Measurement of the 13C spin–lattice relaxation time of the non-crystalline regions of semicrystalline polymers by a cp MAS-based method

The non-crystalline 13C spin–lattice relaxation times of atactic and isotactic polypropylene and those of an ethylene–1-octene copolymer of low crystallinity have been measured by classical inversion and saturation recovery methods as well as by a cp MAS-based pulse sequence. The latter is a saturation recovery-type sequence that involves cross-polarization. It samples preferentially the soft non-crystalline regions of semicrystalline polymers. The method is found to be useful in determining T1C of the amorphous regions of semicrystalline iPP at room temperature. It is found that the atactic PP molecule and the non-crystalline iPP regions have the same average segmental relaxation rate. The T1C of some of the carbons investigated was <T1H and the experimental recovery curves showed complex exponential behavior from the contribution of a transient nuclear Overhauser effect (NOE) to the 13C magnetization. Moreover, the experimental data were fitted with a double exponential function obtained from solving the Solomon equations. The fitting leads to T1C in very good agreement with the values obtained by classical inversion or saturation recovery sequences. The same T1C value was obtained with the cp-based sequence when transient NOEs were eliminated by saturation of the proton magnetization during the delay period. The hexyl branches of the ethylene copolymer lead to an increased average backbone C–C intermolecular distance in the non-crystalline regions compared to those of the linear polyethylene chain and, thus, to a higher backbone methylene segmental mobility.