High resolution 1H solid state NMR studies of polyethyleneterephthalate

Abstract

Molecular motions and spatial properties of the solid polymer polyethyleneterephthalate have been investigated using high resolution ¹H solid state NMR techniques. The longitudinal spin relaxation time T₁ρ of protons (¹H) in the rotating frame was measured for a spin locking field ranging from 5 to 20 G. The decay of the ¹H magnetization indicated the existence of two distinct T_1ρ’s and their field dependence shows that they are associated with two mobile phases of the polymer. The ¹H magnetization also relaxes under the dipolar narrowed Carr–Purcell (DNCP) multipulse sequence with two dintinct T_1y relaxation times. The ratios T_1y’s and T_1ρ’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 individual T_1y relaxation of the resolved methylene and aromatic protons. These two species of protons were found to relax with the same T_1y’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 two T_1ρ’s and the two T_1y’s. The long T_1ρ corresponds to the short T_1y whereas the short T_1ρ corresponds to the long T_1y. Communication between the two spatial phases via spin diffusion was also observed in this experiment by monitoring the recovery of the ¹H magnitization associated with the short T_1ρ after it has been eliminated during the spin locking. The total ¹H magnetization is allowed to equilibrate in the laboratory frame for a variable time much shorter than T₁ after the spin locking field has been turned off. The spatial relationship between the two phases is discussed.