Abstract
A series of colorless copolyimide films with high thermal stability and good solubility are synthesized from (trifluoromethyl)biphenyl-4,4’-diamine (TFMB) with different 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) to 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropane (6FDA) dianhydride mole ratios through one-pot solution polycondensation. These copolyimide films exhibit excellent optical transparency (T400 > 90% and λ0 ~305–333 nm) with a thickness of 15 μm and good solubility in most organic solvents. The excellent optical properties are mainly attributed to the low inter- and intra-molecular charge transfer interactions due to the alicyclic structure and the strong electronegative CF3 groups. The glass transition temperature increases from 332 to 352 °C with increasing HPMDA content in the copolymers, while the thermal decomposition temperature is improved with increasing 6FDA content. These results indicate that the copolyimide films can be successfully utilized in the development of novel heat-resistant plastic substrates for the optoelectronic engineering applications.
Highlights
Polyimide (PI) has been extensively applied in fields like aerospace, microelectronics and optoelectronics owning to its excellent combination properties, such as high thermal stability, low dielectric constant, excellent chemical resistance and mechanical properties [1,2,3].Conventional aromatic polyimide films generally exhibit deep coloration generated by the formation of intra- and inter-molecular charge transfer complex (CTC) due to their highly conjugated molecular structures, which significantly limits their applications in which optical transparency is very important, such as optoelectronics devices [4,5,6]
A series of homopolyimides and copolyimides were synthesized from TFMB diamine and two dianhydrides, 6FDA and HPMDA in DMAc with different HPMDA/6FDA mole ratios through a one-pot method
With the increasing content of alicyclic monomer (HPMDA), the molecular weight of the polyimide decreased due to the low reactivity of HPMDA compared to 6FDA
Summary
Polyimide (PI) has been extensively applied in fields like aerospace, microelectronics and optoelectronics owning to its excellent combination properties, such as high thermal stability, low dielectric constant, excellent chemical resistance and mechanical properties [1,2,3]. Combining fluorinated aromatic monomers with alicyclic monomers is an effective way to achieve semi-alicyclic PIs with desired optical transparency, thermal stability and solubility. Two-step thermal imidization is unsuitable for semi-alicyclic PI synthesis due to the low reactivity of alicyclic monomers, which results in the low molecular weight of the semi-alicyclic PI and brings difficulty for film-forming [32]. For this problem, one-pot solution polycondensation was developed [33]. Most of the obtained films possess excellent overall performances, which expand their applications in optoelectronic engineering
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