**2. Chemical generalities of polyimide**

### **2.1 Thermal stability**

The thermal stability of a polymer is defined as its ability to withstand high temperatures without initiating degradation processes such as thermolysis. It is usually evaluated by thermogravimetric analysis. This method consists in measuring the mass loss of a material either as a function of temperature or as a function of time at a constant temperature. By convention, a polymer is said to be thermostable if it can be used without losing its properties for 1000 hours at 300°C, 10 hours at 400°C, and a few minutes at 500°C. In the case of polyimides, the decomposition temperature generally appears between 500°C and 600°C. However, their typical

### **Figure 3.**

*Maximum working temperature for the main developed polymers over the last century including polyimides (*reproduced and modified from *[1]).*

maximum usable temperature is usually between 250°C and 275°C, as shown in **Figure 3** where all the main thermostable polymers are also displayed for comparison. Thus, polyimides appear as the thermostable polymer having the highest maximum working temperature.

It has been shown that the increase in the number of benzene rings in polyimide monomer macromolecules contributes to increase their degradation temperature [1]. However, the degradation temperature can also be affected by the presence of low thermostable chemical bonds in the macromolecular structure like the CdOdC ether group [3]. Finally, a few studies even present thermal stability as high as 300°C that report on the potential use of polyimides in high temperature electronic applications for electrical insulation purposes [4, 5].

### **2.2 Synthesis and imidization reaction**

The two-step synthesis method is the simplest and most commonly used method to obtain polyimides in industry. In 1955, Edwards et al. were the first to synthesize polyimides (PI) from polyamide salts [6]. Endrey was the first to successfully synthesize high molecular weight aromatic polyimides [7]. In the method described, the synthesis is carried out in two stages.

The first one is to prepare a polyamic acid (PAA) solution, which is the precursor of polyimide. The synthesis of PAA takes place via the reaction between two precursor monomers, a dianhydride and a diamine at room temperature and in polar aprotic solvents such as N-methyl-2-pyrrolidone (NMP), N,Ndimethylformamid (DMF) or N,N-dimethylacetamid (DMAc).

PAA is then cyclodehydrated using a thermal or chemical conversion process, called "imidization," to form the final, insoluble, and infusible polyimide. The steps for the synthesis and imidization of polyimides by this method are presented in **Figure 4**, where dRd and dR<sup>0</sup> d represent the radicals of the dianhydride and diamine monomers, respectively.

### **2.3 Main radicals for dianhydride and diamine precursor monomers**

Many varieties of PAA can be synthesized leading to hundreds of different polyimide combinations. **Tables 1** and **2** list the main radicals dRd and dR<sup>0</sup> d of the dianhydride and diamine monomers, respectively, marketed for the synthesis of PAA. *Polyimide in Electronics: Applications and Processability Overview DOI: http://dx.doi.org/10.5772/intechopen.92629*
