**4.2 Balance of the desired properties of CPI film for TaFE applications**

Extensive studies have been made to improve the transmittance of CPI film. The major strategies to improve the transmission properties of light with the balance of other properties are by employing fluorine containing monomers [63–67], aliphatic monomers [68, 69], non-coplanar monomers [70, 71], adjusting conjugated


### **Table 7.** *The specifications of CPI film for transparent and flexible electronics.*

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

electrochromophores which limit the formation of charge transfer complex [72]. The influence of these monomer molecular changes on the transmittance and thermal stability can be observed more clearly in PI with a similar structural unit [11] (**Figure 3**). As shown in **Figure 3**, introducing fluorine atoms, alicyclic moieties, and flexible main chain is a very effective way to improve the transparency of CPI. The transparency will grow with more of these groups added into the molecule. However, the thermal stability decreases fast with the increase of the transparency of CPI, which is what the researchers have tried for years to overcome. With the deepening of the research of CPI, a unique kind of monomers-isomerized monomers have drawn people's attention. **Figure 4** has shown that changing a CPI monomer to an isomerized one will enhance not only the transparency of the CPI but also the thermal property, which is different from other CPI monomers [73]. This means this kind of noncoplanar structure have no or less influence in the rigidity, and therefore will not sacrifice the mechanical and thermal properties. So it is suggested that using the noncoplanar in the main chain as well as bringing in fluorine atoms, alicyclic groups in the molecular design may be a very useful way to achieve a better comprehensive performance under the synergistic action of all the factors.

As discussed in the previous section, it is very promising to obtain CPI with good mechanical and thermal properties by using noncoplanar structure cooperating with fluorine and alicyclic groups. But these factors are associated with negative effects to the applications as gas barriers and transparent packaging materials. They will lead to worse water/oxygen permeability, even though the water/oxygen permeability is always a pain point of CPI. The usual methods to solve this problem is coating barriers like SiNxOy [74] and Al2O3 [75] or using composite fillers like graphene [75], montmorillonite [76], and other nanosheet fillers. Nanocomposite filler method is more practical in industrial production and can reduce the thermal expansion of CPI while keeping its transparency and thermal properties, so it is a more promising way in the future.

### **Figure 3.**

*The molecules of CPIs with a similar structure but different transmittance and thermal stability, which suggest the influence of the main chain, fluorine atoms, and alicyclic groups.*

**Figure 4.** *The molecules of CPIs with isomeric structures have different transmittance and thermal stability.*

In general, it is thought prospective to produce CPI with high comprehensive performance by using nanosheets as the filler to modify CPI with good thermal and mechanical properties. This kind of PI can be prepared by using the noncoplanar in the main chain as well as bringing in fluorine atoms, alicyclic groups.

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