(649i) Sorption of Gaseous and Liquid Penetrants in Polyethylene Measured By Low Field NMR

Properties of final polymer materials and reaction rate in catalytic reactors (slurry and fluid reactors) depend, among other phenomena, on the sorption of organic solvents (gas or liquid) into the polymer particle during the manufacturing process. The dynamics of such a sorption and consequently polymer morphological changes are in our focus. Our results primarily improve quantitative understanding of thermodynamics in polyolefin with gas and liquid diluent systems and may contribute to optimization of production processes, thus, realize significant resource and energy savings for the manufacturer.

As a model system, we consider the sorption of gas ethylene (30 bar), propylene (11 bar) and liquid n-hexane into polyethylene. We chose these organic solvents to simulate manufacturing process conditions. Polyethylene contains two easily distinguishable phases: rigid and amorphous. Rigid phase is formed by a crystalline phase (30-70 wt.% of the sample) and an interphase (max 5 wt.% of the sample).

We confirm the broadly accepted fact that the penetrant sorbs only into the amorphous phase. However, our results show that the crystal phase content is not inert to changes. It is also influenced by the sorption. The crystallinity of sample decreases with the increasing pressure of organic solvent (and so with the increasing concentration of sorbed penetrant). We propose that the sorption process, even when induced by the low soluble ethylene, influences the crystallinity of PE by pulling the polymer chains out of lamellae stacks.

These changes are measured by a low-field nuclear magnetic resonance, which is also known as a time-domain NMR (TD-NMR). TD-NMR has a high potential for a broad area of applications. For example, it is already successfully employed in the quantitative analysis of water content in fat (Solid Fat content method, SFC) or water in crude oil. Moreover, the speed of its measurement meets the requirements for utilizing it as an online sensor. Therefore, we decided to test its capabilities for measuring the sorption dynamics. To observe the sorption process and morphological changes, we utilized Solid-Echo pulse sequence. The evaluation procedure was based on the modification of SFC method.

The changes of PE inner morphology and composition of phases during the sorption process in organic solvent environment influence the final properties of polymer particles and their thermodynamic behaviour. The understanding of these changes and their dependencies on different conditions is important for the future studies and for the optimization of manufacturing process.