Polyolefin Resin Grade Verification via Attenuated Total Reflectance Spectroscopy
ATR-FTIR verifies polyolefin grades by measuring specific infrared absorbance ratios to quantify comonomer content, branching density, and copolymer structure.
Evanescent Wave
Meaning
Electromagnetic field that propagates along an optical interface and decays exponentially with distance from the boundary. An evanescent wave is generated when light undergoes total internal reflection inside a high-index crystal in contact with a polymer sample. This localized field penetrates a fraction of a micrometre into the resin, enabling surface-specific spectroscopy.
Penetration Depth
Light absorption occurs only within the shallow zone reached by the electromagnetic field. The penetration depth of the evanescent wave is governed by the angle of incidence, the excitation wavelength, and the refractive indices of both the crystal and the polymer. This depth typically ranges from zero point five to two micrometres.
For thin coatings or surface modifications, this restricted interaction volume avoids interference from the bulk substrate, ensuring that only the outermost skin of the moulded part is analyzed.
Spectral Analysis
Fourier transform infrared instruments rely on this localized phenomenon to collect clean spectra from highly absorbing materials. Contact with the sample allows the evanescent wave to transfer energy to specific molecular vibrations within the polymer. This transfer produces the attenuated signal that is recorded by the detector.
Without this close contact, no useful spectral information can be retrieved.
Boundary Failure
Poor contact at the sample interface prevents the localized field from interacting with the resin. When surface roughness or rigid geometry creates a gap, the evanescent wave decays entirely in the air before reaching the polymer surface. This results in extremely weak bands and high noise levels.
Soft elastomers conform easily, but hard glass-reinforced resins require high mechanical force to ensure proper coupling.