Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. I. Effects of crystallinity and atactic fraction

Spin–lattice relaxation time T₁ and spin–spin relaxation time T₂ were measured at 40°C on the isotactic polypropylene films of varying preparations and thermal history. T₁ increases with increasing crystallinity and two T₁'s appear for the samples annealed at elevated temperatures (>120°C). These variations in T₁ are well interpreted in terms of the spin diffusion and decoupling of the mobile protons with immobile ones. A free induction decay following a 90° pulse is the superposition of three different decay curves, one of which is exponential and other two are nonexponential. There is an increase in T_2a with increasing crystallinity, which is indicative of the enhancement of the chain mobility in the amorphous region. There are differences between the crystallinity calculated from density and the fraction of crystalline region, F_c, obtained by the NMR method, which can be explained by the existence of the microparacrystals and the stress imposed on the amorphous chains on rapid cooling. On the other hand, there is a gradual lowering in T₁ and a considerable increase in T_2a as an atactic fraction is increased. The increase in atactic fraction also results in a decrease in the amount of the isotactic amorphous chains in the amorphous region.