Evaluation by XPS of the contribution of an atmospheric plasma treatment on the extreme surface chemistry of a PET

Characterize and quantify with XPS the functionalization of the extreme surface of polymers by plasma (impacting the first nm)


 


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.).

In order to increase the surface energy of these polymers, specific treatments can be implemented (chemical or physical treatments: corona, or atmospheric plasma, 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.

Functionalization by atmospheric plasma makes it possible to overcome the vacuum and to process polymer films in motion (roll to roll process).


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. 

X-ray photoelectron spectroscopy (XPS) is perfectly suited to the surface characterization of polymeric materials because it provides the elementary and chemical composition (except for H and He) of the extreme surface over 3 to 10 nm. 

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).

TESCAN ANALYTICS has more than 20 years of expertise in the use of XPS for the surface analysis of all types of materials, insulators or conductors... With the latest generation instruments, our team of experts works with all industrial sectors.


 

Objective of the analysis


Evaluation by XPS of the contribution of a plasma treatment on the extreme surface chemistry of a PET polymer.
 

Sample preparation


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.

Results

XPS data is collected with a monochromatic AlKα source. The flyover 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.

Summary


In this example, it has been demonstrated that XPS is a suitable tool to determine and quantify the impact of plasma treatment on the extreme surface chemistry of polymer films.

XPS is the optimal method for validating functionalization performed by plasma treatment and monitoring its stability.

The possibility of carrying out the analysis in grazing detection proved to be very useful for highlighting the elementary and chemical signature of the thin layer (2 - 3 nm) of functionalization induced by the plasma.


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...

For more information on polymer analysis with XPS or our other techniques, ask us.

Other complementary techniques can be used to study the molecular chemical nature of plasma grafting (TOF-SIMS) as well as the morphological modifications induced at the extreme surface of the polymer (AFM) or the thickness of nanometric deposits (TEM).