“侧链偶极”聚酰亚胺薄膜电介质的分子设计与性能调控

High dielectric constant and low dissipation factor are difficult to obtain simultaneously, as well as dielectric property and heat resistance due to their mutual implication. To meet the requirements of dielectric properties and high-temperature reliability for film capacitor, polyimide dielectrics with side chain dipoles are designed and prepared by introducing cyano group (-CN, 3.9D) or sulfone group (-SO2-, 4.3D) with large dipole moment into the polymer side chain in this project. Specifically, high dielectric constant is obtained owing to the more sufficient orientation of dipoles in the side chain under the alternating electric field. Low dissipation factor is reached due to the free volume that facilitating dipoles’ low-friction orientation generated near the end of side chain. At the same time, the structures of the main chain is also tailored. High breakdown strength is maintained through the introduction of bridging group between the aromatic rings to break the large π-π conjugation. In addition, by regulating the content of aromatic, alicyclic and aliphatic structures, self-healing property is improved and heat resistance is maintained. The molecular simulation method is adopted to calculate the free volume and dipole density in polyimide dielectric. The mechanism of how dipole moment, dipole density and dipole orientation behavior function on dielectric constant, and dipole orientating restriction act on dissipation factor is revealed in detail. The correlation between molecular structure and property is also clarified. The results would provide significant theoretical basis for the design and preparation of advanced high-temperature polymer dielectrics.

为满足高温薄膜电容器对聚合物电介质的介电性能和温度可靠性的要求，解决高介电常数与低损耗因子难以兼得，介电性能与耐热性能互相牵制的问题，本项目从分子设计出发，将大偶极矩极性基团氰基（-CN，3.9D）或砜基（-SO2-，4.3D）引入聚合物侧链，制备“侧链偶极”聚酰亚胺薄膜电介质。侧链偶极在交变电场下能更充分地取向，提高介电常数；侧链链端附近产生的自由体积可减小偶极取向内摩擦，降低损耗因子。同时，调控聚酰亚胺主链结构，通过向芳环之间引入桥联基团破坏大规模π-π共轭，保持高击穿场强；通过调控脂环、脂肪和芳香结构含量，降低碳氢比，改善自愈性，并保持优良耐热性能。采用分子模拟手段计算聚酰亚胺电介质中的自由体积和偶极密度，深刻解析偶极矩大小、偶极密度及偶极取向行为对介电常数的作用机制，及偶极取向难易对损耗因子的影响规律，揭示分子结构对性能的内在决定关系，为耐高温聚合物电介质的设计和制备提供理论基础。

为满足电动汽车、电磁武器、深井钻探等应用领域中薄膜电容器对其中聚合物电介质的介电性能和温度可靠性的要求，本项目从分子设计出发，将大偶极矩极性基团（偶极）氰基（-CN，3.9德拜）或砜基（-SO2-，4.3德拜）引入侧链，制备“侧链偶极”聚酰亚胺电介质。具体研究内容为：（1）含极性侧链二胺单体的分子设计与合成；（2）“侧链偶极”聚酰亚胺电介质薄膜的制备；（3）“侧链偶极”聚酰亚胺电介质的分子结构与介电性能关系研究。由于聚合物侧链的极性基团在交变电场下能更充分地取向，有利于提高介电常数；芳环之间的桥联基团可破坏大规模π-π共轭，从而保持高击穿场强；同时，聚酰亚胺中的芳杂环结构赋予其优异的耐热性能。因此，获得的侧链偶极电介质材料中，最高介电常数为6.56，最高击穿场强高于500MV/m，玻璃化温度为162~208°C，具有优异的介电性能和耐热性。并且，基于密度泛函理论（DFT）和分子动力学（MD），通过计算分子极化率、偶极矩以及范德华体积，获得理论极化率密度和偶极矩密度，从介电本质层面剖析偶极性质对介电响应的作用机制。并且，研究发现，偶极矩密度与介电常数正相关。该理论规律可广泛推广至无定型极性聚合物，用于预测其介电常数，从而提高分子设计效率，缩短电介质材料研发周期。总之，通过分子设计并制备系列“侧链偶极”聚酰亚胺电介质，结合理论模拟手段，获得了分子结构对微观介电参数的影响，进而对介电响应和介电性能的作用机制，为新型耐高温聚合物电介质材料设计和制备提供理论基础。