**2.1 Preparation mechanism for polyamic acid (PAA)**

The aromatic PI film can only be synthesized from the solvent route using a twostep method. In the first step, PAA solution is synthesized using a reaction between dianhydride (pyromellitic dianhydride PMDA) and diamine (4,4'-oxydianiline ODA) at room temperature as shown in **Figure 6**. The dipolar aprotic solvents, such as N-methyl pyrrolidone (NMP) and N, N-dimethylacetamide (DMAc) are used to synthesize PAA solution. In the second step, PAA solution is converted into the final PI films after applying thermal imidization to evaporate solvent as explained in **Figure 7**. To make sure that PMDA did not absorb any moisture, it was heated for 2 h at 150°C. ODA was added into the beaker and mixed with DMAc for half an hour. Mechanical stirrer was used to stir the solution. PMDA was added into the solution in three parts, and the solution was further stirred for 24 h to get yellow color high molecular weight PAA solution as shown in **Figure 6**.

The reaction between the monomers to obtain viscous PAA solution is strongly dependent on the precise measurements. The following discussion will highlight important factors to select the monomers, solvents, and reaction conditions to get better results and avoid side reactions. The formation of the PAA solution may also involve some reversible reactions, which leads to the opening of the anhydride ring. Despite that, the forward rate of reaction is larger than the reverse rate depending on the purity of reagents. The molecular weight of the PAA product is also relying on the rate of difference between the forward and reverse reactions. Reverse reaction provokes when carboxyl group strikes to the adjacent polyacid group [6]. Some reagents can be used to stop the reverse reaction and provokes a forward reaction. The amino group basicity and PMDA electrophilicity in different solvents can change the equilibrium constant, and the reaction is exothermic; therefore, it should carry out at room temperature to minimize the equilibrium constant effect.

**Figure 6.** *Synthesis process of PAA solution.*
