Test Methodology Using Raman Instrumention to Analyze Polymer Multilayer Films
Co-extrusion and lamination are polymer processing technologies capable of producing multilayer films with improved barrier and mechanical properties. Current analytical methods used to control the quality and composition of these films during and after production include Nuclear Magnetic Resonance (NMR). and differential scanning calorimetry (DSC). Raman spectroscopy is a valuable addition to these techniques because it provides definitive molecular information and it is highly sensitive to small changes in molecular backbone and branching configurations.
In this second post in the series, we’ll examine a test methodology using Raman instrumentation to analyze polymer multilayer films that demonstrates why Raman technology is ideal for polymer identification.
Cross-sectional Analysis
Most multi-layer samples were prepared for microscopic analysis by immobilizing the film in epoxy followed by carefully microtoming into micrometer-thin slices. This is necessary for an absorption technique such as infrared spectroscopy in order to prevent over-saturation of the bands. However since Raman is an emission technique, sample thickness is not a critical concern. The microtome slices can be much thicker, since only the cross-sectioned end will be analyzed. Samples can also be simply cross-sectioned, placed cut side up on a glass slide, and immobilized using a small piece of double-sided adhesive tape or putty. In this experiment, the microscope was focused on the sample edge and a line map constructed.
Depth Profile Analysis
The depth profile line maps were the easiest to collect. The film was simply immobilized on a slide and the depth profile map collected using a precision motorized stage with a Z-axis controller. The stage was moved in 1 μm steps and resolution of better than 2 μm was observed.
Analysis of Layer Composition and Thickness
Initial depth profile observations of the sample revealed the presence of six layers. Some of the layers were tens of microns thick, while others were much thinner. At the interfaces between the thinnest layers (1 – 3 microns) it was difficult to resolve the spectra of the individual layers. A line map was also constructed across a cross-section of the sample.
Layer composition and thickness was estimated from both the cross-section map and from the confocal depth profile of the sample. Using a Raman microscope, when samples are mapped using sufficiently small steps, layers of the order of 500 nm can be detected. The sample was placed cut end up on a reflective glass slide and analyzed using a line map across the sample. In order to use a line map to calculate the thicknesses of the constituent layers of a multi-layer sample, it is convenient to select a spectral feature that is unique to a specific layer and calculate a profile of this feature across the line map. This profile was constructed using the corrected peak area of a Raman band centered at 1727 cm-1 in a spectrum of polyester.
The profile shows that this band occurs only in the sixth layer of the sample. The thickness of the layer can be estimated by measuring the full width at half maximum of the profile of this band and was calculated to be 5 μm.
Similar results were obtained after performing a depth profile analysis of the sample and calculating the thickness of the polyester layer using the same peak at 1727 cm-1. A similar approach was used to calculate the thickness of the other five layers in the sample. The composition of the six layers was identified by using spectral search to compare the spectra from the layers with those in a High Resolution Raman Polymer Library. The calculated values match very well with the reported values and the results were comparable with those obtained using DSC, and took less time to obtain because Raman microscopy is a single technique analysis. The spectral matches could have been enhanced by building a specific library against which to search the data and using spectra collected from the specific polymer formulations in use.
Learn more about the Raman instrument used to perform this analysis, and be sure to click on the Resources tab to access more Raman Spectroscopy application notes.
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