Stretching very thin polymer films

Erik Brok and Martin Schmiele from KU are in a good mood as data is coming in.

Just before most of Europe went into coronavirus-shutdown, scientists from University of Copenhagen and Tetra Pak travelled to the DESY synchrotron in Hamburg to perform experiments on thin polyethylene films used in Tetra Pak packages. The primary objective was to measure the nanostructure of the films directly and in real time while the films were undergoing mechanical deformation.

In a recently completed LINX project, similar films were measured during stretching, using the small-angle X-ray scattering (SAXS) laboratory instrument at the Niels Bohr Institute, by placing dog bone shaped polymer films in a tensile testing apparatus that can be integrated with the SAXS measurements. In this setup, mechanical data is collected simultaneously with the information about the nanostructure from SAXS.

In the measurements in Copenhagen, long measurement times were necessary because of the extremely weak scattering of the films that are only about 20 micrometers thick. This meant that the time resolution of the experiments was on the order of one hour, which made real time stretching or relaxation phenomena impossible to study. With the high flux available at P03 MINAXS beamline at the DESY synchrotron, it was possible to improve the time resolution from one hour to perhaps a second, enabling real time studies of structural changes during mechanical deformation.

A polyethylene film in the tensile testing apparatus. The apparatus records the stress and strain as the film is stretched. In an X-ray experiment, the X-ray beam comes through the small hole in the middle of the metallic plate. Scattering data is collected every second. Click to see a video of the film being stretched until it ruptures.

The experiments at DESY were challenging, and it was an exhausted but happy crew that exited after around 40 hours of non-stop measurements. The experiments were far from a standard setup at the beam line. Despite this, data were obtained for a large series of Tetra Pak’s samples produced with several polymers and measured under various stretching conditions. The next step will be to analyze the large amount of data that was collected to determine the key structural parameters and to combine this with the mechanical information obtained at the same time. The ultimate goal is to get a thorough understanding of the process induced polymer morphology and in the long-term input the structural data into simulation tools at Tetra Pak to predict the properties of materials. The knowledge from scattering (nanoscale) and the input in the modeling (micrometer scale) are approaching each other, and in the future, information obtained from SAXS measurements could be valuable input for the modeling tools. Ultimately this information could be a useful part of predicting material behavior in industrial processes, such as those in a Tetra Pak filling machine. For now, the scientists from University of Copenhagen will concentrate on analyzing the large amount of obtained data from their home offices. Stay tuned for the results!

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