In situ density determination of polyethylene in multilayer polymer films using Raman microscopy

Importance of the Topic

Polyethylene (PE) is one of the most produced plastics worldwide, with tunable density directly affecting mechanical, barrier, and processing properties. Accurate density measurement in multilayer films is critical for quality control, product development, and regulatory compliance in sectors such as packaging, pharmaceuticals, and consumer goods.

Objectives and Study Overview

This study aims to develop a non-destructive, in situ method for quantifying PE density within single-layer and multilayer films using confocal Raman microscopy combined with Partial Least Squares (PLS) regression. The approach targets rapid analysis without the labor-intensive sample preparation required by traditional ISO and ASTM methods.

Methodology and Instrumentation

A total of 25 PE samples (12 pellets, 13 films) spanning densities from 0.8598 to 0.9620 g/cm3 were analyzed. A Thermo Scientific DXR2 Raman Microscope equipped with a 532 nm laser (2 mW for pellets/films; 5 mW for depth profiling) and objectives of 10× (50 µm slit) and 50× (25 µm confocal pinhole) was used. Spectra were acquired for 30 s per point and processed in OMNIC software. TQ Analyst software normalized spectra using CH2 bending (1422–1452 cm–1) peak area and developed a 3-factor PLS model over 1400–1500 cm–1. Model validation employed five independent standards.

Main Results and Discussion

• Distinct spectral differences in CH2 stretching (2848 vs 2882 cm–1) and bending/twisting (1416 vs 1440 cm–1) enable discrimination of HDPE, LDPE, and LLDPE.
• The 3-factor PLS calibration achieved R²=0.9914, RMSEC=0.00360 g/cm3 and RMSEP=0.00432 g/cm3, with all samples within ±0.81% of actual density.
• Application to an unknown film predicted 0.9014 g/cm3 versus the actual 0.9008 g/cm3.
• Confocal depth profiling of a transdermal nicotine patch identified two distinct PE layers: Layer 1 at 0.9150 g/cm3 (LDPE/LLDPE) and Layer 2 at 0.9583 g/cm3 (HDPE), matching expected functional roles.

Benefits and Practical Applications

• In situ density determination eliminates tedious microtoming and solvent-based layer separation.
• Applicable to both pellet and film samples, including complex multilayer structures.
• Supports rapid, non-destructive quality control and reverse engineering workflows.

Future Trends and Potential Applications

• Integration with advanced chemometric algorithms and machine learning for broader polymer systems.
• Extension to other physical properties such as crystallinity and molecular orientation in situ.
• Potential use in real-time process monitoring and high-throughput screening in industrial settings.

Conclusion

Confocal Raman microscopy combined with PLS regression provides an accurate, efficient, and non-destructive method for quantifying PE density in multilayer films. The approach simplifies workflows, reduces sample preparation, and offers robust performance across diverse sample formats.

Reference

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