Characterization of a polyethylene terephthalate by XPS

Polymeric materials are ubiquitous in our daily lives: from food packaging to biomaterials, electronics, medical devices and automobiles... They are used in a wide range of industrial and consumer products for their physical properties.

The surface properties of these materials are fundamental in defining the performance of the objects they are made of and in meeting the very specific applications for which they are used.

Polypropylene and polyester are used as textiles in medical prostheses. It is therefore important that their surfaces have biocompatible and antibacterial properties in order to limit or even eliminate the adhesion of potential bacteria and therefore the risk of infection for the patient.

Each industry has its own prerogatives that require specific characteristics for polymers: modification of surface roughness, hardness, reactivity with the environment...

X-ray photoelectron spectroscopy (XPS) is well suited for surface characterization of polymeric materials because it provides the elemental and chemical composition (except for H and He) of the extreme surface over 3 to 10 nm, or even more in profile mode.
The vast majority of polymeric materials are insulating, so it is essential to use an efficient charge compensation when acquiring high resolution spectra with monochromatic X-rays.

The surface is irradiated with X-ray photons from a monochromatic source. The atoms of the sample emit electrons called photoelectrons with energies specific to each element and its environment. The spectra obtained show the binding energy (subtraction of the X-ray energy from the kinetic energy of the emitted electrons) as a function of the intensity.

BIOPHY RESEARCH has an expertise of more than 20 years in the use of XPS on all types of materials, insulating or conducting... With latest generation instruments, our team of experts works with all industrial sectors.

XPS data are collected with a monochromatic AlK_α source. The fly-through spectrum is shown in Figure 1.

Cleaning with the Argon Clusters source allows the total removal of Silicon and reveals only the presence of Carbon and Oxygen visible in the low resolution spectrum at ~285 eV and ~530 eV respectively, with an O/C atomic stoichiometry close to 0.4.

The energy resolution of the fly-through spectrum does not allow the determination of the chemical composition at the extreme surface of the film. The identification of carbon or oxygen functional groups requires the acquisition of spectra in High Resolution mode.

The high-resolution spectra of the C1s peaks of carbon and O1s of oxygen are shown in Figures 2 and 3, respectively. The relative distributions of the different chemical forms detected on the carbon and oxygen spectra are presented in Tables 1-2. These distributions are in perfect agreement with those expected for a polyethylene terephthalate (Tables 3-4) confirming the importance of using the argon Clusters source for the removal of surface contamination without deteriorating the chemistry of the polymer material.

Also note the detection of π → π transitions between 291 and 296 eV on the C1s spectrum of carbon and between 536 and 542 eV on the O1s spectrum of oxygen reflecting the presence of an aromatic structure.

In this example, it was shown that XPS is a useful tool to evaluate the surface elemental composition as well as the chemical nature of a polymer film.

XPS is the optimal method for evaluating the surface composition of polymers even when they are contaminated.

It was also highlighted the contribution of the Argon Clusters source for the cleaning of extreme surface contamination while preserving the chemical speciation of the material.

In other works, it was shown the strength of XPS for the evaluation of the effect of plasma treatments on the chemistry (functionalization, grafting) of extreme surface of polymers.

For more polymer analysis applications with XPS or our other techniques, see our non-exhaustive list by material here or ask us for information.

Other complementary techniques can be used to study the molecular chemical nature of the polymer (FTIR and TOF-SIMS) and its surface topology (AFM).