Converting Magazine

Eldridge M. Mount

Measuring treatment, Part 3
January 25, 2008

We have been discussing how to measure treatment and so far have discussed the simplest and some more sophisticated methods all targeted at measuring the wetability or surface energy of the surface. But in reality this is not what the treatment has done to the substrate surface; it is just the outcome of the treatment process, whichever one is used, flame, corona or plasma. So what is it that treatment really does? It oxidizes the surface, replacing the carbon hydrogen bond (C-H) with various carbon oxygen bonds, C-OH the alcohol bond, C=O the aldehyde or carbonyl bond and HO-C=0, the acid bond. The acid bond indicates the breaking of the carbon chain indicating overtreatment. What the oxygen does is to change the polarity of the surface. How much oxygen as well as the chemistry of the oxygen determines the polarity and hence the wetability and the strength of the surface. We will speak more of the importance of the surface chemistry in later postings. Now, we are interested in determining the surface chemistry and the surface oxygen concentration.

But how do we know what the surface chemistry and total surface oxygen concentration is? This can be determined by a special chemical analysis called X-Ray Photon Spectroscopy (XPS), or as it was originally termed, electron spectroscopy for chemical analysis (ESCA). This is a special analysis that is very surface-sensitive. In this method, a sample is placed in a high-vacuum chamber and bombarded with monochromatic X-rays. The X-rays interact with the uppermost atoms of the surface and eject electrons from the innermost electron shells surrounding the nucleus. Because the polymers are principally composed of carbon, the XPS spectra we will be interested in is that of the carbon and oxygen atoms.

The atoms' chemical environments, i.e. the nature of the chemical bonds comprising the molecules, determine the binding energy required to remove the electron from the surface atom. By carefully measuring the kinetic energy of the ejected electrons, the atoms of the surface may be determined from the binding energy, and the relative concentrations of the atoms may also be determined. Electrons can escape only from the uppermost layer of the sample making this a very surface-sensitive method, (upper 50 nm of sample). In its simplest form, the spectra peaks represent the binding energy of the atoms, but without further detailed analysis no chemical functionality can be determined. For instance, the various carbon-oxygen binding energies are very close together and in low-resolution spectra appear as a single peak. For polymers the C 1s electron spectrum is used, and for treated surfaces ratioed with the O 1s electron to determine the molecular concentrations of oxygen in the treated surface. 

Surface Chemical Composition: As described above, the XPS spectrum contains a single peak for carbon-oxygen species on the surface, which is not resolved in terms of the contributions of individual surface-chemical species. However, it is possible to obtain a calculated value of the surface composition by estimating the relative contributions of each chemical group and performing a curve fit to reproduce the actual peak shape. This approach, while not exact, represents a practical approach to characterizing the treated surface and makes use of model compound studies. It has been widely used in literature studies of polymer-metal surface interactions. It is not possible to differentiate between alcohol and ether groups with this method, which is why the results can be considered speculative[i]

The chemical functionality of a treated surface may also be determined from direct chemical derivitization and analysis of the surface. This is a complex and time-consuming approach, but classical chemical analysis technique where specific reagents are reacted with a surface to form unique chemical groups, which may then be analyzed by several methods such as XPS and other spectroscopic methods. Whie making its own set of assumptions about relative reactivity, etc., in combination with XPS, it allows the determination of alcohol, acid, ketone and aldehyde groups directly. By varying the reactant chemicals it is also possible to determine the amount of enolizable carbonyl groups and by a difference the amount of ether linkages. 

In Part 4, I will show the relationship between the surface oxygen concentration and treatment levels.

Posted by Eldridge M. Mount on January 25, 2008 | Comments (0)