XPS investigation of plasma modified polyethylene surfaces

Li Jiashen, Sheng Jing, He Fei, Ding Huili
(Department of Material Science and Engineering, Tianjin University, Tianjin 300072, China)

Received  Apr. 3, 2001; Sponsored by National Natural Science Foundation of China (No.59733070).

Abstract Polyethylene (PE) was submitted to CO₂, N₂ and Air low temperature plasma. The chemical composition of the modified polymers was investigated by X-ray photoelectron spectroscopy (XPS). It was found that the surface of the modified polymers had some functional groups, such as -CO-O, -C-O-, -CO-NH₂, -C-NH, besides original bonds. These groups may improve the soakage and adhesion of the polymers. There are some oxygen-containing groups but little nitrogen-containing groups on the surface of PE without plasma treatment. The amounts of oxygen and the nitrogen combined with the PE surface are heavily affected by plasma conditions. N₂ plasma has to work with CO₂ or air plasma to introduce more nitrogen-containing groups on the PE surface.
Key words Low temperature plasma XPS Surface modification Polyethylene

1. INTRODUCTION   
The industrial use of polyethylene is always increasing, but taking into account its low surface energy, this polymer requires modification before it can be applied in some special fields^{[1, 2]}. Several methods have been developed to modify polymer surface for improved adhesion including mechanical treatments, wet-chemical treatments, corona discharge, and glow discharge plasma^[3-5]. Basic objectives of any such treatment are to remove surface contamination, to roughen surface, and to increase surface reactivity. With plasma, it is possible to selectively add different functional groups to the polymer surface. The plasma treatment can also alter the polymer surface by changing the surface roughness and possibly causing some crosslinking^[6-8].
    A plasma treatment is a way to introduce some functional groups on the surface of polymer^{[9, 10]}. Plasma is a mixture of electrons, ions, and radicals. These species disappear in the processes of electron-positive ion recombination, and the radical-radical recombination. The rate constants of these reactions are in an order of 10^-7cm³/s and 10^-33cm³/s, respectively. When these species bombard on the surface of polymers with high energy, some chemical reactions happens^[11-13].
    In this work, polyethylene (PE) was submitted to CO₂, N₂ and Air low temperature plasma and the treated polymer surfaces were studied with x-ray photoelectron spectroscopy (XPS).

2. EXPERIMENTAL
2.1 Materials
Polyethylene (PE) was  obtained from Yanshan Petrochemical Company, Beijing, China. It was washed prior to the surface modification experiments.
2.2 Plasma Reactor
A special reactor for the plasma treatment was used. The reactor consists of a columnar stainless steel chamber (210mm diameter, 150mm height) with a vacuum gauge (M713) and a vacuum system of a rotary pump (4L/s). In the stainless steel chamber, there is a sample stage positioned at 0 to 50mm from the copper electrodes for the rf power input. The vacuum system can depress a pressure in the reaction chamber to an order of 10Pa. The schematic diagram of the reactor is shown in Figure 1.

Fig. 1 Schematic representation of the plasma reactor.

2.3 Plasma Treatment
PE grains were mounted on a given sample stage in the reaction chamber. The sample stage was 20mm from the upper copper electrode. The reaction chamber was evacuated to approximate 50Pa. Afterward, CO₂ or N₂ whose flow rate was adjusted to 20cm³/min by a mass flow controller was introduced into the reaction chamber. The plasma treatments were performed at a given radio frequency (rf) power of 100W for a given time of 120s. In order to compare with other plasma treatments,  PS and PE were treated in the reaction chamber without other gases.
2.4 X-ray Photoelectron Spectra
XPS spectra of the surface of the treated PE grains were obtained on a Perkin-Elmer PHI1600ESCA System using a nonmonochromatic MgK_α X-ray source (1253.6 eV). The power was 250W and the background pressure in the analytical chamber was 2.8×10^-10Pa. A size of the X-ray spot was 2mm diameter, and a take-off angle of photoelectrons was 90 degrees with respect to the sample surface. The XPS spectra were referenced with respect to the 285.0eV carbon 1s level observed for hydrocarbon to eliminate charge effects.

3 RESULTS AND DISCUSSION
Fig. 2, 3 and 4 were C_1s, N_1s and O_1s spectra of PE measured by XPS, respectively. From the results, we found that there were some O groups on the surfaces after they were treated with the air, vacuum, CO₂ and N₂ plasmas. The quantities of O on the PE samples were 18.62%, 16.32%, 20.75% and 11.63%, respectively. According to the bonding energy of C_1s and N_1s, we know that the O groups were CO O and C O . There were two kinds of N groups, i.e., CO NH₂ and C NH . The introducing of some polar groups on the PE surface can increase the polarity of PE surface.
3.1 The influence of air plasma on PE surfaces
It can be seen form the XPS spectra (Fig. 5) that there are some O_1s spectra (no N_1s spectrum) without any plasma treatments. In air, the ability of O₂ to bond with polymer is stronger than that of N₂. After treated with air plasma, PE had both O and N spectrum (Table 1). Although  the reaction chamber was evacuated up to the pressure 50Pa, there was still air remained in it. In order to investigate the influence of the residual air on the polymer surfaces, we treated PE without importing any other gases. However, we surprisingly found that the content of N on the PE surface was higher than that of the polymers treated by the air plasma.

Table 1 Summary of surface analysis for plasma-modified PE

Fig. 4 O_1s Spectra of PE measured by XPS

Fig. 5 The influence of air and air plasma on PE surface
(a) Unmodified  (b) Air plasma-modified  (c) Ample air plasma-modified

3.2 The influence of CO₂ plasma on PE surfaces
Contrast the result of CO₂ plasma with the results of air plasma, we can find that the value of O_1s increased sharply and the value of N1s also got the max. From the XPS spectrum, it was found that there was a peak for the carbonyl group (C=O) at the 288Ev in the C_1s spectrum (Fig. 2) and there was also a diagnostic peak for the carbonyl group (C=O) at the 532Ev in the O_1s (Fig. 3) spectrum. It testified that there was some O that took part in the reactions. Paradoxically, there was a N_1s peak in the XPS spectrum, but, in fact, we did not introduce N₂ into the reaction chamber. The only reason for this phenomenon is that the residual N₂ took part in the reaction. There was also an interesting result that although there was less N₂ in the CO₂ plasma than in the Air plasma, the area of N_1s peak in the CO₂ plasma result is more than two times of that in the air plasma result (from 3.04% to 6.65%). The reason maybe that the CO₂ plasma induced the excitation of N₂ plasma. So the efficiency of excitation and reaction of N₂ plasma was accelerated.
3.3 The influence of N₂ plasma on PE surfaces
Compared with other plasmas, the N₂ plasma can introduce less N and O groups on the PE surface than other plasmas. In other words, the N₂ plasma had a poor ability to react with PE surface. It needs other plasmas to get a good effect.

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