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ISSN:1369 7021 © Elsevier Ltd 2009
APRIL 2009 | VOLUME 12 | NUMBER 4
38
Analysis of multi-layer
polymer films
Polymer multi-layer films are used in a variety of industries. In
food packaging, for example, polymer laminates are used not only
to protect the food, but also to retain aroma and flavors, and to
extend shelf life. Multi-layer films are produced using co-extrusion
and lamination techniques. Some of the problems that can occur
during film manufacture include the introduction of defect
particles and separation of the layers. Current analytical methods
used to examine the materials during and after production include
NMR, DSC and FT-IR. Vibrational spectroscopy is a valuable
addition to these techniques because it provides definitive
molecular information. Raman spectroscopy is complementary to
FT-IR and offers advantages that include higher spatial resolution
and easier sample preparation.
Dispersive Raman spectroscopy offers many advantages over
other techniques such as FT-Raman. In comparison with commercial
FT-Raman microscopes which have a typical resolution less than 5 μm,
dispersive Raman spectroscopy uses visible (400 – 785 nm) lasers for
sample excitation which permits higher spatial resolution (better than
one micron). In addition, Raman emission is proportional to 1/λ4,
offering much greater sensitivity. Raman spectroscopy is sensitive
to both chemical and physical properties and its unique selection
rules generate a molecular fingerprint that is well suited to material
identification.
Experimental
For this particular application, the Thermo Scientific Nicolet™
Almega™ XR dispersive Raman spectrometer was used to analyze the
layers of a polymer multi-layer film. The Thermo Scientific OMNIC™
software suite provided the analytical tools used to estimate film
thickness and composition.
Confocal analysis
The confocal design of most Raman spectrometers makes it possible to
target individual layers in a multi-layered sample. The pinhole aperture
at the entrance to the spectrograph allows only the Raman scatter
from the focal point of the objective to reach the detector (Fig. 1).
By changing the position of the microscope stage in the vertical
direction, confocal analysis can be used for depth profiling and can
permit the analysis of individual layers of a sample while requiring little
or no sample preparation. This technique is very effective for rapid
identification of sample layers.
Visible microscopy
The Raman microscope is a powerful research microscope with
advanced viewing capabilities that include brightfield and darkfield
illumination, visual polarization and differential interference contrast
(DIC). DIC is useful for observing multi-layer films as it improves
Polymer multi-layer films are used in a variety of industries. It is
important both to the manufacturers of polymer films and to the
industries using these films that the quality and composition be strictly
controlled. The confocal analysis and high spatial resolution of Raman
microscopy make this technique ideal for identifying the source and
identity of defects and inclusions in polymer films.
Paulette Guillory, Tim Deschaines and Pat Henson
Thermo Fisher Scientific, Madison, WI USA
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Analysis of multi-layer polymer films APPLICATION
APRIL 2009 | VOLUME 12 | NUMBER 4
39
the contrast between different layers and helps to define the layer
boundaries. By rotating a Nomarski prism in the DIC optics, the
changes in the light interference result in pseudo three-dimensional
images, giving rise to differing regions of color in the sample. In a
polymer multi-layer material, this allows the user to selectively analyze
different layers in a cross-section of the sample.
Sample preparation
Cross-sectional analysis
Most multi-layer samples are prepared for microscopic analysis by
immobilizing the film in epoxy followed by carefully microtoming into
micrometer-thin slices. This is necessary for an absorption technique
such as infrared spectroscopy, in order to prevent over-saturation of
the bands. However since Raman is an emission technique, sample
thickness is not a critical concern. The microtome slices can be much
thicker, since only the cross-sectioned end will be analyzed. Samples
can also be simply cross-sectioned, placed cut side up on a glass slide,
and immobilized using a small piece of double-sided adhesive tape or
putty. For this application the microscope was focused on the sample
edge and a line map constructed.
Depth profile analysis
The depth profile line maps were the easiest to collect. The film was
simply immobilized on a slide and the depth profile map collected
using a precision motorized stage with a Z-axis controller. The stage
was moved in 1 μm steps and resolution of better than 2 μm was
observed.
Results
Initial depth profile observations of the sample revealed the presence of
six layers. Some of the layers were tens of microns thick, while others
were much thinner. At the interfaces between the thinnest layers
(1 – 3 microns) it was difficult to resolve the spectra of the individual
layers. A line map was also constructed across a cross-section of the
sample.
Analysis of layer composition and thickness
Layer composition and thickness can be estimated from both the
cross-section map and from the confocal depth profile of the sample.
When samples are mapped using sufficiently small steps, layers of the
order of 500 nm can be detected. The sample was placed cut end up
on a reflective glass slide and analyzed using a line map across the
sample.
In order to use a line map to calculate the thicknesses of the
constituent layers of a multi-layer sample, it is convenient to select a
spectral feature that is unique to a specific layer and calculate a profile
of this feature across the line map.
The composition of the six layers was identified by using spectral
search to compare the spectra from the layers with those in the
Thermo Scientific high resolution Raman polymer library. The
calculated results are shown in Table 1 and are compared with data
showing the actual thickness and composition of the sample.
The calculated values match very well with the reported values and
the results were comparable with those obtained using differential
scanning calorimetry (DSC), but took less time to obtain because
Raman microscopy is a single technique analysis. The spectral matches
could have been enhanced by building a specific library against which
to search the data, using spectra collected from the specific polymer
formulations in use.
Conclusion
Dispersive Raman microscopy is an excellent technique for the analysis
of multi-thin-layer polymer materials. The spatial resolution of
dispersive Raman spectrometers enables the analysis and detection of
layers less than one micron in thickness. The confocal optical design
allows for quick identification of the layers, and the excellent spatial
resolution in the x, y dimensions permits a more rigorous examination
of the material by cross-sectioning. Although not demonstrated in this
article, dispersive Raman microscopy is also ideal for the analysis of
defects and intrusions in the film.
Experimental Results Reported
Layer Thickness Composition Thickness Composition
1 1.5 mils (40 μm) Polyethylene 1.61 mil (40.9 μm)
LLDPE, linear
low density
polyethylene
2
0.1 mils (3 μm)
Polyethylene/
vinyl acetate
copolymer
0.08 mil (2.03 μm) Polyethylene 1
3 0.2 mils (7 μm) Vinyl OH 0.27 mil (6.86 μm)
Ethylene vinyl
alcohol with ~48
mole % ethylene
4 0.7 mils (20 μm) Nylon
0.79 mil (20.1 μm)
Nylon 6 with a
small amount of
nylon 6,6
5 0.1 mils (3 μm) Polyethylene 0.09 mil (2.29 μm)
Ethylene alpha-
olefin copolymer
6 0.2 mils (5 μm) Polyester 0.14 mil (3.56 μm) Polyester
Table 1: Multi-layer polymer sample: thickness and
composition of the layers – experimental and reported
results
Fig. 1 Confocal analysis. The pinhole aperture at the focal plane of the
microscope permits only the Raman scatter originating from the focal point of
the objective to fall on the detector. (a) the detector collects Raman scatter
from the top sample layer; (b) after raising the sampling stage, the detector
now collects Raman scatter from the middle sample layer; (c) the detector
collects Raman scatter from the bottom layer.
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