**2.9 Thermal imidization of a polyamic acid (PAA)**

The PI films can be cured of the PAA solution after applying thermal imidization. After casting the PAA solution onto the substrate, thermal steps with temperature from 100 to 350°C were used to evaporate the solvents. According to the literature, various thermal steps and different temperature ranges have been utilized to achieve 100% imidization from PAA to PI. Two main thermal imidization ways are as follows:

1.Baking films with slowly increasing in temperature up to 350°C according to the flexibility and Tg of the PI film.

### **Figure 8.**

*Film thickness variation at different spin coater speeds (a) PAA solution high molecular weight and (b) PAA solution slight less molecular weight.*

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

2.Start baking PAA with the temperature 100°C for 1 h, then baking at 200°C for 1 hour, then baking at 300°C and keeping for 1 h and slowly cool down to room temperature.

Several arduous factors are involved in the above simple-looking thermal imidization process to predispose the degree of imidization of PI films. During imidization, the remaining solvent at the later stages determines the stability of PI films. In early stages, the imidization process is faster, and several dynamic changing in physical properties happen due to the basicity of the amide solvent to accept protons and the amicable conformation of the amic acid to increase the mobility of the reacting functional groups [6]. At later stages, the rate of imidization slows down due to the decyclohydration of the open amic acid group into the closed imide ring, which decreases the chain mobility, and the Tg approaches the reaction temperature [6].

### **2.10 Determination of the degree of imidization using FTIR**

When one monomer reacts with other monomer, it forms a carbon chain of the polymer. In our case, a single unit of two monomers such as diamine (ODA) and dianhydride (PMDA) reacts with each other to form a single unit of polyamic acid (PAA) solution. In this reaction, an oxygen atom of diamine reacts with the hydrogen atom of dianhydride, and the hydrogen atom of dianhydride reacts with the carbon atom of diamine to give us a repeated unit of PAA. After obtaining PAA, thermal imidization was applied to cure PI and PI/ SiO2 films.

Fourier transform infrared spectroscopy (FTIR) is a promising tool to determine the degree of imidization despite a minor error in measurement due to its sensitivity to chemical change. The bond composition of PI atomic structure was studied by FTIR spectroscopy to analyze the chemical bonds present in PI. The bands most frequently utilized are imide absorption bands near 1780 cm<sup>−</sup><sup>1</sup> (C═O asymmetrical stretching), 1380 cm<sup>−</sup><sup>1</sup> (C▬N stretching), and 725 cm<sup>−</sup><sup>1</sup> (C═O bending). This study describes the chemical characterization of half cured and fully cured PI and PI/SiO2 films. The range of wavelength used was from 650 to 4000 cm<sup>−</sup><sup>1</sup> . The PI subatomic structure is composed of a molecular chain containing the functional groups such as aromatic rings, amide rings, and some noncyclic ether rings. These functional groups have discrete chemical bonds such as C▬N▬C, C═O, benzene (C6Hn), ether link (C▬O▬C), and ▬OCH2▬CH2 deformation. The functional groups of PI polymer chains provide the level of linkage with subatoms as well as the linkage with silicon oxide nanoparticles. Imide group contains imide carbonyl in-phase and out-of-phase stretching, the C▬N▬C axial, transverse, and out-of-phase stretching. The absorbance peaks at 1371, 1112, and 721 cm<sup>−</sup><sup>1</sup> indicate the transverse and axial stretching of C▬N▬C bond, and the reasons of 1781 and 1720 cm<sup>−</sup><sup>1</sup> peaks are in-phase and out-of-phase stretching of C═O in imide ring. Aromatic ring is divided into tangential, radial skeletal, and outof-plane vibrations. The peak at 1590 cm<sup>−</sup><sup>1</sup> relates to tangential C▬C vibrations. The peak at 1280 cm<sup>−</sup><sup>1</sup> is tangential phenyl ring vibrations. The other 1480, 1168, 1087, 1011, 843, and 788 cm<sup>−</sup><sup>1</sup> peaks describe about the tangential, radial skeletal, and out-of-plane bending vibrations of C6Hn in aromatic ring. The peaks at 1234 and 1454 cm<sup>−</sup><sup>1</sup> relate to the bond of ether link (C▬O▬C), and 1416 cm<sup>−</sup><sup>1</sup> peak corresponds to ▬OCH2▬CH2 deformation. **Figure 9(a)** and **(b)** shows the FTIR spectra of changes in the bonds of half cured and fully cured PI films at different temperatures and time to determine the degree of imidization to obtain imide ring.

**Figure 9.** *(a) Half cured PI film at different temperature and time and (b) zoom version of main function group peaks.*

In fully cured PI films, as shown from **Figure 9**, the bonds are grouped into amide rings, aromatic rings, and noncyclical stretching based on atom vibrations [18]. The intensity of the absorption band at 3463 cm<sup>−</sup><sup>1</sup> corresponding to the stretch vibration of ▬OH bond.
