The use of confocal raman microspectroscopy in depth profiling polymer films

Investigator: Joanne Dixon
Supervisors: Dr Chris Sammon, Dr Steve Spells

Initial project objectives

• to prepare PVC and polystyrene films with surface specific cross-linking and/ or biologically active groups, and to characterise the depth profile of the modification using Raman microscopy in either confocal mode or by physical cross-sectioning
• to determine the refractive index of the modified and unmodified films using the MacCrone and/ or Becke line techniques, and to calculate the effect refractive index has on the confocal Raman profiles of those films when compared with the cross-sectional data

Background to project objectives

The surface properties of polymers are important in a wide range of contexts, including ad-hesion, gas transport and bio-compatibility. At the same time it is often vital to retain the desirable bulk properties of the polymer, such as tear resistance, modulus and tensile strength.

Figure 1

Surface specific chemical mod-ification of polymer films can be achieved by careful control of the reaction parameters, and confocal Raman microscopy, a powerful, non-destructive technique for characterising the modifier depth profile, can be used to enhance the development of surface modified polymers that will be much more versatile in their commercial applications.

Raman microscopy technique

Raman microscopy is a technique that provides useful information about the vibrations molecules undergo when a monochromatic light source (e.g. a laser) is shone onto the surface of a sample. The Raman microscope can be used in confocal mode by focusing the laser beam through a normal optical microscope to particular depths of the polymer, to give information on the depth, type and amount of modification present.

Figure 1 illustrates Raman spectra of a surface specific modification in a 100 micron film. It can be seen that the modification is restricted to about 20 microns of both of the films surfaces.

One complication of the confocal Raman technique is that the apparent depth of analysis, is not the actual depth due to differences in the refractive index of the sample as illustrated in Figure 2. This problem can be overcome by using equation 1 once the refractive index of the film has been measured.

Where   = the apparent focal point of the laser.

And NA (numerical aperture) requires the refractive index (n) to be calculated.

In collaboration with the Instituto de Cienncia y Tecnologia de Polimeros (Madrdid) we have also examined the extent of the surface modification of a polymeric membrane. The polymer (PVC) was modified according to the following reaction scheme

The individual Raman spectra are shown. As is indicated substitution of a chlorine with an aromatic amine introduces two new highly characteristic bands ~ 1600 wavenumbers.

The Raman depth profile clearly shows the extent of the 'surface' modification in a 100 µm thick film