The use of Raman confocal methods for depth analysis of multilayer polymer films

Polymer multilayer films play an increasingly important role. Such films are used, for example, for food protection, as packing or insulating material, etc.

Confocal Raman microscopy is a well suited tool for three-dimensional characterization of polymer films. The confocal setup provides the highest spatial resolution and has the ability to visualize any chemical composition within polymer films. The Raman image (distribution of components) can be generated by scanning the focused laser beam across the defined area. Spectral depth profile can be measured by moving focus position in a sample.

Principle of confocal detection is described in Fig.1. The pinhole aperture rejects the residual scattered rays originated from any out-of-focus points on the sample

We made a confocal Raman mapping in depth direction (XZ mapping) on a multilayer polymer film. A variety of Raman spectra measured in depth of the sample is shown in Fig.2. Each spectrum represents a specific chemical composition within the polymer film. Different polymer layers are clearly observed and shown as the color coded image in Fig.2. The investigated film contains three layers.

The image analysis software provides the ability to measure thickness of layers within an image. Thus, the thickness of the first layer can be estimated as 11 µm, and the second one as 7 µm.

Conclusion:

We have demonstrated capabilities of Raman Confocal Microscopy for in-depth analysis of polymers. The distribution of each chemical component in multilayer polymer films can be visualized with high spatial resolution.

Fig.1. The principle of confocal detection.

Fig.2. Depth profile (XZ Raman confocal imaging) of a multilayer polymer film. Different polymer layers are clearly observed: polypropylene (red), acrylic adhesive layer (green), polyethylene terephthalate (blue).