Solid-State ¹³C NMR Analyses for the Structure and Molecular Motion in the α Relaxation Temperature Region for Metallocene-Catalyzed Linear Low-Density Polyethylene

The structure and molecular motion of metallocene-catalyzed linear low-density polyethylenes (MLLDPE) have been investigated in the α relaxation temperature region by solid-state 13 C NMR spectroscopy. Fully relaxed dipolar decoupling (DD)/MAS 13 C NMR spectra are well resolved into the crystalline, 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 °C, while the chemical shift anisotropy (CSA) spectra stay almost unchanged from the line shape in the rigid state. This fact indicates, in good accord with the previous results, that the 180° flip motion around the molecular chain axis occurs above 60 °C in the crystalline region with the rate of about 10 5 Hz. Moreover, the segmental exchange between the crystalline and noncrystalline regions, which will be closely associated with the multistep forward and backward 180° flip motions, is clearly confirmed for MLLDPE with butyl branches at 80 °C by 2D 13 C exchange NMR spectroscopy. A simple one-dimensional random walk simulation is also performed to elucidate the chain diffusion process that allows the exchange of the crystalline and noncrystalline segments. A T 1C analysis for the crystalline component transferred within 1 s by the chain diffusion from the noncrystalline region is further made by a newly developed pulse sequence.