Application Compendium - Agilent Technologies

To construct these materials, adhesive (tie) layers are
often required between two adjacent but chemical
incompatible layers. Typically these incompatibilities
are between materials with differing polarities, such as
nylon and polyethylene.
The adhesives typically have intermediate polarity or
contain functional groups with an affinity to both polar
and non-polar layers and hence act as good binding
material. Such adhesive layers in laminates can be very
thin, e.g., between 2 to 10 microns.
Polymer laminates can range in complexity and
thickness from those containing only two layers to
more than 10 layers (not including adhesive layers).
With total cross-sectional thicknesses ranging from
200 microns, polymer laminates are
used in a variety of packaging applications, which are
employed in industries such as food and
pharmaceuticals.
What are the analytical challenges/requirements
for polymer laminates?
With ever increasing manufacturing sophistication
enabling more complex and thinner laminate structures
to be produced, 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
are wide and varied and include a range of optical
microscopy techniques, thermal techniques (such as
differential scanning calorimetry) and various
spectroscopic techniques.
In particular, Fourier Transform Infrared (FTIR)
microscopy has proven most useful for the analysis of
polymer laminates. This has resulted from the core
application of FTIR spectroscopy in the identification
and characterization of polymers, combined with the
ability to obtain this information from small areas.
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When applied to polymer laminate analysis, FTIR
microscopy is typically performed in transmission mode
and requires that the total sampled thickness be within
a certain limit. For polymeric materials, this is typically
10–20 microns. Preparing thinly sliced polymer and
polymer laminate materials at a thickness of
10–20 microns presents some challenges. Typically,
dedicated (and often expensive) specialized cutting
devices such as microtomes are required. Even then,
the cut samples are often difficult to handle due to
curling or difficulties with static stick. To minimize
these effects, samples can be embedded in resin before
cutting and microtomed together within the resin
support (Figure1). This unfortunately adds another
material with a complex IR spectrum to the sample.
Once cut, if the sample is flat, it can be placed in a
sandwich between infrared transparent windows and
sampled in transmission mode. However, because of
internal reflections between the front and back surfaces
of the sample, ―fringing effects‖ can commonly be
observed. This results in a sinusoidal baseline during
such measurements.
With these issues and sampling preparation steps
aside, transmission FTIR 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 for transmission mode FTIR
microscopy are about 10–15 microns.
In comparison to transmission mode, the use of micro
attenuated total reflectance (ATR) as the mode of
analysis removes the requirement for samples to be a
certain thickness, so samples no longer need to be
thinly cut. However, as ATR requires intimate contact
with the samples, there are still some important sample
preparation requirements. Primarily, the sample must
be flat and smooth to ensure that there is full and
complete contact across the ATR measurement’s field
of view. Additionally, and of paramount importance to
the detection of ultrathin layers, micro ATR FTIR