Nitrogen Plasma Modification and Chemical Derivatization of Polyethylene Surfaces – An In Situ Study Using FTIR‐ATR Spectroscopy

Abstract

Chemical derivatization reactions of nitrogen plasma‐treated surfaces with aromatic aldehydes, such as the prototypic 4‐trifluoromethyl‐benzaldehyde (TFBA), have for a long time been considered selective for primary amines. Results of an in situ study using FTIR‐ATR spectroscopy challenge the validity of this assumption: Modification of polyethylene surfaces by afterglows of dielectric barrier discharges in nitrogen–hydrogen mixtures with subsequent hydrogen/deuterium exchange or TFBA derivatization suggests that the latter does not follow the commonly assumed reaction scheme.

Critical remarks on chemical derivatization analysis of plasma‐treated polymer surfaces and plasma polymers

“…Even if one rules out exotic and less stable configurations and considers just structures obtained by combining two (or in one case three) of the more classical “building blocks” NH and CN, one ends up with the following configurations, all leading to different chemical reactivities toward aldehydes and hydrazines or isothiocyanates as well (Figure ). There are two reasons to focus the speculations on structures with nitrogen atoms inserted into the polymer chain: Aside from an argument based on IR spectral evidence (missing of spectral changes expected for H/D exchange in primary ketimine, i.e., >CNH moieties) one can formulate a quite simple, reasonable chemical mechanism by which the combination of a ground state nitrogen atom with a CH* radical center on the polymer chain would form secondary aldimine configurations CHN within the chain . Secondary amine groups CH 2 NH could then hypothetically be formed by hydrogenation, i.e., additions of two hydrogen atoms to CHN.…”

“…In the opinion of the authors, the potential of ATR FT‐IR spectroscopy as a method for a quantitative analysis of plasma‐modified surfaces is not fully realized as yet. FT‐IR spectrometers are available in many laboratories and the theoretical concept of the “effective thickness” for an ATR configuration, defined by the angle of incidence, polarization, and the relevant refractive thicknesses, makes a quantitative analysis straightforward . (Interested readers may also consult a recent review article on the utilization of ATR FT‐IR and FT‐IR techniques in general for the analysis of plasma processes at materials interfaces …”

PMMA Surface Functionalization Using Atmospheric Pressure Plasma for Development of Plasmonically Active Polymer Optical Fiber Probes

“…n O2 )Án O2 ) -1 = 0.1 ms-in this case only a fraction of 5Á10 -5 of the starting O atom concentration n 0 (0) would still be present after 1 ms. For this reason, a DBD in nominally We think that effects of VUV radiation cannot be excluded because DBDs in virtually pure Ar are known to be quite efficient sources of excimer radiation at 126 nm. However, results of recently performed unpublished experiments on the etching of ultrathin silk protein films in flowing post-discharges from DBDs in Ar (purity 6 N), monitored in situ by ATR-FTIR spectroscopy, lead us to the conclusion that VUV radiation is not the predominant agent here: For these experiments the set-up described earlier by Klages et al [44] was used. With comparable gas flow velocities as used in the present study, the protein etch rates were in the order of 0.015 nm/s.…”

“…Interestingly, most significant changes in the 1500-1700 cm -1 region are obtained by the Ar Plasma Chem Plasma Process treatment, followed by N 2 , while the other gases do not yield significant absorption bands in this region. While it cannot be excluded that a secondary reaction with the ambient after exposure to the post-discharge may play a role in generating electrophilic sites, the presence of imines or carbonyl groups formed during plasma exposure [44] may be responsible for the electrophilic reactivity of the N 2 -treated sample. The case of ''pure'' Ar will be discussed below.…”

“…Carboxylic acid groups are generally not reactive enough for that. Probably the best way to study this issue further would be in situ ATR-FTIR spectroscopy during the exposure of the surface to streaming DBD post-discharge atmospheres and subsequently to amine vapor [44].…”