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 C-13 spin-lattice relaxation times of atactic and isotactic polypropylene and those of an ethylene-l-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 T-1C 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 T-1C of some of the carbons investigated was <T-1H and the experimental recovery curves showed complex exponential behavior from the contribution of a transient nuclear Overhauser effect (NOE) to the C-13 magnetization. Moreover, the experimental data were fitted with a double exponential function obtained from solving the Solomon equations. The fitting leads to T-1C in very good agreement with the values obtained by classical inversion or saturation recovery sequences. The same T-1C 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. (C) 2002 Elsevier Science Ltd. All rights reserved.