**2.1 Fluorinated CPIs**

Examples of aromatic CPIs are largely dominated by fluorinated ones, as incorporating the charge negative fluorine atoms into the electron-donating diamine will suppress the formation of CTC and therefore increase the transparency of PI films. Furthermore, due to the larger volume of fluorine atoms, the free volume between the molecules are also increased which improves the dielectric properties of PIs. Despite the improvement of both optical and dielectric properties, the incorporation of fluorine atoms might lead to the lowering of mechanical strength and glass transition temperature (Tg).

### **2.2 Alicyclic CPIs**

A more effective approach to synthesize CPIs is to use nonaromatic monomers either in diamines or dianhydrides. An alicyclic compound is an organic compound that contains one or more all-carbon rings, which may be either saturated or unsaturated but do not have aromatic character. By adopting the alicyclic moieties in the main chain of PI, the probability of undergoing inter- or intramolecular charge transfer (CT) is lower, which leads to an improvement in the optical properties of PIs. However, the incorporation of alicyclic unit generally reduces thermal stability and mechanical strength and both are crucial factors for PI. Therefore, it's important to consider the proportion of alicyclic structure without too much compromise on the thermal and mechanical properties.

### **2.3 Noncoplanar colorless PIs**

Incorporation of noncoplanar structures into polymer chains is considered as one of the effective ways to improve the optical transparency of PIs without too much compromise of their thermal stability and mechanical strength. The nonplanar structure reduces the strong interaction and tight stacking of the molecular chains preventing the forming of CTC.

### **2.4 Other colorless PIs**

Besides the monomers listed, there are also other structures that can be introduced into the PIs to improve the optical transparency of PI films. For instance, sulfonyl-substituted PIs exhibit good combined quality, including good thermal stability and high optical transparency within the visible light region. This is due to

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


**Table 2.** *Key monomers for fluorinated CPIs.*

the existence of charge-accepting sulfur atoms in sulfonyl suppressing the formation of CTC. Another example is incorporating meta-substituted diamine to increase the free volume which lowers the interaction between molecules and effectively reduces the CTC. Moreover, hyperbranched polymer is considered to have low probability of forming CTC due to their asymmetrical spherical loose structures with defect (**Table 6**).
