Resolving Micron to Sub-Micron Layers in Polymer Laminates

A wide range of industries, from food to pharmaceuticals, use polymer laminates. When multiple polymeric layers of different physical properties, chemistry, and thickness are adhered together, desired chemical can be attained to suit various packaging applications. Moreover, continuous technological improvements and manufacturing sophistication enable the production of more complex and thinner laminate structures. As a result, there is always an ever-growing demand on laminate analysis for failure analysis, competitive reverse engineering, and product quality control.

The analytical devices for laminate analysis are differential scanning calorimetry (DSC), optical microscopy, Raman spectroscopy, and Fourier Transform infrared spectroscopy (FT-IR). In particular, confocal Raman microscopy provides various benefits. Raman spectroscopy is sensitive to both physical and chemical properties, and its unique selection rules produce a molecular fingerprint that is appropriate for material identification. Confocal Raman microscopy frequently uses short-wavelength visible and NIR (400–785 nm) lasers for sample excitation that renders improved sensitivity. This is because Raman signal intensity is inversely proportional to the 4^th power of the laser wavelength. In addition, the spatial resolutio n is inversely proportional to the laser wavelength. The resolution changes according to the wavelength, for example, the shorter the laser wavelength, the higher resolution.

In this article, two cases of polymer laminate analysis integrated with a Thermo Scientific DXR2 Raman Microscope are demonstrated. The implications of instrument configurations, such as magnification objective, laser wavelength, and pinhole size, on lateral spatial resolution are discussed in detail.

Materials and Methods

Two polymer laminate samples, A and B, were microtomed and cut into microfilms. These films were then placed on glass slides and flattened by adding pentane. Next, the glass slide was placed on the microscope stage insert for Raman mapping. A Thermo Scientific DXR2 Raman Microscope was employed for all analyses. A 532 nm laser was used, with the laser power set at 5 mW. A 25 μm confocal pinhole and a 100× objective were used. The Raman line map (27.4 μm) was gathered with a 0.2-μm step size and includes 138 spectra. The Raman area map (3 μm [X] x 20 μm [Y]) was gathered with 1 μm (X) and 0.2 μm (Y) step sizes and includes 584 spectra. Thermo Scientific OMNIC for Dispersive Raman software suite was used for data acquisition and processing.  

Results and Discussion

Identification of polymer laminate layers using Raman microscopy can be done by confocal depth profiling and cross-section analysis. Although depth profiling is beneficial in that it needs little to no sample preparation, cross-section analysis through line/area mapping provides superior spatial resolution that lends itself to the identification of micron to sub-micron layers in polymer laminates.

Image 1. (A) Video image of microfilm A showing the region where Raman line mapping was performed (red line); (B) Raman line map of microfilm A; (C) representative Raman spectra of each layer. PE = polyethylene, PP = polypropylene, PVA = poly vinyl alcohol; and (D) Raman correlation profile obtained using a PVA reference spectrum.

The video image of microfilm A and the location where the Raman line mapping was performed (red line) is illustrated in Figure 1A. Figure 1B shows the corresponding Raman line map. The Raman line map is shown in a 2-D contour plot, revealing the compositional changes across various layers (y-direction). The rainbow color scheme of the contour plot reflects the intensity of the Raman peaks, with blue denoting the lowest intensity and red the highest. Seven distinct layers were identified by library search based on the Raman spectral changes in the vertical direction: the layers 1, 3, 5 and 7 represent polyethylene (PE), the layers 2 and 6 represent polypropylene (PP), and the layer 4 represent polyvinyl alcohol (PVA). Figure 1C shows the representative Raman spectra for those layers. There was no cross contamination between the adjacent layers, which demonstrated a sufficient spatial resolution to differentiate adjacent layers. Based on the full width at half maximum (FWHM) method (Figure 1D), the thickness of the thinnest layer (Layer 4) was estimated to be 1.2 μm.

The 3-D Raman area images of microfilm B is shown in Figure 2A-2C. PP, PE, and PVA spectra were carefully selected from the map as the references to create the Raman area images with correlation profiling. The red color signifies a high correlation with the reference material and the blue color denotes low correlation. Just like microfilm A, seven layers are clearly identified in microfilm B as well: the layers 1, 3, 5 and 7 are PE, the layers 2 and 6 are PP, and the layer 4 at the middle is PVA. In order to estimate the thickness of layer 4, a line map was first extracted from the area map, which is delineated by the white, dashed line in Figure 2C. The extracted line was then exposed to correlation profiling using a PVA spectrum from the map as the reference, and the result is displayed in Figure 2D. The sharp peak at ~ 14.5 μm corresponds to the layer 4, with an estimated thickness of abuot 0.4 μm based on the FWHM of the profile peak.³ It should be noted that the aforementioned methodology largely depends on the spectral quality as well as the spectral differences between adjacent layers. Therefore, the layer thickness derived from the correlation profile should be regarded as an estimate. Techniques such as scanning electron microscopy (SEM) are required for precise measurement of layer thickness.

Image 2. 3-D Raman correlation images for microfilm B. (A)-(C): Raman correlation images for PE, PP, and PVA, respectively. (D) Raman correlation profile obtained using a PVA reference spectrum on a line extracted from the area map (white dashed line on C).

Diffraction-limited resolution in optical microscopy is empirically assessed by the Rayleigh criterion shown below, where λ denotes the laser wavelength, η is the refractive index of the immersion/mounting media, d is the Rayleigh criterion, and N.A. is the microscope objective numerical aperture.

As specified in the equations, both axial and lateral Rayleigh criteria are directly proportional to the wavelength, but inversely proportional to the objective numerical aperture. The axial Rayleigh criterion is also proportional to the refractive index of the material being investigated.

For samples that are optically denser than air, the axial resolution is generally 4-6 times lower than the lateral one. For a 100× objective (N.A. = 0.90) and a 532 nm laser, assuming the refraction index as 1.5 for the laminates, the theoretical axial resolution and lateral spatial resolution are around 2 μm and 0.4 μm, respectively. However, this resolution can be decreased by many factors such as scattering of the laser/Raman photons and interaction with interfaces in the sample. The pinhole size is another important consideration in instrument configuration. The confocal pinhole functions as a spatial filter, by enabling the Raman spectrometer to look into a smaller spatial domain than with a traditional configuration without the pinhole, attenuating the out-of-focus regions of the sample.² The current study involved the combination of a 100× objective with an N.A. of 0.9, a 532 nm laser, and a 25 μm confocal pinhole which enabled the resolution of a 0.4 μm-thick PVA layer, approaching the theoretical limit of the lateral spatial resolution.

Conclusions

In this article, the analysis of polymer laminates using area mapping and Raman line has been demonstrated. In either case, seven layers were detected. A properly configured Raman microscope made it possible to resolve a thin polymer layer of thickness (≈0.4 μm) that is very close to the theoretical limit of spatial resolution.

References and Further Reading

1. Guillory P., Deschaines T. Henson P, Materials Today, 2009, 12(4), 38-39.
2. Rzhevskii A, Ibrahim M, Ramirez J, Macisaac C., Considerations and Techniques for Optimizing Raman Spectral and Spatial Information, Thermo Scientific White Paper 52699, 2015.
3. Guillory P., Deschaines T., Henson P. Confocal Raman microscopy analysis of multilayer polymer films, Thermo Scientific Application Note 51718, 2008

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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