1H magnetic relaxation times in isotactic polypropylene crystallized at high temperature. Effect of molecular weight

The temperature dependence of ¹H spin-lattice, T₁(H), and spin-spin, T₂(H), relaxation times of isotactic polypropylene with relatively low molecular weight, M?_w = 1.95 × 10⁵, and ultra-high molecular weight, M?_w = 1.78 × 10⁶, crystallized at high temperature (155° C) for a long time (200 h), was measured with a broad line pulse spectrometer. T₁(H) for the ultra-high molecular weight sample is shorter than that for the low molecular weight sample, while T₂(H) for the ultra-high molecular weight sample is longer than that for the low molecular weight sample. From the analysis of free induction decay (FID), three components of T₂(H), i.e. T_2c(H), T_2m(H) and T_2a(H), were obtained. These relaxation times are associated with the crystalline, intermediate and amorphous regions, respectively. Here the intermediate regions are the regions in which chain molecules have intermediate mobility between that of crystalline and amorphous regions. A decrease in signal intensity of methylene, methine and methyl carbons was measured as a function of delay time following a 90° pulse before cross-polarization. From the slope of a log (intensity) versus t² plot a relaxation time associated with rigid regions of proton nuclei, T_2r(H), was obtained, where the rigid regions are the regions in which cross-polarization occurs easily. The value of T_2r(H) lies between that of T_2c(H) and T_2m(H), indicating that the intermediate regions act as the rigid regions so far as cross-polarization is concerned.