Microwave-assisted synthesis of high thermal stability and colorless polyimides containing pyridine

A novel aromatic diamine containing pyridyl side group, 4-pyridine-4,4-bis(3,5-dimethyl-5-aminophenyl)methane (PyDPM), was successfully synthesized via electrophilic substitution reaction. The polyimides (PIs) containing pyridine were obtained via the microwave-assisted one-step polycondensation of the PyDPM with pyromellitic dianhydride (PMDA), 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA), 3,3’,4,4’-diphenylether tetracarboxylic dianhydride (ODPA) and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). Contrarily to the reported similar PIs, these PIs exhibit much higher thermal stability or heat resistance, i.e. high glass transition temperatures (Tgs) in the range of 358-473 °C, and the decomposition temperatures at 5% weight loss were over 476 °C under nitrogen. They can afford flexible and strong films with tensile strength of 82.1-93.3 MPa, elongation at break of 3.7%-15.2%, and Young's modulus of 3.3-3.8 GPa. Furthermore, The PI films exhibit good optical transparency with the cutoff wavelength at 313-366 nm and transmittance higher than 73% at 450 nm. The excellent thermal and optical transmittance can be attributed to synthesis method and the introduction of pyridine rings and ortho-methyl groups. The inherent viscosities of PIs via one-step method were found to be 0.58-1.12 dL g−1 in DMAc, much higher than those via two-step method. These results indicate these PIs could be potential candidates for optical substrates of OLEDs.