**3.3 Introducing nitrile groups in the PI backbone**

Due to the high polarity of nitrile groups, special classes of PIs containing nitrile units for piezoelectric and other dielectric applications have previously been studied [11]. Kakimoto et al. [12] reported that attaching a polar -CN side group to a PI could increase its εr. Treufeld et al. [11] found that adding a CN dipole to the PI main chain had two main effects: Firstly, because the -CN group is directly connected to the main chain in a 90° configuration, the motion is hindered and generates significant friction with randomly stacked adjacent chains, so dipole motion (such as wobble) is introduced into the PI sample; secondly, by adding the - CN group, the PI becomes more polar, easily contaminated by impurity ions, thereby improving ion mobility. Furthermore, it has been shown that the presence of three nitrile groups on the diamine unit is more effective in improving the ε<sup>r</sup> than one nitrile group. Wang et al. [13] studied and synthesized a series of PIs from a diamine synthesized with three nitrile groups (as shown in **Figure 4**) and four commercial dianhydride starting materials. All PIs showed a high Tg, thermal stability and excellent mechanical properties; the PI had a ε<sup>r</sup> of 4.7 resulting from the introduction of three highly polar nitrile groups.

## **3.4 PI grafted with phthalocyanine oligomer at amino terminal**

Unlike ordinary composite materials, graft polymers, with good properties, were synthesized by Chen et al. [7]. The copper phthalocyanine oligomer (o-CuPc,

**Figure 5.** *Synthetic copper phthalocyanine oligomer [7].*

**Figure 6.** *Schematic diagram of graft reaction [7].*

shown in **Figure 5**) is a semiconductor material with unique electrical properties (ε<sup>r</sup> > 103) and good thermal stability, used widely in organic optoelectronics, the dye industry, catalysis, electrochromism, and electroluminescence display and other fields. The design and synthesis resulted in a high-ε<sup>r</sup> all-organic polymer material, that is, a CuPc-PI homogeneous block copolymer was prepared (see **Figure 6**). The CuPc-PI also showed low dielectric loss, high breakdown strength, high Ue, high thermal stability, and good mechanical properties; its overall performance was higher than the direct use of o-CuPc/PI composites obtained using CuPc as the conductive filler [7].
