Highly transparent polyimides derived from 2-phenyl-4,6-bis(4-aminophenoxy)pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene: preparation, characterization, and optical properties

To investigate the structure-property relationships of polyimides containing pyrimidine or pyridine moieties, two diamine monomers, 2-phenyl-4,6-bis(4-aminophenoxy) pyrimidine and 1,3-bis(5-amino-2-pyridinoxy)benzene, were designed and synthesized. The diamines reacted with different commercially available aromatic dianhydrides to yield a series of heterocyclic polyimides via a classical two-step polymerization method. The polyimides, PI-1 and PI-6, exhibited excellent solubility in strong polar solvents, such as N, N-dimethylacetamide, N, N-dimethylformamide and N-methyl-2-pyrrolidone. The flexible, strong and transparent polyimide films were formed with UV-visible absorption cut-off wavelengths at 344-399 nm. Glass transition temperatures (T-g) of the polyimides PI-(1-6), derived from PBAPD and BADB with dianhydrides, in the range of 212-260 degrees C were obtained by DSC, and the temperatures for 5% wt loss of the polyimides in nitrogen and air atmospheres were obtained from TGA in the range of 430-503 degrees C and 422-465 degrees C, respectively. Moreover, the polyimide films showed a low moisture absorption of 0.18-1.38% and outstanding mechanical properties with tensile strengths of 81-111 MPa, an elongation at break of 4.3-8.7%, and a tensile modulus of 1.3-2.4 GPa. The coefficients of thermal expansion (CTEs) of the polyimides ranged from 28 to 62 ppm degrees C-1.