Oxygen doped polymers: an 1H NMR spin-lattice relaxation study

Selective adsorption of oxygen on aromatic rings acts as a strong relaxation contrast agent. The effect is maximal at rather low temperatures, in the 80-160 K range, where a well-defined minimum is observed. The positions of the minima and the relative value of the spin-lattice relaxation times 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 carried out, leading to a best fit treatment of relaxation data. The proton-proton dipolar term at high temperatures and the proton-oxygen scalar term at lower temperatures provide the major contribution to spin-lattice relaxation. From the full theoretical treatment, the best fit of experimental data on polymorphous polystyrenes permits the evaluation of a number of physicochemical parameters. The activation energy for the phenyl ring libration is obtained, which is different for each polymorphous form. Moreover, for each aromatic polymer, the maximal number of adsorbable oxygen molecules is obtained, giving a scale of polymers suitable to act as oxygen scavengers.