*1.3.3 Nanoparticle dispersion and distribution*

The homogenous dispersion and distribution of nanoparticles are the major factor to improve nanocomposite dielectric material properties. A vital part of preparing

**Figure 3.** *Change in composite material properties with respect to particles' size.*

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

**Figure 4.** *Fillers' types, sizes, and shapes.*

nanocomposites is the nanoparticle dispersion; they have heterogeneous surfaces, which cause variability in contact with the polyimide matrices. For better dispersion, several methods have been used such as the orientation of nanoparticles by applying an electric field, chemical treatment by using coupling agents, and plasma treatment. All these methods are adopted to solve the problems of heterogeneous agglomeration and the compatibility between the polyimide matrix and the nanoparticles. The nanoparticle mixing techniques such as mechanical milling, ultrasonication, high-speed stirrer, liquid dissolving, or heat melting can also affect the nanoparticle dispersion level. The size of particles and filler loading percentage can also influence the dispersion level [17]. Particles smaller than 80 nm tend to agglomerate and form more significant chunks of particles. The intermolecular forces keep nanoparticles together. The following particle parameters can influence the overall properties of polymer composites.


The interlinking of the PI and the nanoparticles depends on the functional groups and the surface energy of PI and nanoparticles. Some hydroxyl (OH)

## *Polyimide for Electronic and Electrical Engineering Applications*

functional groups can be formed on nanoparticles after surface modification, which provide a better interface and tightly bound with the PI matrix. To modify nanoparticle surface, different methods have been presented in recent papers such as deposition reaction modification, chemical surface treatment modification, high energy such as plasma source modification, and intercalation modification [14]. Affinity and polarity compatibility can also be used to create materials with homogenous dispersion of nanoparticles. For better interlink between the silica nanoparticles and the polyimide matrix, the surface of polar silica nanoparticles is modified using the KH550 coupling agent. Silica nanoparticles are hydrophilic, and polyimide is nonpolar, which is not compatible with mixing; therefore, the surface of the silica is modified to make it hydrophobic with hydroxyl (OH) functional groups on its surface, which are easy to bond with aromatic polyimide functional groups [14]. This surface modification is adopted to ensure success in the application. The interphase region around nanoparticles can be controlled using surface treatment [14].
