Cross-Sectional and Depth-Profiling Analysis of Multilayer Films Using the AIRsight Infrared Raman Microscope

Cross-Sectional and Depth-Profiling Analysis of Multilayer Films Using the AIRsight Infrared Raman Microscope — Summary

Significance of the topic

Multilayer polymer films are widely used in food and pharmaceutical packaging to provide combinations of mechanical strength, heat resistance, light shielding and barrier properties (oxygen/moisture). Accurate identification of layer composition, thickness and interlayer adhesive materials is essential for product development, failure analysis and quality control. Combining complementary spectroscopic techniques that provide both molecular specificity and spatial resolution accelerates characterization and reduces sample preparation time.

Study objectives and overview

This application study demonstrates how the AIRsight infrared Raman microscope—an integrated instrument that enables infrared (IR) and Raman spectroscopy on the same sample area without moving the specimen—can be used to:

• Perform cross-sectional mapping of multilayer packaging films by IR transmission and Raman spectroscopy.
• Measure layer thickness from microscope images.
• Perform Raman depth-profiling (through-thickness analysis) without microtomy to detect thin layers and adhesives.
• Combine IR and Raman data to build a robust laminate model and identify thin interlayers not readily observed by a single technique.

Methodology and measurement conditions

Two complementary measurement strategies were applied: cross-sectional mapping on microtomed film sections and non-destructive Raman depth-profiling on intact film. Key aspects of the analytical approach were:

• Cross-sectional preparation: microtome sections ~10 µm thick were produced to expose the film stack for transmission IR mapping and high-resolution Raman mapping.
• IR mapping: transmission FTIR mapping with a 10 × 10 µm aperture, 8 cm-1 resolution, 64 averaged scans and 100 × 150 µm mapping area. Spectral similarity mapping (color gradient from high to low similarity) was used to visualize material distribution.
• Raman mapping (cross-section): a 50× objective with 785 nm excitation, spot diameter ~5 µm, CCD detection. Mapping area ~90 × 150 µm with 5 µm vertical × 10 µm horizontal step size to achieve higher lateral resolution than IR.
• Raman depth-profiling: a 100× objective with a ~3 µm laser spot, depth steps of 3 µm and lateral step size 10 µm; reported depth resolution ~7.5 µm. A large area (140 × 150 µm) was profiled in approximately 20 minutes.

Instrumentation

The study used Shimadzu instrumentation and ancillary sample-preparation tools. Principal items and settings included:

• AIRsight infrared Raman microscope (integrated IR and Raman on a single stage).
• IRTracer-100 Fourier transform infrared spectrometer (T2SL detector) for IR transmission mapping.
• 785 nm laser excitation for Raman; CCD detector; objectives 50× and 100×.
• Microtome (HistoCore AUTOCUT R, Leica Microsystems) for preparing ~10 µm cross-sections.
• Typical mapping apertures and step sizes: IR aperture 10 × 10 µm; Raman spot diameters ~5 µm (50×) and ~3 µm (100×); mapping areas and step sizes as described above.

Main results and discussion

Combined IR and Raman analysis revealed a laminated film composed primarily of PET, nylon and polypropylene (PP), with thin adhesive interlayers identified as alkyd resin by Raman:

• Layer structure (from cross-section measurements and image-based thickness): a four-layer sandwich was initially observed with nominal thicknesses of ~14 µm (outer PET), ~100 µm (core nylon), and two additional 14 µm layers (PP and nylon) in the sequence PET / nylon / PP / nylon. IR chemical maps assigned PET, nylon and PP reliably; boundary regions showed mixed spectra consistent with very thin additional layers.
• Raman cross-sectional mapping provided higher lateral resolution and detected an alkyd-resin adhesive present at the boundaries between layers. Raman peak intensity in the 1020–980 cm−1 region was used to map the alkyd presence.
• Raman depth-profiling on the intact film detected an additional PET layer (a fifth layer) that was apparently removed or lost during microtome sectioning, demonstrating the non-destructive depth-profiling advantage. Depth profiling also detected alkyd-resin signals between certain layer interfaces but not consistently across all interfaces—likely due to the limited axial (depth) resolution, lateral resolution dominance, and reduced signal quality at depth due to scattering and fluorescence.
• IR spectroscopy was advantageous for bulk polymer identification and for materials that exhibit fluorescence under Raman excitation; Raman afforded higher spatial resolution and sensitivity to thin adhesives and interlayers.

Benefits and practical applications

The integrated AIRsight workflow provides multiple operational benefits:

• On-stage switching between IR and Raman avoids sample repositioning and improves co-registration of spectral maps.
• Raman depth-profiling eliminates the need for destructive cross-section preparation for many analyses, saving time and reducing artefacts introduced by microtomy.
• Combining IR and Raman data increases chemical identification confidence—IR handles fluorescent/prone-to-fluorescence polymers well, while Raman detects thin adhesive layers and provides higher spatial resolution.
• Chemical imaging (spectral similarity, peak-height mapping, multivariate methods) gives spatially resolved composition maps that are directly useful for materials development, QC, and failure analysis in packaging and barrier film R&D.

Future trends and potential applications

Key directions and opportunities arising from this work include:

• Improved axial resolution for Raman depth profiling via confocal enhancements, objective/immersion optics, or multi-photon approaches to better resolve ultrathin interlayers at depth.
• Advanced multivariate and hyperspectral fusion techniques that combine IR and Raman datasets to improve discrimination of chemically similar polymers and thin adhesives.
• Automation and high-throughput mapping protocols for routine QC of multilayer films in production environments.
• Integration with other modalities (e.g., SEM, X-ray microtomography) for correlative analysis when morphology and composition must be jointly interpreted.

Conclusion

The AIRsight infrared Raman microscope enables complementary, co-registered IR and Raman mapping of multilayer packaging films and supports non-destructive Raman depth-profiling. This combined approach accelerates layer identification and thickness assessment, reveals thin adhesive interlayers that may be missed by IR alone, and reduces sample preparation time when depth-profiling is used. The ability to rapidly generate chemical images and to switch between modalities on a single stage makes the system well suited for R&D and quality control of complex laminate films.

References

1. Belle, J. M.; Stokes, D. L.; Vo-Dinh, T. Direct characterization of the phthalic acid isomers in mixtures using surface-enhanced Raman scattering. Analytical Chemistry 1990, 62, 1349.
2. Shimadzu Corporation. Multilayer Film Analysis Using the AIRsight Infrared Raman Microscope, Application News No. 01-00465; First Edition April 2026.