典型高分子材料的固体核磁共振研究

本论文通过固体核磁共振（NMR）谱及动力学参量的测量，并结合X-射线衍射技术和DSC测量等研究了两种典型高分子材料的相结构、链的运动以及相与相之间的关系.  乙烯-醋酸乙烯共聚物( EVA) 是最主要的乙烯共聚物之一. 研究发现，EVA的相组成非常复杂，共有5个不同的组分. 除了PE中所观察到的常规单斜晶相和刚性的正交晶相外，我们发现还存在第三个晶相分量－运动性较强的晶相（SOCP，可能是转动相）. 它不仅拥有自己的熔点，而且它的化学位移和分子运动性不同于刚性正交晶相（LOCP）. 另一方面，非晶相也由两种不同的分量组成：运动受限的各相异性的非晶界面相和高度可动的橡胶型的非晶相. 我们进一步详细研究了EVA中的晶区链动力学和非晶区的低温冻结行为. 实验发现，在正交晶相中，高分子链以180° flip-flop方式运动，同时伴随沿链方向的平移型跳跃运动，并引起正交晶相和非晶相之间的长程链扩散，通过NOE的测量证实了这种相间链扩散的存在，并进一步通过实验证实这种相间链扩散是一种受限扩散而不是自由扩散. 同时非晶相的两个组分具有不同的低温冻结行为：当温度低于-弛豫转变温度时，橡胶型的非晶相中的长程分子运动被冻结，但仍存在分子的局域运动；而界面非晶在低温时冻结成一种有序取向结构，并用质子自旋扩散实验证实该有序结构与正交晶相相邻近.  少量纳米级片层状粘土分散在聚合物中就可赋予材料许多优异的性能，我们用固体NMR技术对EVA/REC复合材料的结构和其中粘土的分散性质进行研究，发现上述复合材料中所形成的晶体类型不仅依赖于各组分的性质还依赖于所形成的复合材料的类型.  偏氟乙烯/三氟乙烯共聚物（P（VDF-TrFE））是最主要的铁电高聚物之一. 我们利用变温固体¹⁹F MAS NMR 谱及弛豫数据的测量详细研究了电子辐照对P(VDF-TrFE)共聚物的分子结构、构型、运动性以及相变等的影响. 发现，电子辐照不仅改变了分子链段的构型和运动性，同时也改变了局部分子化学结构. 电子辐照促使铁电相向顺电相(或者非晶相)转变，与此同时诱发了富含VDF和含-TrFE链段从全反式的构型到混合的反式-旁式构型的转变. 电子辐照加剧顺电区域中的分子运动而在高温熔融态中(>100 ℃），分子的运动反而受限.

Typical Polymers Studied by Solid-State NMR

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.