**1. Introduction**

Electrical insulations are the key components for electric motors, which are used in space crafts and electric trains. Polyimide (PI) films as an insulating material are used in such motors, which are commercially available in single- and multicoated forms. Over the past few years, global market has shown a great interest in the application of nanodielectrics, especially in the field of electrical insulating materials. Various research results have claimed that polymer nanocomposite materials can improve dielectric properties for electrical insulation applications [1–4]. The key role played by the nanoparticle dispersion and interface region are essential

parts of these improvements. Further conditions to advances are the size and type of nanoparticles properly chosen and distributed into the polymer matrix. Initially, it seemed like magic that everything is possible by using nanodielectrics, which later proved wrong after understanding the exact working principles of polymerbased nanocomposites, though several questions still need to be solved [5]. This has motivated the author to explore in this field further and find those principles by using experimental and simulation work on improving the dielectric properties of polyimide-based nanocomposite.

This chapter begins with a description of the physical properties of polyimide and its derived nanoparticles. Afterward, a detailed synthesis process of polyimide nanocomposite single and multilayer films is outlined leading to the synthesis process optimization. Polyimide nanocomposites are the leading component in the advancement of electric motors and generator's insulating materials. However, nanoparticle dispersion is the primary concern to improve polymer nanocomposite dielectric properties. In this work, polyimide-based nanocomposite single- and multilayer films are synthesized and characterized in detail. The preparation of polyimide nanocomposite is a complex process with many variables involved. Therefore, it is vital to know the right chemistry when dealing with it. Several methods were probed before an optimal synthesis process was found. A detailed synthesis process optimization is described at the end of this chapter to understand all variables that can alter the dielectric properties of the polyimide nanocomposite films.

### **1.1 Polyimide insulation films and their physical properties**

Thermo-oxidative polyimide films by DuPont are in the market since the 1960s. These low dielectric constant thin films are highly corona resistive, are thermally stable, and have higher breakdown strength electrically and mechanically [6]. Such unique characteristics have made polyimide films to use in large industrial applications such as aerospace, automotive, and microelectronic devices. The fire resistance property of PI and low dielectric constant has made it possible to isolate metal lines and reduce electromagnetic interference effect in electronics and signal processing devices [6]. Polyimide is also used in electric motor insulation for high-speed trains. Polyimide is a high-temperature organic class of polymer that is mechanically robust and thermally stable based on stiff aromatic backbones [6]. The main functional groups in polyimide structure are aromatic ring, amide, and ether groups. There are several monomers and methods available to synthesize the polyimide. Therefore, a slight change in the monomer's structure and synthesis process can alter the physical properties of PI films significantly. Polyimides are chemically closed structure polymers that are nonreactive to many chemicals such as solvents and oils. Polyimides are intrinsically resistive to heat and flame retardants. PI is also resistive to acids but avoids to use in alkalis and inorganic acid environment. The remarkable radiation resistant property of PI has made it an ideal material to use in outer space radiation environment and in nuclear reactors, where PI is used alone as well as in composite forms. The changes in the dimension of material per 1°C rise in temperature are called the coefficient of thermal expansion. PI exhibits higher values of thermal expansion coefficient than other polymers. PI undergoes numerous phase changes to 400°C during the imidization process from polyamic acid solution to thin solid films. PI is a thermally stable polymer that has a very high value of Tg and only 5% weight loss above 400°C [6]. Therefore, it is a very suitable material for packaging applications. PI films have higher mechanical strengths. The stress-strain results have sown that the flawless PI films have mechanical strength in between 100 and 200 MPa and the elongation at break in between 10 and 25% [6]. The mechanical vibrations in electric motors and metal conductor's contact in electronic packaging applications can cause severe damage to

*Synthesis Process Optimization of Polyimide Nanocomposite Multilayer Films, Their Dielectric… DOI: http://dx.doi.org/10.5772/intechopen.91206*

the mechanical strength of PI films. If the films are brittle and the applied force due to mechanical vibrations crosses the fracture limit, then the internal cracks or defects can break the insulation. The brittleness of PI films can be controlled during the synthesis process by using different monomers and imidization temperature and time [6]. The physical properties of polyimide films at room temperature are shown in **Table 1**.

## **1.2 Properties of polyimide in electrical engineering**

### *1.2.1 Dielectric constant and dielectric loss*

When the dielectric material is subjected to an electric field, it becomes polarized due to the movement of induced and permanent electric dipoles [7]. For ideal insulation, the movement of dipoles should be zero or very low to block the conduction current. The value of dielectric constant (*ε*′) defines the polarization ability of dielectric material. The movement of dipoles in an alternating electric field causes the loss of energy known as a dielectric loss (*ε*″). The conduction loss and dielectric loss are two significant losses that are responsible for energy loss in a dielectric material. The movement of charges determines the conduction loss, while the movement of dipoles determines the dielectric loss, the movement of dipoles causes the energy dissipation as the polarization switches its direction in an alternating electric field. The polarization lags the alternate electric field to produce heat, and dielectric loss increases at the relaxation frequencies. Therefore, the value of dielectric constant reduces quickly at relaxation frequencies because the polarization is not able to keep pace with the alternating electric field, as illustrated in **Figure 1**. An efficient insulating dielectric material blocks the conduction with a minimum dissipation of energy. The materials with a higher value of dielectric constant usually have a higher dielectric loss. The energy loss in dielectrics can be used to heat the food in a microwave oven. The orientational polarization in water frequency is utilized for this process, which is close to the relaxation or resonance frequency. It means water molecules absorb a lot of energy, which later dissipated to heat the food. The dielectric constant of polyimide films varies from 3.0 to 3.8 according to the structure and composite fillers added into it [7].

The relative permittivity is composed of two parts: the real part denoted as *ε*′ and the imaginary part indicated as *ε*″. The ratio of these two values is defined as the dissipation factor and given as follows:



**Table 1.** *Properties of polyimide.*

**Figure 1.** *Dielectric loss vs. frequency.*

Typically, PI films have dissipation loss in between 0.001 and 0.02 [8]. The low tan *δ* value indicates that PI loses less electrical energy. The low dielectric constant and low dielectric loss make PI films suitable to use in electrical signal packaging applications to avoid signal interference.

### *1.2.2 Conduction current*

Conduction current attributes to different polarization and depolarization processes happening inside the material. The complete polarization process can be presented as Eq. (2) [9].

$$
\dot{\mathbf{u}}\_p = \dot{\mathbf{u}}\_i + \dot{\mathbf{u}}\_a + \dot{\mathbf{u}}\_c \tag{2}
$$

where *i*i is the instantaneous current due to the displacement polarization, *ia* represents the relaxation polarization current, and *ic* presents the conduction current due to the conductivity of the specimen. The Simons and Tam theory represents that the depolarization current is a superposition of different relaxation processes depending on the trap levels [9]. There are several polarizations due to dipole relaxation process that may take place in a dielectric material as follows:


### *1.2.3 Dielectric breakdown strength*

The breakdown strength is the ability of dielectric material to oppose electric field stresses without any insulation breakage or passing a certain amount of leakage current. The value of dielectric breakdown strength can be found as
