Surface analysis

The technique (also named ESCA = Electron Spectroscopy for Chemical Analysis) is based on the irradiating the sample with X-rays and detecting the emitted electrons and their energy distribution. Because of the electrons mean free path (ca. 2 nm) 95 % of the information come from the top 10 nm of the surface.

XPS can be used to quantify the surface elemental composition (except for hydrogen and helium) and provides information about binding partners of the atoms which allows to draw conclusion about the chemical structure of polymers.

For further information on x-ray photoelectron spectrometry please see here

Test liquids can be used to probe the surface properties of materials. They form a characteristic angle on the boundary between the solid, the liquid, and the vapor. In many cases valuable information can be obtained about physical chemical surface properties of materials by this technique.

Our Krüss DSA100 instrument is used for contact angle measurements with up to 4 test liquids. The surface free energy and their components are determined with a number of different methods including dipers-polar and acid-base models.

For further information on contact angle goniometry please see www.polymer-analysis.com/CAG

Most spectroscopic techniques are based on the comparison of the signal which passed the sample with a reference signal. If the difference between the two signals becomes small because the concentration of the species generating the signal alteration is low, the resulting signal becomes more and more covered by noise. In contrary, fluorescence spectroscopy detects an absolute signal, i.e. the spectrum is not generated by the comparison with a reference. This is the reason why fluorescence techniques can be extremely sensitive.  

Coupling fluorescence dyes selectively to certain functional groups at the surface of a polymer allows to correlate the fluorecence intensity with the concentration of the functional group. With this kind of derivatization the concentration of hydroxyl, carboxyl, carbonyl, and amino groups can be determined.

With state of the art equipment we are able to record the fluorescence spectrum of a femtomolar fluorescein solution. This value corresponds to a surface concentration in the order of 10^-16 mol/cm². In a practical experiment, this limit is shifted toward higher concentration mostly due to defraction of the exciting light in the sample and fluorescent contaminations. However, a realistic concentration limit of about 10^-12 mol/cm² is well in the sub-monolayer range.

For further details please see here.

This technique can only be used for surface analysis if the amount of the bulk material is small enough. Plasma deposited layers have been analyzed on potassium chloride pellets. Special thin film samples (some 10 nm thick) permit to analyze also surface alterations created by a low pressure plasma.

For further details please see here.

Plasma treated polymer surfaces are chemically very heterogeneous what renders the analysis of the chemical structure very complicated. The identification and determination of particular functional groups can be improved greatly by the use of specific reactions, so called derivatizations. This principle can be applied to a number of instrumetal techniques. In combination with XPS, a derivatization reaction introduces atoms which have not been on the analyzed surface before (see reaction schemes). With IR spectroscopy the derivatization can help to identify secondary structures. The specific coupling of long alkyl chains in combination with contact angle goniometry permits a very senstive detection of the coupling group.

Fluorescence spectroscopy is not surface sensitive. But for non fluorecent bulk materials the labelling of surface functional groups with fluorescent dyes can be used to quantify these groups.