Colorless PI

Polyimide (PI) film has been broadly used in industries due to its high mechanical strength, thermal stability, low coefficient of expansion (CTE), etc.

However conventional PI film has yellowish to brown color which limits its application in industries, such as display, optoelectronics, aerospace et. al.

Table 1. Key features for a-Si TFTs, LTPS TFTs,  OTFT, and IGZO TFTs

Table 1 shows the temperature requirements for the fabrication of different types of TFTs which drives the pixels in displays (reference 1). A polymer film with Tg higher than 300C is needed to survive the processing conditions of TFT fabrication. PET, PEN, and PC have good optical transparency but they can not survive over 200C. It is desirable to have film materials with excellent optical properties, together with high temperature resistance; among them colorless PI (CPI) are one of the best candidates. Other options can be PEI, PAI, for example.

donor/acceptor Structure

Figure 1. Donor/acceptor structure

PI is synthesized by dianhydrides and diamines as shown in typical Kapton-H structure. Dianhydride moiety is considered as electron acceptor while the diamine moiety is donor. Once donor and acceptor are in each other’s vicinity, donor/acceptor pair or charge transfer complex (CTC) forms in either intramolecular or intermolecular fashion with suitable packing, as shown in the Figure 2.

Figure 2. Intra- and inter-molecular charge transfer complex (Reference 2)

The Energy diagram of the CTC can be shown in Figure 3.

Energy diagram of CTC
Figure 3. CTC energy level diagram

Donor (diamine moiety) and acceptor (dianhydride moiety) can have their own ionization potential (or HOMO or upper edge of valence band) and electron affinity (or LUMO or lower edge of conduction band); their own energy gap (IP-EA) can be large (>3.3 ev) and have no absorption in visible range. But once they form CTC, as shown in Figure 3, the CTC’s absorption edge is determined by donor’s IP and acceptor’s EA, which might be less than 3.3 ev; consequently the PI may have absorption peaks in visible range and present colors.

Therefore, the strategies to make colorless PI will be either to avoid the formation of CTC or to tune the IP and EA’s value to let the energy gap larger than 3.3 ev, with the consideration of C as shown in equation 1 in Figure 3.

Strategies to make CPI

Bent/Distorted monomer structure

Planar structure facilities (1)the delocalization of electrons in the conjugated structure (2) dense packing and CTC formation accordingly. In other words, the effective conjugation length is longer than non-planar structure, even the repeating single-double bond units are same. The different position of substitution, o-, m-, p-, leads to different degree of molecular bent or distortion. The distortion level can roughly consider as 0->m->p-, assuming the side group is same. Bulky side chain can cause more torsion or bending, reducing conjugation length and loose packing to reduce CTC formation.

Fluorinated Materials

Fluorinated materials

Introducing fluorinated side groups, e.g. -CF3, can move IP downward and EA upward, enlarging the band gap of CTC; and the bulky -CF3 groups also can reduce the possibility of the formation of CTC structure due to steric effects. The above are examples of fluorinated PI monomers or polymers. (reference 1, 2, and 3)

Alicyclic structure

Non conjugated alicyclic structure

The introduction of alicyclic structure to make CPI is straightforward, since the alicyclic structure has no conjugation or the IP-EA is very high, as shown the above Figure.

Meanwhile, other conditions are also critical for making CPIs, e.g.

  • Impurity of monomers
  • Kind of solvent used in synthesis
  • Curing conditions（temperature, atmosphere）
  • Oxidized residue solvents
  • Thermal decomposition products（in imidization or annealing stages）

Reference

  1. Journal of Industrial and Engineering Chemistry 28 (2015) 16–27
  2. RSC Adv., 2015, 5, 57339
  3. Prog. Polym. Sci. 26 2001, 259-335