Crystalline phases in ethylene copolymers studied by solid-state NMR and DSC

摘要

The structures and thermal properties of the multiple ordered phases in ethylene-octene and ethylene-butene copolymers have been studied using a combination of solid-state NMR and DSC. Three types of the ordered phases, namely the orthorhombic, monoclinic, and rotator (or ordered mobile phase), have been found to coexist in these two ethylene copolymers. Our experimental results demonstrate that the slow-spinning solid-state CP/MAS ~(13)C NMR provides a convenient method to discern the NMR signals of the three different ordered phases and to measure the ~(13)C chemical shift tensors of orthorhombic and monoclinic phases. The measurements of ~(13)C chemical shift tensors and magnetic relaxation times show that monoclinic and orthorhombic crystal phases have similar chemical shift anisotropy with 180° flip-flop segmental movement. The chemical shift anisotropy and segmental mobility in the rotator phase, on the other hand, are different from those in the orthorhombic and monoclinic phases. DSC results illustrate that a low-melting-point phase forms during room-temperature aging and melts at temperature slightly above the room temperature. The apparent correlation between the low-melting-point phase and the rotator structure is revealed by a combination of variable-temperature solid-state CP/MAS NMR spectra with a slow-spinning rate and DSC measurement. It is thus suggested that the rotator formation induced by room-temperature aging is a common phenomenon for the ethylene copolymers with different sizes of side groups.