Synthesis And Characterization Of Colorless Polyimides Based On Triarylmethane

Attributed to their outstanding thermal properties,polyimides(PIs)are preferred transparent film with high heat resistance.However,conventional aromatic PIs are usually infusible even at high temperature or insoluble in common organic solvents.Furthermore,they exhibit deep color and poor transparency due to the chain rigidity and charge transfer complexes(CTC)formed between the inter/intra-molecular chain,which limits their applications in flexible display,flexible solar cells and other fields.Therefore,it has been an important issue to balance the optical properties and others.In order to clarify the relationship between the molecular structures and properties of PIs,the different type of substitutions have been investigated.The main contents and conclusions are summarized as follows.(1)Three triarylmethane-based diamines with pyridine,benzonitrile or phenol substitution(PyDPM,CyTPM and HyTPM)were synthesised,followed by the polymerization with 3,3',4,4'-biphenyltetracarboxylic dianhydride,3,3',4,4'-diphenylether tetracarboxylic dianhydride(ODPA)and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride(6FDA)under microwave irradiation to obtain of 9 types of PIs.These PIs exhibit excellent solubility in various organic solvents.They can afford flexible films with tensile strength of 75.5~105.5 MPa and elongation at break of 3.6~18.4%.Furthermore,these PIs possess high thermal stability with a glass transition temperature of over 359 oC and a decomposition temperature at 5% weight over 476 oC under nitrogen.In addition,the PI films based on PyDPM and CyTPM show prominent optical transparency with cutoff wavelengths lower than 400 nm,and the transmittance higher than 82% at 450 nm.It has been found that the introduction of pyridine or benzonitrile substitution can simultaneously improve the thermal properties and optical transparency of the PIs.(2)Two triarylmethane-based diamines containing phenoxy and phenyl side groups(PhOTPM and PhTPM)were synthesised,followed by the polymerization with pyromellitic dianhydride(PMDA),BPDA,ODPA and 6FDA under microwave irradiation to obtain 6 types of PIs.These PIs exhibit excellent solubility in various organic solvents.They also possess high thermal stability with a glass transition temperature of over 272 oC and a decomposition temperature at 5% weight over 523 oC under nitrogen.The PIs based on PhOTPM show excellent optical transparency with cutoff wavelengths lower than 400 nm,and the transmittance higher than 84% at 450 nm.The result show that the introduction of a phenoxy group or a phenyl group at the meta position can improve optical transparency and hydrophobicity of PIs.Moreover,the dielectric constant of obtained polyimides can be lowered to 2.87 by introduction of phenyl side groups.