Resolving micron-sized layers in multilayer films with Raman microscopy by cross-section analysis and confocal depth profiling

 

 

Multilayer polymer films, composed of different materials, are used in a variety of industrial applications. The analysis of these multilayers is important to support the production, to identify defect and perform failure analysis, to understand delamination problem due to adhesion issue (lack or non-uniform distribution of the adhesive) and for reverse engineering investigation. Here, we use Raman microscopy by cross-section analysis and confocal depth profiling to investigate the chemical composition of two multilayers films. It is also shown how Layers, the new LabSpec6 software app from HORIBA for cross-section and confocal analysis, can efficiently provide such information, allowing a detailed identification of the chemical composition and spatial distribution of each component within a multilayer structure.

Multilayer polymer films, produced using co-extrusion and lamination processes, exhibit characteristics and properties not achievable by their individual constituent monolayers. These composite materials are widely used in several industrial applications, including consumer product and food packaging, as they are strong and flexible thus able to preserve their contents. Each layer of the complex structure provides a specific functionality to the full film, acting as oxygen, moisture, dust, microbes, or light barrier. Typically, conventional polymers like ethylene vinyl alcohol (EVOH), ethylene vynil acetate (EVA), polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET) and polyamide (PA) are employed to produce multilayer structures. Since the design of such structures has become more complex and the manufacturing specifications have become tighter, there is an increasing need of new hardware and software solutions that can easily and straightforwardly characterize these multilayer films. In this respect, the solution should provide both chemical and thickness information and moreover it should consider that nowadays the new multilayer structure has an overall thickness that is decreasing (few tens of microns) and that the number of layers is increasing (10 to 15 layers) with respect to the past.

 

Confocal Raman microscopy is the perfect technique to probe multilayers polymer film and the combination of Raman microscopy together with our customized polymer vice (Figure 1) and the “Layers” software app of LabSpec6 is the perfect solution. Raman microscopy combines the advantages of Raman spectroscopy allowing chemical identification of the constituents with those of optical imaging (i.e., optical microscopy) showing a high spatial resolution image of the films and of the different layers. By using Raman microscopy, the layer identification (chemical nature and thickness) of multilayer polymer film can be made by cross-section analysis or by confocal depth profiling. The advantage of confocal measurement is that it requires no or minimal sample preparation for samples that cannot be cross-sectioned and allows defect analysis without the need to bring the defect to the surface by sectioning the sample, which can lead to loss of defects. Cross-section Raman mapping, on the other hand, allows the best lateral resolution ensuring the analysis of layers down to sub-micron thickness.

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