The micro attenuated total reflectance (ATR) chemical imaging of polymers, in particular polymer laminates, typically requires
significant pressure to ensure good sample-to-ATR crystal contact. For thin cross-sectioned materials, ensuring structural
rigidity against this pressure requires significant sample preparation, such as resin embedding, cutting, and polishing. Such
procedures are tedious, require overnight resin curing, and carry the added risk of cross-contamination. Presented here is
a novel method of ultralow-pressure micro ATR Fourier transform infrared (FT-IR) chemical imaging that removes the need for
any structural support and allows samples to be measured "as-is" (no embedding) with direct contact with the ATR crystal.
Polymer laminates are film structures consisting of two or more layers adhered together to make a structure. The polymeric
materials forming these laminates have varying thickness (from a few micrometers to >100 Ám), which can influence a variety
of properties, including chemical, mechanical, and barrier (for example, impervious to oxygen or moisture) properties. To
construct these materials, adhesive (tie) layers are often required between two adjacent and chemically incompatible layers.
Typically, these incompatibilities are between materials with differing polarities, such as nylon and polyethylene. The adhesives
usually have intermediate polarity or contain functional groups with an affinity to both polar and nonpolar sides and hence
act as good binding material. Such adhesive layers can be very thin, ranging from ~2 Ám to ~10 Ám. Polymer laminates can range
in complexity and thickness from only two layers to well over 10 layers (not including adhesive layers), with total cross-sectional
thicknesses ranging from <50 Ám to >200 Ám. Polymer laminates can be used in a variety of applications ranging from food packaging
to pharmaceutical packaging.
Figure 1: An example of a polymer film, held by clips embedded in a polished resin block.
With ever-increasing manufacturing sophistication enabling the production of more complex and thinner laminate structures,
the analytical challenges to ensure good product quality control, troubleshooting, or the reverse engineering of competitive
products are also increasing in complexity.
The analytical tools available to analyze such laminates include a range of optical microscopy techniques, thermal techniques
(such as differential scanning calorimetry), and various spectroscopic techniques.
In particular, Fourier transform infrared (FT-IR) microscopy has proven most useful for the analysis of polymer laminates.
This suitability resulted from the core application of FT-IR spectroscopy for the identification and characterization of polymers
in general, combined with the ability to obtain this information from small areas.
When applied to polymer laminate analysis, FT-IR microscopy is typically performed in transmission mode and requires that
the total sampled thickness be within a certain maximum thickness limit, which is usually 10–20 Ám for polymeric materials.
To prepare thinly sliced polymer and polymer laminate materials at <15–20 Ám presents some challenges. Often, dedicated (and
often expensive) specialized cutting devices are required, such as microtomes. Even then, the cut samples are often difficult
to handle because of curling or difficulties with static stick. To minimize these effects, samples can be embedded in resin
and microtomed together within the resin support before cutting. This, unfortunately, adds another material with a complex
IR spectrum to the sample. After the sample is cut, if it is flat, it can be placed in a sandwich between IR-transparent windows
and sampled in transmission mode. Even still, because of sample front and backside inter-reflections, "fringing effects" can
commonly be observed, which present themselves as a sinusoidal baseline.
With these issues and sampling preparation steps aside, transmission FT-IR microscopy is a relatively simple technique to
obtain spectra from small areas. It does however suffer from one major limitation — spatial resolution is relatively poor,
especially when compared to optical microscopy techniques. Typical spatial resolution limits are about 10–15 Ám.