13C magnetic relaxation times in isotactic polypropylene

Abstract

13C spin-lattice relaxation time, T 1, 13C and 1H spin-lattice relaxation times in the rotating frame, T 1ρ( 13 C) and T 1ρ( 1 H) , and the dipolar dephasing behaviour of methylene, methine and methyl carbons of annealed(monoclinic) and quenched(smectic) isotactic polypropylene films were measured using the CP/MAS method. Two T 1s were observed for methylene and methine carbons in the annealed and quenched samples but one T 1 for methyl carbons in both samples. T 1s for methylene and methine carbons in the annealed sample are longer than those in the quenched sample, whereas T 1 for methyl carbons is nearly the same for annealed and quenched samples. There is, however, a small but a definite difference between T 1 of upfield and downfield peaks of methyl carbons in the annealed sample: T 1 for the downfield peak is ca 15–20% shorter than that for the upfield peak. The carbon spin-lattice relaxation time in the rotating frame, T 1ρ( 13 C) , shows rather complicated manner; for the quenched sample, two T 1ρ( 13 C) s were obtained for all carbons but, for the annealed material, one T 1ρ( 13 C) was obtained for methine carbons and the downfield peaks of methylene and methyl carbons and two T 1ρ( 13 C) s were obtained for the upfield peaks of methylene and methyl carbons. For the proton spin-lattice relaxation time in the rotating frame, T 1ρ( 1 H) , two components were observed for all carbons of both samples. There is no large difference in the dipolar dephasing behaviour for annealed and quenched samples and the signal intensity decays as exp[-( t/ T′ 2) 2] for methylene and methyl carbons but as exp(- t/ T′ 2) for methyl carbons.