Influence of gas penetrans on polyethylene crystallinity observed by low field NMR

Polyethylene (PE) has a broad range of applications requiring rather different grades of used material differing also by their inner structure. PE inner structure contains two easily distinguishable phases: crystalline and amorphous. In this paper, we present a simple method of measurement of crystallinity (crystal content in sample) using low-field nuclear magnetic resonance. Moreover, we measured the crystallinity under different pressures of organic penetrant in order to observe sorption influence on inner PE structure. We confirm the broadly accepted fact that the penetrant sorbs into the amorphous phase. However, our results, measured at 35 degrees C, show that also the crystal phase is influenced by the sorption and with increasing pressure of organic penetrant (and so with increasing penetrated amount) the crystallinity of the sample decreases. We performed polymer-gas equilibrium measurements in ethylene and propylene environment to simulate catalytic polymerization conditions. We observed decrease in the crystallinity up to 9.5% at 30 bar ethylene pressure and 6.2% decrease at 11 bar propylene pressure. We propose, that the sorption process, even when induced by low soluble ethylene, decreases the crystallinity of the PE by various mechanisms.