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

Significance of the topic

Multilayer polymer films are ubiquitous in food and pharmaceutical packaging because layer stacking imparts distinct barrier, mechanical, thermal and optical properties. Rapid, chemically specific methods for identifying layer composition and thickness are essential for material development, failure analysis and quality control. Combining infrared (IR) and Raman microspectroscopy on a single platform enables complementary chemical identification and spatial mapping without repeatedly repositioning samples, while Raman depth-profiling can probe layer structure with minimal sample preparation.

Objectives and study overview

This application study evaluated the AIRsight infrared–Raman microscope for cross-sectional mapping and Raman depth-profiling of a multilayer packaging film. Goals were to (1) determine layer composition and thickness, (2) compare information content and spatial resolution of IR versus Raman mapping, and (3) demonstrate non-destructive depth-profiling as an alternative or complement to microtome cross sections.

Methodology

The work combined cross-sectional transmission IR mapping on a microtomed film section with Raman mapping of the same section and Raman depth-profiling on the intact film. Key experimental choices optimized spatial and spectral contrast: IR mapping used a 10 × 10 µm aperture and fine mapping steps; Raman mapping used 50× and 100× objectives with 785 nm excitation to access high lateral resolution and perform confocal depth scans. Chemical imaging used peak-height measures and spectral-similarity metrics to visualize component distribution.

Instrumentation used

• IRTracer-100 (FTIR spectrometer) with AIRsight infrared–Raman microscope platform enabling IR and Raman analysis without moving the sample.
• Detectors: T2SL for IR transmission; CCD for Raman.
• Microtome (HistoCore AUTOCUT R, Leica Microsystems) for preparing 10 µm cross sections.
• Raman optics: 785 nm excitation, objectives 50× (5 µm spot) and 100× (3 µm spot), confocal depth profiling capability.

Analysis conditions (selected)

• IR mapping: 8 cm⁻¹ resolution, 64 averaged scans, SqrTriangle apodization, 10 × 10 µm aperture, mapping area 100 × 150 µm.
• Raman mapping: exposure 1.0 s per point, 50×/100× objectives, laser spot diameters ~5 µm and 3 µm, mapping areas up to 140 × 150 µm, depth step 3 µm (depth resolution ~7.5 µm).

Main results and discussion

• Layer geometry and thickness: Camera-based distance measurements on the cross section identified four layers with thicknesses of approximately 14 µm, 100 µm, 14 µm and 14 µm (from outside to inside).
• IR cross-sectional mapping: Transmission IR chemical imaging identified the major polymeric constituents—outermost layer as polyethylene terephthalate (PET), middle layer as polypropylene (PP), and second and fourth layers as nylon. Boundary zones showed mixed spectra consistent with very thin interlayers or intermixing below the IR lateral sensitivity.
• Raman cross-sectional mapping: Raman mapping with 50× objective confirmed PET, nylon and PP layers and, critically, revealed the presence of very thin alkyd-resin adhesive layers at interfaces that IR could not clearly resolve. Raman peak-height mapping (1020–980 cm⁻¹ region) was used to image the alkyd distribution.
• Raman depth-profiling on intact film: Non-destructive confocal Raman depth scans (100×, 3 µm lateral spot, 3 µm depth steps) completed a large-area map (~140 × 150 µm) in ~20 minutes. Depth profiling detected an additional PET layer (a fifth layer) that was apparently lost during microtome sectioning, demonstrating an advantage of non-sectioned analysis. Alkyd resin features were detected at some interfaces but not uniformly; detection was limited by reduced axial resolution with increasing depth and by light scattering and signal attenuation.
• Complementarity: IR shows strengths for fluorescent or weakly scattering plastics (better signal for some polymers), whereas Raman provides higher lateral resolution and sensitivity to thin adhesive/interlayer materials. Combining both modalities yields a more complete and reliable laminate characterization.

Practical benefits and applications

• Single-platform IR and Raman measurements avoid sample relocation, improving spatial correlation between datasets and speeding analysis workflows.
• Raman depth-profiling reduces or eliminates the need for microtoming for many applications, saving time and avoiding artefacts introduced by sectioning.
• C hemical imaging permits visualization of component distributions across the film, aiding failure analysis, complaint investigation, new film design and routine QC of multilayer laminates.
• Higher lateral resolution of Raman mapping enables detection of thin adhesive or tie layers that may be invisible to IR mapping; conversely, IR mapping can outperform Raman when sample fluorescence is problematic.

Future trends and potential applications

• Improved confocal and axial resolution: advances in objective design, higher numerical aperture optics and adaptive optics could enhance depth resolution for Raman profiling and improve thin-layer detection.
• Wavelength selection and fluorescence suppression: use of longer-wavelength lasers (e.g., 1064 nm) or time-gated Raman could reduce fluorescence background and extend Raman applicability to highly fluorescent polymers.
• Enhanced data analysis: multivariate, machine-learning and hyperspectral unmixing approaches will increase automation, quantitative interpretation and discrimination of similar polymer chemistries and thin interlayers.
• SERS and surface-enhanced methods: for trace additives or adhesives, surface enhancement strategies may permit detection of components at very low concentrations or in extremely thin films.
• Integration into production QA: faster mapping and automated spectral classification could allow in-line or at-line monitoring of laminate structure during manufacturing.
• Correlative multimodal workflows: coupling IR/Raman maps with microscopy (optical/SEM) and mechanical or barrier testing will deepen structure–property correlations for film design.

Conclusions

The AIRsight infrared–Raman microscope effectively combined complementary spectroscopic modalities to characterize multilayer packaging film. IR transmission mapping provided robust identification for several polymer layers, while Raman mapping and confocal depth-profiling resolved thin adhesive interlayers and revealed an additional PET layer lost during sectioning. The ability to switch modalities on the same stage and to perform depth-profiling without destructive sample preparation accelerates multilayer analysis and supports development and QC workflows in packaging materials research.

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.