**3. CPI film production**

In general, polymer film manufacturing techniques include several types, such as melting extrusion, casting, and blowing. There are many factors that affect the choice of polymer film production process, including physical and chemical properties, color and appearance requirements of polymer resins, and the existing capabilities of film production equipment. For crystalline polymer resins with clear melting points, such as PET and PEN, non-solvent melt extrusion technology is mainly used. For amorphous polymers with low to medium melting points, such as


*Design, Fabrication, and Application of Colorless Polyimide Film for Transparent and Flexible… DOI: http://dx.doi.org/10.5772/intechopen.93428*

**Table 3.** *Key monomers for Alicyclic CPIs.*

PC and PES, melt extrusion and solution casting are used. As PIs are amorphous polymers with high melting points due to their hard molecular skeleton, the solvent casting process [47] (as shown in **Figure 1**) is usually the best choice, especially in the laboratory.

The solvent casting process to prepare colorless polyimide film in the laboratory can be realized by two conventional film-forming routes, PAA and organo-soluble PI route [48–54]. Similar to the preparation of ordinary PI film, the PAA method to produce CPI just needs to add salivation, drying, stretching, and other steps in the middle of the two steps of PI polymer production, synthesis of polyamide acid, and imidization. In detail, the monomers of dianhydride and diamine are first polymerized in DMAc or NMP to form a PAA solution. A PAA film is then formed by casting the solution on a clean substrate. Finally, an imidization treatment is performed to produce a colorless PI film. Because PAA membranes are prone to

**Table 4.** *Key monomers with large substituent group or side group.*

*Design, Fabrication, and Application of Colorless Polyimide Film for Transparent and Flexible… DOI: http://dx.doi.org/10.5772/intechopen.93428*

### **Table 5.**

*Key monomers in bended main chain of CPIs.*

thermal decomposition or hydrolysis, freshly synthesized PAA is usually used to prepare PI films. High-temperature curing can cause obvious coloring, so the preparation of CPI film should be carried out under vacuum or inert atmosphere. The curing process includes both solvent evaporation and dehydration imidization. In

*Sulfonyl-substituted, meta-substituted, and hyperbranched monomers.*

### **Figure 1.**

*Production of solvent cast films, adopted from [47].*

most cases, the temperature of the imidization process needs to reach 300–350°C [55], under which, even in a vacuum or inert atmosphere, the PI system is prone to discoloration. At the same time, microdefects such as pinhole cracks are also prone to occur during the dehydration process. Therefore, in the preparation process of the colorless PI film, the PAA route is rarely used. The organo-soluble PI solution is only applicable to polyimides that can be dissolved in organic solvents. This method uses a stable high-solid polyimide solution to form a film on a clean substrate.
