Materials and Methods

A high pressure vessel from SITEC (Sieber Engineering AG, Maur, Switzerland) was used for the experiments. This was originally built for studying aroma compound migration and permeation (RICHTER, LANGOWSKI 2005). Inside the pressure vessel a polymer sample (ø = 0.7 cm) can be fixed with a retainer (Figure 4-1), so separating the pressure cell into two chambers. The upper chamber has a volume of 5.4 cm³, the bottom chamber a volume of 6.7 cm³. Both chambers can be filled and drained separately with different fluids (e.g. oxygen saturated water and water with reduced oxygen concentration). The temperature inside the vessel was measured with a thermocouple (Fe-CuNi Type J) and could be regulated from 20°C to 50°C with the help of a heating jacket mounted around the vessel. Both chambers were pressurised simultaneously by a common manual spindle pump. The maximum pressure was 2,500 bars and the pressure build-up and pressure release rates were around 2,000 bar/min. A pressure sensor made by EBM Brosa Messgeräte (Tettnang, Germany) was integrated into tubing next to the vessel. The pressure transmitting fluid was the sample water itself. In the bottom chamber a sensor spot was fixed on the inside of a sapphire window (thickness 1 cm) for optical measurement of the fluorescence quenching by the oxygen present in the cell. The sensor spot PSt3 is commercially available from PreSens - Precision Sensing GmbH (Regensburg, Germany). It operates within a specific measurement range (0 to 1,400 µmol/L dissolved oxygen) and has a resolution of 6%. The sensor spot is illuminated from the outside with blue light by an optical fibre connected to the Fibox 3 LCD Trace measurement system (PreSens - Precision Sensing GmbH Regensburg, Germany).

The oxygen measurement system was calibrated before starting the experiments and after every 100,000 data points. A two-point calibration was carried out at atmospheric pressure by first adding sodium sulphite (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) to the water to create an oxygen-free environment. The second calibration point was taken with 100% air-saturated water. At 23°C and 1,013 mbar pressure, air-saturated water contains 280 µmol oxygen per litre water (calculated using Equation 4-7, SANDER

2015).

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Figure 4-1 Schematic presentation of the pressure cell [according to RICHTER,LANGOWSKI (2005); Copyright Tobias Richter. Reproduced with permission of Der Weihenstephaner 44, 85 (2005).]

For the experiments, water with reduced oxygen concentration was prepared by boiling and subsequent cooling water to 23°C in hermetically sealed glass containers. This enabled oxygen concentrations of down to 40 µmol/L to be reached (approx. 20% of the oxygen saturation of water under atmospheric conditions). To produce water with different concentrations of dissolved oxygen, the cooled water was stirred under atmospheric conditions for varying periods of time. For the preparation of air-saturated water, the water was stirred for at least 30 minutes. The oxygen concentrations were validated before every experiment by take measurements with the sensor in the pressure vessel. All the experiments were performed with demineralised and boiled water to prevent microbiological contamination. Additionally, the system was cleaned between every experiment with RIMASAN®-AQ N-34524 (Tensid Chemie GmbH, Muggensturm, Germany) to avoid microbiological growth and hence oxygen attrition.

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Preliminary experiments were carried out to check the reproducibility and reliability of the sensor reaction. As the vessel has two sapphire windows it was possible to fix two sensor spots inside the vessel, one spot on each window. With two independently working measuring devices the reduction of the two phase angles with the applied pressure could be monitored independently. Data for testing for hysteresis and temperature dependence were collected at the same time as the determination of the oxygen response signal correction.

4.2.2 Method for determining the dependence of the oxygen response signal on the applied pressure

For these experiments, water with varying initial oxygen concentrations (40 µmol/L <

ciO2 < 260 µmol/L) was prepared and filled into the vessel. To check whether air bubbles remained in the system, the pressure was first increased to 20 bars and held for a few minutes (Figure 4-2 and Figure 4-3). At this low pressure the sensor works within standard parameters and additional dissolved oxygen immediately increases the displayed oxygen concentration. In such an eventuality, the experiment was interrupted and restarted. Otherwise the pressure was increased gradually up to a maximum pressure of 2,000 bars in steps of 200 bars (Figure 4-2). After every step the pressure was held for a few minutes until the response signal was stable. The pressure release was also performed in steps to check for possible pressure hysteresis. The temperature was kept constant in each experiment at respectively 23°C, 25°C, 27°C and 29°C. In order to obtain a correlation curve describing the raw data as real values, a mean oxygen concentration at every pressure step as a function of the initial oxygen concentration ciO2 was calculated.

Curve fitting was performed with the Curve Fitting Toolbox of MATLAB R2013b. A polynomial function was chosen as the regression model and the order of the polynomial function was selected, considering the sum of squared errors due to error (SSE) and the coefficient of determination.

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Figure 4-2 Stepwise increase in pressure leads to a simultaneous decrease in the displayed oxygen concentration cdO2.

4.2.3 Oxygen permeation experiments

For the permeation measurements, the bottom chamber was filled with water having a reduced concentration of oxygen. After mounting the polymer sample, the upper chamber was filled with air-saturated water. To overcome the low diffusion coefficient of oxygen in water, two magnetic stirrers (one in each chamber) ensured the homogeneous distribution of oxygen in the water during the permeation experiments. All the permeation experiments were carried out with low density polyethylene (PE-LD) film samples of 50 µm and 100 µm thickness made from additive free granulate (Lupolen 3020 K by LyondellBasell, Wesseling, Germany) on a Collin lab extrusion line at the Fraunhofer Institute for Process Engineering and Packaging IVV in Freising, Germany.

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