Monitoring Nylon-6,6 Thermochemistry by Solid-State NMR

Abstract

Solid-state 13C and 15N NMR spectroscopy was used to study the structure and thermochemistry of nylon-6,6. Experiments were performed on samples of nylon-6,6 heated in the presence of labeled adipic acid and hexamethylenediamine (HMD) monomers as well as labeled bis(hexamethylenetriamine) (BHMT), a molecule previously implicated as a potential nylon cross-linker. The uptake and subsequent reaction of labeled materials were characterized by a variety of solid-state NMR techniques. Reaction of the central (α) nitrogen of BHMT to form an amide occurs very rapidly. With time, transamidation causes this position to become indistinguishable from the bulk amide nitrogen in nylon. Rotational-echo double resonance (REDOR), a solid-state NMR experiment that permits the quantitative measurement of 13C−15N labeled chemical bonds, was used to detect the formation and breaking of carbon−nitrogen bonds due to polymerization and transamidation. The reaction of adipic acid and hexamethylenediamine (HMD) and the transamidation reaction between the resulting oligomers and bulk nylon were studied by heating nylon-6,6 in the presence of [13C]adipic acid and [15N]HMD and characterizing the resulting polymers with REDOR. The combination of stable isotope labeling and NMR spectroscopy illustrated in this work should be applicable to the study of a wide variety of polymers.

Modeling of Polyamide 66 Solid State Polymerization: Drawing a Chemical Reaction Scheme

“…This conclusion is also supported by the fact that Reaction (12) was the only one observed in PA 6, which in turn shows a much smaller tendency to form gel than PA 66 . Despite that, tertiary amides ( N , N ‐disubstituted amides, final product of Reaction12 and 13) have been detected for PA 66 as well as tertiary amines . Although tertiary amines can be generated for short residence times (≈3 h) in this medium (Reaction 11), they have been detected only under conditions of excess of amine groups.…”

“…These results motivated many researchers to interpret that the nitrogen of secondary amines (first step in Reaction (11–13) in Table ) act as the main branching point. Actually, Garbow et al demonstrated by 13 C NMR that the di‐substituted nitrogen of BHMT reacts rapidly to form tertiary amides (a branching structure) over the polymerization course (Reaction 12).Analogously, the same group performed other polymerization runs between 280 and 290 °C, as: (i) adding 5‐oxononane‐1,9‐dicarboxylic acid to PA 66, (ii) adding carboxyl groups in excess into a model compound, and finally, (iii) polymerizing PA 66 samples containing carboxyl at both ends (samples ) . This procedure did not cause gelling in 3.5 h in the first case, gelled only after 35 h in the second case and did not form gel in the last one.…”

“…Thus, this reaction was also incorporated in our modeling approach.The brief overview of PA 66 literature discussed above shows a very large set of by‐products and chemical reactions over the polyamidation course, not always chemically identified unequivocally due to inherent complexity and misleading application of experimental techniques. For example: (i) quantification techniques (such as mass spectrometry) that are not intrinsically identification techniques (such as NMR and IR) applied independently led different authors to attribute distinct species (cyclohexane, 1‐hexene, and CP) to the same spectral peaks; (ii) low‐selective methods may overestimate amine end groups in PA 66 medium, such as titration; and iii) the method of marking carbon isotopes has been rarely combined with NMR, with exception for Garbow et al Therefore, the set of chemical reactions selected to represent the PA 66 SSP process presented in the next section is strictly based on compounds, the formation of which is relatively well accepted, such as CP, CO 2 , NH 3 , H 2 O, and amine groups. Focussing on SSP experimental studies, provided by Soto‐Valdez and Gramshaw, Edel and Etienne, and Mackerron and Gordon, they exhibit relevant amounts of one or more of these compounds, as well as alkylated CP.…”

“…These results motivated many researchers to interpret that the nitrogen of secondary amines (first step in Reaction (11–13) in Table ) act as the main branching point. Actually, Garbow et al demonstrated by 13 C NMR that the di‐substituted nitrogen of BHMT reacts rapidly to form tertiary amides (a branching structure) over the polymerization course (Reaction 12).…”

%Solid-State Nuclear Magnetic Resonance

“…Differences in solid-state NMR of four different forms of isotactic poly(4-methyl-1-pentene) were explained by packing and conformational effects (161). Solid-state NMR was used to monitor thermochemistry of nylon-6,6 heated in the presence of labeled adipic acid and hexamethylenediamine (162).…”