High resolution1H solid state NMR studies of polyethyleneterephthalate

摘要

Molecular motions and spatial properties of the solid polymer polyethyleneterephthalate have been investigated using high resolution1H solid state NMR techniques. The longitudinal spin relaxation timeT1rgr; of protons (1H) in the rotating frame was measured for a spin locking field ranging from 5 to 20 G. The decay of the1H magnetization indicated the existence of two distinctT1rgr;rsquo;s and their field dependence shows that they are associated with two mobile phases of the polymer. The1H magnetization also relaxes under the dipolar narrowed Carrndash;Purcell (DNCP) multipulse sequence with two dintinctT1yrelaxation times. The ratiosT1yrsquo;s andT1rgr;rsquo;s deviate significantly from the expected theoretical values. The combined experiment with magic angle spinning and the DNCP sequence followed by homonuclear dipolar decoupling reveals the individualT1yrelaxation of the resolved methylene and aromatic protons. These two species of protons were found to relax with the sameT1yrsquo;s, thus implying that spin diffusion must have taken place under the homonuclear dipolar decoupling multipulse. The qualitative description of spin diffusion under homonuclear decoupling is given. The combined experiment with spin locking and the DNCP sequence yields the correspondence between the twoT1rgr;rsquo;s and the twoT1yrsquo;s. The longT1rgr;corresponds to the shortT1ywhereas the shortT1rgr;corresponds to the longT1y. Communication between the two spatial phases via spin diffusion was also observed in this experiment by monitoring the recovery of the1H magnitization associated with the shortT1rgr;after it has been eliminated during the spin locking. The total1H magnetization is allowed to equilibrate in the laboratory frame for a variable time much shorter thanT1after the spin locking field has been turned off. The spatial relationship between the two phases is discussed.