**2. Commercial PI**

Kapton PI is an aromatic PI film that has been commercially available since the mid-1960s. Due to its continuous operating temperature of 300–350°C, it is widely used as a high-temperature wire and cable insulation material. At 25°C and 1 kHz, Kapton's *ε<sup>r</sup>* is 3.1, but it drops to 2.8 at 300°C. Despite its good thermal stability, Kapton PI cannot be applied to capacitor films because it is difficult to manufacture films with a thickness of <12 μm, and problems of carbonization during breakdown. Consequently, research is required to find other PIs with superior dielectric properties. SIXEF-44 is a fluorinated PI (from Hoechst Celanese) prepared from 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) and 2,2-bis(4-aminophenyl)hexafluoropropane (4,4<sup>0</sup> -6F diamine). This fluorinated PI has a *ε<sup>r</sup>* of 2.8 at 1 kHz,*Tg* of 323°C, and a change in ε<sup>r</sup> of less than 10% over a temperature range of �55 to 300°C. Other aromatic PIs include: perfluoropolyimide (PFPI; developed by TRW), prepared from the perfluoroisopropylidene diamine of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF) and pyromellitic dianhydride (PMDA); and Upilex-S (from ICI), prepared from 3,3<sup>0</sup> ,4,4<sup>0</sup> -biphenyltetracarboxylic dianhydride (BPDA) and p-phenylenediamine

*High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

**Figure 1.** *Heat-resistant PI polymers used as capacitor dielectrics [9].*

(p-PDA). The *Tg* of PFPI is >300°C, the ε<sup>r</sup> is 3.1 at 25°C, and decreases to 2.9 at 300°C; Upilex-S has a Tg of 355°C, ε<sup>r</sup> of 3.3 at 25°C and 1 kHz, and remains stable at 300°C. Most of these aromatic PIs affect practical applications due to processing difficulties. A polyetherimide (PEI) called Ultem is modified by the addition of flexible moieties such as ether bonds and alkyl groups in the polymer backbone; it is synthesized from the disodium salt of bisphenol A and 1,3-bis(4-nitrophthalimido)benzene. After development, the PEI film can attain a thickness of 5 μm by melt extrusion and stretching. In order to give PEI flexibility, ether bonds and alkyl groups are added, which results in the following changes (compared to PI): the *Tg* reduces to circa 215°C; the ε<sup>r</sup> increases to 25 at 200°C, and the εr over 100 Hz–10 kHz is 3.1 [9]. Some of the heat-resistant PI polymers used as capacitor dielectrics are shown in **Figure 1**.

## **3. Structure modification of PI**

The relationship between the dielectric properties of PI and molecular structure can be studied by changing the structure of the aromatic tetracarboxylic dianhydride and diamine monomers used to prepare PI. However, the preparation of the aromatic tetracarboxylic dianhydride is often complex and the yield is low while the synthetic method for phenyl-substituted aromatic diamine is relatively simple, diverse, and high yield. Consequently, modifying the structure of the aromatic diamine monomers has become the primary choice to improve the properties of PIs [10].

### **3.1 Introduction of bispyridine groups in PI chains**

Peng et al. [3] used 5,5<sup>0</sup> -bis[(4-amino)phenoxy]-2,2<sup>0</sup> -bipyridine (BPBPA) diamine monomer (as shown in **Figure 2**) and different dianhydrides [BPDA, PMDA, 3,3<sup>0</sup> ,4,4<sup>0</sup> -Benzophenonetetracarboxylic dianhydride (BTDA), 4,4<sup>0</sup> - Oxydiphthalic anhydride (OPDA)] to give a series of bispyridyl-containing PIs using a two-step synthesis. The bipyridyl unit enhanced the electronic polarization and coupling: The polarized PI had a ε<sup>r</sup> of ≤7.2, the dielectric loss was ≥0.04, and the energy density was ≤2.77 J cm�<sup>3</sup> . At the same time, it demonstrated good thermal and mechanical properties.

**Figure 2.** *Dipyridyl-containing diamine monomer [3].*

### **3.2 Introduction of sulfonyl group in PI backbone**

Tong et al. [4] studied the relationship between molecular structure and properties using a range of modified PIs. In this study, the εr was increased by introducing sulfonyl groups, the loss factor was reduced by introducing flexible bonds, and the Tg was increased by retaining the aromatic structure. The resulting sulfonyl-containing PI with different flexible connections gave high ε<sup>r</sup> (4.50–5.98), low loss coefficients (0.00298–0.00426), high breakdown strength (mostly at 500 MV m�<sup>1</sup> or more) and high heat resistance (Tg: 244–304°C) (**Figure 3**).

For the anhydride 3,3<sup>0</sup> ,4,4<sup>0</sup> -diphenylsulfonetetracarboxylic dianhydride (DSDA-mDS, each repeat unit contains two -SO2-, which has the highest dipole density), the ε<sup>r</sup> was not as high as expected but it can be seen that the two -SO2 units improved the stiffness of the overall chain, hindering the rotation of the dipole. Therefore, in addition to the dipole moment and dipole density, the "effective" dipole is another important factor affecting the value of the εr. Compared to ortho-symmetric OPDA-mDS, para-symmetric PI (OPDA-pDS) was more effective for PI with sulfonyl group (OPDA-pDS) in the diamine moiety (para-para bond). The symmetric structure and low free rotation energy barrier facilitate the alignment of the excimer: The ε<sup>r</sup> increased to 5.98; the dielectric properties were stable at 150°C; the discharge energy density and charge and discharge efficiency increased to 7.04 J cm�<sup>3</sup> and 91.3% at 500 MV m�<sup>1</sup> respectively [4].

**Figure 3.** *Synthesis of sulfo-containing PI [4].*

*High-Temperature Polyimide Dielectric Materials for Energy Storage DOI: http://dx.doi.org/10.5772/intechopen.92260*

**Figure 4.** *Diamine monomer containing three nitrile groups [13].*
