Atmospheric gas concentrations in polymers

Abstract

Magnetic susceptibility measurements have been performed to determine the concentration of molecular oxygen in these polymers. Both low density polyethylene and polystyrene have diamagnetic susceptibilities of the order of 10⁻⁶ at liquid helium temperatures; whereas, high density polyethylene has a paramagnetic susceptibility of the order of 10⁻⁶ at liquid helium temperatures. As oxygen was removed from the diamagnetic samples, the susceptibility became more diamagnetic by an amount com parable to the original susceptibility. On the other hand, the paramagnetic sample became less paramagnetic by an amount comparable to the original value of the susceptibility as oxygen was removed. In all three polymers, the change in susceptibility was attributed to the removal of approximately 10¹⁸ oxygen molecules per cubic centimeter of the sample. In addition, the more amorphous samples were seen to have the highest concentration of oxygen molecules.

Paramagnetic oxygen molecules, present as impurities, have been detected by pulsed nuclear magnetic resonance techniques, magnetic susceptibility measurements, and mass measurements in polystyrene, linear polyethylene, and branched polyethylene. The spin-lattice relaxation time (T₁) of the protons in the polymers changes from about one second to hundreds of seconds when oxygen is removed from the samples by gentle baking in a vacuum. The baked samples are subsequently exposed to air for certain intervals of time, and then T₁ is again determined. This technique has yielded information concerning the relative-rates at which molecular oxygen diffuses into the three polymers. In fact, molecular oxygen is seen to diffuse much more rapidly into the two polyethylene samples than into the polystyrene sample.

The mass measurements were done in such a way that in addition to determining the increase in mass due to oxygen and nitrogen, an increase in mass due solely to water vapor was detected and measured. The mass measurements were consistent with the pulsed nuclear magnetic resonance data and showed good agreement with the susceptibility measurements. In addition, considering the difference in amorphous content of the three samples investigated, the gain in weight due to water vapor is in general agreement with what is expected.