Oxygen Adsorption on aromatic polymers: 1n 1H NMR relaxation study

In aromatic polymers, oxygen selectively adsorbed on aromatic rings acts as a strong relaxation contrast agent. The effect is maximal at rather low temperatures, in the 80-160K range, where a well defined minimum can be observed. The position of the minimum and the relative value of the spin-lattice relaxation time are modulated by the chemical nature of the polymer, by its packing (polymorphism), by the crystalline vs. amorphous ratio and by the maximal amount of adsorbed oxygen. A full theoretical treatment of relaxation parameters has been done, leading to a best fit treatment of relaxation data. The proton-proton dipolar term at high temperature and the proton-oxygen scalar term at lower temperatures give the major contribution to spin-lattice relaxation. From the full theoretical equation, the best fit of experimental data on polymorphous polystyrenes leads to a large number of physico-chemical parameters. The activation energy for the phenyl ring libration was obtained, different for each polymorphous form. Moreover, for each aromatic polymer, the maximal number of adsorbable oxygen molecules can be obtained, giving an index for polymers suitable to act as oxygen scavengers.