Studies On The Cross-Polarization Kinetics Of Polyethylene Crystals

Two models of cross-polarization kinetics were summarized on the basis of previous studies: classical I-S model and I-I*-S model. The two models fit for different kinds of samples. The most of samples are described by classical I-S model because of strong I-I homonuclear dipolar interaction and weak I-S heteronuclear dipolar interaction. The cross-polarization build-up curve increases monotonously in this case. In a minority of case, the oscillation occurs in the CP build-up curve, which is described by I-I*-S model. The reason is strong heteronuclear dipolar interaction is dominant, and weak homonuclear is subordinate. In that case the magnetization simply oscillates back and forth between I* and S spins at a rate determined by the dipolar coupling between them.The cross-polarization kinetics between ¹H and ¹³C of the crystalline regions of elongated polyethylene fiber was studied. It was found that, under moderate magic angle spinning, the CP build-up curves of both orthorhombic and monoclinic crystals of polyethylene can be described by I-I*-S model. For the fiber sample aligning with the MAS axis of the rotor, the CP build-up curves of both orthorhombic and monoclinic crystals exhibit oscillation, while those of the unoriented samples exhibit monotonic increase. The results demonstrate that, to a certain extent, both orthorhombic and monocline crystals are oriented along the direction of elongation. The degree of orientation of the monoclinic crystal is higher than that of the orthorhombic crystal. Thus, the measurement of the kinetics of cross-polarization can be employed as a new technique to determine the degree of orientation of polymer chain.