Polymer materials, used by everyone and for everything, are very often treated on the extreme surface in order to give them chemical functionalities allowing them to meet the desired properties of use. Surface treatments have the advantage of providing additional characteristics without altering the volume properties of polymeric materials (films, textiles, etc.). Polypropylene, polyethylene and polyester are used in particular in the form of films from a few tens to a few hundreds of µm in thickness. These materials have a low surface energy, thus limiting their use for a large number of applications (printing, bonding, etc.).
Functionalization by atmospheric plasma is an innovative and "clean" process. It is considered energy efficient since the gas is not heated, very little effluent is produced and no solvent is used. The interactions between the polymer and the highly reactive species of the plasma gas will modify the surface chemistry by grafting new functionalities.
Atmospheric plasma is therefore a complementary solution to traditional processes, particularly with regard to the treatment of very large surfaces or materials that are not compatible with vacuum.
Since the vast majority of polymeric materials are electrically insulative, effective charge compensation is paramount in order to obtain high-resolution artifact-free spectra. In XPS, the surface of the sample is irradiated by X photons from a monochromatic source. Atoms of the first few nanometers emit photoelectrons with energies characteristic of each element as well as their chemical environment. The spectra obtained most commonly show binding energy (difference between X-ray energy and photoelectron kinetic energy) as a function of intensity (number of photoelectrons emitted).
One cm2 samples were cut from the center of a reference untreated PET (Poly Ethylene Terephthalate) film and a plasma treated PA-PECVD (Pressure Atmospheric - Plasma Enhanced Chemical Vapor Deposition) PET film. They are then attached to the stage and introduced into the analysis chamber. The XPS analysis was performed in grazing detection (angular analysis with a detection angle of 75°) in order to reduce the analysis depth to ~ 2 - 3 nm (instead of 10 nm in normal detection), thus increasing the sensitivity to the "thin" layer deposited by plasma treatment.
XPS data is collected with a monochromatic AlKα source. The survey and high-resolution spectra (C1s of carbon, O1s of oxygen and N1s of nitrogen) are shown in Figure 1 (before plasma) and Figure 2 (after plasma). The overflight spectrum makes it possible to determine the elementary chemical composition at the extreme surface of the film. The identification of carbon, oxygen or nitrogen functional groups grafted by the plasma requires the acquisition of spectra in High Resolution mode. Elemental analyzes show the detection of carbon and oxygen at the extreme surface of the untreated PET film. The measured O/C ratio (0.4) is consistent with the expected O/C stoichiometry for a PET reference [C10O4H8]n. After plasma treatment, nitrogen is detected at 2.2 atomic % and the measured O/C ratio (0.45) is higher than that of the untreated reference, reflecting surface oxidation. It is then possible to qualify and quantify the different functions present on the high resolution spectra. After plasma treatment, the transitions π → π (shake-up) are still observed on the C1s spectra of carbon and O1s of oxygen, indicating that the thickness of the plasma functionalization is < ~2-3 nm and/or n does not affect the aromatic structure of PET. Plasma leads to nitrogen grafting, mainly in the form of N-C=O amide functions (~ 60% of total nitrogen). In addition, N-C amine, N(CO)2 imide and C=O carbonyl functions are also detected and quantified.
In other works, the strength of XPS for the evaluation of the homogeneity of a treatment on substrates mimicking biological materials has been shown. Plasma opens the way to a wide spectrum of technological solutions for improving the barrier and water-repellent properties of technical products in the textile sector...