We encounter plastic films several times a day. Walk through a grocery store and you’ll likely see dozens of examples of plastic packaging. Low density polyethylene (LDPE) film acts as a gas barrier, so you’ll likely find it in meat section packaging chicken or ground beef. Over in the chips and snacks aisle, you might find biodegradable bags made from aliphatic polyesters and aliphatic-aromatic co-polyesters. These rapidly break down, with some microbial help to, CO2 and water in a matter of weeks. The ubiquitous clear plastic bottles are generally made of thermoformed polyethylene terephthalate (PETE) and are 100% recyclable.
While many polymer films are a half of a mil or two thick–think about plastic shopping bags–thinner films have their special uses in various industrial and biomedical applications. They also are likely to be composed of a number of layers of different materials. While several techniques are used in characterizing the thickness or morphology of thin polymer laminate films, identifying the chemical components of each individual layer can be a tricky business.
Molecular spectroscopy is one of the go-to techniques for chemical identification of polymeric materials. Infrared spectroscopy is capable of seeing down to about 2 microns in thickness–with special accessories and some sample prep magic. Raman spectroscopy, however, is a wizard at looking through layers of disparate materials in a standard configuration. Identifying layers of polymer laminates with Raman microscopy can be done by confocal depth profiling and cross-section analysis. While depth profiling is advantageous in that it requires little to no sample preparation, cross-section analysis via line/area mapping offers superior spatial resolution, lending itself to the identification of micron to sub-micron layers in polymer laminates.
In our recently published Application Note, “Leveraging the lateral spatial resolution of a confocal Raman microscope to resolve micron to sub-micron layers in polymer laminates,” we identify seven layers of a polyethlene, polypropylene and polyvinyl alcohol thin film sandwich and resolve a thin polymer layer with a thickness (≈0.4 μm) that is close to the theoretical spatial resolution limit of Raman microscopy.