10 Oct 2019

Oxygen Diffusion in Polymer Films | Spectroscopy Guides

Polymer films have an enormous number of uses ranging from gas separation, food packaging, protective layers, and membranes for oxidative degradation. There is also a large variety in types of polymer, so there is also a significant difference in how they react to the various factors, with some being better suited for high temperatures, or others being able to handle a greater amount of mechanical stress.

One of these factors is the presence of oxygen in the surrounding atmosphere and how permeable to this oxygen the polymer is. With the significant effect that oxygen can have on the state of a foodstuff, being able to determine how oxygen diffuses through polymer films is incredibly important.

food packaging

Polymers are used extensively in food packaging, particularly as films that protect the food, while still allowing it to be seen by the consumer. However, unlike in glass and metal packaging, these plastic films are permeable to various degrees to small molecules that may be present in the food in the form of gases and vapors. These low molecular weight compounds could be coming from the food as aromas, flavors, and additives, or they may be outside the film as atmospheric gases and water vapor. 

The rate of oxygen diffusion through a polymer is particularly important as if effects many of the processes that influence the flavor and final quality of food. For example, fruit will ripen if exposed to oxygen, but if that oxygen is removed and replaced with nitrogen, the fruit will remain at this unripe state1. Fruit that is picked in one country and then transported around the world would have to be picked a long time before it was ripe, or perish during the journey, but this method allows it to arrive in store ready condition.

|This is just one example of how oxygen affects food, and all types of food behave differently to changing oxygen partial pressure. The study of how permeable these films are to oxygen is therefore vital to both producers and sellers. This has resulted in a continuously changing field of study, with new techniques constantly being refined and developed. A few of the more prominent ones are outlined below. 

Early experiments on the rate of oxygen diffusion through a particular polymer used a pressed film sample and equipment to take either gravimetric (change in sample weight), barometric (change in ambient gas pressure), or volumetric (change in ambient gas volume) measurements, to detect any change in the rate of oxygen diffusion through the sample.
Modern techniques are more reliant on inherent properties of the material, analyzing parameters like thermal conductivity, and using colorimetric or ionization-based detectors to monitor any gases that may pass through the film.

A technique that has seen increasing use is that of polymer-supported luminescence-based oxygen sensors2. These sensors hold several advantages in that they do not consume any oxygen in order to measure, and they allow a high spatial resolution in all materials. They operate through “quenching”, where the quencher (oxygen) collides with the sensor in an excited state and energy is transferred. This excites the oxygen to a singlet state and the indicator molecule is unable to luminesce, and the measurable signal decreases. By measuring this signal intensity against oxygen partial pressure, a value for the diffusion rate may be obtained. 

A similar principle to the one above may also be applied to fluorescence3. In this case, a dyed polymer film is monitored using a fluorescence spectrometer, and any changes in the fluorescence of the film may be linked to oxygen quenching at its surface. This technique offers several advantages over the traditional polymer membrane methods First is that it uses a significantly smaller area of film to reduce the possibility of inhomogeneities and thickness variation in the film. Second, the parameter that defines the sensitivity can be chosen from 0 to infinity, and finally, it allows the diffusion coefficient to be measured in viscous polymer liquid.

Although there have been numerous changes in how the diffusion rate is measured, the typical form of the polymer has not. Specac provides a range of presses that allow for the production of polymer films in micron to millimeter thickness, in order to cover all the differing techniques in the determination of oxygen diffusion in polymer films.
With thermoplastic polymers forming at an array of temperatures, you should have access to equipment that covers as much of this temperature range as possible. Specac’s High-Temperature Film-Maker Kit is ideal for fabricating polymer thin films at temperatures of up to 400°C. Able to exert a pressure of up to 2 tons, films of the desired thickness can be produced simply and consistently4.

If space is a concern then there is also the Specac Mini-Film Maker, which can produce the same high-quality thin films but with a reduced footprint5. Perfectly suited to smaller laboratories it enables quick and easy fabrication of polymer and plastic materials for oxygen diffusion analysis, with pressing between ambient to 250°C. This allows films of a constant and reproducible thickness over the range 50 to 500 microns with a formed diameter of 15mm.

Browse our spectroscopy film-makers here

If you are working with oxygen diffusion in polymer films, or any measurements with plastic thin films, and would like to know more about Specac’s range of presses, please visit their website at www.specac.com.

References

  1. Miller, K. S. & Krochta, J. M. Oxygen and aroma barrier properties of edible films: A review. Trends Food Sci. Technol.8, 228–237 (1997).
  2. Pénicaud, C., Guilbert, S., Peyron, S., Gontard, N. & Guillard, V. Oxygen transfer in foods using oxygen luminescence sensors: Influence of oxygen partial pressure and food nature and composition. Food Chem.123, 1275–1281 (2010).
  3. Kneas, K. A. et al.Method for measuring oxygen diffusion coefficients of polymer films by luminescence quenching. Anal. Chem.74, 1111–1118 (2002).
  4. Specac. High Temperature Film Maker. Available at: https://www.specac.com/en/products/sample-prep/films/film-makers/high-temp.
  5. Specac. Mini-Film Maker | Polymer Film Maker. Available at: https://www.specac.com/en/products/sample-prep/films/film-makers/mini.