Films and coatings have important applications in various fields (coatings, coatings on medical devices, drug delivery, thin films for food protection etc.). Therefore, an efficient technique for the investigation of these materials is very important. The powerful tool, Confocal Raman Microscopy, allows non-destructive imaging of the chemical composition of heterogeneous components within a coating or film. Apart from providing the highest spatial resolution, the confocal setup also allows the acquisition of depth profiles that are perfectly suited to the three-dimensional characterization of polymer coatings and films without sample preparation.
WITec CRM - Combining Confocal Microscope with Raman Spectroscopy
The WITec Confocal Raman Microscope alpha300 integrates a high-transmission Raman spectroscopy system and a highly sensitive confocal microscope. Thanks to a resolution of 200 nm at each image pixel, a complete spectrum is typically acquired in just 10 to 100 ms. Only the computer memory limits the number of image points (spectra). A characteristic image consists of 10,000 (100 x 100) to 65 536 (256 x 256) spectra. By integrating over a specific Raman line or a region in all spectra, an image is generated from this multi-spectrum file. Various properties such as center of mass, peak-width, or peak position of certain Raman lines can be calculated from a single measurement, as all spectra are stored in memory. The focal plane can be moved in the z-direction when performing either xz scans or generating x-y image stacks in the z direction for depth profiling measurements.
Polymer Coating Used for this Investigation
In this experiment, the inner polymer coating of an orange juice container was examined with the alpha300 R by conducting an x-z scan with a scan range of 50 µm x 100 µm at 200 x 120 pixels (= 24 000 spectra) using a 100x objective (NA=1.25). A 532 nm Nd:Yag laser was used for excitation. The time taken for the acquisition of one spectrum was 50 ms.
Four distinct spectra can be observed within the acquired multi-spectrum file (see Figure 1 left). A specific chemical compound within the polymer film is represented by each spectrum. With the integrated software tools, the distribution of each compound can be visualized by analyzing all the acquired spectra. Shown in Figure 1 (right) is the resulting color-coded image, clearly showing that the coating contains 5 different layers although only 4 components are involved. Also, the software makes it possible to measure the distances within the image, indicating that all layers differ in thickness.
Figure 1. Raman spectra (left) and Raman image (right) of the inner coating of an orange juice container.
WITec CRM - Ideal Tool for Heterogeneous Layered Sample Analysis
This article has shown why the WITec Confocal Raman Microscope alpha300 R is a suitable tool for the analysis of heterogeneous layered samples on the sub-micrometer scale. As demonstrated in the example, different kinds of polymer layers in the film can be clearly resolved and matched with their associated Raman-Spectrum. In-depth information on the structure of multilayered polymer coatings or films can thus be achieved using the depth profiling capabilities of the Confocal Raman Microscope.
This information has been sourced, reviewed and adapted from materials provided by WITec GmbH.
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