关键词: 固体NMR, 高分子材料, EVA, EVA/REC复合材料, P(VDF-TrFE)共聚物

Abstract: In this dissertation, various solid-state NMR spectra and NMR relaxation measurements, together with XRD and DSC, have been used to elucidate the structures and molecular dynamics in two typical polymers.
Poly (ethylene-co-vinyl acetate) (EVA) is one of the most important ethylene copolymers. It has been found that the phase structure of EVA is very complex and there are five physically distinct components in all present EVA samples. Besides the familiar immobile orthorhombic and monoclinic crystalline phases as detected in PE, a third crystalline phase-mobile crystalline phase SOCP (possibly the rotator phase) was detected by solid-state NMR and then confirmed by DSC. Such a third crystalline phase has not only the welldefined melting point of its own, but also has different molecular mobility and different chemical shift from the orthorhombic phase (LOCP). In addition, the amorphous phase also contains two components: an anisotropic interfacial amorphous phase and a melt-like amorphous phase. Then, we investigated in more detail the chain dynamics in crystalline phase and the low-temperature freezing behavior of amorphous phase in the EVA. And we found that the motion of CH₂ chain in rigid orthorhombic phase is the 180° filp-flop motion around its chain axis, accompanied by the simultaneous hopping motion along its axis, which causes the chain diffusion between orthorhombic and amorphous phases. This chain diffusion is evidenced by NOE measurement. Furthermore, we confirmed that the chain diffusion between two phases was restricted rather than free. The low-temperature freezing behaviors of the two amorphous components are different: below-relaxation temperature, although the long-range movement of chains in the melt-like amorphous phase is frozen, the local segmental motion of several CH₂ units still occurs, while the interfacial amorphous phase is completely frozen into an orientation-ordered interphase below-relaxation temperature. 
Compared with conventional polymers, the corresponding nanocomposites typically exhibit remarkable improvement in materials properties, while the filler content is just very low. We used solid-state NMR and XRD to characterize the structure and clay-dispersion quality of the EVA/REC composites and we found that the crystalline forms of said composites depend not only on the nature of the components used but also on the type of the corresponding composites.
Poly (vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) is one of the most important ferroelectric copolymers. We have investigated the effects of electron irradiation on molecular structure, conformation, dynamics and phase transition of P(VDF-TrFE) copolymers with 80 mol% VDF film by variabletemperature (VT) solid-state ¹⁹F NMR. It has been found that the electron irradiation changes not only the configuration and mobility of molecular chains but also the molecular chemical structure. Electron irradiation converts alltrans conformation into dynamically mixed trans-gauche conformation in both VDF-rich and TrFE-containing segments, accompanied by a simultaneous ferroelectric to paraelectric (or amorphous) transition. Moreover, electron irradiation enhances the molecular motion in paraelectric regions while the molecular motion in high-temperature amorphous melt (>100 ℃) is more constrained in those irradiated films.

Key words: solid-state NMR, macromolecular material, EVA, EVA/REC composite, P(VDF-TrFE) copolymer