Amines in Polymers Quantified by X-ray Photoelectron Spectroscopy (XPS)

Figure 1:How an afm works.Image from oxinst.

A method was proposed to identify and quantify amines in polymers by utilizing the energy distribution of the N1s atomic orbital through X-ray photoelectron spectroscopy (XPS). The energy states of the nitrogen orbitals were obtained and correlated with the atomic chemical states of amines via their formation energies (FE).

Plasma-polymerized pyrrole and allylamine were used as test polymers for this method, as their structures may contain primary, secondary, and tertiary amines. However, additional unsaturated and oxidized amines were also detected in the test polymers, which are attributed to the synthesis energy [2,3].

The percentage contributions of each amine type, as determined by the method, showed:

• Secondary amines contributed the most.
• Followed by unsaturated and tertiary amines.
• The lowest contributions came from primary amines or oxidized amines detected during the analysis.

This novel method can be applied not only to polymers but also to other solid-state materials, provided they are sufficiently stable to withstand the X-ray beam of the XPS instrument.

Figure 1: a Energy distribution of the N1s orbital and fitted curves in PPy-PAl-I synthesized at 40 W. b Participation of amines as a function of the synthesis power[1].

Table 1. Energy of the chemical groups used in the work

FE is formation energy. BE is binding energy.

Formation Energy Considerations

Atomic bond energy depends on various factors. Table 1 lists the formation energies of different chemical states and chemical bonds. The following average formation energies (in eV) were used:

Thus, the total FE values for different types of amines were:

• Primary amine (C–N–H₂): 10.7 eV
• Secondary amine (C₂–N–H): 9.8 eV
• Tertiary amine (C₃–N): 9.1 eV

📌 Note: The formation energy decreases as the number of hydrogen atoms in the structure decreases.

Peak Fitting and Spectral Assignment

The N1s energy distribution in the polymers was fitted using five Gaussian peaks:

• 3 peaks correspond to typical amine types
• 2 peaks correspond to other amine-related states

Example: Copolymer Spectrum at 40 W

To illustrate the correlation between fitted Gaussian peaks and chemical states, Figure 1a shows the fitted N1s spectrum of a copolymer synthesized at 40 W, along with:

• Percentage contributions (area percentage) of each fitted peak
• Elemental composition (C1s, N1s, O1s) shown in the inset

The peaks were labeled left to right based on increasing formation energy.

Binding Energy Assignments

The N1s energy distribution was fitted with Gaussian curves (FWHM = 1.2 eV). The resulting peaks were assigned as follows:

Application and Results

The method was tested on:

• Polypyrrole
• Polyallylamine
• Their copolymers

All synthesized and doped via plasma iodination.

Five nitrogen chemical states were identified and quantified. Their percentages varied with synthesis energy:

• Secondary amines showed the highest percentage
• Primary or oxidized states the lowest
• Tertiary and unsaturated amines were intermediate

1.Olayo, M.G., Alvarado, E.J., González-Torres, M. et al. Quantifying amines in polymers by XPS. Polym. Bull. 81, 2319–2328 (2024). https://doi.org/10.1007/s00289-023-04829-y

2.Manakhov A, Cechal J, Michlícek M, Shtansky DV (2017) Determination of NH2 concentration on 3-aminopropyl tri-ethoxy silane layers and cyclopropylamine plasma polymers by liquid-phase derivatization with 5-iodo 2-furaldehyde. Appl Surf Sci 414:390–397. https://doi.org/10.1016/j.apsusc. 2017.04.127

3.Ryssy J, Prioste-Amaral E, Assuncao DFN, Rogers N, Kirby GTS, Smith LE, Michelmore A (2016) Chemical and physical processes in the retention of functional groups in plasma polymers studied by plasma phase mass spectroscopy. Phys Chem Chem Phys 18:4496–4504. https://doi.org/10.1039/c5cp05850c