Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. II. Effects of stretching ratio and temperature

Proton spin–lattice, T₁, and spin–spin, T₂, relaxation times of uniaxially stretched polypropylene film were measured at 40°C using a wide line pulse spectrometer operating at 19.8 MHz. T_1l, the longer T₁, increases almost linearly with increasing stretching ratio, and T_2a, T₂ of the amorphous region, decreases gradually as the stretching ratio is increased. These results can be interpreted in terms of the increased constraints to molecular motion in the amorphous region. The fraction of the rigid protons in the sample, F_c, increases with increasing stretching ratio, while the crystallinity calculated from the density, X_d, does not change largely. The difference between F_c and X_d, therefore, increases as the stretching ratio is increased. This indicates that the physical structure of the highly stretched sample is far from the ideal two-phase model. The influence of the stretching temperature was also investigated. There are only slight increases in T_1l and in F_c for the samples stretched in a temperature range from 80°C to 150°C, whereas the considerable increase in T_2a occurs. The most notable change introduced at a high temperature stretching is the increase in the chain mobility in the amorphous region.