta et al [ 103 ] and Bernardo et al [ 104 ] supported this statement that H‐bonds are stronger in the α form.…”

“…Despite the above contradictions, the presence of true γ crystals-generally imperfect and metastable-has been clearly shown by vibrational spectroscopy in PA6 spun fibers and cast films, and PA6/clay nanocomposites as well. [37,43,51,[81][82][83][84][85][86][87][88][89][90][91][92][93][94] Illers et al [37] initially reported investigations on the α and γ major crystal forms of PA6 and its mesophase via conventional infrared spectroscopy. Rotter and Ishida [83] later reported clear identifications of these ordered forms via Fourier Transform Infra-Red spectroscopy (FTIR) and the amorphous phase as well.…”

“…From C 13 and N 15 NMR measurements both Hatfield et al [ 107 ] and Konishi and Ito [ 108 ] also observed that H‐bonds are stronger in the γ form than in the α form. Combining FTIR and molecular dynamics Brela et al [ 93 ] concluded moreover that H‐bonds are more stabilized in the γ form than in the α form.…”

P(N-Phenylmaleimide-Alt-Styrene) Introduced with 4-Carboxyl and Its Effect on the Heat Deflection Temperature of Nylon 6

“…The P(CPMI-alt-St) could induce the transformation of the α crystal type to the γ crystal type because the 4-carboxyl (-COOH) formed a hydrogen bond with the amino (-NH 2 ) in nylon 6, inducing nylon to form a γ crystal. Mateusz Z. Brela [25] studied the hydrogen bond dynamics in nylon 6 crystals using IR spectroscopy and molecular dynamics, focusing on the differences between α and γ crystal forms, and found that hydrogen bonds were dynamically favored in the γ form of nylon 6.…”