Biexponential C spin-lattice relaxation behavior of the crystalline region of ethylene copolymers and its origin

In this work, two series of ethylene-dimethylaminoethylmethacrylate (EDAM) and ethylene-methyl acrylate copolymers (EMA) with different comonomer content were studied by high-resolution solid-state ¹³C NMR spectroscopy. Biexponential ¹³C spin-lattice relaxation behaviors of the crystalline region were observed for all copolymer samples either melt-quenched or isothermally crystallized. The relative content of the shorter ¹³C T₁ component to that of the longer ¹³C T₁ component was found to increase with comonomer content. By employing a new pulse sequence which can be considered as a combination of Goldman-Shen's and Torchia's pulse sequences, it was demonstrated that the shorter ¹³C T₁ component is corresponding to the intermediate part of the crystalline region. The thickness of the intermediate part is estimated to be about 0.85 nm.