Solid-state C-13 NMR analyses for the structure and molecular motion in the alpha relaxation temperature region for metallocene-catalyzed linear low-density polyethylene

The structure and molecular motion of metallocene-catalyzed linear low-dens ity polyethylenes (MLLDPE) have been investigated in the alpha relaxation t emperature region by solid-state C-13 NMR spectroscopy, Fully relaxed dipol ar decoupling (DD)/MAS C-13 NMR spectra are well resolved into the crystall ine, crystalline-amorphous interfacial, and rubbery amorphous components at different temperatures above room temperature. Prominent broadening of the crystalline resonance line in the DD/MAS spectra is observed above 60 degr ees C, while the chemical shift anisotropy (CSA) spectra stay almost unchan ged from the line shape in the rigid state. This fact indicates, in good ac cord with the previous results, that the 180 degrees flip motion around the molecular chain axis occurs above 60 OC in the crystalline region with the rate of about 10(5) Hz. Moreover, the segmental exchange between the cryst alline and noncrystalline regions, which will be closely associated with th e multistep forward and backward 180 degrees flip motions, is clearly confi rmed for MLLDPE with butyl branches at 80 degrees C by 2D C-13 exchange NMR spectroscopy. A simple one-dimensional random walk simulation is also perf ormed to elucidate the chain diffusion process that allows the exchange of the crystalline and noncrystalline segments. A T-1C analysis for the crysta lline component transferred within 1 s by the chain diffusion from the nonc rystalline region is further made by a newly developed pulse sequence.