Proton Spin-Lattice Relaxation in Aromatic Polymers

The most striking requirement for the NMR imaging of an object is that this object should be made with components having different NMR parameters. In most cases these parameters are due to the presence of a fluid and to its mobility, thus measurable parameters are proton signal intensity contrasted in T1 or T2. From the glass transition temperature T(g), by decreasing the temperature, all nonaromatic polymers as well as any well degassed polymers, show a continuous increase of their proton spin-lattice relaxation, which at low temperature is usually larger than 10-20 sec. However, due to O2 molecules selectively adsorbed on aromatic rings, non-degassed aromatic polymers show a marked shortening of the proton spin-lattice relaxation. This effect is maximal at rather low temperature, where T1 can be shorter than 1 msec, and in many known cases shorter than 500 msec. Since the amount of sorbed-O2 is a function of the chemical nature of the polymer, the type of crystallinity (polymorphism of semi-crystalline polymers), the crystalline-amorphous ratio, and so on, a careful study of T1 relaxation as a function of the temperature can define optimal conditions for T1 contrast. Examples regarding polymorphism in syndiotactic polystyrene, butadiene-styrene block copolymers and blends, and poly(phenylene) oxide, will be discussed.